Symposium EP07 : Phase-Change Materials and Their Applications---Memories, Photonics, Displays and Non-von Neumann Computing

Phase change media utilize a remarkable property portfolio including the ability to rapidly switch between the amorphous and crystalline state, which differ significantly in their properties. This material combination makes them very attractive for data storage applications in rewriteable optical data storage, where the pronounced difference of optical properties between the amorphous and crystalline state is used. This unconventional class of materials is also the basis of a storage concept to replace flash memory. This talk will discuss the unique material properties, which characterize phase change materials. In particular, it will be shown that only a rather small group of materials utilizes a unique bonding mechanism (‘Bond No. 6’), which can explain many of the characteristic features of crystalline phase change materials. Different pieces of evidence for the existence of this novel bond will be presented. This insight is subsequently employed to predict systematic property trends and to explore the limits in stoichiometry for such memory applications. It will also be demonstrated how this concept can be used to tailor the electrical and thermal conductivity of phase change materials. Yet, the discoveries presented here also force us to revisit the concept of chemical bonds and bring back a history of vivid scientific disputes about ‘the nature of the chemical bond’.

Finite element models are useful for understanding and designing phase change materials and devices. Models capturing crystal nucleation, growth, and amorphization; phase, temperature, and electric field dependent material parameters; and interfacial effects have been reported with each model achieving some level of accuracy and efficiency [1]–[6]. 2D simulations with a fixed out-of-plane depth can capture stochastic nucleation and dynamic grain boundary effects in thin structures but may not appropriately model current densities or thermal profiles. 2D rotational simulations correctly model electrothermal profiles in inherently symmetric scenarios but cannot model discrete nucleation. 3D simulations can model crystallization dynamics or electrothermal profiles most accurately, but computational complexity limits fully coupled simulations of both. We explore the role of dimensionality on accuracy and computational cost with our finite element model which captures phase change, stochastic nucleation, temperature and electric field dependent material parameters, and interfacial effects. We analyze 2D, 2D rotational, and 3D simulations of crystallization, electrical reset and set, and ovonic switching. For 2D modeling, we introduce a variable out-of-plane depth which improves eletrothermal accuracy. We find 2D rotational simulations appropriate for ovonic switching in some scenarios but show that 3D simulations are required to properly model filament dynamics even in some apparently symmetric cases. Specifically, we observe a filament form in the center of an ovonic threshold switch, migrate to the outside of the device, and continuously revolve around the device edge, suggesting instability in a symmetric device.

[1] Z. Woods and A. Gokirmak, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part I—Effective Media Approximation,” IEEE Trans. Electron Devices, vol. 64, no. 11, pp. 4466–4471, Nov. 2017.
[2] Z. Woods et al., “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part II--Discrete Grains,” IEEE Trans. Electron Devices, pp. 1–7, 2017.
[3] A. Faraclas et al., “Modeling of Thermoelectric Effects in Phase Change Memory Cells,” IEEE Trans. Electron Devices, vol. 61, no. 2, 2014.
[4] J. P. Reifenberg et al., “The Impact of Thermal Boundary Resistance in Phase-Change Memory Devices,” IEEE Electron Device Lett., vol. 29, no. 10, pp. 1112–1114, Oct. 2008.
[5] H. L. Lung et al., “A novel low power phase change memory using inter-granular switching,” in 2016 IEEE Symposium on VLSI Technology, 2016, pp. 1–2.
[6] Y. Yin et al., “Finite Element Analysis of Dependence of Programming Characteristics of Phase-Change Memory on Material Properties of Chalcogenides,” 2006.

In a world of increasing demand for energy-efficient and portable devices, the need for fast, non-volatile memory continues to grow. The advent of interfacial phase change memory based upon van der Waals (vdW)-bonded GeTe/Sb₂Te₃ superlattices (SL) has yielded order of magnitude faster switching rates and lower energy consumption compared with comparable alloy-based devices and now constitutes a major new research area in phase-change memory (PCM). Unlike conventional alloy-based PCM where the SET and RESET states correspond to the crystalline and amorphous phases, in SLs both the SET and RESET states remain crystalline. While in earlier work the superior performance of SLs was attributed with the reduction of entropic loses associated with the quasi one-dimension motion of Ge atoms across GeTe/Sb₂Te₃ interfaces, recent experimental transmission electron microscopy studies have revealed that the GeTe and Sb₂Te₃ blocks intermix during the growth of the GeTe phase challenging the initial conclusion and raising new fundamental issues. In the current work, we suggest that the switching process is associated with the reconfiguration of vdW gaps along with concomitant deviations in the local stoichiometry from the GeTe/Sb₂Te₃ quasibinary alloys. The proposed model resolves the existing controversies and at the same time explains why the large conductivity contrast between the SET and RESET states is unaffected by Ge/Sb intermixing while providing a new perspective for the development of SL-based PCM. The proposed concept of vdW gap reconfiguration bay also be applicable to designing a wide range of engineered two-dimensional solids.

In this work, we discuss theoretical predictions for the amount of energy required to drive a structural phase transformation in two-dimensional materials via electrostatic gating. Structural phase-change materials are of great importance for their applications in energy and information storage devices. Typically, thermally driven structural phase transitions are employed in phase-change memory to achieve lower programming voltages and potentially lower energy consumption than mainstream nonvolatile memory technologies. However, the energy consumption and waste heat generated by such thermal mechanisms is often not optimized, and could present a limiting factor to widespread use. The potential for electrostatically driven structural phase transitions has recently been predicted and subsequently experimentally reported in some two-dimensional materials, providing a novel and athermal mechanism to dynamically control the properties of the materials in a nonvolatile fashion while achieving potentially lower energy consumption. In this work, we employ DFT-based calculations to make the first theoretical comparison of energy consumption required to drive a phase transition for the thermally-driven and electrostatically-driven mechanisms. Determining theoretical limits in monolayer MoTe₂ and thin films of Ge₂Sb₂Te₅, we find that the energy consumption per unit volume of the electrostatically driven phase transition in monolayer MoTe₂ at room temperature is at most 9% of the adiabatic lower limit of the thermally driven phase transition in Ge₂Sb₂Te₅. Furthermore, experimentally reported energy consumption of Ge₂Sb₂Te₅ is 100–10,000 times larger than the adiabatic lower limit, leaving the possibility for energy consumption in monolayer MoTe₂-based devices to be several orders of magnitude smaller than Ge₂Sb₂Te₅-based devices.

On-chip all-optical switching of phase-change materials allows for non-volatile, sub-diffraction limit, and low insertion-loss reconfigurable photonic devices. This novel platform has already enabled non-volatile multilevel memories [1], an optical synapse [2], 1x2 optical switches [3], and on-chip computing [4] despite limited knowledge on the actual mechanism governing the phase switching. In this talk, we present a computational analysis of the switching behaviour of Ge₂Sb₂Te₅ placed onto photonic waveguides and compare with experimental findings. In particular, we study the precise and reliable control of the amorphization and crystallization processes by means of evanescent field coupling between the confined mode and phase-change material. Furthermore, we study of the unique deterministic control of intermediate states, which enables the multilevel operation in this type of device. From a better understanding of the phase switching process, we then propose optimized parameters and geometries to improve the operation speed and energy consumption. The fundamentals of the switching mechanism offer a clearer perspective on the applicability and limitations of phase-change materials in on-chip reconfigurable optics and unconventional computing architectures.

[1] C. Ríos, M. Stegmaier, P. Hosseini, D. Wang, T. Scherer, C.D. Wright, H. Bhaskaran, W.H.P. Pernice. Integrated all-photonic non-volatile multi-level memory. Nat. Photonics. 9, 725–732. 2015.
[2] Z. Cheng, C. Ríos, W.H.P. Pernice, C.D. Wright, H. Bhaskaran, On-chip photonic synapse, Sci. Adv. 3 (9), e1700160. 2017.
[3] M. Stegmaier, C. Ríos, H. Bhaskaran, C.D. Wright, W.H.P. Pernice, Nonvolatile All-Optical 1X2 Switch for Chipscale Photonic Networks, Adv. Opt. Mater. 5 (1), 1600346, 2017.
[4] J. Feldmann, M. Stegmaier, N. Gruhler, C. Ríos, H Bhaskaran, C.D. Wright, W.H.P. Pernice. Calculating with light using a chip-scale all-optical abacus. Nat. Commun. In press, 2017.

Terahertz (THz) wave is a powerful experimental tool for studying a metal-insulator transition in phase change materials [1]. In addition, a pulse of THz wave can be used for fast material control technique by exploiting unique light-matter interaction without interband electronic transition. Indeed, sub-picosecond threshold phase switching has been realized in Ag-In-Sb-Te by application of intense THz field [2]. In this study, we irradiated thin films of crystalline (cubic) Ge₂Sb₂Te₅ (GST) and GeTe/Sb₂Te₃ interfacial phase change memory (iPCM) material with intense THz pulse train by employing a free electron laser (FEL). This FEL emits intense THz macropulses which consist of train of ∼150 micropulses with 4 THz center frequency, 10 ps duration, and 37 ns interval. It was found that a significant volume expansion due to peculiar damage or precursor of ablation was formed at the center of the THz-irradiated area in GST samples. A surface profile measurement revealed that this volume expansion is completely different from conventional amorphization in the context of the phase change material study. Furthermore, a fine ripple patterns, whose spatial periods are smaller than excitation wavelength, were observed at the outside of the expanded area. This ripple pattern is thought to be a sort of laser induced periodic surface structure (LIPSS). Among LIPSSs, the ripples overbid in this study can be classified as low-spatial-frequency LIPSS (LSFL) by considering the fact that the refractive index of cubic GST is relatively high and hence the effective wavelength at the sample is estimated to be comparable to the spatial period of ripples. In general, the threshold fluence of laser-induced phase change is lower than that of a damage. However, any consequence of phase change was not induced when the fluence was lower than the damage threshold. This result manifests that the application of intense electrical field introduced by a THz pulse train cannot induce amorphization nor further crystallization in cubic GST under the current experimental condition and implies that electrically- and photo-excited incubation states play an important role for amorphization in GST as previously reported [3,4]. On the other hand, in iPCM sample, a different THz-induced pattern without ripple was observed. We attributed this result to the difference in the in-plane electrical conductance.

[1] V. Bragaglia et al., Scientific Reports 6, 28560 (2016).
[2] P. Zalden et. al. Phys. Rev. Lett. 117, 067601 (2016).
[3] P. Fons et. al. Phys. Rev. B 82, 041203 (R) (2010).
[4] D. Loke et. al. Science 336, 1566 (2012).

Among various materials utilized in nanophotonics and nanoelectronics, an increasing attention has been given to the phase change materials (PCMs) with emerging application in resistive switching memories and reconfigurable metadevices. PCMs are desirable due to their reversible phase shift, high endurance, fast switching, and large data retention. Ge2Sb2Te5 (GST) as a chalcogenide PCM is a nonvolatile and bi-stable material with two distinct properties at amorphous and crystalline states.
Owing to the phase transition of GST by both optical and electrical stimuli, it has been integrated into optoelectronic circuits, nonvolatile memories, and tunable photonic metadevices. We introduce a GST-based metamaterial programmable perfect absorber (MPA) and resistive switching memory (ReRAM) with metal-insulator-metal (MIM) structure that contains a sandwiched GST as a high-dielectric material between a metal-coated substrate and square-patterned arrays of the metal electrode. The bottom electrode of ReRAM works as a mirror to eliminate the transmittance, while the patterned metallic layer (top electrode of ReRAM ) acts as plasmon-resonators for MPA. The nanoarrays convey an electric reply relevant to the grounded bottom electrode by intensely coupling to the electric field at a specific resonance frequency. Additionally, the MPA shows promising ReRAM characteristics with reliable and reproducible multi-level storage ability. This work is an attempt to fabricate a reconfigurable and programmable MPA and ReRAM in a simple MIM structure. The structure is analogous to Fabry-Perot (FP) etalon with an ultrathin cavity according to the top electrode arrayed nanoparticles. Furthermore, the reliability of the GST-based nonvolatile device is confirmed by measuring the data retention. The multifunctional nanodevice based on GST has promisingly satisfied the ReRAM and MPA functional requirements. Detailed simulation, fabrication, and characterization of the ReRAM and MPA will be discussed in this presentation.

