Effects of Si Doping on the Electronic Structure and Conductivity of Pure and Off-Stoichiometric Ge₂Sb₂Te₅ Cystrals: First-Principles Investigation

We calculate the electronic structure and electrical conductivity of pure and Si-doped off-stoichiometric Ge₂Sb₂Te₅ cubic crystals, using the relativistic Korringa-Kohn-Rostoker method based on the multiple-scattering theory. These crystals are described by a rock-salt unit cell, in which the chemical disorder is taken into account through the coherent potential approximation. The accuracy of the results is verified by comparing, for several compositions, the density of electronic states calculated with this method and with a method that uses Kohn-Sham wave functions in big supercells. The calculated Bloch spectral function shows the dispersion of the electron states and its modification with the atomic disorder. We show the chemical-composition dependence of the electrical conductivity, and discuss it in terms of the concentration of carriers and of the modification of their scattering by atomic disorder. These results can be used to model Ge-Sb-Te phase-change-material samples, the microstructure of which consists of grains with different compositions, each grain being described by a different value of the conductivity.