The gallium-site donors germanium and silicon in gallium phosphide

Abstract

The node in the Bloch part of the electron wave function expected for a Ga-site donor in GaP removes the usual valley-orbit splitting and associated chemical shift. However, the T 2 ground state can still show a small spin-valley splitting into Γ 8 and Γ 7 states, as previously verified for the Sn donor. We find that the optical properties of the Ge and Si donors deviate appreciably from this “normal” behaviour. The Ge donor is anomalously deep, E D ∼ 202 meV, yet binds an exciton by ∼63 meV consistent with the Haynes rule for neutral donors in GaP. We find that this exciton possesses the large oscillator strength, f∼3.5 × 10 -3, Zeeman and piezo-optical splittings characteristic of a Γ 6, 1s(A 1) ground state, like a P rather than Ga-site donor. However, f and the exciton localization energy are consistent with expectation for E D ∼ 200 meV, as measured from the lowest set of X conduction band minima, if we assume a symmetric A 1-like wave function. A possible explanation for this unexpected result is advanced. The much shallower Si donor, E D∼82 meV, binds an exciton by only ∼ 14 meV, also consistent with the Haynes rule. By contrast, we find this Ga-site donor to be normal except that our Zeeman and piezo-optical results indicate an inverted spin-valley splitting, about 25% of that for the still shallower Sn donor. We also discuss the numerous low-lying excited states, some anomalous phonon replicas in the Ge and Si donor bound exciton spectra and the magneto-optical properties of a sharp line near 2.24 eV, attributed to the decay of excitons bound to (S) p-(Ge) p donor-acceptor associates.