Ider Ronneberger

Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing

Fast phase change with no preconditions Random access memory (RAM) devices that rely on phase changes are primarily limited by the speed of crystallization. Rao et al. combined theory with a simple set of selection criteria to isolate a scandium-doped antimony telluride (SST) with a subnanosecond crystallization speed (see the Perspective by Akola and Jones). They synthesized SST and constructed a RAM device with a 700-picosecond writing speed. This is an order of magnitude faster than previous phase-change memory devices and competitive with consumer dynamic access, static random access, and flash memory. Science, this issue p. 1423; see also p. 1386 Computer-aided materials design helps to identify a subnanosecond phase-change random-access memory material. Operation speed is a key challenge in phase-change random-access memory (PCRAM) technology, especially for achieving subnanosecond high-speed cache memory. Commercialized PCRAM products are limited by the tens of nanoseconds writing speed, originating from the stochastic crystal nucleation during the crystallization of amorphous germanium antimony telluride (Ge2Sb2Te5). Here, we demonstrate an alloying strategy to speed up the crystallization kinetics. The scandium antimony telluride (Sc0.2Sb2Te3) compound that we designed allows a writing speed of only 700 picoseconds without preprogramming in a large conventional PCRAM device. This ultrafast crystallization stems from the reduced stochasticity of nucleation through geometrically matched and robust scandium telluride (ScTe) chemical bonds that stabilize crystal precursors in the amorphous state. Controlling nucleation through alloy design paves the way for the development of cache-type PCRAM technology to boost the working efficiency of computing systems.

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 Ag4In3Sb67Te26 (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.

Magnetic Properties of Crystalline and Amorphous Phase-Change Materials Doped with 3d Impurities

First-principles study of the structural and magnetic properties of cubic and amorphous phase-change materials doped with 3d impurities. We find that Co- and Ni-doped Ge(2) Sb(2) Te(5) is non-magnetic, whereas Cr- and Mn-doped Ge(2) Sb(2) Te(5) is magnetic and exhibits a significant magnetic contrast between the two phases in the ferromagnetic configuration. These results are explained in terms of differences in local structure and hybridization of the impurity d-orbitals with the host states.

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, nonvolatile 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

Metadynamics Simulations of Nucleation

This chapter offers an overview of recent applications of the metadynamics method to the study of nucleation and related phenomena. In the first section, the classical nucleation theory and the metadynamics method are introduced. The second section is devoted to applications, including computational studies of the surface tension, which affects the size and energy of criticial nuclei, and investigation of crystal nucleation from the amorphous and supercooled liquid state.