It was recalled that, by using the theoretically calculated point defect total-energy values of Baraff and Schlüter, an amphoteric defect model had been proposed by Walukiewicz in order to explain a large number of experimental results. The suggested amphoteric defect system consisted of 2 point defect species that were capable of transforming into each other. These were the doubly negatively charged Ga vacancy, VGa2-, and the triply positively charged defect complex, (AsGa + VAs)3+. Here, AsGa was the antisite defect of an As atom which occupied a Ga site and VAs was an As vacancy. When they were present in sufficiently high concentrations, the amphoteric defect system, VGa2-/(AsGa + VAs)3+, was supposed to be able to pin the GaAs Fermi level at about the Ev + 0.6eV level. This required that the net free energy of the VGa/(AsGa + VAs) defect system be a minimum at the same Fermi-level position. A quantitative study was made, of the net energy of this defect system, which was in accord with the individual point-defect total-energy results of Baraff and Schlüter. It was found that the minimum net defect-system energy was located at about Ev + 1.2eV instead of at Ev + 0.6eV. This cast doubt upon the validity of the amphoteric defect model. A simple criterion was proposed for the determination of the Fermi-level pinning position, in the deeper part of the band-gap, as being due to 2 oppositely charged point defect species.

C.H.Chen, T.Y.Tan: Applied Physics A, 1995, 61[4], 397-405