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, 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. Subsequently, it will be shown that disorder in certain phase change materials is a second turning knob for the properties of crystalline phase change materials. It will be demonstrated how this concept can be used to tailor the electrical and thermal conductivity of phase change materials.

The binary phase change memories, which take advantage of the resistive contrast between the amorphous and the crystalline phases, will eventually give way to the high-capacity multi-state memories, in which both phases can reach various resistive states. Recent studies demonstrated that these multiple resistive states can be achieved in crystalline phase change materials (PCMs), by manipulating the vacancies in Ge-Sb-Te (GST) system and controlling the disorder with different annealing temperatures [1, 2]. In lieu of the heat treatment, our large-scale ab initio molecular dynamics (AIMD) simulations show that pressure can also tune the disorder in crystalline PCMs. Our models contain 1008 atoms of rocksalt-like GeSb₂Te₄ with random distribution of vacancies, and the AIMD simulations were performed at various temperatures and pressures for about 100 ps. We observed that, by lowering the migration energy for the anti-site hopping (e.g., Te ions jump into the Ge/Sb layers and Sb ions hop into the Te layers), the high pressure increases the compositional disorder due to the accumulation of these anti-site ions, resulting in some localized electronic states near the bottom of the conduction band. On the other hand, the disorder leads to the unoriented displacement of the anti-site ions, distorting and destabilizing the lattice near them [3]. The disorder-induced electron localization triggered by pressure will pave the way for the understanding and development of the multi-state memory devices. And the random distortion of lattices due to the compositional disorder offers a new mechanism that may contribute to the amorphization of crystalline PCMs at high pressure [4].
[1] T. Siegrist, P. Jost, H. Volker, M. Woda, P. Merkelbach, C. Schlockermann, M. Wuttig, Nat. Mater., 10 (2011) 202-208.
[2] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P.H. Dederichs, J.Y. Raty, M. Wuttig, S. Blugel, R. Mazzarello, Nat. Mater., 11 (2012) 952-956.
[3] A.V. Kolobov, M. Krbal, P. Fons, J. Tominaga, T. Uruga, Nat. Chem., 3 (2011) 311-316.
[4] S. Caravati, M. Bernasconi, T.D. Kuhne, M. Krack, M. Parrinello, Physical Review Letters, 102 (2009) 205502.

Recently, we have shown that crystal-amorphous transformation in phase-change materials can be achieved through a defect-based pathway, which involves creation of defects such as anti-phase boundaries (APBs) in GeTe and dislocations in GST (1,2), their migration and accumulation at a region of local inhomogeneity creating a defect template, followed by amorphization along the template. Crystalline GeTe, shows bonding hierarchy, which results in it being ferroelectric. It is unclear, however, as to how these extended defects interact with preexisting ferroelectric domain boundaries, and relax the domains in GeTe during the crystal-amorphous pathway. By studying these interactions it will be possible to uncover atomic scale information about the bond distortions that are directly responsible for amorphization. It must be noted using conventional techniques such as piezo-force microscopy (PFM) is very difficult in GeTe, owing to its large conductivity. Hence we developed an optical second harmonic generation (SHG) polarimetry technique on GeTe to obtain quantitative information about the ferroelectric domains and their evolution.
Second harmonic generation (SHG), i.e., conversion of two photons with frequency omega; into a new photon with frequency 2omega;, is a powerful technique to probe structure of non-centrosymmetric materials. However, the availability of neither the non-linear SHG coefficients in GeTe nor the standard single-domain samples to obtain these coefficients complicated the analysis. To overcome this issue, we carefully designed the polarimetry experiments on nanowires grown along <110> direction, with the fundamental wave vector (k) along <111> direction, and an analysis procedure to give information about the ferroelectric domain fractions and the non-linear SHG coefficients. We measured the polarized SHG intensity (at 0^o, 60^o, 120^o) by varying the fundamental polarization between -180^o and 180^o, fit them to a second order sinusoidal function (A₁sin²x+A₂cos²x+A₃sin2x), and extracted the domain fractions and material constants from the fitting parameters. We were able to obtain consistent set of material constants (d₃₁/d₁₅, d₃₃/d₁₅) for our analysis of 15 different nanowires, proving the validity of this analysis. SHG signal from the nanowires was sensitive to 16 domains: 8 ferroelectric each having a polarization along a <111> direction, and 2 stacking variants along each domain polarization. Our experiments for the first time predicted the existence of stacking twins, which we verified using TEM at appropriate tilting conditions; and this further validates the analysis. We will extended this to study the ferroelectric domain evolution during crystal-amorphous transformation in memory devices, to study how the interaction of APBs with domains invert the polarization of certain domains and their influence on electronic and structural phase transitions.
P.Nukala, et al., Nanolett14, 2201 (2014)
S.W.Nam et al., Science336, 1561 (2012)

In an effort to obtain new chalcogenide materials with a higher crystallization temperature and better stability several alloys were investigated, e.g. Ge_xTe_1-x[1]. Among these alloys, Ge₃₆Te₆₄ presents very interesting characteristics with a crystallization temperature of 250°C, suitable for automotive applications. To ascertain the role played by the local atomic arrangement in the amorphous state we have studied structural, optical and electrical properties of amorphous chalcogenide alloys by means of Selected Area Electron Diffraction (SAED), spectroscopic ellipsometry, electrical measurements and Raman spectroscopy. Amorphous films of Ge₃₆Te₆₄, Ge₅₀Te₅₀ and Ge₂Sb₂Te₅, 40 nm thick, were fabricated at room temperature (RT) by sputtering on a SiO₂/Si substrate. To avoid non homogeneity in the local stoichiometry as deposited samples were implanted with Ge+ at 130keV with 10¹⁴ ion/cm² and at 20keV with 5x10¹³ ion/cm². Finally, some samples were melt quenched (MQ) by irradiation of the amorphous film with a pulsed (12 ns) Nd:YAG laser (lambda;=532 nm). From SAED analyses Radial Distribution Function has been measured for each sample estimating a bond angle γ in the range of 100°divide;108° for Ge₃₆Te₆₄ and Ge₅₀Te₅₀, while γ asymp;94° for Ge₂Sb₂Te₅. Electrical measurements show that Ge₃₆Te₆₄ resistivity is two order of magnitude higher than that of Ge₅₀Te₅₀ and Ge₂Sb₂Te₅ films. By measuring film resistance at several temperatures we estimated activation energies of 0.40eV, 0.36eV and 0.38eV, for Ge₃₆Te₆₄, Ge₅₀Te₅₀ and Ge₂Sb₂Te₅, respectively. Higher Ge₃₆Te₆₄ resistivity can be ascribed to the reduced number of homopolar Ge-Ge bonds, with respect to the other studied alloys, corresponding to a reduction of energy states inside the gap. From spectroscopic ellipsometry we calculated the dielectric function. In comparison with Ge₅₀Te_50, Ge₃₆Te₆₄ is characterized by a lower optical absorption coefficient, in agreement with the activation energy measured for resistivity. To gain insights on the structure of the amorphous network, a model (300 atoms) of amorphous Ge₃₆Te₆₄ has been generated by quenching from melt within molecular dynamics simulations based on Density Functional Theory [2]. Comparison is made with experimental structural, vibrational and optical data. The local bonding geometry turns out to be mostly octahedral-like, albeit with a coordination number lower than six (defective octahedral). About 20 % of Ge atoms are, however, in a tetrahedral bonding geometry. As opposed to the Ge₅₀Te₅₀ composition, few long Te chains (6-9 atoms) are found in the model of amorphous Ge₃₆Te₆₄. The Te chains do contribute to the Raman spectrum with a peak at about 125 cm^minus;1.
[1] E. Carria, A.M. Mio, S. Gibilisco, M. Miritello, C. Bongiorno, M.G. Grimaldi, E. Rimini. J. Electrochem. Soc. (2012), 159, H130minus;H139.
[2] S. Caravati, M. Bernasconi, M. Parrinello, J. Phys. Condens. Matter 22 (2010) 315801.

In recent years, we have deployed a liquid cell for use in a transmission electron microscope and we have used it to monitor the growth and assembly of colloidal nanocrystals.Using this tool, we have observed unexpected mechanisms of growth and we have been able to observe many specific pathways for nanocrystal assembly that previously were not easy to anticipate.In addition, we can directly observe the trajectories for chemical and structural transformations of individual nanoparticles.

Ultrathin semiconductors (3-30 nm) on metals constitute color filters which absorb selective wavelength ranges of the incident light. We demonstrate that these absorption resonances depend strongly on the refractive index of the semiconductor. Consequently, these coatings are not only attractive for static but also for tunable color devices. Ultrathin Ge₂Sb₂Te₅ (18 nm) deposited on gold can be switched reversibly between two colors with femtosecond laser pulses. The optical contrast is enhanced compared to optical dense phase-change materials and the position of the absorption resonance is tunable via the thickness of the phase-change material. Color switching is even feasible if the phase-change material is replaced by a conventional group IV semiconductor, whose amorphous and crystalline phases are optically less distinct. These structures hold significant promise for optical data storage: A lack of optical contrast, which makes new or sometimes established materials unfavorable for novel optical data storage applications, may be overcome by exploiting this effect. Furthermore, the mechanical properties of these ultrathin films are investigated because they are attractive for non-volatile flexible display applications.

