Symposium Organizers
Simone Raoux IBM Almaden Research Center
Arthur (Art) H. Edwards Air Force Research Laboratory/VSSE
Matthias Wuttig I. Physikalisches Institut IA
Paul J. Fons Advanced Institute of Industrial Science and Technology
P. Craig Taylor Colorado School of Mines
Symposium Support
Air Force Office of Scientific Research (AFOSR)
IBM Almaden Research Center
G1: Theory of Phase Change Materials
Session Chairs
Tuesday PM, March 25, 2008
Room 2007 (Moscone West)
9:30 AM - *G1.1
Market Opportunities and Challenges for Phase-change Memory.
Stefan Lai 2 , Stephen Hudgens 1
2 , Ovonyx, Inc., Rochester Hills, Michigan, United States, 1 , Ovonyx Technologies, Inc., Santa Clara, California, United States
Show AbstractThe growth of battery-powered, portable entertainment and communications applications has fueled a huge and increasing demand for low-power nonvolatile memory storage. This is currently being met with stored-charge NAND and NOR Flash memory, but clear scaling limits are approaching. Many alternative nonvolatile memories are being investigated, but, among these, phase-change technology is receiving the greatest attention and it is widely regarded as the most promising. Phase-change memory is being considered not only as a Flash memory replacement, but for development of a “fusion” or unified memory to replace both Flash and volatile DRAM, which is facing its own scaling limits. The advantages of phase-change technology in addressing these new and emerging markets, the current status of phase-change memory commercialization, and the challenges it will face going forward will be reviewed.
10:15 AM - **G1.2
Computer Simulation of the Phase-change Cycle of GST-225.
Stephen Elliott 1 , Jozsef Hegedus 1
1 Chemistry, University of Cambridge, Cambridge United Kingdom
Show AbstractWe have simulated for the first time, by ab initio molecular dynamics, the complete phase-transformation cycle (liquid-crystal, liquid-amorphous-crystal) of the phase-change (PC) memory material Ge2Sb2Te5 (GST-225). We have observed that rapid cooling of the simulated melt leads to an amorphous product, whereas slow cooling results in the metastable rocksalt crystal. Furthermore, crystallization to the same structure is observed to occur on annealing the quenched amorphous model to temperatures below the melting temperature. We have observed crystal-nucleation events in the simulated liquid that have been identified as the creation of connected near-regular square fourfold rings, the basic structural units of the rocksalt structure. These crystal nuclei are invariably found to be quenched into the amorphous state on rapid cooling of the simulated melt. This observation therefore explains why GST materials crystallize so readily
10:45 AM - G1:Theory
BREAK
11:15 AM - **G1.3
Tight-Binding Theory of Phase-Change Materials.
Walter Harrison 1
1 Applied Physics, Stanford University, Stanford, California, United States
Show Abstract11:45 AM - **G1.4
Phase Change Alloys for Non-volatile Memories - Designing New Storage Materials from First Principles.
Wojciech Welnic 1 2 3 , Daniel Lusebrink 4 , Daniel Wamwangi 4 , Michael Gillessen 5 , Richard Dronskowski 5 , Silvana Botti 1 2 , Lucia Reining 1 2 , Matthias Wuttig 4
1 Laboratoire des Solides Irradies, Ecole Polytechnique, Palaiseau France, 2 , European Theoretical Spectroscopy Facility (ETSF), Palaiseau France, 3 , European Synchrotron Radiation Facility (ESRF), Grenoble France, 4 I. Physikalisches Institut IA, RWTH Aachen, Aachen Germany, 5 Instituts für Anorganische Chemie, RWTH Aachen, Aachen Germany
Show AbstractIn recent years, non-volatile solid state memories have in many applications replaced magnetic hard disk drives. A promising candidate for the next generation of non-volatile memories is the phase change random access memory (PCRAM). The PCRAM utilizes phase change materials (PCM) already successfully employed in rewritable optical data storage. The storage concept of these materials is based on a unique combination of properties: On one side they show a rapid phase transition (~10-100ns) from a metastable crystalline to the amorphous phase upon heating. On the other side this phase transition is accompanied by a fundamental change in the electronic and optical properties between the two phases unknown from commonsemiconductors such as Si, Ge or GaAs.Despite the increasing technological interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. In this work we use density-functional theory (DFT), to reveal the origin of the unusually high vacancy concentration of up to 10% found in GeSbTe based PCM's. We show that the vacanices and local distortions stabilize the rocksalt-like crystalline structures. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge-Te and Sb-Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by experimental data. Finally we provide an outlook on new methods to study the correlation between local structural and electronic properties in PCM's.
