Symposium MD4 : Phase-Change Materials and Applications

The mechanism in phase change materials involves three different states of a defined alloy: crystalline, amorphous and liquid. Non-crystalline phases of phase change materials are at the center of this presentation. The first part is devoted to phase change materials in the liquid state. Diffraction and inelastic scattering measurements performed on a wide range of Te-based alloys [1-3] are presented. It is shown that the liquid structure, in addition to be a good starting point to describe the amorphous phase, provides useful information on the suitability of an alloy to be a good phase change material. Moreover, it serves as an important input for first-principles molecular dynamics calculations. The second part is devoted to the amorphous state of phase change materials. An experimental strategy is proposed to follow the local order changes in the amorphous phase up to the crystallization. The method consists in using differential scanning calorimetry simultaneously coupled with x-ray absorption spectroscopy. In spite of being far from working conditions, it is shown that such measurements, close to the thermodynamic equilibrium, are clear indicators of the cycling ability of a given composition [4].

References

[1] Steimer C., Coulet M.-V., Welnic W., Dieker H., Detemple R., Bichara C., Beuneu B., Gaspard J.-P., Wuttig M. Advanced Materials, 20, 1–6 (2008).
[2] Micoulaut M., Coulet M.-V., Piarristeguy A., Johnson M.R., Cuello G.J. , Bichara C., Raty J.-Y., Flores-Ruiz H., Pradel A. , Physical Review B 89, 174205 (2014)
[3] Flores-Ruiz H., Micoulaut M., Coulet M.-V., Piarristeguy A., Johnson M.R., Cuello G.J. , Pradel A. , Physical Review B 92, 134205 (2015)
[4] Zalden P., Aquilanti G., Prestipino C., Mathon O., André B, Wuttig M., Coulet M.-V. Journal of Synchrotron Radiation, 19, 806−813 (2012).

The working principle of non-volatile phase-change memory (PCM) devices involves phase transitions between amorphous and crystalline phases, mediated by the liquid phase in the case of amorphization. Therefore, understanding the physics involved in such phase transitions has been one of the key research topics in studying PCM materials so far. Intensive efforts have been devoted to finding the unique structural, and electronic, signatures of PC materials that differentiate them from other non-PCM materials: the identification of such signatures would provide a crucial insight into the mechanism of the phase transitions, thereby helping to enhance our understanding of PC materials, and eventually to develop new materials for PCMs.
A structural signature of the prototypical PC materials, Ge-(Sb-)Te [G(S)T], may be identified from their amorphous and crystalline structures. Metastable crystalline G(S)T forms a distorted rock-salt cubic structure. Although some portion of the Ge atoms are known to be tetrahedrally coordinated in the amorphous form, most of them seem to adopt an octahedral coordination at elevated temperatures according to the bond-angle distributions obtained from ab initio molecular-dynamics (AIMD) simulations. For this reason, (defective) octahedral local coordination has been considered as a structural signature of G(S)T PC materials which would enable, for instance, ultrafast crystallization and a large contrast in electrical resistance in the course of phase transitions. In this talk, we will discuss the various structural units centred at each constituent element in G(S)T PC materials (including octahedral and tetrahedral units), which are identified from crystalline and amorphous models generated from AIMD simulations using definitions based on bonding characteristics rather than interatomic-separation cutoffs. In addition, implications for the unique dynamical characteristics of PC materials, such as fast crystallization speeds, will be discussed in terms of the observed dynamical behaviour of the structural units constituting G(S)T materials.

Phase-change materials (PCMs) are among the leading candidates for new non-volatile memory technologies. Miniaturization is of tremendous interest for such new applications, and each step towards a smaller scale increases the importance of interfaces and surfaces. In this work the low-index surfaces of one of the prototype materials, GeTe, are investigated using DFT simulations.
In recent years the GeTe(111) surface has been studied by experiment and theory alike [1–3]. Apart from physical deposition of thin films there lies great potential in a chemical way to synthesize GeTe nanostructures. In this regard, there is a need for new theoretical surface models which, if successful, could be used to predict crystal shapes and promote the understanding of experimental data such as given in ref. [4].
For the first time, we created a full set of ab initio surface models for all low-index surfaces of GeTe by investigating the ratio of surface energies for clean surfaces. Not only were the experimentally observed octahedral crystals successfully recovered by those new models, but also additional surface–ligand-interactions were studied keeping the wet chemical route in mind.
To do so, several representative ligands were chosen and their influence on surface energies and crystal morphology was examined extensively. By delving into different halides, possible surfactants, and solvents, a diverse picture of possible interactions is described, supporting chemical intuition and assisting the understanding of experimentally observed structures.

[1] L. V. Yashina, R. Püttner, V. S. Neudachina, T. S. Zyubina, V. I. Shtanov, M. V. Poygin, J. Appl. Phys. 2008, 103, 094909.
[2] V. L. Deringer, M. Lumeij, R. Dronskowski, J. Phys. Chem. C 2012, 116, 15801–15811.
[3] R. Wang, J. E. Boschker, E. Bruyer, D. Di Sante, S. Picozzi, K. Perumal, A. Giussani, H. Riechert, R. Calarco, J. Phys. Chem. C 2014, 118, 29724–29730.
[4] M. R. Buck, I. T. Sines, R. E. Schaak, Chem. Mater. 2010, 22, 3236–3240.

Chalcogenide phases are of enormous technological importance nowadays because they serve as, for instance, phase-change [1] and thermoelectric materials [2]. Antimony telluride, whose properties are part of this presentation, has turned out to be a topological insulator [3].
We present the results of density-functional theory-based investigations regarding the lattice dynamical and related properties of several chalcogenide materials, such as antimony telluride, Sb₂Te₃ [4], antimony selenide, Sb₂Se₃ [5], and germanium selenide, GeSe [6], using the ab initio force-constant method [7]. Thermodynamic properties, such as volume expansion and constant-pressure heat capacities, are available in the framework of the quasi-harmonic approximation [8].
In particular, we will present simulation results of the behavior of chalcogenides under pressure, with a special focus on stabilization of selected polymorphs. As an indicator of dynamical stability, we utilize the absence of so-called imaginary frequencies which destabilize the crystal structure. For example, we prove the stabilization of rock-salt type germanium selenide under pressure by lattice dynamical calculations [6].
Another important task is the investigation of atomic force constants, which give valuable information on atomistic interactions in chalcogenide materials. On-site force constants of Sb₂Te₃ from first principles calculations [4] are generally in good agreement with experimental ones determined from nuclear inelastic scattering experiments [9]. Recently, we introduced the calculation of bond-projected force constants, exemplarily for antimony selenide [5], which seems to be a promising tool for the understanding of vibrational processes in chalcogenide materials.

References
[1] M. Wuttig, N. Yamada, Nature Mater. 6, 824–832 (2007).
[2] G. J. Snyder, E. S. Toberer, Nature Mater. 7, 105–114 (2008).
[3] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang S.-C. Zhang, Nat. Phys. 5, 438–442 (2009).
[4] R. P. Stoffel, V. L. Deringer, R. E. Simon, R. P. Hermann, R. Dronskowski, J. Phys.: Condens. Matter 27, 085402 (2015).
[5] V. L. Deringer, R. P. Stoffel, M. Wuttig, R. Dronskowski, Chem. Sci. 6, 5255–5262 (2015).
[6] V. L. Deringer, R. P. Stoffel, R. Dronskowski, Phys. Rev. B 89, 094303 (2014).
[7] A. Togo, I. Tanaka, Scripta Mater. 108, 1–5 (2015).
[8] R. P. Stoffel, C. Wessel, M.-W. Lumey, R. Dronskowski, Angew. Chem. Int. Ed. 49, 5242–5266 (2010).
[9] D. Bessas, I. Sergueev, H.-C. Wille, J. Persson, D. Ebling, R. P. Hermann, Phys. Rev. B 86, 224301 (2012).

Disorder-induced electron localization and metal-insulator transitions (MITs) have been a very active research field starting from the seminal paper by Anderson half a century ago. However, pure Anderson insulators are very difficult to identify due to ubiquitous electron-correlation effects. Recently, an MIT has been reported in crystalline phase-change GeSbTe (GST) compounds, which appears to be exclusively disorder driven as indicated by electrical transport measurements [1,2] and density functional theory (DFT) simulations [3,4]. All these works rely on key assumptions about structural properties of GST. Here we report a direct atomic scale chemical identification experiment on the rocksalt structure obtained upon fast crystallization of amorphous GST. Our results firmly establish the two-sublattice structure resolving the distribution of chemical species, and unequivocally demonstrate the existence of atomic disorder on the Ge/Sb/vacancy sublattice. Moreover, we identify a gradual vacancy ordering process upon further annealing that was predicted to trigger the transition to extended electronic states. These findings not only provide a structural underpinning of the observed Anderson localization, but also have implications for the development of novel multi-level data storage within the crystalline phases.

[1] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater. 10, 202-208 (2011).
[2] H. Volker, P. Jost, M. Wuttig, Adv. Funct. Mater. DOI: 10.1002/adfm.201500830 (2015).
[3] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J. Y. Raty, M. Wuttig, S. Blügel, R. Mazzarello, Nat. Mater. 11, 952-956 (2012).
[4] W. Zhang, M. Wuttig, R. Mazzarello, Sci. Rep. 5, 13496 (2015).

Phase-Change Materials (PCMs), mainly represented by (GeTe)_x-(Sb₂Te₃)_y (GST) alloys, are used for high-density data storage in optical media and for solid-state non volatile memory [1]. Recently, it has been shown that multi-layered crystalline phase change memories, such as interfacial PCM (iPCM) can have improved functional properties [2, 3]. The high-ordered GST layers in the trigonal structure are characterized by the presence of van der Waals (vdW) gaps in between two planes of Te.
In this work we compare the structures of GeSbTe thin films deposited by Molecular Beam Epitaxy (MBE) and by sputtering, followed by annealing at temperatures in the range 110°C-350°C, in order to investigate the mechanism involved in the ordering of vacancy site in the vdW layers.
Analyses were performed by means of High Angular Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM). This technique directly relates the micrograph contrast to the atomic number (Z-contrast), allowing a straightforward interpretation of the images.
In the metastable rock-salt phase, formed at low temperature (150°C), no vdW gaps have been observed, with vacancies randomly distributed in the 4a Wyckoff site together with Ge and Sb.
In the high temperature annealed samples, either deposited by MBE or sputtering, van der Waals (vdW) gaps are clearly seen between two Te planes [4, 5], indicated by the strongest Z-contrast signal. In these samples, we also clearly distinguish compositional disorder, as suggested by the presence of different stacking sequences. In particular, we observed blocks of 7, 9 or 11 planes, corresponding to Ge₁Sb₂Te₄, Ge₂Sb₂Te₅ and Ge₃Sb₂Te₆, respectively. In agreement with Rutherford Backscattering Spectrometry (RBS), Ge-rich films (23.5%) contain more Ge₃Sb₂Te₆ blocks with respect to Ge₁Sb₂Te₄ blocks.
Along the stacking sequence, for all the stoichiometries, different plane spacings are clearly distinguished. The width of the vdW gap amounts to 2.7 Å, the planes adjacent to vdW gap are characterized by a spacing of 1.6 Å, while the next plane lies at a distance of 2.1 Å. This difference might be associated to the strength of the bonds Te-Te (vdW gap) and Te-Ge/Sb.
In the sample annealed at intermediate temperature, partially depleted Ge/Sb layers have been detected. With the increasing of the annealing temperature or the degree of order, Sb-rich layers appear next to the Te-Te planes.

