Computer Simulation of the Phase-change Cycle of GST-225
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1072-G01-02
Computer Simulation of the Phase-change Cycle of GST-225 Stephen Elliott, and Jozsef Hegedus Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom ABSTRACT We have simulated, by ab initio molecular dynamics (MD), the entire phase-change (PC) cycle in Ge-Sb-Te (GST) materials. Rapid quenching from the melt results in the amorphous state but, on slower cooling, the models crystallize into the metastable rocksalt crystalline phase. We have also found that the amorphous models crystallize on being thermally annealed. Significant numbers of connected near-planar 4-membered (square) rings exist in the liquid state and are quenched into the amorphous solid. These crystal seeds are responsible for the rapid, homogeneously-nucleated crystallization behaviour observed in GST PC materials. INTRODUCTION Storage of information is essential in this computer age. In the case of non-volatile memories, rewriteable optical data-storage technology has been established for many years in CD and DVD formats, relying on the rapid, and reversible, laser-induced PC transformation between amorphous and crystalline states of GST materials, and the optical-reflectivity contrast between these phases, to store bits of information. In addition, a new electronic non-volatile memory technology is currently being developed to replace existing FLASH memories, in which voltage-pulse-induced PC transformations between amorphous and crystalline phases of GST materials are produced, and information discrimination is based on the electrical-conductivity contrast between the two phases. Aspects of the PC transformation processes in GST materials that are not fully understood include the reasons for the PC reversibility, the apparently homogeneous crystalnucleation behaviour and the very rapid (ns) crystallization exhibited by GST materials. In order to answer these questions, we have performed ab initio MD simulations to try to simulate the crystallization behaviour, and to obtain structural information about the quenched amorphous state. SIMULATION METHOD Constant-volume MD simulations were carried out using the Vienna Ab-initio Simulations Package (VASP) [1]. We used the projector-augmented wave method and the PBE exchange-correlation functional [2,3]. The plane-wave cut-off was set at 175 or 250 eV. The outer s and p electrons were treated as valence electrons. Simulations were made of a variety of GST compositions, along the GeTe-Sb2Te3 tie-line, i.e. the compositions Ge2Sb2Te5 (225) and GeSb2Te4 (124), and also GeTe and Ge10Sb56. For the 225 simulations, for example, the cubic simulation boxes (with periodic boundary conditions) contained 63 or 72 atoms, at a density of
6.11 g/cm3, chosen to be slightly smaller than the experimental room-temperature value of the density of the metastable rocksalt structure (6.2 g/cm3 [4]). Two quenching protocols were used to create solid models from the liquid state: either a linear cooling ramp or a series of isothermal anneals, each followed by discontin
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