Switchable and nonlinear metamaterials, with properties surpassing those of natural media, will underpin the next stage of the photonic technological revolution, providing a functional platform for nanoscale ‘meta-devices&’, and it has been seen recently that all-dielectric architectures can deliver metamaterial functionalities free from the high resistive losses inherent to noble metal frameworks. Phase-change media take us a step further by providing for optically-driven, non-volatile switching, tuning and reconfiguration of meta-devices. We report here on recent advances in the development of versatile, planar photonic chalcogenide metamaterials to provide a new generation of nanoscale optical switching and memory devices.

Storage concepts employing the resistance state of phase change memory cells have matured in recent years. These concepts take advantage of the pronounced resistivity contrast between the amorphous and crystalline phase of the active phase change material ¹. An easy storage and tracking of multiple resistance states per cell has so far been impeded by the phenomenon of resistance drift exhibited by the amorphous phase of the employed materials ². Several studies on amorphous phase change thin films as well as memory cells have shown that the switching behavior as well as the resistance drift behavior of the amorphous phase depends strongly on the employed material ^3,4. As both aforementioned phenomena are related to charge transport gathering information regarding the electronic density of states of different phase change materials is crucial.
In order to gain insight into the density of states of different phase-change materials, we study the Dark- and Photoconductivity in thin film devices based on GeTe, Ge₂Sb₂Te₅ and Ag₄In₃Sb₆₇Te₂₆. By varying the temperature in a range of 300 K to 100 K we are able to measure changes in the activation energy of the dark conductivity of ~50 %. In addition, different photoconductivity regimes are explored by varying both temperature and light intensity over a wide range.
We discuss our results obtained for Ge₂Sb₂Te₅ and GeTe in light of measurements reported in literature ^5,6. Furthermore, we attempt to explore the reason for the different drift behavior of Ag₄In₃Sb₆₇Te₂₆ by describing charge transport in the framework of a specific electronic density of states supported by our experiments.
1. Wuttig, M. & Yamada, N. Phase-change materials for rewriteable data storage. Nature Materials6, 824-832 (2007).
2. Papandreou, N. et al. Drift-Tolerant Multilevel Phase-Change Memory. Memory Workshop (IMW), 2011 3rd IEEE International 1-4 (2011).
3. Krebs, D. et al. Threshold field of phase change memory materials measured using phase change bridge devices. Applied Physics Letters95, 082101 (2009).
4. Wimmer, M., Salinga, M., Dellen, C., Kaes, M. & Wuttig, M. Drift of activation energy in amorphous phase change materials. MRS Spring Meeting 2013 (2013).
5. Krebs, D., Bachmann, T., Jonnalagadda, P., Dellmann, L. & Raoux, S. Changes in electrical transport and density of states of phase change materials upon resistance drift. New Journal of Physics16, 043015 (2014).
6. Luckas, J. et al. The influence of a temperature dependent bandgap on the energy scale of modulated photocurrent experiments. Journal of Applied Physics110, 013719 (2011).

Development of phase-change memory and recent advanced studies beyond von-Neumann architecture, such as spike-timing-dependent plasticity, rely on detailed knowledge of the reversible amorphous-to-crystalline transition. The temperature dependence of atomic mobility (expressed, for example, in terms of shear viscosity), and of crystal nucleation and growth are of practical interest as crystallization is rate-limiting for the write operation, and of fundamental interest for chalcogenides, such as Ge-Sb-Te, for which the supercooled liquid has particularly low glass-forming ability. Recent studies on as-prepared and melt-quenched amorphous phase-change materials (PCM), in different sample configurations, i.e. thin films, sandwiched films and memory devices (the two latter taking into account the influence of stresses on viscous flow in the liquid), have confirmed that the crystal growth rate has a temperature dependence that is strongly non-Arrhenius, and that the maximum rate ~10⁰ m s^-1 occurs at homologous temperatures (~0.7-0.8T_m) much lower than for typical strong liquids and good-glass formers such as silica, which shows the growth-rate maximum at ~0.9T_m. PCM supercooled liquids have high values of the kinetic fragility index, and a corresponding breakdown in Stokes-Einstein relation between diffusivity and viscosity has been identified around the glass-transition temperature (T_g). Given these characteristics, the crystallization of PCM, always thought to be rather special among the variety of glass-forming liquids, is fast, with kinetics close to those for supercooled glass-forming metals at high temperature, while being very sluggish at around T_g. This remarkable transition of ten orders of magnitude in growth rate (10^-10-10⁰ m s^-1), in a somewhat narrow temperature range (0.4-0.55T_m), is difficult to describe with any current theory, where deviations of only three orders of magnitude can be understood between Turnbull&’s ‘collision-limited&’ growth (without thermal activation) in metals below T_g, and diffusion-limited growth. Understanding of PCM crystallization may be approached by comparing with the kinetics in a range of metallic glass-forming systems.

In the last few years atomistic simulations based on density functional theory (DFT) have provided useful insights on the properties of chalcogenide alloys of interest for phase change memories (PCM). Still, large simulation cells and long simulation times beyond the reach of fully DFT simulations are needed to address several key issues of relevance for PCM operation. To overcome these limitations, we have developed an interatomic potential for the prototypical phase change compound GeTe by fitting a large DFT database with a neural network (NN) scheme [1]. Large scale NN simulations allowed us to get insights on the fragility of the supercooled liquid [2] and its crystallization kinetics [3]. In this talk, we will discuss the connection between the fragility of the supercooled liquid and the structural relaxations below the glass transition which are responsible for the drift in the electrical resistance of the amorphous phase. We will show that the same features giving rise to structural relaxations in the glass are also responsible for the emergence of structural and dynamical heterogeneities in the supercooled liquid phase. Dynamical heterogeneities are in turn the source of the breakdown of the Stokes-Einstein relation between viscosity and atomic mobility in the supercooled. This allows for the persistence of a high atomic diffusivity at low temperatures whic ultimately leads to the high crystallization speed exploited in the devices. Crystal growth velocities at the interface between liquid and crystalline GeTe in slab geometries will be discussed as well.
[1] G. C. Sosso, G. Miceli, S. Caravati, J. Behler, and M. Bernasconi, Phys. Rev. B 85, 174103 (2012).
[2] G. C. Sosso, J. Behler, and M. Bernasconi, Physica Status Solidi B 249, 1880 (2012).
[3] G. C. Sosso, G. Miceli, S. Caravati, F. Giberti, J. Behler, and M. Bernasconi, J. Phys. Chem. Lett. 4, 4241 (2013).

For phase change materials (PCMs) used in optical or resistance-based memory, crystallization of the amorphous phase is the data-rate-limiting step of the switching process and must be achieved within nanoseconds. The knowledge of crystallization kinetics is critical to the understanding phase transformations in PCMs, but basic quantities, such as crystal growth rates, are difficult to measure experimentally during highly-driven laser- or current-induced crystallization. The crystallization rates that occur under such conditions are many orders of magnitude greater than what may be observed during in situ crystallization experiments using conventional microscopy techniques (e.g. thermionic- or field-emission-based TEM, AFM, optical microscopy).
We use the dynamic transmission electron microscope (DTEM), a photo-emission TEM capable of nanosecond-scale time-resolved electron imaging and diffraction, to study the crystallization kinetics of chalcogenide-based PCMs during laser heating. In the DTEM experiments, up to nine images may be captured during laser-induced crystallization of amorphous phase change materials. Each frame consists of an electron image with nanosecond-scale time resolution (variable down to 15 ns). Individual growing grains are tracked during growth and accurate measurements of growth rate are made even when the growth front velocity exceeds 3 m/s. The DTEM has been used to study laser-induced crystallization in PCMs, such as Ge₂Sb₂Te₅ [1] and stoichiometric GeTe [2,3]. In this talk, progress on the study of crystallization kinetics in GeTe system will be described as well as the application of the technique to other PCMs, such as Sb-rich Ge-Sb-Te alloys.
Since direct measurement of the temperature is not experimentally accessible during the laser crystallization experiments, finite element analysis simulations are used to model the rapidly changing spatial and temporal temperature profiles during laser heating. The modeled temperature profile allows us to connect our experimental observations to models of crystal growth.
This work performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
References
[1] M. K. Santala et al., Journal of Applied Physics111 (2012) 024309.
[2] M. K. Santala et al., Physica Status Solidi B249 (2012) 1907.
[3] M. K. Santala et al., Applied Physics Letters102 (2013) 174105.

Chalcogenide phase-change materials have important applications in optical storage devices and electronic non-volatile memories, due to their ability to switch rapidly and reversibly between the crystalline and the amorphous state upon heating and to the pronounced optical and resistivity contrast between the two states. The GeSbTe compounds lying on the pseudobinary line GeTe-Sb₂Te₃ are the most widely studied family of PCMs.
In this work, we have investigated crystal growth of Ge₂Sb₂Te₅ at 600 K by first-principles molecular dynamics based on density functional theory. We have performed simulations of two different crystallization processes. In the first set of simulations, we have considered models containing a planar amorphous-crystalline interface and we have studied crystal growth at the interface. In the second set, we have investigated the growth of small crystallites generated using an enhanced sampling technique called metadynamics. The use of this method has enabled us to circumvent the computationally demanding problem of simulating the stochastic processes of nucleation.
We have obtained two independent estimates of the crystal growth velocity from the two sets of simulations. We have found that the growth velocity is of the order of 1 m/s in both cases. This value is compatible with recent experimental work. Our simulations also indicate that, upon fast crystallization from the amorphous phase on a subnanosecond time scale, a disordered cubic phase is formed, in which vacancies form randomly distributed clusters.