12:15 PM - G1.5
First Principles Study of Models for Amorphous Ge-X-Y Compounds.
Arthur Edwards 1 , Andrew Pineda 1
1 , AFRL, Kirtland AFB, New Mexico, United States
Show Abstract12:30 PM - G1.6
Massively-parallel Density Functional Simulations of the Disordered Te-based Phase-change Materials.
Jaakko Akola 1 2 , R. Jones 2
1 IFF, Forschungszentrum Jülich, Jülich Germany, 2 Department of Physics, University of Jyväskylä, Jyväskylä Finland
Show AbstractPhase-change materials are of immense importance for optical recording and computer memory, but the structure of amorphous phases and the nature of rapid amorphous-to-crystalline transition pose continuing challenges. We have used massively-parallel density functional (DF) calculations for characterizing the amorphous structure of several Te-based materials: GeTe (50:50), Ge15Te85 (15:85), Ge2Sb2Te5 (2:2:5, DVD-RAM compound), Ge8Sb2Te11 (8:2:11, Blu-Ray), and Te itself. DF methods are free of adjustable parameters, and their combination with molecular dynamics has has a major impact in materials science and chemistry. The demands on computational resources have restricted previous DF simulations on chalcogenide materials to relatively small unit cells (<100 atoms) and time scales (~10 ps). Here we present results for systems that are much larger in both number of atoms (216-630 atoms) and time scale (hundreds of picoseconds). This significant improvement enables us to mimic the experimental quenching process and characterize the amorphous structure.Atoms on GeTe-based materials can generally be classified into atomic types A (Ge,Sb) and B (Te), with strong AB alternation. The main structural motif of such materials is a four-membered ABAB ring ("ABAB square"), and the bonding angles are predominantly octahedral. The rapid amorphous-to-crystalline transition can be viewed as a re-orientation of disordered ABAB squares as the metastable NaCl structure corresponds to the perfectly ordered case. There are deviations from the the 8-N rule for coordination numbers, with Te having near three-fold coordination. All alloys (including Te itself) display a marked long-range order of Te atoms. Ge atoms are predominantly fourfold coordinated, but - contrary to recent speculation - tetrahedral coordination is found in only approximately one-third of the Ge atoms. The average coordination number of Sb is between 3 and 4, and the local environment of Ge and Sb is usually "distorted octahedral" with AB separations from 3.2 to 4 Å in the first coordination shell. The number of Ge-Ge bonds is significantly greater in GeTe than in 2:2:5. Vacancies (voids) in the disordered phases of these materials provide the necessary space for the phase transition to take place. The vacancy concentration in 2:2:5 (11.8%) is greater than in GeTe (6.4%), which is consistent with the better phase change performance of the former. Furthermore, inspection of the void centers reveals that they have a long-range ordering, and that they repel each other. The Te-rich compounds exhibit enhanced vacancy concentrations (26-27% for 15:85) as the overall coordination of the material decreases. Unlike in the GeTe-based materials, the tetrahedral Ge atoms dominate in 15:85, and the eutectic material comprises corner and edge sharing GeTe4 tetrahedra that are embedded in Te.
12:45 PM - G1.7
Theoretical model of Crystal Nucleation in PCM NANO-Glasses.
