Kinetics of holes optically excited from theAsGaEL2 midgap level in semi-insulating GaAs

Electron paramagnetic resonance (EPR) has been used to study the growth of the dominant acceptor EPR signals FR1 and GR2 in undoped semi-insulating GaAs under illumination as well as their spontaneous intensity changes when the light is switched off. These studies locate the FR1 level at ${\mathit{E}}_{\mathit{v}}$+270 meV and show that the GR2 level is shallower. In addition, they give detailed information about the kinetics of hole transfer from the (0/+) ${\mathrm{As}}_{\mathrm{Ga}}$ EL2 midgap level to the compensated acceptors and about thermalization of acceptor bound holes. All observations are shown to be a natural consequence of the known optical and thermal properties of the midgap level, and it is suggested that this is also true for the recently observed persistent hole photoconductivity in semi-insulating GaAs.