PHOTOLUMINESCENCE OF QUASI-DIRECT TRANSITIONS IN DISORDERED IN1-XGAXPGRADED GAP ALLOYS

We have examined the photoluminescence and photoluminescence kinetics of a series of In1-xGaxP alloys in an effort: 1) to elucidate the elec tronic structure of the conduction band versus alloy composition, espe cially near the direct-indirect crossover; 2) to determine precisely t he composition of the direct-indirect crossover, and its temperature d ependence; and 3) to understand the nonradiative decay mechanism and i ts temperature dependence. We find that the fundamental bandgap is onl y determined by the Gamma(1c) and X-1c states in samples with Ga-compo sitions ranging from 0.58 to 0.75, and that the 2-K direct-indirect cr ossover from Gamma(1c) to X-1c occurs at x = 0.69 and is not strongly temperature-dependent. Further, we find, in agreement with our spectro scopic ellipsometry measurements at room temperature, that the mixing near crossover is rather complicated and leads to the previous observa tion of quasi-direct transitions, Our combined photoluminescence and s pectroscopic ellipsometry measurements have therefore clearly resolved the controversy regarding the bandgap crossover, This has strong impl ications for the realization of InGaP-based efficient light-emitting d evices with emission at higher energies.