HH3: Phase-Change Memory
Chair: Huai-Yu Cheng
Chair: Dong-Ho Ahn
- Wednesday AM, April 23, 2014
- Moscone West, Level 3, Room 3005
8:30 AM - *HH3.01
The Evolution of PRAM Technology and Applications
Memory Business, Samsung Electronics, Hwasung-city, Kyunggi-Do, Republic of Korea.Show Abstract
PRAM( Phase-change Random Access Memory ) has been spotlighted due to its fantastic characteristics such as good shrinkability, high endurance, long-time retention and so on. Finally PRAM has been commercialized for mobile phone. Naturally, main interest for PRAM is moving from possibility of production to its usage and aplication-driven technology. In this presentation, I will share my idea about the evolution of PRAM applications which will be evolved into cost effective interface, innovative system and brain-inspired computation with respect to efficiency of energy-saving. And the advanced technologies related to the evolution of applications will discussed too.
Effective Interface: PRAM can be operated by bit-by-bit and at low voltage compared to conventional Flash memories. These advantages make PRAM possible to share the interface of central processing unit (CPU) and DRAM with system’s simplicity and effectiveness. This kind of application is being commercialized in some IT systems. Furthermore, global researchers and developers are focusing on other advantages such as fast latency and better reliability in order to find out novel applications .
Innovative System: Legacy computer architectures consist of CPU, DRAM as main memory, and NAND or HDD as storage. Unfortunately there has been a big latency gap between memory (<100ns) and storage (~100us), which masks the system be complicated and inefficient. When introducing the higher speed and better endurance of PRAM to reduce the latency gap, a new fusion technology so called memorage (a combination word of memory and storage) is emerging. This innovative fusion system can make computing system more cost-effective, more energy-saving, and much faster.
Brain-Inspired Computation: PRAM has very recently started to search new application for brain-inspired computing, which belongs to the kind of fusion technologies between electronics and neuroscience. This may cause paradigm shift in computation method from von Neumann type to cognitive-based algorithm . I will show that phase change memory cell array is able to emulate synaptic functions by introducing novel core algorithms, which can realize more energy-saving IT systems as way of making PRAM to be adaptable in neuromorphic applications.
 Youngdon Choi et. al., ISSCC digest of technical papers (2012) 46.
 M. Suri et. al., IEDM digest of technical papers (2012) 235.
9:00 AM - HH3.02
Effect of Baseline Voltage on the Set Dynamics of Phase Change Memory Devices
Electrical & Computer Engineering, University of Connecticut, Storrs, Connecticut, USA; 2,
, IBM T. J. Watson Research Center, Yorktown Heights, New York, USA.Show Abstract
Phase Change Memory (PCM) utilizes the large electrical resistivity contrast between the amorphous (high resistance) and the crystalline (low resistance) phases of chalcogenides. These materials can be reversibly and rapidly switched between the two phases by self-heating via electric pulses . Understanding the crystallization dynamics during set operation is critically important. A baseline (offset) voltage plays an important role in set operation achieved by melting the amorphous region and utilizing growth-from-melt templated from the crystalline regions .
In this work, we experimentally investigate the effect of baseline voltage on crystallization behavior of nanoscale Ge2Sb2Te5 (GST) line cells by applying a voltage pulse (melting pulse) with varying baseline voltages. After the melting pulse, the GST wires stay partially molten (retention of a current carrying filament) and recrystallize, or they resolidify as amorphous, depending on the applied baseline. Our electro-thermal models using finite element simulations also show the same current-time characteristics and capture the filament formation and crystallization dynamics. Details of the electrical measurements, simulation results, and analysis will be presented.
 H. -. P. Wong, S. Raoux, S. B. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi and K. E. Goodson, "Phase Change Memory," Proc. IEEE, vol. 98, pp. 2201-2227, 2010.
 D. Loke, T. Lee, W. Wang, L. Shi, R. Zhao, Y. Yeo, T. Chong and S. Elliott, "Breaking the speed limits of phase-change memory," Science, vol. 336, pp. 1566-1569, 2012.
 M. Cassinerio, N. Ciocchini and D. Ielmini, "Logic Computation in Phase Change Materials by Threshold and Memory Switching," Adv Mater, 2013.
9:15 AM - *HH3.03
Thermal Transport in Phase Change Memory Materials
, Stanford University, Stanford, California, USA.Show Abstract
Nanoelectronic devices - and their constituent materials and interfaces - present some of the most promising and challenging opportunities for the study of thermal transport in the solid state. Phase change memory devices provide a particularly compelling example owing to the importance of both electrons and phonons in one of the crystalline phases of the functional chalcogenide material (e.g., 1). Additional complications (and opportunities) arise from the impacts of thermoelectric transport and thermal boundary resistances on the key figures of merit of memory devices, such as the time and energy required for bit writing and the bit stability (e.g., 2).
This presentation will describe a multi-year investigation of thermal transport in GeSbTe compounds using thermoreflectance and Joule-heating methodologies as well as transmission electron microscopy, x-ray diffraction, and a novel “micro thermal stage” that facilitates incomplete transitions between the crystalline and amorphous phases. While the findings have been primarily helpful with the design of phase change memory, they are also enabling more exploratory research on brain-inspired computing, field-programmable gate arrays, and a solid-state thermal switch.
1Wong, Raoux, Kim, Liang, Reifenberg, Rajendran, Asheghi, Goodson, "Phase Change Memory," Proceedings of the IEEE 98, 2201-2227 (2010).
2Lee, Asheghi, Goodson, "Impact of Thermoelectric Phenomena on Phase-Change Memory Performance Metrics and Scaling," Nanotechnology 23, 205201 (2012).
9:45 AM -
10:15 AM - *HH3.04
Crystal Growth in GeSb Thin Films and Electrical and Structural Characterization of PRAM Line Cells
Zernike Institute for Advanced Materials, University of Groningen, Groningen, Netherlands.Show Abstract
The presentation will focus on two main topics: 1. Crystallization kinetics of GexSb1-x films (50 and 200 nm thick with x = 6, 7, 8, 9 and 10) studied using mainly high speed optical microscopy; 2. Electrical and structural characterization of PRAM line cells using a 2-point probe technique and scanning and transmission electron microscopy.
In part 1 Alloys ranging from Ge6Sb94 to Ge10Sb90 were studied and it was found that crystallization properties change dramatically with small changes in Ge concentration. Crystal growth rates were measured over five orders of magnitude (100 nm/s to over 1 cm/s). Using a viscosity model a fragility m=60 for Ge8Sb92 and Ge9Sb91 was found and based on this model extrapolation of the experimental data led to a maximum crystal growth rate of ~15 m/s at a temperature of ~90% of the melting temperature. Moreover, the large influence of moderate stresses on crystal growth was directly observed by using a four point bending stage in combination with high speed optical microscopy. Finally, two competing growth modes were found in Ge8Sb92, Ge9Sb91 and Ge10Sb90 alloys with growth rates that differ more than an order of magnitude while external conditions are kept constant.
In part 2 the evolution of many characteristics of single line cells during their cycling up to 100 million cycles is shown. We also present extensive work done on the intricate time and temperature dependence of the amorphous resistance of the line cells as influenced by drift. Finally, the correlation between electrical properties and the nanostructure of the line cells is shown.
10:45 AM - HH3.05
Probing the Crystal Growth Velocity of Melt-Quenched Phase-Change Materials within a Memory Cell
Le Gallo1, Daniel
, IBM Research - Zurich, Rueschlikon, Switzerland.Show Abstract
Chalcogenide materials undergoing reversible crystal-to-amorphous phase transitions continue to play a key role in information technology. In particular, phase-change memory (PCM) has emerged as the most promising new nonvolatile solid-state memory technology. A key property that makes these materials attractive for such applications is the crystallization process that occurs at the nanometer length scale and the nanosecond time scale. Moreover, in spite of this fast crystallization rate at elevated temperatures, the melt-quenched amorphous state is stable at lower temperatures for several years. Given the length and time scales associated with this unique crystallization process, experimental measurements are extremely challenging.
Most experimental investigations have been limited to low temperatures, at which the crystallization rate is small. A recent effort to measure the crystal growth rate using ultrafast differential scanning calorimetry was reported by Orava et al. . These measurements were conducted on as-deposited phase-change materials. A more recent effort involved the use of laser-based time-resolved reflectivity measurements . But both techniques were limited in terms of the temperature range over which crystallization rate was measured.
In this talk, we present the measurement of crystallization rate with a PCM cell using electrical means. This approach is capable of measuring the crystallization rate down to nanometer dimensions as well as on the nanosecond time scale thanks to, respectively, the dimensions of the PCM cell and the fast thermal dynamics associated with such cells. We present a consistent description of the temperature dependence of the crystal growth velocity in the glass as well as the super-cooled liquid from room temperature up to the melting temperature. Isothermal measurements are conducted to measure the growth velocity in the glass state, which is shown to follow an Arrhenius relationship. The melting temperature, where the crystallization rate is zero, and the temperature at which this rate is maximum are estimated by exploiting the spatial and temporal evolution of temperature in the PCM cell created by the application of suitable voltage pulses. A cryogenic probe station to vary the ambient temperature and finite-element models to support the experimental findings are used. The overall temperature dependence of the growth velocity is quantified using these experiments in combination with an estimate of the evolution of the effective amorphous thickness as a function of the application of voltage pulses.
 J. Orava et al., Nature Materials 11, 279 (2012)
 M. Salinga et al., Nature Communications 4, 2371 (2013)
11:00 AM - HH3.06
A Platform for Studying the Scaling of a Phase Change Material Memory Bit
, IBM T.J. Watson Research Center, Yorktown Heights, New York, USA.Show Abstract
We report on a new platform for studying the scaling of a phase change material (PCM) memory bit, enabling electrical as well as structural characterization of a PCM cell formed between point electrodes with gaps as small as 2 nm. In this work the platform was used to study memory bits made of Ge2Sb2Te5 (GST) with dimensions as small as 10×10×2 nm. A record low ISET current of 0.4 μA was obtained.
