Single pulse time-resolved X-ray scattering with the ultrabright and ultrashort X-ray pulses from the LCLS free electron laser has been used to study the phase transition dynamics in laser-excited phase change materials (GST225, AIST, and GeSb). For the frist time a complete picture of the structural changes during switching at an atomic level and on the relevant time-scales could be obtained. The response was found to be very similar for all investigated materials involving fast melting on sub-ps to ps time-scales, resolidification into an intermediate amorphous state within a few ns, eventually followed by the formation of the final amorphous or crystalline state on a tens of ns to µs-scale. These time-scales imply purely thermal mechanisms for reaching the final state despite the high degree of electronic excitation initially created by fs laser irradiation and are most likely determined by the nucleation and growth kinetics for the given material and sample geometry. Based on these results concepts will be discussed to use time-resolved X-ray scattering for a quantitative determination of nucleation and growth rates as a function temperature, parameters which are critical to understand and optimize the switching behavior in real devices.

Phase change memory devices are conventionally switched between a crystalline and amorphous phase by current-driven heating of the material. Numerous models attempt to explain the field-driven threshold switching process, whereby the conductivity of the amorphous material increases considerably under high electric fields. Knowledge of the exact nature of this threshold switching effect will help to understand the fundamental mechanisms of the switching process itself. We have applied single-cycle terahertz pump pulses of 300 femtosecond duration, with electric fields on the order of the DC threshold fields, and probed the time-resolved near-bandgap optical transmission of thin films of GeSbTe, GeTe, and GeSnTe alloys, in a repetitive non-switching regime. The amorphous films exhibited a transient decrease in transmission scaling with the square of the instantaneous terahertz field, consistent with a field-induced electrical breakdown process. This was followed by a weaker long-lived reduction in transmission, lasting hundreds of picoseconds, consistent with field-induced heating studied in optical pump-probe measurements. On the other hand, the crystalline sample had a much weaker time-zero response followed by a stronger long-lived reduction in transmission. These studies help to unravel the first steps in the electric field driven switching process in these materials.

Phase-change materials as symbolized by the prototypical phase-change alloy Ge₂Sb₂Te₅ (GST), experience large changes in physical properties in response to structural change. Typically this has utilized for memory applications via the switching of GST between the metastable crystalline and amorphous phases. The dramatic property differences that occur from the transition are due to a large change in bonding properties. The harnessing of such changes has enabled the development of optical storage (DVD-RAM) as well as the next generation of nonvolatile storage, phase-change random access memory (PC-RAM). Information storage via structure as opposed to charge is predicted to allow the future scaling of devices far beyond the limits of charge-based devices. The presence of a bonding hierarchy of long and short bonds and p-orbital based bonding in GST has been suggested theoretically to offer an ultrafast pathway to structural rearrangement via selective excitation of particular phonon modes[1]. In particular, such studies suggest that there may be energetic pathways between the two bonding states that do not require quenching from the melt, but are attainable due to excited state effects and strictly local rearrangement of bonds. Experimentally, it has been shown that light can enhance structural switching in GST [2,3] and that the use of ultrafast lasers to selectively generate coherent phonons can give rise to non-thermal structure changes. While most experimental results on coherent phonon generation have been based upon all optical pump-probe experiments, the advent of the free electron laser allows the direct probing of atomic rearrangement due to ultrafast optical pulses. Using the free-electron laser SACLA at SPring-8, we have observed the atomic rearrangement due to coherent phonon generation in GST in response to a 800 nm 30 fs CPA laser pump using a 10 fs, 8 keV ultrafast hard x-ray free-electron laser probe pulse. Here we use ultrafast diffraction to observe changes in lattice dynamics due to the generation of coherent acoustic and optical phonons by the ultrafast optical pump beam. While acoustic lattice dynamics have been modeled by solving the dynamical equations of motion for the lattice in one dimension and using the subsequent strain fields to compute x-ray diffraction intensity as a function of time using dynamical x-ray theory, there are clear indications of excited state effects that are not present for longer excitation pulses. Here we explore the possibilities of using excited state effects in combination with coherent phonon movement to switch atomic configurations on ultrafast time scales.
[1] A. V. Kolobov et al., Nature Chem., 3, 311 (2011).
[2] P. Fons et al., Phys. Rev. B, 82, 041203 (2010).
[3] K. Makino et al., Opt. Express, 19, 1260 (2011).
ACKNOWLEDGEMENT: The authors would like to acknowledge the support from the X-ray Free Electron Laser Priority Strategy Program of MEXT, Japan.

The rapid growth of the scientific interest in phase-change memory (PCM) is in large part due to its desirable characteristics such as scalability, fast writing speed and data retention. An exciting possibility for PCM is the use of non-equilibrium switching techniques. To date there have been several reports in the literature that underscore this possibility. One possible way implement non-thermal phase switching is the usage of coherent phonons dynamics. In particular by selective excitation of particular phonon modes that can give rise to a desired local change in coordination of particular atoms, it may be possible to switch between the two bonding states of PCM directly without an intermediate phase. The effective usage of this technique requires careful study that allows the following of atom dynamics on extremely short time scales.
We have carried out time-resolved ultrafast X-ray diffraction measurements at the SPring-8 Angstrom compact free electron laser (SACLA). The main goal of the current study is to explore atomic dynamics in epitaxial films of Ge2Sb2Te5 at the sub-picosecond time scale in a response to irradiation with an ultra-short (40 fs) laser pump pulse. Ten femtosecond x-ray pulses were then employed to probe collective atomic motion by monitoring transient changes in the diffraction signal. The effective time probed was varied by delaying the optical pulses with respect to the x-ray probe pulses using an optical delay line. Diffraction data was monitored by a fast readout CCD system was read out for each pump-probe pair and individually saved. In the course of data processing, the signal intensities from each diffraction pattern for specified time delays were integrated over a region of interest and plotted as a function of time. Every measurement was repeated 250 times to improve the statistics of the result. During the experiment there were two main complications making appropriate data acquisition a rather challenging task. The main challenge was signal to noise ratio optimization. In addition, data conversion, subsequent optimization of input parameters (CCD and I0 detector signals thresholds and offsets), useful data sampling and averaging required considerable computing to reduce the multiple terabyte data sets to usable data.
Among the observed transient diffraction patterns features a large drop in diffraction peak intensity occurred within a few picoseconds of the pump laser hitting the sample, while peak intensity fluctuations starting after the drop and an additional peak at lower Qz appeared after a few ps. Although the current talk will focus upon picosecond order response, in future work we will use femtosecond laser irradiation on shorter time scales as a tool for ultrafast switching back and forth between the “set” and “reset” states in Ge2Sb2Te5. These efforts will require sophisticated data process algorithms which will take into account time jitter correction.

Phase-change electronic memory utilizes the electric field-induced reversible structural change in chalcogenide materials to switch between crystalline and amorphous phases to store information in a rapid and non-volatile manner. In spite of extensive investigations of the phase-change switching phenomena, the underlying mechanisms involved in the relationship between structural and electrical properties in phase change materials are quite complex and their elucidation requires the continued development of new experimental and theoretical tools.
My group while working on the switching studies in phase change nanowires in the past several years realized that conventional understanding of melt-quench based amorphization process needed to be revisited. Nanowires, due to their long lengths (few micrometers) do not typically reach high enough temperatures required for conventional melting of the material. Furthermore, due to their cylindrical geometry, conventional melting would lead to the formation of an amorphous shell around the crystalline core, which cannot explain the rapid and abrupt resistance switching as always observed. By utilizing in situ transmission electron microscopy (TEM) technique to observe the nanowire devices with high spatial resolution while being electrically switched, we have shown that the application of electrical pulses introduces disorder in the form of dislocations, which migrate along the direction of hole-wind force and jam at a region of a local inhomogeneity, thus creating a defect-template. This defect template aids the low power switching of the device with the amorphization domain cutting through the entire cross-section of the nanowire (as against core-shell), which explains the abrupt resistance switching. We have now shown the generality of this concept on Ge2Sb2Te5 and GeTe nanowire systems. Using a simpler GeTe system, and utilizing both real and fourier space TEM imaging techniques, we have shown the evolution of intrinsic point defects to extended planar defects, and their jamming process. Through a detailed structure-property correlation, we also show that the accumulation of disorder changes the electronic properties of GeTe while still maintaining the single-crystalline long-range order, prior to solid-state amorphization- suggesting the role of electronic instabilities in the phase change process. The implication of these studies towards the design of low-power memory devices will be discussed.

