Electron-Capture ("Internal") Luminescence from the Oxygen Donor in Gallium Phosphide

We report the infrared radiative capture of electrons at ionized oxygen donors in gallium phosphide. The radiative transition occurs between a shallow excited state (ionization energy 51.5±2 meV) and the deep $1s\left(A_1\right)$ ground state (ionization energy 893±2 meV) of the donor. The uniaxial stress dependence of this transition suggests that the excited state is the doublet $1s\left(E\right)\mathrm{}$. The luminescence is predominantly due to phonon-assisted transitions. Appreciable fine structure has been resolved in the vibronic sideband. Some of this structure involves phonons characteristic of pure gallium phosphide, but strong replicas associated with in-band-resonance modes of the oxygen impurity atom are also seen. The energies of the local modes are appreciably reduced when ${\mathrm{O}}^{16}$ is replaced by ${\mathrm{O}}^{18}$. In addition, the no-phonon transition energy decreases slightly by an amount anticipated from the no-phonon shift of donor-acceptor pair spectra involving the oxygen donor. The oxygen electron-capture luminescence is quenched in crystals containing ≳5×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ neutral acceptors and does not saturate as rapidly as the donor-acceptor pair luminescence. Two Auger transitions are suggested to account for these properties. Weak absorption lines due to no-phonon and phonon-assisted interbound-state excitations within the oxygen donor have been detected through measurements of the excitation spectrum of the oxygen luminescence in $n$-type crystals. A tentative assignment of the principal no-phonon absorption lines is given.