We track the changes in optical and structural properties of phase change materials during the ultrafast amorphization of the cubic phase of Ge2Sb2Te5 though a novel single-shot optical and electron diffraction setup enabling us to correlate optical and structural changes.
By measuring the time-resolved change of the dielectric function, we see that the real part shows a resolution/limited, 100 fs, collapse to the amorphous phase value after photoexcitation, whereas the imaginary part shows a slower evolution. Furthermore, we see that the energy transfer to the lattice occurs on a slower 2 ps timescale with complete amorphization occurring after approximately 5 ps.
Our results show that ultrafast excitation transiently depopulates electrons from their resonantly bonded ground state. The crystalline state then becomes unstable; however, crystallinity persists despite the loss of resonant bonding until the electrons have transferred their energy to the lattice. These results show the primary events that occur during ultrafast amorphization and represent the limiting timescale for amorphization driven by optical pulses.

To date Ge₂Sb₂Te₅ (GST) and related phase-change alloys have been employed as the memory element in optical disks and more recently in electrical phase-change memory. The large optical /electrical contrast between RESET and SET phases of GST has been the basis for optical and electrical memory. To date the RESET phase has been formed by a melt-quench process from the crystalline SET phase. In both cases, the limiting factor for switching rate has been the speed of crystallization from the melt-quenched glassy phase. More recently with the advent of interfacial phase-change memory (iPCM), it has been demonstrated that by confining the switching processes to one dimension, it is possible to eliminate the large entropic losses encountered with the conversion of the glassy amorphous phase into the crystalline phase by switching between crystalline RESET and SET phases. Electric field effects have been demonstrated to play an important role in the iPCM switching process. As the difference in optical and electrical properties originates from bonding differences between the SET (resonant bonding), and RESET (covalent bonding) phases, it is interesting to speculate on what the most energy/time efficient atomic motion that can result in a corresponding change in bonding character. If the selective excitation of coherent optical phonons that can drive the phase transition can be achieved, the ultimate speed limit of the transition process may be on the order of 3 THz as derived from the period of an optical phonon in GST. To provide insight from direct observations of lattice dynamics on the tens-of-femtosecond time scale, we have generated coherent phonons in GST using an ultrafast 30 fs optical pump laser and have directly probed lattice dynamics directly using diffraction by means of a 10 fs 10 keV x-ray probe with a variable delay produced by the free-electron laser facility SACLA. The approximately 250 fs jitter of the free-electron laser source was compensated for by a timing monitor based 10 micron thin GaAs cross correlator to allow for sub-50 fs time resolution. This time-resolution allowed for the separation of electronic and thermal excitation effects. The lattice dynamics associated with both optical and acoustic phonon response in the system is reported and compared with the pure optical response measured in an all optical coherent phonon pump-probe experiment. In addition the effects of resonant pumping using a pair of optical pump pulses separated by an optical phonon period is reported.
ACKNOWLEDGEMENT: The authors would like to acknowledge the support from the X-ray Free Electron Laser Priority Strategy Program of MEXT, Japan.

In the present work, we hypothesized and experimentally verified the existence of a non-thermal switching process in phase change memory devices. Phase change materials (PCM) are capable of switching rapidly and reversibly between the two structural states: amorphous and the crystalline. There is a substantial contrast in both electrical and optical properties between the two states. Consequently, these materials have been developed for the practical application in optical data storage and electronic memories. Electrical phase change random access memory (PCRAM) devices were fabricated. The crystal to amorphous, RESET, switching process was studied as a function of pulse power and pulse duration by measuring the electrical resistance transients during the switching pulse. A coupled electric-field to temperature finite element (FE) model was used to gain information about the transient temperature profile in the PCRAM cell. We found that both electric field and thermal energy play an important role in amorphization of the Ge2Sb2Te5 PCM. Furthermore, for short (t<5 ns) electrical pulses our experimental measurements show a volatile switching action whilst our FE model implies that the resistance switching is not a result of heating. These results further support the amassing evidence for non-thermal switching processes in phase change materials and may lead to a new type volatile/non-volatile memory.

In phase-change memory, the structure-property relation is critical to understanding and controlling the memory switching process. Here we correlate the local electrical and structural properties of phase-change In₂Se₃ nanowires, by performing high-resolution transmission electron microscopy and scanning Kelvin probe microscopy on the same nanowires. This approach reveals a direct correlation between a presence of dislocations and a high local electrical resistance in nanowires subject to the RESET switching (i.e. switching from low to high electrical resistance state). This correlation indicates that the RESET switching, commonly understood as the amorphization process, can occur entirely via the generation of dislocations at temperatures much below the melting point. From a fundamental perspective, this discovery provides new insight into the critical structure-property relation in phase-change materials and the important role of dislocations. Practically, since the RESET switching is commonly considered as the most energy-consuming process that requires heating above the melting point, our findings suggest that a more energy-efficient phase-change memory can be realized based on In₂Se₃.

Phase change material is what enables phase change memory technology. Therefore, important characteristics of phase change memory devices such as read/write speed, programming current, endurance, and retention are largely determined by the phase change material. This puts numerous requirements on phase change material such as appropriate electrical resistivity, high crystallization temperature, fast crystallization speed, and low drift coefficient and it becomes challenging to find a phase change material which satisfies all of them. Recently, it has been demonstrated that metallic surfactant layer can stabilize the resistance of phase change memory cell by providing alternative current path to amorphous region in the phase change material which suffers from temporal drift, large temperature dependence, and large noise. This leads to multi-level cell (MLC) performance improvement for given phase change material and relieves some of challenging requirements for phase change material.

Phase-Change Memories (PCM) are today considered the most mature among emerging non-volatile memory technologies. However, to be eligible for embedded applications, the data reliability at high temperature has to be improved. The development of new alloys based on Ge-rich GST has recently enabled to enhance the stability of the high resistance state RESET with some drawback on the stability of the low resistance SET state [1, 2]. In this paper, we use TEM observations and low frequency noise (LFN) measurements to study the resistance drift of the SET and RESET states of Ge-rich based PCM devices. For the first time, we show that the drift of the polycrystalline SET state is accompanied by a decrease of the LFN, as opposite to that of the amorphous RESET state. The results are successfully interpreted in the framework of recently published models accounting for the drift of the SET and RESET states.
In fact, in the case of the RESET state, the structural relaxation (SR) of the disordered amorphous phase results in a decrease in the number of traps which are responsible for the electrical conduction, thus increasing both the resistance and the normalized noise S_I/I² [3, 4]. As for the polycrystalline SET state, the resistance drift has been recently interpreted as the result of the SR of the “residual disordered regions localized at the grain boudaries between the crystalline grains or in some confined region along the conduction path” [2]. The decrease of S_I/I² of the polycrystalline material leads us to the conclusion that S_I/I² originating from the crystalline grains is also impacted by the SR of the disordered regions and that it decreases. This decrease is understood by the carrier number fluctuation noise model, in which the carriers flowing within the crystalline grains are submitted to traping-detrapping by the nearby localized traps when passing in the vicinity of the disordered regions, thus inducing conductivity fluctuations [5]. During the SR of the residual disordered regions, the decrease of the number of traps located in the disordered regions lowers the interfacial trap density and, by turn, the carrier number fluctuations in the crystalline grains, thus leading to a decrease of S_I/I². Our results demonstrate that the LFN of the polycrystalline material is dominated by carrier number fluctuations in the grains due to trapping-detrapping processing the disordered regions interfacial defects, as opposite to the LFN of the amorphous material, which results from the bulk properties of the amorphous phase [6].
Acknowledgements: This work has been partially supported by the LabEx Minos ANR-10-LABX-55-01.
References:
[1] P. Zuliani et al., IEEE TED, 60(12) pp4020-4026 (2013).
[2] N. Ciocchini et al., IEEE TED, 61(6) pp2136-2144 (2014).
[3] D. Ielmini et al., JAP (102) 054517 (2007).
[4] D. Ielmini et al., IEDM, pp939-942 (2007).
[5] Z. Fang et al., IEEE TED, 59(3) pp850-853 (2012).
[6] G. Betti Benventti et al., JAP (106) 054506 (2009).

We present the structure and integration of a novel confined PCM cell prepared utilizing ALD PCM deposition and that employs a metallic surfactant layer to stabilize the high (and intermediate) resistance state time- and temperature-dependent resistance drift and noise. We discuss both the integration of a testable device based on ALD materials and the implications of the metallic layer for MLC phase change memory technology. The ALD GST material is integrated in an ULVAC Entron multi-chamber cluster that allows for in-situ processing--deposition of amorphous ALD PCM and metallic layer materials without a vacuum break. Further, we discuss the role that the metallic surfactant layer plays as an alternative read current path to the region blocked by the amorphous phase change material. Thus, the resistance instabilities such as resistance temporal drift and noise in the amorphous phase change material can be avoided. This results in a better multi-level phase change memory performance with longer retention time and less number of program-and-verify iterations. The data here focuses on ALD GST deposition, fill characteristics, and the stabilization of time- and temperature-dependent resistance noise that the metallic layer provides during device operation.

