Excitonic transitions in GaAs/GaxAl1−xAs quantum wells observed by photoreflectance spectroscopy: Comparison with a first-principles theory

Abstract

Excitonic transitions in GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multiple quantum wells are observed using photoreflectance spectra obtained with a novel double-monochrometer spectrometer. The first pass of the monochrometer produces the infrared probe beam while the second pass is used as a synchronous band-pass filter, resulting in a better signal-to-noise ratio than the low-pass filter conventionally used, and allowing simultaneous collection of the photoluminescence spectrum. The photoreflectance data are analyzed using a multiband effective-mass theory which includes valence-band mixing and assumes a first-derivative modulation of the built-in field as the dominant modulation mechanism. The entire photoreflectance spectrum is simultaneously fit using a first-principles theory in which only the exciton linewidth and value of the built-in electric field are not determined by the theory. Full advantage is thus taken of the signal intensity information, in contrast to comparisons currently encountered in the literature where positions of excitons are usually compared to values predicted by a finite-square-well calculation, without valence-band mixing effects. The theory is shown to have much success in explaining the observed photoreflectance signal from a 200-Å multiple quantum well at 77 K, including the qualitative line shape for any given exciton.