Conductive 10 nm wide and 10 nm thick Pd nano-wire electrodes were fabricated over a thin silicon nitride membrane using e-beam lithography and a lift-off process. The 50 nm thick silicon nitride film served as a TEM membrane to allow the imaging of the PCM bit before and after setting of the bit by passing electrical current. Optical lithography and metallization was used to define probe pads with connections to the nano-wires. The nano-wires were coated with a thin Al2O3 layer and gaps cut with dimensions as small as 2 nm were made in each of the nano-wires using a focused high-energy electron beam. An amorphous GST film was blanket deposited over the structure by physical vapor deposition (PVD) from a GST 225 target. The deposited GST was verified by TEM to fill the gap that was cut in the nano-wires. Since the nano-wire electrodes were coated with Al2O3 prior to the gap formation, electrical contact to the deposited PCM material took place only at the exposed metal in the cut gap. This allowed the fabrication of PCM memory bits with dimensions as small as 10×10×2 nm.
Current-Voltage (I-V) sweeps forcing current and measuring voltage were taken to set the PCM bits. A record low set current (ISET) of 0.4 μA for switching the device to the low resistance, crystalline state, was achieved. The formation of a crystalline phase change material that bridges the gap between the wires was verified in the switched devices by TEM. Additional DC current sweeps verified that the bit maintained the SET state.
11:15 AM - HH3.07
Role of Inelastic Electron-Phonon Scattering in Ultra-Scaled Phase Change Material Nanostructures
, University of Washington, Seattle, Washington, USA.Show Abstract
One of the most attractive merits of PCM technology is its superb scalability. The state-of-art scaling experiments have shown that the sub-2 nm PCM nanostructures still can keep phase change properties . Simulations and experiments have shown that the PCM devices, downscaled to 3.8 nm  and 6 nm  respectively, can offer enough ON/OFF ratio. The promising scaling scenario unveiled in these studies have spurred intensive interests to develop ultra-scaled PCM devices, to offer ultra-dense data storage solutions. It is known that inelastic electron-phonon scattering plays an important role in determining electron transport properties of bulk PCM (tens of nm or larger) [4-5]. But the understanding of its role in ultra-scaled PCM (sub-10 nm) is still missing. It is important to investigate this problem, in order to offer deeper physical insight for designing ultra-scaled PCM devices.
In this work , we study the electron transport in ultra-scaled amorphous GeTe sandwiched by TiN electrodes. The electronic structure of the TiN-GeTe-TiN model is obtained using density functional theory; the electronic transport is simulated using non-equilibrium Green’s function (NEGF) ; and the inelastic electron-phonon scattering is accounted for using Born approximation. Our results show that, though the measured transport properties (e.g. current-voltage curve, negative differential resistance, and threshold switching) of ultra-scaled PCM highly resemble those of bulk PCM, their governing physical mechanisms are totally different. In bulk PCM devices, it is known that the transport is largely diffusive (Ohm’s Law), partially due to inelastic electron-phonon scattering. Furthermore, the inelastic electron-phonon scattering plays a crucial role to excite the trapped electrons so that they can participate in the transport (Poole-Frenkel Law) [3-4]. In contrast, we find that the electron transport in the ultra-scaled PCM nanostructures is largely elastic. For the ultra-scaled 6 nm GeTe ultrathin film used in our simulation, less than 5% of the energy carried by the incident electrons is transferred to GeTe lattice when they are transported from source to drain. This indicates that, in the ultra-scaled PCM nanostructures, the inelastic electron-phonon scattering exerts limited influence on the electron transport process and the total current. Therefore, the Poole-Frenkel effect does not play a role in ultra-scaled PCM.
. Raoux, S et. al, J. Appl. Phys., 103, 114310, 2008.
. Liu, J. et. al, J. Appl. Phys., 113, 063711, 2013.
. Kim, S. et. al, IEEE Trans. on Electr. Dev, 58, 1483, 2011.
. Ielmini, D, Phys. Rev. B, 78, 035308, 2008.
. Ielmini, D, et. al, J. Appl. Phys., 102, 054517, 2007.
. Liu, J., Xu, X., and Anantram, M.P., “Analysis of the role of inelastic electron-phonon scattering in ultra-scaled GeTe ultrathin film”, (in preparation).
11:30 AM - HH3.08
Electrochemical Preparation of Nanostructured Phase Change Random Access Memory Devices
de Groot2, Andrew
Chemistry, University of Southampton, Southampton, United Kingdom; 2,
Electronics and Computer Science, University of Southampton, Southampton, United Kingdom.Show Abstract
The miniaturization of memory devices is one of the major driving forces in the development of ever faster, more efficient and more compact electronic devices. However, pushing towards smaller, more densely packed memory bits can cause the overlap of neighboring data points in the classical thin-film mushroom cell geometry of phase change random access memory (PCRAM) devices. Therefore, different architectures, such as incorporating the phase change material into a nanostructured insulator, have to be considered. The preparation of materials inside nanostructured substrates proves challenging for the widely used sputtering techniques and new ways of preparing the phase change materials must therefore be investigated.
We present an alternative materials preparation approach based on the electrochemical deposition of phase change materials, such as SbTe and GeSbTe from soluble precursors that provide sources for the individual elements. Electrodeposition as a preparation technique is already well established in the electronics industry through the Damascene process which is used for the preparation of copper vias in microchips. In the case of phase change materials the electrochemical approach allows the direct deposition of the binary or ternary alloy materials into a nanopatterned device substrate consisting of a TiN bottom electrode covered by a patterned SiO2 insulator. The bottom-up nature of electrodeposition prevents the issue of void formation during material deposition. Besides the advantages provided by direct materials preparation within nanopatterned substrates the electrochemical approach also allows tuning of the material composition, for example, by adjusting the amount of the Ge, Sb and Te precursors in the deposition bath.
We will present an electrochemical system used for the preparation of the SbTe and GeSbTe phase change materials and characterize the prepared materials using scanning electron microscopy (SEM), energy/wavelength dispersive X-ray spectroscopy (EDX / WDX) and X-ray diffraction (XRD). The materials are electrodeposited either as a thin film for materials characterization or within nanopatterned substrates prepared by electron-beam lithography. The control over the material composition within the GeSbTe composition triangle will be discussed. We will also present methods for the accurate filling of nanopatterned substrates with hole sizes ranging from a few micrometers down to less than 50 nanometers. The electrical switching and the related quality of the obtained materials within the nanopatterned substrates will be discussed.
Chair: Dong-Ho Ahn
- Wednesday PM, April 23, 2014
- Moscone West, Level 3, Room 3005
1:30 PM - HH4.01
Stoichiometric GaSb - A Candidate for Fast and Pb-Free Soldering Reflow Complying Phase-Change Memory
Cheng1 2, Simone
Raoux1 3, Khanh
, IBM/Macronix PCRAM Joint Project, Yorktown Heights, New York, USA; 2,
Emerging Central Lab, Macronix International Co. Ltd., Hsinchu, Taiwan; 3,
, IBM T. J. Watson Research Center, Yorktown Heights, New York, USA; 4,
, IBM Almaden Research Center, San Jose, California, USA.Show Abstract
Phase change material is at the heart of phase-change memory (PCM) technology. Today almost all phase change memory IC’s still use Ge2Sb2Te5 (GST-225) inherited from optical disk technology, even though poor high-temperature data retention due to its low crystallization temperature (Tx~150 oC) inhibit its use in some new applications such as automotive. Furthermore, many embedded system applications embrace a pre-coding procedure where system code data are pre-programmed into the non-volatile memory (NVM) before the chips are soldered onto printed circuit board. It is also impossible to pass the rigors of withstanding the 260 C soldering process using traditional GST-225 material.
Thin films were prepared by PVD sputtering from a compound Ga50Sb50 target. Crystallization temperature and electrical contrast were measured by in-situ resistivity measurements in van der Pauw (vdP) geometry during continuous heating in nitrogen atmosphere. The stoichiometric compositions of Ga50Sb50 is characterized with very high crystallization temperature indicating an excellent amorphous stability of this material. Ga50Sb50 shows two to five orders of magnitude resistance difference depending on the final heating temperature. Crystallization times were measured using a custom-made static laser tester. It was found that the stoichiometric alloy has an unusual inverse optical contrast compared to typical phase-change materials where the crystalline phase has lower reflectance compared to the amorphous phase. Moreover, Ga50Sb50 showed a very short crystallization time of around 20 ns.
The 30 nm GaSb and 30 nm TiN top electrode thin films were deposited into PCM devices with the bottom electrode size of 40 nm. A lift-off process was used to fabricate prototype mushroom PCM test devices. Almost 2 orders of magnitude SET/RESET resistance window was successfully achieved. 60 ns fast SET speed was demonstrated in the devices which is consistent with the fast switching performance from optical laser testing. PCM devices were programmed into the SET and RESET state, respectively, and exposed to the 260 oC solder bonding temperature cycle. The RESET and SET resistance both slightly increase but were still clearly separated after heating.
These results indicate that GaSb material with the stoichiometric composition is a promising candidate for phase-change memory by combining fast crystallization speed and good thermal stability.
1:45 PM - HH4.02
GaSb-Based Phase Change Alloys as Candidates for Phase Change Memory
Xiong3 4, Eric
Putero5 6, Vanessa
Coulet5 6, Christophe
Muller5 6, Carsten
IBM/Macronix PCRAM Joint Project, IBM T. J. Watson Research Center, Yorktown Heights, New York, USA; 2,
Emerging Central Lab.,, Macronix International Co., Ltd.h Center, Hsinchu, Taiwan; 3,
Department of Electrical Engineering, Stanford University, Stanford, California, USA; 4,
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Champaign, Illinois, USA; 5,
IM2NP, Campus de Saint-Jérôme, Aix-Marseille Université, Marseille, France; 6,
Campus de Saint-Jérôme, CNRS, IM2NP - UMR 7334, Marseille, France; 7,
Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf e.V., Dresden, Germany.Show Abstract
Phase change materials are at the core of phase change memory (PCM) technology. They possess a unique combination of physical properties that allow storing and retaining data.