The most common phase-change memory (PCM) structure, mushroom cell, is asymmetric along the current path resulting in asymmetric heating and amorphization of the cells. The effect of thermoelectric transport in these structures is observed when current direction is reversed which leads to different amount of self-heating, hence molten volume ^1-4 in the active region. It has been recently demonstrated that when the current flow is directed from top (large) to bottom (narrow) electrode (positive voltage configuration), thermoelectric heating (Peltier heat at the junction and Thomson heat within the phase-change material) helps Joule heating ⁴. When the current direction is reversed, thermoelectric cooling within the active volume weakens the effect of Joule heating resulting in a smaller molten, hence amorphous volume. Thus, one voltage polarity is found to be favorable for a more efficient PCM RESET operation (crystalline to amorphous phase transition).
For this study, we investigate the effect of thermoelectric heating within a mushroom PCM cell with 10 nm critical dimension for various bias conditions (polarity, amplitude and duration) by performing electrothermal simulations. For the positive polarity configuration, appropriate electrical pulse amplitudes are used to achieve the same RESET resistances for various pulse durations (0.1 to 100 ns). Shorter duration pulses require larger amplitudes to achieve the same amorphous, hence RESET resistance. Then the same electrical pulse configurations (amplitude and duration) are used for the negative polarities resulting in smaller amorphous volumes, hence RESET resistances. The major finding of this study is that the difference between the RESET resistances for opposite voltage polarities is smaller for shorter duration, larger amplitude pulses owing to domination of J²-dependent Joule heating over J-dependent thermoelectric heat. These results suggest that symmetric operation of PCM cells is possible when large amplitude, short duration pulses are used.
References:
1. Pandian, R., Kooi, B. J., Palasantzas, G., De Hosson, J. T. M. & Pauza, A. Polarity-dependent reversible resistance switching in Ge-Sb-Te phase-change thin films. Appl. Phys. Lett.91, 152103 (2007).
2. Suh, D.-S. et al. Thermoelectric heating of Ge₂Sb₂Te₅ in phase change memory devices. Appl. Phys. Lett.96, 123115 (2010).
3. Padilla, A. et al. Voltage polarity effects in Ge₂Sb₂Te₅-based phase change memory devices. J. Appl. Phys.110, 054501 (2011).
4. Faraclas, A. et al. Modeling of Thermoelectric Effects in Phase Change Memory Cells. IEEE Trans. Electron Devices61, 372-378 (2014).

We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate Van der Waals approximation. New insight is provided on the stability of homopolar GeGe bonds and tetrahedral Ge bonding, in relation with the resistance drift phenomenon, that is investigated experimentally using photothermal deflection spectroscopy experiments.
Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb₂Te₃ composition line of the GeSbTe phase diagram.
Since the structural relaxations concerned with the drift take place on long time scales, the task of understanding them to limit their consequences is not a simple one. We successfully achieved this goal by developing new approaches to overcome a series of hurdles.
A first problem is that directly generating an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atom (typically a few hundreds), and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging.
A second aspect is that GST alloys are known to display complex bonding mechanisms, for which the simple chemist&’s “octet-rule” does not apply, leading a long series of controversies, concerning in particular the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed.
We can then identify their fingerprints in the available structural experimental data and assess the stability of these local environments to obtain information of the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments that are presented here.
Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal.

The crystallisation kinetics, electrical properties, and the refractive index of the well known GeTe-Sb2Te3 phase change data storage materials are commonly tuned by adjusting the composition or alloying with other elements. However, the effect of strain has not been exploited as a means to design the properties of phase change materials, yet in microelectronics material research, ?strain engineering? is the principal technique used to enhance the performance of metal oxide semiconductor field-effect transistors (MOSFETs). In this presentation we will explore the effect of strain on the properties of phase change materials and phase change superlattice structures. The talk will encompass our latest results from experiment and simulations whilst also projecting forward to discuss the potential of using strain as a design parameter to optimise the properties of phase change data storage materials.

A central issue of nanoelectronics concerns their fundamental scaling limits, that is, the smallest and most energy-efficient devices that can function reliably. Unlike charge-based electronics, memory devices based on phase change materials (PCMs) and metal oxides (RRAM) are more immune to leakage at nanoscale dimensions [1]. In order to probe the scalability of these materials, we developed novel approaches to build PCM nanowires and RRAM crossbars with individual carbon nanotube (CNT) electrodes [2,3]. With diameters ranging from 1-5 nm, CNTs are the smallest electrodes available, enabling us to probe memory bits of sub-10 nm dimensions. By utilizing CNTs in this context, we are able to reduce the programming current and power of such memory devices by more than 100× compared to previous state-of-the-art. We have also demonstrated data storage with graphene electrodes and we are currently examining the benefits of integrating PCM with other 2D materials such as MoS2. Finally, we will describe some recent results concerning our studies of thermoelectric effects in PCM devices, and their importance for nanoscale memory elements [4].
[1] S. Raoux, F. Xiong, M. Wuttig, E. Pop, MRS Bulletin39, 703 (2014)
[2] F. Xiong, E. Pop, et al., Nano Letters13, 464 (2013)
[3] C.-L. Tsai, F. Xiong, E. Pop, M. Shim, ACS Nano7, 5360 (2013)
[4] K.L. Grosse, E. Pop, W.P. King, J. Appl. Phys. 116, 124508 (2014)

Phase change memory (PCM), which exploits the phase change behavior of chalcogenide materials, affords significant advantages over conventional memories. However, one of the key challenges for PCM is the requirement of high programming current density. In this talk, we show how a bottom-up approach based on block copolymer self-assembly can resolve the chronic issue of power consumption in phase change memory. Si-containing block copolymers were induced to self-assemble to form a thin nanostructured SiO_x layer that locally blocks the contact between a heater electrode and a phase-change material. The writing current was decreased five-fold (corresponding to a power reduction by 1/20) as the occupying area fraction of SiO_x nanostructures is increased from a fill factor of 9.1% to 63.6%. On the other hand, resistive random access memory (ReRAM), which shows two or more stable resistance states through electrical stimuli, also has strong potential to be one of the next generation nonvolatile memories. Its key concern for practical applications, however, is large fluctuations of switching voltages and resistances caused by random growth/rupture of conductive filaments. We incorporated the SiO_x nanostructures at the interface between a resistive switching material (NiO) and a top electrode (Pt/Ti) to control the locations where the CFs were formed. The self-assembled SiO_x in unipolar NiO resistive memories can extensively modify the electric field distributions to significantly improve the reliability of switching behavior. Growth and rupture of metallic Ni filaments in the amorphous NiO were directly observed through transmission electron microscopy and elemental mapping analyses, demonstrating that the uniformly distributed SiO_x nanostructures can guide resistive switching operation by selective filament growth/rupture.

The thermal disturbance between neighbouring data points has become a major bottleneck for the further miniaturization of phase change memory devices. Incorporation of phase change materials into nanostructured insulators can provide immunity to this program disturb by introducing thermal isolations between adjacent cells. However, deposition of phase change materials into nanostructured substrates can be challenging for the widely used sputtering technique and other deposition approaches therefore need to be investigated. Electrodeposition as an alternative method for the deposition of phase change materials is presented in this work. Being a well-established technique in the electronics industry for the preparation of copper vias in microchips, electrodeposition benefits from a natural advantage of selective deposition and allows direct filling of the materials into nanostructures.
This research reports, for the first time, the electrodeposition of ternary GeSbTe phase change materials from a single, tuneable, non-aqueous electrolyte bath. It offers easy control over the deposited material composition by adjusting the deposition conditions and concentration of the Ge, Sb and Te precursors in the deposition bath and the deposition of all compositions within the GeSbTe ternary composition triangle is possible. The relation between the electrolyte concentration and the material composition will be discussed. More importantly, this electrochemical approach allows the selective deposition of GeSbTe into conductive nanopatterned substrates with TiN as a bottom electrode and SiO₂ as insulator. Accurate filling of feature sizes ranging from a few hundred nanometers down to below 50 nanometers will be presented. Good electrical switching behaviour is obtained from the deposited Ge₂Sb₂Te₅ phase change material. Devices with 200 cycles of endurance and 3 orders of magnitude resistance change will be demonstrated.

At the beginning of this millennium, in early 2000, few disruptive technologies had been proposed to replace the industry standard Non-Volatile Memory (NVM) technology and to enlarge the Flash application base. A widely accepted statement was that if any technology will succeed, it will materialize in the next decade. Among these proposals, it can be noted that only Phase-Change Memories (PCM) has demonstrated the capability to reach the mass volume manufacturing maturity and to be available as a commercial product.
PCM devices based on Ge2Sb2Te5 (GST) chalcogenide alloy provide a new set of features interesting for novel applications, combining features of NVM and DRAM. Moreover the cell performance can be tuned in order to respond to requirements specific for each application. For example it is the case for automotive applications, where PCM devices with very high temperature retention capabilities are required. At the core of this performance tuning lies the choice of suitable phase-change alloys that are capable to provide improvements over standard GST performance.
Aim of this paper is to provide a review of the methodologies adopted to select phase -change alloys suitable to address specific application requests. The desired bulk properties of thin film chalcogenide alloys will be analyzed and the main issues related to their adoption for fabricating PCM devices will be presented. Finally the role of the high temperatures, currents and electric field employed in PCM device operations and their impact on the material properties will be discussed.