The talk overviews experimental and theoretical studies of terahertz (THz) radiation induced second order opto-electronic phenomena in three dimensional topological insulators (TI). Two kinds of direct photocurrents are discussed: the photogalvanic [1-5] and the photon drag effect [6].

It will be shown, that photogalvanic spectroscopy, like previously used for study of DF in graphene [7], is caused in (Bi,Sb)Te based 3D TIs by asymmetric scattering of the Dirac fermions, driven in the applied alternating electric field. It will be shown, that photogalvanic spectroscopy opens up new opportunities for probing Dirac fermions in (Bi,Sb)Te based 3D TIs even in materials with substantial conductance in the bulk. It allows insight into the scattering details and mechanism of high frequency conductivity of the surface states [2,6], can be applied to study carrier dynamics, to map the domain orientation and to verify the homogeneity of the electronic properties of the surface states [5]. An advantage of photogalvanic spectroscopy is that it probes, due to symmetry arguments, only the surface states - even at room temperature, where classical magneto-transport techniques can be hindered by a high bulk carrier concentration. The competing photon drag effect - resulting from the additional transfer of the light momentum to charged carriers - is caused by the dynamical momentum alignment by time and space dependent radiation electric field and includes different scattering probabilities for different half periods of the electromagnetic wave [6]. The nonlinear transport phenomena are discussed in terms of a model, a phenomenological and a microscopic theory. The latest state of the art in this field, possible applications and an outlook will be presented.


[1] J. W. McIver et al., Nature Nanotech.7, 96, (2012).
[2] P. Olbrich et al., Phys. Rev. Lett. 113, 096601, (2014).
[3] K. N. Okada et al., Phys. Rev. B 93, 081403 (2016).
[4] Y. Pan et al., arXiv:1706.04296v1 (2017).
[5] H. Plank et al., J. Appl. Phys. 120, 165301 (2016).
[6] H. Plank et al., Phys. Rev. B 93, 125434 (2016).
[7] M. M. Glazov and S.D. Ganichev, Phys. Rep. 535, 101 (2014).

Optical phase change materials (O-PCMs) are a unique class of materials which exhibit extraordinarily large optical property change (e.g. refractive index change Dn > 1) when undergoing a solid-state phase transition. These materials, exemplified by Mott insulators such as VO₂ and chalcogenide compounds, have been exploited for a plethora of emerging applications including optical switching, photonic memories, reconfigurable metasurfaces, and non-volatile display. These traditional phase change materials, however, generally suffer from large optical losses even in their dielectric states, which fundamentally limits the performance of optical devices based on traditional O-PCMs. In this talk, we will discuss our progress in developing O-PCMs with unprecedented broadband low optical loss and their applications in novel photonic systems, such as high-contrast switches and routers towards a reconfigurable optical chip – the optical analog of electronic field-programmable gate arrays (FPGAs).

Maintaining the indoor temperature of industrial and residential buildings consumes a large portion of the energy budget in developed countries, ranging from 20% to 40%. This energy consumption is particularly high in regions of the globe that experience large swings in the environmental temperature from the winter to summer months. To improve efficiency, it is highly desirable to harvest solar radiation in the winter and reflect it in the summer—an impossibility for materials with fixed thermal and optical properties. Using switchable materials to maximize the energy efficiency of buildings throughout the year has a significant environmental and economic advantage to this sector.

Windows are a particularly promising architectural component for targeting efficiency gains. For example, a significant amount of heat is lost through windows in the winter season—as much as 25% in the US and 50% in northern China. “Low-E” coatings can be used to reduce heat transfer, but cannot be actively switched to make use of the near-infrared solar spectrum in winter months. Electro-chromatic smart windows are difficult to fabricate on curved surfaces, require a continuous electric field in the “on” state, and often have unwanted colouration, whereas VO₂-based windows contain toxic materials. A smart window technology is needed that is inexpensive, switches in a non-volatile manner, and minimizes changes in visible colouration in both states.

Here, we experimentally demonstrate a smart window which uses a thin-film coating containing a chalcogenide-based phase-change material. Our thin-film coating is able to modulate near-infrared reflection while maintaining neutral-colouration of transmission at visible wavelengths. This transition is non-volatile and only requires energy when switching between the two states. We find experimentally that the total modulation of the near-infrared solar energy is more than a factor of two, while the energy in the visible spectrum varies by only 20%. Additionally, we show that in the far-infrared these thin-films serve as low-emissivity coatings, reducing thermal radiation from a building’s interior during the winter seasons. These combined properties result in a smart window that is simple, affordable, and aesthetically pleasing—three aspects which are crucial for successful adoption of green technology.

Dopants such as nitrogen, oxygen, transition metal and oxide have been demonstrated to effectively enhance the phase change properties of phase change material such as thermal stability, endurance and power consumption. [1] Usually, with dopant incorporation in phase change materials, the dynamic resistance in both phases can be raised several orders because of the grain refinement and formation of nitride in grain boundary. The reset current will be reduced as a result. The thermal retention also be greatly improved since some dopants have the ability to modify the crystallization kinetics, giving rise to a sizable increase in crystallization temperature. For example, N doped GeTe was confirmed to exhibit lower power consumption and better data retention property. The crystallization temperature of GeTe film increases markedly from 187 to 372 °C by moderate N doping and its high data retention is around 240 °C for 10 years, which is suitable for high temperature products and systems. [2] In this work, N dopant has been firstly used in the anomalous phase change material Cr-Ge-Te ternary compound which shows an inverse phase transition behavior, i.e., higher resistance crystalline phase and lower resistance amorphous phase. [3] The Cr-Ge-Te ternary compound has the advantages of high crystallization temperature and larger crystalline resistance in comparison with conventional phase change material Ge₂Sb₂Te₅. So, we expected that N would enhance the performance of the Cr-Ge-Te compound.
Cr-Ge-Te ternary compound was deposited on a SiO₂ (100 nm)/Si substrate by RF magnetron co-sputtering using Cr, Ge and Te target. Nitrogen flow rate was controlled by changing N₂ gas flow rate from 0 to 0.9 SCCM. Resistance of the films as a function of temperature (R-T) was in-situ measured in a two-point probe method. The nitrogen content was determined by XPS. The crystallization temperature was measured from DSC. Simple memory cell was fabricated using a traditional lithograph technique based on N doped Cr-Ge-Te film (NCrGT).
By N doping, we found that phase change behavior of Cr-Ge-Te can be tuned by N content. When N content was in a relatively higher region, the resistance in the amorphous phase becomes higher than that in the crystalline. The crystallization temperature and thermal stability was also enhanced by N doping. The NCrGT based phase change memory cell was also demonstrated to show a typical switching behavior.
[1] Lee T H, Loke D, Elliott S R. Microscopic Mechanism of Doping-Induced Kinetically Constrained Crystallization in Phase-Change Materials[J]. Advanced Materials, 2015, 27(37): 5477-5483.
[2] Peng C, Wu L, Rao F, et al. Nitrogen incorporated GeTe phase change thin film for high-temperature data retention and low-power application[J]. Scripta Materialia, 2011, 65(4): 327-330.
[3] Hatayama S. et al. Phase change behaviors of Cr-Ge-Te compound thin film. Proceeding of the 28^th Symposium on Phase Change Oriented Science (PCOS2016). 71 (2016).

Temperature measurement is essential to a variety of scientific experiments and technological application. Quantities of thermometers which were used for thermal sensing at macroscopic length scales have been developed and produced. However, it’s still challenging to do the in situ and quantitative temperature measurement of nanoscale objects with a convenient and simple approach. In this work, we demonstrate a new type of optically-readable VO₂ nanowire-based thermometer¹. By changing the conditions of reaction, the hydrothermal synthesis of intrinsic H-doping VO₂ (M) nanowires has been achieved. During the hydrothermal reactions, the concentration of hydrogen doping can be adjusted by changing the concentration of reductive agent and filling ratio. After annealing treatment, the dopants or vacancies in as-grown hydric VO₂ nanowires can be redistributed or eliminated. Because of the hydrogen doping through hydrothermal fabrication and the hydrogen engineering via a post-annealing process, the single-domain VO₂ nanowires obtains a unique axially-gradient phase transition behavior, which makes the advanced thermometer possible. The optically-readable VO₂ nanowire-based thermometer is user-friendly and appropriate to microscopic size. What’s more, it also has ultra-high relative sensitivity (≈17.4%/K) and temperature resolution(≈0.026K) via optical microscope. It can even achieve an extremely high resolution of (≈10^-5K) when combining with transmission electron microscope (TEM). The advanced thermometer we demonstrated enables the sensitive monitoring of the thermal environment of small space or the temperature of even nanoscale structure which greatly facilitates the nanoscale scientific experiments and technological application.


References
(1)Guo, H.; Khan, M. I.; Cheng, C.; Fan, W.; Dames, C.; Wu, J.; Minor, A. M. Vanadium Dioxide Nanowire-Based Microthermometer for Quantitative Evaluation of Electron Beam Heating. Nat. Commun. 2014, 5.

Ferroelectrics, as phase change materials, have long been studied for non-volatile phase change memories due to spontaneous and electrically switchable polarization. However, for electrical switching in ferroelectrics, an intractable problem is fatigue on account of the happening of electric-field-driven processes such as defect generation and movement. Electrical switching is also known to cause complex memristive dynamics, leakage and even dielectric break-down problem. For these reasons, it is desirable to seek out non-electrically switching of ferroelectric domains to meet the needs of high-performance storage. In fact, novel information storage concept has been put forward to store data in the mechanical state of a ferroelectric thin film. Recently, mechanical switching of ferroelectric polarization has been demonstrated in experiments. The strain gradient, generated by the tip of an atomic force microscope, affects the polarization bi-stability through the so-called flexoelectric effect. Nevertheless, it is still not clear if this mechanical switching behavior realized in experiments could be solely attributed to flexoelectricity. In fact, epitaxial strain has influences on the polarization bistability of ferroelectric tunnel junctions (FTJs). For asymmetric FTJs, the surface asymmetric effect, caused by the different bonding environment at the two surface of the ferroelectric nanofilm, can lead to broken degeneracy of the two polarization states by declining the double-well free energy, making one polarization state metastable. In this paper, we performed ab initio calculations to demonstrate the effects of epitaxial strain on polarization bi-stability and pinpoint its role in the mechanical switching. The results reveal that the metastable polarization state in ferroelectric thin film maintains stability at a relative compressive epitaxial strain; however, it would become unstable at a relative tensile strain due to an increase in the critical thickness of polarization bi-stability. To verify such strain-dependent polarization bistability and include the surface asymmetric effect, we constructed a phase-field model for Pt/(TiO₂-BaO)_m/SrRuO₃, with the surface parameters fitted from the ab initio calculations. The fitting results are in good coincidence with that of the ab initio calculations. Phase field simulation result shows that local switching of the metastable polarization state can occur if the strain state of the FTJ can be controlled. In practice, a relatively tensile strain might be exerted to the FTJs by a local tip force. Depending on the FTJ layer stacking sequence, the epitaxial strain effect on polarization bi-stability can either promote or offset the flexoelectric switching in asymmetric FTJ systems. These results provide us a deeper understanding of mechanical control of phase change memories.

