The authors discuss the interband transitions of AlxGa1-xAs alloys with emphasis on the energy region from 4 to 5.5 eV. Analysis of the dielectric-function spectra obtained by spectroscopic ellipsometry at low temperatures and their numerically calculated derivatives yields critical-point energies, broadenings, amplitude, and phase for the valence-to-conduction-band transitions E'0 E'0+ Delta '0, E2(X), E2( Sigma ), and E2(P), where X and Gamma are the usual symmetry points in the Brillouin zone (BZ), and P designates a region in the Gamma XUL plane of the BZ. They discuss the experimental results and the identification of the interband transitions using the band structure and the dielectric function of GaAs and AlAs as obtained from the linear muffin-tin orbitals method and a virtual-crystal approximation for the electronic structure of the AlxGa1-xAs alloys. The observed inversion of the relative strengths of the E2(P) and E2( Sigma ) transitions with increasing Al content (x) is attributed to changes in the parallelism of the energy bands. They reassign the structure previously attributed to E2(P) to E2( Sigma ), and vice versa. A decrease in the strength of the E'0 transitions is also observed for large x, whereas the main contribution to the dielectric response is shown to arise from transitions along the Delta direction. The dependence on x of the energy position and the strength of the interband transitions is also given and discussed.
 

Physical Review B, 41 2959-65, 1990.