MODULATION EFFECTS NEAR THE GAAS ABSORPTION-EDGE

We show that the large features often found below the band gap in modulated spectra of GaAs samples are linked not only to impurities but also to back-surface reflection effects. Their proximity to the band edge depends on the geometry of the cavity in which the multiple reflections occur, the impurity species, and the temperature. Impurity contributions to the energy-dependent absorption coefficient become manifest in a multiple-reflection-enhanced reflectance edge. Externally applied perturbations modulate this reflectance edge, thereby producing first-derivative-like spectral features. At 5 K the extrema of the impurity-related features roughly correspond to the energies associated with donor and acceptor states. However, at room temperature they disagree significantly and therefore do not specify an impurity species with certainty.