Ga-Sb alloys have recently be proposed as phase change materials for PCM because of their very fast switching speeds but several other material parameters are not optimized for PCM such as the low resistances in the amorphous and crystalline phases and for some alloys the low crystallization temperature.
Here we report on a new class of phase change materials which are designed by starting from the stoichiometric Ga:Sb=50:50 material and adding various materials to it. Stoichiometric Ga:Sb=50:50 has unusual properties because its electrical properties are similar to other phase change materials, but its optical contrast and mass density change behavior is opposite to most phase change materials [1-3]. The crystalline phase has a lower reflectivity than the amorphous phase and the mass density of the crystalline phase is also lower than that of the amorphous phase, as opposed to most other phase change materials. Here we report on the properties of alloys which were deposited by mixing materials that show typical phase change behavior including Sb, Ge and Si into Ga:Sb=50:50.
Addition of Sb over a wide range produces materials with with very fast recrystallization times on the order of 15 ns, but reduced crystallization temperature with increased Sb content. In particular, the Ga:Sb=30:70 material is interesting because it shows no mass density change upon crystallization but still has high electrical contrast . This makes it a great candidate for improved cyclability because void formation upon cycling is one of the main failure mechanisms of PCM and believed to be caused by mass density change upon switching. The optical contrast changes from negative (Ga:Sb=50:50) to three distinct levels of reflectivity for Ga:Sb=30:70 to positive contrast for the materials with high Sb content.
Adding Si or Ge increases the crystallization temperature but also re-crystallization times. Ge has the strongest effect leading to a crystallization temperature that can be as high as 450 °C (about 220 °C for Ga:Sb=50:50), but this is accompanied by an increase in the re-crystallization time from 20 ns of Ga:Sb=50:50 to 80 ns of materials with about 50 atomic % Ge added. The alloys with Ge and Si added show the negative optical contrast observed for Ga:Sb=50:50, indicating that with the right amount of Ge and Si added also alloys with no mass density change might be found. These experiments show that by modifying phase change material composition the properties can be tuned over a wide range and optimized for specific applications.
 Cheng et al., EPCOS Proc. p. 103, 2011
 Raoux et a., Physica Status Solidi 249 (2012) 1999
 Putero et al., Appl. Phys. Lett. (submitted 2013)
 Putero et al., Appl. Phys. Lett. Mater. (submitted 2013)
2:00 PM - HH4.03
Phase Transition in GaSb Alloys: Phase Segregation and Mass Density Change
IM2NP CNRS UMR7334, Aix Marseille Université, MARSEILLE, France; 2,
Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials Research, Dresden, Germany; 3,
IBM/Macronix PCRAM Joint Project, IBM T. J. Watson Research Center, Yorktown Heights, New York, USA.Show Abstract
Materials design and optimization are at the core of the Phase Change Random Access Memory (PCRAM) technology and often seemingly conflicting materials requirements need to be met for a successful integration. For a prototypical phase change material such as Ge2Sb2Te5 which enabled the development of CD and DVD optical storage , the amorphous phase exhibits low optical reflectivity, low mass density and very high electrical resistivity. In contrast, the crystalline phase shows high optical reflectivity, high mass density and low electrical resistivity. Unfortunately, large optical contrast is also associated with a large mass density change  leading to void formation in PCRAM cells that represents one of the main failure mechanisms .
In order to optimize phase change materials for PCRAM application Ga-Sb binary system was investigated by tuning the composition between Ga:Sb=9:91 (in atomic %) and Ga:Sb=45:55. Combined in situ sheet resistance measurements and synchrotron X-ray scattering techniques performed during heating enabled demonstrating a reduced crystallization temperature while Sb content increases. Besides, the electrical contrast increases with increasing Sb content and the resistivity in both the amorphous and crystalline phase decreases.
Whatever the composition, X-ray diffraction showed an elemental segregation evidenced by the detection of two crystalline phases, the rhombohedral Sb phase and the cubic GaSb phase. For alloys close to the stoichiometric composition a stepwise crystallization was observed  during a temperature ramp with the GaSb phase crystallizing first and the Sb one crystallizing at higher temperature. In contrast, the reverse sequence was observed for higher Sb concentrations with Sb phase crystallizing first. In between, it was shown that the Ga:Sb=30:70 alloy undergoes a simultaneous crystallization of both phases. Finally, in situ X-ray reflectivity revealed a very interesting change as Sb concentration decreases: while the mass density increases upon crystallization in Ga:Sb=9:91 (as usually observed in most of the phase change materials), it decreases in GaSb close to stoichiometric composition. At the crossroad, the composition Ga:Sb=30:70 exhibits upon crystallization neither change in mass density nor change in film thickness. This result is of primary importance for memory applications since the lack of density change may considerably reduce the mechanical stresses as the PCM programmable volume is cycled between amorphous and crystalline states.
 M. Wuttig and N. Yamada, Nat. Mater. 6, 824-32 (2007).
 Y. Saito, Y. Sutou, and J. Koike, Appl. Phys. Lett. 102, 051910 (2013).
 C.-F. Chen et al., in Int. Mem. Work. Monterey, CA, IEEE (2009).
 M. Putero et al., Appl. Phys. Lett. submitted, (2013).
2:15 PM - HH4.04
Ab-Initio Molecular-Dynamics Simulations of Ga-Sb Phase-Change Materials
Chemistry, University of Cambridge, Cambridge, United Kingdom.Show Abstract
A large number of potential phase-change materials have been have produced promising results experimentally; however, only Ge2Sb2Te5-based PCMs have been significantly explored so far using ab initio molecular-dynamics (AIMD) simulations. We present the first AIMD study of the full melt/quench/anneal cycle for Ga -Sb PC materials, for compositions ranging from the near-eutectic alloy Ga16Sb84, to stoichiometric GaSb. The primary changes in local environment associated with crystallisation are demonstrated, and correlated with increasing Ga content. For Ga16Sb84, it is shown that crystallisation is characterised by the transition of Ga atoms from a tetrahedral to octahedral-like coordination. In GaSb, the opposite transition occurs for Sb atoms, from octahedral-like to tetrahedral coordination. The electronic density of states and the optical reflectivity are calculated for each phase and demonstrate good agreement with experimental results.
2:30 PM -
HH5: Resistance Drift
Chair: Huai-Yu Cheng
- Wednesday PM, April 23, 2014
- Moscone West, Level 3, Room 3005
3:00 PM - *HH5.01
Material Engineering for New Phase Change Memory Applications
, Micron Semiconductor Italy, Agrate Brianza (MB), Italy.Show Abstract
In order to continue with the PCM roadmap, after entering into the mass production phase since last year, new advanced technology development must be faced. Considering alternative non volatile memory technologies, PCM is still one big player that is competitive through different memory segment. Nevertheless in order to be effective through different application scenarios, new phase change materials need to be evaluated on top of the well know GST225. Embedded segment and in particular automotive market is continuously demanding for larger retention capabilities without losing other electrical performances (low programming current, good read window, fast setability, high endurance…) still preserving the scalability aspect.
Aim of this talk is to describe the starting point of actual PCM product inside general memory trend, highlight main issues for future technology nodes and show the main strategy to physically and electrically screen alternative phase change materials to face new markets, in particular embedded one, maintaining focus on wall architecture.
Finally some highlight on different concept like chalcogenide superlattice will be discussed as alternative way to overcome specific PCM limitations that would enable PCM as a very low power memory.
3:30 PM - HH5.02
TEM Study of the Low Resistance State in GST Ge Rich and N Doped PRAM Devices
, CEA-LETI, Minatec Campus, Grenoble, France; 2,
, STMicroelectronics, Technology R&D, Agrate Brianza, Italy.Show Abstract
Phase change memories (PCM) are one of the most promising technology to fulfill the requirements of non-volatile memories, both for stand-alone and embedded applications. However, some challenges still remain to enlarge the application spectrum, and one of them is the low resistance SET state drift observed under soldering reflow conditions. Recently, the material GST-Ge45%-N4% has attracted lots of attention thanks to the excellent trade-off it offers between data retention and SET-RESET performances. In this study, we present a thorough investigation of both the electrical performances and physico-chemical characteristics of germanium-rich GST devices doped with nitrogen, providing for the first time a full insight of the devices integrating this specific material.
We first proceed to several material characterizations on GST thin films with variation of Ge content and nitrogen. The resistivity measurements as a function of temperature reveal the benefits of Ge enrichment on the thermal stability of the amorphous phase while the kissinger plots of GST-Ge45%-N4% also shows a boost of more than 60% of the activation energy. In addition, DRX studies show the segregation of cubic Ge at 400°C and formation of hexagonal GST at 550°C.
The GST devices with different Ge and N contents are then electrically characterized. The R-I curves obtained show a decrease of the RESET current in GST-Ge45%-N4% devices up to 33%, and an increase of the SET resistance. The data retention measurements display a drastic increase of the high resistance RESET state stability with Ge and N doping, but also highlight the loss of stability of the low resistance SET state at high temperatures, strongly worsen by doping. The influence of pulses fall times is also investigated and reveals that a resistance lower than the SET state can be reached by applying longer fall times than standard square pulses. The drift curves of this so-called SETMIN state exhibit a smaller drift than the SET state, and has been proven to be stable under soldering reflow conditions without affecting the functionality of the devices, which can still be re-programmed in the RESET & SET state afterwards.
TEM studies focus on the GST-Ge45%-N4% material : three devices are electrically programmed in the SETMIN, SET & RESET states and prepared for TEM observations by FIB etching. HR-TEM images enable shows the amorphous volume on top of the heater resulting from the RESET operation. The crystalline nature of both the SET and SETMIN states is also confirmed. EELS cartographies are then performed, providing information on the elements distribution. Those cartographies point out the segregation of elements in the SETMIN state, showing a rejection of germanium above the heater. Further analysis of diffraction patterns gathered on multiple points of interest confirm the formation of a crystalline phase above the heater which differs from the crystalline structures identified in the rest of the PCM layer.
