Phase Change Materials from First Principles

Abstract

Phase change materials are widely used in optical information storage (DVD) and are the active part of the most promising non-volatile memories of new concept, the Phase Change Memory (PCM) device, which exploits the large change in conductivity between the crystalline metallic phase and the insulating amorphous phase of a chalcogenide film. Ge2Sb2Te5 (GST) is the material of choice for PCM applications due to its best performances in terms of speed and stability. In spite of the great technological importance of GST and related materials, their microscopic structures and the detailed mechanism of the phase transformation are largely unknown. In this project, we will investigate by ab-initio molecular dynamics simulations GST and related binary systems (GeTe, GeSb, Sb2Te3). Density functional based Born-Oppenheimer molecular dynamics (BO-MD) simulations will be performed with the QUICKSTEP code which employs a hybrid basis set of Gaussian functions and plane waves. This scheme is particularly suitable for the simulation of large systems. It is also the basis for the new BO-MD scheme (T. Kühne et al., Phys. Rev. Lett. 98, 066401 (2007)) that will be used in the present project. Models of the amorphous phase of GST and related binary systems will be generated by quenching from the melt. To shade light on the reversible crystal-to-amorphous transition we will simulate by metadynamics the pressure induced amorphization of GST, recently demonstrated experimentally. The availability of a microscopic model of the amorphous phases will aid clarifying the electronic properties of these materials exploited in PCM devices. The knowledge of the detailed structure of amorphous GST and related binary compounds and of the mechanism of their reversible amorphization might provide critical insight for the search of new better performing materials in this class, e.g. by doping (which will be investigate as well) or change in composition, for PCM applications.