Y. Kryukov 1 , M. Mitra 1 2 , I. Karpov 2 , V. Karpov 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 2 , Intel Corporation, Santa Clara, California, United States
Show AbstractCrystal nucleation underlies important operations of chalcogenide phase change memory (PCM). The existing understanding of crystal nucleation in PCM is based on the classical nucleation theory neglecting statistical fluctuations of microscopic parameters in the disordered structure of a glass. This works presents a theoretical analysis that takes such fluctuations into account and predicts their effect on nucleation rate and statistics vs. material parameters, temperature, electric field, and device dimensions.Our theory accounts for statistical fluctuations in energy changes accompanying microscopic structure transformations that underlie the nucleation. Because of the large number, N>>1 of such transformations involved in a single nucleation event, the corresponding nucleation barrier becomes a random quantity, whose additive nature makes its statistical distribution Gaussian, with the average and dispersion proportional to N. The statistical distribution of nucleation barriers includes “soft” regions where the barriers are relatively low and nucleation is exponentially faster than the average. We introduce the concept of the optimum barrier that compromises between the exponentially high transformation rate and the exponentially small probability corresponding to low nucleation barriers. These optimum barriers are shown to be temperature and electric field dependent (the latter due to the field-induced nucleation). We describe the corresponding dependencies of the crystal nucleation kinetics. The statistics of nucleation induction times turns out to be log-normal with parameters that are temperature and field dependent.For the case of switching, not only a random nucleation barrier has to allow for transformation to occur at a given time, but, in addition, the system must not have any lower barriers that would allow such transformation in the earlier times. Combination of the corresponding probabilities leads to a significantly non-Gaussian statistics of switching events where delayed switching is strongly suppressed and the incubation times do not obey the log-normal distribution. We describe this distribution analytically and show that it has a nontrivial dependence on area and thickness. In conclusion, our consideration offers predictions and a theoretical framework for statistical analysis of parameters in chalcogenide phase change memory devices.
Symposium Organizers
Simone Raoux IBM Almaden Research Center
Arthur (Art) H. Edwards Air Force Research Laboratory/VSSE
Matthias Wuttig I. Physikalisches Institut IA
Paul J. Fons Advanced Institute of Industrial Science and Technology
P. Craig Taylor Colorado School of Mines
G6: Phase Change Materials - Applications I
Session Chairs
Thursday AM, March 27, 2008
Room 2007 (Moscone West)
9:30 AM - **G6.2
Interlayer Atomic Zipper with Large Optical and Electrical Transition in SbTe Alloy.
Junji Tominaga 1 , Paul Fons 1 , Takayuki Shima 1 , Masashi Kuwahara 1 , Osamu Suzuki 1 , Alexander Kolobov 1
1 CAN-FOR, AIST, Tsukuba Japan
Show AbstractChalcogenides, especially among them, germanium-antimony-tellurium (GeSbTe) and antimony-rich tellurium (R-SbTe) with some additives if necessary, are the most important and useful alloys currently applied to recordable optical storage (rewritable digital versatile disc, DVD-RW or DVD random access memory, DVD-RAM) and nonvolatile random access memory (phase change random access memory, PCRAM). In 2004, the phase transition mechanism of GeSbTe was first revealed, and made it clear that an amorphous state is not totally at random but has an order in local, and Ge atom can easily transit between its octahedral and tetrahedral positions. There is a small potential wall between the two states. In contrast, no theoretical analysis has been proposed in SbTe alloy because a Ge-free system. In this paper, using local density approximation (LDA), SbTe-11R (Te: 27.3 at%) and SbTe-9P (Te: 33.3 at %) crystal lattices were first constructed and compared with the experimental results. Then, the models were modified under several different stresses. We found that SbTe-11R induces two phase-transitions at around 12 GPa (compressive) and –6 GPa (tensile)while SbTe-9P has also two phase transition at around 18 GPa and -3 GPa. Especially in the negative pressure, c-axis was isotropically expanded than the other axes, giving a large refractive index change and electronic conductivity. We report that coherent (uni-axial) melting by breaking a sigma bond between Sb2Te3 and Sb superlattices is the origin of the phase transition to induce large change on physical properties.
10:00 AM - **G6.3
Prospective Of Phase-Change Memory.
