A Computational Study of Oxygen Contamination in Sb 2 Te 3
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A Computational Study of Oxygen Contamination in Sb2Te3 John E. Boyd, Arthur Edwards, and Andrew C. Pineda Space Vehicles Directorate, Air Force Research Lab, AFRL/VSSE, 3550 Aberdeen Ave SE, Kirtland AFB, New Mexico, 87117 ABSTRACT We present first principles electronic structure calculations of oxygen substitutional defects in the Sb2Te3 layered crystalline system and a model of amorphous Sb2Te3 using density functional theory (DFT). Our calculated formation energies for oxygen substitutional defects at Sb sites are above 2 eV, so most of our results are on the Sb2Te3-xOx [x = .0074 - .20] system, where one of two inequivalent Te sites are instead occupied by a single oxygen atom with formation energies between -1.2 eV and .2 eV. Defect formation energies for the system show a preference for oxygen atoms on the Te1 site at low concentrations that switches to the Te2 site at high concentrations at approximately 6 atomic percent. In agreement with experiment, we find that oxygen does widen the band gap, even at relatively low concentrations. INTRODUCTION Sb2Te3 has been studied extensively in recent years1-6, both for its thermoelectric properties and for its relationship to other chalcogenide glasses. It is the other end of a stoichiometric tie line connecting GeTe and the current state of the art phase change electronics material, Ge2Sb2Te5. Thus a quantitative understanding of Sb2Te3 can lead to important insights for phase change materials. Several experimental groups have investigated the transport and structural properties of Sb2Te3, as well as the doping of Sb2Te3 with various materials to create diluted magnetic semiconductors.1-3 Other defects have also been studied, but of particular interest for this work has been the work of J.K. Olson et al.. that examines the role of oxygen in amorphous GeTe, Sb2Te3, and Ge2Sb2Te5.4 Along with finding that oxygen consistently comprises a few At. % of these materials, they found that a faster growth rate led to decreased oxygen concentration, and that the optical gap decreased with faster growth rates. It is not clear whether the opening of the band gap is the effect of oxygen concentration or other factors affected by growth rate, such as an onset or decline in crystallization. To our knowledge, nobody has performed calculations studying oxygen in crystalline or amorphous Sb2Te3, nor any of the aforementioned chalcogenide glasses for that matter. However, other crystalline calculations have been performed that at least benchmark the performance of the level of theory we used. Recently, Thonhauser et al. have studied stress induced defects5 (anti-site, exchanges, and vacancies) in crystalline Sb2Te3, as well as the impact of pressure on transport phenomena6. They found that, while the spin orbit interaction had a significant effect on the band gap, formation energies were virtually unchanged. They also concluded that a 135 atom unit cell was able to represent a single antisite defect to within numerical error. METHOD AND THEORETICAL BACKGROUND Crysta
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