1. M. Wuttig, N. Yamada, Nat. Mater. 6 , 824 (2007).
2. J. Tominaga, R. Simpson, P. Fons and A.V. Kolobov, Appl. Phys. Lett. 99, 152105 (2011).
3. Tominaga et al., Sci. Technol. Adv. Mater. 16, 014402 (2015).
4. T. Matsunaga et al., Acta Crystallogr., Sect. B: Struct. Sci., 60, 685 (2004).
5. E. Rotunno, L. Lazzarini, M. Longo and V. Grillo, Nanoscale 5, 1557 (2013).

Phase change materials with composition along the pseudo-binary line GeTe-Sb₂Te₃ are characterized by a stable phase with trigonal structure, in which well ordered vacancy layers are formed. By crystallizing the material at temperature around 150°C or through current or laser pulses, the metastable rocksalt structure is formed, instead, with structural vacancies randomly distributed in the Ge/Sb sublattice. It has been shown in literature that the properties of GeSbTe alloys can be tuned by modifying the degree of order in the material. For example, it has been shown that the conversion from an insulating into a metallic material can be obtained through the reduction of disorder by thermal annealing or by changing the composition.
In this work we study the opposite route, starting from the crystalline structures and introducing disorder by ion irradiation. Ge₂Sb₂Te₅ (GST) samples, sputter deposited at room temperature in the amorphous phase, have been annealed at 200°C or 350°C, in order to form the rocksalt or the trigonal phase, respectively. Then the samples were irradiated at room temperature and at 70 K using 150keV Ar⁺ ions at different doses.
The phase transitions have been studied by in-situ monitoring of the reflectivity changes as a function of the dose. X-ray Diffraction measurements and Transmission Electron Microscopy have been employed for the structural characterization and the electrical properties have been investigated by ex-situ electrical measurements.
Under irradiation, as the dose increases, the reflectivity of the GST films exponentially decreases and reaches a minimum value corresponding to a completely amorphous material. Amorphous phases formed under different irradiation conditions and starting from different crystalline phases have been studied.
Large differences in the threshold dose for amorphization have been observed between the metastable and the stable phase, by changing the temperature during irradiation. At 70 K both the phases become amorphous at a dose of 5x10¹³ cm^-2, whilst at room temperature the complete amorphization of the stable trigonal phase occurs at a dose of 2x10¹⁴ cm^-2, double than that required for the rocksalt phase.
Ion irradiation also affects the electrical properties of the material: the temperature dependence of the resistance of the trigonal GST as a function of irradiation dose exhibits a metal-insulator transition well below the amorphization threshold (around 10%).
By structural analyses we found that, before amorphization, ion irradiation induces a transition from the trigonal stable phase to the rocksalt structure. By further increasing the irradiation dose the formed rocksalt phase converts into the amorphous phase.
The results underlie the relevant role played by the disorder in the phase stability and indicate ion irradiation as a powerful technique to tune the electrical and structural properties of phase change materials in a well reproducible way.

Chalcogenide phase-change materials have important applications in electronic non-volatile memories, due to the pronounced resistivity contrast between their crystalline and amorphous state. Fast and reversible switching between the two phases can be induced by current pulses. The GeSbTe compounds lying on the pseudobinary line GeTe-Sb₂Te₃ are the most widely studied family of phase-change materials. Recently, it has been shown that a metal-insulator transition occurs in crystalline GeSb₂Te₄. The transition is of Anderson type and is triggered by the ordering of the vacancies upon thermal annealing. In this work, we investigate the localization properties of electrons in selected crystalline GeSbTe compounds with varying GeTe content by first-principles methods. We consider realistic models containing up to 1150 atoms. We also include excess vacancies, which are needed to account for the large carrier concentrations determined experimentally. We show that the compounds with a high concentration of stoichiometric vacancies (e.g. GeSb₂Te₄ and Ge₃Sb₂Te₆) possess states at the Fermi energy localized inside vacancy clusters. On the other hand, in GeTe-rich models the region of localized states shrinks due to the low probability of having vacancy clusters, whereas the excess vacancies lead to a shift of the Fermi energy across the mobility edge. As a result, these models exhibit metallic behavior. These findings indicate that a stoichiometry-controlled metal-insulator transition occurs in GeSbTe, in agreement with recent experiments.

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 application 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 resonant bonding, a particular flavour of covalent bonding, which can explain many of the characteristic features of phase change materials. This insight is employed to predict systematic property trends and to explore the limits in stoichiometry for such memory applications. It will be demonstrated how this concept can be used to tailor the electrical and thermal conductivity of phase change materials.

Ge₂Sb₂Te₅ (GST) is the most common chalcogenide materials serving as the programming layer of phase-change memory (PCM). Since the signal recording of PCM is induced by the electrical heating, the electromigration (EM) behaviors and thermal properties of GST are hence the key issues affecting the device reliability. This work studies the EM characteristics of pristine GST and cerium-doped GST (Ce-GST) strips under pulse bias and their influence on the operational properties of PCM devices.
The GST layers with the thickness of 70 nm were deposited on Ti pads by sputtering method and, with the aid of focused ion beam technique, the GST strips with the width of 70 nm was formed in order to simulate the nano-scale geometry of practical PCM devices. The mean-time-to-failure (MTTF) test under pulse bias was then performed to evaluate the EM behaviors of GST strips. Analytical results revealed the increase of peak current density (j_p) and duty cycle (d) of pulse bias accelerates the EM failure whereas the frequency (f) of pulse bias insignificantly affects the EM lifetime of GSTs when f > 10 MHz. Moreover, at f > 10 MHz, the EM behaviors of GSTs under pulse bias can be depicted by the “average current model”, i.e.,MTTF = C''j_avg^-2exp(E_a/k_BT), where j_avg = j_p×d and E_a is the activation energy of EM process. The MTTF tests at various temperatures found the E_a values are 0.63 eV and 0.56 eV for GST and Ce-GST, respectively. The lower E_a values compared with those obtained by direct-current MTTF test indicated the enhancement of surface diffusion during EM process due to the nano-scale sample geometry and the skin effect. As to the lower E_a value for Ce-GST, it was ascribed to the grain refinement which amplifies the short-circuit diffusion and accelerates the EM failure in doped sample. Scanning electron microscopy observed the failure occurs at the cathode side of GST strips, implying the dominance of electron-wind force effect during EM process. However, the element segregation deduced by energy dispersive spectroscopy indicated the electrostatic force might also involve in the EM failure of GSTs due to the relatively high j_p of our MTTF test.

Phase-change memory (PCM) containing chalcogenides as the programming layers has been recognized as one of the next-generation nonvolatile memory due to its promising features of operational speed, data storage capacity, scalability, endurance and compatibility to complementary metal-oxide-semiconductor technology. In the issue of scalability, PCM is a highly scalable memory technology and one of the methods to accomplish such a purpose is the formation of chalcogenide programming layer with nanowire (NW) geometry. This work prepares the single-crystalline GeTe (GT) and Ge₂Sb₂Te₅ (GST) NWs by using the vapor-liquid-solid process so that their feasibility to PCM fabrication can be explored in the future.
The NW samples were prepared by first depositing 4-nm thick Au film on SiO₂/Si substrate via sputtering, annealing at 600°C for 1 min to form the Au nanoparticles with 30 nm in diameter, and transferring to a three-zone tube furnace to serve as the catalyst for NW growth. Subsequently, the GT or GST NWs were grown at various conditions of ambient pressures, temperatures and carrier gas flow rates by placing the GT or GST powders at the high-temperature side of furnace to serve as the precursor.
Analytical results indicated that, at the pressure of 1 torr, the GT NWs with the diameters ranging from 200 to 300 nm and the length up to 30 μm could be achieved at the gas flow rate/temperature conditions of 0 sccm/425-475°C, 50 sccm/425-450°C, 100 sccm/425-475°C, and 1000 sccm/400-450°C. Similarly at the pressure of 1 torr, the GST NWs with the diameters in range of 150 to 800 nm and the length up to 40 μm could be achieved at the flow rate/temperature condition of 50 sccm/370-460°C. X-ray diffraction indicated GT NWs are of the rhombohedral GT phase whereas GST NWs are of the hexagonal GST phase. Transmission electron microscopy revealed the single-crystalline structure for both GT and GST NWs. Moreover, the GT NWs grew along the c-axis direction of the rhombohedral lattice and the Ge and Te elements uniformly distribute along NWs with the stoichiometric ratio of 1:1.

Keywords: Chalcogenides, GeTe, Ge₂Sb₂Te₅, Nanowire, Phase-change memory, Vapor-liquid-solid process.

Recent advancements in thin film growth techniques have demonstrated the ability to grow large grained high aspect ratio (lateral dimension/thickness) thin films using thin-film vapor-liquid-solid (TF-VLS) growth.^[1] The key to growing these large grained films is the fact that the confinement of the film by a substrate and a coating enables one to control the nucleation rate. Using this knowledge, InP films with grains as large as 100 microns have been realized for films that are approximately 3 microns thick. The question immediately arises: Can the thin film VLS process be used to grow extremely thin films only a few layers thick? Answering this question requires a detailed understanding of nucleation processes in extremely thin films.
In order to explore this question, classical nucleation theory was applied to the solidification of a melt confined between two flat surfaces. Using analytical and finite element techniques, the critical nucleus shape and energy were calculated as function of the film’s thickness and the relevant bulk and interfacial free energies governing the system. Typical homogenous nucleation and heterogeneous nucleation were considered, along with heterogeneous nucleation with a catenoid-like shape. It was found that the nucleation rate can vary by orders of magnitude, and in certain situations be heavily suppressed, thus exhibiting melting point suppression. Using the nucleation rates, approximate grain sizes were calculated as a function of the film’s thickness, the system’s energy parameters, and the system’s undercooling.

References
R. Kapadia, Z. Yu, H.-H. H. Wang, M. Zheng, C. Battaglia, M. Hettick, D. Kiriya, K. Takei, P. Lobaccaro, J. W. Beeman, J. W. Ager, R. Maboudian, D. C. Chrzan, and A. Javey, Scientic Reports 3, 2275 (2013).

Acknowledgement
This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231

Recently, interfacial phase change memory (iPCM) materials consisting of GeTe and Sb₂Te₃ layers [1] have received considerable attention because of not only its low phase switching energy but also the topological insulating properties including peculiar phenomena such as huge magnetoresistance [2] and anomalous magneto-optical Kerr-rotation [3]. Since topological insulator possesses Dirac cones which can interact with long-wavelength lights, iPCM materials as well as graphene are potential candidates for terahertz (THz) applications. For development of THz technologies including THz imaging, high-sensitivity 2D array sensor is required. Compared with graphene, iPCM material has high scalability and phase change nature which lead to the modification of electronic structure and hence this material is promising for THz wave detection.

Here, we report the results of THz time-domain spectroscopy (TDS) and the demonstration of THz detection with topological iPCM material. The iPCM sample investigated was a thin film of [(GeTe)₂(Sb₂Te₃)₄]₈. Also Ge₂Sb₂Te₅ alloy (GST) is used for comparison. The THz transmission spectra of the samples were measured with a THz-TDS system. Photoconductive-type light detection system was employed and THz-induced change in the applied bias voltage (0.2 V) was recorded by a digital oscilloscope as a function of time.

