Effect of annealing on Fermi level pinning of low-temperature molecular-beam epitaxial GaAs

The Fermi level position in low temperature (LT) GaAs is studied by photoreflectance (PR). The experiments show that the Fermi level in both the as-grown and the annealed LT-GaAs is firmly pinned, however, the pinning position 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 the result of a high degree of charge compensation of 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 is found to be at E c - 0.57 eV.