Symposium Organizers
Stefania Privitera, Consiglio Nazionale delle Ricerche (CNR)
Harish Bhaskaran, University of Oxford
Eric Pop, Stanford University
Yuta Saito, National Institute of Advanced Industrial Science and Technology (AIST)
ED11.1: Structure and Stability
Session Chairs
Raffaella Calarco
Wei Zhang
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 131 C
11:30 AM - *ED11.1.01
Theoretical Prospects for Two-Dimensional Phase Change Materials
Evan Reed 1 , Yao Li 1 , Karel-Alexander Duerloo 1 , Yao Zhou 1
1 , Stanford University, Stanford, California, United States
Show AbstractSingle-layers of some transition metal dichalcogenide compounds have the potential to exist in more than one crystal structure. I will discuss our theoretical work combined with DFT-based calculations to identify several single layer materials and their alloys including MoxW1-xTe2 that have potential to exhibit structural phase changes under stress and strain states, temperature changes, and electrical conditions. Our calculations indicate that phase boundaries of MoxW1-xTe2 can be tuned from hundreds of degrees C to room temperature and below by adjusting the composition. I will discuss the conditions under which mixed phases are expected to be thermodynamically stable. Some monolayer phase changes exhibit large electronic contrast, bringing an exciting new application space to monolayer materials ranging from information storage to electronic and optical devices.
12:00 PM - ED11.1.02
The Structural Origins of the Non-Resonantly Bonded Phase-Change Material Ga2Te3
Paul Fons 1 2 , Alexander Kolobov 1 2 , Milos Krbal 3 , Kirill Mitrofanov 1 , Junji Tominaga 1 , Tomoya Uruga 2
1 , National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan, 2 , Japan Synchrotron Radiation Institute SPring-8, Mikazuki, Hyogo, Japan, 3 Faculty of Chemical Technology, University of Pardubice, Pardubice Czech Republic
Show AbstractGa2Te3 is a a phase change material that exhibits many favorable attributes. Similar to the prototypical alloy Ge2Sb2Te5, the resistivity difference between amorphous and crystalline states is over 103 Ω-cm, there is a large optical contrast between the two phases, the amorphous phase exhibits a larger band gap than the crystalline phase, and the crystalline phase has very low thermal conductivity. The crystallization temperature, however, is significantly larger than that for Ge2Sb2Te5 at 450°C leading to decade long retention times for temperatures over 100°C. In a break from the existing paradigm, unlike Ge2Sb2Te5, the crystalline phase of Ga2Te3 is characterized by tetrahedral bonding with a lack of resonant bonding. Based upon x-ray absorption measurements and ab-initio simulations, we propose a structural model differing from the conventional resonant bonding paradigm in which the differences in bonding between the two phases arise from a large concentration of Ga vacancies and the presence of both primary and secondary bonding with a key factor behind the structural stability of the two phases being attributable to the polyvalency of Te atoms due to the presence of lone-pair electrons. In the crystalline phase Te atoms are shown to be sp3 hybridized with a coordination number three. In addition to the primary bonds, the presence of secondary Te-Te bonds is demonstrated to lead to tetrahedral coordination about Ga vacancies. The existence of primary and secondary bonding in the crystalline phase is supported by observation of local structure using x-ray absorption spectroscopy. In contrast in the amorphous phase, Te atoms are not sp3 hybridized and assume mixed coordination in the absence of secondary bonding with the presence of “wrong” bonds acting to further stabilize the phase. Details of the transformation mechanism will be discussed.
12:15 PM - ED11.1.03
Glass Transitions, Semiconductor-Metal (SC-M) Transitions and Fragilities in Ge-V-Te (V=As, or Sb) Liquid Alloy—The Difference One Element Can Make
Shuai Wei 1 , Garrrett Coleman 2 , Pierre Lucas 2 , C. Austen Angell 1
1 , Arizona State University, Tempe, Arizona, United States, 2 , University of Arizona, Tucson, Arizona, United States
Show AbstractGlass transition temperatures (Tg) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of more covalent system. Comparison of Tg vs bond density in the three systems Ge-As-chalcogen differing in chalcogen i.e. S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e. in the order STg becomes systematically weaker, with a crossover at = 2.26. When the more metallic Sb replaces As at greater than 2.26, incipient metallicity rather than directional bond covalency apparently gains control of the physics. This leads us to an examination of the electronic conductivity and semiconductor-to-metal (SC-M) transitions, with their associated thermodynamic manifestations, in relevant liquid alloys. The thermodynamic components, as seen previously, control liquid fragility and cause fragile-to-strong transitions during cooling. We tentatively conclude that liquid state behavior in phase-change materials (PCMs) is controlled by liquid-liquid (SC-M) transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation to GeTe stoichiometry indicates that the SC-M transition lies about 20% below the melting point, suggesting a parallel with the intensely researched "hidden liquid-liquid (LL) transition", in supercooled water. In the water case, superfast crystallization initiates in the high fragility domain some 4% above the TLL which is located at ~15% below the (ambient pressure) melting point.
12:30 PM - ED11.1.04
Copper-Doped Chalcogen Glasses for Access Devices Applications—A First-Principles Study
David Guzman 1 , Alejandro Strachan 1
1 , Purdue University, West Lafayette, Indiana, United States
Show AbstractUsing ab initio molecular dynamics based on density functional theory, we characterized the structural, electronic, and diffusive properties of copper-doped germanium-based chalcogenide glasses. These mixed ionic-electronic conduction exhibit threshold switching and are suitable for 3D cross-point memory array access devices. The amorphous network is characterized using a maximally localized Wannier functions charge center correction to the distance cut-off based bonding analysis. We find that the optical band gap increases going from tellurium to sulfur and quantify how the addition of Cu induces the localized states in the gap. Based on the network analysis we find specific atomic arrangements to have the most detrimental effects on gap states. From an application point of view, we find a tradeoff is necessary between increasing band gaps (desired for voltage margin) and ionic mobility (desired for fast switching).
ED11.2: Correlation between Structure and Properties
Session Chairs
Raffaella Calarco
Stefania Privitera
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 131 C
2:30 PM - *ED11.2.01
Structural and Electronic Properties of Ultrathin Films of Phase-Change Materials
Ider Ronneberger 1 2 , Riccardo Mazzarello 1 2 , Wei Zhang 3 , Matthias Wuttig 2 4
1 Institute for Theoretical Solid State Physics, RWTH Aachen University, Aachen Germany, 2 JARA-FIT and JARA-HPC, RWTH Aachen University, Aachen Germany, 3 Center for Advancing Materials Performance from the Nanoscale, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an China, 4 I. Physikalisches Institut, RWTH Aachen University, Aachen Germany
Show Abstract
Chalcogenide phase change materials show an unconventionally large electrical and optical contrast between the crystalline and the amorphous phase, which is exploited in optical data storage and electronic memory devices [1]. The optical contrast stems from the resonance bonding displayed by the crystalline bulk phase, as opposed to standard covalent bonding found in the amorphous phase [2].
The continuous miniaturization of phase-change memory devices calls for an understanding of the properties of these materials in the quasi two-dimensional limit. The confinement in one dimension should lead to considerable changes in the bond lengths and other physical properties, including the electronic properties. Indeed, experimental evidence for these changes has been recently found in epitaxially grown ultrathin GeTe layers [3].
In the present study, we investigate thin layers of selected chalcogenide phase-change materials with varying thickness by first principles calculations based on density functional theory. We compare the corresponding structural changes, as well as the changes in the electronic properties, as a function of film thickness. Our study provides a bridge between the bulk phase of these materials and the two-dimensional limit, which has recently been explored theoretically [4,5].
3:00 PM - ED11.2.02
Metal-Insulator Transition and Carrier Dynamics in GeSbTe Phase Change Materials Investigated via Electrical Transport, Terahertz and Raman Spectroscopy
Valeria Bragaglia 1 , Fabrizio Arciprete 1 2 , Karsten Holldack 3 , Alexander Schnegg 4 , Raffaella Calarco 1
1 , Paul Drude Institute, Berlin Germany, 2 , Università di Roma “Tor Vergata”, Rome Italy, 3 , Helmholtz Zentrum Berlin fur Materialien und Energie GmbH, Institute for Methods and Instrumentation in Synchrotron Radiation Research, Berlin Germany, 4 , Helmholtz Zentrum Berlin fur Materialien und Energie GmbH, Institute for Nanospectroscopy, Berlin Germany
Show AbstractIn addition to the amorphous-to-crystalline transition in GeTe-Sb2Te3 alloys (GST) which is widely used for data storage applications, experimental results on polycrystalline alloys evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we make use of fundamental advance in the fabrication by molecular beam epitaxy of ordered alloys. We assess the degree of ordering by x-ray diffraction and explicitly correlate it with the MIT by means of electrical transport [1]. We further tune the ordering in a controlled fashion attaining a large range of resistivity. A combination of Terahertz and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered epitaxial alloys [2]. By studying the crystallization process upon annealing, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a MIT is clearly identified to occur at the onset of the transition between the disordered to the ordered cubic phase.
THz spectroscopy is also employed to study dynamic response in a- and c- GST upon laser excitation, in order to investigate carrier dynamics on very short time scales (fs to ps). As opposed to a- and polycrystalline-GST which display only a short recovery time of few ps after carriers excitation, ordered c-GST response displays a short recovery time of 0.85 ps followed by a long one of 90 ps, suggesting that vacancy layers in an ordered c-GST might play a role as dissipation channel for photo-induced free carriers [3]. Eventually, the pump-probe measurements on c-GST and GeTe/Sb2Te3 based superlattices are compared, suggesting that different physical mechanisms drive the ultrafast dynamics in the two material systems.
[1] V. Bragaglia et al. Scientific Reports 6, 23843 (2016) doi:10.1038/srep23843
[2] V. Bragaglia et al. Scientific Reports 6, 28560 (2016) doi:10.1038/srep28560
[3] V. Bragaglia et al. Appl. Phys. Lett. 109, 141903 (2016) doi:10.1063/1.4963889
3:15 PM - *ED11.2.03
Nanoscale Characterization of Crystalline Phase-Change Materials for Novel Applications
Antonio Mio 1 , Stefania Privitera 2 , Oana Cojocaru-Miredin 1 , Min Zhu 1 , Valeria Bragaglia 3 , Fabrizio Arciprete 3 4 , Raffaella Calarco 3 , Emanuele Rimini 2 , Matthias Wuttig 1
1 , RWTH Aachen University, Aachen Germany, 2 , IMM-CNR, Catania Italy, 3 , Paul-Drude-Institut für Festkörperelektronik, Berlin Germany, 4 , Dipartimento di Fisica, Università di Roma “Tor Vergata”, Roma Italy
Show AbstractPhase-Change Materials (PCMs) are used since the 80s, for high-density data storage in optical media [1]. Lately, the use of these alloys has been extended to solid-state non-volatile memories. Very recently it has been shown that multi-layered crystalline phase change memories, arranged in superlattice, can exhibit improved functional properties [2]. Moreover, in the last years, it has been argued that many PCMs, which intrinsically possess low thermal conductivity, also exhibit promising thermoelectric figure of merit, providing possible applications for heat energy harvesting.
The first uses of PCMs in optical media rely on very fast (<100ns) phase transitions and on the pronounced optical contrast between the amorphous and crystalline state. The origin of this large optical contrast is due to the unique chemical bonding in the crystalline phase, named resonant bonding [3], which could be correlated with the low thermal conductivity [4].
Additionally, the development of superlattices in PCMs, multi-level memories and thermoelectrics requires a precise control of the nanoscale inhomogeneities (i.e. phase separations, defects, grain boundaries, precipitates) that must be engineered or avoided.
With the aim of a better insight of the role of interfaces and chemical bonding in PCM for these new applications, we show a nanoscale study performed correlating different techniques, such as High Angular Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) and Atom-Probe Tomography (APT).
A part of this study is devoted to investigate the transition between rock-salt and trigonal phases in Ge2Sb2Te5, fabricated on SiO2 or Si (111). HAADF STEM directly relates the micrograph contrast to the atomic number (Z-contrast), permitting a straightforward interpretation of the images. Therefore, high-intensity Te planes and lower intensity Ge/Sb layers can be directly identified. Van der Waals gaps formation and differences in cation-anion plane distance, related to the bonding between these two layers, can be observed in several transition steps. This allows a straightforward inspection of the vacancy ordering dynamics, related to the conversion into the stable trigonal phase, for different morphology of the initial rock-salt phase. This structural information is correlated with Raman spectroscopy response and electrical measurements. As a result, according to the substrate and to the crystallization path, we distinguish upon resistance values characterized by different degree of structural order, necessary for the development of multi-level phase change data storage.
1. M. Wuttig, N. Yamada, Nat. Mater. 6 , 824 (2007).
2. R. E. Simpson R E, et al. Nat. Nano. 6, 501 (2011).
3. K. Shportko et al., Nat. Mater. 7, 653 (2008).
4. S. Lee et al., Nat. Commun. 5, 3525 (2014).
3:45 PM - ED11.2.04
Atomic Defects in Hexagonal GeSbTe Compound
Wei Zhang 1 , Jiangjing Wang 1 , Ider Ronneberger 2 , Riccardo Mazzarello 2
1 , Xi'an Jiaotong University, Xi'an China, 2 , RWTH Aachen, Aachen Germany
Show AbstractGeSbTe (GST) compounds are phase change materials with important applications in electronic memory devices due to the pronounced contrast of electrical resistance between amorphous and crystalline states [1, 2]. Typically, a metastable rocksalt phase is observed after fast crystallization of amorphous GST, and upon further thermal annealing, the metastable rocksalt phase transforms into stable hexagonal phase. Recently, the stable phase becomes important for data storage as it extends the resistance window by another three orders of magnitude enabling a new way of multiple-level data storage [3, 4]. This work rely on key assumptions about structural properties of hexagonal GST. Here, we report thorough investigations of the structural properties of the layered GST compound at atomic scale by means of Scanning Transmission Electron Microscopy (STEM). We identify several common defects, namely, chemical disorder, stack twins and quintuple-layer, nonuple-layer stacking faults, and an unusual defect, we also study their effects on the energetics and electronic properties through Density Functional Theory (DFT) simulations. These findings do not only provide a structural understanding of GST compound, but also have implications for the development of novel multi-level data storage with larger resistance windows.
[1] M. Wuttig, N. Yamada Nat Mater 6, 824 (2007).
[2] W. Zhang, V. L. Deringer, R. Dronskowski, R. Mazzarello, E. Ma, M. Wuttig, MRS Bulletin 40, 856 (2015).
[3] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater. 10, 202 (2011).
[4] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J. Y. Raty, M. Wuttig, S. Blügel, R. Mazzarello, Nat. Mater. 11, 952 (2012).
ED11.3: Memories and Thermal Effects
Session Chairs
Daniele Ielmini
Veronique Sousa
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 131 C
4:30 PM - *ED11.3.01
Thermoelectric Effects in Confined Phase Change Memory Devices
Nicola Ciocchini 1 , Mario Laudato 2 , Paolo Fantini 3 , Andrea Lacaita 2 , Daniele Ielmini 2
1 , Intel Corp, Santa Clara, California, United States, 2 , Politecnico di Milano, Milan Italy, 3 , Micron Technology, Vimercate Italy
Show AbstractIn Phase Change Memory (PCM), the memory cell is subject to strong thermal and electrical stress during programming, with temperature gradient in the order of 50 K/nm and current density in the order of 10 MA/cm2. Due to these extreme conditions, thermoelectric effects play an important role in PCM operation. A deep understanding of thermoelectric effects may allow a physics-based design of the cell structure and materials to optimize programming speed/energy and reliability. The objective of this work is to study the polarity-dependence of PCM characteristics, including crystallization, melting, electrical switching/holding, and ion migration. Electrical characterization was performed on mushroom devices with confined bottom electrode and Ge2Sb2Te5 (GST) as active layer. The PCM characteristics show slower kinetics at negative voltage, which we attribute to thermoelectric effects of electrically-induced heating. We show a universal correlation of positive/negative kinetics, which we reproduce by modelling Thomson and Peltier heating in the PCM device.