The results of TDS measurements indicate that the GST alloy sample is nearly transparent for THz wave. Indeed, the photon energy of the THz pulse currently used is smaller than the band gap of the alloy sample. On the other hand, we found that the transmittance of the [(GeTe)₂(Sb₂Te₃)₄]₈ sample is lower than that of the alloy sample. The FT spectra of the time-domain signals revealed that iPCM absorbs THz waves uniformly over the frequency range of our THz wave (0.5 ~ 3.5 THz). These results can be attributed to the existence of Dirac cones in the topological insulating surface and interfaces due to the multi-layered structure of iPCM. Then, photoconductive measurement were carried out and we observed the THz-induced decrease in the sample resistance. Therefore, we concluded that our iPCM-based THz detection system is capable of THz detection. Furthermore, nonlinear enhancement of the signal was observed when the power of the incident THz pulse was increase.

[1] R. E. Simpson et al., Nature Nanotech. 6, 501 (2011).

[2] J. Tominaga et al., APL 99, 152105 (2011).

[3] D. Bang et al., Sci. Rep. 4, 5727 (2014).

For perovskite transition metal oxides, oxygen vacancy has been the key ingredient for the defect engineering, since it plays a central role in determining the crystal field and consequent electronic structure which leads to important electronic and magnetic phase transitions. In this study, we investigated the physical properties of epitaxial SrTiO₃ (STO), SrRuO₃ (SRO), and CaRuO₃ (CRO) thin films by systematically introducing cation and/or oxygen vacancies during the pulsed laser epitaxy (PLE) growth. In particular, we employ the preferential collision of ionic species with the oxygen molecules inside the vacuum chamber during the PLE growth which results in a drastic difference in the plume propagation. For ABO₃ perovskite oxides, it is known that the lighter cations scatter more easily compared to the heavier ones at high oxygen partial pressure (P(O₂)). Using this preferential scattering, we can systematically control the cation-deficiency in epitaxial thin films. For the STO thin films, we independently introduced Sr and oxygen vacancies in the system and separately identified their roles. P(O₂) and oxygen flow rate was critical in controlling the oxygen vacancies which resulted in insulator-to-metal transition. On the other hand the plume dynamics was responsible for introducing the Sr vacancies, which was responsible for the cubic-to-tetragonal phase transition. For the SRO thin film, a drastic change in the electronic band structure was observed across the orthorhombic-to-cubic structural phase transition, which could be induced by introducing RuO vacancies. The elemental vacancies and larger in-plane compressive strain further suppressed the ferromagnetic T_C to below 150 K with increased electric resistivity in tetragonal SRO thin films. The control of multiple phase transitions in complex oxides exploiting selective vacancy engineering provided by PLE opens an unprecedented opportunity for understanding and tailoring the complex oxide thin films.

Pulsed laser deposition (PLD) is a technique which utilizes a high energy pulsed laser ablation of targets to deposit thin films on substrates in a vacuum chamber. The high-intensity laser pulses create a plasma plume from the target material which is projected towards the substrate whereupon it condenses to deposit a thin film. Here we investigate the properties of vanadium oxide thin films prepared utilizing two variations of the pulsed laser deposition (PLD) technique: femtosecond PLD and nanosecond PLD. Femtosecond PLD (f-PLD) has a significantly higher peak intensity and shorter duration laser pulse compared to that of the excimer-based nanosecond PLD (n-PLD). Experiments have been conducted on the growth of thin films prepared from V2O5 targets on Si wafer, sapphire and glass substrates using f-PLD and n-PLD. Characterization using SEM, XRD and Raman spectroscopy shows that the f-PLD films have significantly rougher texture prior to annealing and exhibit with an amorphous + nano-crystalline character whereas the thin films grown using n-PLD are much smoother and highly predominantly amorphous. The surface morphology, structural, vibrational, and chemical- and electronic-state elemental properties of the vanadium oxide thin films, both prior to and after annealing to 450 °C, will be discussed.

Germanium as a key material in modern material science and electronic industry possesses several phases under pressure. The particularly interesting allotrope is the ST12 structure as it persists at ambient conditions. However, its electronic band structure and bandgap have been discrepantly predicted for both direct and indirect types with a gap varying from 0.5 to 1.4 eV. On the other side, experimental examinations are difficult and rare primarily due to lack of large bulk samples. The sizes of previously reported pure ST12 Ge samples range from a few nm to in the order of ten micron meter in dimension, preventing definitive experimental measurements. The transition temperature from the ST12 phase to diamond structure (Ge-I) was also reported differently from 200 ^oC to 800 ^oC. In this work we report both experimental and theoretical results for the ST12 phase. We report synthesis of phase pure bulk ST12 Ge with the size of a few millimeters through the multi-anvil press method. Powder X-ray diffraction confirmed the structure. We also report the first low-frequency Raman spectra. The optical and temperature-dependent electrical transport measurements show ST12 Ge to be a semiconductor with a fundamental indirect bandgap ~ 0.6 eV and a direct optical transition ~ 0.8 eV, in agreement with our first principles calculations based on density functional theory with hybrid functionals. We also report that the metastable ST12 phase starts to transform into the thermodynamically stable cubic diamond phase at 200 ^oC.

Phase change memory (PCM) has recently emerged as one of the most promising candidates for beyond FLASH memory technology with excellent density/speed capabilities. Prototypical phase change materials lie along the pseudo-binary line of GeTe-Sb₂Te₃. In particular, Ge₂Sb₂Te₅ has been extensively investigated due to its fast crystallisation and good compatibility with semiconductor fabrication.
Over the past decades, the stoichiometric germanium telluride (GeTe) has received much attention as an alternative phase change material due to its simple composition and structure. Several elements such as C, N, Bi, and Cu have been employed to improve the performance of GeTe material by doping. As a favoured doping element, however, oxygen has been rarely reported in doped GeTe materials.
In this work the GeTeO thin films are proposed and the phase change behaviour of the films as a function of oxygen concentration is presented. It reveals that the crystallisation temperature increases with oxygen doping, and the failure of rapid phase transition feature with excessive oxygen incorporation is attributed to the oxides formation. The rhombohedral structure is preserved in the doped crystalline films with significantly suppressed crystal grains. The oxygen content dependence of change in film mass density upon crystallisation exhibits an anomalous behaviour with expanding volume in the crystalline state. Indeed, a zero density change composition upon phase transition has been discovered. These results suggest that the oxygen-incorporated GeTe composition with good thermal stability and non-density change upon crystallisation is a very promising candidate for high-reliable phase change memory application.

The concept of “fragility” proposed by Angell has proved to be useful in analysing the temperature-dependence of kinetics in glass-forming liquids [1]. Phase-change random-access memory relies on the reversible crystalline/glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching, at high temperature, with high resistance to crystallization for non-volatile data retention near to room temperature. In phase-change media such as nucleation-dominated Ge₂Sb₂Te₅ (GST), these conflicting requirements are met through the highly “fragile” nature of the temperature dependence of the viscosity of the supercooled liquid [2]. In contrast, in the growth-dominated medium Ag-In-Sb-Te, the crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. It has been suggested that this is evidence for a fragile-to-strong crossover on cooling the liquid [3], and evidence for a similar crossover in other chalcogenide liquids has recently been examined [4]. Such a crossover has many consequences for the interpretation and control of phase-change kinetics in chalcogenide media, helping to understand the distinction between nucleation- and growth-dominated crystallization, and offering a route to designing improved device performance. We review the studies on crystallization rates in chalcogenides of interest for phase-change memory. Comparison with other glass-forming systems proves to be interesting, and can, of course, be helpful in selecting other potential phase-change systems [5,6].

[1] CA Angell: Formation of glasses from liquids and biopolymers. Science 267 (1995) 1924–1935.
[2] J Orava, AL Greer, B Gholipour, DW Hewak & CE Smith: Characterization of supercooled liquid Ge₂Sb₂Te₅ and its crystallization by ultra-fast-heating calorimetry. Nature Mater. 11 (2012) 279–283.
[3] J Orava, DW Hewak & AL Greer: Fragile-to-strong crossover in supercooled liquid Ag-In-Sb-Te studied by ultrafast calorimetry. Adv. Funct. Mater. 25 (2015) 4851–4858.
[4] S Wei, P Lucas & CA Angell: Phase change alloy viscosities down to T_g using Adam-Gibbs-equation fittings to excess entropy data: A fragile-to-strong transition. J. Appl. Phys. 118 (2015) 034903.
[5] J Orava & AL Greer: Fast and slow crystal growth kinetics in glass-forming melts. J. Chem. Phys. 140 (2014) 214504.
[6] AL Greer: New horizons for glass formation and stability. Nature Mater. 14 (2015) 542–546.

A striking anomaly in the viscosity of Te₈₅Ge₁₅ alloys from the work of Neumann et al. is reminiscent of the equally striking comparison of liquid tellurium and water anomalies documented long ago by Kanno et al. In view of the power laws that are used to fit the data on water, we analyze the data on Te₈₅Ge₁₅ using the Speedy-Angell power-law form, and find a good account with a singularity T_s only 25K below the eutectic temperature. However, the heat capacity data in this case are not diverging, but instead exhibit a sharp maximum. Applying the Adam-Gibbs viscosity equation to calorimetric data, we find that there is a fragile-to-strong liquid transition at the heat capacity peak temperature, and then predict the "strong" liquid course of the viscosity down to T_g at 406 K. The extrapolation is in good agreement with a direct measurement of fragility near T_g using differential scanning calorimetry. The encouraging agreement prompts discussion of relations between water and phase-change alloy anomalies. In addition, the glass transition of phase-change materials revealed by calorimetric studies appears to exhibit an analogous behavior to that of water, which suggests that T_g for the phase-change materials should be left open.

Chalcogenide phase change materials (PCMs) are showing outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, more recently, in non-volatile resistive memories in Phase Change Random Access Memories. The latter are the most promising candidate to replace the current FLASH memories at CMOS technology nodes under 28 nm. The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In that context, numerous studies have been conducted to probe interface and size effects on the PCM crystallization, mostly on thin films (in the sub-100 nm thickness range) capped with various materials. Nowhere in this abundant literature is any effect of the capping layer reported for film thicknesses in the 100 nm range.
In this contribution, optical reflectivity, ellipsometry, resistivity, STEM and XRD analysis on 100 nm thick Ge₂Sb₂Te₅ (GST) and GeTe films will reveal a completely new picture of PCM thin film crystallization. Indeed, they demonstrate the crucial role of interface engineering and show how it affects its crystallization mechanism. In a recent paper [1], we reported that Ta cladding yields a significant increase of T_x with respect to literature values for GST and GeTe films. The report remained an isolated case, suggesting a particular role of Ta. We will unambiguously demonstrate that the crystallization temperature T_x of the prototypical GeTe and GST compounds can significantly vary as a function of their surface state, whatever the capping layer. Thus, these findings provide a novel and easy way to increase T_x and would open the route to new 3D memory cell architecture with improved data retention.
Whereas Ge₂Sb₂Te₅ has been, and is still, the subject of many publications during the last decades, the present results reveal that its crystallization mechanism was not understood. We will show that heterogeneous crystallization, reported so far, results from an interface effect and we demonstrate that Ge₂Sb₂Te₅ crystallizes at a higher temperature through bulk nucleation. Interface engineering, through suitable capping layer deposition methods, yields a spectacular increase in T_x by 20°C in GST and 50°C in GeTe above the values (~150°C in GST and ~180°C in GeTe) that have been reported in the literature for more than 30 years.
The present findings demonstrate that interface engineering allows to select the crystallization mechanism, and hence control the stability of the amorphous phase in PCMs. In addition to phase change materials, it could be applied to preventing crystallization in other glassy systems. Finally, such novel results will be discussed since they raise serious questions about all previous literature results about interface effects on crystallization of PCMs.