Electrical wind force is an important element in switching behaviors of phase-change materials. It is hypothesized that the electrical wind force influences the degree of order-disorder existing in phase-change materials, by coupling with the motions of one-dimensional defect structures, namely, dislocations. We discuss electrical wind force-driven behaviors occurring in phase-change materials. First, we report mass transport behaviors when DC voltage biases are applied in line-shape Ge2Sb2Te5 (GST) devices. As the electrical current density reached 3-4 MA/cm2 by DC voltage bias, a directional mass transport was identified by forming asymmetric surface morphology. Joule-heating by the current density raised the temperature up to ~300 oC, implying that the mass transport of GST occurs in hexagonal phase (solid state) regime. Second, we extend the understanding about the presence of electrical wind force to the electrical switching behaviors of GST. We studied the effect of electrical voltage pulses on crystalline-to-amorphous phase transition of GST by in situ transmission electron microscopy (TEM). The electrical voltage pulse plays a critical role by creating dislocations through heat shock process: The rising edge of the pulse produces vacancies by heating, whereas during rapid cooling, atomic vacancies are condensed into dislocation loops. As the dislocations feel the electrical wind force, they become mobile and glide in the direction of hole-carrier motion. The continuous increase in the density of dislocations moving unidirectionally leads to dislocation jamming, which eventually induces the crystalline-to-amorphous phase transition. We interpreted it through one-dimensional traffic model in which an increase of the dislocation density exceeding a certain threshold point induces a catastrophic jamming of dislocations. Density functional theory (DFT) calculations of generalized-stacking-fault (GSF) energy showed that the basal plane of GST hexagonal phase provides favorable pathways of dislocation motions. Our understanding about the dislocation-templated amorphization suggests that outstanding capability of incorporating dislocations attributed to GST-layered structure is one of the origins of the fast switching behaviors.

Recently, Nam et al., [1] have demonstrated a novel low-power defect-template based solid-state amorphization route in Ge2Sb2Te5 single-crystalline nanowires. Using in situ transmission electron microscopy (TEM), it was shown that the application of electrical pulses introduces disorder in the form of dislocations, which migrate along the direction of hole-wind force and jam at a region of a local inhomogeneity, thus creating a defect-template, which aids the low power amorphization of the device. Here, we first demonstrate a defect-templated amorphization mechanism in GeTe nanowire system using similar in situ TEM techniques, proving the generality of this mechanism in nanowire phase-change memory. Furthermore, through a structure-property analysis we show that electrical pulses introduce disorder in the form of anti-phase boundaries (APB) in single-crystalline GeTe, and the migration and onset of jamming of these APBs at a local region aids efficient amorphization. To further reduce the power required for amorphization in this defect-templated approach, it would be desired to pre-disorder the single-crystalline GeTe nanowires and study their switching properties.
We used high-energy (2.2 MeV) He2+ ion-bombardment with varying dosages on single-crystalline, defect-free GeTe nanowire devices using a tandem ion-accelerator but ensuring to not amorphize the devices. The nature of introduced disorder was revealed to be vacancy clusters and APBs using TEM, whose density increases with increasing dosage; and the device resistivities at different dosages were measured to quantify the introduced disorder. We show that during subsequent switching events, the recrystallized phases are at least as disordered as the starting state at different dosages. The switching currents and voltages were observed to scale with the size of the nanowire device at all the dosages, following similar scaling laws as observed for conventional thin-film devices. We further demonstrate that with increasing pre-disorder, the switching voltages remarkably reduce for devices of similar dimensions. By pre-disordering the nanowires, the energy cost incurred for amorphization is just from the defect migration process and not defect creation, thus reducing the activation barrier and power required for the phase-change process. Therefore, the low-power defect templating strategy for amorphization is demonstrated to be a scalable strategy, whose power-efficiency can further be remarkably improved via pre-disordering the crystalline phase.
1. Nam, S.-W., et al., Science 336, 1561 (2012)

Unlike conventional solid-state devices, phase change memory (PCM) devices experience large variations in temperature and the material dynamically changes between the amorphous, crystalline and liquid phases. The gradual transitions from the amorphous to crystalline phase and between the different crystalline phases lead to local variations in material states that depend on the thermal history. Hence, modeling of PCM operation requires a component that dynamically tracks the local crystallinity state of the material.
The changes in the phase-change material can be modeled using a finite element tool by generating crystalline domains and growing them in time based on the theoretically predicted nucleation-growth dynamics [1]. However, this is computationally intensive approach as the boundaries of the nucleated domains need high-density meshing for the calculations. Alternatively, the crytallinity state of individual mesh points can be tracked and get switched between the amorphous and crystalline states in a binary fashion. However, this mesh-based approach requires a uniform mesh as variations in mesh sizes in the structure would result in variations in grain sizes. Hence, this is also computationally intensive.
We have developed a model that uses a crystal density approach where each point is tracked with a variable that represents a crystal density, rather than a binary switch between amorphous and crystalline states. Hence, the approach is immune to variations in mesh and allows non-uniform meshing of a device geometry. The model is calibrated to increase the crystal density based on the temperature-dependent nucleation-growth parameters of Ge2Sb2Te5 [1] as the material crystallizes, and rapidly decrease the crystal density upon melting. The reset-set cycling behavior of the phase change memory cells simulated for several cycles using this model is very much in-line with the experimental findings.
This model is built upon the electro-thermal models we have developed in our group which accounts for temperature-dependent material parameters, thermoelectric effects, thermal boundary resistances and field dependent electrical conductivity (dielectric breakdown) leading to threshold switching [2,3].
[1] G. W. Burr, et al. “Observation and modeling of polycrystalline grain formation in Ge2Sb2Te5” Journal of Applied Physics 111, 104308 (2012); doi: 10.1063/1.4718574
[2] A Faraclas, N Williams, A Gokirmak, H Silva, “Modeling of Set and Reset Operations of Phase-Change Memory Cells”, IEEE Electron Device Letters, 1-3
[3] N Kan'an, A Faraclas, N Williams, H Silva, A Gokirmak “Computational Analysis of Rupture-Oxide Phase-Change Memory Cells” IEEE Trans. Electr. Dev., 60 (5), 1649-1655

The rapid amorphous-to-crystalline transition is the key feature of phase change materials (PCM) which enables their function in commercial rewriteable optical disks (DVDs, Blu-ray Disc) and nonvolatile computer memory (PC-RAM). The structural phase transition is rapid, reversible, and accompanied by changes in the optical and electrical properties. The amorphous structures of PCMs are difficult to determine, and theory has played an important part in their characterization [1]. The nature of the ultra-fast crystallization mechanism remains the subject of much study and speculation, and density functional (DF) / molecular dynamics (MD) simulations have been performed recently to provide more insight. However, DF methods face severe limitations in terms of system size and time scale which call for vast computational resources and effective methods in order to improve the description of the crystallization process.
We have performed large scale density functional simulations (460 atoms for several nanoseconds) of crystallization in the prototype "nucleation dominated" phase change material Ge2Sb2Te5 (GST-225) and can provide details of the changes in order as crystallization proceeds. Our first DF/MD simulations [2] were on a 460-atom sample at 500, 600, and 700 K for up to 600 picoseconds, where crystallization was promoted by fixing the structure of a crystalline "seed" (58 atoms, 10% vacancies). The second simulation was also on a 460-atom sample at 600 K for 1.2 ns, but with no constraints on the geometry. Most recently, a further set of three recrystallization simulations were carried out at 600 K up to 3 ns, each, in order to collect more data on the stochastic process, and in particular nucleation. The density was adjusted during the simulation to allow for the difference between the amorphous and crystalline forms. Crystallization occurs in stages and was complete within 0.3-3.0 ns for successful attemps, depending on the simulation parameters (constraints, temperature) and sample history.
The structural changes were monitored by calculating the pair distribution functions, appropriate order parameters, the number of "wrong bonds" (bond pairs that do not occur in the ordered form), the variation in the cavities, and the electronic density of states. The simulations reveal the course of the crystallization process at the atomistic level, and the important stages include pre-structuring, nucleation, percolation, and final (rapid) collapse to the crystalline state. The orientation of the crystalline nucleus plays a role when the system starts to interact with its replica (other nuclei). Interestingly, the behavior of the individual elements (Ge, Sb, Te) is temperature-dependent.
[1] J. Akola and R.O. Jones, phys. stat. sol. (b) 249, 1851 (2012).
[2] J. Kalikka, J. Akola, J. Larrucea, R. O. Jones, Phys. Rev. B 86, 144113 (2012).

