Absorption, refractive index, and birefringence of AlAs-GaAs monolayers

Abstract

The band structure of artificial periodic crystals grown by molecular‐beam epitaxy and consisting of sequential AlAs and GaAs monolayers differs slightly from Al_xGa_1−xAs random alloys having a comparable average Al content. The measured direct band gaps are up to 110 meV higher than the alloys, and the indirect gaps up to 70 meV higher. The measured dispersion of the refractive index agrees best with the theoretical dispersion curve of an alloy composition having a higher Al content than in the monolayer. This is consistent with the higher band gap of the monolayer. The alloy and monolayer exhibit a birefringence resulting from strain‐induced anisotropy of the lower and upper cutoffs of the complex part of the dielectric constant. The strain results from the differential thermal contraction of the layers and the GaAs substrate on cooling from the growth temperature to room temperature. The birefringence of the monolayer sample contains an additional contribution resulting from the periodic structure.