Chalcogenide glasses (ChGs) are widely used in electrical and optical memory because of their high refractive index, transparency in infrared (IR) region and thermally driven amorphous-to-crystalline phase change. Recent advancements in ChGs application in electronics and photonics devices demand glasses with high crystallization temperature (T_c) and high refractive index (n). In that aspect, Ge containing binary ChGs are promising candidates because of their high coordination number and relatively strong chemical bonds. In this paper, thermally induced phase change of Ge_xSe_100-x (x = 40, 33, 20) glasses from amorphous to crystalline condition and temperature dependency of their optical properties (refractive index, optical band gap, extinction coefficient) have been studied. The studies were carried out on thermally evaporated thin ChG films over silica substrates. Differential Scanning Calorimetry for different heating rates (10, 20, 40 K/min) have been carried out in order to obtain data about the crystallization temperature (T_c) of the bulk glasses, which was used as a benchmark for the annealing temperature for crystallization of the films. The heating of the thin films was performed in a rapid thermal annealing system at various target temperatures up to the T_c. The temperature was raised at a rate of 150 K/min and the samples were kept at the target temperature for 1 min and then cooled naturally. Refractive indices and extinction coefficients of annealed samples were measured in an ellipsometer using light of 270 nm to 1650 nm at three different incident angles 50°, 60°, 70°. From the ellipsometer data, optical bandgaps (E) have been calculated using Tauc procedure. In amorphous ChG thin films, the refractive indices at their transparent zone (> 600nm light) are observed to follow the order n₄₀>n₃₃>n₂₀ (n_x = refractive index at x) and optical bandgaps are observed to follow a similar trend E₄₀>E₃₃>E₂₀. The Se rich compositions show a decrease in n with increasing of the temperature up to some minimum, after which at further increase of the temperature n rises in parallel with the temperature. An exception of this dependence are the data obtained for the Ge rich Ge₄₀Se₆₀ films_, for which only the decrease of n with temperature has been observed. The surface morphology study of Ge₄₀Se₆₀ films using scanning electron microscopy (SEM) confirmed formation of crystals on the thin film surface. The obtained results are discussed based on the structural specific of the studied glasses and the possibility for application of the observed effects for optical recording.

Phase Change Memory (PCM) is an emerging dense, high speed, and scalable non-volatile memory that utilizes resistivity contrast of the crystalline and amorphous phases of phase change materials such as GST (Ge₂Sb₂Te₅). The large resistance contrast between the amorphous and crystalline states in PCM enables storage of multiple bits per cell (MLC) for ultra-high density memory.¹ However, a major difficulty for MLC using PCM is the resistance drift that is observed in these materials and devices, which causes mixing of different resistance levels in the long run.

The resistance of the amorphous GST increases with time and eventually decreases due to recrystallization.² The upward resistance drift has often been explained using structural relaxation, without accounting for the electrostatic perturbations caused by the crystals nucleated in the amorphous matrix.³ According to our hypothesis, both the upward and downward resistance drift can be explained by nucleation and growth of crystallites: The nuclei composed of narrow band-gap FCC inside the wide bandgap amorphous matrix are potential wells that trap positive free carriers; locally perturbing the potential profile and lead to charge carrier depletion around the nuclei. This decreases the conductivity of the surrounding amorphous material and increases overall resistance. Once the crystallites grow large enough to initiate percolation transport, resistance starts to decrease.

We have investigated the drift of resistance in amorphous GST due to nucleation using our finite element model that simulates the amorphization-crystallization dynamics in COMSOL Multiphysics.^4,5 The depletion region around the charged nuclei is modeled by introducing a thin (~2nm thick) and highly resistive ring around each nucleus. As the number of seed crystallites increases, there is an initial increase of resistance, but as they grow larger, a percolation path is formed and resistance drops. Computational results showing first upward and then downward resistance drift and resistance fluctuations at various temperatures will be presented.

References:
¹ N. Papandreou, A. Pantazi, A. Sebastian, M. Breitwisch, C. Lam, H. Pozidis, and E. Eleftheriou, in 2010 IEEE Int. Conf. Electron. Circuits, Syst. ICECS 2010 - Proc. (2010), pp. 1017–1020.
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 7, 16625 (2015).
³ D. Ielmini, S. Lavizzari, D. Sharma, and A.L. Lacaita, Tech. Dig. - Int. Electron Devices Meet. IEDM 939 (2007).
⁴ Z. Woods and A. Gokirmak, IEEE Trans. Electron Devices 64, 4466 (2017).
⁵ Z. Woods, J. Scoggin, A. Cywar, L. Adnane, and A. Gokirmak, IEEE Trans. Electron Devices 64, 4472 (2017).

Vanadium dioxide (VO₂) undergoes a first order structural phase transformation at a critical temperature (T_c) of 340 K. The structure converts from a low temperature insulating (monoclinic) phase to a high temperature metallic (tetragonal) phase, along with significant changes in the electronic, optical, and thermal properties. Through the transition the electrical resistance has been known to change over 4 orders of magnitude over a narrow temperature range. The phase transition is highly dependent upon both the process parameters and the nature of the film substrate interface. Using pulsed laser deposition (PLD) we were able to optimize the deposition parameters by isolating 3 unique polymorphs of VO₂, namely the, monoclinic M1 phase, triclinic T phase, and the metastable tetragonal A phase as determined by XRD. By depositing on 3 common substrates; thermally oxidized SiO₂ on Si, p-type Si <100>, and c-plane sapphire (0001) we demonstrate the ability to fine tune the metal-insulator transition (MIT). HRTEM was used to determine the growth behavior of the thin films, and to determine, if any, the orientation relationship between film and substrate. An orientation relationship of <001>(010)VO₂ || <100>(0001)Al₂O₃ was established for VO₂ grown on sapphire, while a native oxide on the Si substrate, revealed by HRTEM, and the amorphous nature of thermal SiO₂ prevented growth of a preferential orientation. AFM analysis of the samples revealed the island like growth behaviour of VO₂ on Si and SiO₂, and layer-by-layer mode on sapphire indicating significantly different stress minimization techniques were present. Interfacial strain can lead to significant increase (tensile strain) or decrease (compressive strain) in the absolute position of the MIT, while grain size will affect the onset and severity of the transition. Deposition temperature, which controlled the degree of grain growth, resulted in modulation of the MIT with respect to the grain size. The MIT observed for our sapphire system was larger and over a narrower temperature range than both the Si and SiO₂ systems. The ability to controllably deposit and manipulate the MIT of VO₂ thin films is of paramount importance for integration into device fabrication and application.

Owing to their dramatic ultrafast reversible metal insulator transition (MIT), vanadium dioxide (VO₂) thin films are of significant interest for numerous potential applications, such as ultrafast electro-optical switching devices, Mott field-effect transistors, and optical waveguides ^[1-3]. Since THz transmittance dramatically changes across this first-order structural phase transition, it indicates good application potential for THz device. In general, VO₂ film has a high phase transition temperature and a wide hysteresis width, especially on silicon substrates, which restrict the development of VO₂-based devices. For practical application the MIT properties have to be tailored. In this work, We have deposited undoped, Hf doped VO₂ and Hf-Nb co-doped VO₂ thin film on high-purity single-crystal Si substrates by DC reactive magnetron sputtering method and investigated their switching properties at THz range. Finally, the excellent combined switching properties were obtained by Hf and Nb co-doping. In particular, the Hf-Nb codoped VO₂ film exhibits a small hysteresis width around 5.8 °C, a low phase transition temperature down to 47.1 °C and a high THz transmission modulation depth of 84%, which is promising for THz modulation applications ^[4]. This work provides a feasible solution to the design and fabrication of VO₂ films with suitable MIT properties for THz device.

References:
[1] A. Cavalleri, C. Toth, C.W. Siders, J.A. Squier, F. Raksi, P. Forget, J.C. Kieffer, Femtosecond Structural Dynamics in VO2 during an Ultrafast Solid-Solid Phase Transition, Phys. Rev. Lett., 87 (2001) 237401.
[2] M.N. Ferdous Hoque, G. Karaoglan-Bebek, M. Holtz, A.A. Bernussi, Z. Fan, High performance spatial light modulators for terahertz applications, Opt. Communications, 350 (2015) 309-314.
[3] L. Liu, L. Kang, T.S. Mayer, D.H. Werner, Hybrid metamaterials for electrically triggered multifunctional control, Nature Communications, 7 (2016) 13236.
[4] X. Wu, Z. Wu, C. Ji, H. Zhang, Y. Su, Z. Huang, J. Gou, X. Wei, J. Wang, Y. Jiang, THz Transmittance and Electrical Properties Tuning across IMT in Vanadium Dioxide Films by Al Doping, ACS Appl Mater Interfaces, 8 (2016) 11842-11850.

Our objective was to investigate new ways to tune the properties of chalcogenides. We investigated two different chalcogenide materials, Ge₂Sb₂Te₅ and Ag_x(Sb₂S₃)_1-x. The Ge₂Sb₂Te₅ material was investigated in a phase change random access memory cell. The effect of a voltage pulse on the cell resistance was measured whilst the cell was in the SET (crystalline) state. At voltage levels below that necessary to RESET (amorphise) the cell, we found that the cell resistance increased during the first 7 ns of the voltage pulse. On longer time scales the cell’s resistance returned to the its initial resistance. Our analysis shows that the resistance transient is not due to capacitive effects and can be explained by a model that includes both phonon scattering and charge carrier excitation. We find that this non-phase change switching effect is highly repeatable and can be cycled more than 10⁹ times. We believe, therefore, that this effect could present a route to performing high speed volatile memory switching and non-volatile phase change switching in the same physical memory cell.

The electric field also has a substantial impact on the crystallisation temperature of Ag_x(Sb₂Te₃)_1-x. The crystallisation temperature of Sb₂Te₃ is lowered by adding up to 7% Ag into the amorphous material. We find that Ag ions are readily driven into the amorphous Sb₂Te₃ structure by applying an electric voltage to Ag electrodes that interface the Sb₂Te₃ film. Sb₂Te₃ crystallises in to an orthorhombic structure with an Sb-S bond length of 2.54 Å [1]. Our X-ray absorption measurements showed that the Ag-S bond length in Ag doped Sb₂Te₃ is 2.34 Å. Since the Ag-S bonds are shorter than the Sb-S, they are likely to be more stable and this implies that smaller crystal nuclei are likely to form in Ag doped Sb₂Te₃. This theory is supported by the an observed reduction in the crystal nuclei incubation time when Ag ions are forced into the Sb₂Te₃ structure with an applied electric field of 200 kV/m [2]. These results show that applying electric fields can be used to enhance the crystallisation kinetics of Ag-doped Sb₂Te₃.

In conclusion, the results presented in this poster show that electric fields can play an important role in crystallisation of chalcogenides and the effects should be exploited in memory and computational processing technologies.

This work was funded my the Singapore Ministry of Education under the auspices of projects MOE2017-T2-1-161 “Electric-field induced transitions in chalcogenide monolayers and superlattices” and T1MOE1703 “Advanced Intelligent Materials (AIM)”. We are also grateful for EXAFS beam time at SPring-8 BL01B1, proposal number: 2014A1244.

[1] V.S Tanryverdiev et al, Russ. J. Inorg. Chem. 41:1492–1495, 1996
[2] W Dong et al, Thin Solid Films, 616:80–85, 2016.

