Michael C. Shaughnessy, Josh D. Sugar, Norm C. Bartelt, Jonathan A. Zimmerman
In this paper, we use high-temperature experiments coupled with \textit{ab-initio} modeling of diffusion to study Au in Bi2Te3. This combined methodology enables us to elucidate key aspects of the mechanisms of diffusion and the solubility behavior of the Au-Bi$_2$Te$_3$ system. Using electron microscopy and energy dispersive spectroscopy, we observe fast motion of Au into Bi$_2$Te$_3$ and concentrations above the previously reported solubility limit, sufficient to deplete many microns of Au on the Bi$_2$Te$_3$ surface after 165 hours at 350$^\circ$C. We calculate defect formation energies and diffusion barriers within DFT to develop an atomistic understanding of fluxes of Au into Bi$_2$Te$_3$. We identify an interstitial mechanism responsible for the observed rapid anisotropic diffusion and provide an estimate of the heat of transport, Q*, associated with temperature-gradient driven diffusion. The calculated diffusivity along the Te-Te double layer plane is in good agreement with experiment, while the much slower cross-plane diffusivity requires a non-interstitial mechanism. The low formation energies of substitutional defects suggest these types of defects may be active during diffusion and may explain the high observed concentrations.
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http://arxiv.org/abs/1305.0528
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