Band Structure of Indium Gallium Phosphide and Indium Phosphide Under Hydrostatic Pressure.

A photoluminescence study is presented of the hydrostatic pressure and temperature dependence of the electronic band structure of InP and unordered In _{rm 1-x}Ga_ {rm x}P over the entire composition range. A structural and compositional analysis of sixteen samples (0.0 < x < 0.67) was made using X-ray diffraction, electron probe microscopy and transmission electron microscopy. Using these methods, it was possible to eliminate all samples which showed evidence of severe band-tailing or spontaneous compositional ordering. Once a series of good samples was selected, the direct ( Gamma) band gap composition dependence was determined by fitting nine samples to Cardona's quadratic expression: E_{rm g}^ Gamma = 1.423 + (1.446)x - (0.662)x(1-x). Once the low-temperature, ambient-pressure composition dependence of the direct band gap was determined, hydrostatic pressure was applied to all samples and the composition dependence of the direct (Gamma) and indirect (X), linear (alpha) and non-linear (beta) pressure coefficients were determined. It was shown, for the first time, that the linear coefficients for both gaps increased with increasing Gallium x, consistent with Phillips electronegativity rule. No significant curvature was detected for the X gap for all samples in which the X edge was observed. Furthermore, by extrapolating the pressure dependence of the X gap to ambient pressure for the series of samples, we confirm a compositional bowing of the X gap which can be described by E_{rm g}^{ rm X}(x) = 2.435 - (0.478)x + (0.400)x^2. In this way, we determine the X gap of InP to be 2.435 eV at ambient pressure. Finally, the L conduction band minimum is identified by its pressure coefficient as the lowest conduction band at a composition of approximately 67%. In addition to these optical measurements, a structural phase transition was observed for one alloy sample (x = 0.072) deposited on InP at 104 kbar. In addition to the InGaP study, extensive high -pressure low-temperature PL measurements were made on InP before, during, and after its first-order structural phase transition at 106.5 kbar. In addition to presenting the direct linear (alpha^Gamma) and non-linear (beta^Gamma) pressure coefficients, we offer additional evidence that no bandcrossing ever occurs in InP, as proposed by Kobayashi. This evidence comes in the form of (1) luminographs, (2) PL splitting, which is believed to be evidence of valance band splitting, and (3) an alloy extrapolation which predicts bandcrossing significantly above the structural phase transition.