FERMI LEVEL PINNING IN LOW-TEMPERATURE MOLECULAR-BEAM EPITAXIAL GAAS

The Fermi level position in low temperature (LT) GaAs has been studied by photoreflectance (PR). By etching the LT-GaAs to a different thickness, we find the Fermi level in the as-grown as well as the annealed LT-GaAs is firmly pinned. The pinning position, however, occurs at different energies: 0.47 eV below the conduction band edge for the as-grown samples and 0.65 eV below the conduction band edge for the annealed samples. The pinning in the as-grown LT-GaAs is believed to be the result of a high degree of charge compensation by deep levels, while the pinning in the annealed LT-GaAs is due to the depletion of carriers by the Schottky barrier at the metallic As precipitates. From the measured Fermi level and ionization ratio of As antisites, the (0/+) donor level of the As antisite in LT-GaAs is, for the first time, determined at E(c)-0.57 eV.