We study the diffusion of metal atoms into phase change chalcogenides. Recently, phase change materials have been investigated for active photonics applications. They have been applied in tuneable polarisation-independent perfect absorbers, optical imaging devices, nano-displays, active nano-photonics, and reconfigurable optical circuits. However, many recent publications do not consider reactions and diffusion at the interface between the metal and chalcogenide layers, which may occur. The diffusion influences the properties of the phase change layer, such as the crystallisation kinetics and optical constants[1-2]. Here, we study the interface between the phase change material, Ge₂Sb₂Te₅, and different metal layers using X-ray reflectivity (XRR) and reflectometry of metal/phase change chalcogenide stacks. We find that a diffusion barrier layer, such as Si₃N₄, can help to prevent the interfacial diffusion of the metal and Ge₂Sb₂Te₅ layers.

We are particularly interested in phase change material tuned plasmonic structures, where metals such as Au, Ag, and Al are commonly interfaced with Ge₂Sb₂Te₅. Our XRR analysis showed that heating Si/Au/Ge₂Sb₂Te₅ samples results in severe interfacial roughening. However, inter-diffusion is readily prevented by a 10 nm thick Si₃N₄ layer deposited between the Au and Ge₂Sb₂Te₅ layers. We find that in this structure the XRR fringes are present even after annealing at a temperature of 573 K. This indicates that Si₃N₄ prevents the interfacial damage between metal layers and the Ge₂Sb₂Te₅. Since metallic diffusion into Ge₂Sb₂Te₅ may affect the crystallisation temperature and optical properties, we expect this structure to exhibit stable and cycleable optical switching.

We conclude that Al, Au, and Ag readilly diffuse into Ge2Sb2Te5 when the structure is heated to 573 K. However, the diffusion can be prevented by adding a Si₃N₄ diffusion barrier layer, which indicates that the diffusion barriers must be included in phase change material tuned plasmonic devices.

This research work was supported by the Singapore Ministry of Education project T1MOE1703 “Advanced Intelligent Materials (AIM)” and the A-star Singapore-China joint research program (grant: 1520203155).

[1] Pandian R, Kooi B, De Hosson J, Pauza A. Influence of capping layers on the crystallization of doped SbxTe fast-growth phase-change films. Journal of Applied Physics. 2006;100(12).
[2] Kozyukhin S, Kudoyarova V, Nguyen H, Smirnov A, Lebedev V. Influence of doping on the structure and optical characteristics of Ge2Sb2Te5amorphous films. physica status solidi (c). 2011;8(9):2688-2691.

In the last decades, the use of electronic systems expanded in many areas of the modern society: networking, mobile devices, personal computers, cloud storage as well as social media are parts of everyone life, becoming strong drivers for the semiconductor market increase. Up to now, memory demand has been satisfied by the optimization and miniaturization of standard memory technologies, i.e. DRAM and NAND Flash. However, with the increasing demand of enhanced cell performance and the reduction of the cell dimension much more difficult than in the past, the usual scaling approach is becoming insufficient. Despite originally considered as a Flash memory replacement, today the phase change memory (PCM) has already arose as the ideal candidate to be used as SCM featuring the required intermediate performances to fill the gap between NAND and DRAM in the system. To this purpose, PCM could solve the cost dilemma and deliver a 3D solution capable of bridging, at least partially, the wide gap that separates it from the incumbent 3D NAND architectures.

Research of emerging nonvolatile memory (NVM) technologies, such as phase-change memory (PCM) and resistive memory (RRAM) have been motivated by exciting applications such as storage class memory (SCM). SCM would fill up the performance gap between access memory, like DRAM, and the storage memory, like NAND. To be able to serve as SCM, PCM needs to achieve reasonably high endurance (>3x10⁸ cycles) and read or write access time of < 100 ns is required [1].
The development of phase-change materials for PCM follows the materials originally developed for phase-change rewritable optical storage in the past two decades. Today almost all phase change memory IC’s still use Ge₂Sb₂Te₅ (GST-225) inherited from optical disk technology. Although GST-225 is a fast switching material it suffers large volume change when melting thus limiting cycling endurance. Thus far, attempts to improve the endurance must sacrifice switching speed. Additionally, SCM technology requires PCM densely packed in vast “crosspoint” arrays with selecting devices to achieve high density and high endurance performance. A suitable selector is of primary importance. Ovonic Threshold Switching (OTS) selector is one of the promising candidates, that is based, as PCM is, on chalcogenide materials [2] giving it analogous physical and electrical properties. Chalcogenides based on Te-As-Ge-Si system have been demonstrated for a selector device since 1968 [2] and have been the primarily studied materials. However, insufficient cycling endurance and low thermal stability remains a key hurdle that inhibits these materials to be used in a large crosspoint arrays. During this talk I will focus on these two key aspects of PCM technologies for SCM applications.
In the first part of this talk, we will show the phase change material by engineering the doping into GST. Our 128Mb test chip has demonstrated 20 ns SET speed, 1E9 cycling endurance and 65% reset current reduction for engineered material compared to GST-225.
In the second part of this talk, we will show the improvement of TeAsGeSi OTS material by incorporation of Se and another dopant (<5 at%). Modification of chalcogenide composition show conflicting requirements where high V_th materials always show low I_OFF with strong dependence on the composition. Therefore, additional thickness and process temperature control are needed to further adjust the selector performance for different materials. By optimizing the composition, thickness and process temperature, a thermally stable patterned selector device that exhibited reliable switching characteristics even after 350^oC/30 mins annealing, compatible with IC Back-End-Of-Line (BEOL) temperature budget is demonstrated. It shows more than 10¹⁰ cycling endurance at 420 uA ON-current, low I_OFF (~1.9 nA @ 1V), moderate V_th (2.2V), which is suitable for SCM applications.
[1] R. F. Freitas and W. W. Wilcke, IBM J. Res. & Dev., 52, 439 (2008).
[2] S. R. Ovshinsky, Phys. Rev. Lett., 21, 1450 (1968)

Chalcogenide materials exhibit a unique portfolio of properties which has led to their wide use for non-volatile memory applications such as optical data storage or more recently Phase-Change Random Access Memory [1]. Chalcogenide glasses (CGs) exhibit a high transparency window in the IR range and large optical nonlinearities offering unique opportunities for elaboration of innovative mid-IR components [2]. Besides, a huge nonlinear behavior of conductivity is observed in some CGs under electrical field application. Such CGs appear to be promising materials for innovative OTS (Ovonic Threshold Switching) selector elements in 3D resistive memory arrays [3]. Indeed, among the different selector technologies developed in the last years [4], the OTS selector technology showed the capability to overcome key issues for crossbar application, as very recently demonstrated in the Intel/Micron Optane^TM memory technology [5]. The OTS mechanism discovered in the 60’s [6] consists in the switching between a high resistance (OFF state) and a low resistance state (ON state) when the voltage applied on the CG exceeds a critical value (threshold voltage V_th). When the current is reduced below the holding current density, J_h, the selector recovers its high resistance state. However, the underlying physical mechanism is still under debate with, up to now, two main classes of models, one involving a purely electronic effect [7] and the other invoking structural changes under field application [8]. In that context, we investigate the origin of the OTS effect by means of a structural analysis of some prototypical and state-of-the-art Ge/Sb/Se-based OTS glasses. The structure of selected thin films, differing significantly by the amplitude of the OTS effect and the performance of OTS devices, is studied by means of Fourier Transform Infrared (FTIR) and Raman spectroscopy as well as X-Ray Absorption Spectroscopy. As a result, we elucidate the role of Sb and N doping in changing the structure of Ge₃₀Se₇₀ glass, which leads to improved OTS selector performance. Finally, from this structural analysis and original ab initio molecular dynamics simulations, we will propose a new scenario explaining the origin of the OTS mechanism in such state-of-the-art CGs.

References
[1] P. Noé et al., PC Materials for NVM devices: From Technological Challenges to Materials Science Issues, Topical review in Sem. Sc. And Tech. (2017). http://iopscience.iop.org/article/10.1088/1361-6641/aa7c25
[2] D. Tsiulyanu et al., Sensor. Actuat. B-Chem. 223, 95-100 (2016).
[3] A. Verdy et al., IEEE 9th Int. Mem. Work. 2017, IMW (2017).
[4] [C. Kügeler et al., Solid State Electron. 53, pp. 1287-1292 (2009).
[5] http://www.techinsights.com/about-techinsights/overview/blog/intel-3D-xpoint-memory-die-removed-from-intel-optane-pcm/
[6] S. R. Ovshinsky, Phys. Rev. Lett. 21 (20), 1968.
[7] D. Ielmini, Y. Zhang. J. Appl. Phys. 102, 054517 (2007).
[8] I. Karpov et al., Appl. Phys. Lett. 92, 173501 (2008).

Interfacial phase-change memory (iPCM) based on chalcogenide superlattice structures was initially proposed as an improvement of conventional phase-change memory providing lower switching energy due to minimization of thermal losses attributed to the switching process [1]. Recently, it was reported that chalcogenide superlattices show some additional effects [2-5], which potentially could broaden the application of iPCM. In the current work, the electrical switching behavior of chalcogenide GeTe/Sb₂Te₃ superlattice-based iPCM devices was studied at elevated temperature and under external magnetic field.
iPCM devices showed resistance switching between the SET and the RESET states as a result of applying short (100∼500 ns) voltage pulses at room temperature. Switching was characterized also at elevated temperature (up to 200°C) and it was found that at around 150°C an initial RESET state could not be obtained and that the device resistance approached the SET resistance level. Major features of the observed behavior were explained by thermally driven effects in the superlattice structure, similar to chalcogenide alloy case, while some of the observed differences were attributed to the structural peculiarities of the former. The same experiments were repeated with the external magnetic field (0.1∼0.5 T), applied to the devices during switching. While for the room temperature case there were no additional switching effects detected, at ∼160°C both SET and RESET resistances shifted to the new, intermediate level and remained there during further annealing to 200°C and consequent cooling to ∼150°C. This result implies that an external magnetic field at high temperature can lead to a new structural condition of the iPCM superlattice, which, in turn, leads to the appearance of an additional resistance level in the device. The possible mechanism of the observed effect was reported earlier [5]. It was found that such a process also could lead to appearance of additional resistance effects at room temperature and during electrically induced switching.
In conclusion, the effect of an external magnetic field at elevated temperature on the performance of switching of iPCM devices based on GeTe/Sb₂Te₃ superlattices was found. A combination of the thermal, electric and magnetic field treatment can lead to permanent changes in the structure of the superlattice that may be utilized in a new type of memory.
This work was supported by JST-CREST (JPMJCR14F1). 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] R. E. Simpson et al., Nat. Nanotech., 6, 501-505 (2011).
[2] J. Tominaga et al., Appl. Phys. Lett. 99, 152105 (2011).
[3] J. Tominaga et al., Sci. Technol. Adv. Mater. 16, 014402 (2015).
[4] Y. Saito et al., ACS Appl. Mater. Interfaces, 9, 23918 (2017).
[5] J. Tominaga et al., Adv. Funct. Mater., 27 (40), 1702243 (2017).