3:45 PM - HH5.03
Unified Modeling of Electrically Induced Crystallization in the Filamentary Regime of Phase Change Memory Devices
DEIB, Politecnico di Milano, Milano, Italy.Show Abstract
Phase change memory (PCM) is considered as one of the most promising candidate for future non-volatile memory (NVM) technology . Among the emerging NVMs, PCM is also the first which has reached the industrial maturity . PCM devices are based on the reversible crystalline-amorphous transition in a chalcogenide material such as Ge2Sb2Te5 (GST). While the crystalline (set) state is stable, the amorphous (reset) state is metastable, showing spontaneous thermally-activated crystallization. Crystallization can be obtained either by increasing the ambient temperature or by electrical pulses, where the T increase due to Joule heating leads to phase transition in the 10 ns timescale. For this reason, crystallization plays an important role not only in data retention  and programming speed , but also in reset transition , read disturb  and program disturb .
In this work, we propose a unified finite element model able to predict electrically induced crystallization in PCM devices. The simulated structure is a mushroom cell with confined bottom electrode contact (heater). Literature values for electrical and thermal conductivities were used. To correctly describe Joule-heating in PCM from ambient temperature to melting point (around 900 K), also the T-dependence of GST electrical and thermal conductivities were taken into account . The model relies on continuity and Fourier equations for electrical and thermal transport, while a first order differential equation describes the thermal activated evolution of the crystalline fraction. The fragile nature of GST glass was modeled by considering the non-Arrhenius-activated kinetic constant driving crystallization equation . To model the reset state during set transition, we have described threshold switching by the formation of a highly conducting filament within the amorphous cap . The model is able to predict the measured resistance R evolution in set experiments even at extremely low currents near the hold current Ih (around 40 μA), which is the minimum current necessary for the self-sustaining threshold switching mechanism. In addition, the model also describes crystallization in the subthreshold regime (I < 5 μA), where T is below 500 K and R decay in the 1000 s time-scale. This unified approach is important to predict read disturb effect in PCM devices, which must be thoroughly understood to avoid data loss under repeated read operations .
 H.-S. P. Wong, et al., Proc. IEEE 98 2201 (2010)
 G. Servalli, et al., IEDM Tech. Dig., 113-116 (2009)
 D. Mantegazza, et al., IEDM Tech. Dig. 311 (2007)
 D. Loke, et al., Science 336, 1566 (2012)
 D. H. Kang, et al., Symp. VLSI Tech. Dig., 96 (2007)
 A. Pirovano, et al., IEEE TDMR, 4 (2004)
 M. Boniardi, et al., IEDM Tech. Dig. (2013)
 A. Faraclas, et al., IEEE VLSI, 78-83 (2012)
 N. Ciocchini, et al., Trans. Electron Dev. 60, 3767-3774 (2013)
4:00 PM - HH5.04
Modeling Microstructural Evolution During Multi-Level Switching in Ge2Sb2Te5 from Electrical Characterization
Hong1, Seung Jae
Electrical, Electronic, and Control Engineering, Hankyong National University, Anseong-si, Gyeonggi-do, Republic of Korea.Show Abstract
Multi-level-cell (MLC) operations of phase change memory (PRAM) require stable intermediate resistance states to minimize data errors during the lifetime of the device. It has been investigated that the amorphous state of phase change materials (e.g., Ge2Sb2Te5) possesses an inherent meta-stability, which results in resistance drift phenomena with the elapsed time after the onset of programming. Intermediate resistance states, a mixture of amorphous and crystalline phases, also exhibit the resistance drift, whose power law exponent is approximately proportional to the modeled volume fraction of the amorphous phase. Moreover, the volume fraction itself is not the only parameter that affects the resistance drift but the microstructural morphology is another control parameter for the resistance drift.
To investigate the microstructural morphology of the phase change material during multi-level resistance switching, we have analyzed line-shapes and their temperature dependences of current-voltage characteristics. These analyses led to a model of microstructural evolution during multi-level switching, which states filamentary nucleation, coalescence of nuclei in one dimensional shapes, and cross-sectional areal growth. To confirm this modeling, we have compared resistance drift characteristics in each regime of microstructures. Finally, we suggest some switching strategies to reduce resistance drift characteristics in intermediate resistance levels, and suggest some cell geometries for a potential improvement in resistance drift characteristics. We expect that our proposed modeling and suggestions would provide a promising route to reliable MLC PRAMs.
4:15 PM - *HH5.05
Relaxation in Phase Change Memory Cells
, IBM Research - Zurich, Rueschlikon, Switzerland.Show Abstract
Even though phase change materials have found their way into commercial products for memory applications, some fundamental problems linked to the relaxation and crystallization kinetics remain to be solved. The inherent relaxation of the amorphous structure for example causes an increase of resistance over time, called resistance drift. This prompts a major challenge for the implementation of multilevel storage which is necessary to achieve high storage densities. On the other hand a precise knowledge about the crystallization kinetics is necessary to develop materials which allow the balancing act of ultra fast write operations but long data retention at the same time.
Previous studies[1,2] have linked changes in the density of states with the resistance increase and found an empirical relation for the time evolution of the activation energy of conduction for a constant annealing temperature Ta: Ea(t,Ta) = E0 + κTa ln((t+t0)/t1). The resistance R = R0 exp(Ea/(kBT)) increase follows with this time evolution a power law as experimentally observed. The models that have been proposed to explain this power law dependence by structural relaxation[3,4,5] are all based on a uniform distribution of activation energies for structural relaxation. This however seems unphysical since structural relaxation processes in glasses are typically attributed to α-relaxations following a single activation.
In this work, we present temperature dependent resistance measurements and demonstrate how temperature and time dependence of drift can be decoupled. By describing the relaxation process with an order parameter and a single activation energy for structural relaxation we find a differential equation that can describe experimental data with arbitrary temperature and time evolution of the resistance. Furthermore we find that the empirical relation for the activation energy of conduction is a limiting case for constant temperature of this differential equation.
In order to find potential candidates to represent the order parameter in a microscopic picture, molecular dynamics simulations have been carried. They show that there is a correlation between an increase of the optical band gap and a tendency towards a local structure that reassembles the local order in the crystal. This result suggests that the processes occurring during relaxation are related to the crystallization kinetics.
 Krebs et al., Journal of Non-Crystalline Solids 358, 2412 (2012)
 Oosthoek et al., Journal of Applied Physics 112, 084506 (2012)
 Karpov et al., Journal of Applied Physics 102, 124503 (2007)
 Ielmini et al., Applied Physics Letters 92, 193511 (2008)
 Fantini et al., Applied Physics Letters 102, 253505 (2013)
4:45 PM - HH5.06
High Speed, High Temperature Electrical Characterization of Meta-Stable Phases and Crystallization Dynamics of Ge2Sb2Te5
ECE, University of Connecticut, Storrs, Connecticut, USA; 2,
, IBM Watson Research Center, Yorktown Heights, New York, USA.Show Abstract
Phase change memory (PCM) is the most recent non-volatile memory technology in the marketplace as a flash memory alternative and has the potential to become a non-volatile DRAM replacement. PCM devices work based on electrical resistivity contrast between highly resistive amorphous and highly conductive crystalline phases of phase change materials. A small volume of a phase change material (active region) switches between amorphous and crystalline phases by suitable electrical pulses. These devices experience melting and resolidification in nanoseconds time-scale and their active region reaches ~ 1000 K during the operation. Hence, high-speed and high-temperature characterization of these materials is crucial.
In this study a set of high-speed high-resolution and long duration electrical measurements were performed on nanoscale Ge2Sb2Te5 (GST) line cells in a 125 K- 673 K temperature range. Electrical resistivities of metastable amorphous (above ~400 K) and metastable fcc (face centered cubic) (above ~550 K) GST were extracted. The resistance drift in amorphous phase in 250 K - 500 K temperature range and crystallization dynamics immediately after amorphization at elevated temperatures are characterized. Details of the measurement technique and results will be presented.
HH6: Poster Session
- Wednesday PM, April 23, 2014
- Marriott Marquis, Yerba Buena Level, Salons 8-9
8:00 PM - HH6.01
Simultaneous Seebeck and Electrical Resistivity Characterization of Ge2Sb2Te5 Thin Films
Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, USA.Show Abstract
Ge2Sb2Te5 is the most studied phase change material for phase-change memory applications. However, temperature dependent electrical characterization of the material is not complete. In this work, we have developed a high-temperature thin-film electrical characterization setup that allows simultaneous measurement of Seebeck coefficient and electrical resistivity up to melting temperature. Our results in repeated annealing and cool-down cycles show a consistent Seebeck and electrical resistivity behavior in temperature. The correlations between these two parameters for Ge2Sb2Te5 in a wide temperature range will be presented and possible explanations of carrier transport in GeSbTe compouds will be discussed.
8:00 PM - HH6.02
Improvement of Gap-Filling Ability of Sb-Te Thin Film by the Screen Plasma-Enhanced Atomic Vapor Deposition
Jeong1, Doo Jin
Material Science, Yonsei University, Seoul City, Republic of Korea.Show Abstract
Since the development of dynamic random access memory (DRAM) and FLASH, there have been huge advances in memory devices that have been pioneering a new era of information and technology in society. However, the development of new non-volatile memories is needed to overcome the limitations of the existing DRAM and NAND flash.1, 2 Phase change random access memory (PCRAM) has attracted many attention as the candidates for next generation memory3. However, several critical issues related to operational power consumption need to be resolved in order for PCRAM to become a universal memory. To solve this issue, many studies have reported with regard to the electrical resistivity and melting temperature of phase change materials4. In addition, trench-structure cells have been investigated to minimize thermal energy loss and thermal cross-talk among adjacent memory cells5,6. In this research, a new screen remote plasma-enhanced atomic vapor deposition (SPEAVD) technique was studied for depositing Sb-Te phase change materials inside trench structures with high aspect ratios. We have investigated the new deposition process to increase step coverage, which is a critical problem in the deposition on small trench sizes. In our work, The theoretical model of the screen mechanism including the concepts of a plasma sheath and sticking coefficient was considered. Plasma sheath was introduced to control the flow of ions and electrons for redistribution the energy of the plasma in the reaction chamber. Our research team confirmed the screen effect of the plasma by observing the gap-filling characteristics of the screen remote-plasma enhanced AVD and the direct remote plasma-enhanced AVD. We expect this research to provide a new deposition method that allows for the fine control of step coverage and other characteristics.