We performed ab initio molecular dynamics simulations of crystallization of the phase-change materials Sb2Te and Ag, In-doped Sb2Te (AIST).
Since crystallization of small amorphous bits of these compounds is dominated by crystal growth, we considered large amorphous and supercooled liquid models containing a crystallization seed and investigated the growth of the seeds at different temperatures. The models were generated by very fast quenching from the melt. Each model contained 810 atoms.
We show that, at temperatures below 550 K, the computed growth velocities for AIST are much higher than the experimental values obtained from time-resolved reflectivity measurements. This discrepancy mostly stems from the large differences between the calculated and experimental diffusivities at these temperatures. We suggest that these deviations are due to the different quenching rates, in combination with the very high fragility of the supercooled liquid phase of AIST.
At high temperatures close to 600 K, simulations are in much better agreement with experiments. These high-temperature simulations indicate that AIST and Sb2Te possess a sharp crystalline-liquid interface and that the presence of Ag and In impurities leads to a reduction in the crystal growth velocity, which is due to a decrease in the sticking coefficients at the interface.

The new generation of non volatile memories for data storage is based on a unique property of systems known as phase change materials, i.e. the super fast (ns) and reversible phase transition between the disordered and the crystalline phases. One of the reasons of the fast crystallization of GeTe-based phase change materials is the very high atomic mobility of the supercooled liquid phase, even close to the glass transition temperature. This feature is in turn a consequence of the fact that GeTe is a fragile liquid, i.e. it shows a breakdown of the Stokes-Einstein relation (SER) that relates viscosity and diffusion in the hydrodynamic regime.
In this work we investigate by large scale molecular dynamics simulations the microscopic origin of the breakdown of the SER in GeTe. To this end we employed an interatomic potential based onto a Neural Network framework that allows to overcome the limitations of conventional first principles calculations in terms of system size and timescale.
Our findings demonstrate that the breakdown of the SER is due to the presence of dynamical heterogeneities in the atomic motion. We quantified as a function of temperature the spatial extent of domains of slow/fast moving particles. The most mobile particles tend to cluster in domains that contain a significant number of chains of homopolar Ge-Ge bonds. The number and length of these chains increases with supercooling, boosting the atomic mobility and then the fast crystallization of GeTe-based phase change materials. We also found a certain degree of cooperative motion in this system, which is due to both first-shell correlated and string-like motion. Finally, we investigated the role of the domains of most immobile and mobile particles during crystallization, finding that mobile particles tend not to crystallize, but instead to flow around immobile (crystalline) nuclei facilitating the atomic rearrangement at the liquid-crystal interface.

Recently, chalcogenide-based phase change materials (PCMs) (GeTe,Ge2Sb2Te5, and etc.) have been used in memory devices such as optical memory disks and phase-change random-access memory [1]. These materials show a fast, reversible phase transition between crystalline and amorphous. To understand the crystallization process, Elliott [2] found that connected square rings are the basic building blocks in Ge2Sb2Te5 and their density can be used to detect the crystallization. It is also noticed that the umbrella-flip of Ge atoms between a tetrahedral and octahedral positions can play an important role in the transition [3].
Here we analyzed the crystallization of amorphous GeTe by an ab-initio density functional molecular dynamics simulation of atomic diffusion and the motion of square rings. A supercell of 128 atoms is used with different quenching rates from 100 K/ps to 1 K/ps and annealing temperature varied from 300K to 600K. The pair distribution function (PDF) becomes crystal-like. The sample crystallizes after annealing for 30-150ps from different runs.
First we explore the flipping of a Ge-Te bonding during the bulk crystallisation, where Ge and Te bonds exchange with each other. The transition state is studied using the nudged elastic band method. The energy barrier found to be 0.3-0.5eV depending on the neighbor environment. The difference between the two bonding states is 0.1-0.2eV.
The collective flipping of a layer of Ge sites as in an interfacial GST structure [4] is explored. The energy barrier is found to be 1.8 eV, much larger than the single flipping.
1. M Wuttig et al, Nature Materials. 6, 824 (2007)
2. J. Hegedus, S. R. Elliott, Nature Materials. 7, 399 (2008)
3. A. V. Kolobov et al, Nature materials. 3, 703 (2004)
4. R. E. Simpson et al, Nature Nanotech 6, 501 (2011)

Antimony plays a major role in many of the phase change materials (PCM) used in digital storage technology. Since their discovery [1], PCMs have been enormously successful as optical storage, including rewritable DVD and Blu-ray Disc, where GeSbTe alloys have been the materials of choice for many years. Recent years have also seen much interest in using PCM in nonvolatile memory devices[2] and the first products have been on the market for a few years. The applicability of these materials is based on the existence of two phases (amorphous and crystalline) with remarkably different optical and/ or electronic properties and the possibility of cycling rapidly (~10 ns) and repeatedly ( >1000000 times) between them. It is important for archival purposes that both phases be stable over extended time and temperature ranges[2]. While significant work has been performed on PCMs recently, there remain open questions about elemental Sb itself that deserve attention and may enable us to understand the function of PCMs better.
We have investigated previously both GeSbTe and AgInSbTe compounds and their crystallization properties[3,4] using combined density functional/ molecular dynamics (DF/MD) simulations. Furthermore, we have studied the properties of elemental Te, which like Sb, can exist in amorphous form but crystallizes readily around room temperature [5]. This work has now been extended to investigate the crystallization of amorphous antimony. The simulations have been performed using the CPMD program with the PBEsol approximation to the exchange-correlation energy. The simulation cell contained 588 Sb atoms in order to minimize effect of periodic boundary conditions and to improve statistics, and we discuss the structural properties of the amorphous and crystalline phases. These properties have allowed us to develop a model Hamiltonian to describe the system and perform kinetic Monte Carlo simulations of crystallization and amorphization on time and length scales presently inaccessible to DF/MD simulations.
[1] J. Feinleib et al. Applied Physics Letters 18, 254 (1971) and references therein.
[2] H. -S. P. Wong et al.Proceedings of the IEEE 98, 2201 (2010)
[3] J. Akola and R.O. Jones, physica status solidi (b) 249, 1851 (2012).
[4] T. Matsunaga et al. Nature Materials 10 ,129 (2011)
[5] J. Akola and R.O. Jones, Physical Review B 85, 134103 (2012).

Chalcogenide phase-change (PC) materials have been successfully used as the active layer in optical data storage media, and more recently in electronic PC memory (PC-RAM). The materials show many properties that are all critical to the performance of these memories such as reversible nanosecond timescale switching, phase stability at room temperature, and large optical and electrical contrast between the phases. The static properties of the material, as well as the phase-change kinetics are commonly tuned by adjusting the composition of the PC material.
The Ge2Sb2Te5 phase-change material lies on the GeTe-Sb2Te3 pseudobinary tie-line, and it is one of the commonly used materials in prototype PC memory devices. Pressure and stress have been shown to influence the crystallization temperature of various PC materials, including Ge2Sb2Te5. This indicates that it might be possible to tune the material properties by strain engineering, which would add a new "degree of freedom" that is independent of the material's composition to the design of PC materials with specific properties.
As PC-RAM devices are scaled to smaller dimensions to meet the demands of high density data storage, the influence of stress from electrode and capping materials will play an increasingly important role in the PC-RAM device performance. It is therefore critical to understand ways in which stress imposed from the electrode materials can be used to improve the performance of phase change materials. Herein, the effect of uniaxial stress on the Ge2Sb2Te5 recrystallization rate is studied by performing density functional (DF) / molecular dynamics (MD) simulations using the Vienna Ab-initio Simulation Package (VASP). The simulation model is a 108 atom amorphous structure that is under tension in one axis and compressed in the perpendicular axes. The atomistic evolution of the DF/MD simulation is monitored at 600 K, a temperature which permits crystallization of unstressed Ge2Sb2Te5 models. The crystallization is monitored with bond orientational order parameter, and changes can be seen in pair distribution functions and bonding topology.

