Electronic Structure and Conductivity of Off-stoichiometric and Si-doped Ge 2 Sb 2 Te 5 Crystals from Multiple-Scattering Theory
Résumé
The transport properties of the phase-change material Ge2Sb2Te5 can be tuned by controlling its atomic structure and concentration of charge carriers. Moving away from the "225" stoichiometry or doping with atoms of different chemical species are major methods to reach this aim. The transport properties of these doped samples are challenging to study experimentally, since their crystalline phase generally possesses a complicated microstructure, consisting of grains with different compositions. They are also challenging to investigate by first-principles methods based on the calculation of Kohn-Sham wave functions, as larger supercells are needed to describe the unavoidable chemical disorder among Ge, Sb, dopant atoms, and vacancies. In this work, we perform first-principles calculations of the electronic structure and electrical conductivity of off-stoichiometric or Si-doped cubic Ge2Sb2Te5 crystals, using the spin polarized relativistic Korringa-Kohn-Rostoker (KKR) method based on the multiple-scattering theory. The doped crystals have all been described with a rock-salt unit cell, in which the chemical disorder is taken into account through the coherent potential approximation (CPA). The accuracy of the results obtained using this method is verified by comparing, for several crystal compositions, the density of electronic states calculated with this method and with a method that uses Kohn-Sham wave functions and big supercells. We calculated the Bloch spectral function, which shows the dispersion of the electron states and its modification with the deviation from the 225 stoichiometry, silicon doping, and chemical disorder. We describe the composition dependence of the electrical conductivity, which we discuss in terms of the concentration of charge carriers and of the modification of their scattering by the intrinsic chemical disorder in the crystal. These results can be used to model real samples, the microstructure of which consists of grains with different concentrations of Ge, Sb, or Si atoms, each grain being described by a conductivity that depends on its composition.
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