 A.L. Lacaita, Solid-state Electron. 50, 24 (2006)
 S. Hudgens and B. Johnson, MRS Bull. 29, 829 (2004)
 M. H. R. Lankhorst, B. W. S. M. M. Ketelaars, and R. A. M. Wolters, Nat. Mater. 4, 347 (2005)
 L. Wu, M. Zhu, Z. Song, S. Lv, X. Zhou, C. Peng, F. Rao, S. Song, B. Liu, S. Feng, J. Non-Cryst. Solids 358, 2409 (2012)
 W. Wang, D. Loke, L. Shi, R. Zhao, H. Yang, L.T. Law, L. T. Ng, K. G. Lim, Y. C. Yeo, T. C. Chong, and A. L. Lacaita, Sci. Rep-UK. 2, 360 (2012)
 F. Pellizzer and R. Bez, IEEE. ICICDT (2012) 6232857
8:00 PM - HH6.03
Mass Transport in as Deposited and Structural Relaxed Amorphous GeTe Thin Films
Cocina1, Antonio Massimiliano
D'Arrigo2, Maria Grazia
Grimaldi1 3, Emanuele
Physics Department, Università di Catania, Catania, Italy; 2,
, IMM-CNR, Catania, Italy; 3,
, MATIS-IMM-CNR, Catania, Italy.Show Abstract
Amorphous state in chalcogenides is influenced by the sample preparation and processing (e.g. sputtering, melt-quenching, ion implantation, priming, pre-annealing). In particular, amorphous samples, during annealing at temperature well below the crystallization and even at room temperature, exhibit structural relaxation and defect annealing.
The relaxation strongly influences the electrical conductivity and it has been recently shown that the mobility gap for conduction has a correlation with the incubation time for crystallization.
However, it is still not clear if these relaxation effects are determined by local bond rearrangements, defect annealing or local atomic mobility changes.
In order to better investigate the structural relaxation processes, we have studied the diffusion of Au in amorphous GeTe, comparing as deposited and pre-annealed samples (annealed for several hours at 120°C in Ar atmosfere). Such a pre-annealing treatment produces the amorphous relaxation and it has been recently reported to be very effective in reducing the nucleation rate, therefore increasing the crystallization temperature.
Amorphous GeTe 100 nm thick films were deposited at room temperature (RT) by DC sputtering on a SiO2/Si substrate. Then, Au film, 25 nm thick, was sputtered on both as deposited and pre-annealed GeTe layers. To induce the diffusion of gold in GeTe, annealings were performed at 110°C for 3 and 7 hours and at 115°C for 1 hour. X-ray Diffraction confirmed that all the thermal-treated GeTe films retain the amorphous phase. The samples have been analyzed with 2.0 MeV Rutherford backscattering spectrometry (RBS), by cross-sectional STEM HAADF micrographs and EDX microanalysis.
Gold is found to be a very fast diffuser in as deposited amorphous GeTe, even at room temperature, with diffusion coefficient extimated to be larger than 10-16 cm2s-1 at 25°C and 10-14 cm2s-1 at 110°C.
The situation is completely different in the pre-annealed GeTe film, where a concentration of 1-2% of Au into the film was observed after maintaining the samples for few months at room temperature (≈25°C), but no further Au migration was observed after subsequent annealing at 110°C and 115°C.
Such a result is a clear indication that the structural reordering induced by the pre-thermal treatment inhibits the Au transport in the layer. Quite interestingly, this behavior differs from that found in amorphous silicon, where thermal treatment enhances the subsequent Au diffusion. The difference should be associated probably to a different diffusion mechanism. Indeed, in amorphous silicon the defects act as traps for Au and therefore the defect reduction enhances the diffusion. In the case of GeTe it seems that Au diffusion is mediated by a large amount of void defects, available in the as deposited film. This diffusion is almost completely suppressed after relaxation, indicating that the defects mediating the Au transport have been eliminated.
8:00 PM - HH6.04
Dynamic Crystallization Model for Ge2Sb2Te5 Nanostructures
Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, USA.Show Abstract
We present a model for the crystallization of Ge2Sb2Te5 (GST) thin films and nanostructures by simulating nucleation and growth of crystal grains. Typically, dedicated crystallization models only examine a 2D rectangular area of material in which the material is being heated uniformly throughout the sample [1-3]. Our model is developed to simulate crystallization of GST in any structure in 2D or 3D under arbitrary heating conditions where a thermal gradient and/or a transient may be present. Simulations are performed in COMSOL Multiphysics with the integration of a MATLAB function to handle the logistics of nucleation and grain growth. This approach will allow for the modeling of dynamic crystallization during device operation, allowing for updates of the electrical and thermal conductivities of the material as it crystallizes.
This model calculates a spatial map of nucleation rate in the GST for each time-step in the simulation using temperature dependent nucleation rate . Crystal nuclei are generated via a probability function based on the nucleation rate map, and are then grown into crystal grains according to temperature dependent growth rate data obtained from the literature [1,3]. Nucleation is most significant at lower temperatures (~ 500 K) and diminishes at higher temperatures where growth is most significant (~ 870 K) for GST. This model can be used to predict the crystallization of GST structures during device operation, capturing the stochastic nature of nucleation and growth processes and distribution of the grain sizes.
 S. Senkader and C. D. Wright. "Models for phase-change of GeSbTe in optical and electrical memory devices". Journal of applied physics 95 (2004): 504.
 K. B. Blyuss, et al. "Master-equation approach to the study of phase-change processes in data storage media". Physical Review E 72.1 (2005): 011607.
 G. W. Burr, et al. "Observation and modeling of polycrystalline grain formation in Ge2Sb2Te5”. Journal of Applied Physics 111.10 (2012): 104308-104308.
8:00 PM - HH6.05
Morphological and Electrical Characterization of Non-Standard Composition Chalcogenide Alloys in the Amorphous to Crystal Transition
Institute for Microelectronics and Microsystems (IMM), CNR, Catania, Italy; 2,
Process R&D, Micron Semiconductor Italia s.r.l., Agrate Brianza, Italy.Show Abstract
The GST system has been of interest since its structural transformations are associated with memory switching properties. In phase change memories (PCM) and rewritable optical disks data are recorded by switching the material from the amorphous to the crystalline phase. The heating of the material via laser or electrical current pulses of proper intensity and duration induces the reversible phase transitions. This working principle causes electrical stress due to the high current density and thermal cycling and it produces variations in the cell electrical characteristics. The aging effects are observable in the stoichiometry changes  and in voids formation . The stoichiometry variations, produced by the high electric field, are localized near to the anode and cathode regions of the cell. In particular from recent studies, it was observed an increase of Germanium and Antinomy concentration in proximity of the cathode and a concentration increase of Tellurium in proximity to the anode areas. There is then a quite relevant interest in the investigation of new alloys with better characteristics. In this work we investigated the morphological and electrical characteristics of two chalcogenide compounds: Ge15Sb49Te37 [3 ] and Ge14Sb35Te51. Thin amorphous films, about 50 nm thick, were deposited at room temperature on thermally grown Silicon Nitride using a rf magnetron co-sputtering from elemental targets. The concentration of Ge, Sb and Te in the alloy was varied by changing the applied rf powers. The temperature resolved reflectivity curves recorded during the annealing of the two non stoichiometric alloy and a constant heating ramp of 6 °C/min was used. The enhancement of reflectivity observed during the annealing process is correlated to the transition from the amorphous to the crystalline phase. The crystallization temperature of the two non stoichiometric alloy Ge15Sb49Te37 was 190 °C, and Ge14Sb35Te51 was 165 °C. The crystallization of two alloys, Ge15Sb49Te37 and Ge14Sb35Te51, films has been studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD) and was correlated to the optical reflectivity and to electrical sheet resistance. Comparison of the XRD patterns for GST, Ge15Sb49Te37 and Ge14Sb35Te51 thin films revealed that the crystallization behavior of the non stoichiometric films were quite different from that of GST. In the case of the Ge15Sb49Te37 film, no metastable FCC phase was observed, there was a phase transition from the amorphous phase directly to the HCP phase. Instead the Ge14Sb35Te51 composition films show at 200° C the coexistence of FCC and HCP phase and at 400° C the presence of a X-peak compatible with a rhombohedral symmetry.
1 K. Kim and S. J. Ahn, IEEE Reliability Physics Symposium Proceedings, 2005, 157-162.
2 G. W. Burr et al. J. Vac. Sci. Technol. 28 (2), 223-262.
3 SANG-OUK RYU Journal of ELECTRONIC MATERIALS, Vol. 37, No. 4, 2008
8:00 PM - HH6.06
Crystallization Kinetics of Sb2S3 Thin Films
Krbal2, Xin Yu
Engineering Product Development, Singapore University of Technology and Design, Singapore, Singapore; 2,
Faculty of Chemical Technology, University of Pardubice, Pardubice, Czech Republic; 3,
Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore, Singapore.Show Abstract
Antimony Trisulfide (Sb2S3) has a band gap, which due to quantum confinement effects, is tunable from 1.5-2.2 eV. Consequently this material has been predominantly investigated for use in photovoltaics. Herein, the phase change properties of Sb2S3 films are investigated for new applications in data storage and nano photonic switches.
Sb2S3 films with thickness ranging from 20 nm to 1000 nm were fabricated by pulsed laser deposition and RF sputtering. The crystal nucleation and crystal growth phase transition kinetics in Sb2S3 thin films is studied as a function of film thickness, deposition parameters, interfacial materials and other external stimuli.
The presentation will describe a few experiments that demonstrate control the crystallization of Sb2S3 films using a variety of different external parameters.
