Time decays of donor-bound excitons in GaAs under pressure-inducedΓ-Xcrossover

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≲41.3 kbar) and indirect band gaps. Measured lifetimes for direct donor-exciton states ($D_{\Gamma }^{+}$,X) are uniformly fast, being ≲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 (20–100 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 Γ-X transition illustrates the importance of band structure for shallow, bound states in multivalley semiconductors.