PHOTOACOUSTIC SIGNALS OF N-TYPE GAAS-LAYERS GROWN BY MOLECULAR-BEAM EPITAXY ON SEMIINSULATING SUBSTRATES

Piezoelectric photoacoustic (PA) measurements of molecular-beam-epitaxial (MBE) -grown GaAs layers were carried out in the temperature range from 90 to 290 K. A broad D band with a maximum near 1.3 eV and a sharp Q peak at 1.485 eV have been observed in the 90-K spectra. They vanish in a presence of secondary light illumination. By comparing with optical-absorption spectra, it is considered that the D band is due to electron transitions involving EL2 deep defect levels in the GaAs substrate. The PA signal is considered to be enhanced by the presence of the electric field at the interface between the MBE layer and the substrate. The Q peak is attributed to electron transitions from shallow acceptors such as carbon in GaAs. Observed photoinduced changes in the spectra are explained by a reduction of the electric field in the depletion region which is induced by optical carrier generation.