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 n