[1] Ghezzi, G. E. et al., Appl. Phys. Lett. 104, 221605 (2014).

Glass formation in phase-change materials is ideally suited for the requirements of a memory device. At ambient conditions their glasses are sufficiently stable to enable data retention for tens of years, whereas at elevated temperature crystallization becomes so fast that small volumes in a memory cell could be crystallized in less than a nanosecond [Loke, D. et al., Science (336)6088, p1566 (2012)]. These 16 orders of magnitude difference in crystallization speed are observed within a temperature window of only 300 K and ensure a low power consumption for switching. Despite their technological relevance, little is know about glass formation in these materials - mostly because of the inherent requirement to quench the glass at cooling rates in excess of ~4*10⁹ K/s. Then, resolving the transient properties during the cooling process of at most 10 ns becomes experimentally challenging and for most phase-change materials, not even the glass transition temperature is commonly agreed upon [Orava, J., Adv. Funct. Mater., 25, 30, 4851 (2015)]. The first step toward design rules for crystallization dynamics is to compare glass formation in various phase-change materials.
Using a combination of femtosecond optical and x-ray scattering techniques we are able to atomically resolve both, glass formation and the crystallization process in various phase-change materials, e.g. Ag₄In₃Sb₆₇Te₂₆ (AIST) and Ge₁₅Sb₈₅. During the quenching of their liquid states, a significant change in the x-ray scattering patterns occurs. This change corresponds to a structural difference between the liquid and amorphous states and might be crucial for the understanding of the aforementioned temperature dependence of crystallization dynamics. If, however, the quench is too slow, the formation of Bragg reflections from individual crystalline grains is observed. This allows directly following the crystallization dynamics at different temperatures under quasi-isothermal conditions. We also present an all-optical technique to measure crystal growth velocities up to temperatures close to the melting point. Using this technique we find that amorphous AIST obtained by sputter deposition crystallizes with growth velocities two orders of magnitude slower than those of the glass obtained by melt-quenching. This difference is qualitatively retained up to highest temperatures and underlines the impact of fragility. Crystal growth fronts in AIST propagate with up to 2.5 m/s in the as-deposited and 110 m/s in the melt-quenched glass [Zalden, P. et al., Chem. of Mater., 27, 5641 (2015)].

Phase-change memory (PCM) devices are expected to play a key role in future computing systems as both memory and computing elements. Hence, a comprehensive understanding of the change in the current/voltage (I-V) characteristics of these devices with time and temperature is of considerable importance. In this talk, we present a unified drift model to predict the I-V characteristics at any instance in time and at any temperature.

The drift model consists of a structural relaxation model [1] coupled to an electrical transport model [2]. In a PCM cell, the RESET operation consists of melting and rapidly quenching the phase-change material, creating a low-ordered highly-stressed amorphous state. The structural relaxation model describes the evolution of this state towards an energetically more favorable “ideal glass” state through an order parameter Σ. Σ takes values between 0 and 1, with 1 denoting the fully unrelaxed state and 0 the “ideal glass” state. The key idea is that the atomic configuration relaxes collectively as a group, rather than individual defects relaxing and taking no further part in the relaxation process.

An electrical transport model is then used to compute the I-V characteristics, linking them with the state of relaxation. In this model, the conductivity is calculated based on the transport of free carriers emitted from ionizable defect centers creating Coulomb potentials. The activation energy for carrier emission, E_a, decreases upon the application of an electric field F (Poole – Frenkel effect) by an amount that depends on the distance s between two defect centers (intertrap distance).

The model was validated on large sets of experimental data for an extensive range of time (10 decades) and temperatures (180 – 400 K), different phase-change materials and a collection of 4k cells from a PCM chip.

[1] Sebastian et al., Proc. IRPS, MY.5.1, 2015
[2] Le Gallo et al., New J. Phys. 17, 093035, 2015

Chalcogenide-based phase-change memory (PCM) is one of the potential candidates for the next generation non-volatile electronic memory owing to its promising features such as high-speed, better endurance, scalability, low power operations and longer data retention (1,2,3). A resurgence of interest has therefore been devoted recently towards achieving improved and reliable programming characteristics with higher storage densities in order to meet larger demand for high-density memory devices (4,5). However, one of the major challenges in the non-volatile memory technology is the enhancement of data storage capacity. One of the possible solutions to enable high-density is multi-level storage, which can be achieved by controlling multiple, stable resistance levels between low-resistance crystalline to high-resistance amorphous states of phase change materials (6). Therefore, a larger contrast (more than four orders of magnitude) in resistances is the key requirement to enable multiple states. In this regard, many phase-change materials were investigated to provide multi-bit capability by means of suitable electrical pulses (7,8).
We present here, the electrical characterization of InSbTe phase change memory device for multi-bit data storage application. Temperature-dependent sheet resistance measurements of InSbTe films display larger resistance contrast (more than six orders of magnitude) between the amorphous and crystalline phases, with an improved thermal stability revealing its capability for multi-bit programming. Also, high crystallization temperature of InSbTe (~290 C), validates better data retention as compared with most of the successful phase change materials. Furthermore, we show a correlation between programming currents with multiple resistance levels of InSbTe phase change films using current controlled electrical switching characteristics. The sub-threshold conduction, threshold switching and set operation were systemically studied using the applied current ranging from 500 nA to few 100s of µA. Parameters at threshold switching event were validated using current controlled switching and field-controlled switching behavior. In addition to these, structural aspects of intermediate resistance levels were identified using temperature-dependent X-ray diffraction.

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Electrical wind force is an important element in electrical switching behaviors of phase-change materials. It has been reported that the electrical wind force influences the motions of dislocations, which determines the degree of order-disorder states existing in phase-change materials. In this presentation, we discuss electrical wind force-driven behaviors occurring in phase-change materials. In the first part of the presentation, we report atomic mass-transport behaviors as DC voltage biases are applied in line-shape Ge₂Sb₂Te₅ (GST) devices. As the electrical current density reached 3-4 MA/cm² by DC voltage bias, a directional mass transport was identified by forming asymmetric surface morphology on the line-shape GST devices, such that hillock structures are created in the middle of the line whereas void structures are formed at the entrance of (+) electrode side of the line-shape device. The mass transport of GST occurs as a solid-state behavior by electrical wind force that leads physical scattering between electrons and atoms of GST compound followed by momentum transfer. In the second part, we extend the understanding of the roles of electrical wind force to electrical switching behaviors of GST. We studied the effects of electric voltage pulses on crystalline-to-amorphous phase transition of GST by in situ transmission electron microscopy (TEM). Electrical voltage pulse plays a critical role by creating dislocations through heat shock process: Rising edge of the pulse produces vacancies by heating, whereas during rapid cooling, atomic vacancies are condensed into dislocation loops. As the dislocations feel the electrical wind force, they become mobile and glide in the direction of hole-carrier motion. Continuous increase in the density of dislocations moving unidirectionally leads to dislocation jamming, which eventually induces the crystalline-to-amorphous phase transition. We interpret it through one-dimensional traffic model in which the increase of dislocation density exceeding a certain threshold point induces a catastrophic jamming of dislocations. Our understanding about dislocation induced amorphization suggests that the transition from crystalline to amorphous in phase-change materials may not require a melting process.

Chalcogenide materials (GeSbTe) represent the active core of PCM. In fact the PC compound is the main player in defining the electrical characteristics, thermal behavior, performance and reliability of PCM devices [1]. The comprehension of the mechanisms involved in endurance is crucial since those ones are linked with the material stability [2, 3] after electrical/thermal stress during lifetime. Hence the focus on elemental migration in PCM as a function of an applied stress is being pursued for researching the root causes of both the electrical parameters alteration during cycling [4, 5] and the PCM failure mechanisms. A material change, suggested by the deviation of the RESET current and the programmed resistance levels on cycling was confirmed by EDX analyses pointing out a Sb-richer active volume [4]. The stuck open failure has been reproduced on custom structures and studied by EDX, highlighting a phase separation of GST into a Sb, Te-rich phase close to line rupture and a Ge-rich phase along the temperature profile ascribed to incongruent melting of GST [6]. Other contributions study the behavior of GST versus electric field in linear cell structures, pointing out the accumulation of Te at the anode and Ge and Sb at the cathode due to higher electron affinity of Te [7], whereas Ge and Sb behaving as cations [8]. Such mechanisms are shown to be sensitive to voltage polarity, in molten GST, while mostly sensitive to current wind force in crystalline GST [7]. Reverse polarity tests have shown a Te-rich region close to the BEC (anode) with void formation; coming back to direct polarity partially recovers device functionality [9]. Direct and reverse polarity effects have been studied on the Wall architecture also, highlighting Te field-driven migration, Ge migration to the outer boundary of the molten region and Sb accumulation within the hottest zone with slight dependence on electric field [10]. A migration model accounts for the experimental observables in both polarities [10]. Further experiments done on line-bridge cells [11] allow for the estimation of the element migration in linear cells. All results can be adopted to understand the driving forces of the PCM failure mechanisms.

[1] R. Bez, International Electron Devices Meeting (IEDM) Tech. Dig. pp. 89 – 92, 2009
[2] D. Kang, et al., Applied Physics Letters, 95, 011904, 2009
[3] T.-Y. Yang, et al., Applied Physics Letters, 95, 032104, 2009
[4] B. Rajendran, et al., Symp. on VLSI Tech. Dig., pp. 96 – 97, 2008
[5] M. Boniardi, et al., IEEE Electron Device Letters, 34, 7, pp. 882 – 884, 2013
[6] S.-W. Nam, et al., Applied Physics Letters, 92, 111913, 2008
[7] D. Kang, et al., Applied Physics Letters, 95, 011904, 2009
[8] T.-Y. Yang, et al., Applied Physics Letters, 95, 032104, 2009
[9] A. Padilla, et al, Journal of Applied Physics, 110, 054501, 2011
[10] L. Crespi, et al., Proc. of International Memory Workshop, 2015
[11] G. D’Arrigo, et al., Materials Research Society Spring Meeting, Y5.06, 2015

Phase-change memory (PCM) is an emerging storage technology based on the unique combination of phase-change materials properties. These materials show an amorphous and a crystalline phase with substantially different properties, and it is possible to switch the material repeatedly and rapidly between these two phases. The large difference in electrical resistance between the two phases is used in PCM to store information. PCM offers a very wide range of materials and compositions, allowing materials to be developed for specific application requirements. For example, embedded system memories require materials with much better data retention at high temperatures, either to maintain stored data after soldering or for automotive applications. A material offering both ultrafast SET speed (<10 ns) and very high endurance (10¹²) could potentially be used for DRAM replacement. Storage Class Memory (SCM), which would sit just below DRAM in the memory hierarchy, requires reasonably high endurance (>3x10⁸ cycles) and read and write access times of 100~300 ns.
The development of phase-change materials for PCM has relied on 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, even though high reset current and poor data retention at elevated temperature make it difficult for new applications such as automotive. Many material modifications were made to try to solve these issues, however conflicting material properties between switching speed and thermal stability or switching speed and endurance performance still persisted. In the first part of this talk, we will show various novel phase change materials which properties are engineered and optimized for specific applications based on Ge-Sb-Te (GST) alloys.
Although a significant improvement by engineering GST materials, no GST based material can meet all the requirements: data retention, solder bonding thermal budget and fast speed. In particular the fundamental conflict between switching speed and data retention is very difficult to solve without exploring new materials beyond GST based systems. In the second part of this talk, we will discuss new phase-change material based on pseudobinary GaSb-Ge system. The resulting new phase-change material has demonstrated fast switching speed of 80 ns, long endurance of 1G cycles and excellent data retention that survives 250 ^oC-300 hrs. The 10 years-220 ^oC data retention is the best ever reported. It is also the fastest material that can pass the solder bonding criteria for embedded automotive applications.
In this talk, we will give a comprehensive overview of the materials properties with a focus on materials for high temperature applications, for ultra-fast switching and for maintaining the code after solder bonding thermal budget. The large-scale PCM demonstrations will be also shown in this talk.

