Symposium Organizers
Paul Fons National Institute for Advanced Industrial Science and Technology
Kris Campbell Boise State University
Byung-ki Cheong Korea Institute of Science and Technology
Simone Raoux IBM T. J. Watson Research Center
Matthias Wuttig I. Physikalisches Institut der RWTH Aachen
H1: Theory I
Session Chairs
Tuesday PM, April 06, 2010
Room 2009 (Moscone West)
9:15 AM - **H1.1
Electronic and Optical Properties of Ge-Sb-Te Phase Change Materials: A Simulation Point of View.
Jean-Yves Raty 1 , Celine Otjacques 1 , Rengin Pekoz 1 , Engin Durgun 1 , Jean-pierre Gaspard 1 , Matthieu Micoulaut 2 , Christophe Bichara 3
1 Physics Department, University of Liege, Sart-Tilman Belgium, 2 Laboratoire de Physique Theorique de la Matiere Condensee, Universite Pierre et Marie Curie, Paris France, 3 CINAM-CNRS, Universite Aix-Marseille, Marseille France
Show AbstractDuring the last years, many simulations were performed to model and explain the behavior of phase change materials. The bulk amorphous phase of GST’s has been extensively studied by ab initio molecular dynamics yielding structural and electronic properties compatible with the available experiments. In this work, we present the results of a series of simulations of GeTe-Sb2Te3 pseudobinary amorphous systems and address the relation between the electronic properties (localization, conductivity) of the amorphous phase and those of the relevant crystal phase. We determine the constraints in the amorphous according to new counting procedure and show how these correlate with the phase change properties.We also show results of simulations of 2D (wires) and 1D (dots) GeTe systems, showing the dependence of the optical contrast with the surface properties. Additionally, the crystalline GeTe nanoplatelets are shown to exhibit unusual ‘ferrotoroidic’ polarization patterns, making them of potential interest for 3-states data recording.
9:45 AM - **H1.2
Understanding Phase-change Materials in Ge-Sb-Te System by First Principles Methods.
Zhimei Sun 1 , Jian Zhou 1 , Andreas Blomqvist 2 , Naihua Miao 1 , Baisheng Sa 1 , Borje Johansson 2 , Rajeev Ahuja 2
1 Department of Materials Science and Engineering, Xiamen University, Xiamen China, 2 Deparment of Physics and materials Science, Uppsala University, Uppsala Sweden
Show AbstractPhase-change materials are getting more and more attention due to their potential applications in next generation memory devices. However, there are many discrepancies exist on the crystalline and amorphous structures as well as the mechanism of the fast reversible phase transition between the two states. These fundamental issues are important for tailoring the properties of phase-change alloys as well as for searching new phase-change materials with better performances. In this talk, recent results of the present authors, which include ab initio and ab initio molecular dynamics studies on the crystalline and amorphous structures [1-5], the random occupation of Ge and Sb at the same sublattice and its influence on the phase stability and electronic structure as well, pressure induced amorphization by ab initio molecular dynamics, will be presented and discussed. [1] Z. M. Sun et al., Phys.Rev.Lett.102 (2009) 075504; [2] Z. M. Sun et al., Phys. Rev. Lett. 98 (2007) 055505. [3]Z. M. Sun et al., Phys. Rev. Lett. 96 (2006) 055507; [4] Z. M. Sun et al., Appl. Phys. Lett. 93 (2008) 241908; [5] Z. M. Sun et al., Appl. Phys. Lett. 93 (2008) 061913.
10:15 AM - H1.3
Density Functional/Molecular Dynamics Simulations of Phase-change Materials: Characterization of Disordered Phases.
Jaakko Akola 1 2 , Robert Jones 1
1 Nanoscience Center, University of Jyväskylä, Jyväskylä Finland, 2 Department of Physics, Tampere Technological University, Tampere Finland
Show AbstractThe technological applicability of phase-change material (PCM) is based on the rapid amorphous-to-crystalline transition and subsequent changes in optical (and electrical) properties. The structure of the amorphous phase poses the main problem for scientists and is difficult to tackle both experimentally and theoretically. I have described previously results for the Ge2Sb2Te5 (GST-225, DVD-RAM) and GexTe1-x alloys, obtained from massively-parallel density functional (DF) / molecular dynamics (MD) simulations on the IBM Blue Gene supercomputers. The atoms in GeTe-based materials can generally be classified into atomic types A (Ge,Sb) and B (Te), with strong AB alternation, and the main structural motif of such materials is a four-membered ABAB ring (``ABAB square''). Many Ge atoms can be described as ``tetrahedral'' (coexisting with ``octahedral''), Sb and Te coordination numbers deviate from the 8-N rule, and small cavities (voids, vacancies) characterize these materials. The rapid amorphous-to-crystalline transition can be viewed as a re-orientation (nucleation) of disordered ABAB squares supported by the space provided by the cavities, and the metastable NaCl structure corresponds to the ordered case.I discuss the theoretical methods and their limitations, particularly in the context of DF methods and available exchange-correlation functionals. Tellurium is the major component of many PCMs, and elemental Te (with an anomalous density maximum near the melting point) provides a good case to study. I also discuss two technologically important PCMs: Ge_8Sb_2Te_11 (GST-8,2,11, Blu-ray Disc) and Ag_3.5In_3.8Sb_75.0Te_17.7 (AIST, DVD±RW) alloys. The former has been studied using a full melt-quench simulation for the pseudobinary GST-8,2,11 alloy (630 atoms, over 400 ps), while AIST has been modeled in its liquid phase (640 atoms, 850 K). Structural details in amorphous GST-8,2,11 are very similar to GST-225, indicating that the addition of only a few percent of Sb changes the properties of GeTe significantly. The structure factor and pair distribution function of liquid AIST agree well with HEXRD measurements at 589°C (862 K). Medium-range order is evident, and Ag and In atoms (dopants) prefer to be near Te atoms rather than Sb.Finally, I present new results for the as-deposited GST-225 (648 atoms, computer-assisted as-deposition). They show crucial differences between the as-deposited and melt-quenched amorphous samples and highlight the important factors for the amorphous-to-crystalline phase transition.
10:30 AM - H1:Theory
BREAK
11:00 AM - H1.4
Understanding Amorphous Phase-change Materials from the Viewpoint of Maxwell Rigidity.
Matthieu Micoulaut 2 , Jean Yves Raty 3 , Celine Otjacques 3 , Christophe Bichara 1
2 Laboratoire de Physique Théorique de la Matière Condensée, Université Pierre et Marie Curie, Paris France, 3 Physique de la Matière Condensée, Université de Liège, Sart Tilman Belgium, 1 , CINaM CNRS, Marseille France
Show AbstractPhase-change materials (PCMs) have been identified as potential active layers for the next-generation of non-volatile solid-state memory devices, known as Phase Change Random Access Memories (PRAMs). In practice, data storage is achieved by a fast and reversible change between crystalline and amorphous phases, and the major functionalities (optical or electrical resistivity contrast, re-crystallization speed) depend crucially on composition. Despite intensive experimental and theoretical efforts, a generic description of compositional related structural effects in the amorphous phase is still lacking. Here [1] we show that the phase diagram of the most popular PCMs, the Ge-Sb-Te system, can be split into two compositional regions having a well-defined mechanical character: a Tellurium rich flexible phase, and a stressed rigid phase that encompasses the known phase change materials. This new atomic scale insight is based on a mechanical constraint enumeration derived from First Principles Molecular Dynamics simulations and, as it is based on a very general ground, it should open new avenues for the understanding of PCMs from the viewpoint of rigidity. [1] Manuscript available at arXiv.org > cond-mat > arXiv:0909.5080v1
11:15 AM - H1.5
Ab-initio Study of the Structural and Vibrational Properties of Amorphous Phase Change Materials: Sb2Te3, GeTe and InGeTe2.
Marco Bernasconi 1 , Sebastiano Caravati 2 , Riccardo Mazzarello 2 , Elena Spreafico 1 , Michele Parrinello 2
1 Materials Science, University of Milano-Bicocca, Milano Italy, 2 Chemistry and Applied Biosciences, ETHZ, Zurich Switzerland
Show AbstractWe will present results on the structural and vibrational properties of models of the amorphous phase of Sb2Te3, GeTe and InGeTe2, generated by quenching from the melt within ab-initio molecular dynamics simulations. InGeTe2 in particular is of interest for high temperature PCM applications [1]. It turns out that Sb and Te atoms are in a defective octahedral-like geometry similar to that found in the most studied Ge2Sb2Te5 (GST) compound [2,3]. Coexistence of defective octahedral sites with tetrahedral sites for In and Ge are found in InGeTe2 and GeTe. The calculated Raman spectrum of GeTe is in very good agreement with experimental data [4] and displays signatures of all the peculiar local structures of the amorphous phase, namely tetrahedral Ge and defective octahedral sites for a fraction of Ge (mostly 4-coordinated) and for all Te (mostly 3-coordinated) atoms. In particular, the spectrum above 190 cm−1 is dominated by tetrahedral structures, while the most prominent peaks around 120 and 165 cm−1 are mainly due to vibrations of atoms in defective octahedral sites. Concerning medium range order, amorphous Sb2Te3 displays a larger concentration of nanosized cavities with respect to a-GST and a-GeTe which corroborates the previously proposed [3] correlation between the concentration and size of cavities and the experimental crystallization speed. [1] T. Morikawa et al, IEEE Electron Devices Meeting, IEDM 2007, 307-310 (2007).[2] S. Caravati, M. Bernasconi, T. D. Kuehne, M. Krack, and M. Parrinello, Appl. Phys. Lett. 91, 171906 (2007); ibidem, J. Phys. Cond. Matt. 21, 255501 (2009); ibidem, Phys. Rev. Lett. 102, 205502 (2009).[3] J. Akola and R. O. Jones, Phys. Rev. B 76, 235201 (2007).
11:30 AM - H1.6
Resonant Bonding as the Cause of Optical Contrast in Phase Change Memory Materials.
John Robertson 1 , Bolong Huang 1
1 , Cambridge University, Cambridge United Kingdom
Show AbstractGe2Sb2Te5 and similar phase change memory materials are used for both optical and electric non-volatile storage. The optical memory is based on the large optical contrast, the large difference in reflectivity between the amorphous and crystalline state. The optical dielectric constant is 2 – 3 times higher in the crystalline phase than in the amorphous phase, according to data from Shportko et al [1]. There have been numerous attempts to describe the differences in bonding between the two phases, typically by EXAFs, in which the Ge sites change from 6-fold to 4-fold coordinated and the average coordination reduces in the amorphous phase. However, first principles molecular dynamics simulations do not find such a large difference as the experimental interpretations propose. Secondly, photoemission finds only a ~10% difference in the valence band density of states between the phases, not enough to account for the large change in optical properties. We show using simple molecular models that the optical contrast arises from a change in second neighbor coordination, and the loss of ordering of chains of p orbitals present in the crystalline phase. The orthorhombic structure of GeTe is a good model for the amorphous phase and is calculated to have a factor 3 lower dielectric constant than the stable rhombohedral phase. These differences are repeated for the other materials. The contrast arises because ordered p-orbital chains have unusually large optical matrix elements. This also allows us to build simple models to account for the electrical behavior.1 K Shportko, S Kremers, M Woda, D Lencer, J Robertson, M Wuttig, Nature Mats 7 653 (2008)
11:45 AM - H1.7
Design of Phase-change Materials.
Dominic Lencer 1 , Martin Salinga 1 , Blazej Grabowski 2 , Tilmann Hickel 2 , Joerg Neugebauer 2 , Matthias Wuttig 1 3
1 I. Institute of Physics (IA), RWTH Aachen University, Aachen, NRW, Germany, 2 , Max-Planck Institute for Iron Research, Duesseldorf, NRW, Germany, 3 JARA-FIT, RWTH Aachen University, Aachen, NRW, Germany
Show AbstractAt present, considerable effort is put into the development of data storage devices based on the fast reversible switching between an amorphous and crystalline state. Since the device properties are closely related to the properties of the phase-change material employed, material characterization and selection is an issue of crucial importance. This has inspired us to develop material design rules.The identification of such rules was aided by the finding that suitable materials exhibit generic features in the crystalline state regarding their structure and electronic properties. The coordination is usually sixfold save for slight Peierls-like distortions, while at the same time the number of electrons per site is about three. In this situation, unsaturated covalent bonds are formed, giving rise to resonance effects ('resonant bonding'). In this situation we observe the enhancement of the optical dielectric constant of the crystalline state both theoretically by density functional theory (DFT) calculations and experimentally by infrared spectroscopy as a fingerprint of resonant bonding. The contrast between the amorphous and crystalline phase thus originates from the fact that only the crystalline phase possesses the medium-range order required for resonant bonding to occur [1]. Since this is a rather unique bonding mechanism, a map is proposed to identify materials exhibiting this bonding [2]. This map can also be used to predict property trends. We have extended this approach by investigating the impact of the local distortions on the material properties by DFT calculations. In particular, we study the characteristic shape of the energy landscape of the crystalline phase and relate it to the lattice dynamics.[1] Shportko et al., Nat. Mat. 7, 653-658 (2008)[2] Lencer et al., Nat. Mat. 7, 972-977 (2008)
12:00 PM - H1.8
Structural and Dynamic Features from FPMD Calculations of Binary Sb-Te Alloys in Liquid and Amorphous Phases.
Celine Otjacques 1 , Jean-Pierre Gaspard 1 , Jean-Yves Raty 1
1 Département de Physique B5, University of Liège, Sart-Tilman Belgium
Show AbstractThe binary compounds Sb2Te and Sb2Te3 have been used for decades in memories. They are now used in phase-change recording applications, either doped or mixed with other chalcogenides alloys [1]. Many studies have been performed to explain the phase change ability of Ge-Sb-Te alloys, depending on their amorphization/re-crystallization speed, amorphous phase stability and electrical or optical contrast between quench and crystalline phases [2]. The re-crystallization of amorphous marks in phase change materials is either desired (i.e. in case of an erasing operation with heating) or not (if this phenomenon appears as an uncontrolled way at room temperature, making the material instable with time). To understand the suitability of alloys for phase change applications, the activation energy of crystallization and stability with time are thus important parameters. We compare the evolution of structural and dynamical characteristics of Sb2Te and Sb2Te3 alloys, obtained from FPMD simulations, while decreasing temperature from liquid to amorphous phase. A very strong local order is actually observed in the amorphous phase for both compounds, close to the one observed in the crystal phase. This could explain the ability of these two Sb-Te alloys to switch from amorphous to crystal phase so easily. We also present dynamical results that could help to understand why Sb2Te is suitable for phase change applications, while Sb2Te3 is not, due to the too low glass transition temperature for this compound.[1] M. Wuttig and N. Yamada, Nature Materials, 6 (2007) 824-832[2] W. Welnic and M. Wuttig, Materials Today, 11, 6 (2008) 20-27
12:15 PM - H1.9
Density Functional Simulation of Ag in Ge2Se3.
Arthur Edwards 1 , Kristy Campbell 2
1 AFRL/RVSE, Air Force Research Laboratory, Kirtland AFB, New Mexico, United States, 2 , Boise State University, Boise, Idaho, United States
Show AbstractSeveral materials systems containing silver, such as TiO2:Ag, and Ge(x)Se(1-x):Ag, have exhibited large changes in resistance with applied electric fields. These systems hold promise for both digital memory and for continuously variable resistor applications. Ge2Se3:Ag has exhibited especially reproducible device characteristics, including threshold voltage and on/off resistance ratios. Mitkova et al. have studied a-Ge(x)Se(1-x) compounds with photo-diffused Ag. Using XRD, they have identified peaks associated with two crystalline forms of Ag2Se. Furthermore, based on Raman data, they claim that Ge-Ge bonds persist after Ag diffusion in Se-rich glasses. However, Campbell has claimed that Ag modifies the network in Ge2Se3 by breaking Ge-Ge bonds and forming Ag-Ge bonds. We report density functional calculations on isolated Ag and on Ag dimers in Ge2Se3. Using a crystalline model derived from Si2Te3, we have calculated energies of formation of interstitial Ag, Ag(i), of pairs of Ge dangling orbitals and of the Ge interstitial in pure Ge2Se3 and in the presence of a silver ion. We found that interstitial Ag auto-ionizes because the highest occupied state is above the Ge2Se3 conduction band edge. Thus, in the neutral state, Ag(i) donates an electron, leading to itinerant electronic conductivity. Dangling orbitals introduce states near mid-gap, and act as traps for the electrons donated by Ag. From total energy calculations, the energy of formation of a pair of dangling orbitals requires 1.25 eV in the neutral charge state. However, the presence of Ag(i) alters the thermodynamics considerably, because the interstitial is attracted to the dangling orbital site, lowering the total energy of reaction to 0.72 eV. Finally, the lowest energy configuration of a disrupted Ge-Ge bond is found to be a substitutional Ag(Ge) plus a Ge interstitial. It costs only 0.1 eV to transform an isolated Ag interstitial to the Ag(Ge) + Ge(i). In this case the Ag atom forms a strong bond to the neighboring Ge atom. Based on thermodynamics, we predict that Ag moves easily through the lattice, and that it easily disrupts the network connectivity, creating dangling Ge orbitals and other point defects.
12:30 PM - H1.10
Atomistic Origins of the Phase Transition Mechanism in Ge2Sb2Te5.
Juarez L. F. Da Silva 1 3 , Aron Walsh 2 , Su-Huai Wei 3 , Hosun Lee 4
1 Institute of Physics of Sao Carlos, University of Sao Paulo, Sao Carlos, SP, Brazil, 3 Basic Science, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Chemistry, University College London, London United Kingdom, 4 Dept. of Applied Physics, Kyung Hee University, Suwon Korea (the Republic of)
Show AbstractThe fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge2Sb2Te5 has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the meta-stable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high-energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered 4-fold motifs and the long-rangestructural disorder. Intrinsic vacancies, which originate from Sb2Te3, lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate configuration can be obtained experimentally by applying an intense laser beam, which overcomes the thermodynamic barrier from the octahedron to tetrahedron sites. The high figure of merit of Ge2Sb2Te5 is achieved from the optimal combination of intrinsic vacanciesprovided by Sb2Te3 and the instability of the tetrahedron sites provided by GeTe.
12:45 PM - H1.11
Electronic Transport in Nanoglasses of Phase Change Memory.
Mark Simon 1 , Marco Nardone 1 , Victor Karpov 1 , Ilya Karpov 2
1 Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio, United States, 2 , Intel Corporation, Santa Clara, California, United States
Show AbstractOperations of phase change memory (PCM) depend on electronic transport in its constituting inclusions of chalcogenide glasses (CG). The room temperature conduction in bulk CG is dominated by spatially homogeneous band transport [1]. Here, we discuss how the latter assertion changes for the case of very small CG samples deep in the submicron range of the modern PCM dimensions. Our consideration takes into account the effects of pinholes known to dominate transversal conduction of amorphous films in submicron range [2] and described theoretically in extensive work summarized in [3].Pinholes are formed by rare clusters of abnormally close localized states and exhibit transport channels exponentially amplifying local hopping far from the Fermi level. The integral transport due to many different pinholes is dominated by the optimum channels compromising between the exponentially large probability of tunneling between close centers of the cluster and not too small probability of finding such a cluster. The conductance due to such optimum channels can be written as σ=σ0exp(-√L/√a) where L is the channel length, a=a0/ln(gεa03),a0 ~ 1 nm is the electron localization radius, g is the density of the localized states (cm-3 eV-1), and ε ~ 0.01 eV is the characteristic phonon energy. When the electric field Φ is strong enough, the channel length L=EF/qΦ is shorter than the glass thickness l and is field dependent where EF is the distance between the Fermi energy and the mobility edges; otherwise L=l .Our theory leads to multiple predictions including the following: (1) Given the device thickness l, there exists a critical voltage across the device, Vc=EF/ql, below which the conductance σ is field independent and can exponentially depend on l, while it is thickness independent and is exponentially field dependent obeying the Poole-Frenkel type dependencies above Vc. (2) Depending on the material parameters and device thickness, the activation energy of conductivity can be either EF or smaller. (3) Below Vc, the relative fluctuations in conductivity (noise) scale with device thickness as and are voltage independent, while they are thickness independent and voltage dependent above Vc.Three of us (M.A.S, M.N., and V.G.K.) gratefully acknowledge the Intel Corporation grant supporting this research.REFERENCES[1] N. F. Mott and E. A. Davis, Electronic Processes in Non-crystalline Materials (Clarendon Press, Oxford, 1979)[2] M. Pollack and J. J. Hauser, Phys. Rev. Lett. 31, 21 (1973).[3] M. E. Raikh and I. M. Ruzin, in Mesoscopic Phenomena in Solids, edited by B. L. Altshuller, P. A. Lee, and R. A. Webb (Elsevier, 1991), p. 315.
H2: Structure I
Session Chairs
Tuesday PM, April 06, 2010
Room 2009 (Moscone West)
2:30 PM - **H2.1
Interface Engineering of Phase-change Memory Materials.
