Polarized photomodulated reflectivity and photoluminescence studies of ordered InGaP 2 under pressure

Abstract

Spontaneous ordering of ternary alloys grown on misoriented substrates has been of recent interest. Ordering induced band gap reduction, and valence band splittings exhibiting novel polarization properties have been investigated by theory and experiment. In this paper, we discuss polarized photomodulated reflectivity (PR) and photoluminescence studies of MOCVD grown InGaP 2 epilayers lattice-matched to a GaAs substrate. These structures were grown on a (001) face with a misorientation of two degrees along 〈110〉. The high degree of ordering has enabled us to measure accurately the crystal field splitting and additional structure not reported in the PR spectra. For the electric field E parallel to [110] two features in the PR spectra are seen; for E ∥ [1̄10], however, additional features are observed. A comparison with the spectra of disordered samples of the same alloy composition has enabled a determination of the band gap reduction due to ordering. The linewidths of the PR peaks are approximately 5–10 meV which has enabled us to study them in detail as a function of hydrostatic pressure at cryogenic temperatures. The pressure dependence is slightly sublinear with the first order term of 8–9 meV/kbar for pressures well below the Γ-X crossover. Also observed is the indirect level crossing which occurs under pressure at about 40 kbar. A comparison of PR lineshapes at 1 bar is also presented at several commonly used experimental temperatures. The data indicate a substantial change in PR lineshapes, showing that interpretation of reflectivity data for these samples must be carried out carefully.