Phase Change Materials (PCM) show two stable states in the solid phase with significantly different physical properties. They can be switched reversibly by optical or electronical means, which enables their use for storage and logical applications [1]. Using Transmission Electron Microscopy (TEM) the structural change of the phase change material can be imaged with atomic resolution, but the electrical and optical properties, cannot be derived. In order to gain information about the optical bandgap, contributions from free charge carriers or the prevailing bonding mechanism, spectroscopic optical measurements have to be applied [2], which are however limited in spatial resolution to about λ/2.
Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) allows for spectroscopic analyses of different optical properties at the nm-scale [3] without the need for electron-transparent sample preparation. s-SNOM uses a sharp illuminated tip following the sample surface, but can also retrieve sub-surface information. Thus imaging of PCM devices covered with an additional protective capping layer should be possible. By correlating the optical s-SNOM images with transmission electron microscopy images of the same sample, we unambiguously demonstrate the correlation of the infrared optical contrast with the structural state of the phase change material. The investigated sample consists of sandwiched amorphous and crystalline regions of Ag₄In₃Sb₆₇Te₂₆ below a 100 nm thick (ZnS)₈₀ – (SiO₂)₂₀ capping layer. Our results demonstrate the sensitivity of s-SNOM to small dielectric near-field contrasts even below a comparably thick capping layer (100 nm) [4].


[1] D. Lencer, M. Salinga, and M. Wuttig, Adv. Mater. 23, 2030 (2011).
[2] K. Shportko et al., Nat. Mater. 7, 653-658 (2008).
[3] F. Keilmann and R. Hillenbrand, Philos. Trans. R. Soc. A 362, 787 (2004).
[4] M. Lewin et al., Appl. Phys. Lett. 107, 151902 (2015).

For more than 50 years, the capabilities of Von Neumann-style information processing systems — in which a "memory" delivers operations and then operands to a dedicated "central processing unit (CPU)" — have improved dramatically. With the end of Dennard scaling and the consequent slowdown of Moore’s law, the IT industry is turning to Non-Von Neumann computing, and in particular, to architectures inspired by the human brain.
At the same time, new nonvolatile memory technologies (NVM) — such as Phase Change Memory (PCM), Resistance RAM (RRAM), and Spin-Torque-Transfer Magnetic RAM (STT-MRAM) — are emerging. Combined with suitable Access Devices (ADs) – such as based on Mixed Ionic Electronic Conduction (MIEC) – these memories can be integrated into large-scale 3D crossbar arrays and enable Storage-Class Memory (SCM) — an emerging memory category that combines the high performance and robustness of solid-state memory with the long-term retention and low cost of magnetic storage.
Such 1NVM+1AD arrays can also implement Artificial Neural Networks (ANNs). In an NVM-based implementation, device conductances serve as a modifiable “weight” of each “native” synaptic device. This is an attractive application for these emerging devices, because while many synaptic weights are required, requirements on yield and variability can be more relaxed. By applying known examples and adjusting weights using learning algorithms such as backpropagation, the network can be trained for tasks such as image or speech recognition. This is a non-Von Neumann system, where computation is performed at the location of data, and is expected to have performance and power benefits over a CPU or Graphics Processor Unit (GPU) implementation.
In this talk, I will discuss our recent work towards large crossbar arrays of PCMs for non-Von Neumann computing. I will review earlier work on MIEC–based ADs, and show that a bidirectional NVM with a symmetric, linear conductance response and high dynamic range is fully capable of delivering the same high classification accuracies as a conventional system [1]. I will discuss our work on evaluating a large-scale (165000 synapses) PCM-based ANN and quantitatively assessing the impact of PCM non-idealities, such as SET/RESET asymmetry, on network accuracy [2]. I will also present our initial assessments of performance and power benefits of NVM–based Artificial Neural Networks vs. Von Neumann-type implementations [3]. Finally, I will discuss current research on new techniques to achieve state-of-the-art accuracy despite PCM imperfections, and efficient circuit design for massive parallelism.

REFERENCES
[1] G. W. Burr et al., IEEE Trans. Electr. Dev., to appear (2015)
[2] G. W. Burr et al., IEDM Tech. Digest, 29.5 (2014)
[3] G. W. Burr et al., IEDM Tech. Digest, to appear (2015

Phase change memories (PCM) are attracting high interest and current efforts are mainly towards the reduction of the programming currents, where ultrascaling of the memory cells plays an important role; materials suitable for automotive applications (with higher retention temperatures) are also investigated. The possibility to use phase change nanowires (NW) offers the interesting possibility to exploit the scaling properties of PCMs, since their growth can be controlled, along with their diameter, composition and crystallinity; further, In-based chalcogenide materials, such as In₃Sb₁Te₂, are featured by a higher thermal stability with respect to conventional Ge₂Sb₂Te₅.
In this work, phase change In₃Sb₁Te₂ and In-doped Sb₄Te₁ NWs were self-assembled by metal organic chemical vapor deposition (MOCVD), offering high process control, large area deposition and industrial transferability, coupled to vapor-liquid-solid (VLS) mechanism. In₃Sb₁Te₂ and In-doped Sb₄Te₁ NWs were obtained at 325°C under different growth conditions. Both the deposition of single NWs and template filling were explored in order to investigate the possibility of their positioning for future device application. The compositional, morphological and structural analysis of the grown structures was performed and studied as a function of the employed substrates (flat and structured combinations of Si and SiO₂). Single NWs 2-3 μm in length and with diameters as small as 15 nm were obtained. The NWs were subsequently harvested on SiO₂/Si substrates and contacted using an optimized combination of electron beam and ion beam Pt deposition in a focused ion beam system. The IST NW resistivity from dc electrical characterization ranged from 10^-2 to 10^-1 Ω.cm. The NWs electrical switching behavior will be finally discussed.

Bottom-up growth processes of chalcogenide nanowires (NWs), such as metal organic chemical vapor deposition (MOCVD) coupled to vapor-liquid-solid (VLS) mechanism, represent promising tools for the realization of ultra-scaled phase change memory (PCM) devices. So far, efforts on the synthesis and characterization of this class of NWs have mainly focused on the Ge-Sb-Te (GST) system. However, studies based on thin films have shown interesting properties of the alloys of the In-Ge-Te (IGT) system, including high thermal stability, which could lead to memory cells with improved retention. Here, we report, for the first time, on the synthesis of IGT NWs by MOCVD and VLS mechanism, catalyzed by Au nanoparticles (NPs). Compositional, morphological and microstructural properties of the grown structures as a function of the process parameters, including deposition temperature and pressure, precursors partial pressures and NPs sizes were investigated by total reflection x-ray fluorescence (TXRF), scanning electron microscopy (SEM), x-ray diffraction (XRD), high resolution transmission electron microscopy and x-ray microanalysis (TEM-EDX). High-density IGT NWs with lengths of a few microns and diameters down to 15 nm were synthetized on flat Si(100) and Si(111) substrates. Both average TXRF and local TEM-EDX compositional analyses confirm the formation of ternary IGT alloys, while TEM selective area diffraction and XRD indicate that as-grown NWs are crystalline. Moreover, we show that an epitaxial relationship exists between such substrates and the NWs, producing an ordered growth along specific crystallographic directions. The NWs were then harvested and contacted by Focused Ion Beam (FIB) on SiO₂/Si substrates to investigate their electrical properties. Finally, the fabrication of IGT NWs arrays, including ultra-scaled ones, by selective growth inside different types of templates was explored.

Phase-change random access memory (PRAM) has been researched as next-generation memory for over ten years, in addition, stores data in phase-change materials (PCMs), which reversibly change a certain phase to another phase by control of electrical system. PRAM is expected to be built up new application such as neuromorphic system and storage class memory (SCM) with high speed, retention, and low power consumption. Many (PCMs) have been researched, including Te-based chalcogenide with Ge-Sb-Te, In-Sb-Te, Sb-Te, and Ge-Te systems. Nevertheless, discovery of inherent properties in the materials has reached the limit.
The key that makes those more attractive is the development of controllable method, such as doping in intrinsic PCMs. BIST, designed by adding Bi atoms in In₃SbTe₂ (IST-312) alloy, changes the transition temperatures, the activation energy, the lattice distortion, and the thermal property. Transition temperatures shift lower than that of IST-312. Activation energies also are reduced that of IST-312. We investigated the reason why the changes of properties through experiment and DFT calculation. According to energetic stability with enthalpy change, Sb is the most preferred substitutional site among In, Sb,and Te sites. NaCl structure of IST-312 is locally distorted by Bi atom. The specific heat is reduced, resulting in fast transition and low power consumption. Substituted Bi atoms also make many vacancies and hole carriers, In_In^x → V_In^3- + 3h⁺, in addition, the vacancies induce more distortion. Those changes result in the improvement on electrical speed and the low power consumption. In this study, we discuss the changed properties by Bi doping in IST-312 and the reason of those through calculation and experiments.

The recognition of right materials for non-volatile phase-change memory (PCM) applications is driven by the need to determine materials with tailored properties and the aspiration to realize the scientific origin behind their distinct properties. Amorphous chalcogenide glasses display various changes in structural and optical properties during band-gap illumination like, photoinduced change in volume, photodarkening, photobleaching, and photoinduced phase-change (photocrystallization and photoamorphization). Amorphous Selenium (a-Se) is the model material of chalcogenide glass, which can undergo reversible photostructural transformation under light exposure. Here, we experimentally demonstrate for the first time the photoinduced phase change in a-Se due to considerable changes in photostriction (light induced strain). Nano hemispherical a-Se structures of less than 150 nm in diameter and 90 nm thick were deposited on the cleaned SiO₂ cladding layer of the Fiber Bragg Grating (FBG) sensor. The transient photostriction has been manipulated using FBG sensor by measuring the shift in the wavelength of the reflected IR light. A typical FBG based optical sensor can achieve strain sensitivity of approximately 1.2 pm/μ�� and henceforth is ideal for applications where conventional electrical sensors are ineffective. When a-Se is exposed to 532 nm (5 mW) band-gap light, initially larger magnitude of photostriction is observed. This photostriction is found to decrease gradually upon crystallization with time. Subsequently with prolonged exposure, the photostriction gradually increases and reverts back to the amorphous state. These observations reveal that the amorphous and crystalline phases of a-Se possess significantly different opto-mechanical properties. Thus, an optical phase-change can be induced by band-gap excitation in a-Se at room temperature by tailoring photostriction which offer a new approach into the development of efficient nano-scale optical PCM materials.

