Brillouin scattering from opaque semiconducting alloys and thin films

Abstract

Brillouin scattering spectra were measured in MBE grown single crystal films of A1xGal_xAs alloys and AlxGal_xAs -GaAs supedattices on GaAs substrates, and GaAs films grown on miscut Si substrates. To detect the Brillouin shifted light against the strong elastic background present in opaque materials, a tandem multipass Fabry-Perot system was designed and constructed, providing high resolution and contrast. Scattering from both surface and bulk acoustic waves were used to derive information on the elastic and dielectric constants of AlxGal_xAs alloys. The surface acoustic wave velocities along the [100] and [110] axes were measured for a number of aluminum concentrations. In samples that were transparent to the exciting laser frequency, a leaky surface longitudinal wave was also observed. By fitting to this data, the elastic constants as a function of alloy concentration were found to be Cll = 11.88 + 0.05x, C12 = 5.38 + 1.6x, and CM = 5.95 -.8x (xlOlO dyn/cm2). Scattering from bulk acoustic waves was used to measure the complex index of refraction. The results agree very closely with previous ellipsometric data. Spectra were taken on two superlattices and compared to an alloy of the same average aluminum concentration. No differences were seen in the acoustic velocities or the index of refraction. Brillouin spectra taken of GaAs films on Si exhibited peaks due to acoustic waves trapped in the film. The dispersion curves of these modes as a function of wavevector were fitted by using the bulk elastic constants of GaAs and Si. The good fit obtained, and the narrowness of the peaks, are corroborating evidence of the high crystalline quality of these films.U of I OnlyThesi