Luminescence in Intrinsic and Annealed Electron-Irradiated GaAs:Cd

The lasing transition in boat-grown cadmium-doped GaAs is similar to that previously found in GaAs:Zn. In cadmium-doped GaAs, the three excited states of this transition ($C^{\prime }$, $B^{\prime }$, and $A^{\prime }$) lie at energies of 1.484, 1.486, and 1.507 eV, respectively. In addition, there is a broad-band emission peak centered at 1.37 eV. All of these peaks move to lower energies with decreasing excitation intensity. 1.5-MeV electron irradiation causes exponential decreases in the broad-band and near-band-edge intensities with increasing electron fluence for fluences greater than ${10}^{16}$ $\frac{e}{{\mathrm{cm}}^2}$. Isochronal annealing of the irradiated material in the temperature range 50-190°C causes increases in the emission centered about 1.37 eV, but the emission is modified and appears as two peaks (Cd 0, and Cd 1) at 1.369 and 1.337 eV. The temperature dependence of the intensity of the peaks indicate that Cd 1 is a phonon replica of Cd 0. The development of the structure is interpreted as occurring when Ga vacancies migrate and form localized modes that weaken the coupling of the Cd 0 defect to the lattice. Diminution of intensity and loss of structure are observed in the temperature range 190-240°C and result from a decrease in the number of centers giving rise to localized modes. The killer center which degrades the luminesence is believed to be the As vacancy which anneals out at about 240°C. The energy shift of the lasing transition and of the broad-band emission are interpreted to indicate that both bands are the emission envelopes of unresolved donor-acceptor pair transitions. The lasing transition arises from the recombination of holes localized at neutral Cd acceptors, with electrons localized at donors, possibly Si; whereas the broad-band emission is due to pair annihilation involving the same donor and Cd in a different charge state.