Phase change random access memory (PCRAM) is believed to be an alternative to current FLASH-based non-volatile memory with improved scalability. Currently, Ge-Sb-Te (GST) is used for commercial PCRAM, but GST-PCRAM has issues in high-temperature applications and exhibits thermal disturbance between memory cells in dense memory arrays due to the poor thermal stability of amorphous GST. Recently, Sutou et al. found that GeCu2Te3 (GCT) shows higher thermal stability than GST in the amorphous phase. Moreover, the group demonstrated that the reset process in GCT exhibits lower power consumption due to the lower melting point of GST.

As the memory cell size is scaled down, the total resistance of a PCRAM cell becomes dominated by the contact resistance between the phase change material (PCM) and electrode. However, studies of contact resistance in PCRAM remain limited. An understanding of the contact resistivity and its contrast between amorphous and crystalline states is essential for PCRAM applications. Therefore, in this study, the contact resistivity of the newly developed GCT material with a W electrode was investigated using the Circular Transfer Length Method (CTLM).

A CTLM pattern was fabricated by photolithography in order to measure the contact resistivity. Amorphous GCT films were grown on SiO2/Si substrates by co-sputtering of GeTe and CuTe targets. Crystalline GCT films were obtained by heating the as-deposited amorphous film up to 300 °C in an Ar ambient. Based on the obtained results, the effect of contact resistance on memory cell resistance was estimated by calculation of the resistance of a simple cell model.

The contact resistivity of a W/amorphous GCT contact was calculated to be 3.9×10-2 Ω cm2, while that of W/crystalline GCT contact was found to be 4.8 ×10-6 Ω cm2. From a simple calculation based on the obtained results, it was estimated that the resistance contrast ratio of GCT was about 4 times larger than GST for films for 10 nm thick films. For such films, this leads to the total resistance being dominated by the contact resistivity. Therefore, for PCRAM based upon these films, GCT is expected to show not only better thermal stability but also better accuracy during data readout than GST.

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Recently, some of the current members (P.F., K.V.M., K. M., A.K. and M. H.) have used coherent phonon spectroscopy to revealed the existence of a transient photoexcited state in [Ge2Te2/Sb2Te3]20 superlattices with an average composition of Ge₂Sb₂Te₅. [1] The transient state was interpreted to be a consequence of differing local environments for Ge atoms in the structure. Following the ideas suggested by the density functional theory based work on distortion triggered loss of long range order in the prototypical system of GeTe [2], we have investigated the details of transient structural change in an epitaxial sample of Ge₂Sb₂Te₅ using time-resolved x-ray diffraction with sub-picosecond time-resolution. In this talk we demonstrate using an unique combination of femtosecond laser excitation and an ultrashort x-ray probe distinct temporal regions of electronic and thermal effects in the form of a long-lived (> 100 ps) transient metastable state of Ge₂Sb₂Te₅ with diminished interatomic interaction induced by an excitation-induced weakening of resonant bonding.

Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect will allow the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies. We speculate upon the consequences of this newly found excited state on superlattice transformation.

[1] M. Hase, P. Fons, K. Mitrofanov, A. V. Kolobov, and J. Tominaga. Femtosecond structural transforma- tion of phase-change materials far from equilibrium monitored by coherent phonons. Nat. Commun., 6, 09 2015.
[2] A. V. Kolobov, M. Krbal, P. Fons, J. Tominaga, and T. Uruga. Distortion-triggered loss of long-range order in solids with bonding energy hierarchy. Nat. Chem., 3(4):311–316, 2011.

Sb₂Te₃ – GeTe van der Waals (vdW) heterostructures represent the state of the art in phase change memory material technology. Energy efficient data storage is achieved by confining the phase transition to the interface between the two layers, consequently the entropic losses that are associated with the reversible amorphous-crystalline phase transition in conventional Ge-Sb-Te alloys are suppressed. Accordingly these ‘interfacial phase change memory’ (iPCM) heterostructures present a viable route to lower the energy consumption of data storage devices. The full potential of phase change materials extends beyond their current application in data storage. Indeed, the iPCM structure, which is composed from the topological insulator Sb₂Te₃, provides a path toward switchable spintronic devices, whilst conventional phase change alloys are finding a new lease of life in tuneable photonics. We show that the iPCM switching mechanism entails pre-melting of a 0.5 nm thick two–dimensional GeTe crystal plane, and the energy required for the premelt–switching is lowered by applying biaxial strain. We theoretically and experimentally demonstrate that the GeTe 2D crystal strain is dictated by the Sb₂Te₃ layer thickness and consequently the switching energy is readily decreased by simply increasing the thickness of the Sb₂Te₃ layers of the heterostructure. Finally we lay the foundation for a generalisable approach to the design of switchable vdW heterostructures by identifying four critical rules for the heterostructure superlattice design.

Interfacial phase change material (iPCM) heterostructures present a viable route for energy efficient data storage by confining the phase transitions to the interface between the two different two-dimensional (2-D) crystals. We will present strain tuneable phase change memory based on Sb_xTe_1−x–GeTe superlattice structures. The Sb₂Te₃ and Sb₂Te₁ crystals were prepared via self-organised van der Waals epitaxy, which provided the framework to design c-axis oriented superlattice heterostructures. We propose the minimisation of the interfacial free energy-sapping as a thermodynamic argument to favour the triangular Sb₂Te₁ crystals. The in-plane biaxial strain applied to the GeTe layer was sensitive to the thickness of Sb_xTe_1−x layer in the superlattices. We exploited this effect to enable interfacial diffusive atomic switching in iPCM memory devices. The devices showed excellent electrical switching performance with reduced programming energy, extended cyclic endurance, and SET voltage pulses just 14 ns in length. This work paves the wave to phase change materials with properties that can be designed using strain.

Recently, Simpson et al proposed a new kind of phase change memory device based on crystalline GeSbTe superlattices. In this case, the phase transition is between two crystalline structures, rather than between amorphous and crystalline phases, so this type of phase change might consume a lower energy than switching the traditional amorphous GST phase [1]. The high resistance state and low resistance state have been described in terms of four basic structures built on aligned bonds (Kooi, Ferro, Petrov and Inverted Petrov) [2,3]. The switching process has been shown to include a vertical movement of Ge sites through a Te layer, plus a lateral shift of Ge and Te sites to return to one of the basic structure [4].

Detailed EXAF and HRTEM pictures of the CSL structures suggest that the GeTe bilayers tend to be surrounded by TeSbTe triple layers next to the van der Waals’ bonding gap [5-6]. In addition, regions of the HRTEM images show SbTe bilayers shifting between different GeSbTe “blocks”, which must therefore be a relatively low energy process.

Here we study this SbTe bilayers shifting process by density functional supercell calculation. We find that the lower energy path way involves like-atom bond formation.

1. R. E. Simpson et al, Nature nanotechnology. 6 501 (2011)
2. J. Tominaga et al, Advanced Materials Interfaces 1 1300027 (2014)
3. N. Takaura et al, J. Tech Digest IEDM (IEEE), 2014
4. X. Yu and John Robertson, Scientific Reports. 5 12612 (2015)
5. J. Momand et al, E\PCOS 2015. Page 145
6. B. Casarin et al, E\PCOS 2015. Page 159

Phase change random access memory (PCRAM) uses the fast reversible phase transition between amorphous and crystalline states of chalcogenide phase-change materials to record information. The crystalline chalcogenides generally have a metastable rocksalt structure and a stable hexagonal structure, and the rocksalt structure is the normally observed crystalline state during recording. There are significant amount of intrinsic vacancies in rocksalt phase-change materials, for example, 25% and 20% positions are vacant at Na sites in Ge₁Sb₂Te₄ and Ge₂Sb₂Te₅, respectively. These intrinsic vacancies stemming from the stoichiometry of Ge-Sb-Te (GST) alloys play an important role in the fast reversible phase transition between amorphous and crystalline GST alloys. In this work, on the basis of ab initio calculations and ab initio molecular dynamics simulations, we will shed light on the vacancy configurations, i.e., whether the vacancies are ordered or randomly occupying with Ge and Sb atoms at Na sites, as well as their effect on the internal atomic distortion and electronic properties, and the role of intrinsic vacancies on the rapid phase transition. Using Ge₁Sb₂Te₄ as an example, we show that the ordered configuration possesses a lower energy of 43.3 meV/atom than the disordered configuration described by a SQS structure, and a band gap of 0.24 eV is obtained for the ordered state while there are significant amount of states at the Fermi level but a pseudo-gap of 0.13 eV below the Fermi level for the disordered state. Further, the vacancies result in internal distortions of rocksalt GST, where the first-nearest-neighbor bond-angle-distributions (BAD) show deviations from 90^o. In case of ordered vacancies for Ge₁Sb₂Te₄, the peak positions of BAD for Ge, Sb and Te are 91.6^o, 93.3^o and 92.5^o, respectively, while that are respectively 92.8^o, 93.6^o and 90.5^o for the case of disordered vacancies. The coordination numbers for the three species also show broad distribution from two-fold to six-fold coordinations in cubic Ge₁Sb₂Te₄. Finally, the role of vacancies will be discussed.

Chalcogenide superlattice, such as GeTe/Sb₂Te₃, has been proposed for interfacial phase change memory (iPCM) material and succeeded to reduce switching energy. Since this superlattice is composed of coherently-aligned GeTe and Sb₂Te₃ multilayers, fabricating a highly-oriented Sb₂Te₃ seed layer is crucial. Furthermore, in order to utilize such material for novel devices in industry, reliable processes like large area deposition of the uniform film is strongly required. Sputtering is one of the most useful, technologically friendly, and reliable methods for thin film fabrication. In this work, the improvement of the quality of the Sb₂Te₃ film is discussed.
The films were prepared by RF-magnetron sputtering on Si substrates using Sb₂Te₃ and GeTe alloy targets. The degree of orientation was evaluated by x-ray diffraction (XRD) and the microstructures were observed by transmission electron microscopy (TEM). A deposition temperature of the Sb₂Te₃ seed layer was found to be important for fabricating the film not only with highly-oriented but also with a larger lateral grain size. The XRD out-of-plane peak intensities were significantly increased with increasing the thickness of the room-temperature seed layer first and then decreased again for further thickness of the seed layer, indicating an existence of the optimum thickness. The in-plane scans also showed similar results, namely, the thickness of the room-temperature seed layer of around 3~4 nm showed the best quality in terms of the sharpness of the peak. The microstructures of the superlattice films, where the seed layer was deposited at 250^oC and room temperature, were compared by TEM observation. It was clearly observed that coarsening of the crystal grains to the lateral direction was successfully achieved and the grain size of the room-temperature seed layer sample was more than 3~4 times larger than that of the film with the high-temperature seed layer.
Finally we will propose the novel device using the chalcogenide superlattice film. Since Sb₂Te₃ has been attracting great attention as not only phase change material but also topological insulator, it can be speculated that the chalcogenide superlattice equipped with both topological and normal insulating features may possess interesting properties arising from the phase transition. According to the ab-initio simulation based on the density functional theory, we found that the electronic band structure of GeTe/Sb₂Te₃ superlattice can be tuned between a Dirac semimetal and a gapped insulator state that may be utilized as a channel material in a transistor. The fabrication of the superlattice films with highly-oriented and relatively large grains may realize such novel electronic devices.