Stephen Elliott 1 , Jozsef Hegedus 1
1 Chemistry, University of Cambridge, Cambridge United Kingdom
Show AbstractWe describe ab initio molecular-dynamics simulations of the heterogeneously-nucleated crystallization of the phase-change materials, Sb-Ge and Sb, using crystal templates in the simulation cell. Using such interface engineering, the crystallization can be directed to the metastable cubic form (like rocksalt GST), instead of the normal rhombohedral A7 structure. This crystallization process is much faster, and the cubic phase product is more metallic, than to the A7 form. Such engineered phase-change materials may be suitable as a non-volatile replacement of DRAM.
3:00 PM - **H2.2
Epitaxy of Phase Change Materials.
Wolfgang Braun 1
1 , Paul-Drude Institute for Solid State Electronics, Berlin Germany
Show AbstractWhen reducing the size of a phase change memory device active region, its dimensions will ultimately approach the grain size of the crystalline phase. At the same time, the surface of the active material becomes large compared to its volume and the device properties may be dominated by the interaction of the phase change material with the surrounding matrix. It is therefore important to study the effects of interface interactions on the structure and switching properties of phase change materials. At the same time, epitaxial orientation of the film unit cells allows us to perform structural analysis on films with a single crystalline orientation, which is especially important for the metastable cubic form of Ge-Sb-Te phase change materials (GST), a phase that cannot be synthesized in bulk form.We investigate the epitaxial growth of GST on closely matching substrates such as GaSb. GST grown on such substrates in the appropriate temperature window crystallizes in the metastable cubic phase with a single orientation determined by the substrate. On GaSb(001), GST with a stoichiometric deposition flux ratio of 2:2:5 (Ge:Sb:Te) forms amorphous films up to a temperature of around 150 °C. Between approximately 150 and 230 °C, it crystallizes in the cubic phase, with the best epitaxial orientation around 200 °C. At the same time, the growth rate decreases. Above approximately 230 °C, the growth rate is zero.The density of the layers is up to 25 % below the value expected for cubic crystalline GST, indicating that the creation of vacancies in this material costs little energy. The rapid decrease of the growth rate just below 200 °C can be modeled in a simple estimate by a balance between a hypothetical GST vapor pressure and the supplied fluxes. This postulated GST vapor pressure is between the one for Sb and Te, suggesting that GST forms a molecular crystal in which the Ge is more tightly bound in a molecular environment as compared to the binding energy between these units.In situ x-ray diffraction of epitaxial samples grown on GaSb(001) reveal a rhombohedral distortion of the unit cell leading to a characteristic broadening of the reflections. Since the distortion is along the (111) direction, we have begun to grow GST on GaSb(111). This leads to a better crystallinity of the layers since the distortion can now align with the growth direction and relax perpendicular to the surface while maintaining the perfect in-plane symmetry.
3:30 PM - H2.3
Crystallization of Ion Amorphized Ge2Sb2Te5 Thin Films in Presence of Cubic or Hexagonal Phase.
Riccardo De Bastiani 2 , Egidio Carria 1 2 , Maria Grazia Grimaldi 1 2 , Giuseppe Nicotra 3 , Corrado Spinella 3 , Emanuele Rimini 1 3
2 MATIS, CNR-INFM, Catania Italy, 1 Fisica ed Astronomia, Università di Catania, Catania Italy, 3 IMM, CNR-INFM, Catania Italy
Show AbstractChalcogenide materials have been widely used as storage media for optical memory disks and have also been proposed for semiconductor non-volatile phase change random access memory (PCRAM). Studies so far have shown that Ge2Sb2Te5 (GST) has a good combination of electric and phase changing characteristics for PCRAM applications. The phase-change data storage technology is based on the reversible switching between the amorphous and the crystalline phases of chalcogenides alloys. The stability of the amorphous phase in presence of a crystalline substrate is therefore a point of great interest for technological applications. In this work we report on the crystallization kinetics of continuous amorphous GST thin films (50 nm thick) in contact, with a planar interface, with either cubic and hexagonal GST. Sample were prepared by pulsed laser or low energy Ge+ ion irradiation of cubic or hexagonal phase obtained by isothermal annealing at 150 °C and 400 °C respectively. By a suitable choice of the irradiation parameters a thin surface amorphous layer (25 nm thick) on top of crystalline material was obtained. For example, ion irradiation at the liquid nitrogen temperature by 28keV Ge+ at a fluence of 1014 at/cm2, produced a 30nm thick amorphous layer with a sharp amorphous-crystalline (a/c) interface. A similar configuration was realized by 10ns frequency-doubled Nd:YAG pulse at energy density of 50 mJ/cm2. The crystallization of the amorphous layer during isothermal annealing was investigated by in situ time resolved reflectivity measurements (TRR), transmission electron microscopy and by ex situ X-ray diffraction. The experimental results reveal that during isothermal annealing the crystallization of the amorphous layer, generated by laser or ion irradiation, is affected by the adjacent crystalline structure. In particular the regrowth of the amorphized layer for both crystals (cubic or hexagonal) was found to proceed from the a/c interface with the formation of a cubic state at a temperature below that required in the amorphous to crystal phase transition for the Ge2Sb2Te5 composition. The a/c interface plays the role of a continuous region of potential nucleation sites, making the crystallization process more efficient.
3:45 PM - **H2.4
What Makes Ge-Sb-Te Phase Change Alloys Fast and Stable.
Alexander Kolobov 1 3 , Paul Fons 1 3 , Milos Krbal 1 , Robert Simpson 1 , Junji Tominaga 1 , Stephen Elliott 2 , Jozsef Hegedus 2 , Tomoya Uruga 3
1 , AIST, Tsukuba Japan, 3 , SPring8, Sayo Japan, 2 , Cambridge University, Cambridge United Kingdom
Show AbstractIn this talk, we present our recent simulation and experimental results that demonstrate the intrinsic complexity of the melt-quenched amorphous phase. We further argue - based on our results - that the common use of as-deposited films to model the amorphous phase in inappropriate. We propose a new model of the phase-change process that is in excellent agreement with both experimental XAFS results and those of DFT simulations. Our proposed paradigm of the phase-change process can also be applicable to other functional materials.
4:15 PM - H2:Struct
BREAK
4:30 PM - **H2.5
Phase Change in (GeTe)1-x(Sb2Te3)x and Lattice Instability in Average Five Valence Electron Family.
Keisuke Kobayashi 1
1 Beamline Station at SPring-8, Natioanal Institute for Materials Science, NIMS, Sayo, Hyogo, Japan
Show AbstractCrystals with average of five valence electrons (〈V〉) take the form of NaCl cubic, rhombohedral, orthorhombic, and CsCl cubic structures depending on the relative strength of the metallic-covalent-ionic nature in the bonding [1-4]. The basic structure of this family is cubic, which is sustained by resonance bonding of px, py, and pz orbitals [5], with the s2 electrons remaining as lone pairs. This results in the formation of an unstable half filled metallic band. Displacive transitions from 6-fold to the two types of 3-fold phases (rhombohedral and orthorhombic phases) take place by introducing a gap around the Fermi level [6]. Rewriting (GeTe)1-x(Sb2Te3)x (GST) as (GeTe)1-x(Sb+Te)2x(V2-Te)x, where V stands for a vacancy, the pseudo-binary alloys can be regarded as members of the〈V〉 family. Consequently the crystalline-amorphous (C-A) phase change mechanism in GST can be naturally considered to be related to the lattice instability and polymorphism of the 〈V〉 family materials.The valence-band and core-level hard X-ray photoemission spectroscopy results by J. J. Kim et al. [7] show that; 1) the valence band shapes are very similar between the C and A phases, 2) p-states dominate the bonding, 3) band-gap widening takes place upon a C-A phase change, 4) Sb is singly ionized as expected, and 5) only Sb core levels undergo a shift of 0.2 eV toward higher binding energy upon the C-A phase change. All these results give to strong support for a phase change model in which electronic 6 to 3 fold bond reconstruction play dominant role. Reverse Monte Carlo analysis of X-ray diffraction results by Kohara e al. [8], and first principle calculations by Akola and Jones [9] also consistently support the above model. [1] K. L. I. Kobayashi, Y. Kato, and K. F. Kobmatsubara, Butsuri, 31, 253-264 (1976). in Japanese.[2] K. L. I. Kobayashi, et al., Prog. Crystal Growth and Charact. 1, 117-149 (1978).[3] T. Susuki et al. J. Phys. Chem. Solids, 42, 479 (1981).[4] P. B. Littlewood, J. Phys. C: Solid St. Phys., 485513 (1980).[5] Lucovsky, G. & White, R. M., Phys. Rev. B 8, 660 (1973).[6] K. A. Khachaturyan, Phys. Rev. B 36, 4222 (1987).[7] J. J. Kim et al. Phys. Rev. B, 76, 115124 (2007).[8] S. Kohara et al., Appl. Phys. Lett. 89, 201910 (2006).[9] J. Akola et al., Phys. Rev. B 80, 020201(R) (2009).
5:00 PM - H2.6
Amorphization of Crystalline Phase Change Material by Ion Implantation.
Simone Raoux 4 , Guy Cohen 1 , Robert Shelby 2 , Huai-Yu Cheng 3 , Jean Jordan-Sweet 1
4 IBM/Macronix PCRAM Joint Project, IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 1 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 2 , IBM Almaden Research Center, San Jose, California, United States, 3 IBM/Macronix PCRAM Joint Project, Macronix Emerging Central Lab., Macronix International Co. Ltd., , Hsinchu Taiwan
Show AbstractWe have applied ion implantation as a different method of re-amorphization of crystalline phase change material Ge2Sb2Te5 (GST) and studied the amorphization as a function of implanted ion dose. Germanium ions were implanted at an energy of 30 keV, and a dose ranging from 5x1012 to 1015 ions/cm2. The sample consisted of 16 nm GST on 21 nm Al2O3 on Si substrate and was capped with 5 nm SiO2. The Al2O3 layer serves as a heat barrier for laser experiments while the SiO2 layer prevents oxidation and evaporation when the sample is heated. The implantation was done at low current densities to avoid self-annealing and the sample temperature during implantation was close to room temperature. Prior to the Ge implantation samples were annealed at 200 °C for a minimum of 90 s in nitrogen atmosphere to crystallize them to the rock salt phase which was confirmed by x-ray diffraction (XRD). It was found that rather low doses of > 5x1013 cm-2 were sufficient to re-amorphize the GST. Amorphization was determined by XRD as well as resistivity and reflectivity measurements. Samples implanted with doses > 5x1013 cm-2 showed no XRD peaks and had resistivities and reflectivities very similar to as-deposited amorphous samples. Doses of less than 2x1013 cm-2 were not sufficient for amorphization and samples showed XRD peaks, and resistivities and reflectivities very similar to unimplanted crystalline samples. In the intermediate dose range very weak XRD peaks were visible and the resistivity and reflectivity measurement values were between the amorphous and crystalline samples. A static laser tester was applied to measure the crystallization times of material that was (1) as–deposited amorphous; (2) crystallized by annealing, subsequently re-amorphized by melt-quenching using a short, intense laser pulse and re-crystallized by a second pulse at the same location; and (3) crystallized by annealing and re-amorphized by ion implantation. It was found that as-deposited amorphous and high-dose ion implanted samples (1x1015 cm-2) had a longer crystallization time (~200 ns) while melt-quenched amorphous and low-dose ion implanted samples (5x1013 cm-2) had shorter crystallization times ( ~100 ns). This is probably caused by a more complete randomization of the atomic position by high dose implantation while low-dose implantation probably leaves some very small crystallites intact that are too small to produce XRD peaks but can act as nucleation sites during re-crystallization. Time-resolved XRD during heating of the implantation-amorphized samples showed that samples re-crystallize at an increased crystallization temperature (up to about 25 °C higher) with increased dose compared to as-deposited material. This method of amorphization can be applied locally with either a small ion beam or through a mask, and enables studies of the crystallization process by separating nucleation and growth phenomena.
5:15 PM - H2.7
Phase Change Materials - A Model System Demonstrating the Role of Subcritical Nuclei in Phase Transformation.
Bong-Sub Lee 1 2 , Geoffrey Burr 3 , Robert Shelby 3 , Simone Raoux 4 , Charles Rettner 3 , Stephanie Bogle 1 2 , Kristof Darmawikarta 1 2 , Stephen Bishop 2 5 , John Abelson 1 2
1 Materials Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 , IBM Almaden Research Center, San Jose, California, United States, 4 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 5 Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractThe theory of phase transformation is widely used in physics, chemistry, and materials science. Transformation generally begins with nucleation, in which a small number of atoms organize into a new structural symmetry. However, detecting nanometer-scale subcritical nuclei – which develop before the observable phase transformation – has not been possible except on exposed surfaces. We detect the nuclei embedded in a glassy solid using a statistical technique, fluctuation transmission electron microscopy (FTEM), and we determine their role in crystallization using pulsed laser experiments. A phase change material, Ag/In-incorporated Sb2Te (AIST), serves as an excellent model system for this analysis because the phase transformation is limited by the time required to form a single critical nucleus within the laser-heated spot. Our study provides a convincing proof of the predicted development of a size distribution of subcritical nuclei, important information for the development of phase-change memory devices. (Ref: B.-S. Lee et al., Science, Nov. 2009.)
5:30 PM - H2.8
Local Order and Crystallization of Laser Quenched and Ion Implanted Amorphous Ge1-xTex Thin Films.
Egidio Carria 1 2 , Riccardo De Bastiani 2 , Santo Gibilisco 1 2 , Antonio Massimiliano Mio 1 , Maria Miritello 1 2 , Agata Pennisi 1 , Corrado Bongiorno 3 , Maria Grazia Grimaldi 1 2 , Emanuele Rimini 1 3
1 Fisica ed Astronomia, Università di Catania, Catania Italy, 2 MATIS, CNR-INFM, Catania Italy, 3 IMM, CNR-INFM, Catania Italy
Show AbstractThe GeTe system belongs to IV-VI compound semiconductor and it is characterized by fast crystallization and high stability of the amorphous phase. GeTe is the basic ingredient of a class of materials, GeTe–Sb2Te3 ternary alloys, employed as the active medium in optical and electrical data storage devices. For a given alloy composition, the crystallization kinetics is strongly affected by the local atomic arrangements and chemical bonding of the amorphous network. It is important therefore to understand the short range order of different amorphous states achieved by appropriate handling of the materials. In this work we compare the local order, probed by micro Raman spectroscopy, of amorphous Ge1-xTex(with x=0.3, 0.5, and 0.7) prepared by several techniques: rf sputtering, laser quenching and ion implantation. The basic structure of amorphous GeTe was observed after rf deposition even in non stoichiometric film. In laser quenched as well as in ion implanted amorphous the reduction of Ge-rich tetrahedra with respect to the Te-rich tetrahedra occurs.The crystallization kinetics has been investigated by in situ time resolved resistivity. The local order and the morphology of the crystallized samples have been detected by micro-Raman spectroscopy and energy-filtered transmission electron microscopy (EFTEM).The crystallization temperature of non stoichiometric sputtered alloys increases by more than 100 K in with respect to stoichiometric GeTe that crystallizes at 430 K. Ion implanted and laser quenched amorphous exhibit an enhancement of the crystallization kinetics in agreement with the structural modification, observed by Raman spectroscopy, that are very similar to those occurring in the sputtered materials after low temperature annealing prior to crystallization. In particular, during the heating the fraction of the tetrahedral species that constitute the glass structure of films progressively changes, in such a way to form Te-rich tetrahedra at the expense of Ge-rich tetrahedra, promoting the system to a state closer to the crystalline phase. In the annealed samples the final phase consisted of crystalline GeTe and precipitates of the excess element. In order to quantify the precipitation of crystalline Te or Ge phase in GeTe alloys and to disentangle the crystallization from the precipitation process a detailed EFTEM study was performed on as-deposited, melt-quenched and ion irradiated amorphous samples.
5:45 PM - H2.9
Exploring Local Atomic Arrangements in Amorphous and Metastable Phase Change Materials With X-ray and Neutron Total Scattering.
Katharine Page 1 , Luc Daemen 1 , Thomas Proffen 1
1 Lujan Center, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractVery little experimental work has conclusively explored the structural transformation between the amorphous and metastable crystalline phases of phase change chalcogenides. A recent flurry of theoretical work has supported likely mechanisms for the phase transition process in Ge-Sb-Te (GST) compositions and invigorated efforts at probing local atomic arrangements experimentally. The pair distribution function (PDF) formalism of total scattering data provides directly both local structure correlations at low real-space dimensions, and intermediate range order at higher length scales, a distinct advantage for following the relevant phase transition in phase change materials (PCM).A challenge facing the field is the difficulty in distinguishing separate peak contributions to pair correlation functions in amorphous and highly disordered samples. For example, various types of local order have been reported for GexTe1-x phases, including both random mixtures and discrete structural units, and both 4-fold and 6-fold coordination around Ge. We describe our efforts in advancing capabilities for extracting and refining differential or partial pair distribution function data sets by combining neutron and x-ray total scattering, with extensions to isotopic substitution and anomalous x-ray scattering. Our results combining neutron and x-ray scattering for the GexTe1-x series, for example, clearly distinguish Ge-Te and Te-Te contributions in nearest neighbor correlations.Presenting an additional challenge, phase change materials with fast switching speeds (those arguably of greatest technological interest) have stable bulk crystalline phases and do not readily form glasses until reduced to small dimensions. Thin film samples are inherently difficult to probe with conventional crystallographic methods. We demonstrate successful synchrotron x-ray total scattering experiments for PCM thin films with thicknesses between 100 nm and 1 um and describe how chemical short-range order and local bonding environments vary in amorphous, metastable and crystalline GeSb2Te4 films. Total scattering methods for powders and thin films allow for a direct comparison of PCM properties (crystallization temperature, optical contrast between phases, phase change speed, etc.) with observed local structure and motivate further exploration into the atomic configurations enabling this fascinating class of materials.
H3: Poster Session
Session Chairs
Tuesday PM, April 06, 2010
Exhibition Hall (Moscone West)
6:00 PM - H3.1
The Effect of Dopants on the Amorphous Structure of Ge2Sb2Te5.
Eunae Cho 1 , Jino Im 2 , Jisoon Ihm 2 , Dohyung Kim 3 , Hideki Horii 3 , Seungwu Han 1
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 3 Process Development Team, Semiconductor R&D Center, Samsung Electronics, Hwasung Korea (the Republic of)
Show AbstractGe2Sb2Te5 is thought to be a material of choice to be used in the phase-change random access memory (PRAM) because of several technical advantages such as the high speed in the phase transition. However, the low resistivity of crystalline Ge2Sb2Te5 requires high RESET currents to transform the material from crystalline to amorphous. As a way to solve this problem and also to improve the stability of the amorphous phase, doped Ge2Sb2Te5 materials have been actively investigated. For example, it has been shown that the doping with Si and N atoms increases the crystalline resistivity and the crystallization temperature. However, the microscopic origin of these doping effects has not been elucidated sufficiently. In this presentation, we study the effect of Si and N dopants on the atomic and electronic structures of Ge2Sb2Te5 using the first-principles approach. The doped amorphous structures have been obtained through the melt-quench simulations. We find that the dopants tend to stabilize the amorphous phase by enhancing the covalent bonding. Particularly, all Si atoms are tetrahedrally coordinated in amorphous Ge2Sb2Te5. The density of the four-fold ring which is crucial in the crystallization speed is also significantly affected. In addition, the doped Ge2Sb2Te5 materials show a larger band gap than un-doped Ge2Sb2Te5. The experimental observations will be explained based on the present first-principles results.
6:00 PM - H3.10
Phase Change Switching Behaviors in Bi2Te3 Nanowire.
Nal Ae Han 1 , Jeong Do Yang 1 , Sung In Kim 1 , Kyung Hwa Yoo 1
1 Department of Physics, Yonsei University, Seoul Korea (the Republic of)
Show Abstract We have fabricated Bi2Te3 nanowire arrays by using anodic aluminum oxide (AAO) membranes and an electro-deposition method. These Bi2Te3 nanowire exhibited reversible phase change switching behaviors. The initial state was the low resistance "on" state. After applying a high voltage to the nanowires, however, the state was changed into the high resistance "off" state due to a phase change from poly-crystalline to amorphous structure. These results suggest the feasibility of developing Bi2Te3 nanowire based phase random access memory device.
6:00 PM - H3.11
An ab-initio XANES Study of Ge-Sb-Te Alloys.
Milos Krbal 1 , Alexander Kolobov 1 , Paul Fons 1 , Robert Simpson 1 , Steven Elliott 2 , Jozsef Hegedues 2 , Junji Tominaga 1
1 , Center for Applied Near-Field Optics Research (CanFor), National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba 305-8562 Japan, 2 , Department of Chemistry, University of Cambridge, Cambridge CB2 1EW United Kingdom
Show AbstractTheoretical calculations of Ge K-edge x-ray absorption near-edge structure (XANES) using first- principles method have been performed to get insight into the local environment of Ge atoms in amorphous and crystalline Ge-Sb-Te structure obtained by ab-initio molecular dynamic simulations [1]. We demonstrate that the average XANES simulation for the computer generated amorphous structure is in excellent agreement with the experimental measurement and demonstarte that not all Ge atoms have to necessarily switch into tetrahedral symetry sites during the phase transition. On the other hand, for the crystalline Ge-Sb-Te that has been identified as having the rocksalt like structure, our XANES simulations demonstrate that a perfectly ordered fcc configuration does not agree well with experiment. We demonstrate that significant local distortions around the octahedral sites are needed to reproduce the experimental data.[1] J. Hegedus and S. R. Elliott, Nature Materials 7, 399 (2008).