Sumio Hosaka 1 , Naoya Higano 1 , Kazuhiro Ohta 1 , Akihira Miyachi 2 , Hayato Sone 1 , You Yin 1
1 Graduate School of Engineering, Gunma National University, Kiryu, Gunma, Japan, 2 Advanced Technology Research Center, Gunma National University, Gunma Japan
Show AbstractPhase change memory [1, 2], which calls phase change random access memory (PRAM), is expected to be one of post flash memories. The flash memory is used in wide field as nonvolatile memory. The memory, however, has some technical issues such as slow writing etc in the characteristics. In order to solve the issues, there are many works in fast writing, long retention, much endurance, etc in PCM. Although many solutions have been demonstrated, some technical issues are still remained in low power and multi-levels PCM (M-PCM). Here, we will describe about their approaches and experimental results in low power writing, M-PCM, and 1 transistor memory cell.(1) Low power writing in PCM [3]We have proposed a lateral type PCM with series and parallel heaters. This is because we can study how the heaters behave as direct or indirect heating. The PCM is designed with a multi layers stack structure with PC and heater layers as a bridge between the electrodes. We have studied the device by simulations and experiments. In a finite element method (FEM) simulation [4], we analyzed which direct or indirect heating is dominant by relative heater resistivity against the PC material. In our structure, when the heater resistivity was the same as the PC material, low power writing was estimated. When using the heater resistivity higher than the PC material, the low power writing will be hardly achieved.We did the experiments using 2 prototyped devices. One was used with only Ge2Sb2Te5 (GST) PC layer, and the other was used with the N-doped Sb2Te3 (STN) PC layer and TiN heater layer as the bridge. The former is in the small resistivity or without the heater. The set and reset powers were less than 10μW though the endurance was in trouble [3]. The latter is in higher resistivity than PC material. The powers were larger than 10μW though we obtained good endurance.(2) Multi-levels PCM (M-PCM)There are some proposals in M-PCM structures such as normal M-PCM, patterned M-PCM and multilayer M-PCM [5]. Here, we have proposed the patterned M-PCM and the multilayer M-PCM. We will describe them by the structures and calculated and experimental results. We demonstrate the possibility to achieve the M-PCM [6, 7].(3) Possibility of memory transistor for 1 transistor memory cell.
10:30 AM - G6.4
Blu-ray Type Super-Resolution Near-Field Phase Change Disk with Sn-doped GST Mask Layer.
Irene Lee 1 , Xiangshui Miao 1 , Kok Thong Yong 2 , Chee Lip Gan 2 , Luping Shi 1
1 Optical Materials and System, Data Storage Institute, Singapore Singapore, 2 Materials Science and Engineering, Nanyang Technological University, Singapore Singapore
Show AbstractAddition of specific foreign elements into Ge2Sb2Te5 phase change material has been demonstrated to improve the crystallization speed. Sn-doped GST has been found to exhibit ultra-fast crystallization. In this paper, we developed a Sn-doped GST material in the form of Sn7.0Ge20.6Sb20.7Te51.7 phase change material and demonstrated its use as a mask layer in rewritable aperture-type super-RENS disk to increase the carrier-to-noise ratio (CNR) and thermal stability. Crystallization and melting temperatures of Sn-doped GST material are 153 oC and melted at 536 oC, respectively. The activation energy for crystallization estimated by the Kissinger’s equation was found to be 2.54 eV. A large change in extinction coefficient of the material was obtained, which indicated its suitability as a mask layer. Crystallization speed of 50 ns was realized for the as-deposited material upon irradiation by blue laser. Super-RENS disk comprising of Ge2Sb2Te5 as the recording layer and Sn-doped GST as the mask layer was developed and its performances were measured using the Pulstec tester of 405 nm laser beam and numerical aperture of 0.85. A carrier-to-noise of at least 20 dB for 50 nm mark was observed and readout stability of more than 1000 cycles was obtainable.
10:45 AM - G6 Appl1
BREAK
11:15 AM - **G6.5
Recent Advances on the Modeling of Phase Change Materials and Devices.