Ge₂Sb₂Te₅ (GST) has been largely investigated since it shows a good combination of electrical/optical and phase changing characteristics for memory applications [1]. One of the more discussed issue about this material is the stability of the metastable amorphous state that influences data retention in phase-change memory. It has been previously observed that crystalline GST can be rendered amorphous by the application of hydrostatic pressure [2], suggesting that, in a certain regime, pressure can influence the retention of amorphous GST. Nevertheless, it was also noticed that pressures up to 5-6 GPa are expected upon melting of a confined bit due to its expansion caused by temperature increases and amorphization.
In this study, the crystallization of as sputtered amorphous GST has been investigated at several pressures and temperatures by in situ X-ray diffraction (XRD).
GST film, 100nm thick, has been deposited by RF sputtering on 100nm Poly(methyl methacrylate) (PMMA) covering a silicon wafer. GST film has been detached from the substrate dissolving PMMA in Acetone. The free standing film has been loaded on a Diamond Anvil Cell (DAC) using Argon as pressure transmitting medium. X-ray diffraction measurements were performed with a 50W Mo source (lambda;=0.71073A), collimated with a 100um pinhole, and a CCD camera. The system was also equipped with an in situ heater controlled by a PID to set an annealing temperature ranging from 25 to 400°C.
Analyses have been performed for three samples loaded in the DAC at ambient pressure (no pressure, NP), 5GPa (medium pressure, MP) and 12GPa (high pressure, HP). Each sample was annealed up to 300°C by 10divide;20°C temperature step and in situ analyzed by XRD measurements. At each intermediate temperature a XRD spectrum was recorded for 2 minutes to monitor the phase change transition. NP and MP samples crystallize in the fcc crystalline phase at about 130°C (within the experimental error of asymp;10°C) as non free-standing GST film [1] while HP sample does not crystallize up to 300°C. HP sample was then decompressed to 0GPa and annealed to 130°C, when it crystallizes in the fcc phase. Grain size measurements, performed by Transmission Electron Microscopy micrographs, have shown that crystalline grains are smaller compared to the NP annealed sample, where the grain size is about 200 nm. MP sample was also annealed up to 350°C. At this temperature the sample crystallizes in the hcp phase.
The results suggest that pressure can reduce the number of vacancies needed by the system to crystallize in the fcc phase [3] and, therefore, data retention in phase-change memories can be improved by pressure engineering, operating at a pressure ranging from 5 to 12 GPa. The measurements are then extended to cover in more details this range.
[1] Friedrich et al., J. Appl. Phys., 87, 214 4130 (2000)
[2] Kolobov et al., Phys. Rev. Lett., 97, 035701 (2006)
[3] Krbal et al., Mater. Res. Soc. Symp. Proc., 1251, 1251-H04-10 (2010)

Phase Change Random Access Memories (PCRAM) using chalcogenide Phase Change Materials (PCM), such as GeTe and Ge2Sb2Te5, are one of the most promising technologies for next generation non-volatile memories [1-2]. PCM exhibit the ability to switch reversibly between crystalline and amorphous phases with different optical and electrical properties [3]. Resistive memories based on PCM offer fast programming, good cyclability, good data retention and multi-level capability. Their expected very high scalability is one of their most promising properties. The phase transformation between the amorphous and crystalline phases is observed in very thin films (a few nm thick) [4-6] as well as in small clusters [7-8]. In devices, the PCM is in contact with insulators and metallic electrodes (Ta, TiN, Whellip;). Hence interface effects are of a great technological importance. No interface effects have been reported so far in Ge-Sb-Te films with thickness equal or above 100 nm. The measured crystallization temperature Tx in such films is then identified with the bulk material value. By contrast, strong interface effects have been reported when the film thickness is less than 10 nm.
We have investigated the effect of Ta cladding layers on the crystallization of GeTe and Ge2Sb2Te5 (GST). The effect of Ta is a relevant issue since Ta electrodes can be used in PCRAM devices. PCM films of various thickness (10, 30 and 100 nm) encapsulated by Ta have been studied by optical reflectivity, as well as similar films encapsulated by SiO2 or TiN for comparison purpose. We observe an unexpected increase of Tx in the case of Ta cladding layers with respect to the case of SiO2 layers, even for 100 nm thick films. X-Ray diffraction experiments performed on crystallized 100 nm thick GeTe films suggest different crystallization mechanism for Ta and SiO2 cladding layers. The proposed explanation differs from those most often discussed in literature that are based either on interface effects on bulk crystal nucleation [9] or strains exerted by the cladding layers [5].
[1] A. Fantini et al., 2010 IEEE International Electron Devices Meeting (IEDM), 2010, pp. 29.21.21-29.21.24.
[2] G. W. Burr et al., “Phase change memory technology”, J. Vac. Sci. Technol. B 28, 223 (2010).
[3] S. Raoux, M. Wuttig (eds.), “Phase change materials: Science and applications”, Springer, 2009.
[4] N. Ohshima, J. Appl. Phys. 79, 8357 (1996).
[5] R. E. Simpson, M. Krbal, P. Fons, A. V. Kolobov, J. Tominaga, T. Uruga and H. Tanida, Nano Lett. 10, 414419 (2010).
[6] S. Raoux, J. L Jordan-Sweet and A. J. Kellock, J. Appl.Phys. 103, 114310 (2008).
[7] G. E. Ghezzi, R. Morel, A. Brenac, N. Boudet, M. Audier, F. Fillot, S. Maitrejean, and F. Hippert, Appl. Phys. Lett. 101, 233113 (2012).
[8] M. A. Caldwell, S. Raoux, R. Y. Wang, H.-S. P. Wong and D. J. Milliron, J. Mater. Chem. 20, 1285 (2010).
[9] M. Zacharias and P. Streitenberger, PRB 62(12), 8391 (2000).

Phase change material is at the heart of phase-change memory (PCM) technology. Today almost all phase change memory IC&’s still use Ge2Sb2Te5 (GST-225) inherited from optical disk technology, even though poor high-temperature data retention due to its low crystallization temperature (Tx~150 oC) inhibit its use in some new applications such as automotive. Furthermore, many embedded system applications embrace a pre-coding procedure where system code data are pre-programmed into the non-volatile memory (NVM) before the chips are soldered onto printed circuit board. It is also impossible to pass the rigors of withstanding the 260 C soldering process using traditional GST-225 material.
Ga-Sb alloys have been studied as possible phase change materials for PCM because they show high thermal stability of the amorphous phase, high crystallization temperatures, and fast switching. In this study, we systemically explored the thin film properties of stoichiometric GaSb material in terms of usefulness for PCM applications, including crystallization times, crystallization temperatures, crystallinity and resistivity as a function of temperature. The switching and data retention behavior of GaSb was studied as well in prototype PCM devices.
Thin films were prepared by PVD sputtering from a compound Ga50Sb50 target. Crystallization temperature and electrical contrast were measured by in-situ resistivity measurements in van der Pauw (vdP) geometry during continuous heating in nitrogen atmosphere. The stoichiometric compositions of Ga50Sb50 is characterized with very high crystallization temperature indicating an excellent amorphous stability of this material. Ga50Sb50 shows two to five orders of magnitude resistance difference depending on the final heating temperature. Crystallization times were measured using a custom-made static laser tester. It was found that the stoichiometric alloy has an unusual inverse optical contrast compared to typical phase-change materials where the crystalline phase has lower reflectance compared to the amorphous phase. Moreover, Ga50Sb50 showed a very short crystallization time of around 20 ns.
The 30 nm GaSb and 30 nm TiN top electrode thin films were deposited into PCM devices with the bottom electrode size of 40 nm. A lift-off process was used to fabricate prototype mushroom PCM test devices. Almost 2 orders of magnitude SET/RESET resistance window was successfully achieved. 60 ns fast SET speed was demonstrated in the devices which is consistent with the fast switching performance from optical laser testing. PCM devices were programmed into the SET and RESET state, respectively, and exposed to the 260 oC solder bonding temperature cycle. The RESET and SET resistance both slightly increase but were still clearly separated after heating.
These results indicate that GaSb material with the stoichiometric composition is a promising candidate for phase-change memory by combining fast crystallization speed and good thermal stability.