6:00 PM - H3.12
First-principles Investigation on the InSbTe Phase-change Alloys.
Naihua Miao 1 , Baisheng Sa 1 , Jian Zhou 1 , Zhimei Sun 1
1 Department of materials science and engineering, Xiamen University, Xiamen China
Show AbstractChalcogenides, such as ternary Ge-Sb-Te (GST) alloys, are extensively studied for applications as data storage media due to their great optical or electrical contrast between the amorphous and crystalline states. As a non GST phase-change alloy, In3SbTe2 (IST) with a NaCl-type structure was also shown to have potential for reversible optical data storage media with high-speed erasing and long-term data retention [1]. Quite different from GST alloys in which Ge, Sb, and vacancy randomly occupy the Na site, and Te occupies the Cl site, both sites in IST alloys are randomly occupied with In and vacancy in Na site and Sb, Te, and vacancy in Cl site, respectively. As far as we know, few works have been carried out on IST, and most of them are by experiments. In the present study, the phase stability, electronic properties and bond character of IST have been investigated by means of first-principles calculations. The chemical bonding of IST is analyzed by density of states and electron localization function [2]. Our results will present a fundamental knowledge of the InSbTe phase-change alloys. [1] Y Maeda, H Andoh, I Ikuta, and H Minemura, J. Appl. Phys. 64, 1715 (1988). [2] B. Silvi and A. Savin, Nature 371, 683 (1994).
6:00 PM - H3.13
Comparative Simulations and Analysis of Amorphization Current in Phase Change Memory Applied to Pillar and GST Confined Type Cells.
Olga Cueto 1 , Carine Jahan 1 , Veronique Sousa 1 , Jean-Francois Nodin 1 , Salim Syoud 1 , Luca Perniola 1 , Andrea Fantini 1 , Alain Toffoli 1 , Barbara De Salvo 1 , Fabien Boulanger 1
1 Nanotech Department, CEA-LETI MINATEC, Grenoble France
Show AbstractPhase change memory (PCM) is an emerging technology for non-volatile memory devices. The device operation relies on reversible and fast changes of the phases of chalcogenide materials such as Ge2Sb2Te5 (GST). Lowering Ireset, the large current required during the SET (crystalline state with high conductivity) /RESET (amorphous state with low conductivity) transition, is one of the most critical issues of PCRAM technology. In this study, comparative simulations and analysis of Ireset are presented for pillar type and GST confined type structures. The objectives are the selection of optimized structures for reset conditions. The simulations are realized with the PCM model of Sentaurus Device; in this TCAD simulation tool an analytical phase transition model is coupled with an electro-thermal model. Regarding Ireset, the GST confined cell is more efficient but its fabrication requires a Chemical Vapor Deposition step still difficult to achieve. On the contrary, the pillar type structure is compatible with a Physical Vapor Deposition for GST and by the way still existing. As a preliminary work to our study, simulations of the crystalline to amorphous transition are compared to experimental electrical results. Simulations are realized with geometrical or material variations for the two structures in order to lower Ireset. An optimization of the pillar structure regarding Ireset is realized by simulation. For this, 6 parameters (GST height, heater height, heater width, electrical and thermal conductivity of the heater material and volumetric heat capacity of the heater) are chosen as the most significant parameters and a Design of Experiment (DoE) is used to realize the optimization with a reasonably large number of simulations. After this analysis of the pillar structure, we focus on the confined structure: our simulations indicate that for a 300nm GST height and a 50nm active area, Ireset is five times lower in the confined structure than in the pillar structure. Among other results with regard to the confined structure, simulations indicate that Ireset decreases when the height of GST increases. Volume phase fractions and temperature are visible results of our simulations and it is clear that increasing the height of GST corresponds to a better thermal confinement of the area where the phase change takes place; this points out that the shrinking of PCRAM cells can not be simply homothetic and should include extensive thermal analysis for which TCAD is helpful. Finally whatever structure, Ireset lowering can not be realized without taking into account another important issue of PCRAM: keeping a contrast between the amorphous and crystalline resistivity as high as possible. Our study points out that the contrast can be degraded even when Ireset is lowered. This means that a compromise has to be found between a low Ireset and a high contrast.
6:00 PM - H3.14
Microstructural Analysis Upon Annealing Temperature in In-Sb-Te Thin Films Deposited by RF Magnetron Sputtering.
Chung Soo Kim 1 , Eun Tae Kim 1 , Jeong Yong Lee 1
1 Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractMicrostructure of the annealed In-Sb-Te (IST) thin films deposited on a Si (001) substrates by RF magnetron sputtering method were investigated by transmission electron microscopy (TEM). The crystallization and melting temperatures were about 290 and 630°C, respectively, using a differential scanning calorimeter (DSC) at 10°C/min heating rate. From the DSC results, the rapid thermal annealing (RTA) process was carried out at 300, 350, 400, and 450°C for 10 and 60 min. in N2 ambient condition, respectively. At the 300°C-annealed thin films, about 5nm-sized InSb (zincblende, a=6.48Å) grains were randomly crystallized in amorphous matrix throughout the whole thin films. When the annealed at 350°C thin films were grown, InSb grain had more than two {111} twins with twin-plane-re-entrant-edge (TPRE) growth mechanism. The ternary compound In3Sb1Te2 (rock-salt, a=6.13Å) was observed at 400°C annealed thin films. The films were heated up to 450°C, the In3Sb1Te2 phase had an ordered structure which was different atomic sites of Sb and Te, while InTe (TlSe, a=8.45Å, c=7.15Å) phase separation may take place as the composition deviates from In3Sb1Te2.
6:00 PM - H3.15
Defects of Amorphous and Crystalline GeSbTe Materials and Electrical Non-volatile Memories.
Bolong Huang 1 , John Robertson 1
1 Engineering, Cambridge University, Cambridge United Kingdom
Show AbstractElectrical phase change memories work by the large contrast in electrical resistivity between their crystalline and amorphous phases. The crystalline phase of Ge2Sb2Te5 is based on the cubic rocksalt structure, with Te on the B sites and Ge and Sb plus 20% structural vacancies on the A sites. Additional Ge or Sb vacancies lead to the Fermi level being pinned in the valence band, and this leads to the observed high p-type conductivity of the crystalline phase. The high resistivity of the amorphous phase arises because Ef is pinned near midgap. The most plausible model of this is that it is due valence alternation pair (VAP) defects. But the simplest version of this would need a region of local Te structure. We show that other possibilities, such as fluctuations of Ge and Te coordinations between their 3-3 and 4-2 coordinations, including Ge-Ge bonding, are an alternative form of VAPs that will pin Ef and are more likely at this composition.
6:00 PM - H3.16
The Effect of Ge Addition on the Operation Characteristics of a Phase-change Memory(PCM) Device Using Ge-doped SbTe.
Young-wook Park 1 2 , Hyun Seok Lee 2 , Hyung Woo Ahn 2 , Zhe Wu 2 3 , Suyoun Lee 2 , Jeung-hyun Jeong 2 , Doo Seok Jeong 2 , Kyung-woo Yi 1 , Byung-ki Cheong 2
1 Department of material science and engineering, Seoul National University, Seoul Korea (the Republic of), 2 Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Deajeon Korea (the Republic of)
Show Abstract Ge-doped SbTe (Ge-ST) is a promising candidate for the next generation phase-change materials in high speed applications due to its outstanding attributes like as fast growth-dominated crystallization, low electrical resistivity of the crystalline phases. While existing stably as a single δ phase over a wide range of composition, the material displays markedly varying crystallization characteristics with Sb:Te ratio and/or Ge content hence highly tunable PCM performances in terms of speed, retention and so on. We recently examined the effect of varying Sb:Te ratio in Ge-ST, to find that the SET speed of a PCM device becomes enhanced with increasing Sb:Te ratio by virtue of increasingly higher growth speed of crystallites and extremely sluggish nucleation rate. However, the highly fast growth of a high Sb:Te material tends to leave a RESET operation with a tight margin in melt-quenching rate for amorphization. Accordingly, a RESET operation is likely to become unreliable. Herein, we investigated the effects of Ge addition (0 to 13.1 at%) in Ge-ST of a high Sb:Te ratio(~4.4) with regard to the possibility of achieving highly fast SET on the one hand and reliable RESET programming characteristics on the other. From the material characterization, we found that a higher Ge content led to the enhanced amorphous phase stability and retarded nucleation of crystallites but the growth of crystallites remained very fast and with no significant variation regardless of Ge content. From the device characterization, we also found that a higher Ge content could render RESET programming more reliable by its ability of amorphization at a slower melt-quenching while maintaining a very high SET speed. These ostensibly conflicting effects of Ge addition during SET and RESET programming were analyzed on the basis of a Time-Temperature-Transformation diagram and by experiments to examine differences in recrystallization processes involved in the two differing programming operations.
6:00 PM - H3.17
Analysis on Mechanism of Structural Relaxation in Amorphous Ge2Sb2Te5 Doped With Bi and N Using Thermomechanical Measurement.
Ju-Young Cho 1 , Tae-Youl Yang 1 , Young-Chang Joo 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractIn PRAM device application, metastable nature of amorphous Ge2Sb2Te5 (GST) causes reliability issues, such as the structural relaxation. Structural relaxation is resistance drift in reset state, which leads to reducing the stability of programmed resistance levels in PRAM cell. Amorphous GST tends to rearrange atomically to reach more stable state, results in resistance drift. Structural relaxation has been studied by electrical measurement, and the origin is explained by physical model, which is related to decreasing localized state for PF conduction [1]. Structural relaxation is the viscous flow which can be viewed as relaxation of the mechanical stress. Stress relaxation accompanies changes of mechanical properties, especially viscosity. Therefore, structural relaxation of amorphous GST can be analyzed through the changes of viscosity. In addition, with comparing the changes of mechanical and electrical properties when structural relaxation occurs, it is possible to find the clues of correlation between them. Changes of viscosity indicating the stress relaxation were investigated through the thermomechanical measurement. Stress in the film originating from the annealing causes the wafer to be bended, curvature was measured by the laser technique. Effects of various dopants on structural relaxation behavior were also studied. There are 2 types of dopants, interstitial dopant N and substitutional dopant Bi are selected.Ge2Sb2Te5 film with 300 nm thick were deposited on 500 μm Si by DC magnetron sputtering. Ge2Sb2Te5 and Ge2Bi2Te5 targets were co-deposited for Bi-GST. N-GST was prepared with N2 gas flowing during deposition. The stress changes of the film were evaluated by the wafer curvature method with isothermal condition.Stress relaxation measurement data with elapsed time for 10 hours shows relaxation behavior of amorphous state. Tensile stress in the as-deposited film is released with time, the stress changes were transformed to viscosity using expression for relaxation kinetics. In result, changes of viscosity indicating stress relaxation increases exponentially. In comparison with ex-situ resistance measurement in same condition, the clue for possibility of correlation between stress relaxation and resistance drift was given. In the study of N-GST, N doping increases the viscosity, it is likely due to the suppression of atomic migration by N atoms in the interstitial site. Stress measurement for Bi-GST in further work is expected to show different tendency, because Bi forms new bonding, unlike N. Observation of structural relaxation was studied through the changes of viscosity. Correlation between mechanical and electrical properties was also studied. In addition, discovering exact role of dopant can achieve successful selection of dopant for improving PRAM reliability. [1] D. Ielmini et al., IEDM Tech. Dig. (2007) 939-942.
6:00 PM - H3.18
Unipolar Switching in Pt/GeSexTe1-x/Pt.
Doo Seok Jeong 1 , Seo Hee Son 1 2 , Suyoun Lee 1 , Byung-ki Cheong 1
1 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Nanomaterials Science and Engineering Major, University of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractUnipolar switching in amorphous GeSexTe1-x of various Se content (x) sandwiched between Pt top and bottom electrodes was investigated. A GeSexTe1-x film was thermally evaporated on a platinized Si-wafer. Then, Pt top electrodes were sputtered on the grown GeSexTe1-x film with a shadow mask. A staircase voltage sweep was applied in order to electroform a Pt/GeSexTe1-x/Pt stack into a unipolar-switching state. The electroforming resulted in the stack resistance that was barely scalable with the size of top electrode, implying the formation of local conduction path(s) in the amorphous matrix. The conduction path(s) is(are) probably made of crystalline GeSexTe1-x phases. Optical analysis was conducted on stacks in the unipolar-switching state for a direct observation of the conduction path(s).The effect of the Se to Te ration on the unipolar switching as well as on the electroforming behavior was investigated. With decreasing Se content the electroforming voltage and the resistance in the electroformed state decreased. The observed switching behavior was similar to the one of transition metal oxides. However, the observed behavior was quite contrary to the general memory switching due to phase change in phase change materials. Finally, we suggested a possible mechanism for the observed unipolar switching behavior.
6:00 PM - H3.19
Electrical Properties of Phase-transformed Amorphous and Novel High Pressure Polycrystalline Silicon Formed by Nanoindentation.
Simon Ruffell 1 , Kallista Sears 1 , Jodie Bradby 1 , Jim Williams 1 , Andrew Knights 2
1 Electronic Materials Engineering, Australian National University, Canberra, Australian Capital Territory, Australia, 2 Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada
Show AbstractDuring nanoindentation, with a diamond indenter tip, silicon undergoes a series of pressure-induced phase transformations. On loading, diamond cubic Si-I transforms to a metallic phase (Si-II) at a pressure of ~11 GPa. During unloading Si-II further phase transforms to either amorphous silicon (a-Si) or a mixture of high pressure polycrystalline phases (Si-III and Si-XII) depending on the unloading rate. These phases are stable at room-temperature and pressure. The phase transformations can be induced by indentation in crystalline silicon (c-Si) and a-Si matrices to form zones ranging from ~20 nm up to several micrometers in dimension. The high pressure polycrystalline phases have high conductivity which can be controlled over several orders of magnitude through a room-temperature doping process. In contrast, the nanoindentation-induced a-Si is electrically insulating. Through electrical measurements of carrier type, density, and mobility using novel micro-Hall structures, we illustrate both p and n-type doping of the Si-III/Si-XII zones through ion-implantation of phosphorus or boron prior to indentation. Furthermore, these measurements provide the first experimental evidence that Si-XII is a semiconductor. Resistivities can be controlled over several orders of magnitude and are comparable to those in normal poly-Si with similar doping levels. For example, with a boron sheet concentration of 1015 cm-2 a sheet resistance of 450 Ohm/sq. and hole mobility of 17 cm2/V-s are measured. Indentation can also be performed to form electrically insulating a-Si thus allowing room-temperature, maskless writing of electrically conductive and insulating regions in both conductive and insulating silicon matrices. The semiconducting behaviour and the ability to dope the high pressure phases also open up prospects for room-temperature, lithography-free fabrication of a range of devices.
6:00 PM - H3.2
Studies of Ge-Sb-Te Phase Change Materials At and Above Melting Temperatures and Set to Reset Transition of Memory Devices.
Semyon Savransky 1 , Guy Wicker 1
1 , The TRIZ Experts, Newark, California, United States
Show AbstractProgramming of phase change memory usually occurs above the melting temperature Tm but information about phase change materials at such high temperatures is very limited. In this paper we present results of calorimetric studies of Ge2Sb2Te5 and GeSb2Te4 near Tm and electrical studies of Ge2Sb2Te5 above Tm.Melting region of Ge2Sb2Te5 is wider than GeSb2Te4 that reflects in R-I curves of phase-change memory devices for the set to reset transition. From practical point of view it means that a multi-level cell is simple to realize for devices built from phase change materials with a gradual melting region.Electrical resistance of Ge2Sb2Te5 above Tm was studied in RTA oven. The electrical resistance has semiconductor character in the region of temperatures that correspond to degradation-less reset currents. Therefore Ge2Sb2Te5 melt is similar to molten chalcogenide glasses rather than to molten classical semiconductors.
6:00 PM - H3.20
In-situ Raman Scattering Spectroscopy for Super Resolution Effect.
Masashi Kuwahara 1 , Takayuki Shima 1 , Paul Fons 1 , Junji Tominaga 1
1 CAN-FOR, AIST, Tsukuba, Ibaraki, Japan
Show AbstractSuper resolution readout (SRR) phenomenon on optical disks has attracted many researchers not only because its basic physics is interesting but also its application to future optical disk is promising. Concerning the origin of the SRR phenomenon, it became clear that heat generated by a readout laser light plays an important role and the heat changes optical properties of a functional layer in the SRR disk. To understand the SRR phenomenon, a conventional disk drive tester are often used. However, it only evaluates the optical properties of the disk, that is, reproduced waveform, carrier-to-noise ratio (CNR), and reflected (and transmitted) light intensity. No direct information on the structural properties can be simultaneously obtained, although it is important for further understanding of the SRR phenomenon.In this presentation, we report on the development of a combined Raman scattering spectroscopy and disk property measurement system. We have simultaneously measured in-situ the Raman scattering spectrum and the CNR during readout from a SRR optical disk utilizing an antimony resolution enhancing layer. Both the Stokes and anti-Stokes Raman spectrum of the Antimony A1g mode in the range ±150 cm-1 were observed at incident laser powers ranging from 1.0 mW to 4.5 mW. Simultaneously with the onset of SRR for readout powers over 3.0 mW, the integrated intensity of both Stokes and anti-Stokes mode was noted to reduce markedly with increasing laser power. This behavior is attributed to the generation of a disordered (molten) area in the Antimony film within the readout laser spot. Additionally, we discuss the use of the Stokes and anti-Stokes intensity ratio for in-situ temperature determination.
6:00 PM - H3.22
Study of N-doped GeSb Phase Change Material for PCRAM Applications.
Audrey Bastard 1 3 , Sandrine Lhostis 1 5 , Pierre-Eugene Coulon 2 , Caroline Bonafos 2 , Andrea Fantini 5 , Luca Perniola 5 , Sebastien Loubriat 4 , Anne Roule 4 , Emmanuel Gourvest 1 6 , Edrisse Arbaoui 1 , Alain Fargeix 3 , Marilyn Armand 3 , Berangere Hyot 3 , Frederic Fillot 4 , Raluca Tiron 5 , Nevine Rochat 4 , Sylvain Maitrejean 5 , Veronique Sousa 5
1 , ST Microelectronics, Crolles France, 3 DOPT, CEA LETI, Grenoble France, 5 D2NT, CEA LETI, Grenoble France, 2 , CEMES, Toulouse France, 4 DPTS, CEA LETI, Grenoble France, 6 LTM, CNRS/UJF/INPG, Grenoble France
Show AbstractChalcogenide materials have been widely investigated for their application in Phase Change Random Access Memory (PCRAM). In this field research currently deals with the binary compound GeSb [1-2]. Antimony provides fast crystallization characteristics and germanium is known to contribute to a long-term amorphous stability [3]. This material has some advantages for PCRAM application compared to the Ge2Sb2Te5 reference material such as faster crystallization and higher crystallization temperature [4].In this study, the influence of nitrogen doping on the phase transitions characteristics and the electrical properties of GeSb films is investigated. The crystallization temperature and activation energy are found to increase with the nitrogen content, making N-doped GeSb materials even more attractive than GeSb for PCRAM applications. Chemical-physical characterizations such as Fourier Transform InfraRed Spectroscopy, X-Ray Photoelectron Spectroscopy and X-Ray Diffraction are performed to understand the crystallization mechanism of N-doped GeSb. The evolution of the microstructure with temperature is also investigated through High Resolution Transmission Electron Microscopy observations and STEM-EELS measurements.Furthermore, the material behaviour determined from full-sheet samples is correlated with electrical characteristics of the corresponding memory cells.[1] C. Cabral Jr. et al., Applied Physics Letters 93, 071906 (2008)[2] S. Raoux et al., Journal of Applied Physics 105, 064918 (2009)[3] J. Siegel et al., Applied Physics Letters, 72, 20, 3102-3105 (1999)[4] Y.C. Chen et al., IEDM 06 International (2006)
6:00 PM - H3.23
Activated Pulsed Metalorganic Chemical Vapor Deposition of Ge2Sb2Te5 Thin Films Using Alkyl Precursors.
Denis Reso 1 , Mindaugas Silinskas 1 , Bodo Kalkofen 1 , Edmund Burte 1
1 Micro and Sensor Systems, Otto-von-Guericke-University , Magdeburg Germany
Show AbstractGe2Sb2Te5 is one of the most promising materials for the development of non volatile phase-change random access memories (PCRAM). The production of high density memory devices requires deposition techniques for the conformal filling of structures with high aspect ratios. One disadvantage of widely used physical deposition methods like sputtering or evaporation lies in the appearance of shadowed areas due to the straight movement of the deposited particles. This problem can be avoided by the use of chemical vapor deposition.In this work, GST thin films were deposited on different substrate materials (Si, SiO2, Ir) by metalorganic chemical vapor deposition (MOCVD) using a vapor draw precursor delivery system. Precursors were tetraallylgermanium (TAGe), triisopropylantimony (TIPSb) and diisopropyltelluride (DIPTe) for the elements Ge, Sb and Te, respectively. The precursor vapors were injected into N2 carrier gas flows using fast acting dosing valves. Gas phase was activated by a hot wire to allow film deposition at low substrate temperatures down to 250 °C. H2 was added as a reactive gas to enhance the deposition process. The applied total flows (carrier gases and H2) and chamber pressures were 80-200 sccm and 0.5-2.0 mbar, respectively. The surfaces and cross sections of the produced films were examined by scattering electron microscopy, chemical compositions were determined by energy-dispersive X-ray spectroscopy (EDX) as well as X-ray photoelectron spectroscopy (XPS) and the crystal properties were analyzed by X-ray diffraction (XRD). Average growth rates of ~10 nm/min with a maximum of 30 nm/min could be observed. Average surface roughness Ra was between 2.1 and 8.7 nm. Step coverage and trench filling capacities were tested.
