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

We have examined the photoluminescence and photoluminescence kinetics of a series of In1-xGaxP 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 Gamma(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 Gamma(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.