Shallow donors in semi-insulating GaAs and their role in the excitation of the 0.64-eV photoluminescence

Abstract

Detailed studies of the shallow donor levels in semi-insulating (si) GaAs have been carried out with use of photoluminescence excitation (PLE) spectroscopy of the donor-acceptor pair (DAP) band as well as far-infrared magnetoabsorption. These studies show that in si GaAs there exist shallow donors with anomalously large binding energies in a continuous distribution between the effective-mass value (5.8 meV) and \ensuremath{\approxeq}12 meV. This effect most probably results from the Coulomb interaction between the electron bound to the donor and charged impurities which arise naturally as a result of the compensation process in si GaAs. In addition, PLE spectroscopy has been carried out for the deep 0.64-eV photoluminescence band, which has been associated with the presence of midgap centers. The similarity of the PLE spectra for the deep band and the DAP band is interpreted as evidence that the 0.64-eV band is excited by populating shallow donors. By combining these data with measurements of the PL time decay for this band, we conclude that the excitation mechanism for the 0.64-eV PL is the nonradiative capture by a deep center of an electron bound to a neutral donor, and that the most probable recombination mechanism is the radiative capture by the deep center of a hole trapped at a shallow acceptor.