6:00 PM - H3.24
An Optical Study of GeTe-Sb2Te3 Meta-materials.
Robert Simpson 1 , Paul Fons 1 , Alexander Kolobov 1 , Toshio Fukaya 1 , Reiko Kondou 1 , Takashi Yagi 2 , Milos Krbal 1 , Junji Tominaga 1
1 CAN-FOR, AIST, Tsukuba, Ibaraki, Japan, 2 National Metrology Institute of Japan, AIST, Tsukuba, Ibaraki, Japan
Show Abstract The Ge2Sb2Te5 (GST) is a leading candidate material for future electrical phase change RAM applications. Historically, enhancing the performance of GST has been accomplished by the addition of dopants. Recently, it has been shown that as GST undergoes the phase transition from amorphous to crystalline, Ge atoms transit from a tetrahedral co-ordination to a octahedral position. This insight has allowed us to conceive a novel, GeTe-Sb2Te3 alternating layered structure which has the same average composition of Ge2Sb2Te5. The structure is designed such that, for both tetrahedral and octahedral positions, the Ge atoms are in the same local atomic environment as that of regular, composite GST. However the imposed order resulting from the layered structure alters the overall performance of the material. To investigate these new meta-materials, a dual laser optical pump (650 nm)-probe (633 nm), time resolved, static tester and a more conventional digital versatile disc (DVD) tester have been employed. For the static tester, the pump can provide up to 60 mW into a 0.6 um diameter spot with a minimum pulse width of 5 ns. The shorter wavelength of the probe laser allows the centre of the heated spot to be sampled with a 1 ns time resolution. Both transmission of the light through the sample and reflection from the sample surface are measured simultaneously. These measurements have allowed analysis of the crystallization kinetics. In contrast, the DVD tester uses a single 650 nm laser to write (amorphous) marks, erase (crystallize) the marks and, at lower powers, read the reflectivity of the material. Marks of length 500 nm are written into the disk; the maximum disc linear velocity at which erasure can be accomplished is then measured. These two techniques have allowed us to measure a two fold increase in the crystallization rate of the nano-structured meta-materials in comparison with composite GST. In this presentation, we compare optical switching and thermal measurements of composite GST with GeTe-Sb2Te3 meta-materials and report our complementary results from Finite Element Models (FEM) and structural models.
6:00 PM - H3.25
Atomic Structures of the As-deposited Ge2Sb2Te5 Amorphous Films.
Xiaodong Han 1 , Shengbai Zhang 2
1 , Beijing University of Technology, Beijing China, 2 Applied Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractAtomic Structures of the As-Deposited Ge2Sb2Te5 Amorphous FilmsLei Zhang1, Xianqiang Liu1, Xiaodong Han1*, Ze Zhang1, S. B. Zhang21 Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USAThe phase-change materials (PCMs) are a class of materials in which data can be recorded as crystalline and amorphous phases as 0’s and 1’s. The atomic structures of the amorphous and crystalline phases hold the key to the understanding of the extremely fast phase transitions between 0’s and 1’s, as well as the keys for designing new generation PCMs. The as-deposited amorphous thin film is the parent phase, which transforms to the metastable FCC phase by heating. Therefore, a clear understanding of the structure of the as-deposited amorphous phase should give important clues vital for the decoding of the atomic structures of the intermediate FCC phase and the subsequent laser-irradiation induced amorphous phase for recording. Here, we use transmission electron microscopy techniques to obtain the radial distribution function (RDF) of the as-deposited amorphous Ge2Sb2Te5 alloys. Reverse Monte Carlo simulation is used to match the experimental observations. We found that the atoms in the as-deposited films form two types of basic atomic motifs with fixed and distinctive coordination numbers for the Ge, Sb, and Te atoms, respectively, that can be approximated as: motif 1 = Ge-X(Sb,Te)m; motif 2 = Sb-Y(Ge, Te)n where m and n are integers. By comparing the simulated RDF and paired correlation function with the experiments, we suggest that the two motifs in the as-deposited film are inherited by the subsequent FCC phase and the laser-irradiated amorphous phase. Thus, the motifs in the as-deposited films may hold the key to the understanding of the extreme fast phase transition in the Ge2Sb2Te5 alloys.
6:00 PM - H3.26
Atomic Layer Deposition of (GeTe2)x(Sb2Te3)y Films Using Novel Precursors for Phase Change Memory.
Taeyong Eom 1 , Seol Choi 1 , Byung Joon Choi 1 , Sangho Rha 1 , Woongkyu Lee 1 , Cheol Seong Hwang 1 , Moo Seong Kim 2
1 Material science and engineering, Seoul National University, Seoul Korea (the Republic of), 2 , Air products and chemicals korea, Seoul Korea (the Republic of)
Show AbstractPhase change random access memory (PCRAM) is one of the most probable next generation non-volatile memory devices, which utilizes the large difference in the resistances of the crystalline and amorphous phase. The reversible switching between the two states can be achieved by an electrical pulse. However, a high necessary current level for the transition from the crystalline to the amorphous state has been an obstacle for the scaling down of PCRAM. The modified cell structure, where the phase changing material is confined in the hole plug, can significantly reduce the reset current level due to the large reduction of the heat dissipation. For the fabrication of this structure, conformality of phase changing materials on the hole structure is indispensable. Therefore, the growth behavior and characteristics of GeSbTe films by an atomic layer deposition (ALD) method were investigated in this study. GeTe2, Sb2Te3, and their solid solution films were grown at the temperatures between 60 and 180 oC using Ge(OMt)4, Sb(OEt)3, and (Mt3Si)2Te precursors. Without alternative reactant gas, the films were grown by the reaction of the ligand exchange between methyl-silyl ligands from (Mt3Si)2Te and alkoxy ligands from Ge(OMt)4 and Sb(OEt)3. The growth rate of GeTe2 films was saturated with increasing the feeding times of Ge(OMt)4 and (Mt3Si)2Te, respectively. This saturation behavior clearly shows that the films are grown by ALD reaction mechanism. In addition, it was found that relatively long purge time of (Mt3Si)2Te was required for evacuating the remaining precursor. The remaining Te precursor by insufficient purging deteriorated the growth of the films because the Silyl ligands from the (Mt3Si)2Te precursor passivated the functional groups on the reaction surface. The compositions of GeTe2 and Sb2Te3 binary films were constant irrespective of the deposition conditions such as the cycle ratio of Ge or Sb to Te precursors and precursor pulse/purge time. This was originated from the different valence states of each atom because the binding states in the films should satisfy the octet rule. (GeTe2)x(Sb2Te3)y films were also deposited at a growth temperature of 70oC using the same precursors. The Ge:Sb:Te composition ratio of the grown film was 16:21:63 (x=0.6, y=0.4). In order to verify the phase changing property of the grown film, the change in the resistance of the film with respect to the annealing temperature was examined. The resistance of the film suddenly dropped at an annealing temperature of 140oC, suggesting that the film was transformed to the crystalline phase at this temperature.
6:00 PM - H3.27
Effects of Silicon Doping on the Microstructures, Crystallization, and Physical Properties of GeSb9 Films.
Chun-Fan Liu 1 , Yu-Hsun Perng 1 , Chen-Wei Lee 1 , Lih-Hsin Chou 1
1 Dept. Mat. Sci. Eng., National Tsing Hua University, Hsinchu Taiwan
Show AbstractSilicon was doped into GeSb9 (denoted as GeSb9-Si) film to increase the resistivity of crystalline phase and many improved physical properties for phase change memory application were observed after doping. The addition of silicon into the GeSb9 films results in an increase of crystallization temperature from 185°C of undoped to 269°C for 11.9 at.% Si doping and accompanied with a slight reduction of crystallization activation energy (Ec) from 5.4 eV to 4.45 eV. After 1 min’s rapid thermal annealing (RTA) at 280°C, the grazing angle incident x-ray diffraction (GIXRD) analysis reveals that there are preferred Sb orientation phenomenon for samples with Si doping concentration less than 1.7 at.% and the transmission electron microscopy (TEM) observation shows that the grain shape changed from flat-long to equal axis with the width or the size of the grains reduced from about 300 nm of undoped to about 10 nm for 11.9 at.% Si doped samples of 500 Å thickness. Further calculation showed that the lattice parameter increased from 4.48 of undoped to 4.56 Å for films with 11.9 at.% Si. Electron spectroscopy for chemical analysis (ESCA) shows that for as-deposited films, the electron binding energy of Ge 2p3/2 in pure Ge is 1217.0 eV and increases to 1217.3 eV in GeSb9 film and then decreases with Si doping to 1216.8 eV for 11.9 at.% Si film. After heat treatment, the Ge 2p3/2 in pure Ge is still 1217.0 eV and increases to 1217.5 eV in GeSb9 film and then decreases to 1217 eV after Si doping and keeps unchanged with Si doping concentration. ESCA analyses of Sb and Si are also performed. The ESCA information accompanied with the lattice parameter data reveals the binding and position of the three elements both before and after heat treatment and will be discussed in the paper and will assist in the understanding of the physical properties variations with Si addition.. The optical band gaps of amorphous GeSb9-Si films increased with silicon concentration from 0.25 eV of undoped to 1.34 eV for 11.9 at.% Si sample. The resistivity of crystalline phase increased 16.7 times from undoped to 11.9 at.% Si doped film and is much better than N, O or Te doped films, which are only 5.49, 6.85 and 3.29 times for the highest crystalline resistivity reported with doping. The films thickness decrease after RTA annealing are less than 1% for Si doping concentration higher than 1.73 at.%. Static test was also applied to determine the crystallization speed and a slight increase in the crystallization time was observed with Si doping. For 11.9 at.% Si film, it can be crystallized by applying an 8 mW, 21 ns laser pulse of 398 nm wavelength, while 12 ns only of the same laser power is required for pure GeSb9.
6:00 PM - H3.28
Interface Diffusion in Ge2Sb2Te5 Films With TiN and TiW Electrodes.
Yu-Hsun Perng 1 , Lih-Hsin Chou 1
1 Dept. Mat. Sci. Eng., National Tsing Hua University, Hsinchu Taiwan
Show AbstractThis research investigates the diffusion phenomena of the electrode/memory layer or the memory/electrode interface. Four dual-layer-structure samples: Ge2Sb2Te5 (denoted as GST)/TiN, TiN/GST, GST/TiW and TiW/GST were prepared by sputtering on Si substrates. The influence of the two commonly used electrodes, TiN and TiW, on the GST layer, either placed on top or below GST layer, was studied. Nano scale Auger electron spectroscopy (nano-AES) and transmission electron microscopy (TEM) were performed on either rapid thermally annealed (RTA) or traditionally annealed dual-layered samples of varied top electrode thickness (500 and 1000 Å) to study the interface. The results of traditional annealing may simulate repetitive erasing. The TiW cracked significantly for the TiW(500 Å)/GST samples after both RTA and traditional annealing. However a less atomic diffusion and a flatter surface were observed in TiN(500 Å)/GST after heat treatment. All of the as-deposited, RTA annealed and the traditionally heated GST/TiW samples showed a distinct interface with no atomic inter-diffusion with only one exception as stated in the end of the abstract. Even after traditional annealing for 30 min, the distinct interface was remained in GST/TiW sample. These results indicate that TiW layer is functioned as a promising bottom electrode layer and the thinner 50-nm-thick TiN is adopted as the top electrode for improvement in phase-change memory. For TiN(500 Å)/GST samples, the TiN and Te inter-diffused and resulting in a new layer TiNTe. Ge atoms accumulated in the interface between TiN and the TiNTe layer while the Sb atoms diffused down and accumulated on top of the Si substrate after RTA annealing. Whereas for the 10 min’s traditional annealed sample, the Te atoms diffused out due to high vapor pressure. For the thicker TiN top layer (~1000 Å) sample, TiN cracked after RTA annealing and is supposed to be due to thermal stress. The TEM data will also be presented in the paper.
6:00 PM - H3.29
Binary Phase-change Nanowire: Improving Energy Efficiency With Material Scaling.
Bhaskar Nagabhirava 1 , Benjamin Briggs 1 , Bin Yu 1
1 CNSE, SUNY-Albany, Albany, New York, United States
Show AbstractThe improvement of energy-efficiency is an important aspect to make ultra-high-density phase-change memory (PCM) devices. The reduction in programming (especially RESET) current results in less consumed power and fast amorphization of the chalcogenide material, thus enabling energy-efficient phase switching operations. The RESET current is known to depend on melting point, programmable volume/size, and heating efficiency of chalcogenide material and complex interplay of thermodynamic parameters. 1-D phase-change nanowires as the active material in PCM devices offer attractive solutions to overcome larger RESET currents that hinder the broad use of thin film devices. The use of nanowires with small diameters allows the controlling of programmable volume thus reducing the threshold current. In addition, the scaling of nanowire diameter lowers the melting point as it is size-dependent due to quantum effect. Lower melting point results in less power consumption and faster switching speeds during phase transition process. A study of size-dependent phase-transition of chalcogenide nanowire is necessary for understanding and achieving high-density PCM implementation. Here we report study of programming energy of binary nanowire PCMs as a function of nanowire diameter, length and contacting materials. The nanowires are synthesized via metal catalytic vapor-liquid-solid approach. The phase transition behavior and critical device characteristics are probed with nanowire scaling.
6:00 PM - H3.3
Effect of Nitrogen Addition on Electrical Properties, Microstructures and Crystallization Incubation Time of Ge1Sb4Te7 Phase Change Material.
Hyung Keun Kim 1 , Jae Sung Roh 2 , Doo Jin Choi 1
1 Department of Material Science and engineering, Yonsei University, Seoul Korea (the Republic of), 2 Memory R&D Division, Hynix Semiconductors Inc., Icheon Korea (the Republic of)
Show AbstractIn recently, the phase change random access memory (PCRAM) is the focus of attention among the many candidates of next generation non-volatile memory. PCRAM distinguishes the ON/OFF state of memory bits by sensing the resistivity difference of phase change material (PCM). The ON state, namely SET state, is approached by maintaining the temperature upper the crystallization temperature and below the melting point during sufficient time to crystallize the phase change material. The OFF state, namely RESET state is approached by raising the temperature upper the melting point and cooling fast, therefore, the RESET state have disordered atomic arrangement. Measuring the physical properties and studying kinetics of the crystallization process of PCM is the keys for determining the PCRAM performances. In the view of physical properties, selecting the base material for PCM is very important because the approximations of physical properties are determined via selected base material. Also, to modify the physical properties, selecting the impurity and the impurity concentration is very important. In this research, we selected the Ge1Sb4Te7 (GST147) for the base PCM and nitrogen for the impurity. We measured the electrical properties of GST147 and carried out the secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations. By these measurements and analysis, the effects of nitrogen impurity were clearly shown. As increasing the nitrogen concentration, the electrical resistivity was increased and crystallization degrees were decreased. We concluded that this phenomena is very encouraging results to accomplish the low power operation of PCRAM. In the view of kinetic study of the crystallization process, it is very important to observing the crystallization process in extremely short temporal scale. Therefore, we carried out the static test that is very useful method to observe the crystallization degrees in very short temporal scales (~100ns). We observed that the crystallization incubation time decrease with increasing nitrogen concentration. Also, we concluded that the nitrogen incorporated PCM shows the nucleation dominant crystallization process and the PCM without nitrogen shows the growth dominant crystallization process. In summary, we selected GST147 and nitrogen for a base phase change material and impurity material. We measured electrical properties of our PCM and carried out SIMS, XRD and TEM analysis to define the physical properties of GST 147 and modified physical properties by nitrogen addition. Also, we carried out the static test to observe the crystallization process on very short temporal scales.
6:00 PM - H3.30
Conductive Oxide Interlayer between Phase Change Material and Bottom Electrode in PRAM.
Hyun-Goo Jun 1 , Dongbok Lee 1 , Byung-ki Cheong 2 , Ki-Bum Kim 1
1 Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractPRAM (Phase change Random Access Memory) is qualified as a prospective non-volatile memory by fast and stable transition between crystal and amorphous phase in addition with its prominent scalability. Of several issues undergoing improvement, however, decrease of RESET current is critical and partially achieved by various methods such as structure modification, resistance tuning and so on. In this study, it is proposed that thin conductive oxide layer inserted between phase change material and bottom metal electrode can reduce the power and current necessary for phase transformation. The interlayer is expected to have low thermal conductivity originated from oxide for advanced thermal confinement effect and low electrical resistivity enough to prevent from fading resistance difference between crystal and amorphous phase. Numerical calculation indicates that it requires thickness of several nm and resistance lower than 1 ohm m. In order to deposit such thin oxide layer, ALD (Atomic Layer Deposition) is adopted due to its reproducible ability of precise thickness control by source pulsing cycle. Among various oxides, AZO (Al doped ZnO) is suggested because its resistivity is relatively low in comparison to other oxides and easily tunable by changing cycle ratio of Al source to ZnO source. And Al2O3 is known as having lower thermal conductivity when thickness decreases and its resistivity can be tuned by metal doping in ALD process. The operating device with contact side of 150 nm is tested and the effect of such oxide interlayer will be discussed.
6:00 PM - H3.31
Chemical Vapor Deposition of Ge2Sb2Te5 Thin Film for Phase Change Memory.
Seiti Hamada 1 , Takafumi Horiike 1 , Masato Ishikawa 1 2 3 , Hideaki Machida 2 3 , Atsushi Ogura 1 , Yoshio Ohshita 4 , Takayuki Ohba 3
1 , Meiji university, Kawasaki Japan, 2 , Gas-phase Growth Ltd., Koganei-shi Japan, 3 , The University of Tokyo, Tokyo Japan, 4 , Toyota Technological Institute, Nagoya Japan
Show AbstractThis paper describes the chemical vapor deposition of Ge2Sb2Te5 (GST) thin film for phase change memory (PCRAM). Stoichiometric GST film with smooth surface and good step coverage was successfully achieved.Tertiarybutylgerman (t-C4H9GeH3) was selected as a Ge precursor. The Ge precursor such as (CH3)4Ge has a strong Ge-C bond, so the decomposition temperature tends to be high, and a lot of carbon impurities should be incorporated in the films. Low deposition temperature therefor a small amount carbon impurities can be expected by using t-C4H9GeH 3 with one hydrogen atom of dangerous GeH4 substituted by a bulky t-C4H9- group, which should easily be removed. t-C4H9GeH3 is also much safer than GeH4. Triisopropylantimony ((i-C3H7)3Sb) and diisopropyltellurium ((i-C3H7)2Te) were selected as Sb and Te precursors, respectively. Both precursors are lower deposition temperature than those obtained using conventional methyl or ethyl derivatives.The precursor vapors were precisely controlled using H2 carrier gas flow rate, bottle temperature, and pressure in the bottle. After the SiO2/Si or TiN/Si substrate was heated up to the deposition temperature, three precursor vapors were injected into the chamber. The deposition temperature and pressure were varied from 200 to 350°C and from 1 to 50 torr, respectively. The carrier gas flow rates and the bottle temperatures of the Ge, Sb, and Te precursors were varied to control the precursor supplies. The atomic concentrations of Ge, Sb and Te in the films were obtained using X-ray photoemission spectroscopy (XPS) calibrated by an RBS measurement. The film structures were also evaluated by scanning electron microscopy (SEM).XPS results show that the control of the film composition was rather complicated. All three concentrations were varied with changing each one of the three precursor supplies, suggesting some interaction occurred between ((i-C3H7)3Sb) and (t-C4H9GeH3) during CVD reaction process. We also confirmed the smooth surface GST films with good step coverage by applying appropriate deposition pressure and substrate temperture.Finally we successfully controlled the film composition and fabricated the stoichiometric GST film with good conformability by CVD process. We believe this is very important progress for the phase change memory in the next generation.
6:00 PM - H3.32
Crystallization Dynamic of as Deposited and Ion Implanted GeTe Thin Films by Optical Microscopy and Time Resolved Reflectivity Measurements.
