Photoreflectance study of narrow-well strained-layer InxGa1−xAs/GaAs coupled multiple-quantum-well structures

Abstract

We have measured the photoreflectance (PR) spectra at 300 and 77 K of two strained-layer [001] ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs (x≊0.12) multiple quantum wells (MQW’s) with nominal well ($L_Z$) and barrier ($L_B$) widths of (50 Å)/(100 Å) and (30 Å)/(100 Å), respectively, as deduced from the growth conditions. In both samples we have observed a number of features in the PR spectra corresponding to miniband dispersion (coupling between wells) of both confined and unconfined (above the GaAs barrier) transitions. The coupling between wells leads to different transition energies at the mini-Brillouin-zone center (Γ) and the edge (π) along the growth direction. This is the first observation of unconfined features and miniband dispersion in this system. Even though our samples have fairly wide barriers ($L_B$≊100 Å), the coupling between wells is an important effect because of the relatively small confinement energies for x≊0.12. Using the envelope-function approach, we have calculated the various transition energies taking into account both strain and quantum-well effects, including miniband dispersion. Good agreement with experiment is found for a heavy-hole valence-band discontinuity of 0.3±0.05 and ${\mathit{L}}_{\mathit{Z}}$/${\mathit{L}}_{\mathit{B}}$=(52±3 Å)/(105±5 Å) (x=0.11±0.01) and (32±3 Å)/(95±5 Å) (x=0.12±0.01) for the two samples, respectively. The In composition and well and barrier widths are thus in good agreement with the growth conditions. Although the symmetric component of the fundamental light-hole–to–conduction-subband transition is a strong feature, the small observed amplitude of the antisymmetric component for both samples is evidence for the type-II nature of the light-hole–to–conduction-subband transitions.