Near-band-gap CuPt-order-induced birefringence in Al0.48Ga0.52InP2

The order-induced birefringence in the near-band-gap spectral range (0.75 t o 2.5 eV), and its dependence on the degree of ordering eta is reported for Al0.48Ca0.52InP2. Transmission and reflection generalized variable angle s pectroscopic ellipsometry, dark-field spectroscopy, and cross-polarized ref lectance difference spectroscopy (CRDS) are used to determine precisely the room-temperature dielectric functions for polarization parallel and perpen dicular to the ordering direction of a series of spontaneously CuPt-ordered samples grown by meta;organic vapor-phase epitaxy. The CRDS technique is i ntroduced as an approach to sense extremely weak anisotropy at oblique angl es of incidence. The observed order birefringence is treated as "chemical-s tress" induced piezobirefringence. The dielectric function model for piezob irefringence in zinc-blende compounds, and selection rules for the transiti ons from the Gamma(4.5 upsilon), Gamma(6(1)upsilon), Gamma(6(2)upsilon) val ence band states to the Gamma(6c) conduction band states, allow excellent m odeling of the order birefringence in the near-band-gap spectral region. Th us, explicit treatment of the transition-matrix k dependence, as recently s uggested for ordered GaInP2 or GaInAs2, can be avoided. The transition ener gies, strengths, and broadening parameters for the three zone-center transi tions are obtained from analysis of the sample dielectric function tensor. All parameters in the quasicubic perturbation model can be fitted. We find, in excellent agreement with recent theoretical predictions, that the spin- orbit splitting parameter of 76 meV is nearly ordering independent, and tha t the ratio of the crystal-field splitting to the band-gap reduction for th e perfectly ordered alloy amounts to 0.62. The band gap of the disordered c ompound is extrapolated to 2.195 eV. [S0163-1829(99)04647-0].