Al L2,3 core excitons in AlxGa1-xAs studied by soft-x-ray reflection and emission.

Abstract

The Al ${\mathit{L}}_{2,3}$ soft-x-ray reflection and emission spectra of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As have been measured for x between 0.17 and 1.0. The valence-band maximum was monitored via the emission spectra using a fit based on band-structure parameters. The dominant features in the reflection spectra are produced by the ${\mathit{L}}_{2,3}$ core excitons. Since both the reflection spectra and emission spectra were obtained using the same soft-x-ray spectrometer, the energies of the excitons above the valence-band maximum are accurately obtained. The known values of the optical band gap, as a function of x, are then used to determine the core exciton binding energy. The core excitons are found to follow and lie above the conduction-band minimum by 0.3 eV as the Al composition changes. Our results are compared to various core exciton theories and to previous experiments.