Phase change materials are at the core of phase change memory (PCM) technology. They possess a unique combination of physical properties that allow storing and retaining data.
Ga-Sb alloys have recently be proposed as phase change materials for PCM because of their very fast switching speeds but several other material parameters are not optimized for PCM such as the low resistances in the amorphous and crystalline phases and for some alloys the low crystallization temperature.
Here we report on a new class of phase change materials which are designed by starting from the stoichiometric Ga:Sb=50:50 material and adding various materials to it. Stoichiometric Ga:Sb=50:50 has unusual properties because its electrical properties are similar to other phase change materials, but its optical contrast and mass density change behavior is opposite to most phase change materials [1-3]. The crystalline phase has a lower reflectivity than the amorphous phase and the mass density of the crystalline phase is also lower than that of the amorphous phase, as opposed to most other phase change materials. Here we report on the properties of alloys which were deposited by mixing materials that show typical phase change behavior including Sb, Ge and Si into Ga:Sb=50:50.
Addition of Sb over a wide range produces materials with with very fast recrystallization times on the order of 15 ns, but reduced crystallization temperature with increased Sb content. In particular, the Ga:Sb=30:70 material is interesting because it shows no mass density change upon crystallization but still has high electrical contrast [4]. This makes it a great candidate for improved cyclability because void formation upon cycling is one of the main failure mechanisms of PCM and believed to be caused by mass density change upon switching. The optical contrast changes from negative (Ga:Sb=50:50) to three distinct levels of reflectivity for Ga:Sb=30:70 to positive contrast for the materials with high Sb content.
Adding Si or Ge increases the crystallization temperature but also re-crystallization times. Ge has the strongest effect leading to a crystallization temperature that can be as high as 450 °C (about 220 °C for Ga:Sb=50:50), but this is accompanied by an increase in the re-crystallization time from 20 ns of Ga:Sb=50:50 to 80 ns of materials with about 50 atomic % Ge added. The alloys with Ge and Si added show the negative optical contrast observed for Ga:Sb=50:50, indicating that with the right amount of Ge and Si added also alloys with no mass density change might be found. These experiments show that by modifying phase change material composition the properties can be tuned over a wide range and optimized for specific applications.
[1] Cheng et al., EPCOS Proc. p. 103, 2011
[2] Raoux et a., Physica Status Solidi 249 (2012) 1999
[3] Putero et al., Appl. Phys. Lett. (submitted 2013)
[4] Putero et al., Appl. Phys. Lett. Mater. (submitted 2013)

Materials design and optimization are at the core of the Phase Change Random Access Memory (PCRAM) technology and often seemingly conflicting materials requirements need to be met for a successful integration. For a prototypical phase change material such as Ge2Sb2Te5 which enabled the development of CD and DVD optical storage [1], the amorphous phase exhibits low optical reflectivity, low mass density and very high electrical resistivity. In contrast, the crystalline phase shows high optical reflectivity, high mass density and low electrical resistivity. Unfortunately, large optical contrast is also associated with a large mass density change [2] leading to void formation in PCRAM cells that represents one of the main failure mechanisms [3].
In order to optimize phase change materials for PCRAM application Ga-Sb binary system was investigated by tuning the composition between Ga:Sb=9:91 (in atomic %) and Ga:Sb=45:55. Combined in situ sheet resistance measurements and synchrotron X-ray scattering techniques performed during heating enabled demonstrating a reduced crystallization temperature while Sb content increases. Besides, the electrical contrast increases with increasing Sb content and the resistivity in both the amorphous and crystalline phase decreases.
Whatever the composition, X-ray diffraction showed an elemental segregation evidenced by the detection of two crystalline phases, the rhombohedral Sb phase and the cubic GaSb phase. For alloys close to the stoichiometric composition a stepwise crystallization was observed [4] during a temperature ramp with the GaSb phase crystallizing first and the Sb one crystallizing at higher temperature. In contrast, the reverse sequence was observed for higher Sb concentrations with Sb phase crystallizing first. In between, it was shown that the Ga:Sb=30:70 alloy undergoes a simultaneous crystallization of both phases. Finally, in situ X-ray reflectivity revealed a very interesting change as Sb concentration decreases: while the mass density increases upon crystallization in Ga:Sb=9:91 (as usually observed in most of the phase change materials), it decreases in GaSb close to stoichiometric composition. At the crossroad, the composition Ga:Sb=30:70 exhibits upon crystallization neither change in mass density nor change in film thickness. This result is of primary importance for memory applications since the lack of density change may considerably reduce the mechanical stresses as the PCM programmable volume is cycled between amorphous and crystalline states.
[1] M. Wuttig and N. Yamada, Nat. Mater. 6, 824-32 (2007).
[2] Y. Saito, Y. Sutou, and J. Koike, Appl. Phys. Lett. 102, 051910 (2013).
[3] C.-F. Chen et al., in Int. Mem. Work. Monterey, CA, IEEE (2009).
[4] M. Putero et al., Appl. Phys. Lett. submitted, (2013).

A large number of potential phase-change materials have been have produced promising results experimentally; however, only Ge2Sb2Te5-based PCMs have been significantly explored so far using ab initio molecular-dynamics (AIMD) simulations. We present the first AIMD study of the full melt/quench/anneal cycle for Ga -Sb PC materials, for compositions ranging from the near-eutectic alloy Ga16Sb84, to stoichiometric GaSb. The primary changes in local environment associated with crystallisation are demonstrated, and correlated with increasing Ga content. For Ga16Sb84, it is shown that crystallisation is characterised by the transition of Ga atoms from a tetrahedral to octahedral-like coordination. In GaSb, the opposite transition occurs for Sb atoms, from octahedral-like to tetrahedral coordination. The electronic density of states and the optical reflectivity are calculated for each phase and demonstrate good agreement with experimental results.

In order to continue with the PCM roadmap, after entering into the mass production phase since last year, new advanced technology development must be faced. Considering alternative non volatile memory technologies, PCM is still one big player that is competitive through different memory segment. Nevertheless in order to be effective through different application scenarios, new phase change materials need to be evaluated on top of the well know GST225. Embedded segment and in particular automotive market is continuously demanding for larger retention capabilities without losing other electrical performances (low programming current, good read window, fast setability, high endurancehellip;) still preserving the scalability aspect.
Aim of this talk is to describe the starting point of actual PCM product inside general memory trend, highlight main issues for future technology nodes and show the main strategy to physically and electrically screen alternative phase change materials to face new markets, in particular embedded one, maintaining focus on wall architecture.
Finally some highlight on different concept like chalcogenide superlattice will be discussed as alternative way to overcome specific PCM limitations that would enable PCM as a very low power memory.

Phase change memories (PCM) are one of the most promising technology to fulfill the requirements of non-volatile memories, both for stand-alone and embedded applications. However, some challenges still remain to enlarge the application spectrum, and one of them is the low resistance SET state drift observed under soldering reflow conditions. Recently, the material GST-Ge45%-N4% has attracted lots of attention thanks to the excellent trade-off it offers between data retention and SET-RESET performances. In this study, we present a thorough investigation of both the electrical performances and physico-chemical characteristics of germanium-rich GST devices doped with nitrogen, providing for the first time a full insight of the devices integrating this specific material.
We first proceed to several material characterizations on GST thin films with variation of Ge content and nitrogen. The resistivity measurements as a function of temperature reveal the benefits of Ge enrichment on the thermal stability of the amorphous phase while the kissinger plots of GST-Ge45%-N4% also shows a boost of more than 60% of the activation energy. In addition, DRX studies show the segregation of cubic Ge at 400°C and formation of hexagonal GST at 550°C.
The GST devices with different Ge and N contents are then electrically characterized. The R-I curves obtained show a decrease of the RESET current in GST-Ge45%-N4% devices up to 33%, and an increase of the SET resistance. The data retention measurements display a drastic increase of the high resistance RESET state stability with Ge and N doping, but also highlight the loss of stability of the low resistance SET state at high temperatures, strongly worsen by doping. The influence of pulses fall times is also investigated and reveals that a resistance lower than the SET state can be reached by applying longer fall times than standard square pulses. The drift curves of this so-called SETMIN state exhibit a smaller drift than the SET state, and has been proven to be stable under soldering reflow conditions without affecting the functionality of the devices, which can still be re-programmed in the RESET & SET state afterwards.
TEM studies focus on the GST-Ge45%-N4% material : three devices are electrically programmed in the SETMIN, SET & RESET states and prepared for TEM observations by FIB etching. HR-TEM images enable shows the amorphous volume on top of the heater resulting from the RESET operation. The crystalline nature of both the SET and SETMIN states is also confirmed. EELS cartographies are then performed, providing information on the elements distribution. Those cartographies point out the segregation of elements in the SETMIN state, showing a rejection of germanium above the heater. Further analysis of diffraction patterns gathered on multiple points of interest confirm the formation of a crystalline phase above the heater which differs from the crystalline structures identified in the rest of the PCM layer.

