Time decays of donor-bound excitons in GaAs under pressure-induced Gamma -X crossover.

We report on photoluminescence time decays of excitons bound to donors in high-purity, n-type GaAs at He temperatures and high hydrostatic pressures (up to 80 kbar), for both direct (P\ensuremath{\lesssim}41.3 kbar) and indirect band gaps. Measured lifetimes for direct donor-exciton states (${D}_{\ensuremath{\Gamma}}^{+}$,X) are uniformly fast, being \ensuremath{\lesssim}1 nsec, and correspond to either the exciton formation or radiative decay times. For indirect donor-exciton states (${D}_{X}^{0}$,X), the observed lifetimes are much longer (20\char21{}100 nsec), increase with pressure, and reflect a combination of nonradiative Auger decay and band-structure-dependent natural radiative decay. The considerable difference observed between the donor lifetimes over the range of pressures inducing the \ensuremath{\Gamma}-X transition illustrates the importance of band structure for shallow, bound states in multivalley semiconductors.