Phase change memory (PCM) is an emerging non-volatile memory technology with high endurance, high speed, and good scalability. Compared to other nanoscale devices, PCM exhibits high cell-to-cell and cycle-to-cycle programming variability due to the unique amorphous and crystalline structures that form upon each reset and set operation [1]. The variability in amorphous region for cells of similar dimensions programmed to similar resistance levels can be analyzed using tunneling electron microscopy (TEM) imaging [2] but this is a difficult and time-consuming procedure. We have experimentally extracted the amorphized length of multiple PCM line cells by conducting destructive post-reset read operations with gradually increasing amplitudes until further breakdown and amorphization. The amorphized length is calculated using the reported GST breakdown field of 56 MV/m [3] and the applied voltage amplitudes at which each of the breakdowns takes place. Significant variability is observed in the measurements. For example, the amorphized lengths of 5 cells of similar dimensions (width ~130 nm, length ~470 nm, and thickness ~50 nm) amorphized at very similar resistance levels (~16-19 MΩ) vary from 32 to 55 nm. Electrical measurement results along with corresponding locations of void distributions and amorphized regions from the scanning electron microscopy (SEM) images of read disturbed cells will be presented and discussed.

References:
1. Boniardi, Mattia, et al. "Statistics of resistance drift due to structural relaxation in phase-change memory arrays." IEEE Transactions on Electron Devices 57.10 (2010): 2690-2696.
2. Santala, M. K., et al. "Distinguishing mechanisms of morphological instabilities in phase change materials during switching." Thin Solid Films 571 (2014): 39-44.
3. Krebs, Daniel, et al. "Threshold field of phase change memory materials measured using phase change bridge devices." Applied Physics Letters 95.8 (2009): 082101.

Given the explosive growth in data-centric cognitive computing and the imminent end of CMOS scaling laws, it is becoming increasingly clear that we need to transition to non-von Neumann computing architectures. A first step in this direction could be in-memory computing whereby certain computational tasks are performed in place in a specialized memory unit, which we call computational memory.

Phase-change memory (PCM) devices could play a key role as elements of such a computational memory unit. The physical attributes of these devices can be exploited to achieve in-place computation. When organized in a cross-bar configuration, PCM devices can be used to perform matrix-vector multiplications with very low computational complexity [1]. An appealing application of this concept is for the problem of compressed sensing and recovery of high-dimensional signals [2]. This is an application in which the lack of precision arising from the matrix-vector multiplication operations is not prohibitive. However, in other applications such as solving systems of linear equations or training deep neural networks, the lack of precision could be a key challenge. To address this, we propose the concept of mixed-precision in-memory computing in which, through a judicious combination of high-precision processing units and computational memory, we can achieve arbitrarily high precision, while still retaining much of the benefits of non-von Neumann computing [3]. Finally, I will present applications in which the dynamics of PCM devices, such as the crystallization dynamics and the dynamics of structural relaxation, are used to perform computational tasks. Examples include finding factors of numbers in parallel [4] and the detection of temporal correlations between event-based data streams [5].

[1] G.W. Burr et al., “Neuromorphic computing using non-volatile memory”, Advances in Physics: X, 2:1, 89-124, 2017
[2] M. Le Gallo et al., “Compressed sensing recovery using computational memory”, Proceedings of IEDM, 2017
[3] M. Le Gallo et al., “Mixed precision in-memory computing”, ArXiv, arXiv:1701.04279v3, 2017
[4] P. Hosseini et al., “Accumulation-based computing using phase-change memories with FET access devices”, IEEE Electron Device Letters, 36:9, 975-977, 2015
[5] A. Sebastian et al., “Temporal correlation detection using computational phase-change memory”, Nature Communications, 8, article 1115, 2017

Chalcogenide glasses are a sub-family of phase change materials (PCMs) with nonvolatile properties. Germanium Telluride (GeTe) is one the most commonly used PCMs. It has found numerous applications in different electronic and optical systems. GeTe undergo a structural transition between two room-temperature-stable phases, namely amorphous and crystalline states, in response to an external stimulus such as electrical current or laser pulse. Amorphous films are optically transparent and electrically isolative while crystalline GeTe is optically lossy, and electrically conductive. More than six orders of magnitude change in the resistivity and 2x change in optical properties with reliable phase transitions make GeTe a good candidate for both electrical and optical devices. This talk covers the application of GeTe phase change material in both RF switching devices and reconfigurable optical components: Low-loss ohmic RF switches could be very useful in a number of applications such as in cognitive radios, anti-jamming communication systems, low-power electronics, as well as reconfigurable electromagnetic devices. Significant electrical resistivity modulation between amorphous and crystalline states of GeTe results in very low insertion loss switches with high isolations and cut of frequency of a few THz. Among many different methods of GeTe phase transition, joule-heating with an electrical pulse applied either through the device itself or a separate heater is the most common approach. In both direct and indirectly heated GeTe RF switches, two RF ports are separated with a GeTe patch, which is transitioned between its amorphous (Open) and crystalline (Short) phases with application of an external heat through a separate DC port. Heat pulses with several μs width could switch RF signals with several mW of average power. This talk overviews the design methodology of GeTe RF switches and discusses their application in reconfigurable filters. In addition, the talk discussed new approaches to implementing tunable optical components, such as shutters, modulators, and color filters using GeTe material. Such components are of utmost interest in a variety of applications. The GeTe-based ultra-high contrast optical shutter can find application in atomic-clock assemblies. In addition, incorporating GeTe in a Fabry–Pérot cavity, we have realized a zero static-power color reflector. These zero static power passive displays, which only consume power when switching colors, are among most important alternatives to active devices, especially in ultra-low power displays with low refresh rate. The talk included details of the design and characterization results of these optical components.

Recently, phase-change materials have been explored for reconfigurable RF and electro-optic applications, which include non-volatile RF switches [1], tunable photonic devices [2], and non-volatile optical memory devices [3], for instance. Low-loss, linear RF switches are integral parts of wireless RF front-ends for antenna and filter switching, for instance. Common key features of RF switches include cost, low insertion loss, high isolation, excellent linearity, power handling, easy integration with conventional semiconductor technologies, reliability and packaging. Among various RF switch technologies, Si-based technology is widely used for wireless networks below ~5 GHz. For instance, a matured RF-SOI switch has a R_on×C_off value of ~250 femtosecond with switch FOM of 1/(2*π* R_on×C_off) ~0.6 THz. Its RF performance at mm-wave frequencies would be compromised.
In this talk, we report on the first SbTe phase-change material RF switches with a refractory TiW heater in a planar configuration. With the planar layout and heater reliability, a record switching cycle endurance of >300K was demonstrated. With on-state resistance of 0.5 ohm*mm and off-state capacitance of 75 fF/mm, the RF switch figure-of-merit (FOM) is 4.1 THz, which is 6-7 times better than state-of-the-art RF switches, including RF silicon-on-insulator technology. With further layout optimization, SbTe phase-change material RF switches could be a potential candidate for future RF switch technology.


[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] K.-A. Son et al., “Phase-change GeTe for Photonic Applications”, MRS Spring Meeting, 2017
[3] E. Kuramochi and M. Notomi, “Optical memory: Phase-change materials”, Nature Photonics, pp. 712-714, 2015

In a conventional field effect transistor device, the gate voltage induced electric field across the dielectric layer controls the conductivity of the channel between source and drain. In case an excitation in forms of heat, light, and magnetic field is able to significantly modulate the conductivity of the channel material controllably, a transistor gated thermally, optically, and magnetically can be rationally proposed and designed. Phase change materials provide a suitable platform for the study of the abovementioned devices.

A prototype “thermally” gated transistor is proposed, designed, fabricated, and characterized. A vanadium dioxide nanowire was employed as the channel of the transistor. Multiple Ti micro-heaters were fabricated around the nanowires as multi-gate structure with various gating capabilities. The transport properties of the “thermal” transistors were measured under various “thermal” gating powers and at different temperatures. The results were compared to the I_d-V_d and I_d-V_g characteristics of regular electrically-gated field effect transistors. This work is important for fundamental research as well as potential applications in specific areas.

Reference
1. Z. Yang, C. Ko, and S. Ramanathan, Oxide electronics utilizing ultra-fast metal-insulator transitions, Annual Review of Materials Research 41, 337 (2011).
2. Z. Yang and S. Ramanathan, Breakthrough in Photonics 2014: Phase change materials for photonics (Invited), IEEE Photonics Journal 7, 0700305 (2015).

Resistance drift of the amorphous and on-oxide phase change memory (PCM) devices raises a significant concern in the long term reliability of these cells when they are used in computer memory [1]. The resistance of amorphous Ge2Sb2Te5 (GST) cells and other amorphous phase change memory devices slowly increases over time until a certain point at which it starts decreasing due to crystallization[2]. Resistance drift depends on temperature, original programmed resistance, the waveform used to reset the devices, and the shape and dimensions of the cells[2]-[3].
In this work we compare the resistance drift behavior of suspended and on-oxide GST line cells. The suspended cells are coated by a 15 nm layer of silicon nitride after being suspended in air, while the on-oxide cells rest on silicon dioxide and only the sides and the top surface are capped by 15 nm of silicon nitride. The results, obtained over several months, show that the suspended GST wires have a noticeably larger resistance drift coefficient as compared to the on-oxide wires, and also exhibit significantly more variable resistance behavior. The increase in drift coefficient and variability may be associated with differences in the mechanical stress of the suspended wires compared to the on-oxide ones as well as the different interfacial properties of these devices. Experimental results from a large number of devices of varying dimensions will be presented and discussed.

References:
[1] M. Rizzi, et al. “Role of mechanical stress in the resistance drift of Ge2Sb2Te5 films and phase change memories.” American Institute of Physics, 2011.
[2] N. Noor, et al. “Phase-Change Materials and Their Applications—Memories, Photonics, Displays and Non-von Neumann Computing.” 2017 MRS Spring Meeting and Exhibit, MRS, 2017.
[3] N. Noor, et al. “An experimental study on waveform engineering for Ge2Sb2Te5 phase
change memory cells.” 2015 MRS Fall Meeting and Exhibit, MRS, 2015.

Non-volatile memory (NVM) and memristor arrays with cross-point structure have shown great potentials for 3D data storage and neuromorphic applications and been actively developed. However, the sneak path problem remains a major bottleneck that limits the progress towards large scale integration with effective operation. Thin film selector is a key element to overcome the sneak path problem. Despite significant efforts have been devoted to developing thin film selector till now, none could fully match the performance of various NVMs. An ideal thin film selector is expected to exhibit properties of high nonlinearity, large ON/OFF ratio, appropriate threshold and holding voltage, high speed, large hysteresis window, bi-directional switch, and CMOS back-end process compatibility. Recently we have introduced a thin film selector by combining high ion-mobility element and high defect density glassy chalcogenide, which demonstrates near-ideal properties, such as high non-linearity (< 5 mV/dev), ultra-low leakage (< 10 fA), and bidirectional operation [1-3]. A unique operation scheme was proposed to utilize the large hysteresis window to remove the voltage matching constrains between resistive memory and selector, and achieve low voltage operation. In this talk, we will report the latest progress and its applications. In particular, the underneath mechanism of the selective switching behavior will be discussed with experimental direct observation and modeling, which could provide a guideline for thin film selector design.
References:
[1] Hongxin Yang et al. VLSI 2015, T130-131. [2] Wei He et al. EDL, dio:10.1109/LED.2016.2641018. [3] Xinglong Ji et al. EPCOS 2017.

Operation speed is a key challenge in phase-change random-access memory (PCRAM) technology, especially for achieving sub-nanosecond high-speed cache-memory. Commercialized PCRAM products are limited by the tens of nanoseconds writing speed, originating from the stochastic crystal nucleation during the crystallization of amorphous Ge2Sb2Te5. Here we demonstrate an alloying strategy to speed up the crystallization kinetics. The new compound we designed allows a writing speed of only 700 picoseconds without pre-programming in a large conventional PCRAM device. This ultrafast crystallization stems from the reduced stochasticity of nucleation. Controlling nucleation through alloy design paves the way for the development of cache-type PCRAM technology to boost the working efficiency of computing systems.

