Optical Study of Strained and Relaxed Epitaxial InxGa1-xAs on GaAs

Photoreflectance (PR) at different temperatures and spectroellipsometry (SE) at room temperature were used to study, in a systematic and complementary way, the optical response of a series of strained and relaxed InxGa1−xAs (x<0.15) epilayers. All the samples were grown by molecular‐beam epitaxy on GaAs, both with and without a GaAs cap layer, which in the thinnest samples determines a single‐quantum‐well configuration. The effects of the strain on the optical structures E0, E1, and E1+Δ1 observed in the 1.2–3.3 eV photon‐energy range were analyzed by fitting standard critical points (CP) line shapes to the PR and SE spectra. The CP experimental energies versus x were compared with the relations obtained in the framework of the elastic strain theory and, in the quantum‐well structures, of the envelope‐function scheme. The excellent agreement between experiment and theory allowed us to determine, independently and only by optical techniques, the strain ϵ and the composition x values, which compare well with those measured by x‐ray diffraction. Additional information concerning the critical thickness for the pseudomorphic growth and the residual strain in quasirelaxed layers was achieved.