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 E0 and a crystal-field valence-band splitting C. Theoretically, both quantities are, to first order, quadratic functions of the long-range order parameter , 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+C. We find experimentally and theoretically that the intensity of this RD spectral feature is proportional to (E0) 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.