Phase change memory (PCM) is considered as one of the most promising candidate for future non-volatile memory (NVM) technology [1]. Among the emerging NVMs, PCM is also the first which has reached the industrial maturity [2]. PCM devices are based on the reversible crystalline-amorphous transition in a chalcogenide material such as Ge2Sb2Te5 (GST). While the crystalline (set) state is stable, the amorphous (reset) state is metastable, showing spontaneous thermally-activated crystallization. Crystallization can be obtained either by increasing the ambient temperature or by electrical pulses, where the T increase due to Joule heating leads to phase transition in the 10 ns timescale. For this reason, crystallization plays an important role not only in data retention [3] and programming speed [4], but also in reset transition [5], read disturb [6] and program disturb [7].
In this work, we propose a unified finite element model able to predict electrically induced crystallization in PCM devices. The simulated structure is a mushroom cell with confined bottom electrode contact (heater). Literature values for electrical and thermal conductivities were used. To correctly describe Joule-heating in PCM from ambient temperature to melting point (around 900 K), also the T-dependence of GST electrical and thermal conductivities were taken into account [8]. The model relies on continuity and Fourier equations for electrical and thermal transport, while a first order differential equation describes the thermal activated evolution of the crystalline fraction. The fragile nature of GST glass was modeled by considering the non-Arrhenius-activated kinetic constant driving crystallization equation [9]. To model the reset state during set transition, we have described threshold switching by the formation of a highly conducting filament within the amorphous cap [9]. The model is able to predict the measured resistance R evolution in set experiments even at extremely low currents near the hold current Ih (around 40 mu;A), which is the minimum current necessary for the self-sustaining threshold switching mechanism. In addition, the model also describes crystallization in the subthreshold regime (I < 5 mu;A), where T is below 500 K and R decay in the 1000 s time-scale. This unified approach is important to predict read disturb effect in PCM devices, which must be thoroughly understood to avoid data loss under repeated read operations [6].
[1] H.-S. P. Wong, et al., Proc. IEEE 98 2201 (2010)
[2] G. Servalli, et al., IEDM Tech. Dig., 113-116 (2009)
[3] D. Mantegazza, et al., IEDM Tech. Dig. 311 (2007)
[4] D. Loke, et al., Science 336, 1566 (2012)
[5] D. H. Kang, et al., Symp. VLSI Tech. Dig., 96 (2007)
[6] A. Pirovano, et al., IEEE TDMR, 4 (2004)
[7] M. Boniardi, et al., IEDM Tech. Dig. (2013)
[8] A. Faraclas, et al., IEEE VLSI, 78-83 (2012)
[9] N. Ciocchini, et al., Trans. Electron Dev. 60, 3767-3774 (2013)

Multi-level-cell (MLC) operations of phase change memory (PRAM) require stable intermediate resistance states to minimize data errors during the lifetime of the device. It has been investigated that the amorphous state of phase change materials (e.g., Ge2Sb2Te5) possesses an inherent meta-stability, which results in resistance drift phenomena with the elapsed time after the onset of programming. Intermediate resistance states, a mixture of amorphous and crystalline phases, also exhibit the resistance drift, whose power law exponent is approximately proportional to the modeled volume fraction of the amorphous phase. Moreover, the volume fraction itself is not the only parameter that affects the resistance drift but the microstructural morphology is another control parameter for the resistance drift.
To investigate the microstructural morphology of the phase change material during multi-level resistance switching, we have analyzed line-shapes and their temperature dependences of current-voltage characteristics. These analyses led to a model of microstructural evolution during multi-level switching, which states filamentary nucleation, coalescence of nuclei in one dimensional shapes, and cross-sectional areal growth. To confirm this modeling, we have compared resistance drift characteristics in each regime of microstructures. Finally, we suggest some switching strategies to reduce resistance drift characteristics in intermediate resistance levels, and suggest some cell geometries for a potential improvement in resistance drift characteristics. We expect that our proposed modeling and suggestions would provide a promising route to reliable MLC PRAMs.

Even though phase change materials have found their way into commercial products for memory applications, some fundamental problems linked to the relaxation and crystallization kinetics remain to be solved. The inherent relaxation of the amorphous structure for example causes an increase of resistance over time, called resistance drift. This prompts a major challenge for the implementation of multilevel storage which is necessary to achieve high storage densities. On the other hand a precise knowledge about the crystallization kinetics is necessary to develop materials which allow the balancing act of ultra fast write operations but long data retention at the same time.
Previous studies[1,2] have linked changes in the density of states with the resistance increase and found an empirical relation for the time evolution of the activation energy of conduction for a constant annealing temperature Ta: Ea(t,Ta) = E0 + κTa ln((t+t0)/t1). The resistance R = R0 exp(Ea/(kBT)) increase follows with this time evolution a power law as experimentally observed. The models that have been proposed to explain this power law dependence by structural relaxation[3,4,5] are all based on a uniform distribution of activation energies for structural relaxation. This however seems unphysical since structural relaxation processes in glasses are typically attributed to α-relaxations following a single activation.[6]
In this work, we present temperature dependent resistance measurements and demonstrate how temperature and time dependence of drift can be decoupled. By describing the relaxation process with an order parameter and a single activation energy for structural relaxation we find a differential equation that can describe experimental data with arbitrary temperature and time evolution of the resistance. Furthermore we find that the empirical relation for the activation energy of conduction is a limiting case for constant temperature of this differential equation.
In order to find potential candidates to represent the order parameter in a microscopic picture, molecular dynamics simulations have been carried. They show that there is a correlation between an increase of the optical band gap and a tendency towards a local structure that reassembles the local order in the crystal. This result suggests that the processes occurring during relaxation are related to the crystallization kinetics.
[1] Krebs et al., Journal of Non-Crystalline Solids 358, 2412 (2012)
[2] Oosthoek et al., Journal of Applied Physics 112, 084506 (2012)
[3] Karpov et al., Journal of Applied Physics 102, 124503 (2007)
[4] Ielmini et al., Applied Physics Letters 92, 193511 (2008)
[5] Fantini et al., Applied Physics Letters 102, 253505 (2013)

Phase change memory (PCM) is the most recent non-volatile memory technology in the marketplace as a flash memory alternative and has the potential to become a non-volatile DRAM replacement. PCM devices work based on electrical resistivity contrast between highly resistive amorphous and highly conductive crystalline phases of phase change materials. A small volume of a phase change material (active region) switches between amorphous and crystalline phases by suitable electrical pulses. These devices experience melting and resolidification in nanoseconds time-scale and their active region reaches ~ 1000 K during the operation. Hence, high-speed and high-temperature characterization of these materials is crucial.
In this study a set of high-speed high-resolution and long duration electrical measurements were performed on nanoscale Ge₂Sb₂Te₅ (GST) line cells in a 125 K- 673 K temperature range. Electrical resistivities of metastable amorphous (above ~400 K) and metastable fcc (face centered cubic) (above ~550 K) GST were extracted. The resistance drift in amorphous phase in 250 K - 500 K temperature range and crystallization dynamics immediately after amorphization at elevated temperatures are characterized. Details of the measurement technique and results will be presented.

Ge2Sb2Te5 is the most studied phase change material for phase-change memory applications. However, temperature dependent electrical characterization of the material is not complete. In this work, we have developed a high-temperature thin-film electrical characterization setup that allows simultaneous measurement of Seebeck coefficient and electrical resistivity up to melting temperature. Our results in repeated annealing and cool-down cycles show a consistent Seebeck and electrical resistivity behavior in temperature. The correlations between these two parameters for Ge2Sb2Te5 in a wide temperature range will be presented and possible explanations of carrier transport in GeSbTe compouds will be discussed.