GeTe-Sb₂Te₃ superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb₂Te₃, contradicting previously proposed models in the literature. For this purpose [GeTe(1nm)-Sb₂Te₃(3nm)]₁₅ superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the films actually consist of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb₂Te₃ and rhombohedral GeSbTe. Moreover, by annealing the films at 300 and 400 °C, the superlattices reconfigure into bulk rhombohedral GeSbTe, demonstrating that this van der Waals structure is thermodynamically favored. Even in case we try to introduce sharp interfaces between GeTe and Sb₂Te₃ by using growth interrupts, the tendency to form rhombohedral GeSbTe is still observed. These results are explained in terms of the bonding dimensionality of GeTe and Sb₂Te₃ and the strong tendency of these materials to intermix. Still, such superlattices without sharp GeTe-Sb₂Te₃ interfaces show clear signatures of the interfacial phase-change memory behavior, e.g. an order of magnitude lower switching power than when GeSbTe is used in the memory cells. The present findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application.

Phase change materials (PCMs) are nowadays used as the active material for non-volatile solid-state memories [1]. One impressive achievement has been accomplished when it was realized that PCM memory cells based on superlattices (SLs), structures made of alternating GeTe and Sb₂Te₃ layers, showed dramatically improved performance in terms of reduced switching energies with ultra-low energy consumption, improved write-erase cycle lifetimes, and faster switching speeds [2].
High-resolution transmission electron microscopy investigations carried out on molecular beam epitaxy (MBE) grown SLs have shown that the constituting layers intermix at the interfaces, forming GeSbTe (GST) blocks. Moreover, such intermixing cannot be easily controlled during the growth, resulting in an intrinsic loss of the vertical ordering in the SL where different GST compositions are randomly present along the stacking. The electrical properties of such SLs are however very similar to that of the published GeTe/Sb₂Te₃ SLs.
Now it is interesting to verify whether nominal GST/Sb₂Te₃ SLs would also offer switching advantages. This would facilitate the material optimization as severe requirements to avoid intermixing would no longer be necessary. Furthermore, such SLs would potentially enable to discriminate between the role of heterointerface quality and SL ordering during the phase transition. In fact, the replacement of GeTe layers with GST ones, which are structurally closer to Sb₂Te₃, could prevent the intrinsic intermixing characteristic of GeTe/Sb₂Te₃ SLs, allowing to better control the vertical ordering. Moreover, since the dispersion in the composition along the stacking is suppressed, it would be possible to engineer SL structures by changing the composition of the GST layer.
To this end, here we present the MBE growth and characterization of GST/Sb₂Te₃ SLs on Sb passivated Si(111) substrates. The SLs, together with Sb₂Te₃ and GST calibration samples, have been characterized by high resolution X-ray diffraction, Raman spectroscopy and low temperature four point probe electrical measurements. The switching functionality has been characterized by dedicated pulsed current-voltage measurements in memory cells.
[1] S. Raoux et al., Chem. Rev. 110, 240 (2010).
[2] R.E. Simpson et al., Nat. Nanotechnol. 6, 501 (2011).

Chalcogenide phase change materials are important for data storage applications since they combine scalability and fast switching speed with low power dissipation. The conception of a new superlattice device (CSL), based on alternating layers of GeTe and Sb₂Te₃, allowed for a step forward in memory technology in terms of efficiency and switching speed¹. However, an optimized crystalline quality of these structures is vital for unraveling their unique properties and simplifying the model study. In the present work, crystalline GeSbTe (GST) and CSL films with both single in-plane and out-of-plane orientations are epitaxially grown by molecular beam epitaxy on silicon substrates. A detailed study of the active phonon modes has been realized by means of polarization resolved micro-Raman spectroscopy technique. The more ordered GST shows narrower peaks and out of plane lattice vibrations as the vacancies are ordered into layers. By performing Raman at low temperature (10 K), blueshift of the peaks is observed as the effect of strain during the cooling process. Most importantly, the broadening of the peaks is reduced and several finer features are finally visible in agreement with literature data from theoretical calculations. The Raman spectrum of the CSLs is richer in features, and it resembles the Sb₂Te₃ spectrum with additional modes from both GeTe and GST alloys. The presence of the latter is also confirmed by X-ray diffraction techniques. The effect of the excitation power, the different ratio of Sb₂Te₃ and GeTe and some growth treatments in the stacking are studied. Moreover, in order to verify the structure stability, the CSL samples have been annealed post growth leading to the formation of ordered GST₁₂₄, as shown by the clear out of plane modes.

1- Simpson, R. E. et al. Interfacial phase-change memory. Nature Nanotech. 6, 501–505 (2011).

The strong confinement and enhancement of light when coupled to surface waves or nanoparticles is key for various applications in nanophotonics such as sensing, imaging or other devices that enable the manipulation of light fields. In the mid-infrared spectral range, metallic nanoantennas and materials supporting surface phonon polaritons (SPhPs) can be used as building blocks of such devices. In both cases, the optical functionality is usually only obtained at a fixed wavelength, determined by the geometric design and the material properties.

In the first part of this talk, I will present our latest results on active mid-infrared plasmonics, i.e. the tuning of nanoantennas resonances via variation of the refractive index n of an embedding medium based on phase-change materials (PCMs) [1,2]. PCMs offer a huge contrast in the refractive index n due to a phase transition from amorphous to crystalline state, which can be thermally, optically or electrically triggered. I will show thermal and optical large-area switching of IR antenna resonances as well as individually addressable switching of single structures. Application potential for switchable chirality [3] and switchable detectors [4] will also be presented.

In the second part, I will introduce Phonon-Polariton-based IR antennas made from polar dielectrics which exhibit lower losses and larger Q-values compared to metallic nanoantennas. Specifically, we employ a PCM as a switchable dielectric environment for loading the SPhPs [5]. This allows us to realize all-optical, non-volatile, and reversible switching of the SPhPs by controlling the structural phase of the PCM. We experimentally demonstrate that single nanosecond (ns) laser pulses can locally switch an ultra-thin PCM (down to 7 nm, k_p > 70k₀, k₀ = 2π/λ) in quartz. This offers a new, elegant way to prepare all-dielectric, optically rewritable SPhP resonators without the need of complex fabrication methods.
Our approach of combining PCMs and SPhPs opens up new possibilities for non-volatile, rewritable and active nanophotonics, in particular for re-configurable, digital and memory metamaterials, flat optics and metasurfaces.

[1] A. U. Michel, T. Taubner, et al. Nano Letters, 13, 3470 (2013).
[2] A. U. Michel, P. Zalden, T. Taubner et al. ACS Photonics, 1, 833−839 (2014).
[3] Yin, X. et al. Nano Lett. 15, 4255-4260 (2015).
[4] Tittl, A. et al., Advanced Materials 27, 4597-4603 (2015)
[5] P. Li et al, submitted

Beam steering devices have gained extensive interests in the fields of optical interconnects, communications, displays and data storages.However, the challenge lies in obtaining an ultrafast beam steering structure in the optical regime. Here, we propose phase-array-like plasmonic resonators based on metal/phase-change materials (PCMs)/metal trilayers for all-optical ultrafast beam steering in the mid-infrared (MIR) region. We numerically demonstrate an angle beam steering of 11° for transmitted wave (front lobe) and 22° for reflected wave (back lobe) by switching between the amorphous and crystalline states of the PCM (Ge₂Sb₂Te₅). A photothermal model is used to study the temporal variation of the temperature of the Ge₂Sb₂Te₅ film to show potential for switching the phase of Ge₂Sb₂Te₅ by optical heating. Generation of the beam steering in this structure exhibits a fast beam steering time of 3.6 ns under a low pump light intensity of 2.6 μW/μm². Our design possesses a simple geometry which can be fabricated using standard photolithography patterning and is essential for
exploiting the ultrafast beam steering in various applications in the MIR regime.

Control of localized surface plasmon resonance (LSPR) excited on metal nanostructures has drawn attention for applications in dynamic switching of plasmonic devices. As a reversible active media for LSPR control, chalcogenide phase-change materials (PCMs) such as GeSbTe (GST) are promising for high-contrast robust plasmonic switching. We demonstrated the LSPR switching of individual Au nanospheres on a GST thin film by alternating irradiation by a femtosecond pulse laser for amorphization and a continuous wave laser for crystallization [1]. The metal-dielectric-metal nanosandwich structure is an effective approach for enhancing LSPR switching contrast. The LSPR peaks of the hybridized modes, particularly the magnetic dipole mode, shift dramatically with the refractive index of the dielectric material. Recently we obtained a large peak shift for the magnetic dipole mode in a single Au nanorod (NR)/GST/Au film sandwich structure [2].
Owing to the plasticity and the threshold behavior during both amorphization and crystallization of PCMs, PCM-based LSPR switching elements possess a dual functionality of memory and processing. Integration of LSPR switching elements so that they interact with each other will allow us to build non-von-Neumann computing devices. As a specific demonstration, we discuss the implementation of a cellular automata (CA) algorithm into interacting LSPR switching elements [3]. In the model we propose, PCM cells, which can be in one of two states (amorphous and crystalline), interact with each other by being linked by a AuNR, whose LSPR peak wavelength is determined by the phase of PCM cells on the both sides. The CA program proceeds by irradiating with a light pulse train. The local rule set is defined by the temperature rise in the PCM cells induced by the LSPR of the AuNR, which is subject to the intensity and wavelength of the irradiating pulse.
We also investigate the possibility of solving a problem analogous to the spin-glass problem by using a coupled dipole system, in which the individual coupling strengths can be modified to optimize the system so that the exact solution can be easily reached [4]. For this algorithm, we propose an implementation based on an idea that coupled plasmon particles can create long-range spatial correlations, and the interaction of this with a phase-change material allows the coupling strength to be modified.
[1] Appl. Phys. Lett. 103, 241101 (2013). [2] Appl. Phys. Lett. 106, 031105 (2015), [3] Adv. Opt. Technol. 2015, 150791 (2015). [4] Appl. Phys. A, 10.1007/s00339-015-9338-2 (2015).

Thus far, applications of Phase Change Materials have been mostly limited to non-volatile digital memories. Recently, however, we have explored new approaches towards using phase-change materials in optoelectronic and photonic applications, particularly in ultra-high resolution display components as well as potential solutions in smart glazing applications. Switching with electrical pulses gives rise to hybrid electro-optical applications whereby, we are able to work with a new optoelectronic framework within thin-film cavities to modulate colors of thin flexible films in pixels as small as 100 nm [1]. Indeed, such applications can take advantage of decades of research into optimizing both the cycling reliability as well as long term stability of such materials readily, while also having particularly low power requirements (for example, no refreshing required in displays that use such technologies). Thus, the combination of photonics and phase change materials is a developing field that could lead to future novel implementations, for instance on-chip accumulative photonic computing [2, 3], ultrathin flexible bistable displays, and electro-optical sensors, filters and modulators.
[1] P. Hosseini, C.D. Wright and H. Bhaskaran, Nature 511, 206 (2014)
[2] C. Rios, M. Stegmeier, P. Hosseini, CD. Wright, H. Bhaskaran and WHP. Pernice, Nature Photonics, doi:10.1038/nphoton.2015.182 (2015)
[3] P. Hosseini, A. Sebastian, N. Papandreau, CD. Wright and H. Bhaskaran, IEEE Electron Device Letters vol. 36, no. 9 (2015)

Recent reports have shown that the implementation of chalcogenide phase change materials (PCMs) in RF systems enables world class performance in the form of low loss, high isolation and circuit reconfigurability. The RF phase change switch approach is to independently heat the PCM from an external source, much like the gate on a FET supplies an electric field between the source and drain, creating a 4-terminal, inline phase change switch (IPCS). Using this approach, RF switches with a cutoff frequency (F_co) of 12.5 THz have been demonstrated. Since ON-state resistance directly impacts this RF figure of merit, significant development efforts to reduce the PCM crystalline resistivity were undertaken. It is known that “resonant” bonding forms the basis of many PCM properties, including crystalline resistivity. It is also known that this bonding modality is very sensitive to middle- and long-range order. The aim of this work, then, was to correlate morphological properties which may affect order and disorder in GeTe with the resultant electrical properties. Film deposition conditions including power, pressure and substrate temperature were utilized as a means of achieving varied microstructures. Resultant film properties were evaluated via XRD, STEM, RBS, Hall measurements and a heated probe stand. It was found that microstructural improvements can enable a >50% reduction in crystalline GeTe resistivity, which directly translates to a 50% improvement in F_CO. The framework of morphology and electrical property interdependence will be presented in detail and applied to observations of microstructural evolution in pulsed devices.