5:00 PM - ED11.3.02
Electrothermal-Dynamic Granular Materials Modeling of Phase Change Memory and Logic Devices
Ali Gokirmak 1 , Zachary Woods 1 , Nadim Kan'an 1 , Adam Cywar 1 , Jake Scoggin 1 , Helena Silva 1
1 , University of Connecticut, Storrs Mansfield, Connecticut, United States
Show AbstractFinite-element modeling of phase change devices require integration of electrothermal models with dynamic materials models that can capture phase changes during set and reset operations. The materials models can use (i) an effective media approximation, which uses temperature dependent nucleation and crystallization rates assuming uniform nucleation probabilities, or (ii) a granular model that captures the stochastic nature of nucleation process and keeps track of grain orientations, and capture formation of grain boundaries. The latter approach allows introduction of grain boundary physics and gives more insight to device-to-device and cycle-to-cycle variations.
We have implemented both of those models and integrated them with SPICE models in COMSOL Multi-Physics. We are able to simulate various waveforms that lead to set and reset operations, cycle the devices in a single continuous simulation and model thermal cross-talk between different terminals and devices. We use the same granular models to simulate formation of the initial nano-crystalline structure during processing or laser crystallization.
We have analyzed two terminal memory elements as well as multi contact phase change logic devices that significantly reduce the CMOS footprint for memory access circuitry using these models.
The models, integration of the phase change memory and logic elements with CMOS for increased functionality, and the insights we gain from these studies will be presented.
5:15 PM - ED11.3.03
Thermal Modelling of Phase Change Memory Cells for Extraction of the Cell Temperature during Crystallization
Faruk Dirisaglik 1 , Gokhan Bakan 2 , Burak Gerislioglu 2 , Zoila Jurado 3 , Lindsay Sullivan 3 , Aykutlu Dana 2 , Chung Lam 4 , Ali Gokirmak 3 , Helena Silva 3
1 , Eskisehir Osmangazi University, Eskisehir Turkey, 2 , Bilkent University, Ankara Turkey, 3 ECE, University of Connecticut, Storrs, Connecticut, United States, 4 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractPhase change memory (PCM), an emerging memory technology in the market, offering a high-speed, scalability, and non-volatility. Reversible and rapid switching is achieved by melting and quenching (reset) in the orders of ns, resulting in the amorphous state (highly resistive), and by annealing or by growth-from-melt (set) to reach the crystalline state (higly conductive) which requires much longer time and depends on the cell temperature reached during the pulse. Hence, the temperature-dependent crystallization process of the phase-change materials at the device level has to be well characterized to achieve fast PCM operations. One of the main challenges is determining the cell temperature during crystallization. Here, we demonstrate extraction of the temperature distribution on lateral PCM cells using measured current-voltage (I-V) characteristics and thermal modeling during set operation. The extracted cell temperatures are in the range of 600-700 K that are in agreement with the measured set (low-resistance state) resistances, being lower for higher cell temperature. The effect of the thermal properties of materials on the extracted cell temperature is also studied. The demonstrated study provides promising results for characterization of the temperature-dependent crystallization process within a cell1,2.
1. G. Bakan, B. Gerislioglu, F. Dirisaglik, Z. Jurado, L. Sullivan, A. Dana, C. H. Lam, A. Gokirmak, and H. Silva, J. Appl. Phys., (in press).
2. F. Dirisaglik, G. Bakan, Z. Jurado, S. Muneer, M. Akbulut, J. Rarey, L. Sullivan, M. Wennberg, A. King, L. Zhang, R. Nowak, C. Lam, H. Silva and A. Gokirmak, Nanoscale, 2015, 7.40: 16625-16630.
5:30 PM - ED11.3.04
Atomic Diffusion in Laser Irradiated Ge Rich GeSbTe Thin Films
Stefania Privitera 1 , Veronique Sousa 2 , Corrado Bongiorno 1 , Gabriele Navarro 2 , Chiara Sabbione 2 , Egidio Carria 3
1 IMM, Consiglio Nazionale delle Ricerche, Catania Italy, 2 Leti, CEA, Grenoble France, 3 , STMicroeletronics, Catania Italy
Show AbstractPhase change memories based on GeSbTe alloys are usually not suitable for embedded memory applications, due to the low crystallization temperature. It has been shown that the high temperature stability can be improved by properly doping the GeSbTe material. During device operation, however, the original composition, optimized to ensure a good stability, may change, due to the atomic migration induced by the high temperature and/or by the electric field. These variations may lead to programming and retention performance degradation or even to device failure.
It is therefore important to understand the extent of the atomic migration and its impact on the stoichiometry, as well as on the crystallization properties. In order to decouple the effect of the high temperature from that of the electric field, in this paper we have irradiated Ge rich GeSbTe thin films by laser. In this way we are able to separate the thermal diffusion of the atoms from the field induced electromigration.
The Ge rich GeSbTe films were annealed at 400°C after deposition, in order to obtain the crystalline structure. The laser irradiation has been performed by using a 600 ns pulsed laser operating at different energy densities, employed to obtain the melting and quenching of the material.
By using Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) we have evaluated the atomic elements distribution in the amorphous phase for different irradiation energy values. Depending on the irradiation energy density, complete or partial amorphization of the film can be obtained, indicating high sensitivity to the temperature profile.
Using reflectance measurements and physical-chemical analyses we have also studied the subsequent conversion into the crystalline phase, determining the relationship between the irradiation energy and the crystallization properties.
ED11.4: Poster Session
Session Chairs
Harish Bhaskaran
Stefania Privitera
Wednesday AM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - ED11.4.01
Seebeck Coefficient, Electrical Resistivity and Derived Thermal Conductivity of Ge2Sb2Te5 Thin Films
Lhacene Adnane 1 , Faruk Dirisaglik 4 , Adam Cywar 1 , Kadir Cil 3 , Chung Lam 2 , Helena Silva 1 , Ali Gokirmak 1
1 , University of Connecticut, Storrs, Connecticut, United States, 4 Department of Electrical and Electronics Engineering, Eskisehir Osmangazi University, Eskisehir Turkey, 3 Department of Electrical and Electronics Engineering, Istanbul University, Istanbul Turkey, 2 , IBM, Yorktown Heights, New York, United States
Show AbstractGe2Sb2Te5 (GST) has been the most studied material so far for phase-change memory applications. However, temperature dependent thermoelectrical characterization of the material, critical to understand device behavior and devise improved designs, is still limited. We will present measurement results for various thickness GST films obtained using a high-temperature thin-film electrical characterization setup we developed, that allows simultaneous measurement of Seebeck coefficient (S-T) and electrical resistivity (ρ-T) up to 650 °C [1]. Repeated annealing and cool-down cycles to and from increasingly higher temperatures allow for characterization of each mixed phase state and show a consistent Seebeck and electrical resistivity behavior as the material crystallizes. S-T characteristics show a metallic behavior with S increasing linearly with temperature. The thermal conductivity of the material can be derived from the S-T characteristics using a phase separation model for composite alloys [2]. The correlations between these three parameters for Ge2Sb2Te5 up to close to the melting temperature will be presented and can give insights into the transport mechanisms in GeSbTe compounds.
References:
[1] L. Adnane, N. Williams, H. Silva, and A. Gokirmak. "High temperature setup for measurements of Seebeck coefficient and electrical resistivity of thin films using inductive heating." Review of Scientific Instruments 86, no. 10 (2015): 105119.
[2] Sonntag, Joachim. "Thermoelectric power in alloys with phase separation (composites)." Journal of Physics: Condensed Matter 21, no. 17 (2009): 175703.
9:00 PM - ED11.4.02
Insights for Crystalline Phase-Change Materials from Computational Studies of Colloidal Crystals
Chrisy Xiyu Du 1 , Greg van Anders 1 , Richmond Newman 1 , Sharon Glotzer 1
1 , University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPhase-change materials that rely on switching between crystalline phases show promise for functional nanophotonic and plasmonic applications. Taking full advantage of this promise requires developing a deeper understanding of the material physics that underlines crystal—crystal transitions. We develop minimal models of crystal—crystal transitions in systems of anisotropic colloids that permit the direct manipulation of local bond order to induce structural transitions. Our model systems are amenable to Monte Carlo and rare event sampling investigations that provide detailed information about the thermodynamics of the transitions. Our results show that, for colloids, a small change in particle shape can induce a crystal—crystal transition on timescale comparable to liquid—solid transitions. These results provide direct guidance for creating colloidal-scale phase-change materials, and our approach, where local bond order can be controlled directly, has the potential to offer new insights for atomic and molecular phase change materials.
9:00 PM - ED11.4.03
Computational Analysis of High Carrier Generation and Its Impact on the Melting and Thermoelectric Effects in Semiconductor Devices
Sadid Muneer 1 , Helena Silva 1 , Ali Gokirmak 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractUnder extreme thermal gradients (~> 1 K/m) significant asymmetric melting of current-carrying highly-doped silicon micro-wires was observed [1]. This bizarre phenomenon was explained by the generation-transport-recombination (GTR) process of the minority carriers in semiconductors at elevated temperatures. The finite element simulation model, using effective media approximation, also supported the claim; however the asymmetry observed in the experiments is significantly larger compared to computational results [1]. As the conventional electrothermal semiconductor theory fails to account for the exponential growth of carrier generation near melting, the carrier concentration, as well as the shift of the hottest spot because of the thermoelectric Thomson effect, was underestimated.
In this work we specifically solve continuity equations for electrons and holes to pinpoint their effects on the GTR phenomenon. Additional generation-recombination mechanisms included in the model sharply increases the carrier concentration at melting. This approach of modeling is a convenient and computationally less demanding way to simulate non-thermal melting of semiconductors, where the material melts below steady state bulk melting temperature [2]. The technique is particularly applicable to modeling phase change memory, which is an emerging non-volatile memory technology where electrical pulses are used to transition between high and low resistance states and where the thermoelectric effect is significant due to extreme thermal gradients at high operating temperatures [3].
Reference
[1] G. Bakan, N. Khan, H. Silva, and A. Gokirmak, “High-temperature thermoelectric transport at small scales: Thermal generation, transport and recombination of minority carriers,” Sci. Rep., vol. 3, no. 5, p. 2724, 2013.
[2] S. K. Sundaram and E. Mazur, “Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses,” Nat. Mater., vol. 1, no. 4, pp. 217–224, 2002.
[3] A. Faraclas, G. Bakan, L. Adnane, F. Dirisaglik, N. E. Williams, A. Gokirmak, and H. Silva, “Modeling of Thermoelectric Effects in Phase Change Memory Cells,” Electron Devices, IEEE Trans., vol. 61, no. 2, pp. 372–378, Feb. 2014.
9:00 PM - ED11.4.04
Vanadium Dioxide Nanowire Crossbar
Bo Hsu 1 , Subhajit Ghosh 1 , Getu Gebre 1 , Zheng Yang 1
1 Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, Illinois, United States
Show AbstractIn this presentation, the growth, fabrication, structural and electrical properties of a crossbar structure composed of two individual vanadium dioxide nanowires are reported. The crossbar junction shows a temperature-tunable and reversible linear-nonlinear I-V characteristics. The potential device applications of the crossbars are discussed.
9:00 PM - ED11.4.05
Enhanced Electrical Switching in Strain Engineered Sb2Te3-GeTe Interfacial Phase Change Memory Cells
Xilin Zhou 1 , Shilong Lv 2 , Yun Meng 2 , Jitendra Behera 1 , Liangcai Wu 2 , Zhitang Song 2 , Robert Simpson 1
1 , Singapore University of Technology and Design, Singapore Singapore, 2 , Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai China
Show AbstractChalcogenide phase change materials such as GeSbTe alloys have been widely used in optical and electrical data storage due to their substantial property contrast between structural phases, fast phase transitions, and good cyclability. Interfacial phase change material (iPCM) present a viable route for further improving the energy efficiency of data storage. The phase transition in iPCM’s is confined to a thin two dimensional layer at the interface between crystalline scaffold layers [1-3]. The electrical switching performances of strain engineered Sb2Te3-GeTe superlattice phase change memories were systematically investigated in this work. The Sb2Te3-GeTe superlattice heterostructures were prepared via self-organised van der Waals (vdW) epitaxy with high crystallographic orientation along the c-axis. We found that the energy barrier for the diffusive Ge atomic switching decreased significantly as more Ge atoms switch into the vdW gap. The interfacial disordering could be further promoted by applying in-plane biaxial strain to the superlattice heterostructures, which in turn increases the likelihood of switching at lower temperatures. The switching time, switching threshold voltage, reset/set voltage/current, and switching cycles of the interfacial phase change memories were all substantially improved by strain engineering. We demonstrate that the strain engineering provides an effective route to design high-performance phase change memories for data storage.
[1] R. Simpson et al. Interfacial phase-change memory. Nat. Nanotechnol. 6, 501–505 (2011).
[2] X. Zhou et al. Phase-change memory materials by design: a strain engineering approach. Adv. Mater. 28, 3007 (2016).
[3] J. Kallika et al. Strain-engineered diffusive atomic switching in two-dimensional crystals. Nat. Commun. 7, 11983 (2016).
9:00 PM - ED11.4.06
An Effect of Various Electrode Materials for Electrical Characteristics of the Ovonic Threshold Switching (OTS) Devices Using the Ge-Se Binary Amorphous Chalcogenide
Hyung-Woo Ahn 1 , Kazumasa Horita 1 , Takahiko Sawada 1 , Kazushi Fuse 1 , Shunpei Ota 1 , Miyaguchi Yusuke 1 , Shun Manita 1 , Takehito Jimbo 1
1 Institute of Semiconductor and Electronics Technologies, ULVAC, Susono Japan
Show AbstractThe threshold type cell selector using amorphous chalcogenide, called as Ovonic threshold switching (OTS) device, have recently attracted much interest due to the simple device structure, the ability of driving high current and etc. From these advantages, it is expected to replace CMOS transistor, p-n diode for the cell select switch in non-volatile memory devices requiring low of sneak current, high driving current and the possibility of 3D stackable memory. Generally, the functional OTS thin films including phase change materials are fabricated by the RF (13.56 MHz) or DC magnetron sputtering process. For the successful implementation as a switching device, it is essential to achieve a method to control the electrical characteristics such as threshold voltage (VTH), on-state current (ION), on-state resistance (RON), and etc. of the OTS devices. Also, it supposed that there are a lot of correlation with the electrode materials.
In this works, we will be discussed how electrode materials engineering can help make high performance OTS device possible. We fabricated the OTS device composed of metal/Ge-Se/metal and all stacks were grown with in-situ process by RF Magnetron Sputter (ULVAC, Entron W300). A remarkable improvement depends on electrode materials and their engineering were found from the threshold switching behavior of devices. For analysis threshold switching behavior, a semiconductor device analyzer (Keysight, B1500A) has been used including pulse generator (Keysight, WGFMU). And TEM (JOEL, JEM-ARM200F) also has been used for observation of the interface characteristics between the electrode and the Ge-Se amorphous chalcogenide materials. The details and results of the experiment will be presented.
