Pair Spectra Involving Si Donors in GaP

We discuss the preparation of Si-C- and Si-Zn-doped GaP crystals and present an interpretation of their emission spectra in the light of a recent theory of donor states in GaP. We find that the no-phonon peak is weak or absent in Si-Zn and Si-C pair spectra, and that the multiple peaks observed arise from transitions induced predominantly by phonons from the point $X$ of the Brillouin zone. This interpretation differs from that of a recent paper by Dean et al. The energy of the weak no-phonon transition agrees with that calculated from the binding energies and a resonable Coulomb pair energy, and the phonon energies equal the known values for the TA, LA, and TO phonons at $X$. We show that the displacement in energy between a pair peak and one of its phonon replicas may under certain conditions differ from the energy of the phonon emitted.