A 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.