Optical anisotropy and spontaneous ordering in Ga0.5In0.5P: An investigation using reflectance-difference spectroscopy

We have applied reflectance-difference spectroscopy (RDS) to the study of optical anisotropy in spontaneously ordered Ga0.5In0.5P grown by metal-organic chemical-vapor deposition. The degree of order in Ga0.5In0.5P has been associated previously with a shift of the band-gap energy ΔE0 and a crystal-field valence-band splitting ΔC. Theoretically, both quantities are, to first order, quadratic functions of the long-range order parameter η, which varies from 0 to 1 for disordered and perfectly ordered Ga0.5In0.5P, respectively. The main RD spectral feature in partially ordered Ga0.5In0.5P is a bulk-induced, asymmetric peak at E0 with a long tail that extends well below E0 and a sharp high-energy cutoff at E0+ΔC. We find experimentally and theoretically that the intensity of this RD spectral feature is proportional to √(ΔE0) and, therefore, is linear with the order parameter. This makes RDS particularly useful for measuring the optical anisotropy of high-band-gap Ga0.5In0.5P. We also compare heterostructures of GaAs and Al0.5In0.5P on Ga0.5In0.5P with uncoated Ga0.5In0.5P in an effort to separate bulk-, surface-, and interface-induced RD spectral features