Andrea Lacaita 1 , Ugo Russo 1 , Daniele Ielmini 1
1 , Politecnico di Milano, Milano Italy
Show Abstract11:45 AM - **G6.6
Highly Scalable Phase Change Memory Cell; A Technological Overview.
Sunglae Cho 1 , Jinil Lee 1 , Hyeyoung Park 1 , Dohyung Kim 1 , Dong-Hyun Im 1 , Young-Lim Park 1 , Heeju Shin 1 , Hyeong-Geun An 1 , Han-Bong Ko 1 , Dong-Ho Ahn 1 , Myungjin Kang 1 , Doo-Hwan Park 1 , Jeonghee Park 1 , Hyun-Suk Kwon 1 , Yongho Ha 1 , Junsoo Bae 1 , Mi-Lim Park 1 , Byoung-Jae Bae 1 , Hideki Horii 1 , Soon-Oh Park 1 , Hee-Seok Kim 1 , U-In Chung 1 , Joo-Tae Moon 1 , Won-Seong Lee 1
1 , Samsung Electronics Co., LTD., Yongin-City, Gyeonggi-Do, Korea (the Republic of)
Show Abstract12:15 PM - G6.7
RESET Resistance Dynamics in Phase Change Bridge Devices.
Daniel Krebs 1 , Simone Raoux 2 , Charles Rettner 2 , Yi-Chou Chen 3 , Geoffrey Burr 2 , Matthias Wuttig 1
1 I. Institute of Physics, RWTH Aachen University, Aachen, Nordrhein-Westfalen, Germany, 2 , IBM Almaden Research Center, San Jose, California, United States, 3 , Macronix International Co. Ltd., Hsinchu Taiwan
Show AbstractPhase Change Memory has become one of the promising candidates for next-generation non-volatile memory. Because of its good scaling behavior, high endurance, potential for multilevel storage, and fast write and access times it could potentially replace Flash memory in the next few years. In order to provide such features, it is critical to be able to place the phase change material in two (or more) well-defined and persistent resistance states. However it has been observed that immediately following a RESET operation from the crystalline to the amorphous state, induced by flooding the material with a high density of charge carriers, the phase change material runs through a brief recovery process during which the resistance increases exponentially [1]. This effect could potentially mandate a time delay (of roughly 100ns) before any read-after-write process can be accurate. Furthermore, after this "short-term" recovery process, a slow resistance drift to higher values over a long period of time can be found. These phenomena can be expected to become especially problematic for multilevel storage, influencing any iterative write strategy as well as the overall useful range of resistance contrasts.In this work, device testing was performed on a unique, combined optical and electrical setup to gain a better understanding of these recovery dynamics. Phase-change bridge devices [2] with widths between 50nm and 450nm, lengths between 50nm and 510nm, and a film thickness (height) of 30nm were fabricated using electron-beam lithography. Short-term recovery was studied by applying either a voltage or a laser pulse while measuring voltage and current across a phase-change bridge device [2]. The results were fitted with various analytical models to gain a microscopic picture about the ongoing mechanism. The resistance drift was explored by DC measurements over minutes and days after placing the device into different low conductivity states using either voltage or laser pulses. Finally, the influence of temperature on the electrical switching and recovery dynamics was tested and compared to the predictions of previously introduced models. It was found that the resistance drift is dependent on both temperature and starting resistance, indicating that the responsible mechanism is temperature-activated and dependent on the degree of crystallization. The models for the short-term recovery in both optical and electrical testing are in good agreement with experiments and with each other. From the sub-threshold region of the electrical testing experiments, trap densities in the order of 3x1018cm-3 are predicted, indicating high defect concentrations in these materials.[1] D. Ielmini, et al., "Recovery and drift dynamics of resistance and threshold voltages in phase-change memories," IEEE Transactions on Electron Devices, 54(2), 308-315 (2007).[2] Y. C. Chen, et al., "Ultra-Thin Phase-Change Bridge Memory Device Using GeSb," IEDM Tech. Dig., 30-3 (2006).
