| dc.description.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. | en_US |