Symposium Organizers
Stefania Privitera, Consiglio Nazionale delle Ricerche (CNR)
Harish Bhaskaran, University of Oxford
Eric Pop, Stanford University
Yuta Saito, National Institute of Advanced Industrial Science and Technology (AIST)
ED11.5: Threshold Switching and Devices
Session Chairs
Stefania Privitera
Veronique Sousa
Wednesday AM, April 19, 2017
PCC North, 100 Level, Room 131 C
9:00 AM - *ED11.5.01
Picosecond Electric-Field-Induced Threshold Switching in Phase-Change Materials
Peter Zalden 3 , Michael Shu 1 , Aaron Lindenberg 1 2
3 , European XFEL GmbH, Hamburg Germany, 1 , Stanford University, Stanford, California, United States, 2 , SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractMany chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging phase- change memory devices. Here, picosecond electric pulses are used to bias amorphous AIST and GST phase-change materials while probing its ultrafast response. Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching takes place within the electric pulse on subpicosecond time scales.
9:30 AM - *ED11.5.02
Ultrafast Electrical Switching Dynamics in Phase-Change Materials
Anbarasu Manivannan 1 2
1 Electrical Engineering, Indian Institute of Technology Indore, Indore India, 2 Materials Science and Engineering, Indian Institute of Technology Indore, Indore India
Show AbstractRapid and reversible switching in phase-change materials have enabled optical data storage products [1] and have also recently demonstrated their capabilities in the next generation high-speed, non-volatile electronic memories [2], solid-state displays [3] and logic devices [4]. Achieving a fast electrical switching, however, remains a key challenge. This talk will present exhaustive experimental results on electrical switching of various phase change materials including ultrafast electrical switching dynamics, voltage dependent transient characteristics and rapid SET operations of GeSbTe and InSbTe phase change materials [5]. Also, a trajectory map for achieving picosecond threshold-switching of Ge2Sb2Te5 cells based on a correlation between the rate of applied voltage and the dynamics of threshold-switching at picosecond-timescale will be discussed.
Furthermore, our recent results demonstrate a distinct characteristic feature of enabling a rapid threshold-switching at a critical voltage known as the threshold voltage in AgInSbTe cells as validated by an instantaneous response of steep current rise from an amorphous off to on state is achieved within 250 picoseconds [6]. Also, the extraordinary nature of threshold-switching dynamics in AgInSbTe cells is independent to the rate of applied voltage unlike other chalcogenide-based phase change materials exhibiting the voltage dependent transient switching characteristics.
REFERENCES:
1. M. Wuttig and N. Yamada, Nat. Mater. 6, 824 (2007).
2. M.H.R. Lankhorst et al., Nat. Mater. 4, 347 (2005).
3. P. Hosseini, C.D. Wright, and H. Bhaskaran, Nature 511, 206-211 (2014).
4. M. Cassinerio, N. Ciocchini, and D. Ielmini, Adv. Mater. 25, 5975 (2013).
5. S.K. Pandey and M. Anbarasu Appl. Phys. Lett. 108, 233501 (2016).
6. K.D. Shukla, N. Saxena, D. Suresh and M. Anbarasu, Sci. Rep. (communicated).
10:00 AM - ED11.5.03
Impact of Ge-Sb-Te Material Engineering for Fast-Switching Phase Change Memory
Huai-Yu Cheng 1 , Wei-Chih Chien 1 , Matt Brightsky 2 , Hsiang-Lan Lung 1 , Chung Lam 2
1 , Macronix International Co., Ltd., Yorktown Heights, New York, United States, 2 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractNew memory technologies are drawing a lot of attention from computer manufacturers, mainly because the traditional hard disks and new solid-state-drives (SSDs) cannot provide the high input/output (I/O) bandwidth required in advanced systems. New storage-class memory (SCM) is proposed to satisfy the system speed/power requirements. Phase-change memory (PCM) seems to be in the best position to serve as SCM due to its high speed and promising endurance performance.
Phase-change materials are at the core of PCM technology. Switching speed is one of the critically important parameters when designing a new phase-change material for PCM because it determines PCM’s possible applications. Sb-based materials with a growth-dominated crystallization mechanism generally show high crystallization speed. They exhibit a different crystallization mechanism compared to Ge2Sb2Te5 (GST-225), which is a popular composition along the GeTe-Sb2Te3 pseudobinary tie line and the most commonly studied material. However, the drawback of Sb-based materials is the low thermal stability of the amorphous phase. Doping extra Sb into GST-225 has also been proposed to improve the SET speed of GST-225. However, the SET speed is depending on the quantitative effect of Sb addition as a result doping extra Sb into GST-225 exhibits no clear trend speed improvement. Te-poor GST alloy was suggested to enhance data retention, but it caused higher power consumption.
In this paper, we will show various phase change materials by composition engineering along Sb and Te tieline based on doped Ge-Sb-Te (GST) alloys. Each doped GST materials are sputtered from a composite GST target, which is made of GST-225, Sb-enriched and Te-enriched GST-225, respectively and a doping element “A”. A reactive sputtering technique to add a second component to GST is adopted. Due to different sputtering yields of each element, the films composition from doped GST-225 compound target naturally becomes slightly doped Te-poor and Sb-rich GST-225 material. Materials with a set of various Te concentration of doped GST and Sb-enriched GST materials were deposited using this technique.
The impact of Sb and Te addition into doped GST-225 materials is comprehensively studied. New materials are evaluated using PCM devices with TiN ring bottom electrode of 20~40 nm diameter. It was found SET speed is significantly improved with increasing Te content. However, it requires higher RESET current due to lower resistivity at crystalline state of material. In addition, it is confirmed that additional Sb into doped GST-225 does not necessarily improve speed. This result is consistent with the one obtained from Sb-enriched GST225 material system without extra dopants. Data retention is improved by additional Sb incorporation. The very fast crystallization speed (~10 ns) for doped GST material, which was designed and deposited by using Te-enriched GST-225 compound target is encouraging and suggests this material for SCM application.
10:15 AM - ED11.5.04
Composition Control of CVD GexSbyTez for Low-Reset Current, Fast-Switching Phase Change Memory
Fabio Carta 1 , Takeshi Masuda 2 , Huai-Yu Cheng 3 , Michael Gordon 1 , Gloria Fraczak 1 , Robert Bruce 1 , Wanki Kim 1 , Koukou Suu 2 , Chung Lam 1 , Matt Brightsky 1
1 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 2 , ULVAC, Inc, Susono Japan, 3 , Macronix International Co., Hsinchu Taiwan
Show AbstractPhase Change Memory (PCM) is a promising candidate for next generation nonvolatile memories. GexSbyTez is the most commonly studied material system and is typically deposited by either PVD, ALD or CVD methods. PCM devices fabricated with PVD processes allow for a readily tunable phase change material composition but require a planar substrate due to PVD’s poor filling capability. A confined PCM cell (as opposed to a contact-minimized PCM cell) allows lower RESET current, has been shown to exhibit extraordinary endurance and allows an increased array density. ALD/CVD techniques can be used to conformally fill high aspect ratio pores (at small pitch), thus avoiding the exposure of any of the surfaces to harsh RIE chemistries. Previous reports have demonstrated a highly reliable PCM technology using an ALD-based GexSbyTez process. Advances in materials is required for further reducing the required programming current. Crystalline resistivity directly impacts the amount of Joule heat that is generated during electrical programming, and thus directly influences the required RESET current of a PCM cell. This study investigates ALD/CVD process tuning with the aim of achieving increased crystalline resistivity of the phase change material which has the potential of reducing the required RESET programming current.
An ULVAC Entron multi-chamber cluster tool enables an in-situ deposition of a metal-nitride liner and phase change material (without breaking vacuum in between) allowing enhanced reliability. A metallic liner tuned to the appropriate resistivity shows the ability to mitigate the resistance drift by separating the read and write current paths within the PCM cell. A metal nitride liner is used as the preferred substrate for this phase change material process development.
The Resistance-vs-Temperature characteristic of the starting GexSbyTez shows relatively low crystalline phase resistivity indicating that a higher RESET current would be required. By tuning the deposition process parameters and stoichiometry of the phase change material (without changing the Ge, Sb and Te precursors) the resultant crystalline resistivity is shown to vary by as much as three orders of magnitude. Using these techniques, a higher crystalline resistivity GexSbyTez material is successfully demonstrated. To investigate the expected impact on write performance, LASER re-crystallization experiments are performed. The phase transformation behavior can be deduced by plotting the change in reflectivity as a function of LASER power and duration. Our data shows that the crystalline resistivity can be increased without detrimentally affecting the crystallization speed. This study demonstrates the ability to improve the CVD deposited phase change material characteristics by process control alone to further reduce the RESET current for high aspect ratio, fast-switching confined PCM.
10:30 AM - ED11.5.05
Transport Properties and Temperature Dependence of Threshold Switching and Self-Oscillation in NbOx Based Devices
Shuai Li 1 , Xinjun Liu 1 , Sanjoy Nandi 1 , Robert Elliman 1
1 Department of Electronic Materials Engineering, Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractThreshold switching, or current-controlled negative differential resistance (CC-NDR), in strongly correlated oxides has attracted considerable research interest due to uncertainty over the switching mechanism. CC-NDR is also of considerable technological interest for applications such as a memory selector elements and voltage-controlled oscillators [1]. A particular focus in recent years has been the development of coupled relaxation oscillators for neuromorphic computing applications, based on the fact that these have been shown to emulate the functionality of computational neurons [2,3]. A range of mechanisms has been proposed to explain threshold switching, including temperature-induced metal-insulator phase transitions and thermally enhanced Poole-Frenkel conduction [4,5]. While there is general consensus that the CC-NDR results from local Joule-heating with sufficiently large biases or currents, details of the mechanism remain unclear.
Here we report on threshold switching in NbOx (x≈2.5) thin films and compare the experimental results with a locally self-accelerated thermal feedback mechanism based on Poole-Frenkel conduction. The devices consisted of TiN (50 nm) /NbOx (70 nm)/Pt (50 nm) heterostructure with top Pt contacts of 100 mm diameter. The electroformed devices were found to exhibit a combination of volatile threshold switching and non-volatile bipolar resistive switching, and therefore comprised a selector/memory (1S1M) structure. Symmetrical threshold switching occurred when the memory element was set to low resistance state using an appropriate current compliance. The switching responses of devices in this regime were studied as a function of substrate temperature to better understand the transport properties. Based on these measurements the low-field activation energy was estimated to be ~0.1 eV for the insulating state and -5.4x10-3 eV for the metallic state. Electrical oscillations was also investigated as a function of temperature to understand the switching dynamics and practical operating limits. The devices were shown to sustain more than 6.5×1012 oscillation cycles and to exhibit stable oscillations up to temperatures of 105 oC. The implications of these results for realising compact, scalable for neuromorphic computing applications is discussed.
References
1. S. Li, X. Liu, S. K. Nandi, D. K. Venkatachalam, and R. G. Elliman, Appl. Phys. Lett. 106, 212902 (2015).
2. S. Datta, N. Shukla, M. Cotter, A. Parihar and A. Raychowdhury, 51st ACM/EDAC/IEEE Design Automation Conference (DAC), San Francisco, CA, 2014, pp. 1-6.
3. M. D. Pickett, G. Medeiros-Ribeiro, and R. S. Williams, Nat. Mater., 12, 114–117 (2013).
4. G. A. Gibson, S. Musunuru, J. Zhang, K. Vandenberghe, J. Lee, C.-C. Hsieh, W. Jackson, Y. Jeon, D. Henze, Z. Li, and R. Stanley Williams, Appl. Phys. Lett. 108, 023505 (2016).
5. M. D. Pickett and R. S. Williams, Nanotechnology 23, 215202 (2012).
ED11.6: Device Reliability
Session Chairs
Huai-Yu Cheng
Anbarasu Manivannan
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 131 C
11:15 AM - *ED11.6.01
Suppressing Drift and Crystallization in Phase Change Memory Operated in Bipolar Mode
Daniele Ielmini 1 , Nicola Ciocchini 1 , Mario Laudato 1 , Mattia Boniardi 2 , Enrico Varesi 2 , Paolo Fantini 2 , Andrea Lacaita 1
1 , Politecnico di Milano, Milano Italy, 2 , Micron, Vimercate Italy
Show AbstractPhase change memory (PCM) can store logic bits in the form of a different amorphous/crystalline phase within a chalcogenide material, such as Ge2Sb2Te5, or GST. PCM shows large window, fast switching, high reproducibility and good cycling endurance. At the same time, the programming energy is generally higher that other memory devices, due to non-filamentary switching and the high temperature for melting (above 600°C). In addition, the amorphous phase of chalcogenide materials is generally affected by structural relaxation inducing drift of electrical properties. Finally, crystallization takes place at medium-high temperature (e.g., 150°C for GST), thus causing critical data retention limitation for nonvolatile memory applications. To suppress drift and crystallization in PCM, novel materials and operation modes should be explored.
In this work, we demonstrate suppression of drift and crystallization in PCM after set/reset operations under bipolar switching. Bipolar switching was demonstrated in GST-based conventional PCM devices, where set transition to low resistance was achieved under positive polarity, whereas reset transition to high resistance was achieved under negative polarity. Bipolar switching was observed irrespective of the initial state and of the polarity of the initial sweep. Pulsed operation below 100 us pulse and cycling up to 10 thousand cycles were demonstrated. By examination of TEM/EDX profiles in the reset state, the bipolar switching could be attributed to ionic migration of Ge and Sb induced by the electrostatic field. As cations migrate toward the top electrode, they leave a depleted region in correspondence of the confined bottom electrode, causing a high defect concentration responsible for the high resistance state. Redistribution of the elemental profile under positive voltage recovers the low resistance state.
Since the high resistance state does not rely on the amorphous phase, no drift was observed for the reset state, making the bipolar-mode PCM very promising for multilevel cell operation. Similarly, no crystallization was observed in the high resistance state, and a residual window of a factor 10 was still observed after 1 hour bake at 260°C. Improved retention can be attributed by slow diffusion of cations in the reset state. These results make the bipolar-mode PCM very attractive for embedded memory requiring high temperature operation, particularly for automotive applications.
11:45 AM - *ED11.6.02
An Insight into the High Temperature Reliability of Ge-Rich and N-Doped GeSbTe Phase Change Memory Devices
Veronique Sousa 1 , Gabriele Navarro 1 , Niccolo Castellani 1 , Julia Kluge 2 , Olga Cueto 1 , Chiara Sabbione 1 , Anne Roule 1 , Vincent Delaye 1 , Nicolas Bernier 1 , Frederic Fillot 1 , Pierre Noe 1 , Luca Perniola 1 , Serge Blonkowski 2 , Massimo Borghi 3 , Elisabetta Palumbo 3 , Paola Zuliani 3 , Roberto Annunziata 3
1 , CEA LETI MINATEC, Grenoble France, 2 , STMicroelectronics, Crolles France, 3 , STMicroelectronics, Agrate Italy
Show AbstractThe high temperature reliability of the Phase Change Memories (PCM) is a key challenge for embedded memory applications. In fact, in a PCM device, two physical phenomena are likely to affect the resistance of the programmed states: first, the crystallization of the amorphous phase, which can result in a decrease of the resistance and hereafter a RESET state failure; second, the structural relaxation of the disordered material, which can lead to an increase of the resistance known as the resistance drift and hereafter a SET state failure. In the past, several papers have highlighted the enhancement of the thermal stability of the programmed states which results from the increase in the Germanium content and from the addition of Nitrogen in the GeSbTe material [1-6]. In particular, the thorough characterization of a 12Mb test vehicle integrating the optimized Ge-rich GST alloy has shown that the data retention can be guaranteed in an extended temperature range. In this paper, we give an insight into the high temperature reliability of these N-doped and Ge-rich GeSbTe PCM devices.
