On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects
Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, an amphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancy VGa2- and the triply positively charged defect complex (ASGa+VAs)3+, with AsGa being the antisite defect of an As atom occupying a Ga site and VAs being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect system VGa2-/(AsGa+VAs)3+ is supposed to be able to pin the GaAs Fermi level at approximately the Ev+0.6 eV level position, which requires that the net free energy of the VGa/(AsGa+VAs) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about the Ev+1.2 eV level position instead of the needed Ev+0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future. © 1995 Springer-Verlag.
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