Antonio Mio 1 2 , Egidio Carria 1 3 , Riccardo De Bastiani 3 , Maria Miritello 3 1 , Giuseppe D'Arrigo 2 , Maria Grazia Grimaldi 1 3 , Emanuele Rimini 1 2
1 Dipartimento di Fisica e Astronomia, Università di Catania, Catania Italy, 2 IMM, CNR-INFM, Catania Italy, 3 MATIS, CNR-INFM, Catania Italy
Show AbstractThe crystallization dynamic of GeTe 50nm thick films, deposited on a SiO2/Si substrate by RF magnetron sputtering, has been investigated by means of in situ time resolved reflectivity measurements (TRR), optical microscopy, transmission electron microscopy (TEM) during annealing in the 149-160 °C temperatures range and ex situ Raman scattering spectroscopy. The study has been performed on as deposited and ion implanted amorphous samples.In the ion implanted films, the as-deposited layer were irradiated at room temperature by Ge+ ions at energy of 130 KeV and fluency of 104 ions/cm2 to provide a nearly uniform energy loss across the sample.TEM images have shown that crystallization occurs only along the free surface of the samples. During annealing the crystalline nuclei grow up to tens of μm before coalescence. Then, nucleation and growth processes have been observed by optical microscopy in a region of about 4.5*104 μm2. Nucleation rates and growth velocities have been measured from images at different temperatures in the range from 149°C to 157°C. The implanted samples show a nucleation rate one order of magnitude higher than the as deposited amorphous films while the growth velocity is a factor three higher in implanted ones. From these data, activation energies of each process (Enucl and Egrowth) have been obtained independently. Similar values Enucl=4.5±0.3 eV and Egrowth=1.8±0.2 eV have been found for both the samples indicating that the mechanism involved in the crystallization dynamics is the same, the increase in nucleation rate and growth velocity is associated to the preexponential factor which is higher in the implanted samples. The irradiation process creates a local atomic arrangement during the collision cascade evolution which aids by a collective process the crystallization kinetics. The structure of GeTe, as determined by Raman spectroscopy, shows that the effects due to the ion irradiation are comparable to that observed in partially crystallized GeTe films. In particular ion implantation induces the reduction of Ge-rich tetrahedra with respect to the Te-rich tetrahedra, promoting the system to a state closer to the crystalline structure.Similar results have been found by TRR measurements in the 150-160 °C temperatures range.
6:00 PM - H3.33
Structural Transitions in the Ge2Sb2Te5 Phase Change Memory Alloy Under Compression.
Ravhi Kumar 1 , Matthew Jacobsen 1 , Andrew Cornelius 1
1 Physics, University of Nevada Las Vegas, Las Vegas, Nevada, United States
Show AbstractWe have investigated the Ge2Sb2Te5 (GST) phase change memory alloy with the hexagonal structure by synchrotron powder x-ray diffraction up to 61 GPa and resistivity up to 20 GPa at room temperature. The high pressure experiments show a pressure induced structural transition from the hexagonal to CsCl type structure around 17 GPa which is reversible. The CsCl type structure is stable up to 61 GPa. The results show that the pressure induced structural behavior observed for the stable hexagonal structure is different from the cubic form.
6:00 PM - H3.34
Ab initio Calculations of Crystalline and Amorphous In2Se3 Compounds for Chalcogenide Phase Change Memory.
Renyu Chen 1 , Scott Dunham 1
1 Electrical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractAb initio calculations of various configurations of In2Se3 compounds are performed to investigate the transition from crystalline to amorphous phase. The structures considered are based on zinc-blende or wurzite structures with 1/3 of In sites vacant as observed experimentally. From extensive calculations for possible vacancy configurations in In2Se3 compounds, a model based on the local coordination of In/Se atoms is built to predict the energetically favorable vacancy ordering structures. Results indicate that in the most stable In vacancy configurations, Se atoms have coordination of either 2 or 3 (In atoms have coordination of 4). Other coordinations lead to significantly higher formation energies. Results from analyzing the total energy and electronic structure of a range of off-stoichiometry, including vacancies, interstitials and anti-site configurations, suggest that the energetically most favorable way to form In-rich material is via incorporation of Se vacancies, while In anti-site is favorable for formation of Se-rich phase. Based on these calculations, predictions are made on how minor stoichiometry deviations impact structural evolution during phase change. The results are further used as input parameters of the kinetic lattice Monte Carlo simulation technique to simulate the evolution of defect structures during annealing.
6:00 PM - H3.35
Nuclei in Phase Change Materials: The Effects of Composition, Processing Conditions, and Interfaces.
Kristof Darmawikarta 1 2 , Bong-Sub Lee 1 2 , Simone Raoux 3 , Robert Shelby 4 , Charles Rettner 4 , Stephen Bishop 2 5 , John Abelson 1 2
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 4 , IBM Almaden Research Center, San Jose, California, United States, 5 Electrical & Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractThe crystallization rate of many amorphous phase change materials depends strongly on the size distribution of embedded subcritical nuclei. We recently reported that a statistical method, Fluctuation Transmission Electron Microscopy (FTEM), detects the presence of nanometer-scale subnuclei in glassy solids (B.-S. Lee et al., Science, Nov. 2009). Here, we demonstrate the effects of composition and processing conditions on these nuclei using a combination of FTEM, atomic force microscopy (AFM), and pump-probe laser analysis. First, we compare the amorphous states of Ge2Sb2Te5 (GST) and Ag/In-incorporated Sb2Te (AIST) after melt-quenching or low-temperature annealing. A rapid quench from melt produces a substantial population of nuclei in GST, but few nuclei in AIST. This explains why the subsequent crystallization rate in melt-quenched amorphous GST is typically higher than in as-deposited (sputtered) amorphous GST. In AIST, low temperature annealing or laser priming can be used to increase the population of nuclei and thus the crystallization rate. Second, we show the effect of nitrogen incorporation on the distribution of nuclei in GST. In the as-deposited state, pure GST includes more nuclei than nitrogen-incorporated GST ([N] = 5 - 10 at. %). This trend agrees with reports of slower crystallization rates in nitrogen-incorporated GST. Finally, we demonstrate the effects of interfaces on the evolution of nuclei, employing various kinds of capping layers on GST.
6:00 PM - H3.4
Characterizations of AgInSbTe and Its Nanocomposite Thin Films for Phase-change Memory Applications.
Yu-Jen Huang 1 , Tsung-Eong Hsieh 1
1 Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractPhase change kinetics of AgInSbTe (AIST) and AIST-SiO2 nanocomposite thin films containing 10 and 15 wt.% of SiO2 (denoted by 90A10S and 85A15S, respectively) with various thickness were investigated by in-situ electrical property measurement, x-ray diffraction (XRD) and transmission electron microscopy (TEM). A cross-point-type phase-change memory device with 160-μm contact hole was also fabricated to evaluate the static current-voltage (I-V) behaviors.In-situ heating XRD found that all types of samples transform from initially amorphous structure to HCP structure and, according to the integrated intensity calculation, the recrystallization temperature (Tx) of sample increases with the SiO2 amount in nanocomposite samples. Further, as revealed by the broadening of XRD peaks, SiO2 embedment resulted in grain refinement in nanocomposite samples. Subsequent TEM characterization confirmed the results obtained by XRD analysis.Exothermal experiment found that, for all samples the values of Tx increase with the increase of film thickness and heating rate while the activation energy (Ea) calculated in terms of Kissinger’s formula increases with the decrease of film thickness. This illustrates the sample dimension affects the progress of phase transition. The Ea increases with the SiO2 amount in nanocomposites with the same thickness, denoting the SiO2 embedment restrains the grain growth of AIST during recrystallization. Isothermal experiment in conjunction with Johnson-Mehl-Avrami (JMA) theory revealed the decrease of Avrami exponent, indicating that the phase transition is prone to be heterogeneous. This is ascribed to the fact that the dispersed SiO2 particles provides additional nucleation sites and thus promotes the heterogeneous feature of phase transition in AIST.Static I-V measurement indicated that the resistance of nanocomposite layers in the low resistive state are higher than that of AIST and the switching threshold voltage of the 90A10S and 85A15S (1.50 V and 1.65 V) are higher than that of the AIST (1.10 V). Dynamic resistance increases with the SiO2 amount, leading to the reduction of writing current. I-V analysis also confirmed the retardation of recrystallization in AIST due to the incorporation of SiO2 and the rise of Ea is able to enhance the thermal stability of amorphous state in phase-change memory devices.
6:00 PM - H3.5
Ce Doped-GeSbTe Thin Films Applied to Phase-change Random Access Memory Devices.
Yu-Jen Huang 1 , Tsung-Eong Hsieh 1
1 Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractMicrostructure, electrical property and phase-change kinetics of cerium (Ce)-doped GeSbTe (GST) thin films applied to phase-change random access memory (PRAM) were investigated. Unlike other metallic dopants reported previously, Ce doping escalated the phase-change temperature without altering the resistivity levels of amorphous and crystalline GST; the resistivity ratio remains the same at about 105 in the samples containing Ce up to 12 at.%. Such a unique electrical behavior greatly benefits the preservation of signal contrast as well as the high-density signal storage and will not cause the increase of writing current of devices. X-ray diffraction (XRD) indicated that Ce doping may stabilize the amorphous status of GST and suppress the formation of hexagonal GST phase after annealing. Transmission electron microscopy (TEM) in conjunction with element mapping revealed that Ce doping refines the grain size in GST via the solid-solution strengthening mechanism. However, intermetallic compounds (IMCs) likely formed in crystalline GST when Ce content exceeded 10 at.%. found that the Increase of recrystallization temperature (Tx) and activation energy (Ea) of phase transition for GST by Ce-doping was verified by Kissinger’s analysis. Isothermal experiment further depicted a distinct improvement on the retention time of doped-GST, illustrating the benefit of data preservation by Ce doping.In the study of PRAM device applications, it was found that the threshold voltage of device containing doped-GST increases with the Ce content. This confirmed the fact that Ce doping into GST effectively retards the crystallization of GST and improve the stability of the amorphous GST. It nevertheless showed that Ce-doping does not eliminate the phase-change reversibility of GST and is indeed feasible to PRAM device fabrication.
6:00 PM - H3.6
Crystallization Dynamics in Nanoglasses of Phase Change Memory.
Marco Nardone 1 , Mark Simon 1 , Victor Karpov 1 , Ilya Karpov 2
1 Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio, United States, 2 , Intel Corporation, Santa Clara, California, United States
Show AbstractData retention is important for all types of nonvolatile memory. While it is limited by charge loss in flash based devices, loss of glassy domains can affect data retention in phase change memory (PCM) resulting from the fact that glass is unstable with respect to crystal nucleation [1]. It is generally assumed that when crystalline nuclei of radius R each occupy volume fraction exceeding the threshold value vc~0.3, they form a percolation cluster shunting the resistive glassy material and causing retention loss.Our analysis takes into account several factors overlooked in the above percolation scenario: 1) small dimensions of glass domains L~10-100 nm (we call them nanoglasses); 2) randomness of glassy structure that makes nucleation barriers W varying between different microscopic regions with probabilistic distribution ρ(W); 3) electric field induced increase in nucleation rate [2]. Some other factors include reduction of R at low temperatures (T), increase in crystallization temperatures for very thin films, and suppression of crystallization for L<2R [3].Nanoglasses can be shunted by L/2R nuclei forming quasi-linear conductive pathways at volume fractions vc, making percolation cluster unnecessary (a similar role of small dimensions was earlier established in another percolation related problem of hopping conduction [4]). We show that such quasi-linear shunting takes place when L is smaller than the critical thickness Lc, and that for Lc, τ, the time of forming a quasi-linear shunt is determined by the thickness dependent barrier Wc where Lc=2Rln(A/R2)/ln(1/vc), τ=τcexp(-Wc/kT), and ∫Wcρ(W)dW=2Rln(A/R2)/L where A is the integral area of the glass part of the device. We have estimated that the critical thickness Lc~50-100 nm is in the domain of practical interest. This mechanism becomes especially important for PCM arrays of N>>1 cells where effectively A→NA>>Aand Lc is larger. This can describe the experimentally observed early failures [1]. Our analysis predicts the statistics and times of such early failures vs. material parameters, cell dimensions, and external or interface related electric fields. Three of us (M.N., M.A.S., and V.G.K.) gratefully acknowledge the Intel Corporation grant supporting this research.REFERENCES[1] B. Gleixner, A. Pirovano, J. Sarkar, F. Ottogalli, E. Tortorelli, M. Tosi, R. Bez, IEEE 07CH37867 45th Ann. Internat. Reliab. Phy. Symp., Phoenix, p. 542 (2007)[2] V. G. Karpov, Y. A. Kryukov, I. V. Karpov, and M. Mitra, Phys. Rev. B 78,052201 (2008)[3]M. Zacharias and P. Streitenberger, Phys. Rev. B, 62, 8391 (2000).[4] M. Pollack and J. J. Hauser, Phys. Rev. Lett., 31, 21 (1973). M. E. Raikh and I. M. Ruzin, in Mesoscopic Phenomena in Solids, edited by B. L. Altshuller, P. A. Lee, and R. A. Webb (Elsevier, 1991), p. 315.
6:00 PM - H3.7
Theory of Field Dependent Crystal Nucleation in Glasses.
Mark Simon 1 , Victor Karpov 1
1 Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio, United States
Show AbstractPhase change memory operates by the switching between an amorphous phase and crystalline phase in a chalcogenide material. Recently, it was argued that the rate of this phase transition exponentially increases under electric field of the typical switching strengths [1]. It was shown that strong fields favor nucleation of strongly asymmetric needle shaped particles stretched along the field lines. Such spheroidal nuclei are described in the terms of radius R and height H>>R. The preceding work has determined the field dependent nucleation barrier for needle shaped particles in strong fields; however the pre-exponential factor of the nucleation rate remained unknown.
In this work, we give a systematic derivation of two-dimensional (2D) nucleation rate corresponding to the R and H degrees of freedom and including both the exponent and the pre-exponential factor. Mathematically, this 2D problem is considerably more complex than that of the classical 1D nucleation evolving with radius R of a spherical particle.
Our approach is based on the Fokker-Planck equation describing evolution of the embryo distribution function f(R,H) for steady state nucleation [2] driven by the field-dependent change in free energy. While emphasizing the strong field regime, our approach successfully describes the entire range of field strengths and determines the conditions for which spherical or needle-like nucleation dominate.
As a result we derive the equation for 2D nucleation rate where the pre-exponential includes properly modified Zeldovich factor known in 1D nucleation theory and is expressed in the terms of growth velocity of post-critical nuclei that is experimentally known [3] for important chalcogenides used with phase change memory applications.
Our results can find applications describing switching of phase change memory due to crystal nucleation in strong fields and simultaneously effects of much weaker fields effects that may be important with the data retention issues.
The authors gratefully acknowledge the Intel Corporation grant supporting this research.
REFERENCES[1] I.V. Karpov, M. Mitra, D.Kau, G. Spadini, Y.A. Kryukov, and V.G.Karpov,Appl. Phys. Lett. 92, 173501 (2008); V. G. Karpov, Y. A. Kryukov, I. V. Karpov, and M. Mitra, Phys. Rev. B 78,052201 (2008).[2] E. M. Lifshitz and L. P. Pitaevskii, Physical Kinetics, Elsevier 2008.[3] J. Kalb, F. Spaepen, and M. Wuttig, Appl. Phys. Lett. 84, 5240 (2004). J. A. Kalb, C. Y. Wen, and Frans Spaepenb, H. Dieker and M. Wuttig, J. Appl. Phys., 98, 054902 (2005)
6:00 PM - H3.8
Growth and Crystallization Behavior of Ge Doped SbxTey Thin Films Deposited by a Plasma-enhanced CVD.
Seol Choi 1 , Byung Joon Choi 1 , Taeyong Eom 1 , Jae Hyuck Jang 1 , Woongkyu Lee 1 , Cheol Seong Hwang 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractPhase change random access memory (PcRAM) utilizes the largely different resistances of the crystalline and amorphous phase change materials, most typically Ge2Sb2Te5, where the phase change was achieved by current pulses. For the application of the PcRAM in universal non-volatile memories, operation speed of PcRAM must be improved. Ge-doped SbTe (Ge-ST) has been reported to have a faster crystallization speed and better thermal stability than Ge2Sb2Te5 for its application to optical memory devices. Accordingly, in this study, the Ge-ST films are deposited by a cyclic plasma-enhanced CVD (PECVD) at 150°C on TiO2/ SiO2/Si substrates using a shower-head type 8 in.-scale plasma-enhanced ALD reactor for applying it to high speed PcRAM. Ge(i-C4H9)4, Sb(i-C3H7)3, and Te(i-C3H7)2 were used as the Ge, Sb, and Te precursors, respectively. The sequence of precursor injection pulses was Sb-Ge-Te (super-cycle) and 1 super-cycle was composed of 3 elemental sub-cycles. Each sub-cycle was composed of the precursor pulse - precursor purge pulse - reduction gas pulse - process purge pulse. Reduction gas was H2-Ar mixture and carrier gas was Ar. Experiments were conducted under the following conditions; Te source feeding time was fixed (0.6s). Sb and Ge source feeding time were varied from 0.5 to 2.5 s and 0 to 4.5s, respectively. With these conditions, ST compositions near Sb7Te3 and Sb2Te3 were achieved. For these two Sb-Te composition materials, Ge was doped and its influence on the crystallization and electrical resistivity were studied. Ge increased the overall film growth rate when its concentration is over 10at%. The composition and crystallographic phase of the as-deposited film affected the resistivity and crystallization temperature of the film. Ge17Sb58Te25 and Ge18Sb53Te29 showed the highest resistivity of ~ 670 Ωcm and crystallization temperature of ~ 195°C. Two base alloys, Sb7Te3 and Sb2Te3, showed different crystallization behavior as Ge was doped. The Ge solubility and crystallization kinetics of the two SbTe alloys provided clues to understand the crystallization behavior of the films at the growth temperature. Surface morphologies observed by FE-SEM showed complicated structures where unidentified particles were included in the amorphous-like matrix. Detailed analysis on the effects of the overall composition on the phase formation of the film and phase change properties by the current pulse will be also reported.
6:00 PM - H3.9
Crystallization Properties of Ge-Sb-Te Phase Change Materials Studied by Time-resolved X-ray Diffraction and Static Laser Tester.
Becky Munoz 1 , Simone Raoux 2 , Jean Jordan-Sweet 3 , Darryl Butt 1
1 , Boise State University, Boise, Idaho, United States, 2 , IBM/Macronix PCRAM Joint Project, Yorktown Heights, New York, United States, 3 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractPhase change materials on the GeTe-Sb2Te3 pseudo-binary line, in particular Ge2Sb2Te5, as well as Ag- and In-doped Sb2Te (AIST), have been widely applied as re-writable optical storage materials. However, their relatively low crystallization temperatures are problematic for phase change random access memory (PCRAM) applications. This is particularly true for automotive applications, where data retention at 150 °C for 10 years is required. The work presented here investigates the crystallization temperatures and scaling behaviors, using time-resolved x-ray diffraction (XRD), and switching times, using laser static testing, of memory alloys with the compositions GeSb2Te3, Ge2Sb5Te4, Ge6Sb4Te5, Ge15Sb85. Time-resolved XRD allowed for a study of crystallization temperatures as a function of film thickness (2, 3, 5, 10 and 50 nm). XRD measurements were able to determine the lower limits in terms of scalability for these phase change materials. We found a critical film thickness below which films no longer crystallized. The alloys GeSb2Te3 and Ge15Sb85, as well as the benchmark material, Ge2Sb2Te5, crystallized for all thin films measured between 2 and 50 nm, while alloys of Ge2Sb5Te4 and Ge6Sb4Te5 crystallized film thicknesses of 3 nm and greater. The second benchmark material, AIST, crystallized film thicknesses of 5 nm and greater. The crystallization temperatures of 50 nm thick films were 129, 236, 250 and 242 °C for GeSb2Te3, Ge2Sb5Te4, Ge6Sb4Te5 and Ge15Sb85, respectively, compared to 156 °C for Ge2Sb2Te5 and 170 °C for AIST, the benchmark materials (all measured at 1 K/s heating rate). Thus, all alloys except GeSb2Te3 show higher crystallization temperatures than the benchmark materials indicating better data retention properties. Increased crystallization temperatures with decreasing film thickness were observed for all alloys. This scaling behavior of the crystallization temperature is beneficial because increased crystallization temperature relates to improved data retention.Static laser testing of the amorphous and crystalline samples provided crystallization times. The laser studies measured the change in reflectivity during the crystallization of as-deposited, amorphous material, and the re-crystallization of melt-quenched, amorphous material. The alloys GeSb2Te3, Ge2Sb5Te4 and Ge15Sb85 showed comparable switching times to that of the benchmark alloys. Ge6Sb4Te5 exhibited long crystallization times, suggesting it would be inadequate for PCRAM applications.In summary, the alloys GeSb2Te3, Ge2Sb5Te4 and Ge15Sb85 have substantially higher crystallization temperatures and crystallization times comparable to benchmark phase change materials making them promising candidates for PCRAM applications.
Symposium Organizers
Paul Fons National Institute for Advanced Industrial Science and Technology
Kris Campbell Boise State University
Byung-ki Cheong Korea Institute of Science and Technology
Simone Raoux IBM T. J. Watson Research Center
Matthias Wuttig I. Physikalisches Institut der RWTH Aachen
H4: Experiment I
Session Chairs
Wednesday AM, April 07, 2010
Room 2009 (Moscone West)
9:00 AM - **H4.1
What Are We Going To Do For The New Applications of Future PRAM?
