Optical study of (Al_xGa_1-x)_0.5In _0.5P/GaAs semiconductor alloys by spectroscopic ellipsometry

The critical points of (Al_xGa_1-x) _0.5In_0.5P semiconductor alloys grown by metal-organic chemical vapor deposition and lattice matched to GaAs have been measured at room temperature in the 1.5-6.2 eV spectral region using spectroscopic ellipsometry. We have performed standard lineshape analysis of the second derivatives of pseudodielectric function data to obtain the standard critical point parameters (peak position, broadening parameter, amplitude, and excitonic phase angle) as a function of x. The properties of the E₁ and the E ₁+Δ₁ spectral structures have been observed for the first time. The anomalously small amplitude of the contribution of E ₁+Δ₁ spectral feature relative to that of E ₁ is attributed to the k-linear interaction between the Λ_4,5^v and Λ₆^v valence bands, which is large in III-P alloys due to the very small spin-orbit splitting. The bowing parameter of Δ₁ is determined and discussed. The critical point parameters smoothly interpolate between those of the two ternary endpoints, Ga_0.5In_0.5P and Al _0.5In_0.5P. These parameters are discussed and compared with those of Al_xGa_1-xAs, (Al_xGa _1-x)_0.5In_0.5As/InP, and In_xGa _1-xAs_yP_1-y /InP.