We first focus on the RESET state, which is demonstrated to be stable up to 240°C. Thanks to the physico-chemical characterization of the RESET device, we show that the strong opposition against crystallization of the RESET state relates to the elemental distribution in the active zone of the PCM device. In fact, following the electrical activation of the device, the composition at the core of the storage element retains a Ge-rich GST alloy exhibiting a relatively high crystallization temperature, which is aligned with the thermal stability of the RESET state. As far as the SET state is concerned, we demonstrate how a programming procedure performed at low current enables to contain the resistance drift. We show that the low resistance drift of the SET state relates to the low number of grain boundaries along the conductive path. Multi-physical simulation results consolidate our interpretation of the experimental observations, showing how the germanium segregation phenomenon and the localization of the threshold switching of the amorphous phase do both impact the elemental distribution and the formation of the polycrystalline structure during programming.
[1] H. Y. Cheng et al., Proceedings IEEE IEDM (2011)
[2] H.Y. Cheng et al., Proceedings IEEE IEDM (2012)
[3] P. Zuliani et al., IEEE TED v60 n12 (2013) p4020
[4] G. Navarro et al., Proceedings IEEE IEDM (2013)
[5] V. Sousa et al., Proceedings IEEE VLSI (2015)
[6] G. Navarro et al., Proceedings IEEE IMW (2016)
12:15 PM - ED11.6.03
Short and Long Time Resistance Drift Measurement in Intermediate States of Ge2Sb2Te5 Phase Change Memory Line Cells
Nafisa Noor 1 , Raihan Sayeed Khan 1 , Sadid Muneer 1 , Lhacene Adnane 1 , Ryanne Ramadan 1 , Faruk Dirisaglik 2 , Adam Cywar 3 , Chung Lam 4 , Yu Zhu 4 , Ali Gokirmak 1 , Helena Silva 1
1 , University of Connecticut, Storrs, Connecticut, United States, 2 , Eskisehir Osmangazi University, Eskisehir Turkey, 3 , Analog Devices, Norwood, Massachusetts, United States, 4 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractPhase change memory (PCM) is an emerging computer memory technology with memory window large enough to allow for intermediate states and hence multi-level cell (MLC) operation for high density storage. Inherent resistance drifts in PCM, however, pose a major challenge for reliability [1] and the drift behavior in fully amorphous, fully crystalline or intermediate states must be well characterized. We have electrically characterized resistance-drift behaviors of intermediate states after partial amorphization of Ge2Sb2Te5 (GST) PCM line cells of different lengths and widths (L: ~300-700 nm, W: ~60-700 nm). The measured GST lines are 50 nm thick. These devices lie on SiO2 over bottom buried metal contacts and are capped by a 10 nm thick layer of Si3N4 [2]. The resistance of these partially amorphized cells has been measured in short time intervals (every 15-20 s up to an hour) and in long time intervals (up to ~73 days [3]) after amorphization. Narrow wires (~60-200 nm) show random resistance drift (upward, downward, or no drift). Wider cells (~400-700 nm), on the other hand, consistently show an upward drift. The stochasticity associated with the resistance drift in narrow cells may be used in hardware security applications. The details on the measurement techniques, results and possible reasons for the different behaviors observed will be discussed.
References:
[1] N. Papandreou, et al. "Drift-tolerant multilevel phase-change memory." 2011 3rd IEEE International Memory Workshop (IMW). IEEE, 2011.
[2] F. Dirisaglik, et al. "High speed, high temperature electrical characterization of phase change materials: metastable phases, crystallization dynamics, and resistance drift," Nanoscale, vol. 7, pp. 16625-16630, 2015.
[3] N. Noor, et al. "An experimental study on waveform engineering for Ge2Sb2Te5 phase change memory cells." 2015 MRS Fall Meetings & Exhibits, MRS, 2015.
12:30 PM - ED11.6.04
Analysis of Resistance State Stability in Ge-Rich PCM Devices under Voltage and Temperature Stress
Julia Kluge 1 2 3 , Gabriele Navarro 2 , Veronique Sousa 2 , Niccolo Castellani 2 , Serge Blonkowski 1 , Roberto Annunziata 4 , Paola Zuliani 4 , Luca Perniola 2
1 , STMicroelectronics, Crolles France, 2 , CEA LETI, Grenoble France, 3 , IMEP-LAHC, Minatec/INPG, Grenoble France, 4 , STMicroelectronics, Agrate Brianza Italy
Show AbstractPhase-Change Memory (PCM) is considered the most promising candidate among emerging non-volatile memories due to its excellent properties such as fast switching speed, long endurance and excellent data retention [1]. Moreover, this technology demonstrated the capability to address multi-level storage applications thanks to innovative sensing schemes [2] and novel write schemes [3]. Recently, it has been shown in Ge-rich Ge2Sb2Te5 (GST) based devices, which feature high data retention performance [4], how the write procedure can improve the programming reliability. It shows how the segregation phenomena and the localization of the electronic switching impact the elemental distribution and the formation of the crystalline structure during SET operation [5].
In this study, we analyze the programming strategies in order to achieve intermediate resistance states in Ge-rich GST PCM. We use a double pulse programming technique to target different resistance states in between the low resistance state (SET) and the high resistance state (RESET). The first pulse is either a SET or a RESET pulse (preprogramming), which is then followed by a second programming pulse with varying amplitude and duration. We show here how the pulse shape impacts the final resistance state distribution of the PCM device. The stability of the so obtained intermediate resistance states is then investigated at high operating temperatures, in particular showing how the drift is impacted by the preprogrammed resistance state of the cell. Furthermore, we analyze the stability of the cells under voltage application up to threshold switching event. The results highlight how a reliable reading operation can be achieved for all the resistance states analyzed at high operating temperature and under voltage stress.
In conclusion, this study confirms that PCM technology based on optimized Ge-rich GST demonstrates the capability of intermediate resistance states programming at high operating temperatures and high reading voltages.
[1] H. Y. Cheng et al., "Novel fast-switching and high-data retention phase-change memory based on new Ga-Sb-Ge material," 2015 IEEE International Electron Devices Meeting (IEDM), 2015, pp. 3.5.1-3.5.4.
[2] J. Y. Wu et al., "Greater than 2-bits/cell MLC storage for ultra high density phase change memory using a novel sensing scheme," VLSI Technology, 2015 Symposium on, 2015, pp. T94-T95.
[3] W. C. Chien et al., "A novel self-converging write scheme for 2-bits/cell phase change memory for Storage Class Memory (SCM) application," VLSI Technology, 2015 Symposium on, 2015, pp. T100-T101.
[4] P. Zuliani et al., "Engineering of chalcogenide materials for embedded applications of Phase Change Memory", Solid-State Electronics, 2015, Vol. 111, Pages 27-31.
[5] V. Sousa et al., "Operation fundamentals in 12Mb Phase Change Memory based on innovative Ge-rich GST materials featuring high reliability performance," VLSI Technology, 2015 Symposium on, 2015, pp. T98-T99.
12:45 PM - ED11.6.05
Improving Phase Change Material-Based RF Switch Reliability via In Depth Morphological Analysis
Matt King 1 2 , Nabil El-Hinnawy 1 3 , Pavel Borodulin 1 , Andy Ezis 1 , Carlos Padilla 1 , Brian Wagner 1 , Evan Jones 1 , Doyle Nichols 1 , Elizabeth Dickey 2 , Jon-Paul Maria 2 , Robert Young 1
1 , Northrop Grumman, Linthicum, Maryland, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractRecent reports have shown that implementing chalcogenide phase change materials (PCMs) in RF systems enables world class performance in the form of low loss, high isolation and circuit reconfigurability. The RF phase change switch approach is to independently heat the PCM from an external source, much like the gate on a FET supplies an electric field between the source and drain, creating a 4-terminal, inline phase change switch (IPCS). Using this approach, RF switches with a cutoff frequency (Fco) of 12.5 THz have been demonstrated. Furthermore operation for >200k pulses has been achieved. In order to continue improving device reliability while maintaining performance, a detailed morphological study of pulsed devices was undertaken. Fortuitously, since the chalcogenide GeTe layer of the IPCS is sandwiched between silicon nitride layers one can prepare plan view TEM foils which capture GeTe morphology throughout the entirety of the RF gap. More typical “vertical cut” TEM cross sections capture information such as the shape of the amorphous region, but lose registry with global effects that occur along RF signal and DC heater control lines. This methodology allows for a direct correlation between GeTe micromorphology and as-measured electrical characteristics. By varying ON and OFF pulse energies and evaluating switches as a function of pulse number, we observe substantial morphological changes. First, direct evidence of templated growth is observed, where GeTe grain size and shape indicate ~100x300nm single GeTe domains grow toward the switch centerline, along the thermal gradient. Furthermore the melt width estimated to be is as large as 600nm for a 900nm RF gap based on a sharp interface between crystallized and as-fabricated GeTe. Finally, in spite of extremely stable ON-state resistance as a function of pulse number a substantial number of voids along the centerline are observed. A phenomenological model will be presented for amorphization and crystallization processed based on observed micromorphology. While these results most immediately impact IPCS device design, this structure in general is a valuable platform for phase change material studies.
ED11.7: Emerging Applications and Devices
Session Chairs
Stefania Privitera
Abu Sebastian
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 131 C
2:30 PM - ED11.7.01
A Study on Stochasticity in Hexagonal Close Packed Ge2Sb2Te5 Nanowires for Possible Physical Unclonable Function (PUF) Implementation
Raihan Sayeed Khan 1 , Nafisa Noor 1 , Aaron Ciardullo 1 , Sadid Muneer 1 , Lhacene Adnane 1 , Faruk Dirisaglik 2 , Adam Cywar 1 , Chung Lam 3 , Yu Zhu 3 , Ali Gokirmak 1 , Helena Silva 1
1 Department of Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 Department of Electrical and Electronics Engineering, Eskisehir Osmangazi University, Eskisehir Turkey, 3 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractProcess variations lead to characteristic differences in electronic circuits that can be utilized as physical unclonable functions (PUFs) for hardware identification. Phase change memory (PCM) devices utilize the resistance contrast between amorphous and crystalline phases in materials such as Ge2Sb2Te5 (GST) to store information [1]. Typically PCM devices utilizing GST as the active material switch between the amorphous and face centered cubic (FCC) phases during operation. The resistance of amorphous and FCC phases drifts over time [2]. The hexagonal closed packed (HCP) phase is the stable phase and does not display drift, making it suitable for PUF implementations [3].
We have studied lithographically defined 20 nm and 50 nm thick GST HCP nanowires with widths ranging from ~50 nm to ~700 nm and lengths from ~330 nm to ~700 nm. Stochasticity of wire resistance with varying length and width and its possible implementations for PUFs will be presented.
References:
[1] H. P. Wong, S. Raoux, S. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi, and K. E. Goodson, “Phase Change Memory,” Proc. IEEE, vol. 98, no. 12, 2010.
[2] F. Dirisaglik, G. Bakan, Z. Jurado, S. Muneer, M. Akbulut, J. Rarey, L. Sullivan, M. Wennberg, A. King, L. Zhang, R. Nowak, C. Lam, H. Silva, and A. Gokirmak, “High speed, high temperature electrical characterization of phase change materials: metastable phases, crystallization dynamics, and resistance drift.,” Nanoscale, vol. 7, no. 40, pp. 16625–30, 2015.
[3] Y. Jung, S.-H. Lee, D.-K. Ko, and R. Agarwal, “Synthesis and Characterization of Ge 2 Sb 2 Te 5 Nanowires with Memory Switching Effect,” J. Am. Chem. Soc., vol. 128, no. 43, pp. 14026–14027, 2006.
2:45 PM - ED11.7.02
Thermal Probe Lithography of GeTe Thin Films
Laura Ruppalt 1 , Adrian Podpirka 2 , Woo-Kyung Lee 1 , Jed Ziegler 1 , Todd Brintlinger 1 , Blake Simpkins 1 , Nabil Bassim 3 , Arnaldo Laracuente 1 , Paul Sheehan 1
1 , US Naval Research Laboratory, Washington, District of Columbia, United States, 2 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 3 Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada
Show AbstractThe ability to reversibly, and non-volatilely, transition chalcogenide phase change materials (PCM) between an electrically conductive, optically absorptive crystalline phase and an electrically resistive, optically transparent amorphous phase suggests the possibility of novel device modalities for emerging applications in photonics, neuromorphic computing, and metamaterials [1]. For nanoscale devices, localized phase transition can be accomplished by the confined application of heat, where a low-temperature treatment induces crystallization and a high-temperature heat pulse, followed by a rapid quench, locally vitrifies the film. Among the most precise means of applying heat are thermal probes, nanometer-sharp scanning probes with built-in Joule heaters capable of reaching high temperatures (>1000 °C) with fast transit times (< microsecond). Here, we demonstrate nanometer-scale patterning of thin film germanium telluride (GeTe) PCM films using a fast-scanning heated-probe atomic force microscope (AFM) equipped with a doped-silicon heatable cantilever. Using this method, crystalline features of arbitrary geometry and varying dimensions are generated in amorphous GeTe thin films, where the extent of crystalline transformation is controlled by varying the write-speed and temperature of the heated tip. The dramatic differences in behavior between crystalline and amorphous GeTe phases result in embedded nanoscale features with strong topographic, electronic, and optical contrast, as verified by AFM, transmission electron microscopy (TEM), and near-field scanning optical microscopy (NSOM). Compared to other methods of nanopatterning, thermal probe lithography offers several advantages. Particularly, fine feature sizes can be achieved without the optics required for laser-based techniques; PCM films can be deposited on arbitrary substrates, removing the requirement of the integrated backside electrode necessary for conductive-AFM patterning and opening possibilities for RF electronics and flexible plastic substrates; and large-scale arrays of heated cantilevers providing high-speed and broad-area coverage have already been developed [2], providing a feasible pathway for wafer-scale manufacturability. Thermal probe lithography is not limited to GeTe, but is extensible to the general class of chalcogenide PCM alloys, suggesting a new approach to the fabrication of nanoscale monolithic PCM-based electronic and optoelectronic devices.
[1]Q. Wang, et al., Nature Photonics, 2016, 10, 60; T. Tuma, et al., Nature Nanotechnology, 2016, 11, 693; A. Tittl, et al., Advanced Materials, 2015, 27, 4597.
[2]P. Vettiger, et al., IEEE Transactions on Nanotechnology, 2002, 1, 39.