Dae-Hwan Kang 1
1 , Samsung Electronics Co. Ltd., Yongin-city, Gyunggi-do Korea (the Republic of)
Show AbstractIt is not one decade until new-concept phase-change random access memory (PRAM) shows up in the fast-growing non-volatile memory market since it is well compatible to existing CMOS fabrication processes and is highly scalable to sub 30nm superior to convention Flash memory. On increasing revenue in the mobile markets such as smart phone and portable multimedia player, major chip makers are now turning their eyes toward finding new applications of future PRAM. In this presentation, for this, we consider some technical prerequisites in terms of memory capacity, programming speed, and/or reliability with the respective on-going research and engineering works. Firstly, current status and issues about novel cell structure, programming current scaling including electrode engineering, and multi-level cell techniques are given to achieve a high memory capacity above 512Mb or 1Gbit. Next, we comment the design of new phase-change materials and set write pulse engineering in order to accelerate a sluggish set operation speed which limits write performance. Finally, we discuss reliability issues such as data retention, write endurance, thermal disturbance and interface stability.
9:30 AM - **H4.2
Development Orientations for Phase-change Memory.
Enrico Varesi 1
1 R&D - Technology Development, Numonyx, Agrate Brianza, Milan, Italy
Show AbstractPhase Change Memory (PCM) technology is demonstrating the capability to enter in the memory market and to provide new features, combining NVM and DRAM aspects, suitable also for new memory applications. The control and the optimization of phase change materials in terms of their physical and chemical characteristics are fundamental key points to achieve suitable performances (programming speed, fail control, data retention) and to enable their integration into multi-Gb devices. This review will focus some of the main development orientations adopted for the process integration and for the chalcogenide materials exploration.
10:00 AM - H4.3
Controlling Charge Transport in Phase Change Materials.
Theo Siegrist 2 1 , Michael Woda 1 , Stephan Kremers 1 , Peter Jost 1 , Philipp Merkelbach 1 , Pascal Rausch 1 , Matthias Wuttig 1
2 Chemical and Biomedical Engineering, Florida State University, Tallahassee, Florida, United States, 1 Institute of Physics (IA), RWTH Aachen, Aachen Germany
Show AbstractPhase change materials are one of the most promising materials for emerging electronic data storage applications. By application of a short voltage pulse they can be rapidly and reversibly switched between the crystalline and amorphous state. The pronounced change of resistance upon crystallization, which often spans more than four orders of magnitude, is particularly attractive, since it should enable multilevel storage. This storage attribute is crucial to compete with ongoing developments in flash memories. In addition, it was recently shown that devices could be reversibly switched in 4 nanoseconds reaching DRAM like speeds. The combination of high switching speeds and non-volatility might even help to establish this memory as a universal memory replacing both Flash and DRAM. This appears feasible if the power consumption upon reading, writing and erasing bits can be minimized. The most power-demanding step is the erasure of a bit that is achieved by local amorphization of a previously crystalline region. The necessary power could be significantly reduced if phase change materials with a higher resistance in the crystalline phase would be available. In this case, heating efficiency is enhanced. Therefore we have focused on a systematic understanding of the charge transport in amorphous and crystalline phase change materials. The goal of these studies is the identification of the dominant scattering mechanism as well as the development of concepts to modify this scattering mechanism.
10:15 AM - H4.4
Enhanced Thermal Efficiency of Ge2Sb2Te5 Phase Change Films Using the Microstructure Modification.
Dongbok Lee 1 , Ho-ki Lyeo 2 , Hyun-Seok Lee 3 , Hyun-Goo Jun 1 , Byung-ki Cheong 3 , Ki-Bum Kim 1
1 Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of), 3 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractPhase change random access memory (PRAM) utilizing chalcogenide materials, most notably Ge2Sb2Te5 (GST), has attracted much attention as the next generation nonvolatile memory device due to its fast switching speed, outstanding endurance, good scalability, and CMOS compatibility. However, in order to further scale down the device size, the reduction of the RESET current is one of the most important issues to be resolved, since the out-put current level of the transistor or diode is also scaled down with the device size. As one of the novel methods to reduce the RESET current, we previously reported the formation of GST nanoclusters (several tens of nanometer in size) self-encapsulated by SiOx, and proposed that these microstructures could be employed in PRAM devices. Furthermore, we suggested the replacement of SiO2 with TiO2 in these microstructures for reducing the SET resistance of a PRAM device. As an additional advantage of the GST-TiOx films, due to larger dissimilarity in the Debye temperature with TiO2 than with SiO2, we can expect that the thermal conductance through an interface is reduced more, and thus the thermal efficiency is enhanced. In this presentation, we report about the enhanced thermal efficiency of phase change layer consisting of a GST-TiOx nanocluster system. Microstructures were analyzed by transmission electron microscopy. Using a homemade static tester with a heating and a probe laser beam, we determined the powers required for melting of the crystalline GST and GST-TiOx films, and the change in real-time optical reflectivity accompanying laser-melting. The thermal conductivities of the GST-TiOx films were measured by time-domain-themoreflectance method. The laser-power for melting was reduced by 27% relative to pure GST likely due to limited thermal diffusion resulting from reduced thermal conductivity being lowered from 0.44 to 0.20 Wm-1K-1. Such notably low thermal-conductivity is explained with thermal-boundary-resistance-effect from the microstructure of the nanoclustered GST-TiOx films.
10:30 AM - H4:Exper
BREAK
11:00 AM - **H4.5
Synthesis and Electrical Characterization of Amorphous GeTe Nanoparticles.
Marissa Caldwell 1 , Simone Raoux 2 , Robert Wang 3 , Delia Milliron 3 , H.-S. Philip Wong 4
1 Chemistry, Stanford University, Stanford, California, United States, 2 , IBM Watson Research Center, Yorktown, New York, United States, 3 The Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, United States, 4 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe potential for scaling to extremely small sizes is often cited as one of the key advantages of phase change memory as it competes as a non-volatile memory technology. One of the major factors limiting phase change memory scaling is the critical dimension at which the active phase change material (PCM) loses the bi-stability of its crystalline and amorphous phases. In order to investigate the properties of PCM at extremely small dimensions, GeTe nanoparticles were synthesized through a colloidal route. The synthesized nanoparticles are 2-7nm in diameter and were characterized by TEM, EDAX and XRD to determine their size, composition and structure. Using XRD with in-situ heating, we were able to probe the dependence on size of the amorphous phase stability. We found that even the largest nanoparticles retain their amorphous phase at temperatures exceeding 300°C, over 150°C higher than the bulk crystallization temperature. By measuring the crystallization temperature of different sized nanoparticles, we found an increase in crystallization temperature as the size decreased, implying the smaller nanoparticles have higher amorphous phase stability. These results imply better lifetime data retention as the programming volume of the PCM scales. In addition, we report electrical resistivity versus temperature measurements demonstrating that nanoparticle films retain the high resistivity contrast required for phase change memory applications. We also report the fabrication of line devices where the PCM was deposited through solution processing of the nanoparticles into a continuous film.
11:30 AM - **H4.6
Electrical Switching Behavior of Phase Change Memory Devices.
Martin Salinga 1
1 I. Institute of Physics, RWTH Aachen University, Aachen Germany
Show AbstractExperimental results from electrical switching experiments on phase change memory cells in a vertical architecture are presented. Recrystallization of amorphized cells have been achieved using voltage pulses with durations in the single nanosecond range. This underlines the aspiration of phase change memories to not only compete with Flash technology but to also move forward into the direction of non-volatile memories with DRAM-like performance. The importance of crystallization kinetics for the understanding of those results will be discussed as well as the switching dynamics that is probed by the time resolved measurements of both the voltage applied to the cell and the corresponding current.
12:00 PM - H4.7
Perspectives of Nanostructured Phase Change Materials for High Speed Non-volatile Memory.
Desmond Loke 1 , Weijie Wang 2 , Luping Shi 2 , Rong Zhao 2 , Hongxin Yang 2 , Kian Guan Lim 2 , Lung Tat Ng 2 , Hock Koon Lee 2 , Yee-Chia Yeo 3 , Tow Chong Chong 2 3
1 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore Singapore, 2 , Data Storage Institute, Singapore Singapore, 3 Department of Electrical and Computer Engineering, National University of Singapore, Singapore Singapore
Show AbstractThe emergence of fast crystallizing materials from the GeTe-Sb2Te3 pseudo-binary line has attracted many researchers’ interest on the fast speed phase change memory technology. Over the years, the progressive scaling of the PCRAM has further improved its switching speed significantly. However, the crystallization speed of the PCRAM is relatively slow and the attempts to improve the switching speed are limited by the intrinsic nature of the phase change materials. These materials typically exhibit faster switching speed at the expense of power and stability. More recently, our research group suggested that the nanosize effect of nanostructured GST materials can significantly increase the phase switching speed, and has demonstrated the advanced switching performance of the Ge2Sb2Te5 nanocells with fast switching speed in the time scale of hundred picoseconds. In this paper, the ultrafast switching capability of the GST and doped-GST are investigated experimentally, and the correlation between the switching speed and the PCRAM cell dimension is demonstrated. When the cell dimension is sufficiently small, ultrashort switching pulse widths less than 1 ns are found capable of switching the GST and doped-GST PCRAM cells, while maintaining their superb power and stable device performances. The material and nanosize effects of the nanostructured phase change materials have been strongly attributed for their ultrafast switching ability. We believed that these effects will provide new mechanism factors to achieve high speed, low power and stable memory devices for important industry applications.
12:15 PM - H4.8
Improved Temperature Dependence of Phase Change Memory Device Using Ge-doped SbTe.
Zhe Wu 1 2 , Youngwook Park 2 3 , Hyung-woo Ahn 2 , Suyoun Lee 2 , Jeung-hyun Jeong 2 , Dooseok Jeong 2 , Kwangsoo No 1 , Byung-ki Cheong 2
1 Material Sicience and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Material Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractPhase Change Random Access Memory (PCRAM) based on Ge2Sb2Te5 (GST) is on the verge of commercialization as a strong candidate for next generation non-volatile memory in place of a flash memory. The present GST-based PCRAM has many promising attributes, but doesn’t seem to offer a high writing speed as expected due to use of a time-consuming writing scheme to obtain a low SET resistance required for good sensing speed and margin. Ge-doped SbTe (GeST) [1] is posed as a promising material to overcome the problem and the GeST-based device was demonstrated recently to provide indeed a faster writing speed and a lower SET resistance than the GST-based device [2]. In the present study, comparison between the two materials was made further with respect to temperature dependence of device characteristics that comprises an important reliability issue by itself. As it turned out, GeST-based device showed significantly improved temperature dependence compared with that of GST-based device. As for device resistances, RESET and SET resistances were found to decrease significantly with increasing operation temperature for GST but respectively displayed smaller and no change with temperature for GeST. A similar trend was observed for dynamic resistance as well, leading to RESET current increasing with operation temperature for GST-devices but almost no change with temperature for GeST-devices. These findings are consistent with the fact that the crystallized fcc-GST is semiconducting but the crystallized Ge-ST is rather metallic in nature. Temperature dependence of threshold voltage was also examined to find linearly decreasing trends with operation temperature for both materials, in support of relaxation semiconductor model for threshold switching in chalcogenide glasses.[1] Martijn H. R. Lankhorst et al., Nature Mat. 4, 347 (2005).[2] Suyoun Lee et al., J. Electrochem. Soc. 156, H612 (2009)
12:30 PM - H4.9
In situ TEM Investigation of Electrically-driven Phase Change Behavior in Ge2Sb2Te5 Nanowire Devices.
Yeonwoong Jung 1 , Sung-Wook Nam 1 , Ritesh Agarwal 1
1 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractPhase change memory (PCM) is regarded as a very promising alternative to conventional memory devices such as DRAM or FLASH owing to its nonvolatility, scalability and fast switching speed. However, the fundamental origin of the fast switching process in PCM has been hard to be achieved, which mostly comes from the difficulties to separate the electric field effect from the high temperature processes. The unique geometry of one-dimensional nanowires (NW) serves as a good platform to investigate the atomic structures during electrically-driven phase change process via in situ transmission electron microscopy (TEM) observation. In NW PCM, the compositions and the grain formations can be easily monitored during the switching process, which offers a great opportunity to explore the underlying mechanism of the electrically-driven phase change behavior.Our study is focused on the direct observations of the electrically-driven phase change behavior through TEM. We utilized an electron transparent SiN membrane on which we assemble vapor-liquid-solid (VLS) grown Ge2Sb2Te5 NWs for simultaneous electrical biasing and structural/chemical analysis. We carried out both current-voltage (I-V) sweeping and programming operations of Ge2Sb2Te5 NW devices and obtained reliable threshold switching and excellent phase switching behavior between crystalline and amorphous states. TEM observation clearly shows that the hot spots region defined in the nanowire devices localize the heat and act as active phase change region. Very clear signatures of amorphous RESET and polycrystalline SET states are observed with HRTEM. We show a direct evidence of nucleation dominant recrystallization mechanism in Ge2Sb2Te5 nanowire devices with electrical switching, consistent with thermally induced process. What is more interesting is the structure of the device just above threshold switching; we have observed the evidence of electric-field assisted nucleation process which guides the crystalline nuclei formation in a correlated manner with the applied electric field, and unlike the random nucleation and percolation behavior typically observed in thermally-induced recrystallization. Similarly, during RESET (amorphization) operation, the recrystallized region next to the amorphous phase has aligned grains attributed to the directional grain growth during melting process. It is believed that the uniform current path in near-melting state may allow the grains to be aligned in the direction of the electric current. Our observations suggest that the electric field plays a unique role during electrically-driven phase change process and may explain the fast switching process observed in these materials. The role of filament formation by electric field during switching will be discussed and the implications of these findings for modeling the electrical properties of phase change materials will be presented.
12:45 PM - H4.10
Pressure-induced Changes in Phase Change Memory Alloys.
Milos Krbal 1 , Alexander Kolobov 1 , Julien Haines 2 , Paul Fons 1 , Claire Levelut 4 , Rozen Le Parc 4 , Michael Hanfland 3 , Junji Tominaga 1 , Annie Pradel 2 , Michel Ribes 2
1 , Center for Applied Near-Field Optics Research (CanFor),National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba 305-8562 Japan, 2 , Institut Charles Gerhardt, UMR 5253 CNRS-UM2-ENSCM-UM1, PMDP/PMOF, Université Montpellier II, Place Eugène Bataillon, Montpellier Cedex 5 France, 4 , Laboratoire des Colloides, Verres et Nanomatériaux, Université Montpellier II, Place Eugène Bataillon, Montlpellier Cedex 5 France, 3 , European Synchrotron Radiation Facility (ESRF), 6 rue Jules Horowitz, Boîte Postale 220,Grenoble France
Show AbstractWe demonstrate, both experimentally and by computer simulation, that while the metastable face-centered cubic (fcc) phase of Ge-Sb-Te becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. Upon higher compression, a body-centered cubic phase is obtained in both cases around 30 GPa. Upon decompression, the amorphous phase is retained for the starting fcc phase while the initial structure is recovered for the starting trigonal phase. We argue that the presence of vacancies and associated subsequent large atomic displacements lead to nanoscale phase separation and loss of initial structure memory in the fcc staring phase of Ge-Sb-Te. We futher compare the amorphous phase obtained via the pressure route with the melt quenched amorphous phase.
H5: Experiment II
Session Chairs
Wednesday PM, April 07, 2010
Room 2009 (Moscone West)
2:30 PM - **H5.1
Understanding Phase Change Memory Reliability and Scaling by Physical Models of the Amorphous Chalcogenide Phase.
Daniele Ielmini 1
1 Dipartimento di Elettronica e Informazione, Politecnico di Milano, Milano, MI, Italy
Show AbstractAfter its first conception in the 1960s, chalcogenide-based phase change memory (PCM) has been revamped during the last decade as a possible nonvolatile memory solution for post 20nm generations. PCM features extremely fast operation and good endurance, however the reliability mechanisms associated with the amorphous state and the ultimate scaling capability are still a matter of debate. More physical insight into the basic mechanisms controlling the memory operation, such as carrier and heat conduction, threshold switching, phase change and structural relaxation are needed to develop accurate device models and physics-based guidelines for material selection.The purpose of this talk will be to clarify the basic reliability issues affecting PCM, including current fluctuations, resistance drift and crystallization in the amorphous chalcogenide material. The reliability mechanisms will be explained by a unified model for the metastable amorphous phase, where minimization of the internal energy occurs through thermal excitation of the structure by many-phonon phenomena. Physically-based models for structure relaxation and crystallization will be discussed, highlighting the common kinetics based on universal entropy/enthalpy quantitative arguments. Sources of variability affecting reliability and scaling will be identified, addressing space/time fluctuations of chemical composition and structural bonds. The impact of composition fluctuation on crystallization time distribution will be shown. A microscopic picture for electrical conduction and switching will be developed, allowing for an understanding of device electrical and program/erase characteristics. Finally, possible scaling showstoppers will be identified and discussed from a physical-modeling perspective.
3:00 PM - H5.2
Effect of Pressure on the Amorphous Structures of Ge2Sb2Te5 and Its Implications to the Mechanism of Resistance Drift.
Seungwu Han 2 , Jino Im 1 , Eunae Cho 2 , Dohyung Kim 3 , Hideki Horii 3 , Jisoon Ihm 1
2 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 1 Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 3 Process Development Team, Semiconductor R&D Center, Samsung Electronics, Hwasung Korea (the Republic of)
Show AbstractIn the phase-change memory (PRAM) cell, the chalcogenide compounds such as Ge2Sb2Te5 (GST) are usually confined in a volume corresponding to the crystalline density and tightly bound by rigid materials such as TiN and W. Because of the disparate densities between the crystalline and amorphous phases of GST, a significant stress results when the amorphous phase is created inside the cell. In the extreme case, if the whole GST material undergoes a RESET process, the amorphous phase will be formed with its volume compressed by 6.5 %. Even though the electronic structures of the chalcogenide glass are known to be sensitive to the applied pressure, the confinement effect on the amorphous GST has not been highlighted much in experimental as well as theoretical studies. In this presentation, we investigate the pressure effects on the atomic and electronic structures of amorphous GST using the first-principles methods. During the melt-quench simulations, the supercell volume was varied from 0.94 to 1.13 times the ideal amorphous volume. As the cell is pressurized, it is found that the energy gap is reduced and the density of localized in-gap states increases. This indicates that the pressurized amorphous GST is more conducting than those made under stress-free conditions. It is also found that compressing amorphous GST causes an increase of four-fold rings, shifting the local order toward that of the crystalline phase. Consistently, the fast re-crystallization process is observed for the compressed amorphous GST. Based on our simulation results, we propose a mechanism for the resistance drift in which the relaxation process in the amorphous GST corresponds to the growth of the crystalline seed inside, thereby lowering the internal stress. Our model can consistently explain the experimental observations on the resistance drift such as the dependence of the drift exponent on the amorphous size.
3:15 PM - **H5.3
Extremely Low Temporal Drift in Phase Change Nanowire Memory Devices.
Ritesh Agarwal 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractI will discuss our efforts in studying reversible crystalline to amorphous phase transitions in chalcogenide nanowires (GeTe, Ge2Sb2Te5), which are becoming important materials for Phase Change Memory (PCM) devices. Of the different memory device concepts being currently explored, PCM devices based on Ge-Sb-Te alloys are very promising for scalable device size, high-speed operation with nonvolatile random accessing capability. However, the top-down nature of thin-film device fabrication leads to scalability issues at sub-100 nm size. Therefore, there is great interest in developing new materials and processing techniques to overcome this barrier. Self-assembled nanowires are particularly promising owing to their sub-lithographic size that is free of etch-induced damage. Reversible phase transitions in single-crystalline nanowire devices scaled down to 20 nm sizes are observed with dramatic reduction in switching currents and power consumption. High-resolution TEM results clearly show that recrystallization occurs via nucleaction dominant mechanism, which follow the classic Avrami type kinetics even at sub-30 nm sizes. Our efforts towards assembling multi-state memory switching devices utilizing the different size-dependent electronic and thermal properties of GeTe and Ge2Sb2Te5 materials in core-shell heterostructured nanaowires will also be discussed. Time dependent drift of parameters such as threshold voltage (Vth) and amorphous state resistance R have important implications because they lead to spontaneous changes in the measurable device parameters used for recording and reading information in PCM devices. Our study on PCM devices based on Ge2Sb2Te5 nanowires show much lower temporal drift of device parameters such as threshold voltage, and amorphous state resistance compared to conventional thin-film PCM devices It has been observed that PCM nanowires exhibit almost one order of magnitude lower drift coefficients in comparison to thin-film devices. We believe that the lower temporal drift observed in nanowire devices is due to their unique geometry which has a large surface area leading to efficient stress relaxation during the formation of the amorphized “dome”. We will present results from experiments where stress in re-introduced into nanowire devices, upon which they display much higher temporal drift behavior. These measurements strongly suggest that temporal drift observed in PCM devices is dominated by stress-relaxation and not due to electronic relaxations involving dynamics of the intrinsic traps. Our studies suggest that phase-change nanowires hold great promise as building blocks for miniaturized memory devices and for in-depth understanding of size-dependent phase transitions in confined geometries in self-assembled nanostructures.
3:45 PM - H5:Exper2
BREAK
4:15 PM - H5.4
A Study on the Temperature Dependence of the Threshold Switching Characteristics of Ge2Sb2Te5.