Among the emerging nonvolatile memory technologies, phase change random access memory (PCRAM) is considered to be one of the most promising thanks to its unique set of features such as high scalability, multi-level storage capability, good data retention and endurance [1]. However, the high power consumption during the RESET operation is the major obstacle for this technology and must be reduced in order to compete with current flash memory technology. It has been demonstrated that by integrating the phase change material (PCM) in high aspect ratio lithographic structures, the heating efficiency is improved leading to a reduced reset current [2]. In that context, a highly conformal deposition process is required.
Our previous work has reported the deposition of stoichiometric, amorphous, and smooth layer of GeTe by Plasma Enhanced - pulsed liquid injection - Chemical Vapor Deposition (PECVD) process [3]. Here, we focus on the deposition of the ternary alloy compound Ge-Sb-Te (GST) which is another promising candidate for PCRAM.
Contrary to atomic layer deposition (ALD) where the GST composition range is very tight [4], various compositions of amorphous Ge-Sb-Te layers can be deposited by PECVD using the commercial organometallic precursors TDMA-Ge, TDMA-Sb and DIP-Te. Both Ge-rich and Te-rich GST films can be obtained thanks to the pulsed injection system. The advantage of this process is the ability of varying stoichiometry, which results in varying phase change characteristics of the deposited PCM.
Thereafter, we present the strong impact of the deposition parameters on GST film properties. We focus on the impact of dual frequency mode on the gap filling of PCM in confined structures. By adding a low frequency power component to the radio frequency power of the plasma, trench structures of high aspect ratio can be successfully filled. The deposited PCM has a T-shape: the gap is perfectly filled and the top is covered by a smooth and homogenous layer. No void or any recess is observed; furthermore Chemical Mechanical Planarization (CMP) is not necessary. In other words, PCM is ready to be integrated in the final memory device.
Finally, electrical characterization of PCM devices integrating active materials GeTe and GST deposited by PECVD is also presented. The electrical properties of these materials are then compared with those obtained for PCMs deposited by the conventional PVD techniques.
[1] G. W. Burr et al., J. Vac. Sci. Technol. B 28 (223), (2010).
[2] S. Kim et al., SEC, 2010 VLSI ; M. Kang et al., SEC, 2011, IEDM.
[3] M. Aoukar et al., MRS spring meeting, San Francisco (2014).
[4] Eom et al., Chem. Mater., 24 (11), pp 2099-2110, (2012).

The market of flash memory has been expanding in this decade by virtue of the rapid bit cost reduction. The bit cost reduction has been realized by cell-size scaling, multi-level technology, and three-dimensional (3-D) stacking [1]. As a result, new applications as digital camera, portable music player, mobile phone, tablet terminals, and solid-state drive (SSD) were created. Recently the bit cost of flash memory has approached to that of hard disk drive, so that enterprise storage system has become another promising application. However, not only conventional planar-type flash but also new 3-D flash will reach their limits of bit cost reduction. In addition to the low bit cost, high-programming throughput is also required for storage device. However, increasing programming throughput of flash memory is difficult because the tunneling current through the insulating film is utilized for programming operation. Therefore, emerging memory that satisfies the market requirements of both low bit cost and high programming throughput will be necessary in the very near future. Accordingly, we have developed novel 3-D chain-cell-type phase-change memory [2]. This device features one-time patterning process of stacked memory, selection device made of poly-Si diode [3], and chemical vapor deposition (CVD) of phase-change material. The thickness of CVD phase-change material was successfully reduced to several nanometers by realizing very smooth surface with root mean square of less than 1 nm. The one-time patterning reduces the number of process steps, and poly-Si diode and thin phase-change material reduce cell size. As a result, newly developed vertical chain-cell type phase-change memory cell driven by poly-Si MOS transistor and poly-Si selection diode enables us to achieve relative bit cost of 0.2 compared to 3-D flash. The thin phase-change layer enables low programming current by increasing the density of in-plane current utilized in our device. Programming time, that is reset time in our device, was 30 ns which was equivalent to the conventional phase-change memory [4]. Low programming current makes it possible to increase the degree of parallel programming, and thus the estimated programming throughput is increased up to 1 GB/s which is over 10 times faster than flash. Novel memory technologies including device structure, process technologies, and device characteristics will be presented and discussed.
[1] H. Tanaka et al., Symp. on VLSI Tech., pp. 14-15 (2007).
[2] M. Kinoshita et al., Symp. on VLSI Tech., pp. 35 - 36 (2012).
[3] Y. Sasago et al., Symp. on VLSI Tech., pp. 24-25 (2009).
[4] H. -S. P. Wong et al., Proceedings of the IEEE, 98, pp. 2201 - 2227 (2010).

Chalcogenide materials continue to play a key role in information technology. In particular, phase-change memory (PCM) has emerged as the most promising new nonvolatile solid-state memory technology. A key property that makes these materials attractive for such applications is threshold switching, which allows the current density in the cell to rise steeply above the so-called threshold switching voltage. This allows the temperature within the cell to rise to very high values (>1000 K) via Joule heating, making the phase-change process (melting and crystallization) possible. However, the details of the threshold switching dynamics and subthreshold conduction, even if first reported almost 50 years ago, are still as of today a matter of ongoing research.
A large number of models were proposed to explain threshold switching in semiconducting glasses, mainly via thermally-induced or solely electronic mechanisms.[1,2] Most of the experimental work in the 1970s was done on thin films, typically with large thermal time constants, and a debate over thermal versus electrical origin of threshold and memory switching was settled.[1,2] In the last 10 years, those models have been revived to explain data in nanometric PCM cells.[3] However, very few experimental data has been obtained on state-of-the-art memory cells to verify those models on a wide temperature range and upon structural relaxation. These dependencies are especially important to assess for memory applications, in order to ensure that the READ operation does not disturb the state of the cell in all application conditions.
In this talk, we present the measurement of the complete time/temperature dependence of the threshold voltage and current in PCM cells. Our approach is capable of measuring the threshold IV characteristics starting from 10 mu;s after RESET, which allows reporting the switching characteristics over more than 6 orders of magnitude in time. The measurements were done at temperatures ranging from 30 °C to 160 °C for different programmed states. We report a nearly constant threshold power behavior upon both time and temperature, as being recently noted as well by a few other groups [4]. The observed constant power behavior suggests that, in such highly thermally efficient cells, one cannot preclude a thermally initiated switching as originally envisioned in the 1970s.[2] We reassess the plausibility of this mechanism using a simple analysis based on our detailed knowledge of the thermal environment and temperature distribution in nanometric cells.[5] A field and temperature dependent description of the conductivity is used supported by FEM simulations of the PCM cell, and comparison to the experimental data is made.
[1] D. Adler et al., J. Appl. Phys., 50(6), 1980
[2] K.W Böer et al., J. Appl. Phys., 41(6), 1970
[3] A. Redaelli, J. Appl. Phys. 103(11), 2008
[4] N. Ciocchini et al., IEEE Trans. Electron Devices, 59(11), 2012
[5] A. Sebastian et al., Nat. Commun., 5(4314), 2014

Since the discovery of the Phase-Change Materials, the threshold switching has been the most intriguing phenomenon concerning these materials. It makes possible the transition from high to low resistivity in the amorphous phase, and this mechanism is what made it possible that Phase-Change Memory is today the most promising technology among the other emerging non-volatile memories [1]. Different models have been proposed to explain the threshold switching, that range from purely electronic explanations to the electric field-driven nucleation theory, but the debate on this topic is still open [2-5]. A recurrent analogy in the models proposed, is the filamentary nature of the switching mechanism with the consequent starting crystallization from the filament generated inside the amorphous volume of the memory cell [3,6]. In the light of our recent results that support this hypothesis [7], we report here a statistical analysis of the switching behavior in state-of-the-art Phase-Change Memory cells. Threshold switching is investigated at different stress voltages and different operating temperatures, enabling the collection of complete distributions of the switching time of the memory devices. The data show a good confidence level with a log#8209;normal distribution, in agreement with the electric field induced nucleation theory. Moreover, no correlation is observed between the switching time and the initial programmed resistance value of the devices. This helps to exclude in our analysis a significant impact of the spread of the thickness of the amorphous region on the variance of the data distributions. The relationship between expected switching time, stress voltage and temperature is then discussed. This analysis becomes fundamental for design optimization and for the reliability assessment of read disturbs, in particular at high operating temperatures targeted in industrial and automotive applications. Standard Ge₂Sb₂Te₅ and innovative phase#8209;change materials are here analyzed and we demonstrate how the threshold switching characterization can be used to optimize the programming and the sensing of Phase-Change Memory.
[1] P. Zuliani et al., IEEE Trans. on Electron Dev. 60, 4020-4026 (2013).
[2] D. Ielmini et al., J. Appl. Phys. 102, 054517 (2007).
[3] V. G. Karpov et al., J. Appl. Phys. 104, 054507 (2008).
[4] K. Kohary et al., Phys. Status Solidi B249 10, 1897-1901 (2012).
[5] V. G. Karpov et al., Appl. Phys. Lett. 102, 236101 (2013).
[6] N. Ciocchini et al., IRPS 2014, 5E.1.1-5E.1.6.
[7] G. Navarro et al., EPCOS 2014.