8:00 PM - HH6.07
Impact of Heater Material on Thermoelectric Heating and Cooling in Phase Change Memory Cells
Electrical Engineering, University of Connecticut, Storrs, Connecticut, USA.Show Abstract
Phase change memory (PCM), having just recently been commercialized in the past few years , has been attracting considerable attention from the scientific community. Investigations on thermoelectric phenomena, both within the phase change layer and at the junctions with the electrical contacts, have shown that asymmetric PCM cells have one operation polarity that is more favorable than the other for heating the active region [2, 3]. In this work, we have used our established thermoelectric PCM model to evaluate the merit of n-type and p-type contacts with a large contrast of Seebeck coefficients. The analysis is performed on Ge2Sb2Te5 mushroom cells. The material properties of the bottom electrode determine the thermoelectric heat release at the junction as well as thermal losses. Hence, the heater material has a significant impact on the device operation dynamics and dependence on the voltage polarity.
 J. Rice, Micron Announces Availability of Phase Change Memory for Mobile Devices: First PCM Solution in the World in Volume Production. 2012. Available:http://investors.micron.com/releasedetail.cfm?ReleaseID=692563.
 A. Faraclas, G. Bakan, N. Williams, A. Gokirmak and H. Silva, "Modeling of Thermoelectric Effects in Phase Change Memory Cells," Transactions on Electron Devices, Submitted with Revisions, 2013.
 A. Faraclas, G. Bakan, N. Williams, A. Gokirmak and H. Silva, "Thermoelectric Effects in Phase-change Memory Cells," Mat. Res. Soc. Spring Meeting, vol. H1.03, 2013.
8:00 PM - HH6.08
Determination of Specific Contact Resistance of Ge2Sb2Te5 Phase Change Materials by Spacer Etched Nanowires
Wang1, C. H. "Kees"
School of Electronics and Computer Science, University of Southampton, Southampton, United Kingdom.Show Abstract
Phase change materials (PCM) based memory device is considered as one of the most promising candidates for next-generation non-volatile solid-state memory. The set and reset states in this device correspond to a low resistance and a high resistance of the cell, which in-turn correspond to the crystalline and amorphous states of the phase change material, respectively. The total resistance of a phase change memory cell, however, consists of the resistance from the PCM and the interfacial contact resistance of the PCM to the electrodes. Although a large amount research has been done on characterization of PCM resistance, little attention is paid to study the contact resistance. Here in this work, the contact resistance of Ge2Sb2Te5 to titanium nitride (TiN) electrode has been characterized in both set and reset states using a nanowire structure obtained from spacer etch. This spacer etch is a novel technique and can be used as a low-cost alternative to E-beam lithography for sub-hundred nanometre nanowire fabrication. Unlike bottom-up technology, it is compatible with current CMOS process and the geometry and location of the nanowires can be precisely controlled. In this case it allows us make long structures with small contact area to separate the resistive contribution of bulk and interface.
A high-insulating silicon dioxide (SiO2) layer was first patterned by photolithography and etched to form a step with a depth of 100 nanometers. A 100 nm layer of Ge2Sb2Te5 was deposited by sputtering and anisotropically etched using an ion beam, leaving a spacer of Ge2Sb2Te5 next to the oxide structure.
Three different lengths (20 μm, 25 μm and 30 μm) of Ge2Sb2Te5 nanowires with same cross-section area (50 nm × 100 nm) were fabricated by space etching process. TiN electrodes with a thickness of 200 nm were then patterned on both sides of the nanowire by lift-off. The electrical characterization reveals the resistivity of the as-deposited Ge2Sb2Te5 nanowire material to be 0.6 Ωm. The specific contact resistance between the TiN electrode and amorphous Ge2Sb2Te5 was extracted to be 3.59×10-6 Ωm^2. Then nanowires were then thermally switched to crystalline state with resistivity of 3.37×10^(-4) Ωm and specific contact resistance of 7.07×10-9 Ωm^2. Even for these very long wires, the Roff/Ron ratio of 1.78x10^3 is partially determined by the contact resistance. These results indicate that for real memory cell layout, the contact resistance is the dominant factor in Ge2Sb2Te5 phase change memory devices.
8:00 PM - HH6.09
Selective Deposition of Phase Change Materials by Chemical Vapor Deposition
Ried2, C. H. "Kees"
School of Electronics and Computer Science, University of Southampton, Southampton, United Kingdom; 2,
School of Chemistry, University of Southampton, Southampton, United Kingdom.Show Abstract
The ever increasing demand for a universal memory which combines rapid read and write speeds, high storage density and non-volatility is driving the development of new memory concepts and materials. Phase change materials based random access memory (Phase Change RAM) has emerged as a leading candidate for the next generation of non-volatile memory. However, the critical issue of thermal cross-talk between adjacent cells when scaling down the cell size is yet to be solved. Growth of the entire device inside a contact hole may be more favourable as it could reduce the thermal cross-talk and simultaneously reduce the power required for the switching operations. The conventional deposition of phase change materials by sputtering does not allow selective deposition and is unable to uniformly fill small holes. Hence, alternative deposition approaches need to be investigated.
Here we report the selective deposition of phase change materials using chemical vapor deposition (CVD) with new, custom-made single source precursors. CVD is well established as a deposition process with most processes using dual or multiple sources, Being able to deposit alloys using single source reagents can be advantageous as it can offer improved stoichiometry and fewer defects, as well as often being safer and easier to handle. More importantly, through these single source reagents area selective deposition behavior by CVD was discovered. This selectivity is observed when depositing materials onto patterned substrates which contain TiN “holes” in a SiO2 film. A set of binary phase change materials, SnSe2, Ga2Te3, Bi2Te3 and Sb2Te3 have all demonstrated highly selective deposition behavior on these substrates.
The characterization of phase change materials will be presented. All as-deposited materials are crystalline and of a high purity. The properties of all materials are studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Raman spectroscopy and Hall measurements. When depositing the materials onto patterned substrates, the deposition occurs preferentially onto the exposed TiN (conductive) area inside the holes while leaving the outside SiO2 (insulator) bare. This selective deposition behavior can be observed in micro-patterned (2 µm to 100 µm) or even nano-patterned (100 nm to 1000 nm) holes. The possible reasons for this selective behavior will be discussed.
8:00 PM - HH6.10
Thermal Boundary Resistance and Their Impact of Heat Transfer Simulation in Phase Change Memory
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan.Show Abstract
Thermal conductivities of chalcogenide thin films, Ge2Sb2Te5(GST) and Ce-doped GST (Ce-GST), were measured by the 3-omaga (3ω) method and their thermal boundary resistances (TBR) properties at the interface of chalcogenide and TiN contact layer were analyzed in terms of the plywood sample structures. Analytical results were consequently implanted in a finite-element simulation in order to analyze the thermal and electrical characteristics of phase-change memory (PCM) devices. The simulation based on a three-dimensional electric and thermal coupling model indicated that the TBR property significantly affects the temperature profile and the heating efficiency of PCM cells subjected to a pulse heating operation.
In PCM cell utilizing doped GST, i.e., Ce-GST, as the programming layer, a better thermal confinement effect was observed when the same amount of heat was generated during programming. This is ascribed to the increment of resistivity in Ce-GST which results in the decrease of programming current in PCM cells and thermal conductivity of Ce-GST layer. In the analysis regarding of TBR effect, dissipation of programming power caused by the inhibition of heat propagation from bottom electrode to programming layer of devices was observed. Simulation indicated the presence of TBRGST/TiN = 7.6×10−8 m2°K/W in PCM cell containing pristine GST leads to a 30% decrement of programming power whereas the presence of TBRCe-GST/TiN = 9.6×10−8 m2°K/W in PCM cell containing Ce-GST leads to a 27% power decrement in comparison with the cases without the TBR effect. The influences of device scale-down and TBR properties on the programming conditions of PCM cells were also examined. It found that the smaller TiN contact width benefits the increment of cell temperature. The TBR effects caused the 16% and 19% increments of set and reset currents in PCM containing GST whereas the increment of set and reset current are 19% and 18% for PCM containing Ce-GST.
8:00 PM - HH6.11
Resistive Switching and Polarity Reversal Phenomena in Ge2Sb2Te5-Based Conductive-Bridge Random Access Memory
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan.Show Abstract
Electrical properties of conductive-bridge random access memory (CBRAM) devices containing Ge2Sb2Te5 (GST) chalcogenide thin films as the solid-state electrolyte, silver (Ag) as the active electrode (AE) and tungsten-titanium as the counter electrode were investigated. The correlations of electrical properties with the microstructures and compositions of CBRAM samples analyzed by conductive-atomic force microscopy, Auger electron nanoscope and transmission electron microscopy are also presented.
Electrical measurement observed no resistive switching behaviors in the samples containing amorphous GST. This implies the crystallinity of GST is essential to the resistive switching behaviors of CBRAM samples and the grain boundaries in polycrystalline GST should play a key role in the evolution of conduction channels since they might serve as the fast transport paths for Ag elements to achieve the resistive switching behaviors. The sample with the best electrical performance (VSet = −0.14 V; VReset = 0.24 V; R-ratio ≈ 511) and satisfactory retention and cycleability properties was achieved by the insertion of ZnS-SiO2 dielectric layer at the Ag/GST interface and a post-annealing at 250°C for 30 min. In addition to the resistive memory characteristics of GST, electrical measurement clearly illustrated the advantage of low operational power of GST-CBRAM devices. Moreover, the polarity reversal phenomenon was observed in the sample containing the ZnS-SiO2 layer. Microstructure and composition analyses indicated that ZnS-SiO2 insertion likely reverses the role of electrodes in the sample when electrical bias is applied, consequently altering the Ag distribution in GST and the polarity of such a CBRAM device. Analytical results also revealed the gradient distribution of Ag in GST and, hence, the formation and rapture of Ag conductive channels in the vicinity of AE/GST interface are responsible to the resistive switching behaviors of CBRAM samples.
8:00 PM - HH6.12
Crystallization Kinetics of Ge2Sb2Te5 and GeTe Phase Change Materials as a Function of Deposition Temperature
Khoo1 2, Hai
I Made3, Nobumichi
Thompson2 3 6, Chee Lip
Gan1 2 3.
Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore; 2,
Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, Singapore, Singapore; 3,
Low Energy Electronic Systems, Singapore-MIT Alliance for Research and Technology, Singapore, Singapore; 4,
Advance Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA; 5,
, Singapore University of Technology and Design, Singapore, Singapore; 6,
Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.Show Abstract
The speed at which phase change memory devices can operate depends strongly on the crystallization kinetics of the amorphous phase. To better understand factors that affect the crystallization rate, we have carried out basic studies of crystallization of Ge2Sb2Te5 and GeTe films as a function of how they are made. We found that the crystallization kinetics varies strongly with the temperature of the substrate on which the film is deposited. We argue that this reveals characteristics of the atomic scale structure of the film, allowing correlation of the atomic scale structure with crystallization kinetics of films with the same composition.
1 µm-thick Ge2Sb2Te5 and GeTe films were deposited by sputter deposition on Si substrates with a thin layer of native SiO2. Different samples were deposited at room temperature, 60oC, 80oC and 100oC.
Crystallization of these films was monitored using x-ray synchrotron radiation (λ=0.124 nm) while samples were annealed at a constant rate of 2oC/min, 5oC/min or 10oC/min from room temperature to temperatures at which they were fully crystalline. The fraction of the film that had transformed was determined as a function of time for different heating rates to provide transformation curves. This allowed calculation of the effective activation energy for crystallization using Kissinger’s method. The apparent crystallization temperature was defined as the temperature at which half of the sample was crystallized. The microstructures of the samples were characterized using scanning electron microscopy, energy dispersive x-ray spectroscopy and transmission electron microscopy.
For Ge2Sb2Te5 samples, the apparent crystallization temperature and the effective activation energy for crystallization were found to decrease with increasing deposition temperatures. Microscopy revealed that as-deposited samples were composed of small crystallites (<10nm) embedded in an amorphous matrix for all deposition conditions, with the number of crystallites increasing with increasing deposition temperature. The shapes of the transformation curves suggest that crystallization occurred though growth of these pre-existing crystallites. The increased number of pre-existing crystallites in films deposited at high temperature causes the apparent crystallization temperature to decrease with increasing deposition temperature.
The apparent crystallization temperature and effective activation energy of GeTe films were also found to decrease with increasing deposition temperature. However, the GeTe samples were found to be amorphous in the as-deposited state. The transformation curves also indicate that nucleation was required for crystallization. We suggest that the decrease in the apparent crystallization temperature with increasing deposition temperature correlates with different degrees of ordering to form sub-critical nuclei in the as-deposited films. This is supported by analysis of the measured transformation curves.
8:00 PM - HH6.13
Evaluation of Se-Doped GeSb Phase Change Material for Multi-Level PRAM Device Cells
Materials Science and Engineering, Yonsei University, Seoul, Republic of Korea; 2,
Process Development Team, Semiconductor R&D Division, Samsung Electronics Co., Ltd., Hwasung, Republic of Korea.Show Abstract
PRAM devices have demonstrated great advantages for next generation memories owing to the fast speed, high retention and high resistance margins between amorphous and crystalline states. Recently, many researches have been carried out to realize PRAM as Flash memory due to its merits on low fabrication costs compared with other resistance based memory, and also its scalability below sub 20 nm design cells. In order to achieve high density of PRAMs for Flash memory applications, MLC (multi-level cells) characteristics should be realized.
In this study, we investigate the Se-doped GeSb as the phase change material for MLC since it shows larger ratio of Rreset/Rset than GeSbTe material. We observed the rapid decrease of sheet resistance from amorphous to crystalline and the higher sheet resistance ratio between amorphous and crystalline states than conventional GST. When the cell structures are fabricated with Se-doped GeSb films, shorter crystallization time and lower SET voltages are demonstrated compared with the one with conventional GST. Based on our results, we conclude that the Se-doped GeSb has the feasibilities as MLC phase change materials for next generation PRAM applications.
8:00 PM - HH6.14
Facile Electrochemical Synthesis of SbxTey Nanowires and Their Electrical Transport Properties
, University of California, Riverside, riverside, California, USA; 2,
Electrochemistry Department, Korea Institute of Materials Science, Changwon, 641-831, Republic of Korea; 3,
Fine Chemical Engineering/Bionano Technology, Hanyang University, Ansan, 426-791, Republic of Korea.Show Abstract
Phase change random access memory (PCRAM) is a great candidate for the next generation computing because of its great scalability, fast speed, and low power consumption. Especially, one-dimensional (1-D) chalcogenide nanostructures (e.g., GeTe, Sb2Te3, AgSbTe) are commercially utilized with the expectation of high performance in virtue of device architecture and excellent phase-transition properties of the materials. So far, extensive researches have fulfilled the field by enhancement of switching speed, endurance, and scalability with conventional techniques. However, none of studies systemically investigates effect of composition whereas the composition change during the switching can additionally affect the PCM performance in addition to amorphous-crystal structure transformation. Moreover successful applications of chalcogenides rely on synthesis techniques that can reproducibly fabricate hierarchical and dimensionally uniform nanostructures with controlled crystallinity and composition in a cost effective manner. Electrodeposition is a high-yield, cost-effective and versatile process that operates near room temperature, has low energy requirements, and is capable of handling complex geometries at a variety of length scales. Furthermore, it is able to synthesize solid solutions and non-stoichiometric alloys based on its non-equilibrium reaction that is difficult using other processes.
In the work, template-directed electrodeposition was utilized to synthesize antimony telluride (SbxTe1-x) nanostructures with well-controlled dimensions, morphology and composition. In order for study on crystal structure and phase transition of synthesized SbxTe1-x nanowires, XRD and TEM analysis were conducted under thermal treatment. In addition, electrical transport properties of individual SbxTey nanowires such as temperature-dependent resistivity, temperature coefficient of resistivity(TCR), activation energy(Ea) and FET mobiltiy were correlated with observed solid-state structural transformation to investigate effects of Sb content and phase.
8:00 PM - HH6.15
Density Functional Simulations of Phase Change Materials in Graphene Sandwich Structures
Department of Physics, Tampere University of Technology, Tampere, Finland; 2,
Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, USA; 3,
, Forschungszentrum Jülich, Jülich, Germany.Show Abstract
Optical recording discs are used extensively as rewritable nonvolatile high-density memory storage media (DVD-RAM, DVD-RW, Blu-ray Disc). The most widely used recording materials are the GeTe-Sb2Te3 (GST) alloys, particularly Ge2Sb2Te5. This material is used also for the phase change RAM (PC-RAM), which is a contender of Flash memory in computers and electronic devices.
In DVDs, laser light induces a transition between the amorphous and crystalline states by heating the recording layer, and the phase change in PC-RAMs is caused by heating that follows an applied voltage. The state is monitored by measuring changes in the reflectivity or resistivity. Low-power operation of phase change memories (PCM) has been difficult to achieve, but one possible solution in a PC-RAM cell is to lower the heat loss by increasing the thermal resistance between the GST layer and the heater electrode. Extremely small dimensions and low set and reset currents have been achieved recently by using carbon nanotubes (CNT) as memory electrodes. The use of CNTs or graphene as the interface material should then reduce the heat loss significantly.
We have studied thin films of GeTe and GST in graphene sandwich structures by simulating the amorphous and crystalline phases of PCM layers. The systems comprised an approximately 17 Å thick layer of GeTe, a 21 Å thick layer of GST, and a single sheet of graphene, where both PCM layers were in rocksalt lattice (111 face). Geometries and orientations were optimized with density functional (DF) (CP2K software) and molecular dynamics (MD) (CPMD software) methods. A partial charge analysis of the optimized structures shows no charge transfer between the PCM and graphene. In addition, the sandwich structure with amorphous GeTe shows graphene undulation, and the PCM experiences the graphene as a physical "wall".
Electronic structures and phonon dispersion curves of the systems were computed with DF method (Quantum Espresso software). The electronic band structure, density of states (DOS) and the partial DOS in crystalline GeTe showed that the Dirac Cone of the graphene is unaltered - no band gap or shifting in relation to the Fermi energy. This is further evidence that there is no charge transfer between the layers. We plan to compare the features of GeTe and GST and also the effect of the phase change.
 J. Akola, R. O. Jones, Phys. Rev. B 76, 235201 (2007)
 J. Liang, R.G.D. Jeyasingh, H.-Y. Chen, and H.-S.P. Wong, 2011 Symposium on VLSI Tech., 100 (2011).
8:00 PM - HH6.16
Ab-Initio Study of the Sub-Threshold Electron Transport Properties of Ultra-Scaled Amorphous Phase Change Material Germanium Telluride
, University of Washington, Seattle, Washington, USA.Show Abstract
The amorphous chalcogenide phase change materials (PCM) have a lot of interesting electron transport properties, e.g. the sub-threshold linear (exponential) current-voltage curve when the bias is low (large), the threshold switching, and the current-voltage curve snap-back. Interestingly, the stat-of-art PCM scaling research has found that these peculiar electron transport properties exist not only in the bulk PCM (tens of nm or larger) measurements, but also in the ultra-scaled PCM nanostructure (sub-10 nm) measurements . However, though the electron transport of the bulk PCM has been well studied [2-3], the understanding of the electron transport of ultra-scaled PCM nanostructures is still missing. It is important to study the electron transport properties of the ultra-scaled PCM nanostructures, which are crucial to enable ultra-dense PCM device technologies.
In this study [4-5], we investigate the sub-threshold electron transport properties of the prototypical PCM GeTe ultrathin film in the amorphous phase, by using the ab initio molecular dynamics, density functional theory, and non-equilibrium Green’s function simulations. Our purely ab initio simulations reproduce the linear (exponential) shape of the measured current-voltage curve for low (large) bias in the sub-threshold region. We find that, when the bias is small, the transmission coefficients are not significantly dependent on the bias. And, the linear shape of the current-voltage curve is a manifestation of the bias window enlarging. In contrast, when the bias is large, the transmission coefficients is significantly bias-dependent. The exponential shape of the current-voltage curve is jointly determined by the bias-induced change of the transmission coefficients and the bias window enlarging. Our simulation results also successfully reproduce other measured physical properties, like the bandgap, intra-bandgap donor-like and acceptor-like defect states, and the p-type conductivity of the amorphous GeTe. We hope our simulation results and conclusions will help better understand the electron transport behavior in ultra-scaled PCM nanostructures, and improve the design of ultra-scaled PCM devices.