Suyoun Lee 1 , Doo Seok Jeong 1 , Jeung-hyun Jeong 1 , Zhe Wu 2 , Young-Wook Park 3 , Hyung-Woo Ahn 1 , Byung-ki Cheong 1
1 Materials Science and Tech. Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 3 Dept. of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractWe investigated the temperature dependence of the threshold switching characteristics of a memory-type chalcogenide material, Ge2Sb2Te5. We found that the threshold voltage (Vth) decreased linearly with temperature, implying the existence of a critical conductivity of Ge2Sb2Te5 for its threshold switching. In addition, we investigated the effect of bias voltage and temperature on the delay time (tdel) of the threshold switching of Ge2Sb2Te5 and successfully described the measured relationship by an analytic expression which we derived based on a physical model where thermally activated hopping is a dominant transport mechanism in the material. These results support a model for the threshold switching in chalcogenide materials where the threshold switching is described to occur when the carrier distribution is disequilibrated to have a high concentration of free carriers near an electrode.
4:30 PM - H5.5
Fermi-level Pinning Effect at the Interface Between Phase Change Materials and Metals.
Lina Wei-Wei Fang 1 , Rong Zhao 2 , Jisheng Pan 3 , Zheng Zhang 3 , Luping Shi 2 , Tow-Chong Chong 1 2 , Yee-Chia Yeo 1
1 , National University of Singapore, Singapore Singapore, 2 , Data Storage Institute, A*STAR , Singapore Singapore, 3 , Institute of Materials Research and Engineering, A*STAR , Singapore Singapore
Show AbstractPhase change random access memory (PCRAM) is a promising next-generation non-volatile memory technology due to its high speed, excellent cyclability, and inherent scaling capability. However, its reset current is high and should be further reduced with innovative materials and device design. Doping Ge2Sb2Te5 with nitrogen has been demonstrated to achieve improved PCRAM performance. As Ge2Sb2Te5 is sandiwiched between metal electrodes, detailed understanding of the fundamental properties that affect carrier transport across the interface between phase change material and metals would be important for further optimization of PCRAM devices. In this work, the band alignment between various metals and Ge2Sb2Te5 as well as with nitrogen-doped Ge2Sb2Te5 is investigated. Fermi level pinning between metals and either crystalline and amorphous states of Ge2Sb2Te5 is investigated. Metals (aluminium, tungsten and platinum) whose vacuum work function spans a wide range from 4.2 eV to 5.65 eV were used. Low and negative hole barrier heights were obtained between the metals and the various nitrogen-doped Ge2Sb2Te5 films. On the other hand, in the crystalline state, the hole barrier was found to increase as compared to in the amorphous state. Changes in the hole barrier height is observed upon addition of nitrogen into Ge2Sb2Te5. In addition, significant metal Fermi level pinning at the interface towards the valence band energy of the amorphous phase change films is observed, leading to low barrier heights and good ohmic contacts. Fundamental interface parameters, including slope material and charge-neutrality level of Ge2Sb2Te5, are derived.
4:45 PM - H5.6
Electrical and Thermal Contact Resistance of the Phase-change Material Cycled by Micro-thermal Stage.
SangBum Kim 1 , Rakesh Jeyasingh 1 , John Reifenberg 2 , Jaeho Lee 2 , Mehdi Asheghi 2 , Kenneth Goodson 2 , H.-S. Philip Wong 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show Abstract Phase change memory (PCM) is one of the most mature candidates for a next generation non-volatile memory. New device structures and materials have been proposed to reduce programming current and power. To accurately evaluate the performance of PCM devices, we need a better understanding of interface properties such as electrical and thermal contact resistances. Interface properties become more dominant compared to bulk properties when PCM device is further scaled down. We show the measurement results on electrical and thermal contact resistances of crystalline phase Ge2Sb2Te5 (GST) from the same GST and metal interface cycled between amorphous and crystalline phase by the micro-thermal stage (MTS). Thermal measurement requires large interface area which cannot be cycled by GST self-heating due to large programming voltage and current requirement. To overcome this, we integrate the contact resistance measurement structures with the metal heater on the MTS to accurately control temperature of the phase-changing material in less than μs. Previously, MTS has been shown to be a valuable tool to measure temperature dependent properties of PCM devices such as drift, threshold switching voltage, and crystallization kinetics. The temperature achieved by the Joule heating of the MTS heater can be up to the melting temperature of the phase-changing material. The integrated lateral heater on the MTS has a small time constant to quench fast enough to program adjacent region of GST into amorphous phase. MTS can crystallize GST as well with a voltage smaller than large threshold switching voltage of large volume of amorphous GST. Electrical contact resistance is measured by the Kelvin contact resistance measurement. Thermal interface resistance is measured either by 3w measurement or laser-reflectance measurement. We further discuss the physics behind the interaction between thermal and electrical contact resistances.
5:00 PM - H5.7
Study of Chalcogenide Materials Interfaces in a PCRAM Cell.
Sebastien Loubriat 1 , Frederic Fillot 1 , Patrice Gergaud 1 , Denise Muyard 1 , Emmanuel Gourvest 3 4 , Laurent Vandroux 1 , Anne Roule 1 , Marc Verdier 2 , Sylvain Maitrejean 1
1 Leti - DPTS - SIDE, CEA Grenoble, Grenoble France, 3 , ST Microelectronics, Crolles France, 4 , Microelectronics Technology Laboratory (LTM), Joseph Fourier University (UJF), CEA/LETI/D2NT/LTM , Grenoble France, 2 , SIMAP (INPG-UJF-CNRS), Grenoble France
Show AbstractThe cyclability of phase-change random access memory (PCRAM) can be limited by the adhesion between the phase-change material (PCM) and the surrounding layers integrated in the memory cell. The change in density due to the phase transitions implies large stresses to the phase-change material which can cause alteration and fracture. A good understanding of the interfaces is then required to increase the adhesion and cyclability. In this study, we focus on 2 critical interfaces: PCM / oxide and TEC (Top Electrode) / PCM.After evaluation of mechanical properties (nanoindentation) and internal stresses, we studied the adhesion of Ge2Sb2Te5, doped or undoped-GeTe materials on different silicon oxides deposited by various techniques (nanoscratch, four point bending). We evaluated the influence of adhesion layers between the PCM and the oxide. Results are discussed through the analysis of oxide chemical bonding and thermodynamic considerations.Concerning the TEC / PCM adhesion, titanium-based barriers have been selected to avoid an inter-diffusion of the electrode and the PCM. Impact of barrier and PCM process on adhesion is studied. Chemical bonding between barrier and PCM is evaluated by X ray Photoelectron spectroscopy (XPS).
5:15 PM - H5.8
Crystallization Process of Ge1Cu2Te3 Phase Change Material.
Yuji Sutou 1 , Toshiya Kamada 1 , Yuta Saito 1 , Neishi Koji 1 , Junichi Koike 1
1 Department of Materials Science, Tohoku University, Sendai, Miyagi, Japan
Show AbstractPhase change random access memory (PCRAM) has been regarded as one of the promising candidates for the next-generation nonvolatile memories because of low production cost and excellent scalability. Generally, a phase change material for PCRAM need to possess a low melting point for low power consumption, a high crystallization temperature for long data retention, and to show crystallization without phase decomposition for excellent repeatability. Currently, Ge2Sb2Te5 compound (GST) is attracting considerable attention as a phase change material for PCRAM. The amorphous GST crystallizes first into a NaCl structure, and then transforms into a more stable hexagonal structure by further heating. However, the GST has a high melting point (~ 630 oC), and its crystallization temperature is a relatively low (~ 150 oC). Therefore, it is required to develop a new phase change material with a low melting point and a high crystallization temperature. According to phase diagrams, the Ge-Cu-Te ternary alloy system has a Ge1Cu2Te3 compound (GCT) with a melting point of about 540 oC, which is much lower than that of the GST. However, there is no study regarding the application of the GCT to the phase change material for PCRAM. In this study, the amorphization and crystallization process of the GCT were investigated. Thin films of amorphous Ge1Cu2Te3 (GCT) with 200 nm thickness were deposited by co-sputtering of GeTe, Te and Cu targets on SiO2/Si substrates. In-situ electrical resistance measurement during heating process of the film was performed by a two point probe method. X-ray diffraction (XRD) analysis was employed for the structural identification of the film using an X-ray diffractometer with Cu-Kα. Transmission electron microscope (TEM) analysis was carried out to investigate the microstructure and to identify crystalline structure. An as-deposited GCT film was confirmed to be amorphous by XRD and TEM. It was found from the in-situ electrical resistance measurement that the as-deposited GCT film showed abrupt electrical resistance change by more than 102 ohm with crystallization at about 200 oC. The crystallization temperature of the GCT was higher than that of GST. The amorphous GCT film directly crystallized into a stable orthorhombic Ge1Cu2Te3 structure. The GCT compound film with a low melting point and a high crystallization temperature holds great promise as a new phase change material for PCRAM.
5:30 PM - **H5.9
A Sub-nanosecond Time-resolved Micro XAFS System for Optical Phase Change Materials.
Hitoshi Osawa 1 , Paul Fons 1 2 , A. Kolobov 1 2 , Toshio Fukaya 1 2 , Tomoya Uruga 1 , Hajime Tanida 1 , Naomi Kawamura 1 , Masafumi Takagaki 1 , Motohiro Suzuki 1 , Yasuko Terada 1 , Junji Tominaga 2
1 , Japan Synchrotron Radiation Institute, Hyogo-ken Japan, 2 Center for Applied Near-Field Optics Research, National Institute of Advanced Industrial Science and Technology, Tsukuba Ibaraki Japan
Show AbstractPhase transitions are used every day in our lives for a variety of purposes. In particular, the use of phase-transitions to encode data using phase-change in re-writable optical memory such as DVD-RAM media has become commonplace. Te-based alloys such as Ge-Sb-Te (GST) and Ag-In-Sb-Te (AIST) are typically used as the recording layer for such applications due to the large difference in optical reflectivity that exists between the amorphous and crystalline states. In these media, data is encoded as amorphous spots on a more reflective crystalline background; amorphous pulses are written by shorter intense bursts of light while the crystalline state is brought about by longer, lower power irradiation.Laser induced amorphization of GST occurs in nanoseconds within devices as observed by time-resolved reflectivity [1], but how fast the structural transition actually takes has remained unclear. From femtosecond pulsed laser experiments, the time scale of laser-induced amorphization has been reported to occur on sub-picosecond time-scales [2]. Purely electronic excitation as generated by femtosecond irradiation is thought to bring about a non-thermal electron distribution out of equilibrium with the lattice. As characteristic time scales for thermalization of the effective electronic temperature to that of the lattice occurs on the order of picoseconds, in the current experiment we have restricted the duration of laser excitation to longer periods to allow for modeling of the amorphization process on technologically relevant time-scales. In the current work, we have developed an optical pump/x-ray probe time-resolved micro x-ray absorption system utilizing a sub-nanosecond excitation laser [3]. X-ray absorption fine structure (XAFS) based techniques are particularly useful for observations of transitions involving the presence of an amorphous phase as the short-range correlations (< 1 nm) observed by XAFS are present in both crystalline and amorphous states allowed measurements of both end states on an equal footing. The optical pump/x-ray probe system was constructed at SPring-8 beamlines BL37XU and BL39XU. A recording pulsed laser (532 nm, 600 ps) was focused to approximately a 20 micron spot and the diametrically opposite x-ray probe was focused to approximately a 2 micron spot using a Kirkpatrick-Baez mirror. XAFS spectra were taken at the Ge edge using fluorescence detection using a fast, gated photomultiplier. Although the x-ray pulse width was 60 ps, the total time resolution of the system is estimated to be 600 ps due to the convolution effects of the laser pump width. Both the measurement system as well as the results of the Ge XAFS measurements will be discussed.[1] C. B. Peng et. al., Appl. Optics 43,4367 (2004), [2] Santo et. al, E*PCOS2009, Aachen, Germany, [3] P. Fons et. al., Jpn. J. Appl. Phys. 46, 3711(2007).
Symposium Organizers
Paul Fons National Institute for Advanced Industrial Science and Technology
Kris Campbell Boise State University
Byung-ki Cheong Korea Institute of Science and Technology
Simone Raoux IBM T. J. Watson Research Center
Matthias Wuttig I. Physikalisches Institut der RWTH Aachen
H6: Applications I
Session Chairs
Thursday AM, April 08, 2010
Room 2009 (Moscone West)
9:00 AM - **H6.1
Minimum Voltage for Threshold Switching in Nanoscale Phase-change Memory.
Dong Yu 1 , Sarah Brittman 2 , Jin Lee 2 , Abram Falk 3 , Hongkun Park 2 3
1 Physics, UC Davis, Davis, California, United States, 2 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 3 Physics, Harvard University, Cambridge, Massachusetts, United States
Show AbstractPhase-change compounds undergoing crystalline-amorphous (order-disorder) phase transitions, are the material bases for nonvolatile optical memory, such as CD and DVD, and they are now being actively investigated for solid-state memory. The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes.
9:30 AM - H6.2
Stress Limited Scaling of Ge2Sb2Te5.
Robert Simpson 1 , Milos Krbal 1 , Paul Fons 1 2 , Alexander Kolobov 1 2 , Junji Tominaga 1 , Tomoya Uruga 2 , Hajime Tanida 2
1 CAN-FOR, AIST, Tsukuba, IBARAKI, Japan, 2 , Japan Synchrotron Radiation Research Institute (JASRI), Mikazuki, Hyogo, Japan
Show AbstractThe limit to which the phase change memory material, Ge2Sb2Te5, can be scaled towards the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge2Sb2Te5 has an increasingly dominant effect on the materials ability to change phase and a profound increase in the crystallisation temperature is observed when the Ge2Sb2Te5 layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress we have maintained the bulk crystallization temperature in 2 nm thick Ge2Sb2Te5 films. These experimental results and further theoretical analysis have allowed us to determine that a minimum cell volume of about 1 cubic nanometer is required for crystallization of Ge2Sb2Te5.
9:45 AM - H6.3
GeTe-filled Carbon Nanotubes for Data Storage Applications.
Cristina Giusca 1 , Vlad Stolojan 1 , Jeremy Sloan 2 , Hidetsugu Shiozawa 1 , S. Ravi Silva 1
1 Advanced Technology Institute, University of Surrey, Guildford United Kingdom, 2 Department of Physics, University of Warwick, Coventry United Kingdom
Show AbstractBy virtue of their unique electronic properties, nanometer-diameter sized single-walled carbon nanotubes (SWNTs) represent ideal candidates to function as active parts of nanoelectronic memory storage devices. Their central cavity has been shown to serve as a template for the controlled growth of one dimensional crystals and a wide range of organic as well as inorganic materials have been used as fillers in SWNTs.We report here a scanning tunnelling microscopy study, coupled with structural investigations by high resolution and scanning transmission electron microscopy of SWNTs encapsulating binary inorganic chalcogenide compound GeTe. Information on the electronic structure of the encapsulated nanowires has been extracted from scanning tunnelling spectroscopy, based on the recorded tunnelling spectra and with the aid of the predicted density of states for the particular nanotubes. The electronic interactions between the filling material and the host nanotube have been examined using ultraviolet photoelectron spectroscopy experiments and the work function of the hybrid system determined. We show that GeTe, a phase change material, currently considered to be one of the most promising materials for data-storage applications, can efficiently be encapsulated within SWNTs. Using SWNTs with diameters as low as 1.4 nm, as templates for creating thinner GeTe nanowires may represent the ultimate size limit in exploring induced phase transitions in nanoscale systems that no other existing synthesis method will be able to challenge. The newly formed system offers significant potential for applications in an emerging non-volatile electronic memory device based on filled carbon nanotubes.
10:00 AM - **H6.4
Bridging the Gap Between Phase Change Material Properties and Phase Change Memory Device Characteristics.
Matthew Breitwisch 1
1 , IBM/Macronix PCRAM Joint Project, T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractAs phase change materials are at the heart of the operation and reliability of phase change memory (PCM) devices, the phase change material properties and characteristics are targeted with specific attributes in mind. Typically, phase change materials are developed and investigated with a wide range of fast characterization techniques utilizing blanket thin films: resistivity measurements, X-ray characterization, laser switching speed, elemental profiling, etc. However, linking these thin film characteristics with PCM device characteristics can often be a challenge. This talk will review the various phase change material characterization techniques and will compare the results to characteristics of fully integrated PCM devices. Finally, this talk will review the current PCM device challenges and the wish list of desired phase change material characteristics.
10:30 AM - **H6.5
The Crystallization Behavior of Ge1SbxTe1 Phase-change Materials.
Huai-Yu Cheng 1 2 , Simone Raoux 1 3 , Matthias Wuttig 4 , Becky Munoz 5 , Jean L. Jordan-Sweet 3
1 , IBM/Macronix PCRAM Joint Project, Yorktown Heights, New York, United States, 2 Macronix Emerging Central Lab, Macronix International Co. Ltd., Hsinchu Taiwan, 3 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 4 I. Physikalisches Institut (IA), RWTH Aachen University, Aachen Germany, 5 Material Science and Engineering Department, Boise State University, Boise, Idaho, United States
Show AbstractSb-based materials with a growth dominated crystallization mechanism generally show high crystallization speed. They exhibit a different crystallization mechanism compared to Ge2Sb2Te5, which is a popular composition along the GeTe-Sb2Te3 tie line and the most commonly studied material. The drawback of Sb-based materials is their low thermal stability, however. The effects of various dopants in eutectic Sb2Te such as Ga, Ge, In and Ag have been extensively studied. In this study, we have prepared and systemically evaluated a different series of Sb-based Ge1SbxTe1 materials with equal amounts of Ge and Te with varying Sbx, x = 1 to 6 in an attempt to identify materials along this isoelectronic tie line which exhibit both high thermal stability and fast crystallization speed.It was found that the crystallization temperatures Tx of Ge1SbxTe1 phase-change materials decreased from 252 oC to 165 oC when x increased from 1 to 6. In addition, the resistance of the amorphous and crystalline states was decreased greatly as well. All of the Ge1SbxTe1 compositions we studied showed higher Tx than Ge2Sb2Te5 indicating a better data retention property. The crystallization time was measured using a custom-made static laser tester. Ge1Sb1Te1 samples showed relatively long crystallization times for both as-deposited, amorphous material and melt-quenched, amorphous material, of approximately 600 ns and 300 ns, respectively. These were significantly slower than the other Ge1SbxTe1 phase-change materials and Ge2Sb2Te5 as well. For the as-deposited, amorphous samples, the crystallization time reduced with increasing x values between 1 and 4.6 of Ge1SbxTe1. Further increase of x did not reduce crystallization time and crystallization times similar to Ge1Sb4.6Te1 were measured for Ge1Sb6Te1. For melt-quenched, amorphous samples, the re-crystallization time reduced monotonously with increasing x and a very short re-crystallization time of 7 ns was found for Ge1Sb6Te1, which was remarkably eight times faster than Ge2Sb2Te5. The crystalline structures of these materials were investigated by time-resolved x-ray diffraction (XRD) during heating from 25 oC to 450 oC. All films were amorphous as deposited and all showed a single rhombohedral phase transition, similar to pure Sb, upon crystallization. Further phase-segregation of these films was evaluated by Auger electron spectroscopy measurements.An increase of the Sb concentration thus not only decreased both the Tx and the resistance in the amorphous and crystalline states, but also reduced the crystallization time greatly. Thus a trade off between speed and thermal stability has to be made when choosing the optimal x. In summary, the higher Tx of all samples in the series of Ge1SbxTe1 materials compared to Ge2Sb2Te5, the larger resistance contrast, and the ultra-fast crystallization times make these materials promising candidates for Phase Change Random Access Memory.
11:15 AM - H6.6
Spectroscopic Ellipsometry and Raman Spectroscopy of Ge-doped SbTe Thin Films.
Jun-Woo Park 1 , Hosun Lee 1 , Tae Dong Kang 2 , Andrei Sirenko 2 , Hyun Seok Lee 3 , Suyoun Lee 3 , Jeung-hyun Jeong 3 , Byung-ki Cheong 3
1 Dept. of Applied Physics, Kyung Hee University, Yong-In, Gyeonggi-do, Korea (the Republic of), 2 Dept. of Physics, New Jersey Institute of Technology, Newark, New Jersey, United States, 3 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractPhase change random access memory (PRAM), utilizing fast and reversible phase changes between high-resistivity amorphous and low-resistivity crystalline phases of chalcogenides, has attracted considerable attention. Presently, commercial developments of PRAMs are based on the use of Ge2Sb2Te5. Ge-doped SbTe (Ge–ST) materials have been utilized successfully in high speed phase change optical recording. They have promising properties, such as lower melting temperatures, higher crystallization temperatures and lower electrical resistivities of the crystalline phases as well as fast growth-dominated crystallization. For these reasons, Ge–ST has been investigated as a good candidate for PRAM. We investigated the optical properties of Ge-doped SbTe (Ge-ST) thin films of three differing compositions grown on Si substrate by RF sputtering method; Ge0.06Sb0.77Te0.17 (Ge-STH), Ge0.05Sb0.70Te0.25 (Ge-STM), Ge0.05Sb0.64Te0.31 (Ge-STL).[1] The films were annealed at 250 °C for crystallization and examined by x-ray diffraction to verify the crystal structures. Our analysis of the x-ray peaks demonstrated that all crystalline thin films have hexagonal structures. Specifically, the peak positions of the crystalline Ge-ST films agree reasonably well with those of the long-period-stacking structures for the δ-phase binary Sb-Te materials.[2] By using Raman spectroscopy, we investigated the phonon modes in the thin films. We observed the phonon modes at 124, 144, 160, 192, 256, and 450 cm-1, and discussed dependence of the phonon peak intensities and frequencies as functions of the Sb:Te composition. By using spectroscopic ellipsometry, we measured the dielectric function of the thin films in the near infrared, visible, and ultra-violet region. By linear extrapolation of the absorption coefficient we determined the optical energy gaps and the band gaps for amorphous and crystalline phase, respectively. The optical gap energies of the amorphous Ge-ST films were found to be about 0.5 - 0.6 eV, whereas the indirect band gap energies of the crystalline films shrank substantially to about 0.15 - 0.2 eV. Note that the literature values for the optical and indirect gap energies of amorphous and crystalline phases of Ge2Sb2Te5 thin films are about 0.8 eV and 0.6 eV, respectively. We estimated the transition (critical point) energies of the amorphous (crystalline) Ge-ST films by taking the second derivative of their dielectric functions and by using a standard critical point (CP) model.[3] The CP energies appear to decrease with annealing. This phenomenon is consistent with the decreasing energy gap estimated by the linear extrapolation of the absorption coefficient.[1] S. Lee et al., J. Electrochem. Soc. 156, H612 (2009).[2] K. Kifune et al., Acta Crystallogr., Sect. B 61, 492 (2005).[3] J. Park et al., Phys. Rev. B 80, 115209 (2009).