Phase Change Memory (PCM) has been the first emerging non-volatile memory technology to experience volume production maturity [1]. In these devices electro-thermal stress during programming operation induces stoichiometric variations that may impact on memory cell performance and reliability. In this work a detailed analysis of the stoichiometric drift induced by programming pulses was performed on test structures. The devices were line bridge cells of different dimensions, fabricated on a 50 nm GST layer by e-beam lithography. The cells were programmed with increasing electrical stress in terms of both voltage and programming pulse duration, as well as cycling up to cell failure. Bi-dimensional concentration maps were obtained by X-ray fluorescence analysis carried out at the European Synchrotron Radiation Facility, Grenoble, France.
The obtained results show that a strong Te-rich phase builds up at the anode, while a Sb/Ge-rich phase is generated within the line core and towards the cathode, with a tendency of Ge to agglomerate with a discontinuous distribution. To account for the size variation of the molten region and its drift on electrical stress, a self-consistent electro-thermal model, including thermoelectric effects, was developed. Key elements of the model are: (i) the segmentation of the device in multiple blocks, each of them with local properties consistent with the local experimental stoichiometry, that is, variations in both the melting point and the electrical resistivity of the alloy [2,3]; (ii) the adoption of a steep rise on the electrical conductivity dependence on temperature when solid/molten phase transition occurs [4].
The model accounts well for the experimental results, highlighting that ion migration induces significant stoichiometry variations within the molten area. Filamentary conduction also takes place, which is more relevant in large cells, consistent with results already available in literature [5].
[1] G. Servalli, IEDM Tech. Dig., 2009
[2] B. Legendre et al., Thermochimica Acta, 1984
[3] S. Bordas et al., Thermochimica Acta, 1986
[4] L. Crespi et al., IEEE Electron Device Letters, 2014
[5] D. Kang et al., Applied Physics Letters, 2009

Inherently, phase change materials (PCM) experience high current density to write or erase information in phase change memory. The harsh operating condition (~ 10⁸ A/cm²) can lead to device failure induced by compositional change and void formation, which is well known reliability problems. In contrast, reading process of device using relatively low current density (~ 10⁶ A/cm²) has been considered to have no direct effect on the stability of PCM because the driving force for migration is insufficient to migrate atoms. In recent studies, however, the resistance of Ge₂Sb₂Te₅ (GST) change to more metallic state even under very low current density has been observed, without accompanying a massive structural change. It is believed to be originated from the inherent vacancies of crystalline GST which is easily rearranged by electric current. It is a highly rewarding task to unravel the resistance change under current stressing based on vacancies, because this phenomena can be a crucial reliability problem in a resistance-based device, in further can be utilized to modulate the conductivity of cubic phase of GST.
In this study, we investigated the resistance change of crystalline GST under current stressing, and compared with thermal test. The GST line structures with 20 mu;m in length, 2 mu;m in width, and 300 nm in thickness were patterned by photo lithography. For the current stressing test, current densities ranged from 0.17 to 1.66 MA/cm² which is called current sample. For the thermal test, temperature ranged from 200 to 350 °C which is called temperature sample. The microstructures were ex-situ analyzed by TEM.
The decreases of resistance of current and temperature samples were observed; as the current density or temperature increased, the resistance decreased to lower values, however, the time dependencies were different. For thermal testing, resistance decreased gradually until tens of hours. Saturation times for isothermal annealing were represented as Arrhenius relation with energy barrier of 0.97 eV. On the contrary, the current-induced resistance change is not a time-dependent and kinetically fast process which is completed in few minutes. Microstructural analysis using TEM was conducted for current sample of 10⁶ A/cm² (at room temperature) and temperature sample of 250 ^oC which had a same final resistance. The temperature sample was hexagonal structure which is the stable phase of GST, however, interestingly the current sample was cubic structure even though it exhibits similar resistance with the temperature sample. These results show the formation of stable cubic GST with high conductivity as much as hexagonal phase by utilizing electrical current stressing. This study provide a new insight about electrical reliability in the crystalline phase of GST, which offers the fundamental knowledge of the mechanisms of current-induced modulation of conductivity in chalcogenide materials.

Phase change materials enable a broad range of electronic/photonic applications, from optical/electrical storage to artificial synapses in neuromorphic systems. Similar to other resistive switches, such as those based on spin-transfer torque and redox transition, the phase change memory (PCM) has been proposed for normally-off digital circuits, where fast, low-power switching can be combined with nonvolatile storage to yield hybrid logic/memory circuits with virtually zero static power consumption. However, to support phase change logic, complete and cascadable Boolean functions must be demonstrated in PCM.
This talk will review phase change logic gates and circuits [1-3]. First, the physical principles used for logic computation, namely threshold switching [4] and additive crystallization [5], will be presented. Prototypical logic gates for NOR, NAND, NOT and XNOR functions will then be shown by a review of their experimental demonstration. Phase change logic will be compared to other similar proposals of normally-off logic circuits, such as those based on memristors and spin torque. The main challenges for a future application space of phase change logic will be finally identified and discussed.
[1] M. Cassinerio, et al., Adv. Mater. 25, 5975 (2013).
[2] Y. Li, et al., J. Appl. Phys. 114, 234503 (2013).
[3] D. Loke, et al., PNAS 111, 13272 (2014).
[4] S. R. Ovshinsky, Phys. Rev. Lett. 21, 1450 (1968).
[5] C. D. Wright, et al., Adv. Mater. 23, 3408 (2011).

Recent reports have shown that the implementation of chalcogenide phase change materials (PCMs) in RF systems enables world class performance in terms 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 world class cutoff frequency (F_co) of 12.5 THz have been demonstrated. In order to achieve such high level performance, significant material development efforts were undertaken. GeTe films were deposited using the DC magnetron sputtering technique and a power-pressure matrix was utilized to minimize sheet resistivity (R_s) and maximize the OFF-ON ratio. Morphological features were evaluated using XRD and SEM. Electrical properties of as-deposited GeTe films were evaluated using 4-point probe measurements and a heated stage. The combined effect of sputtering process parameters on film morphology, material properties and device performance will be presented. Finally, aberration-corrected scanning transmission microscopy (AC-STEM) was used to evaluate the effect of ON and OFF pulses on the GeTe morphology. Critical insights into crystallization and amorphization mechanisms unique to this novel device will also be presented

To date the main applications of phase-change materials and devices have been limited to the provision of non-volatile memories. Recently, however, the potential has been demonstrated for using a phase-change approach for the provision of entirely new concepts in optoelectronics, including phase-change displays, integrated phase-change photonic memories, optical modulation and optical computing [1-3]. Such novel applications are enabled by the ability of phase-change devices to operate in a 'mixed-mode' configuration, where the excitation is provided electrically and the sensing is carried out optically, or vice-versa. Exploitation of this mixed-mode is made possible in phase-change materials due to the large and simultaneous changes that occur in both refractive index and electrical resistivity on transformation between amorphous and crystalline states. In this paper, based on studies part-funded by the NSF Materials World Network, we present recent results of the use of such mixed-mode operation to provide new applications, including a demonstration of phase-change optoelectronics devices that can be used to make ultrathin all-solid-state colour displays of ultrahigh resolution [1], and hybrid integrated phase-change photonic circuits that offer both a low-power, multi-level memory capability and a computing functionality [2,3]. As so often mentioned by the late (and sadly missed) Stanford Ovhinsky at previous MRS meetings [4], phase-change materials have the potential to provide us with so much more than simple digital memory - a potential that we are now beginning to realize and exploit.
[1] P Hosseini, C D Wright and H Bhaskaran, Nature 511, 206 (2014)
[2] C Rios , P Hosseini , C D Wright , H Bhaskaran and W H P Pernice, Advanced Materials 26, 1372 (2014)
[3] C D Wright, Y Liu, K I Kohary, M M Aziz, R J Hicken, Advanced Materials 23, 3408 (2011)
[4] S R Ovshinsky and B Pashmakov, MRS Proceedings 803, 49 (2004)

As the conventional computational paradigm based on the von Neumann architecture reaches its limits in terms of power density, speed and scalability, significant research efforts are invested to create biologically-inspired cognitive systems. In these systems, computation relies on spiking-based communication between large numbers of neurons, and the memory is represented by evolving states of the synaptic interconnections. To understand, mimic and control the learning process, hardware realizations of such synaptronic devices have become a central research topic.
Nanoelectronic devices with memristive properties are promising candidates for the realization of synapses in neural hardware [1]. Recent research efforts have mainly focused on time-based synaptic plasticity, such as Hebbian learning and spike-time dependent plasticity (STDP) [2]. STDP was successfully demonstrated in nanoscale hardware implementations using phase-change technology [3,4,5].
In this talk, we demonstrate that synaptic dynamics, which go beyond the time-based plasticity, play a pivotal role in spiking-based algorithms for unsupervised learning and big data processing. We focus on one such aspect of the dynamics, namely, the weight-dependent plasticity (WDP), which is the ability of a synapse to vary its effective response based on the value of its internal state at the time of the weight change. WDP is needed to impose soft limits on the achievable synaptic weight, and has a profound connection to the computational properties of neural networks [6,7]. Unfortunately, implementing WDP poses a significant additional challenge for the technological realization of synapses, making them even more complex and more costly. In a CMOS circuitry, physical constraints of transistors can be used to evoke WDP efficiently [8]. However, so far few results have been presented that would apply to non-CMOS synaptic devices, and phase-change devices in particular.
We show how WDP can be realized in phase-change-based synaptronic devices. We demonstrate that the resulting dynamics can be finely adjusted by the programming scheme and the hardware design of the cell, and elaborate on the interplay between WDP and STDP. Experimental measurements are shown that demonstrate the various WDP characteristics in on-chip phase-change cells, and a study of WDP-dependent algorithmic properties in single-neuron-based correlation detection is presented.
[1] Di Ventra et al., Proc. IEEE, 2009.
[2] Bi and Poo, Ann. Rev. Neurosci., 2001.
[3] Kuzum et al., Nanoletters, 2011.
[4] Jackson et al., J. Emerg. Tech. in Comp. Sys., 9(2), 2013.
[5] Eryilmaz et al. (2013) Proc. IEDM, 2013.
[6] Song et al., Nat. Neurosci., 2000.
[7] Gütig et al., J. Neurosci., 2003.
[8] Bamford et al., IEEE Trans. Biomed. Circuits Syst., 2012.