3:00 PM - ED11.7.03
Mechanical and Electrical Characterization of CVD-Grown Graphene Transferred on Chalcogenide Phase Change Materials
Giuseppe D'Arrigo 1 , Rita Rizzoli 2 , Meganne Christian 2 , Vittorio Morandi 2 , Corrado Bongiorno 1 , Michele Calabretta 3 , Emanuele Rimini 1
1 , CNR-IMM HQ, Catania Italy, 2 , CNR-IMM Bologna, Bologna Italy, 3 , SSC, Scuola Superiore di Catania, Ctania Italy
Show AbstractNon volatile memories based on Phase Change Materials, e. g. Ge2Sb2Te5 are considered possible candidates in the present market scenario. The Extreme Ultraviolet Lithography (EUVL) technology, actually, allows to scale the memory device structures in the node of 18-20 nm. The capability of increasing the bits in the storage devices is one of the most important requests of the market for Non-Volatile Memory that remains one of the key product segment for the mass production. The scaling possibilities of PCM must be coupled to the need of improving the device characteristics also in terms of power performances. The reduction in power consumption for each bit transcription implies also a reduction of the power losses. In the present work we investigate the physical properties of the mechanically transferred CVD-grown graphene layer on different PCM and the possible utilization of graphene as scaled contact. Different authors [1-2] showed that carbon nanotubes and carbon nanoribbons can be used as electric contacts in a planar PCM with a relevant reduction in Joule losses. Moreover, it was shown [3] that the interposition of a graphene layer between the PCM active material and the metallic via increases the thermal confinement, due to the weak van der Waals interactions at the interface [4-5], thus reducing the programming current. Here we report the mechanical and electrical characterization of CVD-grown graphene transferred on the substrate, in particular on 50 nm thick Ge2Sb2Te5, on 50 nm GeTe and on 50 nm Sb2Te3. The graphene layer was synthesized by C-CVD technique using a Cu foil as a catalyst and it was then transferred on the GST layer by standard Cu foil wet-etching and subsequent release of the polymer-supported graphene layer. The physical characterizations show a multi domain nature of the graphene grown and transferred on chalcogenide layer. The bonding forces between the graphene and GST layer in a complete multilayer stack, namely Ni/Au layer - CVD graphene – PCM layer, were characterized by using the Nano-scratch Tester (CSM instruments). Every sample was tested with the CSM equipment, setting a progressive linear scratch using an initial load of 0.05 mN and ending with a load of 11 mN. The indentation and the scratches were performed using a diamond spherical tip with a radius 1 µm. The electrical contact resistance of the same type of complete multilayers was investigated using the Circular Transmission Line Method (CTLM). The preliminary physical and electrical characterizations indicate the possible advantages of using graphene contacts in PCM structures.
[1] F. Xiong, A. Liao, D. Estrada, Eric Pop Science 29, Vol. 332, Issue 6029, pp. 568-570, 2011.
[2] A. Behanm, et al. Appl. Phys. Lett. 2015, 107, 123508.
[3]C. Ahn et al. Nano Lett. 15, 6809-6814, 2015.
[4] P. A. V. Guzman, et al. IEEE Int.Conf. on thermal and Termomechanical Phenomena in Electronic System 2014, 1385-1389.
[5]Mak, K. F. et al. Appl. Phys. Lett. 2010, 97, 221904.
3:15 PM - ED11.7.04
Ultra-Low Resistance Sn-Based Contacts to GeTe
Hamed Simchi 1 , Kayla Cooley 1 , Suzanne Mohney 1
1 , Pennsylvania State University, State college, Pennsylvania, United States
Show AbstractGermanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention due to its incorporation as an active material for radio frequency (RF) switches, as well as memory and novel optoelectronic devices. Considering the PCM-based RF switches, parasitic resistances from Ohmic contacts can be a limiting factor in device performance. Reduction of the contact resistance is therefore critical for reducing the ON-state resistance in order to meet the requirements of high-frequency RF applications. To engineer the Schottky barrier between the metal contact and GeTe, Sn was tested as an interesting candidate to grade the composition of the semiconductor near its surface. SnTe has a lower bandgap (0.2 eV) than GeTe (0.7 eV). For this purpose, a novel contact stack of Sn/Fe/Au was employed. Au provides low sheet resistance for the probing the electrodes, while Fe was found to be a reasonable barrier for the interdiffusion of Sn and Au layers, and was better than other barriers we tested. Specific contact resistance values were extracted using the refined transfer length method (RTLM) and compared to those of a conventional Ti/Pt/Au stack. The as-deposited Sn/Fe/Au contact provided excellent contact resistance (5.5±1 x10-3 Ω-mm) compared to the Ti/Pt/Au stack (1.6±0.2 x10-2 Ω-mm). The Sn/Fe/Au contacts were stable and their resistance decreased further to 2.3±2 x10-3 Ω-mm after annealing at 200 oC. Assuming that the sheet resistance of GeTe below the contact region did not change significantly and that the transmission line model is still valid, a specific contact resistance of 3.8±2.8 x10-9 Ω-cm2 would be obtained for the annealed contacts, which is the lowest reported value for contacts to GeTe. Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) were used to characterize the interfacial reactions between the metals and GeTe.
ED11.8/ED2.6: Joint Session: Devices for Neuromorphic Computation
Session Chairs
Daniele Ielmini
Shimeng Yu
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 131 C
4:30 PM - *ED11.8.01/ED2.6.01
The N3XT Technology for Brain-Inspired Computing
S. Burc Eryilmaz 1 , Haitong Li 1 , Weier Wan 1 , H.-S. Philip Wong 1
1 Department of Electrical Engineering and Stanford SystemX Alliance, Stanford University, Stanford, California, United States
Show AbstractAdvances in brain-inspired computing are making rapid progress to meet the demands of abundant-data processing using a variety of techniques, including spiking neural networks, hyperdimensional computing using sparse vectors, deep neural nets, deep belief nets, restricted Boltzmann machines, and their variants. It is therefore crucial to create a scalable and flexible brain-inspired technology platform that can support all the essential elements, and can be adapted for a wide variety of neural computational model.
The key elements of a scalable, fast, and energy-efficient computation platform that may provide another 1,000× in computing performance (energy-execution time product) for future computing workloads are [1]: massive on-chip memory co-located with highly energy-efficient computation, enabled by monolithic 3D integration using ultra-dense and fine-grained massive connectivity. There will be multiple layers of analog and digital memories interleaved with computing logic, sensors, and application-specific devices. We call this technology platform N3XT – Nanoengineered Computing Systems Technology. N3XT will support computing architectures that embrace sparsity, stochasticity, and device variability.
In this talk, I will give an overview of nanoscale memory and logic technologies for implementing N3XT. In particular, I give an overview of the use of nanoscale analog non-volatile memory devices for implementing brain-inspired computing [2]. Phase change memory (PCM) and resistive switching memory (RRAM) are used as examples to illustrate the need to co-design, co-optimize the device technology, circuit design, system architecture, and learning algorithms [3].
Acknowledgements: Supported in part by Stanford Non-Volatile Memory Technology Research Initiative (NMTRI), Stanford SystemX Alliance, the National Science Foundation (E2CDA, Expedition in Computing), and STARnet SONIC.
References:
[1] M.M. Sabry Aly, M. Gao, G. Hills, C.-S. Lee, G. Pitner, M.M. Shulaker, T.F. Wu, M. Asheghi, J. Bokor, F. Franchetti, K.E. Goodson, C. Kozyrakis, I. Markov, K. Olukotun, L. Pileggi, E. Pop, J. Rabaey, C. Re, H.-S. P. Wong, S. Mitra, "Energy-Efficient Abundant-Data Computing: The N3XT 1,000X," IEEE Computer, pp. 24 – 33, December 2015
[2] S. B. Eryilmaz, D. Kuzum, S. Yu, H.-S. P. Wong, “Device and System Level Design Considerations for Analog-Non-Volatile-Memory Based Neuromorphic Architectures,” invited paper, IEEE International Electron Devices Meeting (IEDM), paper 4.1, pp. 64 – 67, 2015.
[3] H. Li, T.F. Wu, A. Rahimi. K.-S. Li, M. Rusch, C.-H. Lin, J.-L. Hsu, M.M. Sabry, S.B. Eryilmaz. J. Sohn, W.-C. Chiu, M.-C. Chen, T.-T. Wu, J.-M. Shieh, W.-K. Yeh, J. M. Rabaey, S. Mitra, and H.-S. P. Wong, “Hyperdimensional Computing with 3D VRRAM In-Memory Kernels: Device-Architecture Co-Design for Energy-Efficient, Error-Resilient Language Recognition,” IEEE International Electron Devices Meeting (IEDM), paper 16.1, 2016.
5:00 PM - ED11.8.02/ED2.6.02
Low Voltage Nano-Ionics Based Selector Devices Using Doped HfO2 for Application in 3D Crosspoint Memories
Sushant Sonde 1 2 , Kiran Sasikumar 2 , Yuzi Liu 2 , Jianqiang Lin 2 , Anil Annadi 1 2 , Daniel Rosenmann 2 , Subramanian Sankaranarayanan 2 , Matt Jerry 3 , Nikhil Shukla 3 , Suman Datta 3 , Supratik Guha 1 2
1 , Institute for Molecular Engineering, University of Chicago, Chicago, Illinois, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, United States, 3 , University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractMetal oxides have been explored for applications such as in resistive RAM [1], electronic synaptic devices for neuromorphic computation [2] etc. Recently, enhanced nonlinearity in metal oxide stacks was proposed for possible application as selectors in cross-point memory arrays [3] where they are essential in suppressing sneak leakage currents. Such structures have been studied before from a device performance point of view, with a goal towards low voltage operation [4]. Much, however remains to be understood regarding the underlying materials phenomena and microstructural changes that dictate the operation voltage, the variability and the cycling behavior of these devices.
In this study, we have fabricated and demonstrated low voltage bi-directional selectors using atomic layer deposited HfO2 thin films. The fabricated selectors consist of a cross-bar array with a HfO2 thin film sandwiched between inert (Pt) and electrochemically active (Cu or Ag) electrodes. Filamentary conduction beyond a threshold voltage was verified by varying device areas. It is understood that the switching effect is facilitated by the HfO2 playing the role of a cation conductor for redox active species e. g. Cu+ and Ag+. Through detailed studies of a large set of different samples under different annealing conditions, HfO2 thickness and device structures, we show that it is possible to achieve very low switching voltages of ~0.5 V, tune the hysteresis and ON/OFF ratio, and observe the following key phenomena: (i) the devices can be switched at temperatures down to 20 K, indicating the presence of athermal effects in the field driven microstructural changes; (ii) repeated cycling results in a leaky device that can then be surprisingly restored to initial conditions by driving current through it and (iii) device to device variability is an important issue. We will describe these results and—combined with results from our ongoing molecular dynamic simulations of HfO2:Cu structures, identify the underlying materials phenomena driving these observations related to the cation conduction paths, diffusion mechanism of Cu and Ag in HfO2, time dependent response of the conducting filament to electric field, and lifetime and stability of conduction filaments. We will also describe a simple model for variability for the case of this class of diffusion mediated filamentary breakdown devices, such as threshold switches and resistive random access memory (RRAM) devices that demonstrates that the operating voltage of these devices – related to the energy required to create the resistive state forming “defect” – is inversely proportional to the device to device variability which will fundamentally increase the variability of these devices.
References:
[1] Sawa, A. Mater. Today 2008, 11, 28–36.
[2] Yu S. et al., IEEE TED 2011, 58, 2729-2737.
[3] Alimardani, N., Conley, J.F. Appl. Phys. Lett. 2013, 102, 143501:1–143501:3.
[4] Luo Q. et al., IEEE IEDM, Washington DC, 2015 253-256.
5:15 PM - ED11.8.03/ED2.6.03
Finite Element Modeling of Ovonic Threshold Switch Controlled Phase Change Memory Devices
Jake Scoggin 1 , Zachary Woods 1 , Helena Silva 1 , Ali Gokirmak 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractPhase change memory (PCM) is a high-speed high-density resistive memory technology that utilizes the resistance contrast between amorphous and crystalline phases in phase change materials such as Ge2Sb2Te5 (GST) [1]. PCM can reach 2F2 density when integrated into a crosspoint architecture; however, crosspoint arrays are prone to current sneak paths. Integration of access devices with nonlinear current-voltage characteristics in series with PCM storage elements is therefore necessary for successful crosspoint implementation. Ovonic threshold switches (OTSs) [2] are promising nonlinear access devices for PCM cells [3]. OTSs utilize the threshold switching phenomenon in chalcogenide materials to limit undesired current in PCM devices. Integration of an OTS into a phase change process flow is relatively simple due to the wide use of chalcogenide materials in phase change devices.
We extend our finite element electrothermal model of phase change materials [4], [5] to model OTSs. We then simulate an OTS as an access device for a crossbar-like PCM cell and analyze the affects of the OTS on PCM operation using COMSOL Multiphysics. Analysis includes read/write waveforms, thermal crosstalk between devices, and energy consumption. We find that an OTS in series with a phase change cell provides stable, repeatable switching, and thermal crosstalk between such OTS-PCM cells is low enough to allow for the maximum 2F2 packing of memory elements in a crosspoint architecture.
References
[1] H.-S. P. Wong, S. Raoux, S. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi, and K. E. Goodson, “Phase Change Memory,” Proc. IEEE, vol. 98, no. 12, pp. 2201–2227, 2010.
[2] R. R. Shanks, “Ovonic threshold switching characteristics,” J. Non. Cryst. Solids, vol. 2, pp. 504–514, 1970.
[3] DerChang Kau, S. Tang, I. V. Karpov, R. Dodge, B. Klehn, J. A. Kalb, J. Strand, A. Diaz, N. Leung, J. Wu, Sean Lee, T. Langtry, Kuo-wei Chang, C. Papagianni, Jinwook Lee, J. Hirst, S. Erra, E. Flores, N. Righos, H. Castro, and G. Spadini, “A stackable cross point Phase Change Memory,” in 2009 IEEE International Electron Devices Meeting (IEDM), 2009, pp. 1–4.
[4] A. Faraclas, G. Bakan, H. Adnane, F. Dirisaglik, N. E. Williams, A. Gokirmak, and H. Silva, “Modeling of Thermoelectric Effects in Phase Change Memory Cells,” IEEE Trans. Electron Devices, vol. 61, no. 2, 2014.
[5] F. Dirisaglik, G. Bakan, A. Faraclas, A. Gokirmak, and H. Silva, “Numerical Modeling of Thermoelectric Thomson Effect in Phase Change Memory Bridge Structures,” Int. J. High Speed Electron. Syst., vol. 23, no. 01n02, p. 1450004, Mar. 2014.
Symposium Organizers
Stefania Privitera, Consiglio Nazionale delle Ricerche (CNR)
Harish Bhaskaran, University of Oxford
Eric Pop, Stanford University
Yuta Saito, National Institute of Advanced Industrial Science and Technology (AIST)
ED11.9: Epitaxial Films and Superlattices
Session Chairs
Kirill Mitrofanov
Yuta Saito
Thursday AM, April 20, 2017
PCC North, 100 Level, Room 131 C
9:30 AM - *ED11.9.01
Epitaxial Ultra-Thin GeTe Films
Raffaella Calarco 1 , Rui Ning Wang 1 , Wei Zhang 2 , Jamo Momand 3 , Ider Ronneberger 4 , Jos Boschker 1 , Riccardo Mazzarello 4 , Bart Kooi 3 , Henning Riechert 1 , Matthias Wuttig 4 , Davide Campi 5 , Marco Bernasconi 5 , Marcel Verheijen 6
1 , Paul-Drude-Inst, Berlin Germany, 2 , Xi’an Jiaotong University, Xi’an China, 3 , University of Groningen, Groningen Netherlands, 4 , RWTH Aachen, Aachen Germany, 5 , University of Milano-Bicocca, Milan Italy, 6 , Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractEpitaxial growth for a number of non-conventional semiconductors including GeTe, Sb2Te3 and GeSbTe alloys has revealed some interesting challenges. Especially if it comes to produce ultrathin films of highest possible quality, epitaxial growth is an indispensable technique. Often, however, the deposition of a thin crystalline layer is not successful and an amorphous layer is obtained instead. Here we report and explain such a scenario for the growth of GeTe ultra-thin films.
