ELECTROMODULATION STUDY OF GAAS WITH EXCESS ARSENIC

The authors have used contactless electromodulation (photoreflectance and contactless electroreflectance) to determine the electric field distributions in thin films of as-grown and annealed low temperature GaAs with excess arsenic. The molecular-beam epitaxy fabricated structures used in this study consisted of an n+ (p+) buffer layer with an undoped (U) region followed by an as-grown low-temperature (LT) layer or an annealed low-temperature (ALT) layer. The four types of sequences investigated were LTUN, LTUP, ALTUN, and ALTUP. From the Franz-Keldysh oscillations originating in the electric field in the U region it was possible to show that the Fermi level in LT-GaAs is consistent with a single deep donor at 0.4 eV below the conduction band whereas the ALTUN and ALTUP indicated a common Fermi level pinning position 0.67 eV below the conduction band. Thus, the controlling mechanism in the ALT material is indeed that of Schottky barrier controlled pinning on metallic As precipitates. These results are consistent with previously reported measurements for these materials.