. Kim, S, Bae, B.J., Zhang, Y., et al, IEEE Trans on Electr Dev, 58, 1483-1489, 2011.
. Ielmini, D, Phys. Rev. B, 78, 035308, 2008.
. Ielmini, D, and Zhang, Y, J. Appl. Phys., 102, 054517, 2007.
. Liu, J., Xu, X., and Anantram, M.P., “Ab Initio Simulation of Sub-threshold Electron Transport Properties of Amorphous Germanium Telluride Phase-Change Ultrathin Film”, (in preparation).
. Liu, J., and Anantram, MP, J. Appl. Phys., 113, 063711, 2013.
8:00 PM - HH6.17
Multi-Scale Analysis of the Crystallization of Amorphous Germanium Telluride
, University of Washington, Seattle, Washington, USA.Show Abstract
The state-of-art experiments have found that the crystallization of phase change material (PCM) can be accomplished within about 0.5 ns ; and the PCM crystallization capabilities can be kept in sub-2 nm PCM nanostructures . These unique crystallization properties (ultra-fast phase switching and superb scalability) of PCM are indispensable to enable ultra-fast and ultra-dense nonvolatile PCM memory devices. Also, they have spurred intensive research interests, to understand the governing physics of the PCM crystallization [1,3,4,5]. Though a lot of physical insights have been obtained in previous studies [1-5], the quantitative analysis of the various physical quantities (Gibbs free energy density, interfacial energy density, specific heat capacity, critical formation energy, critical nuclei radius, etc.) that determine the crystallization properties of PCM and the temperature dependence of these physical quantities are still missing.
In this study , these physical quantities and their temperature dependence are calculated and analyzed in a multi-scale way, using density functional theory, ab initio molecular dynamics, and classical thermodynamic theory. Here, we focus on prototypical PCM GeTe. We show that, as temperature rises, the Gibbs free energy density difference between amorphous phase and crystalline phase monotonously decreases; and the amorphous/crystalline interface energy density monotonously increases. We quantitatively unveil the importance of the elastic energy in PCM crystallization, which is largely ignored in the existing PCM crystallization studies. Our analysis reveals that the elastic energy plays an important role in determining various crystallization properties and the ultimate scaling limit of PCM. By omitting the elastic energy, the critical formation energy (critical nuclei radius) will be underestimated by 41.7% (22.4%) and the nucleation rate will be overestimated by 74.2% when the annealing temperature is 600 K. We find that the PCM crystallization properties and the PCM scalability are jointly determined by the Gibbs free energy density difference, interfacial energy density, and elastic energy density. Our results show that the critical nuclei radius of the crystalline cluster is smaller than 1.4 nm when the annealing temperature is lower than 600 K, indicating extremely promising scaling scenario of the PCM technology.
. Loke, D. et al, Science, 336, 1566-1569, 2012.
. Raoux, et al, J. Appl. Phys., 103, 114310, 2008.
. Hegedus, J. et. al, Nature Mater., 7, 399-405, 2008.
. Kalikka, J. et al, Phys. Rev. B, 86, 144113, 2012.
. Matsunaga, T, et al Nature Mater., 10, 129-134, 2011.
. Liu, J. and Xu, X. and Brush, L. and Anantram, M.P., “A Multi-scale Analysis of the Crystallization of Amorphous Germanium Telluride Using Ab-Initio Simulations and Classical Crystallization Theory”, J. Appl. Phys. (submitted).
8:00 PM - HH6.18
Dielectric Functions and Phonons of Ge1-xSex and Ge1-x-ySexAsy Chalcogenide Glasses
, Kyung Hee University, Yongin-si, Republic of Korea; 2,
, Ewha Womans University, Seoul, Republic of Korea; 3,
, Korea Institute of Science and Technology, Seoul, Republic of Korea.Show Abstract
Based on the unique threshold switching phenomenon of amorphous chalcogenide glasses , the Ovonic Threshold Switching (OTS) is being studied for the potential application in cell selector in memory devices. For the development of a reliable OTS device, the chalcogenide glass must have properties such as high crystallization temperature, low threshold voltage, and high on/off ratio, most of which are closely related to the nature of bonding between constituent atoms. In this regard, understanding the correlation between electronic structures and the bonding nature in chalcogenide glasses is very important in material design for implementing the high-performance OTS devices.
We investigated the electronic and vibrational structure of the Ge1-xSex (x≤0.67) and Ge1-x-ySexAsy amorphous thin films by using spectroscopic ellipsometry and Raman spectroscopy. Amorphous thin films of Ge1-xSex and Ge1-x-ySexAsy were prepared on Si substrate by cosputtering method using two or three targets of Ge, Ge0.4Se 0.6, and Ge0.4As 0.6 at room temperature.[2,3] Using spectroscopic ellipsometry, we measured the ellipsometric angles at room temperature in the spectral range of 0.8 - 6 eV, from which we determined the dielectric functions of the amorphous thin films. The Raman data were obtained using defocused 488 nm with 15 mW laser power to prevent photo-induced crystallization. Resonance Raman phenomenon at 514 nm (laser power 0.2 mW) was compared to other laser excitation wavelengths. We determined the optical gap energies and Urbach energies from the absorption coefficients, and found a strong positive bowing effect for the optical gap energy for Ge1-x-ySexAsy where the endpoint binaries were Ge0.50Se0.50 and Ge0.31As0.69. We attributed the strong bowing to the mixing of lone pair electron states in valence bands associated with As and Se atoms. We found that As-As mode in Ge0.31As0.69 was transformed to As(Se1/2)3 mode in Ge1-x-ySexAsy, and finally Se cluster mode in Ge0.5Se0.5 as Se content increased. Particularly, we observed resonant Raman phenomenon from Ge0.38Se0.62 at the laser excitation of 514 nm (2.41 eV). We verified that this laser energy corresponded to the transition energy of Ge0.38Se0.62 using the second derivative of the dielectric function of Ge0.38Se0.62. The Raman peak 169 cm-1 (ethane-like mode of Ge2(Se1/2)6) and 283 cm-1 (stretching modes of Se chains and rings) at off-resonance shifted to 172 cm-1 and 292 cm-1 at resonance, respectively, whereas 200 cm-1 peak (corner-sharing mode of Ge(Se1/2)4) did not shift. The finding of resonant Raman scattering in amorphous films suggests that the electronic interband transitions associated with short range order can cause the resonant Raman phenomenon.
 S. R. Ovshinsky, Phys. Rev. Lett. 21, 1450 (1968).
 H.-W. Ahn et al., Appl. Phys. Lett. 103, 042908 (2013).
 S.-D. Kim et al., ECS Solid State Letters 2, Q75 (2013).
8:00 PM - HH6.19
Effects of Capping Layer on the Structural Volume Change of Ge2Sb2Te5
Liu1, I Made
Riko1, Chee Ying
Khoo1, Chee Lip
Gan1, Leong Kam
MSE, NTU, Singapore, Singapore; 2,
, GLOBALFOUNDRIES Singapore Pte Ltd, Singapore, Singapore.Show Abstract
Phase change materials such as Ge2Sb2Te5 are characterized by large volume change upon crystallization, which could negatively affect the performance and reliability of phase change memory devices. Moreover, the phase change memory cell is confined between metals and dielectric layers, making crystallization-induced structural change more severe. Hence, it is essential to understand the effects of interfacial layers on the structural volume change of the film. In this study, structural change as a result of crystallization of amorphous Ge2Sb2Te5 films by thermal annealing was characterized by atomic force microscopy (AFM). A cross sectional area analysis method is used for evaluation of the structural volume change. Ge2Sb2Te5 film with and without dielectric cap layer were considered. Ge2Sb2Te5 film was sputtered deposited on a four-point test structure for evaluation of phase change through electrical measurement. Reduction in cross sectional area at 130oC and 200oC for SiO2 capped and non-cap films were observed. This reduction corresponds to the phase transitions from amorphous to FCC phase structure and FCC phase to HCP-phase, respectively, as revealed by the X-ray diffraction (XRD). Electrical resistance measurements were also correlated to the phase transitions. The thickness and cross sectional area reduction for SiO2 capped film for phase transition from amorphous to FCC phase structure and FCC phase to HCP phase is relatively higher. This is believed to be associated with impact of the SiO2 film on the crystallization temperature and also possible due to the large thermal expansion coefficient (TEC) difference between SiO2 and Ge2Sb2Te5 where SiO2 has the lowest thermal expansion coefficient (TEC), which is about 1/20 of the Ge2Sb2Te5. AFM allows precise and non-destructive determination of the phase transformation via observing the physical change of the film not only in one dimensional but rather in three dimensions. The present setup allows the understanding of the effect of dielectric cap layer on the volume change of the phase change material by the cross sectional area analysis method. Moreover, the results show the importance of the dielectric layer for the design of phase-change memory devices.
8:00 PM - HH6.20
Development of an Efficient Scheme for Generating Amorphous Structures
Materials science and engineering, Seoul National University, Seoul, Republic of Korea.Show Abstract
Recently, amorphous material has been receiving much attention both theoretically and practically because of varied material properties that are particular distinguishing features unlike crystalline materials. In order to generate the amorphous structure theoretically, various methods have been used; melt-quench method based on molecular dynamics (MD) is the most favored method since it resembles experimental melt-quench procedure. Because the interatomic force fields are not known in many cases, the ab initio approach based on the density functional theory (DFT) is a popular method of choice. However, the computational cost of DFT-MD is too expensive and typical supercells comprise only about 100-200 atoms. For more realistic modeling of amorphous structures, it is critical to simulate on the large cell.
In this work, we propose a new method to generate the amorphous structure using the local geometry obtained from DFT-MD. Although amorphous structures do not have long-range order, they have short-range order at the atomic length scale due to the chemical bonding. By considering the local order such as coordination number, bond length, and the type of the bonded atom, we were able to generate a reasonable amorphous structure with much smaller computational cost than conventional melt-quench method. To improve the medium-range order, an annealing simulation is carried out below the melting point. We compare the final structures obtained by this procedure with melt-quenched structures for various types of materials, and the agreements are reasonably good.