Feng Rao, Wei Zhang, Evan Ma et al. Science (2017) DOI: 1126/science.aao3212

Sb-Te and Bi-Te chalcogenide alloys as well as a range of layered compounds, such as Sb₂Te₃ and Bi₂Te₃, have been gathering more and more attention not only for phase change memory or thermoelectric applications, but as a result of a recent surge of interest in topological insulators and two-dimensional materials. The chalcogenide GeTe/Sb₂Te₃ superlattice in particular has been proposed for use in interfacial phase change memory (iPCM) and has led to a reduction in switching energy compared to conventional alloy-type phase change memory. Since this superlattice is composed of coherently-aligned GeTe and Sb₂Te₃ multilayers, fabricating a highly-oriented film is crucial. Furthermore, in order to utilize such material for novel devices in an industrial setting, reliable processes such as large area deposition of uniform films is critical. Sputtering is one of the most useful, technologically friendly, and reliable methods for thin film fabrication. In this work, the growth mechanism of layered chalcogenide films is discussed.
The crystal structure of the films as well as the degree of orientation was evaluated by x-ray diffraction (XRD) while the microstructure was observed by transmission electron microscopy (TEM). On the basis of the obtained results, we will discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for growth. Finally, we will also show some examples of novel device applications using layered chalcogenide films.

Phase Change Materials (PCM) have been widely recognized as effective building-block for alternative memory devices, where the information bit is stored in the form of two distinct resistive states, namely the high-resistance amorphous phase and the low-resistance crystalline phase. Switching between the different phases is achieved through the heating of the PCM, by means of the application of voltage pulses via the Joule effect. Phase Change Memories have exhibited sub-10 ns switching speed, scalability to sub-10 nm dimensions, extremely good cyclability and endurance, data retention ability as well as integration in large-arrays, thus representing potential candidates for future substitute devices to flash memories. Among the PCM, the investigation on pseudo-binary chalcogenide-based alloys, in particular Ge₂Sb₂Te₅ (GST), has been encouraged because of the combination of their adequate resistivity contrast and rapid switching between the two phases. Nevertheless, some issues still subsist, concerning the reliability of GST-based memories as well as their thermal stability for PCM applications which require quite high working-temperatures, hence motivating further studies. In this context, it has been found that GST can be enhanced by means of Ge enrichment or by doping it with nitrogen or carbon, in order to increase the crystallization temperature and provide a better thermic stability of the amorphous phase. After such modifications, variations of the phase-change mechanism and of the resistance trend are expected. In this work, focus has been put on Ge-enriched N-doped GST materials, especially on their structural properties at the nanoscale and the dynamics of their phase change. To this aim, in-situ Transmission Electron Microscopy (TEM) analyses at different temperatures have been systematically pursued in order to delineate the phase change process during the thermal transient and describe the physics beyond the mechanisms of nucleation and grain growth. The impact of surrounding media on the physical mechanism has been considered in terms of interfacial energy balance and comparisons have been made with the more known Ge₂Sb₂Te₅. Furthermore, ex-situ measurements on samples previously annealed in a hot tube have been realized, in particular X-Ray Diffraction spectroscopy and ex-situ structural TEM characterizations, i.e. diffraction, energy filtered TEM and High Resolution imaging. Hence, TEM-Energy Dispersive X-Ray mapping has been performed to characterize the chemical implications of the phase change mechanism. This study represents a step toward a major understanding of the physical phenomena beyond the phase transformation of PCM at nanoscale resolution, which reveals to be at the basis of the behavior of the final device. The attentive examination of the different stages of the crystallization process may also support the application of GST as multilevel data storage employing more than two states of electrical resistance.

This work assesses the potential of colloidal nanoparticles for phase-change memory technology. Because solution-phase synthesis of phase-change materials (PCMs) is little developed, there are open questions regarding the ability to produce material of sufficient quality and amount to satisfy the needs of memory technology.
We choose binary GeTe PCM nanoparticles as a case study. First, we will present a colloidal synthesis of amorphous GeTe nanoparticles that allows for accurate size control between 4 nm and 10 nm and narrow size distributions < 10%. About 2 g of monodisperse GeTe nanoparticles can be obtained from a single synthesis. Precise size tunability and high chemical yield of GeTe nanoparticles allow systematic study of their size-dependent crystallization and melting phase transitions using high-temperature X-ray diffraction, in-situ TEM heating, and differential scanning calorimetry. Finally, we will show a ligand exchange that enables the removal of insulating organic monolayer from the surface of GeTe nanoparticles.
Our results suggest great prospects for PCM colloids in phase-change memory technology. Monodisperse PCM nanoparticles represent convenient template-free system to study size dependent phase transitions. Furthermore, the PCM colloids can sufficiently reduce the cost of PCM cells and give an access to easy solution-processing of PCM films on flat, flexible, and prepatterned substrates.

GeSbTe (GST) alloys lying along the GeTe/Sb₂Te₃ pseudobinary line exhibit impressive performance as non-volatile memory [1]. Epitaxial GST growth by molecular beam epitaxy (MBE) promoted the understanding of the structural properties, due to the superior thickness and interface quality control. In fact, along with the metastable cubic and stable trigonal crystalline phases, a cubic ordered GST has been recently demonstrated by gradually aligning vacancies in the growth direction [2]. The material is deposited in the stable phase by means of van der Waals (vdW) epitaxy, where the layers are weakly bonded to the substrate. This offers a toolbox for the investigation of 2D structures and the role of vdW gaps, but the possibility of functionalizing vdW bonded layers is still unclear.
In this talk I will first review the progress we made in the MBE growth of crystalline GST, which is dictated by the interplay between composition, phase and ordering. These parameters are correlated by a combination of X-ray diffraction and Raman spectroscopy techniques resulting in a growth phase diagram of Te flux as a function of substrate temperature [3]. I will then report on the usage of vicinal surfaces to induce strain at the step edges allowing for a combination of classical epitaxy and vdW epitaxy in vdW-bonded GST [4]. Ge_xSb₂Te_(3+x) with almost GST124, trigonal phase and improved vacancy layers ordering is obtained for substrate offcut angles between 3-6°. Interestingly, the tilt of the epitaxial layer with respect to the Si substrate crystal axis is well reproduced by the Nagai model, which applies to covalently bonded materials. Finally, I will show Raman investigation of ultrathin GST films and GeTe/Sb₂Te₃ based superlattices.

[1] S. Raoux et al., Chem. Rev., 110, 240 (2010)
[2] V. Bragaglia et al., Sci. Rep., 6, 23843 (2016)
[3] V. Bragaglia et al., unpublished
[4] E. Zallo et al., Sci. Rep., 7, 1466 (2017)

The outstanding properties of chalcogenide phase change materials (PCMs) have led to their successful application in optical memories for a long time and, more recently, in phase change random access memories (PCRAMs) [1,2]. PCMs feature fast and reversible phase transformations between crystalline and amorphous states which have different transport and optical properties. A prototypical PCM is GeTe which thin films crystallize at ≈ 230 °C, governed by “homogeneous” nucleation in the volume of the film [1,2,3]. However, when GeTe is surface-oxidized, its crystallization occurs in a completely different manner: the crystallization is heterogeneous, starts already at ≈ 180 °C, and is slower than that of non-oxidized GeTe thin films. Although this effect has already been reported, a detailed comprehension of its origin at the nanoscale is still missing [3,4]. Here, we will show why surface-oxidation affects the crystallization mechanism of GeTe thin films using advanced scanning transmission electron microscopy (STEM) techniques, energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS).

Aiming at understanding the origin of heterogeneous crystallization in surface-oxidized amorphous GeTe layers, a surface-oxidized 100 nm GeTe film was annealed under Ar atmosphere. The crystallization was followed by optical reflectivity at 670 nm and quenched just after the first sharp optical increase corresponding to heterogeneous crystallization of the surface of the film [3,4]. Subsequent analysis of the film by STEM imaging revealed that only the top 20 nm of the film was crystallized while the bottom of the film remained amorphous. EDS mapping and XPS depth profiling showed how oxidation affects the chemical composition of the film. Using nanobeam precession electron diffraction (NPED) mapping, we identified the different crystallographic structures present near the sample surface. Furthermore, spectrum imaging diffraction (SIdiff) mapping allowed for the direct correlation of chemical and crystallographic information in different regions of the film. Taking all results together, we found that surface-oxidation causes, e.g., separation of the GeTe phase and formation of crystalline species other than GeTe. In conclusion, the presented results will provide clues for elucidating the puzzling aspects of the effect of surface-oxidation on the crystallization of GeTe thin films, opening up novel strategies for interface engineering in order to master crystallization of PCM thin films.

[1] P. Noé et al., accepted manuscript, Topical review in Sem. Sc. And Tech. (2017). http://iopscience.iop.org/article/10.1088/1361-6641/aa7c25
[2] G. W. Burr, et al., IEEE Journal on Emerging and Selected Topics in Circuits and Systems, issue on "Emerging Memories - Technology, Architecture & Applications," 6(2) 146-162 (2016).
[3] P. Noé et al., Acta Mater. 110, 142 (2016).
[4] R. Berthier et al., J. Appl. Phys. 122, 115304 (2017).

Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). And such phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. By combining second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy, it shows that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.

PCRAM is a practical next generation non-volatile memory. Currently, Ge-Sb-Te compound (GST) is widely studied for PCRAM because of its fast phase change speed and excellent reversibility of phase transition. However, GST has a low crystallization temperature of around 160°C which limits data retention at high temperature and shows a high melting point of about 600°C which causes a high amorphization energy. Moreover, with further scaling down of PCRAM cell, thermal disturbance between cells becomes a serious problem. Therefore, it is desired to develop a new PCM which shows high thermal stability in amorphous phase and enables to lower amorphization energy (e.g., low melting point PCM, high electrical resistance PCM in crystalline state etc.)
From the above background, we have studied the effect of doping element, X on the crystallization temperature, Tx in amorphous Ge-Te. Based on the total bonding enthalpy calculation of amorphous X-Ge-Te, we found that transition metal doping is effective to increase the Tx of amorphous Ge-Te. Actually, transition metal doping such as V, Cr, Ni and Cu were experimentally confirmed to increase the Tx of amorphous Ge-Te.
As mentioned above, next generation PCM is desired to show not only high Tx, but also low melting point. Among various transition metal-Ge-Te chalcogenide, we are proposing Cu-Ge-Te compound as a new PCM. In the ternary system, there is a Cu₂GeTe₃ (CuGT) compound with a chalcopyrite-type structure. This compound shows a low melting point of around 500°C. Actually, CuGT shows better thermal stability in the amorphous state than GST and exhibits a fast reversible phase transition with an enough resistance contrast. Also, it was confirmed that CuGT has a lower amorphization energy than GST because of its low melting point. It is noteworthy that CuGT has smaller reflectance change and simultaneously, smaller density change (2~4%) upon phase change than GST. It was found from Hard X-ray photoemission spectroscopy that Cu d electrons are expected to play an important role during the phase change process. Moreover, we also pay attention to Cr-Ge-Te ternary system. In this system, there is a Cr₂Ge₂Te₆ (CrGT) which is known to be a semiconductor compound. Therefore, this compound shows a high electrical resistance in the crystalline state. Interestingly, this compound shows an inverse resistance change upon phase change, i.e., low resistance amorphous and high resistance crystalline states. It was confirmed that CrGT shows a high thermal stability in amorphous state and exhibits a fast reversible phase change with an enough resistance contrast. These results suggest that transition-metal based PCM possessing d-electron bonding are good candidates for future PCM with high thermal stability, large resistance contrast, low density change and fast phase change. In this presentation, we will review the phase change behaviors of TM-Ge-Te (TM: Cu and Cr) compounds in viewpoint of PCRAM application.

