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 Schluter 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 T-Ga(2-) and the triply positively charged defect complex (AS(Ga) + V-As)(3+), with As-Ga being the antisite defect of an As atom occupying a Ga site and V-As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect system V-Ga(2-)/(As-Ga + V-As)(3+) is supposed to be able to pin the GaAs Fermi level at approximately the E(v) + 0.6 eV level position, which requires that the net free energy of the V-Ga/(AS(Ga) + V-As) 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 Schluter, and found that the minimum net defect-system-energy position is located at about the E(v) + 1.2 eV level position instead of the needed E(v) + 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.