In addition to its proven relevance as a phase-change material, germanium telluride (GeTe) is also of great interest for its ferroelectric properties. These are however entirely enabled by the ordered Peierls dimerization of an otherwise centro-symmetric crystal. In the present work we show that an ultra-thin film of GeTe (<2 nm) might not be able to adopt its expected bulk crystalline structure; the Peierls distortion could be suppressed, or could occur in a disordered fashion.[1] This phenomenon is attributed to the confinement in the growth direction and to the influence of the interface. As a direct consequence, a lower limit in the thickness of GeTe thin-films could exist before their ferroelectric properties can be expressed.
[1] R. Wang et al., Scientific Reports, 6, 32895 (2016)
10:00 AM - ED11.9.02
Precise Control of the In-Plane Lattice Parameter in Sb2+xTe3/GeTe Superlattices
Stefano Cecchi 1 , Eugenio Zallo 1 , Fabrizio Arciprete 1 2 , Raffaella Calarco 1
1 , Paul-Drude-Institut für Festkörperelektronik, Berlin Germany, 2 Dipartimento di Fisica, Università di Roma “Tor Vergata”, Rome Italy
Show AbstractPhase change materials (PCM) have attracted in the last years the interest of research as concrete candidates for the development of storage class memories. GeSbTe alloys along the GeTe/Sb2Te3 pseudobinary line are nowadays used as the active material for non-volatile solid-state memories [1]. PCM superlattices (SLs), more complex structures made of alternating GeTe and Sb2Te3 layers, have recently demonstrated dramatically improved performance. [2] Notably, the possibility to engineer SL structures targeting to further increase the switching efficiency [3] could potentially provide a playground for the study of the switching mechanism in SL structures, which is nowadays still debated.
In this sense, materials in the Sb-Te binary system represent an intriguing option. In fact, several antimony telluride phases (Sb2+2nTe3) exist, in which Sb2n blocks are inserted between the quintuple layers which form Sb2Te3. [4] Molecular beam epitaxy (MBE) potentially allows to finely control the excess of Sb, tuning the number of van der Waals gaps per unit length as well as the lattice parameter.
Here we present the structural characterization of Sb2+xTe3/GeTe SLs grown by MBE on Sb-passivated ((√3x√3)R30°-Sb) Si (111) substrates. Starting from stoichiometric Sb2Te3, the composition of Sb2+xTe3 was controlled by increasing the atomic flux of Sb while keeping fixed the other growth parameters. The evolution of the composition from Sb2Te3 to Sb2+xTe3 was studied by X-ray diffraction (XRD) radial scans and Raman spectroscopy. Interestingly, X-ray reflectivity and atomic force microscopy data show a reduction of the Sb2+xTe3 film surface roughness as a function of the excess of Sb, which in turn improves the SL structural quality. XRD asymmetric reciprocal space maps analysis was carried out, demonstrating the systematic control of the SL in-plane lattice parameter. The characterization of the switching functionality will be carried out.
[1] S. Raoux et al., Chem. Rev. 110, 240–267 (2010).
[2] R. E. Simpson et al., Nat. Nanotechnol. 6, 501–505 (2011).
[3] X. Zhou et al., Adv. Mater. 28, 3007–3016 (2016).
[4] K. Kifune et al., Acta Crystallogr. B. 61, 492–497 (2005).
10:15 AM - ED11.9.03
Controlling the Epitaxy of 2D Bonded Sb2Te3 and 3D Bonded GeTe on Si(111)
Jamo Momand 1 , Jos Boschker 2 , Rui Ning Wang 2 , Raffaella Calarco 2 , Bart Kooi 1
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 Departement of Epitaxy, Paul-Drude-Institut für Festkörperelektronik, Berlin Germany
Show AbstractThe 2D bonded Sb2Te3 and 3D bonded GeTe compounds are of particular interest for their recently discovered topological insulator and Rashba-type splitting properties, respectively, as well as for their established applications in thermoelectrics and phase-change memories. To further study the properties of these materials, but also for the study and design of the recently discovered interfacial phase-change memories, highly textured and crystalline films are of paramount importance. In this work the epitaxial matching and texturing of MBE grown Sb2Te3 and GeTe and different Si(111) surfaces is studied using X-ray diffraction and transmission electron microscopy. It is found that, depending on the type of surface passivation, the growth onset proceeds differently in crystallite size and twist orientation, leading to a wide spectrum of crystalline quality. The findings pave way to tune the best parameters for highly textured GeTe or Sb2Te3 growth, as could be implemented in e.g. epitaxial GeTe-Sb2Te3 superlattices. More generally, the findings illustrate the particular significance of substrate-film interface bonding combined with lattice matching for the optimization of epitaxial growth of highly lattice mismatched systems.
ED11.10: Interfacial Phase Change Memories
Session Chairs
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 131 C
11:00 AM - *ED11.10.01
Designing Phase Change Memory Materials
Robert Simpson 1 , Jitendra Behera 1 , Weiling Dong 1 , Bo Tai 1 , Janne Kalikka 2 , Xilin Zhou 1
1 , SUTD, Singapore Singapore, 2 , Tampere University of Technology, Tampere Finland
Show AbstractDesigning materials with specific properties is a formidable challenge. Useful phase change materials, for example, require: a large property contrast between structural phases, fast phase transitions, stability at room temperature, and ovonic threshold switching to allow Joule heating of the amorphous state. In spite of this long property wish list, typically those developing phase change materials resort to expensive, time-consuming, and laborious Edisonian-like trial and error materials discovery and optimisation methods, where useful materials are serendipitous discoveries rather than the norm.
It has been estimated based on the laws of chemical reactivities that there are 10100 new materials waiting to be discovered, optimised, and utilised. Clearly, Edisonian methods are unsuitable for efficient exploration of this enormous compositional design parameter space.
Interfacial phase change memory materials present an opportunity to design materials with specific properties. The typical model for phase transitions in phase change memory alloys involves the crystalline state melting and then rapidly quenching into the amorphous high resistance, low reflectivity state, whilst the amorphous to crystalline transition involves annealing the amorphous material such that the structure rearranges into a lower enthalpy crystalline state. The melt-quench process means that any structural order engineered into the phase change material is soon lost after crystal to amorphous phase transition. Recently, however, a new type of phase change memory structure has been suggested where diffusive atomic transitions are confined to two dimensional layers of GeTe sandwiched in a stable crystalline Sb2Te3 scaffold structure. Since the Sb2Te3 scaffold is stable and only the GeTe layers undergo a phase transition, we can now design the Sb2Te3-GeTe superlattice structures to meet a particular property specification.
In this talk I will present our latest designs of new interfacial phase change materials and the evolutionary design methodology that we used in their optimisation.
T. C. Le and D. A. Winkler. Discovery and optimization of materials using evolutionary approaches. Chem. Rev., 2016.
R. E. Simpson, P. Fons, A. V. Kolobov, T. Fukaya, M. Krbal, T. Yagi, and J. Tominaga. Interfacial Phase-Change Memory. Nature Nanotech., 6:501 – 505, 2011.
J. Kalikka, X. Zhou, E. Dilcher, S. Wall, J. Li, and R. E. Simpson. Strain engineered diffusive atomic switching in two-dimensional crystals. Nat. Commun., 7(11983), 06 2016.
X. Zhou, J. Kalikka, X. Ji, L. Wu, Z. Song, and R. E. Simpson. Phase change memory materials by design: a strain engineering approach. Adv. Mater., 28:3007–3016, 2016.
J. Kalikka, X. Zhou, J. Behera, G. Nannicini, and R. E. Simpson. Evolutionary design of interfacial phase change van der waals heterostructures. Nanoscale, accepted, 2016
11:30 AM - ED11.10.02
Design of Highly C-Axis Oriented Bismuth Chalcogenides for Strain Engineered Interfacial Phase Change Memory
Xilin Zhou 1 , Jitendra Behera 1 , Robert Simpson 1
1 , Singapore University of Technology and Design, Singapore Singapore
Show AbstractInterfacial phase change superlattice materials are regarded as the leading candidate for next-generation non-volatile data storage technology [1, 2]. The first phase change superlattices consist of Sb2Te3 and GeTe two dimensional (2D) crystals that were stacked periodically along the growth axis. Recently, strained engineered van der Waals superlattice heterostructures, which are tuned using either Sb2Te3 or Sb2Te, demonstrated a further mechanism to control the transition speed and switching power of phase change memories [3, 4]. Bismuth chalcogenides, such as Bi2Te3 and Bi2Se3, have been widely investigated in the past decade due to their exceptional thermoelectric and topological insulating properties.
In this work we present strain tuned interfacial phase change memory devices that are based on bismuth chalcogenides superlattice structures. The highly (00l)-oriented Bi2Te3 and Bi2Se3 crystals were prepared via a self-organised van der Waals epitaxy growth technique, which provides the scaffold to form c-axis oriented Bi2Te3-GeTe and Bi2Se3-GeTe superlattice heterostructures. Moreover, the layered structures of composition-spread (Bi2Te3)x(Bi2Se3)(1-x), (Bi2Te3)x(Sb2Te3)(1-x), and (Bi2Se3)x(Sb2Te3)(1-x) films were grown by physical vapour deposition. We will discuss the crystal structure and lattice vibrations on the composition of these layered materials. This work presents new strain, compositional, and structural design of phase-change materials for specific applications in data storage, active photonics, and spintronics.
[1] J. Tominaga, et al. Role of Ge switch in phase transition: Approach using atomically controlled GeTe/Sb2Te3 superlattice. Jpn. J. Appl. Phys. 47, 5763–5766 (2008).
[2] R. Simpson et al. Interfacial phase-change memory. Nat. Nanotechnol. 6, 501–505 (2011).
[3] X. Zhou et al. Phase-change memory materials by design: a strain engineering approach. Adv. Mater. 28, 3007 (2016).
[4] J. Kallika et al. Strain-engineered diffusive atomic switching in two-dimensional crystals. Nat. Commun. 7, 11983 (2016).
11:45 AM - *ED11.10.03
Femtosecond Optical Responses from Topological Phase-Change Materials
Muneaki Hase 1 2
1 , University of Tsukuba, Tsukuba Japan, 2 , CREST, Japan Science and Technology Agency, Kawaguchi Japan
Show AbstractThe interfacial phase change memory (iPCM) material, Ge-Sb-Te based superlattice structure, has shown novel (faster speed and less power-consuming) optical and electrical properties over the conventional Ge-Sb-Te alloyed structures [1]. Recently, it has been argued that under a certain structural condition iPCM structure reveals topological insulating nature [2, 3]. To understand the underlying physical phenomena in iPCM material, we have investigated phonon and spin dynamics as responses to femtosecond optical pulse irradiation, which would determine the local atomic rearrangement and property of topological spin current on the surface. Here we discuss femtosecond structural pre-transformation of iPCM films far from equilibrium monitored by pump-pump-probe coherent phonon spectroscopy under high-density electronic excitation. The Fourier transformed phonon spectra in the excited state shows an appearance of double peak structure, demonstrating ultrafast local atomic modification of iPCM, which can be interpreted to two different atomic coordinations [4]. We also demonstrate that transient magnetization (< 1 ps) by femtosecond laser pulse is possible to occur in iPCM structure as well as in conventional GST225 alloys, although such spin dynamics seems to be different for iPCM and GST225 alloys. [1] R. E. Simpson et al., Nature Nanotech. Vol. 6, 501–505 (2011). [2] J. Tominaga et al., Adv. Mater. Interfaces, Vol. 1, 1300027 (2014). [3] B. Sa et al., Phys. Rev. Lett., Vol. 109, 096802 (2012). [4] M. Hase et al., Nature Commun. Vol. 6, 8367 (2015).
12:15 PM - ED11.10.04
New Insight on Long Range and Local Orders of GeTe/Sb2Te3 Superlattices
Philippe Kowalczyk 1 , Francesco d'Acapito 2 , Cristian Mocuta 3 , Pierre Szkutnik 4 , Nicolas Bernier 1 , Chiara Sabbione 1 , Leila Fellouh 1 , Frederic Fillot 1 , Francoise Hippert 5 , Pierre Noe 1
1 , CEA-Leti, Grenoble France, 2 , CNR-IOM-OGG c/o ESRF, Grenoble France, 3 , SOLEIL, Gif-sur-Yvette France, 4 , LTM, Grenoble France, 5 , LNCMI-EMFL-CNRS, Grenoble France
Show AbstractChalcogenide Phase Change Materials, mainly based on Ge/Sb/Te alloys, have been widely investigated in resistive-switching memory (PCRAM). Their main limitation is related to the high programming currents needed for the switching process. Recently, a new type of memory device (iPCM) has shown significantly improved performance in terms of reduced switching energies [1]. This novel class of chalcogenide-based memories involves a superlattice (SL) structure made of periodically arranged ultra-thin GeTe and Sb2Te3 crystalline layers. The mechanism of the resistance change in iPCM devices is still under debate. A short-range motion of Ge atoms at the interface of GeTe/Sb2Te3 layers is expected to produce a transition between a Dirac semimetal and a ferroelectric material [2]. However, recently HAADF-STEM observations [3] coupled to EXAFS measurements [4] on SL samples grown by MBE tend to indicate that local structure of GeTe does not correspond to the previously proposed structural models. In that context, we will give a new insight on the complex structure of GeTe/Sb2Te3 SL obtained by magnetron sputtering by means of advanced synchrotron characterizations and Raman spectroscopy. XRD allows to get information on the long range structure, whereas EXAFS and Raman spectroscopy will permit to get a better description on the local atomic order. GeTe/Sb2Te3 SL samples were deposited by magnetron co-sputtering at high temperature in an industrial cluster tool. The SL were grown on amorphous Si obtained after Ar sputtering of the surface of 200 mm Si (100) substrates [5]. The thickness of each GeTe layer was optimized at 0.7 nm to perfectly match the (GeTe)2 unit needed for a Dirac semimetal to ferroelectric material transition [2]. HAADF-STEM analysis reveals the good quality of our 24 periods SL samples with the expected lattice periodicity. Then, the structure of the SL samples is studied as a function of the thickness of Sb2Te3 spacing layer, which was fixed at 1, 2, 4 and 8 nm, respectively. Firstly, XRD data evidence a surprising change of the position of diffraction Bragg peaks depending on the Sb2Te3 sub layer thickness. Secondly, the change in the local atomic order within the SL as a function of Sb2Te3 sub layer thickness will be shown thanks to EXAFS and Raman spectroscopy. EXAFS measurements were performed at Ge-K edge and reveal a high atomic ordering around Ge atoms. Significant changes in local order are also evidenced when Sb2Te3 sub layer thickness is increased. From the present study, an alternative picture of the structure of these SL will be given and confronted to theoretical models and to previous experimental results from literature.