12:30 PM - G6.8
Phase Separation Behavior of Ge2Sb2Te5 Line Structure During Electrical Stress Biasing.
Sung-Wook Nam 1 , Cheolkyu Kim 2 , Min-Ho Kwon 1 , Jung-Sub Wi 1 , Hyo-Sung Lee 1 , Dongbok Lee 1 , Dongmin Kang 1 , Tae-Yon Lee 2 , Yoonho Khang 2 , Ki-Bum Kim 1
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Semiconductor Device Lab, Samsung Advanced Institute of Technology, Suwon Korea (the Republic of)
Show AbstractGe2Sb2Te5 is one of the widely employed chalcogenide materials as an information storage medium utilizing its fast reversible switching characteristics between amorphous and crystalline states. This material is already commercialized in an optical data storage media (digital versatile disk: DVD) and now heavily investigated for electrically controlled nonvolatile memory device named as phase change random access memory (PRAM). However, for the successful incorporation of Ge2Sb2Te5 in PRAM, the reliability issues still remain to be resolved, thus the investigation of PRAM failure mechanism has a great importance.Here, we report the phase separation behavior of Ge2Sb2Te5 during electric stress and suggest it as a dominant origin in PRAM device failure. To investigate the electric stress effect on Ge2Sb2Te5, we fabricated Ge2Sb2Te5 multi-line structure (1 µm line width, 100 nm thickness and 5 µm length) by electron beam lithography process, and it is connected to Ti/Au electrode for measuring electric properties. For a given Ge2Sb2Te5 multi-line device, we characterized the electric break-down behavior during voltage driven sweeping. Then, scanning electron microscope (SEM), scanning Auger microscope (SAM) and atomic force microscope (AFM) observations were performed on the electrically failed Ge2Sb2Te5 line structure for the analysis of morphology and compositional variation. Especially, SAM observation shows that the permanently failed Ge2Sb2Te5 line is separated into two different compositional parts, such as a Sb,Te-rich liquid phase and a Ge-rich solid phase. And the volume expansions are observed by AFM, which shows that temperature becomes high enough to induce the mass movements during break-down process. This phase separation behavior is supported by the pseudo binary phase diagram between Sb2Te3 and GeTe, which shows that the incongruent melting process drives the phase separation of Ge2Sb2Te5, under the temperature between 630oC and 650oC. We think that this thermodynamic guidline about the phase separation clarify the main origin of the PRAM device failures.
12:45 PM - G6.9
Investigation of SET and RESET States Resistance in Ohmic Regime for Phase-Change Memory.
Semyon Savransky 1 , Ilya Karpov 1
1 , Intel Corporation, Santa Clara, California, United States
Show AbstractThe resistance difference between glassy (RESET) and polycrystalline (SET) states of Ge2Sb2Te5 (GST) is utilized in phase change non-volatile memories (PCM). In this paper we report results of resistance studies of nano-scale PCM devices.SET resistivity of GST, the contact resistance and their activation energies were obtained from measurements at different temperatures for devices with different GST thickness. RESET resistivity and its activation energy Ea were obtained from similar measurements for the devices programmed with current that lead to resistance saturation. PCM devices resistivity was found to be about15 mOhm*cm for SET state and in the order of 1 kOhm*cm for RESET state at room temperature. Contact resistance values for the devices studied was about 30 uOhm/cm^2 at room temperature with a temperature dependence characterized by 0.073eV activation energy. The activation energies of GST resistivity for SET and RESET devices were distributed around 0.09eV and 0.37eV, respectively. The values of Ea and the resistivity obtained for SET and RESET states were in a good agreement with literature data for thin GST films in FCC and amorphous phases. We discuss in details possible reasons of Ea spread in PCM related to variations in device programming and drift of their resistance.We found that Meyer-Nelder (MN) rule describes well the relationship between Ea and pre-exponential factor for resistivity in both states, SET and RESET. MN characteristic temperatures are approximately 335K and 340K for RESET and SET states, respectively.