Ge-Sb-Te compounds (GST) are well studied as phase change materials (PCMs) for phase change random access memory (PCRAM) applications.^1,2 Since GST shows a fast phase change speed and large resistance contrast between high resistance amorphous and low resistance crystalline phases, GST-based PCRAM shows a fast operation speed and good data reliability. However, the low thermal stability of GST in amorphous phase (i. e. low crystallization temperature of about 150 °C³) makes it difficult to be used in high temperature such as automotive applications. Furthermore, in highly-scaled PCRAM, thermal disturbance between neighboring cells should be considered.
In our previous study, we found that Cr doping into GeTe compound can increase the crystallization temperature of its amorphous phase.⁴ However, the excess Cr doping was found to lead phase separation, deteriorating performance of PCRAM. In Cr-Ge-Te ternary system, there is a compound, Cr₂Ge₂Te₆ (CrGT). If this compound shows a reversible phase transition between amorphous and crystalline states, it may be a good PCM candidate for PCRAM. In this study, the phase change behavior of CrGT was investigated in view points of PCRAM application.
In this study, CrGT film was deposited on SiO₂ (100 nm)/Si substrate by RF-magnetron sputtering using Cr, Ge and Te pure metal targets. The composition of the obtained film was analyzed to be Cr_19.2Ge_20.6Te_60.2 by EDX attached with STEM. The crystal structure of CrGT film was identified by XRD. The resistivity of as-deposited and annealed CrGT films was measured by Van der Pauw method. DSC was employed to estimate crystallization temperature of amorphous CrGT film.
From XRD patterns, it was found that the as-deposited film is amorphous phase, while the film annealed up to 380 °C is crystalline Cr₂Ge₂Te₆ phase. The obtained resistivity of the film annealed up to 380 °C and the as-deposited amorphous CrGT film was 4.1 Ω cm and 0.11 Ω cm, respectively. Consequently, CrGT film was found to show an inverse resistance change between high resistance crystalline and low resistance amorphous phases. In addition, the evaluated crystallization temperature by DSC was 276 °C which is much higher than that of GST. In this presentation, the operation of CrGT-based PCRAM will be discussed.

1. S. Lai, Current status of the phase change memory and its future. Tech. Dig. IEDM 255-258 (2003).
2. M. Wuttig and N. Yamada, Phase-change materials for rewriteable data storage. Nat. Mater. 6, 824-832 (2007).
3. N. Yamada et al., Rapid-phase transitions of GeTe-Sb₂Te₃ pseudobinary amorphous thin films for an optical disk memory. J. Appl. Phys. 69, 2849-2856 (1991).
4. S. Hatayama et al., The doping effect of V, Cr and Ni on the crystallization behavior of GeTe amorphous film. Proc. EPCOS 199-200 (2016).

Memristors made of transition metal oxides like TaO_x and NbO₂ are attracting large interests for the potential application in neuromorphic computing. Some of these materials demonstrate current and temperature induced negative differential resistance (NDR); the physical origin behind such behavior is still not well understood. Joule heating driven thermal runaway is believed to be a source of structural changes and reason for observed NDR for some transition metal oxides based memristors. When a filament is formed in these memristive devices, there is a significant change in resistance leading to corresponding change in Joule heating and temperature distribution in the device. So, the investigation of temperature changes during phase change can help us understand the mechanism behind the NDR. In this study, we explore the temperature changes in TaO_x memristors during current and voltage controlled operation using thermo-reflectance based thermal imaging system. This system is capable of thermal imaging with sub-micron spatial resolution and 100 ns temporal resolution. Using this imaging system, the emergence of localized hot spot during voltage and current controlled operation is studied. We also developed a numerical model to explain the experimental observations. Thermal capacity and thermal resistance is estimated by fitting the developed model against the transient temperature measurements at different states. These material properties is used for more accurate numerical modeling and exploring switching characteristics of TaO_x based memristors.

Crystal structures are important in many fields of material science, however, such structures often exhibit inflexible order. Future technologies will require the creation of smart-materials that can change their properties on demand and exhibit multi-functional responses. Recent research has shown that order can emerge in structures that are maintained far-from-thermodynamic equilibrium through a process of self-organization. Self-organization is frequently observed in living systems, with artificially self-organized structures also displaying life-like behavior, such as with the abilities to self-heal with damage and adapt with their surroundings. In this talk, I will report a new form of non-equilibrium material that is characterized by robust pseudo-crystalline ordering. This order is sparsely periodic, with integer spacings between neighboring elements. Here, the particle-particle interactions that underlie collective ordering are mediated by wave scattering, which is externally tunable by varying the wavelength of a coherent drive. The sparse ordering allows our system to be exceedingly robust to both large mechanical perturbations and a changing environment. Compared to hydrodynamic interactions, which lead to a compact periodic order, wave-scatting provides our system with many different steady-state geometries that can be dynamically switched and reconfigured.

Phase change memory (PCM) has shown great potential in storage in recent years. The main challenge for commercial application of PCM is the power reduction and density increase. In both cases, the tunable disorder in crystalline phase change materials is of great importance. Desirable crystalline state can reduce current density in RESET process and the control of degree of disorder can lead to the development of multilevel memory. The disorder induced carrier localization is important for electrical transport of crystalline phase change materials. The analysis of field effect can directly probe the localized states of carriers and make up the drawback of conductivity and Hall effect measurements. We measured the field effect mobility of cubic crystalline Ge₂Sb₂Te₅ (GST) based on back gate structure and analyzed influence from annealing temperature. We find evidence for that the annealing lifts the mobility edge towards the valence-band edge, delocalizing more carrier.
We observed the MIT of GST in the thermal annealing process and measured field effect of GST with step annealing temperature. We turned our attention to the rather insulating samples for the carrier localization is expected to dominate the electrical transport. The field effect induced current increases significantly with the increase of annealing temperature. And the relationship between the excess carrier mobility and temperature was analyzed. The field effect mobility shows temperature activation, a hallmark of carrier localization. The X-ray Diffraction(XRD) and X-ray photoelectron spectroscopy (XPS) measurement show that GST films used in field effect measurements are all fcc phase.
In crystalline phase change materials, stoichiometric vacancies only contribute to disorder, whereas the carrier concentration is controlled by the excess vacancies. It is hard to distinguish the contribution to conductivity. In the field effect measurements, the change in conductance of semiconductor due to the space charge layer in which the extra carriers are induced. The important role of localized carrier in materials can be directly observed in spite of the high concentration of carrier. Through this method, we can confirm the shit of mobility edge in limited annealing difference on the insulating side of MIT, which is difficult to realize in previous work. The localized carriers were observed to determine the multilevel crystalline states of GST, provides conceptual guidance for designing multilevel memory devices.

The amorphous phase change materials (a-PCM) have many perculiar, yet technologically important, electron transport properties, e.g. single-digit nanometer ultra-scalability and S-shape snapback current-voltage characteristics. These properties are indispensible for PCM device applications. However, despite intensive research effort, the Angstrom-scale microscopic origin of these properties is still under debate. A unified theoretical understanding of the fundamental governing physical mechanism has not emerged yet.

Here, the purely ab-initio methods are used to analyze the electron transport process in prototypical a-PCM germanium telluride (GeTe). In constrast to existing phenomenological electron transport modeling approaches in literature, here the fundamental governing Schroedinger equation is solved in a brute-force way, in order to unravel the Angstrom-scale microscopic origin of the S-shape snapback properties [1].

As a continuation of the author's prior research on low-bias and sub-threshold electron-transport ab-initio analysis [2-3], the analysis here uses density functional theory (DFT), ab initio molecular dynamics (NEGF), nonequilibrium Green’s function (NEGF), and quantum hydrodynamics (QHD) [1]. It is found that the measured peculiar a-PCM electron transport properties are governed by bias-dependent dynamics of local current swirls, which originate from defects-induced electron backscattering and localization. The Angstrom-scale microscopic balance between the defects-induced electron localization and the field-induced electron delocalization is the origin of the linear, exponential, and S-shape snapback current-voltage curve shapes. It is revealed that the threshold switching is a manifestation of quantum percolation. It is shown that the local current swirls are well confined in Angstrom-scale, leading to the promising single-digit nanometer scalability of related device technologies.

[1] Jie Liu, "Microscopic Origin of Electron Transport Properties and Ultrascalability of Amorphous Phase Change Material Germanium Telluride", IEEE Transactions on Electron Devices, Vol. 64, No. 5, May 2017; DOI: 10.1109/TED.2017.2685341
[2] Jie Liu, M.P. Anantram, "Low-bias electron transport properties of germanium telluride ultrathin films", J. Appl. Phys. 113, 063711 (2013); DOI: 10.1063/1.4790801
[3] Jie Liu, Xu Xu, M.P. Anantram, "Subthreshold Electron Transport Properties of Ultrascaled Phase Change Memory", IEEE Electron Device Letters, Vol. 35, No. 5, May 2014; DOI: 10.1109/LED.2014.2311461

Even in their crystalline phases, phase change materials such as Ge₂Sb₂Te₅ (GST) always present a high degree of atomic disorder, which is one of the intrinsic characteristics of these compounds. In the conductive metastable cubic phase of Ge₂Sb₂Te₅, Te atoms occupy all the 4a-atomic sites, while Ge atoms, Sb atoms and vacancies are randomly distributed over all the 4b-atomic sites, with the occupancy rates 2/5, 2/5 and 1/5. Taking such a complicated atomic disorder into account is a challenge when calculating the physical properties of GST materials from first principles. Most of the studies use codes based on the Density Functional Theory (DFT) for calculating the Kohn-Sham wave functions and supercells as needed to mimic this atom disorder. This has a high numerical cost: huge supercells must be used to describe a randomly distributed atomic disorder.
In this work, we show that DFT methods based on the multiple scattering theory and on the coherent potential approximation (CPA) for describing a genuine random atomic disorder can also be used to calculate the physical properties of GST crystals, with a good accuracy and at a far lower numerical cost. We have applied these alternative methods to describe several imperfect cubic phases of Ge₂Sb₂Te₅, such as off-stoichiometric (Ge and/or Sb rich or deficient) crystals, or GST crystals in which few Ge atoms have undergone an umbrella-flip mechanism, from octahedral to tetrahedral positions. For each of these complex systems, we calculate the electron states and deduce the electrical conductivity from the linear response formalism. The correlations we evidence between the electrical properties and the atomic structure of GST crystals would have been difficult to obtain with conventional methods base on supercells. We show that such methods may be used to investigate in details the intringuing properties of this class of materials.

In phase-change memory (PCM), information is stored in a high resistive amorphous or low resistive crystalline phase of a nano-scale volume of a phase-change material. The transition times between amorphous and crystalline states (~50 ns for reset and ~100 ns for set) determine the speed of the memory operation [1]. As crystallization can occur in nanoseconds, rapid measurements of resistance immediately after amorphization are of utmost importance. Our previously reported fast measurements (in μs timescale) of temperature dependent amorphous resistance shows metastable resistivity of amorphous Ge₂Sb₂Te₅ (GST) – the most widely used material for PCM [2]. Interestingly, the metastable amorphous resistivity versus temperature shows a pure exponential behavior, and the thin film molten resistivity falls on the same exponential. Considering Arrhenius behavior of conductivity, this pure exponential decrease in resistivity implies a temperature dependent carrier activation energy for metastable amorphous GST following a quadratic relation. The extracted activation energy has a peak near 450 K. The effective activation energy is also used to calculate the effective trap energy during device operation. Also, by mapping the metastable amorphous activation energy to versus activation energy for mixed phase crystalline GST [3], we estimate Seebeck coefficient (S) for metastable amorphous GST – which is difficult to directly measure at device level because of the requirement to maintain a small known temperature gradient during the measurement. The temperature dependent conduction activation energy and Seebeck coefficient for metastable amorphous GST extracted in this work are critical parameters for accurate modeling of PCM devices [4].

