Optical anisotropy and spontaneous ordering inGa0.5In0.5P: An investigation using reflectance-difference spectroscopy

We have applied reflectance-difference spectroscopy (RDS) to the study of optical anisotropy in spontaneously ordered ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P grown by metal-organic chemical-vapor deposition. The degree of order in ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P has been associated previously with a shift of the band-gap energy Δ${\mathit{E}}_0$ and a crystal-field valence-band splitting ${\mathrm{\Delta }}_{\mathit{C}}$. 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 ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P, respectively. The main RD spectral feature in partially ordered ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P is a bulk-induced, asymmetric peak at ${\mathit{E}}_0$ with a long tail that extends well below ${\mathit{E}}_0$ and a sharp high-energy cutoff at ${\mathit{E}}_0$+${\mathrm{\Delta }}_{\mathit{C}}$. We find experimentally and theoretically that the intensity of this RD spectral feature is proportional to √(Δ${\mathit{E}}_0$) and, therefore, is linear with the order parameter. This makes RDS particularly useful for measuring the optical anisotropy of high-band-gap ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P. We also compare heterostructures of GaAs and ${\mathrm{Al}}_{0.5}$ ${\mathrm{In}}_{0.5}$P on ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P with uncoated ${\mathrm{Ga}}_{0.5}$ ${\mathrm{In}}_{0.5}$P in an effort to separate bulk-, surface-, and interface-induced RD spectral features.