Computational study of the structural, Electronic, optical and thermal properties of Hexagonal and cubic Germanium-antimony-telluride

Abstract

The electronic, optical and thermal properties of hexagonal and cubic phases of Ge2Sb2Te5 (GST) have been calculated using density functional theory (DFT) as implemented in the QUANTUM ESPRESSO computer package. GST is successfully applied in optical memory such as rewritable CDs and is a promising candidate for non-volatile electronic memory. In optical storage, the reflectivity contrast can be optimized towards the ultraviolet spectral range, thereby increasing data storage capacity and doping with nitrogen is one way to achieve this aim. In this study, the reflectivity of pure and nitrogen-doped GST have been computed from the dielectric function, which is obtainable from DFT calculations. We show that nitrogen doped GST has a higher reflectivity contrast in the blue and ultraviolet spectral range and this reflectivity contrast increases with rising nitrogen content for 10-20 at. % doping levels. Because DFT underestimates band gaps of semiconductors and insulators, since it is a ground-state theory and does not take into account many-body effects, the Liouville-Lanczos approach to time-dependent density functional theory (TDDFT) has been employed giving optical band gaps of about 0.48 eV and 0.66 eV for hexagonal and cubic phases, respectively. This is in reasonably good agreement with optical measurements which suggest a value of 0.5 eV for both phases. Analyzing the thermal properties of GST can be useful in validating the structural models such as those used in this study. Thermal properties have been calculated using the quasi-harmonic approximation. The specific heat of both phases is found to exceed the classical Dulong-Petit limit at high temperatures in agreement with experiment. The heat capacity curves are found to exhibit the same trend as experimental curves. The entropy of the hexagonal phase is found to vanish at 0 K, in agreement with experiment.