This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions, and/or findings contained in this article/presentation are those of the author(s)/presenter(s) and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

Distribution Statement "A" (Approved for Public Release, Distribution Unlimited

Ohmic contacts with extremely low resistance and controlled morphologies are required to improve the performance of radio frequency (RF) switches fabricated from phase change materials (PCMs). In this presentation, the role of pre-metallization surface preparation and post-metallization annealing on the resistance of ohmic contacts to GeTe will be discussed. Both refined transfer length method (RTLM) and circular transfer length method (CTLM) test structures were used to extract the specific contact resistance. However, RTLM test structures were found to provide more accurate measurements. Three different pre-metallization surface treatments (HCl, ammonium sulfide, and in-situ argon plasma) were investigated. An in-situ argon plasma treatment prior to metallization was found to provide the lowest specific contact resistance for as-deposited contacts (in the 10^-8 Ω-cm² range), quite similar to contacts prepared using an ammonium sulfide treatment. On the other hand, HCl treatment led to a markedly higher specific contact resistance (in the 10^-7 Ω-cm² range). X-ray photoelectron spectroscopy (XPS) was used to characterize the GeTe surface after each treatment and provided insight into the influence of the surface treatments on specific contact resistance. Finally, annealing can promote interfacial reactions between metals and GeTe. We show how annealing can affect the resistance of the contacts, as well as how phase diagrams we have calculated can be used to understand these reactions and predict which metallizations will provide long-term thermal stability.

Metal surfaces coated with ultrathin lossy dielectrics enable color generation through strong interferences in the visible spectrum. Using lossy phase-change materials for coating further enables tuning colors through crystallization. Here, we study the optical response of surfaces consisting of thin (5-140 nm), phase-changing Ge₂Sb₂Te₅ (GST) films on Al mirrors. Colors achieved using different thicknesses of as-deposited amorphous GST layers turn dim gray upon crystallization at ~150 C. Introducing a lossless dielectric between GST and Al layers reveals Fabry-Pérot resonances which offer a more controllable method of color generation and tuning. The presented color tuning method through changing GST thickness and phase can be used for rewritable high-resolution large-area displays, if the color of the surfaces can be restored through re-amorphization of the phase-change layer. Calculated heating/cooling times of a small pixel element show that both crystallization and amorphization can be achieved using appropriate electrical pulses.

The eminent feature of fast and reversible switching of both optical and electrical properties by phase change enables the wide application of phase change materials (PCMs) in both optical memory and emerging charge based non-volatile memory technologies. Until now, however, there is no reported magnetic device using PCMs. It is still fascinating to have one class of materials which show all controllable optical, electrical and magnetic properties and preferably allow ready integration with the existing silicon technology. Spin degree of freedom has recently been engineered into PCMs by Fe dopant, resulting in magnetic PCMs. It is highly desirable to be able to control the magnetism once the magnetic medium has been prepared and put into use. External electrical field and spin-polarized current have shown the ability to control the magnetism without changing the atomic bonding network. The Fe doped PCMs, on the other hand, demonstrate experimentally the magnetic contrast by phase change, which may suggest an alternative route of fast manipulation of the magnetism.

In this work, we use screened exchange (sX-LDA) hybrid functional to study the single neutral substitutional 3d transition metal (TM) in crystalline GeTe and GeSb₂Te₄ (GST). Previous local density approximation (LDA) study does not describe well localized TM 3d states that they are described as too shallow and over-delocalized, resulting in a large hybridization with the host p states. By curing the problem of LDA by sX-LDA, we find that Fe substitution on Ge/Sb site has its majority d states fully occupied while its minority d states are empty, which is different than previous predicted electronic configuration by LDA. The sX-LDA shows that the host valence band maximum (VBM) is sandwiched in between the majority and minority Fe d states, resulting in antiferromagnetic exchange spin splitting. The host condution band minimum (CBM) is found to have large ferromagnetic exchange spin splitting and its majority component is pulled down to close the VBM to accommodate electrons, if necessary. We provide molecular orbital description to help understand the origin of local and total spin states. From early transition metal Cr to heavier Ni, the majority 3d states are gradually populated until fully occupied and then the minority 3d states begin to be filled.

In order to study the magnetic contrast, we use lower symmetry crystalline GeTe and GeSb₂Te₄ as the amorphous phases, respectively, which has been proposed to model the medium range disordering. We find that only Co substitution in r-GeSb₂Te₄ and s-GeSb₂Te₄ shows magnetic contrast. The experimental magnetic contrast for Fe doped GST may be due to additional TM-TM interaction, which is not included in our model. It can also be possible that these lower symmetry crystalline models, lacking the complex characteristic structural patterns, are not sufficient to characterize the magnetic properties of real 3d TM doped amorphous GST.

Phase change memory, threshold switches, and resistive memory devices exhibit changes in their electrical parameters in response to external bias. The existing understanding attribute these changes to either structural transformations via either direct action of the electric fields or indirect action through creation of the electronic filament causing subsequent material modifications.

The former scenario has been recently developed as the filed induced nucleation (FIN) mechanism in connection with phase change memory applications. However, the same mechanism was shown to explain non-photochemical laser induced nucleation in various materials, phenomena in resistive memories, and metal whisker growth on Sn, Zn, and some other films. Furthermore, it was shown that the same approach can be used to describe operations of threshold switches.

This presentation will review the physics underlying electric field induced structural transformations in various systems with emphasis on the solid state memory devices. The differences between nonvolatile memory and threshold switches operation regimes will be elucidated. Features beyond the nucleation stage of field induced transformations will be described as well.

At the end, we hope to show that the concept of field induced structural transformation describes a set of important phenomena in solid state memory and far beyond that including laser induced structural modifications, problems of metal hydrogen creation, kinetics of metal whiskers and some other.

Storage concepts employing the resistance state of phase change memory cells have matured in recent years. These concepts take advantage of the pronounced contrast in electrical properties between the amorphous and crystalline phase of the active phase change material [1]. A key prerequisite for an accurate tracking and reliable storage of the information in arrays of phase change memory cells is a good understanding of field and temperature dependent charge transport, especially in the amorphous phase.
Charge transport in the subthreshold and threshold-switching regime of phase change memory cells has been frequently studied in the framework of the Poole-Frenkel effect [2-4]. This effect describes how the probability for emission of a charge carrier, bound in the Coulomb potential of a defect, is enhanced by the electric field. For high defect densities, one can expect to have an additional reduction of the emission barrier depending on the spacing between defects i.e. the inter-trap distance [5]. Examining charge-transport models, which invoke this inter-trap distance, we point out that the charge state of any defect in the electronic density of states (DoS) must depend on its occupation. Hence, the inter-trap distance should depend on external parameters governing the occupation of defects in the DoS.
Here, we numerically study how the density of occupied defects in the DoS of Ge₂Sb₂Te₅ [6,7] changes when modified by variation of temperature and illumination with photons.
We complement these calculations with experiments, in which we measure the current-voltage-characteristics of micrometer-scale line-cells composed of as-deposited Ge₂Sb₂Te₅ both in darkness and under light (for temperatures 140 K-300 K and electric fields 0.1-40 V/μm). We find that with decreasing temperature the inter-trap distance increases in darkness whereas it decreases under illumination, in agreement with our DoS calculations.
Since the inter-trap distance can exponentially affect the device current, our findings imply that accounting for the temperature dependence of the defect occupation should greatly improve device modeling.

[1] Wuttig & Yamada, Phase-change materials for rewriteable data storage, Nature Materials 6, 824-832, (2007).
[2] Ielmini and Zhang, Analytical model for subthreshold conduction, J.App.Phys. 102, 054517, (2007).
[3] Betti Beneventi et al., Three-dimensional Poole-Frenkel analytical model for carrier transport in amorphous chalcogenides, J. App.Phys. 113, 044506, (2013).
[4] Le Gallo et al., Subthreshold electrical transport in amorphous phase-change materials, New Journal of Physics 17, 9, (2015).
[5] Hill, Poole-Frenkel conduction in amorphous solids, Phil. Mag. 23, 181, (1971).
[6] Luckas et al., The influence of a temperature dependent bandgap on the energy scale of modulated photocurrent experiments, J.App.Phys. 110, 013719, (2011).
[7] Longeaud et al., On the density of states of germanium telluride, J.App.Phys. 112, 113714 (2012).

We present an overview of insights gained from computational studies aimed at understanding the low bias transport in phase change materials [1][2][3][4]. We model the electrical transport through both amorphous and crystalline Germanium Telluride ultrathin films as a function of thickness. These films are attached to Titanium Nitride source and drain contacts. The molecular dynamics based melt-quench and energy minimization calculations are performed using density functional theory. The structures obtained from these calculations are used to model the electrical transport using the Green’s function approach. Electron-phonon scattering is included within the Born approximation; deformation potentials based on experimental mean free paths are used. The length of GeTe considered in our transport calculations vary from 2 to 6 nm. The calculations of both amorphous and crystalline structures are able to capture many of the experimental features of low bias current-voltage characteristics. . The electrical transport properties of ultrascaled c-PCM are dominated by metal-induced gap states, and the linear and exponential shapes of the subthreshold current–voltage curve of ultrascaled a-PCM are a result of both the bias window enlarging and the bias induced change of transmission

We find that the resistance ratio (between amorphous and crystalline structures) rapidly falls with decrease in GeTe thickness due to metal induced gap states. For a GeTe film thickness of a little larger than 3 nm, the resistance ratio approaches ten. For shorter GeTe film thicknesses, the resistance ratio falls to small enough values that a useful memory device may not be feasible. As the GeTe thickness approaches 6 nm, the resistance ratio increases to values above one hundred.

When electron-phonon scattering is included, for the length scales considered here, less than 5% of the hot electron energy is transferred to the GeTe lattice. As a result, the transport is quasi-ballistic and the heat generated in the GeTe lattice alone is unlikely to cause phase change.

This work was supported by the National Science Foundation under grant number DMR 1006182.

[1] J. Liu, Xu Xu, M. P. Anantram, Journal of Computational Electronics, vol. 13, pages 620-626 (2014)
[2] J. Liu, Xu Xu, M. P. Anantram, Electron Device Letters, IEEE, vol. 35, pages 533-535 (2014)
[3] J. Liu, M. P. Anantram, Journal of Appled Physics, vol. 113, page 063711 (2013)
[4] J. Liu, Xu Xu, Lucien Brush and M. P. Anantram, Journal of Applied Physics, vol. 115, page 023513 (2014)