[1] R.E. Simpson et al., Nat. Nano. 6 (2011) 501 ; [2] J. Tominaga et al., Adv. Mater. Interfaces 1 (2014) 1300027 ; [3] J. Momand et al., Nanoscale 7 (2015) 19136; [4] B. Casarin et al., Sc. Rep. 6 (2016) 22353 ; [5] Y. Saito et al., Phys. Stat. Sol. B 252 (2015) 2151
12:30 PM - ED11.10.05
High-Endurance High-Speed Bipolar Switching of Sputtered GeTe/Sb2Te3 Superlattice iPCM
Kirill Mitrofanov 1 , Yuta Saito 1 , Noriyuki Miyata 1 , Paul Fons 1 , Alexander Kolobov 1 , Reiko Kondou 1 , Junji Tominaga 1
1 , National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan
Show AbstractRecently, phase-change memory (PCM) gained a second wind by a new achievement: it was shown that composite Ge-Sb-Te (GST) can undergo bipolar switching [1], similar to the oxide-based resistive-switching memory. In the current work the use of a GeTe/Sb2Te3 superlattice instead of composite GST was proposed for ensuring high performance bipolar and unipolar switchable memory.
It was found that the fabricated devices based on GeTe/Sb2Te3 superlattices can undergo bipolar switching in addition to unipolar switching with the use of short pulses (from 20 ns to 1 µs) up to 107 times. If after initialization, a device is switched to a SET state, it is possible to switch it to the RESET state by applying a voltage of a positive polarity, but the RESET to SET switching process can be triggered only by applying a negative voltage almost with the same amplitude as for the SET to RESET process (but with opposite polarity). Then, again in order to switch the device to a RESET state one needs to apply a positive voltage etc. This bipolar switching performance was found to be obviously superior to conventional composite GST devices, e.g. ~20 µs for GST [1] and <100 ns for the superlattice devices. The bigger difference between the SET and RESET resistances for bipolar switching over for unipolar switching, as well as a dependence of the switching voltage on the electrical pulse duration, used for bipolar switching, were observed. These features were found in the case of composite GST as well [1], which suggests the presence of possibly the same underlying mechanism. However, while in composite GST the mechanism responsible for switching is a massive migration of Ge and Sb, resulting in the depletion of these atom types around the contact layer interface, in a highly orientated superlattice structure such a mechanism is expected to lead to better switching performance due to vacancies being ordered in van der Waals gaps which provides an efficient path for the migration of Ge atoms.
In conclusion, bipolar switching was obtained in memory devices based on GeTe/Sb2Te3 superlattices. In comparison with similar effects recently reported in the literature for a composite GST, it was found that switching can be performed by electrical pulses which are a factor of 200 faster, and also the cyclability dramatically increased – by more than 3 orders of magnitude.
The work was supported by IMPULSE, AIST and CREST, JST projects. A part of this study was supported by NIMS Nanofabrication Platform in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
[1] N. Ciocchini, M. Laudato, M. Boniardi, E. Varesi, P. Fantini, A. L. Lacaita and D. Ielmini, Sci. Rep., 6, 29162 (2016).
ED11.11: Photonics
Session Chairs
Robert Simpson
Rashid Zia
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 131 C
2:30 PM - *ED11.11.01
Tunable Micro- and Nano-Structured Optical Devices Using Phase-Cange Materials
Valerio Pruneri 1 , Vahagn Mkhitaryan 1 , Miquel Rude 1
1 , ICFO–The Institute of Photonic Sciences, Castelldefels Spain
Show AbstractPhase-change materials (PCMs) are a group of chemical compounds that have two stable phases with very large contrast in their optical and electrical properties. Moreover, phase transitions in PCMs can be triggered by applying an appropriate thermal cycle, which can be supplied by electrical or optical pulses. These unique properties make them interesting for new applications in photonics. In this talk we will show how the PCM Ge2Sb2Te5 (GST) can be used in hybrid structures to demonstrate different functionalities. In particular we show an optical switch in a Si ring resonators covered with GST with 12 dB on/off ratio [1] and modulation of surface-plasmon polaritons in Au/SiO2 plasmonic waveguides, with more than 30% attenuation after crystallization of GST [2]. Using the large contrast in the optical properties we also show how GST can be used to tune EOT resonances in periodic arrays of nanoholes drilled in metallic films [3]. The transmission resonances present in these structures exhibit large shifts (385 nm) after crystallization of GST. Moreover tuning of these resonances in the ultrafast timescale (ps) is also achieved without the need for a phase transition due to a transient change in the electronic configuration of GST, as shown in [4]. Finally, using thin film interference effects and exploiting the high absorption of GST we demonstrate perfect absorbers in the visible and NIR regions of the spectrum [5].
[1] M. Rude, et. al., Appl. Phys. Lett. 103 (2013) 141119. doi:10.1063/1.4824714.
[2] M. Rude et. al., ACS Photonics. 2 (2015) 669–674. doi:10.1021/acsphotonics.5b00050.
[3] M. Rude, et. al., Adv. Opt. Mater. (2016) n/a--n/a. doi:10.1002/adom.201600079.
[4] L. Waldecker, et. al. , Nat. Mater. 14 (2015) 1–6. doi:10.1038/nmat4359.
[5] V.K. Mkhitaryan, et. al., Adv. Opt. Mater. (2016) n/a--n/a. doi:10.1002/adom.201600452.
3:00 PM - ED11.11.02
Phase-Change GeTe for Tunable Photonic Applications
Kyung-Ah Son 1 , Hwa-Chang Seo 1 , Jeong-Sun Moon 1
1 , HRL Laboratories, Malibu, California, United States
Show AbstractPhase-change materials have been used in commercial rewritable optical disks and developed for solid-state non-volatile memories beyond flash memories. Recently, phase-change GeTe is being evaluated for next-generation RF switches [1], leveraging its dramatic (>1 e5) change in electrical resistance depending on its phase: amorphous (high resistance) or crystalline (low resistance), as controlled by electrical pulses. GeTe also shows dramatic change in optical refractive index (N = n + ik) depending on its phase, which can be utilized to build electrically programmable optical switches, meta-structures and optical phase shifters. While silicon can be used for thermo-optic modulators or optical phase shifters in silicon photonics for short-wave infrared (SWIR) LIDAR applications [2], its thermo-optic coefficient is limited to dn/dT of 1.8 e-4 at room temperature.
In this talk, we report on the first thermo-optic characterization of GeTe’s phase change in the short-wave infrared from 25°C to 150°C; in addition to ellipsometry refractive index characterization of its amorphous and crystalline phases from the UV to the THz range. Phase-change GeTe shows a dramatic change in refractive index (Δn >1) depending on its phase within the infrared to THz regime. GeTe also provides a large thermo-optic coefficient at telecommunication wavelengths (dn/dT of 5.5 e-4 at 1.55-µm), which is three times greater than that of silicon. With the increased thermo-optic coefficient, GeTe optical phase shifters are designed with state-of-the-art power × delay product, which is a 10X improvement over silicon thermo-optic phase shifters.
[1] J. S. Moon et al., “11 THz figure-of-merit phase-change RF switches for reconfigurable wireless front-ends,” IEEE MTT-S Digest, pp. 1-3, 2015
[2] J. Sun et al., “Large-scale nanophotnoic phased array”, Nature, pp.195 – 199, 2013
3:15 PM - ED11.11.03
Tuneable Gap-Plasmonic Optical Antennas Enabled by Phase Change Materials
Weiling Dong 1 , Hailong Liu 1 , Robert Simpson 1 , Joel Yang 1 2
1 , Singapore University of Technology and Design, Singapore Singapore, 2 , Institute of Materials Research and Engineering, ASTAR, Singapore Singapore
Show AbstractColour arising from metallic nano structures has been used to demonstrate high resolution static colour images. In this talk, we discuss the influence of an active chalcogenide layer, Ge2Sb2Te5, that is used to tune the plasmonic frequency of metallic nano structures. Ge2Sb2Te5 is a phase change material that exhibits a large optical property contrast between its amorphous and crystalline structural phases, typically tuned by laser or electrical Joule heating. Here, we show that during the amorphous to crystalline transition, the resonant frequency of a nanopillar array composed of Ge2Sb2Te5 and aluminium is significantly red-shifted. This colour switching is demonstrated and measured by colour image analysis and microspectroscopy. The resonant frequencies of the nanostructure can be tuned over a wide range in the visible by changing the dimension of the nano disks and the structural phase of Ge2Sb2Te5. Numerical simulations are consistent with the experimentally measured reflectance, and reveal the gap antennas mechanism underlying the switchable colour. This work paves the way for chalcogenide-tuned plasmonic materials for applications in the next-generation displays and dynamic optical filters.
ED11.12: Optical Devices
Session Chairs
Harish Bhaskaran
Rashid Zia
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 131 C
4:00 PM - *ED11.12.01
Solid State Reflective Displays (SRD)
Peiman Hosseini 1 , Lokeshwar Bandhu 1 , Clement Talagrand 1 , Ben Broughton 1 , Harish Bhaskaran 2
1 , Bodle Technologies, Oxford United Kingdom, 2 Materials, University of Oxford, Oxford United Kingdom
Show AbstractThe unique light modulating properties of chalcogenide based phase change materials have recently been applied to a variety of new and exciting optoelectronic systems far beyond traditional data storage applications. In this presentation we will discuss their use in reflective type display applications where speed, resolution, large colour gamut and flexibility are beyond the technological reach of existing technologies. Reflective type displays provide readily available information to the user without the need for a bright and energy consuming backlight to be constantly on. We will present the unique challenges that must be overcome in order to adapt a material traditionally designed for data storage applications to an information display system. A number of solutions to the aforementioned challenges will be presented and analyzed.
4:30 PM - ED11.12.02
Coding Two-Dimensional Images into Mode Spectrum of Silicon Microcavity Covered with a Phase-Change Layer
Yuya Kihara 1 , Farrabi Sobhi 1 , Daichi Kataiwa 1 , Masashi Kuwahara 2 , Toshiharu Saiki 1
1 , Keio University, Yokohama Japan, 2 , National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan
Show AbstractCoding images or spatial information into a lower dimensional signal space is the most critical process in terms of time, costs and energy efficiency in intelligent information processing, including machine learning and artificial intelligence. In particular in the sparse coding for deep learning, it is essential to find a set of bases which can reconstruct images with a small number of bases. In this study we propose an approach to encode a two-dimensional subwavelength pattern (image) into an infrared optical spectrum. A silicon microcavity is used to generate light confinement modes with a variety of field distributions for a wide wavelength range of resonant peaks. An amorphous phase-change (GeSbTe; GST) film, which covers the top of microcavity, memorizes the two-dimensional pattern projected on the surface of microcavity by allowing partial crystallization and thereby modifies the optical spectrum due to a significant change in refractive index of the phase-change layer.
We performed finite-difference time-domain simulation to calculate absorption spectra of GST (50 nm in thickness) layer and electric field distributions inside the Si microcavity (elliptical cylindrical cavity: 4 μm x 6 μm in lateral dimensions and 2.2 μm in height) for each cavity mode. We illustrated how the absorption spectrum is modified through the partial crystallization of amorphous GST film for different specific spatial patterns. Depending on the degree of matching of the pattern with field distribution of cavity modes, distinct differences in spectrum modification are confirmed.
For experimental demonstration we fabricated a silicon microcavity with a diameter of 10 μm and a height of 2.2 μm by applying a silica microsphere lithography method to a silicon-on-insulator substrate. A GST film with a thickness of 50 nm was deposited on the top surface of microcavity by sputtering coating. In FTIR absorbance spectra for single microcavites with amorphous GST, clear peaks due to resonant scattering and absorption by the microcavity were obtained. After uniform crystallization of the GST layer, deeper and red shifted peaks were observed because of the larger refractive index and extinction of crystalline GST.
These theoretical and experimental results demonstrate the possibility for encoding device applications. In particular, the modification of field distribution for individual cavity modes by partial phase change enables autonomous optimization of dictionary (basis set) learning for sparse coding.
4:45 PM - ED11.12.03
Phase-Change Films for Thermally-Tunable Ultrasensitive Infrared Absorption Spectroscopy
Gokhan Bakan 1 , Sencer Ayas 1 , Erol Ozgur 1 , Kemal Celebi 1 , Aykutlu Dana 1
1 , Bilkent University, Ankara Turkey
Show AbstractThe reversible changes in the optical properties of the phase-change materials have made the rewritable optical storage possible which has revolutionized the dissemination of data since 1990s. After a long gap, many other active photonic applications employing thin-phase films have been demonstrated(1)-(4), mostly because the phase-change materials have been in the spotlight for their recently commercialized application as phase-change memory devices. Here we demonstrate the use of Ge2Sb2Te5 (GST), the most commonly used phase-change material, for thermally-tunable infrared absorption spectroscopy. The demonstrated surfaces consist of patternless thin GST films on Al films/foils and use the uniform field enhancement. 10 nm poly(methyl methacrylate) (PMMA) films on such surfaces are sensed with larger infrared absorption signals than those observed for the plasmonic structures in the literature. Crystallization of the GST layer by simply annealing on a hot plate enables sensing infrared absorption bands at higher wavelengths. A similar functionality, i.e., tuning the optical response, for ultrasensitive infrared absorption spectroscopy platforms has recently been demonstrated using plasmonic surfaces employing patterned graphene by D. Rodrigo et al., Science 349, 2015 and H. Hu et al., Nat. Commun. 7, 2016. In contrast to these recent reports, the demonstrated surfaces offer a much simpler fabrication route and tuning mechanism(5).
References:
(1) Cao, T.; Wei, C.; Simpson, R. E.; Zhang, L.; Cryan, M. J. Sci. Rep. 2014, 4, 3955.
(2) Michel, A.-K. U.; Chigrin, D. N.; Maß, T. W. W.; Schönauer, K.; Salinga, M.; Wuttig, M.; Taubner, T. Nano Lett. 2013, 13 (8), 3470–3475.
(3) Hosseini, P.; Wright, C. D.; Bhaskaran, H. Nature 2014, 511 (7508), 206–211.
(4) Bakan, G.; Ayas, S.; Saidzoda, T.; Celebi, K.; Dana, A. Appl. Phys. Lett. 2016, 109(7), 071109.
(5) Bakan, G.; Ayas, S.; Ozgur, E.; Celebi, K.; Dana, A. ACS Sensors, 2016, 1(12), 1403-1407.
5:00 PM - ED11.12.04
Tunable Dielectric Metadevices Enabled by Phase-Change Materials
Arseny Alexeev 1 , S. Garcia-Cuevas Carrillo 1 , C. Ruiz De Galarreta 1 , E. Gemo 1 , S.V. Makarov 2 , V.A. Milichko 2 , Karthik Nagareddy 1 , A.K. Samusev 2 , I.S. Sinev 2 , C. David Wright 1 , D.A. Zuev 2
1 , University of Exeter, Exeter United Kingdom, 2 , ITMO University, St. Petersburg Russian Federation
Show AbstractDiverse materials readily supplied by nature have been extensively studied since the birth of humanity and found their use in many technology areas, ranging from construction to nanoengineering. Although at the current stage of technological progress we have developed very advanced devices that allowed for significant improvements to our quality of life, further progress requires materials with properties that conventional ones do not possess.
Metamaterials, an artificial media comprised of resonating elements (i.e. metaatoms) structured on the subwavelength scale, have emerged as a brand new material class with properties that can be engineered on demand [Glybovski et al., Physics Reports 634, 1 (2016)]. Various devices which would not otherwise be feasible, such as negative-index superlens or cloaking surfaces, have been created using metamaterials in the recent years. Advanced functionality of the metamaterials-based devices can be achieved if metaatom-wave interaction is controlled externally and thus enabling different operating regimes.
