Electronic structure of (GaAs)1-xGe2: The relative importance of short-range and long-range order
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Xanthakis, JP
(EN)
We calculate the electronic structure of (GaAs)1-xGe2x, for which there are contradicting theories of the short-range structure. To investigate the latter we use the cluster-Bethe-lattice method and the configuration-averaging technique of Gomez-Santos and Verges which can take into account diagonal and off-diagonal disorder as well as disorder in the short-range order parameters. Our results are in good agreement with XPS spectra, and band gap, EXAFS and x-ray diffraction measurements. They point to models in which (i) no As-As or Ga-Ga bands exist, (ii) there is perfect coordination around each atom provided the atoms are not near grain boundaries and (iii) the fraction of antisites is very low for x < 0.2 and then quickly goes to 0.5 at the critical value x(c) at which the zincblende-to-diamond transition occurs. Furthermore we have strong evidence that short-range order is the dominant factor in the formation of not only the band gap, as reported previously, but also the valence band density of states, at least for x < x(c). By comparison with other calculations and experiments we show that the VCA breaks down for this class of alloys, and even the single-site CPA may be inadequate in some cases.
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