Phase-change properties related to anharmonicity of local structure

Abstract

Ge-Sb-Te pseudo-binary compounds are known to be phase-change materials (PCM). Most of these chalcogenide compounds are candidate for future phase-change random access memory (PCRAM) applications since they show abrupt change on crystalline-amorphous phase-change process. For the use in next-generation applications, increase of retention properties and decrease of power needed for phase-change process are required. These phase-change properties depend on various material characteristics, and thermal conductivity is one of them. In this study, to introduce an easier method for evaluating the local structural anharmonicity of phase-change materials, optical pump terahertz (THz) probe experiments were performed. By investigating the phonon behaviours in PCM by this method and comparing them with local structural information extracted from extended X-ray absorption fine structure (EXAFS) on Ge1Sb2Te4 and Ge2Sb2Te5 films, the effects of resonant bonding on lattice anharmonicity and thermal conductivity were determined. As resonant bonding in the local structure get enhanced, local distortion of the system decrease which cause the decrease in anharmonicity. The quantitatively-measured anharmonicity obtained from the optical pump THz probe experiments can be closely related to the structural and electrical properties, thus reflecting well the difference of phase-change properties between Ge1Sb2Te4 and Ge2Sb2Te5 films.