For the last decades, a number of researches in the neuromorphic engineering research field have put vigorous efforts on emulating mammalian brain&’s functionalities. Recently, as a part of this research, basic building blocks of an artificial neural system have been frequently studied, such as artificial neurons and synapses. In this study, we theoretically investigated the neuronal behaviour of neuristor-based leaky integrate-and-fire (NLIF) neurons, which inevitably includes its operational noise caused by variability of the threshold switches that are employed in the NLIF neuron. In general, this noise is not desirable within the framework of the conventional encoding and decoding schemes in digital systems. However, the mammalian brains well function without significant errors even with such noises. We analyze the noise property of the NLIF neuron and demonstrate the probability of reliable operation of the noisy NLIF neurons in terms of their population representation. The reliability of the information conveyed by the neurons is quantitatively evaluated in terms of the standard deviation of the posterior probability distribution. The aforementioned estimation of noise property of the NLIF neuron is experimentally confirmed on unit artificial neuron devices utilizing amorphous chalcogenide-based threshold switches.

Electronic applications rely heavily on material interfaces for their optimum functionality. For example, defects at interfaces can deteriorate the efficiency of light emission for light emitting diodes or reduce the electron mobility and hence the operation speed of metal-oxide-semiconductor field effect transistors. Due to their importance, the interface formation mechanisms between functional materials are well studied and understood.
Materials that are used in conventional (opto)electronic devices are characterized by covalent bonding in three dimensions (3D). In recent years it has become clear that some of the chalcogenide materials used for PCRAM applications are layered materials with covalent bonding in only two dimensions (2D). In order to benefit from the maturity of conventional Si technology, an integration of 2D materials with the 3D material Si is desirable. In addition GeTe/Sb₂Te₃ superlattices consist of alternating 2D/3D materials. This requires a detailed understanding of the mechanisms that determine the interface structure between 2D and 3D materials. We shed some light on this question by investigating Sb₂Te₃ layers grown on top of Si(111) using MBE¹. The study led to the discovery of new epitaxial paradigms for the growth of 2D layered materials onto 3D bonded substrates.
In the present contribution we would like to focus on several surface treatments to allow for the highest control on the structural properties of the deposited materials. We will discuss also the electrical switching properties upon surface preparation.
References
¹ J.E. Boschker, J. Momand, V. Bragaglia, R. Wang, K. Perumal, A. Giussani, B.J. Kooi, H. Riechert, and R. Calarco, Nano Lett. 14, 3534 (2014).

Interfacial phase change memory (iPCM) has been proposed and designed to reduce switching energy loss in phase change memory. As iPCM is composed of coherently-aligned XTe (X: Ge, Sn, Si) and Sb₂Te₃ multilayers, which can be built up as a superlattice, controlling the orientation of each layer is crucial. Even though the first orientation layer of Sb₂Te₃ is required to fabricate a highly-oriented superlattice, the role of the initial Sb₂Te₃ layer has not well been clear. Moreover, since Sb₂Te₃ has been attracting great attention as not only phase change material but also thermoelectric material, or topological insulator, it is more important to understand the deposition mechanism of a highly-oriented Sb₂Te₃ film. In this work, the deposition mechanism of the sputtered Sb₂Te₃ films is analyzed under different conditions in formation.
The films were prepared by RF-magnetron sputtering on a Si single crystal wafer or Si wafers covered with electrode materials. The degree of orientation was evaluated by x-ray diffraction (XRD) and the microstructures were observed by transmission electron microscopy (TEM). It was found that Ar-bombardment of the Si surface significantly improved the quality of the superlattice by the peak intensity of the XRD patterns, indicating the removal of a native oxide film to be effective to form a highly-oriented film. When the superlattice was deposited on a natively-oxidized tungsten, the quality of the film was not satisfactory, resulting in the rough surfaces, and randomly oriented grains. Once the surface of the tungsten oxide is removed by Ar-bombardment just before the Sb₂Te₃ deposition, the superlattice film grows with a highly orientation normal to the substrate. The result suggests that the removal of an oxide from a substrate surface is effective to fabricate a highly oriented film. Based on the results, we propose one possible mechanism about an oriented Sb₂Te₃ film formation.

Owing to the high contrast in optical and electrical properties when switched between their crystalline and amorphous states, GeTe and GeSbTe alloys (GST) have already been used in optical data storage applications for more than a decade now. These materials are currently investigated in the prospect of creating non-volatile, fast, and highly scalable electrical data storage devices.
Simpson et al. have demonstrated that stacking GeTe and Sb₂Te₃ thin films into a superlattice structure (SL) enables a switching mechanism within the crystalline state that greatly reduces the switching energy and improves the cyclability compared to an alloyed GST film [1].
In the present work, GeTe/Sb₂Te₃ superlattices are epitaxially grown by molecular beam epitaxy on silicon substrates and characterized by in-situ reflection high energy electron spectroscopy (RHEED), X-ray diffraction (XRD), and scanning transmission electron microscopy (STEM). Using adequate substrate surface preparation, both out-of-plane and in-plane ordering is achieved, and optimized growth conditions allow the deposition of thin sublayers, down to less than 1nm for GeTe. As the thickness of the GeTe and Sb2Te3 sublayers is decreased, the interfaces between these sublayers play a more salient role and can be more explicitly characterized through XRD.
Incommensurate reflections corresponding to structures alike GST124 and GST225 are observed, highlighting the tendency of GeTe and Sb₂Te₃ towards alloying at the interface. This is unequivocally corroborated by STEM analysis, revealing that GeTe blocks are terminated by Sb₁Te₂, passivating the GeTe dangling bonds and allowing the formation of a van der Waals gap with the neighboring Sb₂Te₃ quintuple layer.
[1] R. E. Simpson et al., “Interfacial phase-change memory”, Nat. Nanotechnol., vol. 6, no. 8, pp. 501-5, Aug. 2011
This work was in part supported by EU within the FP7 project PASTRY (GA 317746)

Innovative phase-change memories based on sputtered Sb₂Te₃/GeTe superlattices (SLs) exhibit reduced switching energies, longer write-erase cycle lifetimes and faster switching speeds than conventional data storage devices employing Ge₂Sb₂Te₅ (GST) alloys. The superior electrical characteristics are attributed to the realization of highly ordered staking sequences [1]. In this perspective, several chalcogenide thin films were grown by molecular beam epitaxy, a technique that is anticipated to improve the PCMs&’ structural perfection allowing for tight control of layer thickness and interface quality. The films grown were analyzed by X-Ray diffraction (XRD) in order to study their structural characteristics. We compared as grown cubic-rhombohedrally distorted GST and GST crystallized from the amorphous phase with SLs of the same nominal average composition. XRD measurements (specular omega;-2theta; scans along the [111] direction) show features not representative of Bragg reflections, which might be related to ordering of vacancies present in the set of GST samples. The features corresponding to the vacancy ordering can be also obtained by annealing the amorphous phase, thus showing a path to induce a transition towards ordering.
Careful simulations of the XRD data were carried out by using the CrystalMaker software package [2] to determine the best stacking sequence which accounts for the observed features, that we assigned to long range ordering of vacancy planes normal to the [111] direction. As the vacancy layers are expected to be inserted between the weakly bonded Te-Te layers, the effects of the Te content as well as the possible partial occupancy by Sb and Ge atoms will be discussed. In addition, the SLs spectra reveal other peaks, which are not Bragg reflections, correlated to the position of the SLs peaks and induced by the superlattice stacking and not by the crystalline lattice. In particular, we identified the presence of some GST225 and GST124 at the SLs interface and by engineering the stacking sequence in the simulated SLs we were able to identify the correct structure of the grown SLs.
[1] R.E. Simpson et al., Nat. Nanotechnol. 6, 501 (2011).
[2] CrystalMaker Software Ltd, Oxford, UK (www.crystalmaker.com).

Phonon modes are fingerprints of lattice properties of materials and allow for a deeper understanding of bonding. Those modes can be studied using different techniques. In literature, many contributions, dealing with Raman scattering of GeTe-SbTe alloys, Sb₂Te₃ and GeTe are found. In contrast, infrared (IR) spectroscopy has been mostly applied for studying Sb₂Te₃ alloys^1,2.
Here, we performed Fourier transform infrared (THz) spectroscopy in transmittance configuration in the spectral range from 20 cm^-1 to 700 cm^-1, in order to investigate soft absorption features in amorphous and crystalline epitaxially grown GeTe-SbTe alloys (GST) with different compositions. In all cases we found distinct soft absorption modes below 100 cm^-1. In order to properly assign these modes to phonons, Raman spectroscopy was performed for comparison. Indeed, we found IR modes that are not observed in the Raman spectra and vice versa. This finding can be understood for GST explaining the observations by the F3m3 space group. This is an indication of a highly ordered GST lattice that preserves the inversion symmetry, like already observed for Sb₂Te₃^1,2. Furthermore, THz spectra of epitaxially grown Sb₂Te₃/GeTe superlattices and of the constituent GeTe and Sb₂Te₃ were measured and an unambiguous assignment of soft phonon modes could be performed.
¹ W. Richter and C.R. Becker, Phys. Status Solidi 84, 619 (1977).
² W. Richter, A. Krost, U. Nowak, and E. Anastassakis, Zeitschrift Fuer Phys. B Condens. Matter 49, 191 (1982).

