Abstract
It has recently been shown that a metal-insulator transition due to disorder occurs in the crystalline state of the GeSb2Te4 phase-change compound. The transition is triggered by the ordering of the vacancies upon thermal annealing. In this work, we investigate the localization properties of the electronic states in selected crystalline (GeTe)x-(Sb2Te3)y compounds with varying GeTe content by large-scale density functional theory simulations. In our models, we also include excess vacancies, which are needed to account for the large carrier concentrations determined experimentally. We show that the models containing a high concentration of stoichiometric vacancies possess states at the Fermi energy localized inside vacancy clusters, as occurs for GeSb2Te4. On the other hand, the GeTe-rich models display metallic behavior, which stems from two facts: a) the tail of localized states shrinks due to the low probability of having sizable vacancy clusters, b) the excess vacancies shift the Fermi energy to the region of extended states. Hence, a stoichiometry-controlled metal-insulator transition occurs. In addition, we show that the localization properties obtained by scalar-relativistic calculations with gradient-corrected functionals are unaffected by the inclusion of spin-orbit coupling or the use of hybrid functionals.
Highlights
IntroductionFirst-principle studies of disordered systems based on Density Functional Theory (DFT) are very rare, since large system sizes are required to unequivocally distinguish between localized states and extended ones and obtain statistically meaningful results
EF lies in the region of localized states and the system is insulating
We show that none of these models exhibits localized states near the Fermi energy
Summary
First-principle studies of disordered systems based on Density Functional Theory (DFT) are very rare, since large system sizes are required to unequivocally distinguish between localized states and extended ones and obtain statistically meaningful results. In the strongly-disordered cubic phase obtained at low annealing temperature, the electronic states at EF are exponentially localized inside vacancy clusters, i.e. regions where the vacancy concentration is much larger than the average value. The formation of a disordered rocksalt phase at low annealing temperature is a property displayed by many GST compounds[23,24,25]. This phase exhibits a large carrier (hole) concentration of the order of 1020 cm−3. It is believed that this property is due to the presence of non-stoichiometric, excess Ge and/or Sb vacancies[26,27,28]
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