We enable tunability, i.e. switching between states with discrete functionality, of dielectric metadevices by combining them with chalcogenide phase-change materials. Under external stimulus, such as voltage or laser pulse, phase-change materials undergo phase transition resulting in a change of their electrical and optical properties [Hosseini et al., Nature 511, 206 (2014)]. Though phase-change materials can be used for creating tunable metamaterials on their own or in combination with metals, their applications are somewhat limited due to the increased losses outside of the infrared region and challenges in reliably switching areas larger than one square micron.
Following the above described concept we model and fabricated a number of hybrid dielectric-phase-change metadevices, such as selective frequency selective filters and dynamic beam steerers.
Symposium Organizers
Stefania Privitera, Consiglio Nazionale delle Ricerche (CNR)
Harish Bhaskaran, University of Oxford
Eric Pop, Stanford University
Yuta Saito, National Institute of Advanced Industrial Science and Technology (AIST)
ED11.13: Optical Devices Based on VO2
Session Chairs
Harish Bhaskaran
Peiman Hosseini
Friday AM, April 21, 2017
PCC North, 100 Level, Room 131 C
9:30 AM - *ED11.13.01
Modulating Emission at Sub-Lifetime Speeds: Phase-Change Materials for High-Speed Sources
Rashid Zia 1 , Sebastien Cueff 1
1 , Brown University, Providence, Rhode Island, United States
Show AbstractThe term “phosphorescence” is generally used to describe the slowly decaying luminescence associated with parity-forbidden and/or spin-forbidden optical transitions. Despite their “slow” and “forbidden” nature, phosphors play an important role in a range of modern device technologies from displays and lighting to lasers, sensors, and telecommunication. Interestingly, the same electronic structure that makes phosphors slow emitters often helps ensure that they are highly efficient. Nevertheless, their slow radiative decay rate is generally perceived as a technological limit for high-speed photonic devices.
In this talk, we will demonstrate how phase-change materials (PCMs) can help transform “slow” phosphorescent emitters into high-speed light sources, while preserving their high quantum efficiencies. Specifically, we will present experimental data that shows dynamic control of light emission orders of magnitude faster than the lifetime limit. In contrast to traditional pump-based modulation of the excited-state population, we use PCMs to control the local optical environment and directly modulate emission into different modes. For example, we will show how the insulator-to-metal transition of vanadium dioxide can be used to directly modulate the multipolar emission of erbium ions at telecom wavelengths. More broadly, we will discuss how PCMs can be used to modulate a range of photonic devices from on-chip sources to adaptive optical sensors.
10:00 AM - ED11.13.02
Probing Metal-Insulator Transitions in VO2 with Ultra-Narrow Carbon Nanotube Electrodes
Stephanie Bohaichuk 1 , Gregory Pitner 1 , Feifei Lian 1 , Jaewoo Jeong 2 , Mahesh Samant 2 , Stuart Parkin 2 , H.-S. Philip Wong 1 , Eric Pop 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 , IBM Almaden Research Center, San Jose, California, United States
Show AbstractVO2 has one of the largest changes in resistance among materials that exhibit a metal-insulator transition (MIT), at a temperature accessible in typical electronics (~65 °C). This is accompanied by a structural phase transition, and by changes in thermal and optical properties, which reverse once the stimulus is removed. Reducing the dimensions of VO2 devices should offer faster, lower power switching. A 10 nm cubic VO2 device could have switching energy <0.5 fJ and sub-nanosecond switching time [1, 2]. However, switching of VO2 at truly nanoscale dimensions has never been characterized.
In this work, we demonstrate for the first time switching of VO2 induced using carbon nanotube (CNT) electrodes. Due to their ~1 nm diameter, metallic CNTs are ideal candidates for probing nanomaterials and nanoscale phase change or MIT, as Joule heaters or electrodes [3]. A CNT can be used as a heater (reaching ~600 °C in air and ~2000 °C in vacuum [4]) to switch VO2 in contact with it.
We fabricate these devices, electrically demonstrating VO2 switching. We grow aligned single-wall CNTs via chemical vapor deposition on ST-cut quartz and transfer them [5] onto thin films of VO2 grown epitaxially on TiO2 substrates. Two-terminal devices are patterned to isolate only one or two metallic CNTs. Two different device geometries are explored, one where the continuous CNT is simply used to heat up the VO2 beyond its transition temperature, and the other relying on a “nanogap” CNT geometry [3] previously used to probe phase change in ~10 nm scale Ge2Sb2Te5 films.
We also developed a 3D electro-thermal finite element model in COMSOL to understand the MIT induced by CNTs on the VO2. The simulations confirm that Joule heating along the length of a CNT on top of a VO2 thin film can easily induce the VO2 transition. In addition, simulations reveal that a metallic region of VO2 is formed directly under the CNT, which widens from a few nanometers to tens of nanometers, and grows along the length of the CNT if the voltage is increased. Once the metallic region extends to the metal contacts to the CNT, a sharp increase in current is simulated and measured.
In summary, we present the first study probing nanoscale metal-insulator transitions in VO2 using CNT electrodes. Such devices examine the limits of the MIT phenomenon, and could lead to improved understanding and more energy-efficient devices relying on MIT.
[1] Y. Zhou et al., Proc. IEEE 103, 8 (2015).
[2] M. Jerry et al., IEEE SNW, June 2016.
[3] F. Xiong et al., Nano Lett. 13, 464 (2013).
[4] A. Liao et al., Phys. Rev. B 82, 205406 (2010).
[5] N. Patil et al., IEEE Trans. Nanotechnol. 8, 498 (2009).
10:15 AM - ED11.13.03
Electro-Thermal Control of Vanadium Dioxide Multilayered Thin Film Phase Change Material by Degenerate Semiconductor for Smart-Device Applications
Aswini Pradhan 1 , Jonathan Skuza 1
1 , Norfolk State University, Norfolk, Virginia, United States
Show AbstractWe demonstrate a large static and dynamic tunability in wavelengths up to 640 nm in Al-doped ZnO (AZO) based transparent conducting degenerate semiconductors by controlling both thickness and applied voltages. This extreme tunability is ascribed to an increase in carrier concentration with increasing thickness as well as voltage-induced thermal effects. These observations could pave the way for optical manipulation for potential transformative opticalapplications. We also demonstrate the high-performance, stable and high-saturated transparent heaters based on AZO showing plethora of applications. The electro-thermal control of AZnO on VO2 multilayered thin films, where the application of a small electric field enables precise control of the applied heat to the VO2 layer to induce its semiconductor-metal transition (SMT). the electro-thermal control of a multilayered AZnO/VO2 thin-film device was demonstrated, where the SMT of the VO2 thin film was induced by applying a small potential (< 3 V) across theAZnO film. This electro-thermal energy provided by the Al:ZnO film can be finely tuned using its transparent conducting oxide properties to significantly control the VO2 SMT and its associated electrical and optical properties. The AZnO film acts as a transparent window and heater, serves as a protective capping layer to the VO2 thin film, and aids in decreasing the VO2 transition temperature. These results have significant impacts on technological applications for both passive and active devices by exploiting this near-room-temperature SMT, particularly active smart devices.
This work is supported by NSF-CREST.
11:00 AM - ED11.13.04
Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films
Clayton Dahlman 1 , Gabriel LeBlanc 1 , Amy Bergerud 1 2 , Delia Milliron 1
1 , University of Texas at Austin, Austin, Texas, United States, 2 , University of California, Berkeley, Berkeley, California, United States
Show AbstractVanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. The low-temperature state is insulating and transparent, while the high-temperature state is metallic and colored in the infrared (IR). In the past few years, alternative stimuli besides temperature have been used to trigger insulator-metal transformations in VO2, including electrochemical gating. This effect has been found to depend on film geometry, orientation and strain, and recent efforts have focused on determining the mechanisms behind the electrochemical transformation in epitaxial VO2 thin films. A novel heterostructure of VO2 has been prepared through solution deposition of vanadium oxide nanocrystals into a mesoporous thin film. V2O3 nanocrystals are synthesized by a colloidal solution method, yielding monodisperse 25 nm particles. Mesoporous VO2 is then produced from spin-coated colloidal V2O3 nanocrystal films upon oxidative annealing. These films demonstrate how interfacial heterostructure of VO2 imparts new electrochemical and optical functionality.
The electrochromic behavior of nanocrystalline VO2 films is explored with in situ optical transmission measurements in a temperature-controlled three-electrode electrochemical cell. As expected from prior gating experiments, electrochemical reduction causes a reversible transition from the insulating, IR transparent monoclinic state to a reduced IR darkened state with higher conductivity and minor structural distortions. However, an unexpected additional transition from this darkened state to a unique insulating, IR transparent phase occurs upon further electrochemical reduction. This sequential insulator–metal–insulator transition has not been reported in previous studies of gated epitaxial VO2 films, and demonstrates a remarkable new functionality attributable to the film’s nanocrystalline mesostructure. To investigate the role of mesoporous structure on this transformation, nanocrystal grain size was varied between 20 and 50 nm. The kinetics of the metal-insulator transition were found to be highly dependent on grain size, such that larger microstructures were unable to transition to the super-reduced insulating phase on experimentally feasible time-scales. This unexpected insulating phase is found to have a rutile-like crystal structure with decreased oxygen stoichiometry (VO2-x) and an expanded, high-symmetry unit cell by X-ray spectroscopy and X-ray diffraction techniques. The role of isotropic nanocrystal interfaces and heterogeneous strain fields on this transformation are explored, and placed in context with recent studies of conventional VO2 thin films and microstructures. This unique preparation of VO2 thin films demonstrates the role of interfacial mesostructure on functional properties in a promising form factor for electrochromic, information storage, or other thin film device applications.
11:15 AM - ED11.13.05
Antenna-Assisted Picosecond Control of Nanoscale Phase-Transition in Vanadium Dioxide
Otto Muskens 1 , Luca Bergamini 2 3 , Bigeng Chen 1 , Dan Traviss 1 , Yudong Wang 1 , Nerea Zabala 2 , Javier Aizpurua 3 , Jeffrey Gaskell 4 , David Sheel 4 , Cornelis de Groot 1
1 , University of Southampton, Southampton United Kingdom, 2 , Department of Electricity and Electronics, FCT-ZTF, UPV-EHU, Bilbao Spain, 3 , Materials Physics Center, CSIC-UPV/EHU and DIPC, San Sebastian Spain, 4 , Materials and Physics Research Centre, University of Salford, Manchester United Kingdom
Show AbstractPhase-change materials offer unique opportunities in tunable optical devices as they can combine very large changes in the dielectric response with fast, multi-level switching. Next to global switching of a phase transition in a material, selective addressing of only a small, nanoscale volume is of interest to achieve low-power devices with small footprint. Nanoantennas offer a unique capability to concentrate energy into nanometer-sized volumes using plasmonic near-field enhancement. In this work, we exploit antenna-assisted effects to achieve selective switching of a phase transition in plasmonic hot-spots [1]. Vanadium dioxide (VO2) provides an ultrafast, reversible insulator-to-metal phase transition (IMT) at only modestly elevated temperatures around 68°C. Experiments on both antenna arrays and single antennas show that antenna-assisted, picosecond optical excitation provides a new pathway to drive the insulator-to-metal phase transition in nanometer-scale pockets of VO2. Antenna-assisted optical pumping results in >20 times lower switching energies and 5 times faster cycling rates compared to bulk VO2. Optical experiments are confirmed by detailed numerical modelling of the picosecond phase transition including antenna-assisted absorption and nanoscale heat transport, showing selective nucleation of the IMT around the antenna end caps that is driven by resonance excitation of the plasmonic antenna mode. Next to providing a selective excitation mechanism, the antennas offer a sensitive readout of the phase transition through its plasmon resonance, providing a modulation of the antenna cross section of up to 100%. We demonstrate multi-level all-optical switching of antenna resonances at over 1 million cycles per second and picojoule energy levels, opening up the possibility for plasmonic memristor-type devices.
[1] O. L Muskens, L. Bergamini, Y. Wang, J. M. Gaskell, N. Zabala, C. H. de Groot, D. W. Sheel, J. Aizpurua, Antenna-assisted picosecond control of nanoscale phase-transition in vanadium dioxide, Light: Science & Appl. 5, e16173 (2016)
11:30 AM - ED11.13.06
Control of Phase Transition Properties of Vanadium Dioxide Thin Film for Thermal Biosensor
Soo Deok Han 1 2 , Bo Yun Kim 2 , Chul Jin Cho 1 , Sang Tae Kim 1 , Seong Keun Kim 1 , Sahn Nahm 2 , Chong-Yun Kang 1 2
1 Center for Electronic Materials, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 KU-KIST School of Converging Science and Technology, Korea University, Seoul Korea (the Republic of)
Show AbstractWith the unique electrical and optical properties accompanied by metal-insulator transition (MIT) near room temperature, vanadium dioxide (VO2) has attracted a large number of potential applications such as a thermochromic glass, optical switches, memristor, FET or sensors. In this study, we demonstrate the possibility of tuning the phase transition properties of VO2 thin films so that they may be used as transducers for thermal biosensors. The requirements for a successful thermal biosensors include tunability of metal-insulator transition temperature (TMI) to the target temperature, preferably within bio-molecule compatible temperature range and minimal thermal hysteresis between cooling and heating for reliable sensing performance. Furthermore, a large difference in electrical resistance during MIT is desired. We fabricated VO2 thin films on a highly textured TiO2 thin film as buffer layer by physical vapor deposition. The tensile stress developed from lattice mismatch on buffer layer and the intermixing of Ti and V via diffusion resulted in the decreased TMI and a thermal hysteresis less than 0.5K. The resistance change during MIT was measured to be in the more than 105 Ω range between the room temperature to 75°C. We also note that the wafer-scale fabrication method used in this work is scalable, especially because only room temperature sputtering and post-annealing was used. With these results, we discuss the feasibility of using VO2 thin films as a novel concept of thermal biosensors with high performance.
11:45 AM - ED11.13.07
Thermal Transistor Based on the Hysteresis of VO2
Jose Ordonez-Miranda 1 , Younes Ezzahri 1 , Jeremie Drevillon 1 , Karl Joulain 1
1 , Institut Pprime CNRS, Futuroscope France
Show AbstractGuiding, amplification, and control of electrical and thermal currents are of critical importance to
efficiently manage the energy resources present in nature. In electricity, this has been done with
diodes and transistors, which have allowed the development of almost all modern electronics,
while the conception of the thermal diode/transistor has recently emerged. These fundamental
thermal devices are based on the dielectric-metal transition of phase change materials (PCMs)
and were reported to provide fine control on heat currents, however up to date, not so much
attention has been put neither on the intrinsic thermal hysteresis of PCMs nor on temperature
control, which are considered in the present work.
The objective of this work is to theoretically demonstrate that a thermal transistor with a PCM
base can be used as a thermal device for heating and cooling. This is done by exploiting the
effect of the PCM thermal hysteresis on the heat fluxes that the base exchanges with the collector
and emitter of the transistor. Based on the principle of energy conservation, we show that these
heat fluxes and the base temperature undergo significant jumps under a small modification of the
heat flux applied to the base. When the collector and emitter of the transistor operate at 350 K
and 300 K, respectively, a temperature jump of +18 K (-5 K) and a coefficient of performance of
58% (32%) are obtained during the heating (cooling) of a VO2 base excited with 208 Wm-2 (63
Wm-2). This sizeable thermal effect is dominated by the photon heat current and could provide an
alternative and/or complement to the present refrigeration and heating technologies involved in
freezers, microwave ovens, and radiators used at home.