Ge-Sb-Te (GST) alloys lying along the pseudobinary (GeTe)_x-(Sb₂Te₃)_1-x tie line exhibit significant contrast in their optical and electrical properties and are used for optical and electrical non-volatile phase change memory (PCM). Recently, interfacial phase change memory (iPCM) consisting of a superlattice-like structure of alternating layers of GeTe and Sb₂Te₃ has received substantial attention because of its low RESET-SET switching energy and its potential topological insulating properties. In this study, we investigated the temperature dependence of optical phonons in the iPCM structure [(GeTe)₂(Sb₂Te₃)₄]₈ and a Ge₂Sb₂Te₅ alloy for comparison. Coherent phonon spectroscopy allows the study of relatively low frequency optical phonon modes (< 200 cm^minus;1). Previously, in GST, a drastic change in coherent phonons behavior due to a temperature-induced phase change was reported. Furthermore, ultrafast phase change induced by manipulation of a specific coherent phonon mode was demonstrated in iPCM structures.
An iPCM film in the RESET state on a 5 nm-thick Sb₂Te₃ growth controlling layer and an as-deposited amorphous Ge₂Sb₂Te₅ alloy film were fabricated on Si substrates using a helicon-wave sputtering system. 20-nm thick ZnS-SiO₂ layers were deposited on top of both samples to prevent oxidation. Optical pump-probe measurements were carried out with a 20 femtosecond laser pulse a center wavelength of 830 nm. The transient reflectivity change was recorded as a function of pump-probe time delay while the sample temperature was varied between 25°C and 180°C at which a phase change occurred in both sample. The pump pulse fluence was maintained below 100 mJ/cm² so as to not increase the sample temperature.
At 25 °C, coherent phonons were observed in both samples. In the alloy sample, two modes were observed at 3.72 THz and 4.75 THz. In the iPCM sample, on the other hand, three modes were observed at 2.11 THz, 3.40 THz, and 5.07 THz. By heating the samples to 180 °C, the amorphous alloy sample transformed into the fcc crystalline state and subsequently the coherent phonons modes became strongly damped. This result is basically consistent with previously reported results. In the iPCM sample, however, the coherent phonon intensity gradually decreased yet oscillations of coherent phonon were clearly observed even at 180 °C even though the RESET state iPCM transformed into the SET state.
This unconventional phonon behavior observed in the iPCM sample may be related to its unique phase change mechanism namely, the quasi one-dimensional transition of Ge atoms at the interfaces between tetrahedrally-coordinated sites and octahedrally-coordinated sites without melting. The moderate temperature dependence on the intensity of coherent phonons suggests that the phase change occur gradually by increasing the temperature.

Recently, Simpson et al [1] 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]. However, the precise phase change process in GeSbTe superlattices is still not clear and even the SET and RESET structures are under debate. Two models have been proposed, based on different analyses of high-resolution electron microscope images and symmetry arguments. In model 1 the transition is from the Ferro to Inverted Petrov states [2]. In model 2, it is between the Petrov and Inverted Petrov states [3].
In this work, we begin with the four basic low energy structure proposed by Tominaga (Ferro, Petrov, Inverted Petrov and Kooi) [4]. We generate the various structures arising from a vertical flip of Ge atomic layers through a Te layer. We note that the resulting structures do not belong to the four basic structures. Thus, we propose that the full switching transition consists of a vertical displacive motion of a plane of Ge atoms to an intermediate state, followed by a lateral diffusive motion of the Ge-Te sublayer to a final, lower energy end structure. The energy barrier for the displacive motion is from 2.59eV to 2.71eV. The energy barrier for the lateral motion is from 0.05eV to 0.39eV, which means this movement will be easily thermally activated.
Through calculating the total energy, band gap and defect formation energies, we evaluate the possibility of direct atomic flipping from SET to RESET structure. We conclude the transition in both model1 and model2 is not only the Ge layer flip along c-axis.
1. R. E. Simpson et al, Nature nanotechnology. 6 501 (2011)
2. Do Bang et al, Scientific Reports. 4 (2014)
3. T. Ohyanagi et al, Applied Physics Letters 104 252106 (2014), K Shiraishi et al, SSDM (2013)
4. J. Tominaga et al, Advanced Materials Interfaces 1 1300027 (2014)

Ga-Sb alloys have recently attracted much attention as potential candidates for Phase Change Random Access Memory (PCRAM) applications [1-3]. They actually exhibit a marked electrical contrast and, as compared to the prototypical material Ge₂Sb₂Te_5, they show a better stability in the amorphous state with higher crystallization temperature [2] and shorter crystallization time (< 15 ns) [4,5]. Besides crystallization time and temperature, the mass density change upon crystallization is a key parameter governing the reliability of PCRAMs. Indeed, few percentages density change induces considerable mechanical stress in memory cells, leading to void formation with subsequent electrical failures. The stoichiometric GaSb films were shown to exhibit an unusual behavior with an increasing thickness/decreasing mass density [6] and a negative optical contrast [3], while its electrical resistance follows the usual behavior observed in PCM upon crystallization. By adding Sb to the stoichiometric Ga:Sb=50:50 compound, the Ga:Sb=30:70 alloy was demonstrated to have no mass density nor thickness change upon crystallization [2]. However, still several other material parameters have to be optimized for PCRAM applications such as the low resistances in the amorphous and crystalline phases and phase segregation.
In order to optimize Ga-Sb alloy for PCRAM application, a new class of materials were studied: starting from the stoichiometric Ga:Sb=50:50 composition, three other doping elements (Ge, Si and Te) were added. Combined in situ sheet resistance measurements and synchrotron X-ray scattering techniques performed during heating were used to study in situ the crystallization process of doped-GaSb alloys.
Addition of Ge or Si was shown to increase the crystallization temperature while the mass density change upon crystallization was reduced and remains negative. Ge-doping has the more marked effect on the crystallization temperature (T_x can reach 400°C compared to 220°C for undoped stoichiometric GaSb) and increases also the electrical contrast. Adding of Te does not have a strong effect on the crystallization temperature but switches the relative change in mass density from negative to positive. These results show that doping of GaSb alloy can be used to tune and optimize their physical properties for PCRAM applications.
[1] Y. Lu et al. J. Appl. Phys. 109 (2011) 064503.
[2] M. Putero et al. APL Mater. 1 (2013) 062101.
[3] H.-Y. Cheng et al. ECS J. Solid State Sci. Technol. 3 (2014) P263-P267.
[4] D.J. Gravesteijn et al. Appl. Opt. 27 (1988) 736-8.
[5] H.-Y. Cheng et al. Appl. Phys. Lett. 98 (2011) 121911.
[6] M. Putero et al. Appl. Phys. Lett. 103 (2013) 231912.

The memory switching cycle of optical and electronic phase-change memory devices involves a time-limiting crystallization mechanism. Materials are selected to enhance the speed of this mechanism, while simultaneously retaining several other criteria to ensure reliable and efficient operation. Due to the lack of detailed knowledge on the crystallization process, the selection of materials for such electronic memory devices is often empirically based on earlier insight from optical switching. At the same time, there are reports on field induced effects that might distinguish the two processes [1,2]. A direct comparison of electrically and optically induced crystallization dynamics is often difficult due to differences in heating pulse shapes and requirements regarding the geometry of the device.
We therefore demonstrate a technique that allows comparing crystallization dynamics not only with the same time scale for heating, but also based on the same device geometry. On the one hand, picosecond duration electric pulses can be generated through THz sources. These pulses heat the material by driving an ultrashort current pulse [3] and eventually induce crystallization of the device. Optical fs-pulses on the other hand generate photoexcited states, which relax through coupling to the lattice on a comparable picosecond timescale [4]. With the heating mechanism taking place on comparable timescales in both excitations, the transient temperature is determined entirely by the thermal design of the device.
Under these conditions with THz excitation we obtain a non-linear scaling of the I-V curve, which shows that our approach through ultrashort electric pulses is consistent with experiments based on far longer pulses. At sufficiently high field strengths, we demonstrate crystallization of the phase-change material Ag₄In₃Sb₆₇Te₂₆ (AIST) and compare this process with results from optically induced crystallization on the same device.
References:
[1] Karpov, I. V., Mitra, M., Kau, D., Spadini, G., Kryukov, Y. a., & Karpov, V. G. (2008). Evidence of field induced nucleation in phase change memory. APL, 92(17), 173501
[2] Kohary, K., & Wright, C. D. (2011). Electric field induced crystallization in phase-change materials for memory applications. APL, 98(22), 223102
[3] Shu, M., Zalden, P., Chen, F., Weems, B., Chatzakis, I., Xiong, F., hellip; Lindenberg, A. (2014). Ultrafast electric-field-induced processes in GeSbTe phase-change materials. APL, 104(25), 251907
[4] Shu, M. J., Chatzakis, I., Kuo, Y., Zalden, P., & Lindenberg, A. M. (2013). Ultrafast sub-threshold photo-induced response in crystalline and amorphous GeSbTe thin films. APL, 102(20), 201903