Photoluminescence of quasi-direct transitions in disordered In1-xGaxP/graded GaP alloys

We have examined the photoluminescence and photoluminescence kinetics of a series of In_1-xGa_xP alloys in an effort: 1) to elucidate the electronic structure of the conduction band versus alloy composition, especially near the direct-indirect crossover; 2) to determine precisely the composition of the direct-indirect crossover, and its temperature dependence; and 3) to understand the nonradiative decay mechanism and its temperature dependence. We find that the fundamental bandgap is only determined by the Γ_1c and X_1c states in samples with Ga-compositions ranging from 0.58 to 0.75, and that the 2-K direct-indirect crossover from Γ_1c, to X_1c occurs at x=0.69 and is not strongly temperature-dependent. Further, we find, in agreement with our spectroscopic ellipsometry measurements at room temperature, that the mixing near crossover is rather complicated and leads to the previous observation of quasi-direct transitions. Our combined photoluminescence and spectroscopic ellipsometry measurements have therefore clearly resolved the controversy regarding the bandgap crossover. This has strong implications for the realization of InGaP-based efficient light-emitting devices with emission at higher energies