References
[1] S. W. Fong, C. M. Neumann, and H.-S. P. Wong, “Phase-Change Memory—Towards a Storage-Class Memory,” IEEE Trans. Electron Devices, vol. 64, no. 11, pp. 4374–4385, Nov. 2017.
[2] F. Dirisaglik, G. Bakan, Z. Jurado, S. Muneer, M. Akbulut, J. Rarey, L. Sullivan, M. Wennberg, A. King, and L. Zhang, “High speed, high temperature electrical characterization of phase change materials: metastable phases, crystallization dynamics, and resistance drift,” Nanoscale, vol. 7, no. 40, pp. 16625–16630, 2015.
[3] L. Adnane, F. Dirisaglik, A. Cywar, K. Cil, Y. Zhu, C. Lam, A. F. M. Anwar, A. Gokirmak, and H. Silva, “High temperature electrical resistivity and Seebeck coefficient of Ge ₂ Sb ₂ Te ₅ thin films,” J. Appl. Phys., vol. 122, no. 12, p. 125104, Sep. 2017.
[4] Z. Woods, J. Scoggin, A. Cywar, L. Adnane, and A. Gokirmak, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part II—Discrete Grains,” IEEE Trans. Electron Devices, vol. 64, no. 11, pp. 4472–4478, Nov. 2017.

Phase change memory (PCM) is a novel nonvolatile memory technology that stores data in a highly resistive amorphous or highly conductive crystalline phase of a chalcogenide such as Ge₂Sb₂Te₅ (GST). PCM devices switch by melting and rapidly quenching crystalline material to amorphous (reset) or heating amorphous material until it crystallizes (set). Accurate modeling of reset and set requires accurate material parameterization over the temperatures experienced during device operation (300 K~1000 K), but high temperature parameters are often difficult to obtain due to rapid (as fast as ~ns) transitions from amorphous to cubic and cubic to hexagonal phases. Where high temperature data is not known, it is extracted from low temperature measurements. For example, room temperature specific heats of amorphous and cubic GST have been measured to be approximately equal, and this room temperature value is often treated as a temperature independent parameter for both phases. However, the measured heat released during crystallization and heat absorbed during melt are not equal, and thus the specific heats of amorphous and cubic GST are thermodynamically required to diverge at some point (likely at the glass transition temperature) [1]. We model high temperature specific heats in GST by enforcing the thermodynamic relationship between specific heat and enthalpy. We use the resultant specific heats coupled with the measured heat of fusion to implement a temperature and phase transition rate dependent heat source which captures at once both a variable heat of crystallization and the heat of fusion. Using our finite element phase change model [2], [3] fully coupled with electrothermal physics, we analyze the effects of a variable heat of crystallization on grain maps in GST due to the exponential dependence of nucleation and growth rates on temperature. We also simulate reset and set operations on mushroom and pillar cell geometries and show more crystallization during reset, longer set times, higher set energies, and lower thermal crosstalk than predicted by temperature independent specific heats and heat of crystallization.
[1] J. Kalb, “Crystallization kinetics in antimony and tellurium alloys used for phase change recording,” 2006.
[2] Z. Woods and A. Gokirmak, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part I—Effective Media Approximation,” IEEE Trans. Electron Devices, vol. 64, no. 11, pp. 4466–4471, Nov. 2017.
[3] Z. Woods, et. al, “Modeling of Phase-Change Memory: Nucleation, Growth, and Amorphization Dynamics During Set and Reset: Part II--Discrete Grains,” IEEE Trans. Electron Devices, pp. 1–7, 2017.

In conventional metals, itinerant electrons carry both charge and heat. As a consequence, electrical conductivity and the electronic contribution to thermal conductivity are typically proportional to each other, known as the Wiedemann-Franz law that is robust in all conventional metals known as Fermi liquids. We found a large violation of this law near the insulator-metal phase transition in metallic vanadium dioxide (VO₂). In the metallic phase, the electronic contribution to thermal conductivity amounts to only 10% of what would be expected from the Wiedemann-Franz law. The results are explained in terms of independent propagation of charge and heat in a strongly correlated electron system, where the electrons move in unison in a new, non-quasiparticle mode. In contrast, chemical doping and artificial introduction of point defects in VO₂ apparently recover its behavior toward normal metals, opening opportunities to discovery of new physics as well as new applications of the metal-insulator transition. Utilizing the phase transition in VO₂ films, we develop a lithography-free, rewritable “meta-canvas” on which nearly arbitrary photonic devices can be rapidly and repeatedly written and erased. Using the meta-canvas, we demonstrate dynamic manipulation of optical waves for light propagation, polarization and reconstruction.

VO₂ shows a metal-insulator phase transition (MIT) at 68°C which is accompanied by a strong decrease in electrical resistivity, change in optical transmittance and reflectance in the near infrared spectral region. The MIT is also marked by a structural phase transition with large anisotropic and abrupt length change by approximately 1% and 2% when the material transforms from either the insulating monoclinic M1 or mechanical stress stabilized monoclinic M2 to the tetragonal rutile phase R. The large change of the VO₂ unit cell volume at the MIT produces in bulk VO₂ crystals huge mechanical stress-strain fields leading to severe deterioration of crystal quality.
In this work nanocrystals of VO₂ buried in SiO₂ were investigated. In contrast to bulk VO₂ crystals VO₂ nanocrystals can be temperature cycled through the MIT without any signs of fatigue effects. The VO₂ nanocrystals were synthesized by sequential ion implantation of the elements vanadium and oxygen followed by a rapid thermal annealing step. We show in this context that chemical phase selective synthesis and positioning of VO₂ nanoclusters buried in thermally grown SiO₂ on Si-substrates and fused silica substrates can be achieved by ion implantation of stoichiometric fluence ratios of the elements V and O.
The MIT of the VO₂ nanocrystals was investigated as a function of temperature by micro-Raman scattering measurements and ellipsometry in the spectral range from UV to the near IR. The temperature dependent dielectric function of the VO₂ nanocrystals embedded in the SiO₂ matrix was determined by applying a Drude-Lorenz oscillator model and using an effective medium approximation. During temperature cycling through the MIT we observe a huge temperature hysteresis with a width of more than 50 K. This hysteresis is characterized by asymmetric super heating and super cooling around the MIT temperature of 68°C of bulk VO₂ material. This can be attributed to the first order phase MIT and the high crystalline quality of the VO₂ nanocrystals by very effectively suppressing seeding effects due to lack of metallic and dielectric phase coexistence in the single VO₂ nanocrystals. First thermochromic optical devices were fabricated demonstrating high contrast switching at different wavelengths [1,2,3].

[1] T. Jostmeier et al., Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposits, Optics Express 24, 15 (1016)
[2] T. Jostmeier et al., Optically imprinted reconfigurable photonic elements in a VO₂ nanocomposit, Appl. Phys. Lett. 105, 071107 (2014)
[3] J. Zimmer et al., Ion beam synthesis of nanothermochromic diffraction grantings with giant switching contrast at telecom wavelengths, Appl. Phys. Lett. 100, 231911 (2012)

In recent years, the need for smart window materials that lower the energy consumption for heating, venting and air-conditioning of buildings has grown immensely. These smart materials undergo a reversible change in physical properties depending on various conditions. A material that fits this description is vanadium dioxide, a thermochromic material that changes from a monoclinic to a rutile phase when heated above a critical temperature. This metal-insulator transition (MIT) leads to the reflection of infrared radiation. By reflecting this, a lower amount of heating-up occurs inside buildings and less cooling is needed.[1] In recent years, interest for this material has grown immensely.[2]
During this work, the main focus is the development of novel and easy methods to synthesize thermochromically active vanadium dioxide nanoparticles. Microwave syntheses were performed and optimized. The influence of various reaction parameters on the morphology, crystal structure and thermochromic properties of the nanosized materials was studied.
After synthesis, these nanoparticles are incorporated in an inorganic matrix. This mixture of nanoparticles and inorganic matrix material is deposited on flexible polymeric substrates. This method implies a facile scale up towards industrial production of smart window films. The inorganic matrix of choice is scratch resistant, which leads to optimized protection and prolonged lifetimes of said polymeric smart window films.

[1] A. Gonçalves. J. Resende. A. C. Marques. J. V. Pinto. D. Nunes. A. Marie. R. Goncalves. L. Pereira. R. Martins, E. Fortunato, Solar Energy Materials and Solar Cells 150 (2016) 1.
[2] M. M. Seyfouri, R. Binions, Solar Energy Materials and Solar Cells 159 (2017) 52.

It is known that reversible pressure-induced insulator-metal (I-M) transition on V₂O₃ at room temperature is accompanied by a jump of the degree of strain, i.e. c/a ratio [1][2]. This property leads one to consider application for switches and transistors [3]. In such devices, it would be required to control the strain of a V₂O₃ film in-situ, which is difficult if the film is clamped to a rigid substrate. In the present study, we transferred a V₂O₃ thin film without a rigid substrate onto a piezoelectric disk, in order to modulate the resistance of the V₂O₃ film, R_V2O3, through its c/a with the piezoelectric effect.
We grew a V₂O₃ thin film by a pulsed laser deposition technique on a mica substrate, then peeled it off by means of a Scotch tape method, and glued it onto a commercial piezoelectric element. On this sample we measured R_V2O3 by a four-probe method as a function of the piezo disk bias, V_piezo. The R_V2O3–V_piezo property showed a butterfly-type curve, which obviously reflected the bias-induced deformation of the piezo material. The change of R_V2O3 was more drastic than the estimation assuming a simple elastic deformation of a solid material. Such resistance modification is supposed to be a sign of the strain-induced I-M phase transition characteristic to V₂O₃, and suggesting a possibility of simple structured piezoelectrically-driven devices.

[1] McWhan et al., Phys. Rev. B 7 (1973) 1920.
[2] Rodolakis et al., Phys. Rev. B 84 (2011) 245113.
[3] Newns et al., J. Appl. Phys. 111 (2012) 084509.

Vanadium dioxide (VO₂) is one of promising materials for electrical/optical switches and sensors because of its temperature-driven metal–insulator transition (MIT) accompanied by significant changes in conductivity and optical properties at above room temperature. Because the MIT is accompanied by a structural phase change, the lattice dynamics and the rearrangement of atomic positions through the change have an impact on changes in electronic states that have to be controlled in device applications. Raman scattering spectroscopy is one of the most popular tools for investigating phonon modes. However, because of the lack of high-quality single crystals or epitaxial thin films of VO₂, the symmetry of Raman-active phonon modes and their Raman tensors remain to be completely elucidated.
In this study, we have investigated the symmetries and the Raman tensors of the phonon modes of VO₂ evaluated by the polarization angular dependence of Raman scattering for epitaxial VO₂ films on MgF₂ substrates. By comparing the tensor elements with a previous theoretical calculation, we identified the two characteristic phonon modes of the V–V dimers, which are believed to play an important role for opening a band gap at the Fermi level in the insulating phase. These findings provide clues to understanding the lattice dynamics of VO₂ which are required for elucidating the mechanism of the MIT in VO₂ and controlling the MIT in practical device applications.