The complex form of donor energy levels in gallium phosphide

Abstract

The conduction band minima of GaP are now recognised to be exceptionally non-parabolic because of the existence of a 'camel's back' form. This has a most serious effect on donor bound states of binding energy 10-20 meV, just the range of the p-states available for a fit by effective mass theory (EMT) for spheroidal conduction band extrema. In the absence of a detailed EMT for such a complicated band structure the authors argue the approximate applicability of a variable gamma model. This model is applied to the present knowledge of donor excited states obtained from electronic Raman scattering, bound exciton 'two-electron' luminescence satellites and direct photoexcitation spectra. The parentage of the excited states observed in these different experimental techniques is critically reviewed. It is shown that the variable gamma model has some merit; in particular gamma is seen to increase with the binding energy of the donor state described especially for the deep 1s state, the expected trend. The first Zeeman data is presented for the donors SiGa, SP and TeP. Several of the SiGa donor p-excited states split as expected although a weak transition to a state unanticipated on the EMT splits lika a p+or- state and magnetic splittings of the p+or- levels are unexpectedly state-dependent.