Optical Properties of the Donor Tin in Gallium Phosphide

The donor Sn substitutes for Ga in GaP. Electrons bound to Ga-site donors have a pseudo-$p$-state representation, in contrast to P-site donors which have simple $s$-like ground states. We report the first optical study of such a substitutional donor in a semiconductor. Analysis of the weak no-phonon structure in donor-acceptor-pair luminescence spectra, characteristic of Ga-site donors, indicates that ${\left(E_D\right)}_{\mathrm{Sn}}=65.\mathrm{}5\pm 1$ meV. Central-cell enhancement of $E_D$ is expected to be small for donors with $p$-like ground states, and Sn is by far the shallowest donor so far identified in GaP. We have seen absorption and luminescence of excitons bound to the neutral Sn donor. The exciton localization energy is ∼ 10 meV, lower than that observed for excitons bound to the deeper P-site donors S, Se, and Te, in rough accord with Haynes's rule. Magneto-optical studies of the principal (lowest-energy) no-phonon component in the Sn exciton luminescence spectrum show that the initial state contains a single unpaired hole, and the final state contains a single unpaired electron with a small negative $g$ factor. The ground state of the Sn donor can be split into a $p_{\frac{3}{2}}$ state (${\Gamma }_8$) and a $p_{\frac{1}{2}}$ state (${\Gamma }_7$) by spin-orbit interaction. The sign and magnitude of the electron $g$ factor indicates that the $p_{\frac{1}{2}}$ state lies lowest for the Sn donor. The $g$ factor for the "orbital" part of the wave function of the Sn donor $p$ state is essentially zero, as expected for a "valley-orbit" $p$ state. This identification is confirmed by the observation that the ground state of the Sn donor is not split by uniaxial stress for any crystallographic orientation of the stress. The magnitude of the ${\Gamma }_7\to {\Gamma }_8$ splitting of the Sn donor ground state is 2.1 ± 0.1 meV, determined roughly from an analysis of nonlinear shifts in the Zeeman pattern of the Sn exciton, and more precisely from the energies of weak "two-electron" transitions in which the donor is left in the ${\Gamma }_8$ state, observed in the zero-field spectra. The decay time from the lowest state of the Sn exciton is 90 nsec, some 2700 times shorter than that determined from the experimental absorption cross section of this transition. This large discrepancy is attributed to the predominance of nonradiative (Auger) recombinations of the Sn exciton.