Monatomic phase change memory

Phase change memory has been developed into a mature technology capable of storing information in a fast and non-volatile way, with potential for neuromorphic computing applications. However, its future impact in electronics depends crucially on how the materials at the core of this technology adapt to the requirements arising from continued scaling towards higher device densities. A common strategy to finetune the properties of phase change memory materials, reaching reasonable thermal stability in optical data storage, relies on mixing precise amounts of different dopants, resulting often in quaternary or even more complicated compounds. Here we show how the simplest material imaginable, a single element (in this case, antimony), can become a valid alternative when confined in extremely small volumes. This compositional simplification eliminates problems related to unwanted deviations from the optimized stoichiometry in the switching volume, which become increasingly pressing when devices are aggressively miniaturized. Removing compositional optimization issues may allow one to capitalize on nanosize effects in information storage

Computational Study of Crystallization Kinetics of Phase Change Materials

Thanks to their outstanding physical properties, phase-change materials (PCM) are considered as one of the most promising active switching materials for future, non-volatile memory applications. As in modern Flash memories, a phase-change random access memory (PCRAM) is able to retain information without external power supply, but at the same time grants considerably faster read and write speeds than flash memories. In such a device, information is stored by utilizing the enormous contrast in the electrical resistance between the amorphous and the crystalline phase of PCM's. Moreover fast and reversible transitions between these phases at elevated temperatures and very high stability at room temperature at the same time make them ideally suited for data storage applications in general, e.g. in optical data storage such as rewritable CD and DVD. The speed for the write operation is mainly controlled by the crystallization rate from the amorphous to the crystalline phase. The experimental investigation of this aspect is however extremely challenging because of the ultra short crystallization times of PCM's at high temperatures. Computer simulations provide an alternative route to study the crystallization in these materials and become increasingly popular in recent years. In this thesis, both the crystallization kinetics as well as the structural properties of prototypical PCM's are studied by a combination of ab initio molecular dynamics (AIMD), based on density functional theory (DFT) and metadynamics (MTD). MTD is a method to enhance the sampling of molecular dynamics (MD) simulations. After a brief review on PCM's and providing the theoretical framework of this thesis as well as the computational details, the crystallization kinetics of Ag4In3Sb67Te26 (AIST) and Ge2Sb2Te5 (GST) is discussed. The former alloy is a growth-dominated PCM and was recently studied experimentally. Employing large models of AIST with planar interfaces, which correspond to the crystalline rim surrounding an amorphous bit, we show that our AIMD simulations of crystallization yield results, which agree very well with the experimental results, and reveal the corresponding microscopic growth kinetics. A quenching rate effect on the dynamic properties of our AIST models is found at low temperatures. The other compound (GST) is a nucleation-dominated material. We considered the crystallization of amorphous GST both from a crystalline rim and from crystalline nuclei, and evaluated the corresponding crystal growth velocity at high temperatures around ~600 K. The crystalline nuclei are generated from separate MTD simulations, demonstrating the ability of MTD to accelerate the occurrence of rare events. A general approach to compute the growth velocity from MD simulations is introduced. In addition, the finite size effects are elucidated using a very large model containing 900 atoms. In the next part, we discuss the results of the MTD simulations of nucleation in amorphous GeTe, using both small (64 atoms) and larger models (216 and 512 atoms). A remarkably small value of the free energy barrier for nucleation is found in the smaller models, which explains the typical fast crystallization observed in such small models (as a consequence of non-negligible finite size effects). On the other hand, the free energy surface (FES) obtained from simulations of the larger models, did not converge completely, albeit the resulting energy scale is in a reasonable range. Thorough analyses of the results are provided, in particular using a new reweighting method. Important insights into methodological aspects of MTD is gained. In the final part, we discuss the simulations of the liquid, the supercooled liquid and the amorphous phase of Sb as well as its doped alloys Ge15Sb85 and In15Sb85. The main structural properties of the liquid and the amorphous phases are determined. A Peierls like distortion is found in amorphous Sb and in amorphous Ge15Sb85, which is more pronounced for the latter one. Additionally, a distinct regime of fast crystallization is identified in the supercooled liquid phase of Sb in the temperature range between ~400 K and ~550 K. Outside this regime of fast crystallization, the crystallization does not occur or occurs more slowly. MTD simulations at ~600 K and ~710 K reveal a substantial reduction of the free energy barrier for nucleation with decreasing temperatures and give further evidence for the instability due to nucleation in this regime

Crystal growth of Ge2Sb2Te5 at high temperatures

Phase-change materials (PCMs) have important applications in optical and electronic storage devices. Ge2Sb2Te5 (GST) is a prototypical phase-change material (PCM) employed in state-of-the-art storage-class memories. In this work, we investigate crystallization of GST at temperatures 600-800 K by ab initio molecular dynamics. We consider large models containing 900 atoms, which enable us to investigate finite-size effects by comparison with smaller models. We use the metadynamics method to accelerate the formation of a large nucleus and then study the growth of the nucleus by unbiased simulations. The calculated crystal growth speed and its temperature-dependent behavior are in line with recent experimental work

Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films

Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few-layer crystalline films of monochalcogenides display robust in-plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first-principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te-based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te-based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few-layers structures offers a powerful tool to tune materials properties for applications in information technology

French, M., Mar. 15, 1978, Part 1. David Taylor interviewing Marcus French regarding boatbuilding.

David Taylor interviewing Marcus French regarding boatbuilding, Winterton, Trinity Bay, Newfoundland. Interview takes place on March 15, 1978. French discusses his family, his work as a fisher, his work in the Overseas Forestry Unit for the British war effort, different types of boats, the parts of a boat, and the methods and materials used in boatbuilding

How fragility makes phase-change data storage robust: insights from ab initio simulations

Phase-change materials are technologically important due to their manifold applications in data storage. Here we report on ab initio molecular dynamics simulations of crystallization of the phase change material Ag(4)In(3)Sb(67)Te(26) (AIST). We show that, at high temperature, the observed crystal growth mechanisms and crystallization speed are in good agreement with experimental data. We provide an in-depth understanding of the crystallization mechanisms at the atomic level. At temperatures below 550 K, the computed growth velocities are much higher than those obtained from time-resolved reflectivity measurements, due to large deviations in the diffusion coefficients. As a consequence of the high fragility of AIST, experimental diffusivities display a dramatic increase in activation energies and prefactors at temperatures below 550 K. This property is essential to ensure fast crystallization at high temperature and a stable amorphous state at low temperature. On the other hand, no such change in the temperature dependence of the diffusivity is observed in our simulations, down to 450 K. We also attribute this different behavior to the fragility of the system, in combination with the very fast quenching times employed in the simulations

Formation of resonant bonding during growth of ultrathin GeTe films

A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) || Si(111): GeTe [-110] || Si[-110] epitaxial relationship rigorously. Hence, in striking contrast to conventional lattice-matched epitaxial systems, a drastic improvement in atomic order is observed above a critical film thickness. Raman spectra show a remarkable change of vibrational modes above the critical thickness that is attributed to a change in the nature of the bonds: While ordinary covalent bonding is found in ultrathin films, resonant bonding can prevail only once a critical thickness is reached. This scenario is further supported by density functional theory calculations showing that ultrathin films do not utilize resonant bonding in contrast to the bulk phase. These findings are important not only for ultrathin films of phase-change materials such as GeTe and GeSbTe, which are employed in phase-change memories, but also for thermoelectrics and topological insulators such as Bi2Te3 and Sb2Te3, where resonant bonding might also have a significant role