11:30 AM - H6.7
Effect of Heat Treatment on Electrical Resistance Change and Crystallization Process of Si-Te Thin Films.
Yuta Saito 1 , Yuji Sutou 1 , Junichi Koike 1
1 Department of Materials Science, Tohoku University, Sendai Japan
Show AbstractGeTe-Sb2Te3 pseudobinary compounds have attracted considerable attention as phase change materials for phase change random access memory (PCRAM). Among these various compounds, Ge2Sb2Te5 has been widely studied for PCRAM because of the fast crystallization speed and the good reversibility between amorphous and crystalline states. However, a high reset current which causes a high-power consumption is necessary because of its relatively high melting temperature (Tm ~ 630 oC). Furthermore, it has been considered that ternary compounds are difficult to control their compositions and a little compositional deviation may have a bad influence on the lifetime cycle of a device. Recently, eutectic binary alloy systems such as Ge-Te and Ge-Sb have been studied as a next candidate of phase change materials for PCRAM. In this work, we focused on Si-Te binary alloy thin films. The Si-Te alloy has a low eutectic point in 85 at.%Te (Tm = 407 oC). Although the Si-Te alloy is known to show the phenomenon of electrical switching, limited work has been done in the eutectic Si-Te alloys from the viewpoint of PCRAM application. Therefore, we report electrical resistance change and crystallization process during heating of the Si-Te binary alloy thin films.Films of amorphous Si-Te with 200 nm thicknesses were deposited by RF sputtering of Si and Te targets on SiO2/Si substrates. In-situ electrical resistance measurements were performed during heating process of these films by two-point probe method at various heating rates. X-ray diffraction (XRD) analysis was employed for the structural identification of thin films using X-ray diffractometer with Cu-Kα. Transmission electron microscopy (TEM) analysis was carried out to investigate the microstructure and to identify crystalline structure. The compositions of these films were confirmed by energy dispersive X-ray spectroscopy (EDX) attached to TEM.The as-deposited eutectic Si15Te85 film was confirmed to be amorphous by XRD and electron diffraction patterns. The resistance of amorphous Si15Te85 film was 101 ~ 102 times higher than that of amorphous Ge2Sb2Te5 film. The Si15Te85 amorphous film showed two steps of resistance decrease during heating. It was confirmed by XRD measurement that the Si15Te85 amorphous film firstly showed a drastic electrical resistance drop at around 180 oC due to the crystallization of Te. This crystallization temperature was higher than Ge2Sb2Te5 film. Interestingly, before the second electrical resistance drop was observed at about 300 oC, the electrical resistance gradually increased with heating from around 250 oC. XRD spectra measured after the second resistance drop indicated the existence of both Te and Si2Te3 crystals. Therefore, the temporal electrical resistance increase before the second drop may be related to the Si2Te3 crystallization process.
11:45 AM - H6.8
Effects of Cu Doping on the Crystallization Properties of Phase Change Materials GeTe and Sb2Te3.
Minghua Li 1 , Rong Zhao 1 , Eng Guan Yeo 1 , Luping Shi 1 , Tow Chong Chong 1
1 , Data Storage Institute, Singapore Singapore
Show AbstractPhase change materials like Ge2Sb2Te3, GeTe, and Sb2Te3 have been widely used for data storage applications due to their sharp contrast in optical reflectivity and electric resistivity when switched between the amorphous and crystalline states. Many dopants have been reported to tune the alloys’ phase change properties effectively. However the reason why doping is able to modify the phase change behavior is still not clear. In this paper, we introduce Cu doped GeTe and Sb2Te3 thin films which show significant modification of the phase change properties. Structure deformation and material parameter change caused by Cu doping were analyzed to address the doping mechanism. As the GeTe and Sb2Te3 are the two important building units in ternary Ge-Sb-Te system, this investigation may help to better understand the doping effects in various GeSbTe phase change materials.Cu-doped GeTe and Sb2Te3 films were deposited on thermal oxidized <001> silicon substrate by magnetron sputtering method. The stoichiometric GeTe and Sb2Te3 targets were used, respectively. Cu doping was performed by inserting a metal Cu layer in between the two GeTe or Sb2Te3 layers. in situ diffusion of Cu was realized and subsequently doped GeTe and Sb2Te3 films were obtained. The doping level was determined by the thickness ratio between the Cu layer and the phase change material layer. In this study, the Cu concentrations were controlled in the range of 0 ~ 12 atm.%.The Cu doping concentrations were confirmed by EDS (Energy Dispersed X-ray Spectrometer), and the Cu in situ diffusion uniformity was evaluated by XPS (X-ray Photoelectron Spectroscopy) depth profiles. XRD (X-Ray Diffraction) patterns show single phase structure for both Cu doped GeTe and Sb2Te3 films, indicating that the Cu ions occupy lattice sites of the phase change materials without any phase segregation in the doping range of this study. Cu doping increases the crystallization temperature Tc while decreases the resistant alteration range. For example in GeTe, 6 atm.% Cu doping increases Tc from 186 °C to 204 °C, and 9 atm.% Cu doping further raises Tc to 225 °C. The electrical conductivity was found to reduce as the increasing of Cu doping concentration. Structure and material property changes caused by Cu doping were characterized and investigated in order to understand the phase change behavior modification.
12:00 PM - **H6.9
Atomic Layer Deposition of Phase Change Material Thin Films.
Mikko Ritala 1 , Viljami Pore 1 , Timo Hatanpaa 1 , Tiina Sarnet 1 , Markku Leskela 1 , Alejandro Schrott 2 , Simone Raoux 2
1 Chemistry, University of Helsinki, Helsinki Finland, 2 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractReaching high integration densities together with low power consumption in phase change memories require that memory cells are scaled into dimensions of a few nanometers only. This in turn requires good conformality from the thin film deposition method used for the phase change material, thereby calling for Atomic Layer Deposition (ALD). The common phase change material germanium antimony telluride (GST) consists of three elements that have been rare to ALD. Tellurium in particular has been lacking precursors that would at the same time be safe and highly reactive as required in ALD. A breakthrough in ALD of GST and related materials was recently made when silyl compounds with a general formula (R3Si)2Te were found as excellent ALD precursors that react efficiently with various metal halides forming the corresponding metal tellurides. Similar chemistry works also for selenides. This presentation reviews our ongoing work on ALD growth and characterisation of GST and other phase change materials.
12:30 PM - H6.10
Influence of Bottom Contact Material on the Selective Chemical Vapor Deposition of Crystalline GeSbTe Alloys.
Alejandro Schrott 1 , Chieh-Fang Chen 2 , Eric Joseph 1 , Matthew Breitwisch 1 , Ravi Dasaka 1 , Roger Cheek 1 , Chung Lam 1
1 , IBM Research, T.J. Watson Res. Ctr., Yorktown Heights, New York, United States, 2 Macronix Emerging Central Laboratory, Macronix International Co., Ltd., Hsinchu Taiwan
Show AbstractDue to the stringent demands imposed by the scaling roadmap of phase change memory devices, it is becoming increasingly important to understand the Chemical Vapor Deposition (CVD and Atomic Layer Deposition (ALD) processes options that can be utilized to fill small but high aspect ratio vias in structures that mimic the relevant features of future memory cells. Towards that goal, the influence of the material that forms the bottom contact of via holes formed in dielectrics, on the chemical vapor deposition (CVD) of crystalline GeSTe alloys has been studied. In this work, the growth is achieved without using any reactive gas or plasma, and the decomposition selectivity of the precursors utilized allows for growing the material starting at the bottom contact, with no deposition on the dielectric top surface. This process ensures a uniform contact of the phase change material with the bottom of the electrode. We used TETRAKIS (DIMETHYLAMINO) GERMANE, TRIS(DIMETHYLAMINO)ANTIMONY and DI-TERT-BUTYLTELLURIDE as precursors and wafer temperatures around 300 degrees C. The vias were formed either in Si oxide or nitride by a RIE process which exposed either TiN or Tungsten as the bottom contact. The vias varied in shape and aspect ratio with the bottom contact diameter ranging from ~ 200 nm to 30 nm, and depth ranging from ~250 to 150 nm. The different approaches used to selectively growth crystalline GeTe, Sb2Te3 and various GeSbTe alloys inside vias, and the effect of via size and bottom contact material on crystalline structure, will be discussed.
12:45 PM - H6.11
MOCVD Growth of Ge-Sb-Te Nanowires by the VLS Process.
Massimo Longo 1 , Claudia Wiemer 1 , Olivier Salicio 1 , Marco Fanciulli 1 2 , Enrico Varesi 3 , Paolo Targa 3
1 Laboratorio Nazionale MDM, CNR-INFM, Agrate Brianza, Milano, Italy, 2 Dipartimento di Scienza dei Materiali, University of Milano Bicocca, Milano Italy, 3 , Numonyx, Agrate Brianza , Milano, Italy
Show AbstractThe Metal Organic Chemical Vapor Deposition (MOCVD), promising for its potentialities for the scaling down of phase change memories in the top-down approach, was adopted for the growth of NanoWires (NWs) of Ge-Sb-Te alloys on SiO2/Si substrates. The self assembling occurred at 400°C on the basis of the Vapor-Liquid-Solid (VLS) process, activated by the presence of the Au metal catalysts species on the substrate surfaces; the employed metalorganic precursors were tetrakisdimethylaminogermanium, trisdimethylaminoantimony and diisopropyltelluride, the process gas was nitrogen.The NW properties were studied in terms of the structural and compositional analysis by X-Ray Diffraction (XRD) and Total Reflection X-Ray Fluorescence (TXRF); morphological aspects were studied by Scanning Electron Microscopy (SEM) observations. The NWs appeared to grow by the epitaxial superposition of crystalline Ge-Sb-Te monolayers; the VLS mechanism was confirmed by the presence of a gold globule at the end of many NWs, limiting the extension of the growing planes.Local area measurements were carried out on single nanostructures by Low and High resolution Transmission Electron Microscopy (TEM) analysis, coupled to Energy Dispersive X-ray (EDX) spectroscopy. The main axis of the grown NWs exhibited a random orientation with respect to the substrate; a long range oriented domain was observed in the single NWs, where the (0 0 l) planes could be identified. NWs exhibited a mean diameter distribution centered on 35 nm and a length up to 1μm. Two types of NWs were identified: i) “big” NWs, featured by mean cross size > 50 nm and aspect ratio (AR = length/mean diameter) distribution centred around 5 and ii) “small” NWs, featured by mean cross size < 50 nm and AR distribution centred around 12. Stacking faults could even bend in some cases the NW axis, yielding a zigzag shape.Measurements indicated that the grown NWs are compatible with both the Ge1Sb2Te4 and Ge1Sb4Te7 compositions and crystallographic phases; the [00l] direction of the hexagonal structure formed an angle close to 35° with the NW axis in the case of a big NW. For a small NW the angle between the [107] direction and the NW axis was measured to be close to 45°.This is the first demonstration of the feasibility of such chalcogenide-based nanostructures for phase change devices by the MOCVD technique.
H7/G14: Joint Session: Applications II
Session Chairs
Thursday PM, April 08, 2010
Room 2011 (Moscone West)
2:45 PM - **H7.1/G14.1
Phase Change Based Memory Devices: Characteristic Behaviors, Physical Models and Key Materials Properties.
Ilya Karpov 1 , DerChang Kau 1 , Gianpaolo Spadini 1 , David Kencke 1
1 , Intel Corporation, Santa Clara, California, United States
Show AbstractPhase Change Memory (PCM) is based on electrically initiated reversible amorphous-to-crystalline phase changes in chalcogenide materials, such as Ge2Sb2Te5 (GST) [1]. We present device characteristics, IV and programming curves, of phase change based memory devices [1,2] and their relation to the device performance. Then we show the results of the studies of the device temporal dependencies, in short and long time scales. We analyze phenomena affecting programming and read such as threshold switching, noise and drift [3,4]. After the data and corresponding models are discussed we identify key materials parameters affecting device performance including variability. Finally we consider how the devices behavior changes with scaling including effects of interfaces such as electrical and thermal resistances [5]. References: [1] S. Lai and T. Lowrey, IEDM Tech. Dig., p.803, 2001.[2] B. Johnson and S. Hudgens, MRS Bulletin, V29, No. 11, p. 829, 2004.[3] I. Karpov, D. Kau, G. Spadini, V. Karpov, Non-Volatile Memory Technology Symposium, p. 1, 2008. [4] M. Nardone, V. I. Kozub, I. V. Karpov, and V. G. Karpov, Phys. Rev. B 79, 165206, 2009.[5] D. Kencke, I. Karpov, B. Johnson, S. Lee, D. Kau, S. Hudgens, J. Reifenberg, S. Savransky, J. Zhang, M. Giles, G. Spadini, IEDM Tech. Dig., p.323, 2007.
3:15 PM - H7.2/G14.2
PCRAM Performances Improvement With Nitrogen Doped GeTe Material for Embedded Applications.
Emmanuel Gourvest 1 2 , Christophe Vallee 2 , Frederic Fillot 3 , Herve Roussel 4 , Luca Perniola 3 , Andrea Fantini 3 , Jean-Claude Bastien 3 , Audrey Bastard 1 3 , Sebastien Loubriat 3 , Anne Roule 3 , Sandrine Lhostis 1 , Sylvain Maitrejean 3
1 , STMicroelectronics, Grenoble France, 2 LTM, CNRS/UJF/INPG, Grenoble France, 3 LETI-Minatec, CEA, Grenoble France, 4 LMGP, CNRS/INPG, Grenoble France
Show AbstractChalcogenide materials are widely used for phase-change data storage based on their high transformation speed between their highly electrically contrasted crystalline and amorphous phase. High temperature applications require moreover high retention time, i.e. stable amorphous phase, at working temperature. Binary compound GeTe have attracted great attention for such applications since it presents high phase transition temperature and good endurance. Improvement of data retention [1] - 10 years at 110°C compared to 10 years at 70°C for conventional Ge2Sb2Te5 - was estimated but keeps out of higher temperature applications specifications. Furthermore it has been shown in a previous work [2] thermal instability of non-stoichiometric GeTe films with post-crystallization of germanium excess. Recently, different works [3-4] related that nitrogen doping favourably modifies crystallization properties of Ge2Sb2Te5 at high temperature. In this study we investigated the influence of nitrogen doping on performance of GeTe thin films. Optical phase-change properties were measured for various nitrogen doped GeTe films and showed significant increase of crystallization temperature and data retention - up to 10 years at 145°C - compared to GeTe ones. Work especially focused on the understanding of the role of nitrogen on crystallisation process using X-ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD). XPS evidenced nitrogen bonding with germanium. For nitrogen doped-Ge rich GeTe films, no post-crystallization of germanium excess was detected by XRD after annealing. These results suggest the formation of Ge-N amorphous phase. Concerning crystal microstructure, crystallite sizes decrease from 40 nm down to 16 nm when nitrogen content increases. Moreover, evolution of stress-free lattice parameter with nitrogen doping was observed. Based on these characterizations, crystallisation mechanisms can be discussed. Additionally, nitrogen-doped GeTe films were integrated in simple test devices on 200 mm wafers. Electrical characteristics showed three orders of magnitude in resistance between amorphous (RESET) and crystalline (SET) states for electrical stress pulse down to 25 ns and a good endurance. This study showed nitrogen-doped GeTe material is a very promising candidate to meet technological requirements for PCRAM high temperature applications. [1] A. Fantini et al. IEEE Proceedings of IMW (2009) pp.66-67. [2] E. Gourvest et al. Appl. Phys. Lett. 95, 1 (2009). [3] S. M. Kim et al. Thin Solid Films 469-470 (2004) 322-326. [4] R. M. Shelby and S. Raoux J. Appl. Phys. 105, 104902 (2009).
3:30 PM - H7.3/G14.3
Electromigration in GeSbTe-based Chalcogenide Materials Under Pulsed DC for Set-stuck Failure.
Tae-Youl Yang 1 , Ju-Young Cho 1 , Young-Chang Joo 1
1 Department of materials science and engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractPhase change memory made rapid progress towards commercializing, but the lack of accurate mechanism of reliability-degradation remains as one of the most significant unsolved problem. In endurace reliability issues, set-stuck is one of the typical failures. This failure is that cell resistance fixed at the set-state, and is caused by the compositional change of Ge2Sb2Te5 due to electric-field-enhanced mass flow during the reset operations. In molten Ge2Sb2Te5, Ge and Sb atoms migrated to the cathode, whearas Te atoms migrted to the anode under electrical bias. This compositional change is induced by the electrostatic-force-induced electromigration. However, the results of the previous study are not enough to predict the failure in real cell because atomic flux cannot be quantified. Study on the inhibition of electromigration based on the understanding of failure mecahnism is required to solve the reliability problem. In this study, we quantified the flux of the constituent elements in the electromigration of molten Ge2Sb2Te5, and confimed the driving force by comparion with the electromigration behavior in molten GeTe and Ge15Sb85. In addition, we also investigated the electromigration suppression by N-doping in Ge2Sb2Te5. Isolated line structure was used for a simple and systematic study on the electromigration. The line was isolated by Mo contact pads to prevented artifacts from a large source/sink of diffusing atoms. Nitrogen was doped in Ge2Sb2Te5 with 3 at.% by N2 gas flowing during the deposition. Electrical bias was applied to the line using DC-type pulse of 7~10 V with long period (~ 10-3 s) enough for the melting by Joule heating. Compositional variation was detected by WDS. Compositional variation by the electromigration was observed as a function of diffusion time in the molten Ge2Sb2Te5. Interdiffusion of Sb and Te atoms occurred prior to the diffusion of Ge atoms. In the comparison of the number of migrated atoms normalized by the initial number concentration, Sb showed the fastest diffusion velocity. The DZ*, which is the parameter for diffusion rate, of Ge, Sb, and Te was calculated from the number of migrated atoms to be 1.13. 1.98, and -1.17 × 10-5 cm2 s-1, respectively. Z* values are also calculated to be 0.28, 0.38, and -0.29. In the electromigration of molten GeTe, we observed the elemental separation into Te at the anode and Ge near at the cathode. However, elemental separation toward the electrodes was not occurred in moltne Ge15Sb85. Based on the results, we confirmed that driving force of the electromigration is the electrostatic-force into each element. In addition, we observed that compositional change by the electromigration was suppressed in the N-doped Ge2Sb2Te5 line. The DZ* values in N-doped Ge2Sb2Te5 will be also calculated by the control of diffusion time. N-doped Ge2Sb2Te5 is expected to retard the endurance failures because the doping increases the viscosity of materials, and decreases the mobility of atoms.
3:45 PM - H7.4/G14.4
Structural and Electrical Switching Dynamics in Phase-change Random Access Memory.
Jasper L. Oosthoek 1 2 , Frans Voogt 3 , Bart Kooi 1 2
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 , Materials Innovation Institute (M2i), Delft Netherlands, 3 Process and Material Analysis, NXP Semiconductors, Nijmegen Netherlands
Show AbstractPhase-change materials, such as Ge2Sb2Te5, have been developed for rewritable CD and DVD applications and are currently intensively studied for future optical and electrical non-volatile memories. Both the technology and also the science of phase-change materials have shown important progress but still offer challenges.The main focus of the presentation will be on the electrical characterization of so-called phase-change line cells. Currently, these memory cells can be switched ~10^8 times before break down, clearly sufficient for replacement of Flash memory. Particular attention is paid to the evolution of the cell properties with the number of switching cycles and to data retention. Interesting cell properties include the temporal drift of the amorphous resistance and the required (minimum) threshold voltage for the amorphous to crystalline phase change. A mechanism is put forward that explains the degradation of the line cells with switching cycles towards the situation that the cell is stuck in the crystalline state.Apart from electrical characterization also crystallization studies will be presented of both phase change thin films and line cell devices based on in situ Transmission Electron Microscopy. A final aim of our work is to combine nano-second switching and electrical characterization with nano-scale imaging of the line cells in the electron microscope.