Isotropic dielectric functions of highly disordered Al_xGa_1-xInP (0 ≤ x ≤ 1) lattice matched to GaAs

Determination of the complex dielectric function and the critical-point energies of (Al_xGa_1-x)_0.51In_0.49P, over the full range of composition x and for photon energies E from 0.75 to 5 eV is reported from variable angle of incidence spectroscopic ellipsometry. Native-oxide effects on the (Al_xGa_1-x)_0.51In_0.49P optical functions are removed numerically. The highly disordered state of the metalorganic vapor-phase epitaxy grown samples is analyzed by transmission electron microscopy. Optical anisotropy investigations revealed that the order-induced optical birefringence is negligible throughout. The augmentation of A. D. Rakić and M. L. Majewski [J. Appl. Phys. 80, 5909 (1996)] to Adachi's critical-point model, i.e., consideration of Gaussian-like broadening function instead of Lorentzian broadening, is used for calculation of the isotropic (Al_xGa_1-x)_0.51In_0.49P dielectric function ∈. The optical functions spectra consistently match the experimental data, whereas previously reported model dielectric functions fail to reproduce the correct absorption behavior of the quaternary, especially near the fundamental band-to-band transition. The results are compared to those presented previously, and influence of spontaneous chemical ordering is discussed.