T. H. Lee

Phase-change random-access memory (PCRAM) is one of the leading candidates for next-generation data-storage devices, but the trade-off between crystallization (writing) speed and amorphous-phase stability (data retention) presents a key challenge. We control the crystallization kinetics of a phase-change material by applying a constant low voltage via prestructural ordering (incubation) effects. A crystallization speed of 500 picoseconds was achieved, as well as high-speed reversible switching using 500-picosecond pulses. Ab initio molecular dynamics simulations reveal the phase-change kinetics in PCRAM devices and the structural origin of the incubation-assisted increase in crystallization speed. This paves the way for achieving a broadly applicable memory device, capable of nonvolatile operations beyond gigahertz data-transfer rates.

By virtue of the ultrashort phase-transition time of phase-change memory materials, e.g., Ge(2)Sb(2)Te(5), we successfully reproduce the early stages of crystallization in such a material using ab initio molecular-dynamics simulations. A stochastic distribution in the crystallization onset time is found, as generally assumed in classical nucleation theory. The critical crystal nucleus is estimated to comprise 5-10 (Ge,Sb)(4)Te(4) cubes. Simulated growth rates of crystalline clusters in amorphous Ge(2)Sb(2)Te(5) are consistent with extrapolated experimental measurements. The formation of ordered planar structures in the amorphous phase plays a critical role in lowering the interfacial energy between crystalline clusters and the amorphous phase, which explains why Ge-Sb-Te materials exhibit ultrafast crystallization.

Crystallization in amorphous materials requires significant atomic diffusion for structural ordering to occur. Vacancies can play a critical role during the crystallization process, although little is known for phase-change materials. Here, using ab initio molecular-dynamics simulations, we have observed how vacancies evolve and influence the crystallization process in Ge${}_{2}$Sb${}_{2}$Te${}_{5}$ (GST) materials. It was found that vacant sites have mostly Te atoms as neighbors. The diffusion of Ge/Sb atoms in the amorphous phase to vacancies at the crystal-glass interface helps in the formation of stable cubic clusters that potentially grow as nuclei for crystallization. Such selective vacancy diffusion with its particular redistribution facilitates the crystal-nucleation process, thereby significantly contributing to the fast speed of crystallization in this material.

Abstract The concept of hypervalency emerged as a notion for chemical bonding in molecules to explain the atomic coordination in hypervalent molecules that violates the electron-octet rule. Despite its significance, however, hypervalency in condensed phases, such as amorphous solids, remains largely unexplored. Using ab initio molecular-dynamics simulations, we report here the underlying principles of hypervalency in amorphous chalcogenide materials, in terms of the behaviour of hypervalent structural units, and its implicit relationship with material properties. The origin of a material-dependent tendency towards hypervalency is made evident with the multi-centre hyperbonding model, from which its relationship to abnormally large Born effective charges is also unambiguously revealed. The hyperbonding model is here extended to include interactions with cation s 2 lone pairs (LPs); such deep-lying LPs can also play a significant role in determining the properties of these chalcogenide materials. The role of hypervalency constitutes an indispensable and important part of chemical interactions in amorphous and crystalline chalcogenide solids.

The spatial distribution of ${\text{Nd}}^{3+}$ ions and ${\text{GaS}}_{4}$ tetrahedral units in Nd-doped Ge-As-Ga-S glasses has been studied by laser spectroscopy and ab initio molecular dynamics (MD) simulations. A sharp increase in ${\text{Nd}}^{3+}$ fluorescence intensities and lifetimes was observed with increasing Ga content, and attributed to the formation of tightly bound ${\text{Nd}}^{3+}$ clusters in Ga-free glasses and the subsequent dissolution of such clusters upon Ga doping. A large modification in ${\text{Nd}}^{3+}$ sites was also identified from low-temperature site-selective excitation spectra, suggesting preferential spatial correlations between ${\text{Nd}}^{3+}$ and ${\text{GaS}}_{4}$ tetrahedra even at low Ga-doping levels. MD simulations of these materials in the liquid state showed a tendency for Ga cluster formation as well as spatial correlations between Nd and Ga atoms consistent with the experimental results. On the basis of this result, a comprehensive structural model for Nd- and Ga-doped sulfide glasses is proposed.