Since the development of dynamic random access memory (DRAM) and FLASH, there have been huge advances in memory devices that have been pioneering a new era of information and technology in society. However, the development of new non-volatile memories is needed to overcome the limitations of the existing DRAM and NAND flash.1, 2 Phase change random access memory (PCRAM) has attracted many attention as the candidates for next generation memory3. However, several critical issues related to operational power consumption need to be resolved in order for PCRAM to become a universal memory. To solve this issue, many studies have reported with regard to the electrical resistivity and melting temperature of phase change materials4. In addition, trench-structure cells have been investigated to minimize thermal energy loss and thermal cross-talk among adjacent memory cells5,6. In this research, a new screen remote plasma-enhanced atomic vapor deposition (SPEAVD) technique was studied for depositing Sb-Te phase change materials inside trench structures with high aspect ratios. We have investigated the new deposition process to increase step coverage, which is a critical problem in the deposition on small trench sizes. In our work, The theoretical model of the screen mechanism including the concepts of a plasma sheath and sticking coefficient was considered. Plasma sheath was introduced to control the flow of ions and electrons for redistribution the energy of the plasma in the reaction chamber. Our research team confirmed the screen effect of the plasma by observing the gap-filling characteristics of the screen remote-plasma enhanced AVD and the direct remote plasma-enhanced AVD. We expect this research to provide a new deposition method that allows for the fine control of step coverage and other characteristics.
[1] A.L. Lacaita, Solid-state Electron. 50, 24 (2006)
[2] S. Hudgens and B. Johnson, MRS Bull. 29, 829 (2004)
[3] M. H. R. Lankhorst, B. W. S. M. M. Ketelaars, and R. A. M. Wolters, Nat. Mater. 4, 347 (2005)
[4] L. Wu, M. Zhu, Z. Song, S. Lv, X. Zhou, C. Peng, F. Rao, S. Song, B. Liu, S. Feng, J. Non-Cryst. Solids 358, 2409 (2012)
[5] W. Wang, D. Loke, L. Shi, R. Zhao, H. Yang, L.T. Law, L. T. Ng, K. G. Lim, Y. C. Yeo, T. C. Chong, and A. L. Lacaita, Sci. Rep-UK. 2, 360 (2012)
[6] F. Pellizzer and R. Bez, IEEE. ICICDT (2012) 6232857

Amorphous state in chalcogenides is influenced by the sample preparation and processing (e.g. sputtering, melt-quenching, ion implantation, priming, pre-annealing). In particular, amorphous samples, during annealing at temperature well below the crystallization and even at room temperature, exhibit structural relaxation and defect annealing.
The relaxation strongly influences the electrical conductivity and it has been recently shown that the mobility gap for conduction has a correlation with the incubation time for crystallization.
However, it is still not clear if these relaxation effects are determined by local bond rearrangements, defect annealing or local atomic mobility changes.
In order to better investigate the structural relaxation processes, we have studied the diffusion of Au in amorphous GeTe, comparing as deposited and pre-annealed samples (annealed for several hours at 120°C in Ar atmosfere). Such a pre-annealing treatment produces the amorphous relaxation and it has been recently reported to be very effective in reducing the nucleation rate, therefore increasing the crystallization temperature.
Amorphous GeTe 100 nm thick films were deposited at room temperature (RT) by DC sputtering on a SiO₂/Si substrate. Then, Au film, 25 nm thick, was sputtered on both as deposited and pre-annealed GeTe layers. To induce the diffusion of gold in GeTe, annealings were performed at 110°C for 3 and 7 hours and at 115°C for 1 hour. X-ray Diffraction confirmed that all the thermal-treated GeTe films retain the amorphous phase. The samples have been analyzed with 2.0 MeV Rutherford backscattering spectrometry (RBS), by cross-sectional STEM HAADF micrographs and EDX microanalysis.
Gold is found to be a very fast diffuser in as deposited amorphous GeTe, even at room temperature, with diffusion coefficient extimated to be larger than 10^-16 cm²s^-1 at 25°C and 10^-14 cm²s^-1 at 110°C.
The situation is completely different in the pre-annealed GeTe film, where a concentration of 1-2% of Au into the film was observed after maintaining the samples for few months at room temperature (asymp;25°C), but no further Au migration was observed after subsequent annealing at 110°C and 115°C.
Such a result is a clear indication that the structural reordering induced by the pre-thermal treatment inhibits the Au transport in the layer. Quite interestingly, this behavior differs from that found in amorphous silicon, where thermal treatment enhances the subsequent Au diffusion. The difference should be associated probably to a different diffusion mechanism. Indeed, in amorphous silicon the defects act as traps for Au and therefore the defect reduction enhances the diffusion. In the case of GeTe it seems that Au diffusion is mediated by a large amount of void defects, available in the as deposited film. This diffusion is almost completely suppressed after relaxation, indicating that the defects mediating the Au transport have been eliminated.

We present a model for the crystallization of Ge₂Sb₂Te₅ (GST) thin films and nanostructures by simulating nucleation and growth of crystal grains. Typically, dedicated crystallization models only examine a 2D rectangular area of material in which the material is being heated uniformly throughout the sample [1-3]. Our model is developed to simulate crystallization of GST in any structure in 2D or 3D under arbitrary heating conditions where a thermal gradient and/or a transient may be present. Simulations are performed in COMSOL Multiphysics with the integration of a MATLAB function to handle the logistics of nucleation and grain growth. This approach will allow for the modeling of dynamic crystallization during device operation, allowing for updates of the electrical and thermal conductivities of the material as it crystallizes.
This model calculates a spatial map of nucleation rate in the GST for each time-step in the simulation using temperature dependent nucleation rate [3]. Crystal nuclei are generated via a probability function based on the nucleation rate map, and are then grown into crystal grains according to temperature dependent growth rate data obtained from the literature [1,3]. Nucleation is most significant at lower temperatures (~ 500 K) and diminishes at higher temperatures where growth is most significant (~ 870 K) for GST. This model can be used to predict the crystallization of GST structures during device operation, capturing the stochastic nature of nucleation and growth processes and distribution of the grain sizes.
[1] S. Senkader and C. D. Wright. "Models for phase-change of GeSbTe in optical and electrical memory devices". Journal of applied physics 95 (2004): 504.
[2] K. B. Blyuss, et al. "Master-equation approach to the study of phase-change processes in data storage media". Physical Review E 72.1 (2005): 011607.
[3] G. W. Burr, et al. "Observation and modeling of polycrystalline grain formation in Ge₂Sb₂Te₅”. Journal of Applied Physics 111.10 (2012): 104308-104308.

The GST system has been of interest since its structural transformations are associated with memory switching properties. In phase change memories (PCM) and rewritable optical disks data are recorded by switching the material from the amorphous to the crystalline phase. The heating of the material via laser or electrical current pulses of proper intensity and duration induces the reversible phase transitions. This working principle causes electrical stress due to the high current density and thermal cycling and it produces variations in the cell electrical characteristics. The aging effects are observable in the stoichiometry changes [1] and in voids formation [2]. The stoichiometry variations, produced by the high electric field, are localized near to the anode and cathode regions of the cell. In particular from recent studies, it was observed an increase of Germanium and Antinomy concentration in proximity of the cathode and a concentration increase of Tellurium in proximity to the anode areas. There is then a quite relevant interest in the investigation of new alloys with better characteristics. In this work we investigated the morphological and electrical characteristics of two chalcogenide compounds: Ge15Sb49Te37 [3 ] and Ge14Sb35Te51. Thin amorphous films, about 50 nm thick, were deposited at room temperature on thermally grown Silicon Nitride using a rf magnetron co-sputtering from elemental targets. The concentration of Ge, Sb and Te in the alloy was varied by changing the applied rf powers. The temperature resolved reflectivity curves recorded during the annealing of the two non stoichiometric alloy and a constant heating ramp of 6 °C/min was used. The enhancement of reflectivity observed during the annealing process is correlated to the transition from the amorphous to the crystalline phase. The crystallization temperature of the two non stoichiometric alloy Ge15Sb49Te37 was 190 °C, and Ge14Sb35Te51 was 165 °C. The crystallization of two alloys, Ge15Sb49Te37 and Ge14Sb35Te51, films has been studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD) and was correlated to the optical reflectivity and to electrical sheet resistance. Comparison of the XRD patterns for GST, Ge15Sb49Te37 and Ge14Sb35Te51 thin films revealed that the crystallization behavior of the non stoichiometric films were quite different from that of GST. In the case of the Ge15Sb49Te37 film, no metastable FCC phase was observed, there was a phase transition from the amorphous phase directly to the HCP phase. Instead the Ge14Sb35Te51 composition films show at 200° C the coexistence of FCC and HCP phase and at 400° C the presence of a X-peak compatible with a rhombohedral symmetry.
REFERENCES
1 K. Kim and S. J. Ahn, IEEE Reliability Physics Symposium Proceedings, 2005, 157-162.
2 G. W. Burr et al. J. Vac. Sci. Technol. 28 (2), 223-262.
3 SANG-OUK RYU Journal of ELECTRONIC MATERIALS, Vol. 37, No. 4, 2008

Antimony Trisulfide (Sb2S3) has a band gap, which due to quantum confinement effects, is tunable from 1.5-2.2 eV. Consequently this material has been predominantly investigated for use in photovoltaics. Herein, the phase change properties of Sb2S3 films are investigated for new applications in data storage and nano photonic switches.
Sb2S3 films with thickness ranging from 20 nm to 1000 nm were fabricated by pulsed laser deposition and RF sputtering. The crystal nucleation and crystal growth phase transition kinetics in Sb2S3 thin films is studied as a function of film thickness, deposition parameters, interfacial materials and other external stimuli.
The presentation will describe a few experiments that demonstrate control the crystallization of Sb2S3 films using a variety of different external parameters.

Phase change memory (PCM), having just recently been commercialized in the past few years [1], has been attracting considerable attention from the scientific community. Investigations on thermoelectric phenomena, both within the phase change layer and at the junctions with the electrical contacts, have shown that asymmetric PCM cells have one operation polarity that is more favorable than the other for heating the active region [2, 3]. In this work, we have used our established thermoelectric PCM model to evaluate the merit of n-type and p-type contacts with a large contrast of Seebeck coefficients. The analysis is performed on Ge2Sb2Te5 mushroom cells. The material properties of the bottom electrode determine the thermoelectric heat release at the junction as well as thermal losses. Hence, the heater material has a significant impact on the device operation dynamics and dependence on the voltage polarity.
References
[1] J. Rice, Micron Announces Availability of Phase Change Memory for Mobile Devices: First PCM Solution in the World in Volume Production. 2012. Available:http://investors.micron.com/releasedetail.cfm?ReleaseID=692563.
[2] A. Faraclas, G. Bakan, N. Williams, A. Gokirmak and H. Silva, "Modeling of Thermoelectric Effects in Phase Change Memory Cells," Transactions on Electron Devices, Submitted with Revisions, 2013.
[3] A. Faraclas, G. Bakan, N. Williams, A. Gokirmak and H. Silva, "Thermoelectric Effects in Phase-change Memory Cells," Mat. Res. Soc. Spring Meeting, vol. H1.03, 2013.

Phase change materials (PCM) based memory device is considered as one of the most promising candidates for next-generation non-volatile solid-state memory. The set and reset states in this device correspond to a low resistance and a high resistance of the cell, which in-turn correspond to the crystalline and amorphous states of the phase change material, respectively. The total resistance of a phase change memory cell, however, consists of the resistance from the PCM and the interfacial contact resistance of the PCM to the electrodes. Although a large amount research has been done on characterization of PCM resistance, little attention is paid to study the contact resistance. Here in this work, the contact resistance of Ge2Sb2Te5 to titanium nitride (TiN) electrode has been characterized in both set and reset states using a nanowire structure obtained from spacer etch. This spacer etch is a novel technique and can be used as a low-cost alternative to E-beam lithography for sub-hundred nanometre nanowire fabrication. Unlike bottom-up technology, it is compatible with current CMOS process and the geometry and location of the nanowires can be precisely controlled. In this case it allows us make long structures with small contact area to separate the resistive contribution of bulk and interface.
A high-insulating silicon dioxide (SiO2) layer was first patterned by photolithography and etched to form a step with a depth of 100 nanometers. A 100 nm layer of Ge2Sb2Te5 was deposited by sputtering and anisotropically etched using an ion beam, leaving a spacer of Ge2Sb2Te5 next to the oxide structure.
Three different lengths (20 mu;m, 25 mu;m and 30 mu;m) of Ge2Sb2Te5 nanowires with same cross-section area (50 nm × 100 nm) were fabricated by space etching process. TiN electrodes with a thickness of 200 nm were then patterned on both sides of the nanowire by lift-off. The electrical characterization reveals the resistivity of the as-deposited Ge2Sb2Te5 nanowire material to be 0.6 Omega;m. The specific contact resistance between the TiN electrode and amorphous Ge2Sb2Te5 was extracted to be 3.59×10-6 Omega;m^2. Then nanowires were then thermally switched to crystalline state with resistivity of 3.37×10^(-4) Omega;m and specific contact resistance of 7.07×10-9 Omega;m^2. Even for these very long wires, the Roff/Ron ratio of 1.78x10^3 is partially determined by the contact resistance. These results indicate that for real memory cell layout, the contact resistance is the dominant factor in Ge2Sb2Te5 phase change memory devices.

The ever increasing demand for a universal memory which combines rapid read and write speeds, high storage density and non-volatility is driving the development of new memory concepts and materials. Phase change materials based random access memory (Phase Change RAM) has emerged as a leading candidate for the next generation of non-volatile memory. However, the critical issue of thermal cross-talk between adjacent cells when scaling down the cell size is yet to be solved. Growth of the entire device inside a contact hole may be more favourable as it could reduce the thermal cross-talk and simultaneously reduce the power required for the switching operations. The conventional deposition of phase change materials by sputtering does not allow selective deposition and is unable to uniformly fill small holes. Hence, alternative deposition approaches need to be investigated.
Here we report the selective deposition of phase change materials using chemical vapor deposition (CVD) with new, custom-made single source precursors. CVD is well established as a deposition process with most processes using dual or multiple sources, Being able to deposit alloys using single source reagents can be advantageous as it can offer improved stoichiometry and fewer defects, as well as often being safer and easier to handle. More importantly, through these single source reagents area selective deposition behavior by CVD was discovered. This selectivity is observed when depositing materials onto patterned substrates which contain TiN “holes” in a SiO2 film. A set of binary phase change materials, SnSe2, Ga2Te3, Bi2Te3 and Sb2Te3 have all demonstrated highly selective deposition behavior on these substrates.
The characterization of phase change materials will be presented. All as-deposited materials are crystalline and of a high purity. The properties of all materials are studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Raman spectroscopy and Hall measurements. When depositing the materials onto patterned substrates, the deposition occurs preferentially onto the exposed TiN (conductive) area inside the holes while leaving the outside SiO2 (insulator) bare. This selective deposition behavior can be observed in micro-patterned (2 µm to 100 µm) or even nano-patterned (100 nm to 1000 nm) holes. The possible reasons for this selective behavior will be discussed.

Thermal conductivities of chalcogenide thin films, Ge₂Sb₂Te₅(GST) and Ce-doped GST (Ce-GST), were measured by the 3-omaga (3omega;) method and their thermal boundary resistances (TBR) properties at the interface of chalcogenide and TiN contact layer were analyzed in terms of the plywood sample structures. Analytical results were consequently implanted in a finite-element simulation in order to analyze the thermal and electrical characteristics of phase-change memory (PCM) devices. The simulation based on a three-dimensional electric and thermal coupling model indicated that the TBR property significantly affects the temperature profile and the heating efficiency of PCM cells subjected to a pulse heating operation.
In PCM cell utilizing doped GST, i.e., Ce-GST, as the programming layer, a better thermal confinement effect was observed when the same amount of heat was generated during programming. This is ascribed to the increment of resistivity in Ce-GST which results in the decrease of programming current in PCM cells and thermal conductivity of Ce-GST layer. In the analysis regarding of TBR effect, dissipation of programming power caused by the inhibition of heat propagation from bottom electrode to programming layer of devices was observed. Simulation indicated the presence of TBR_GST/TiN = 7.6×10^minus;8 m²°K/W in PCM cell containing pristine GST leads to a 30% decrement of programming power whereas the presence of TBR_Ce-GST/TiN = 9.6×10^minus;8 m²°K/W in PCM cell containing Ce-GST leads to a 27% power decrement in comparison with the cases without the TBR effect. The influences of device scale-down and TBR properties on the programming conditions of PCM cells were also examined. It found that the smaller TiN contact width benefits the increment of cell temperature. The TBR effects caused the 16% and 19% increments of set and reset currents in PCM containing GST whereas the increment of set and reset current are 19% and 18% for PCM containing Ce-GST.