Optical-absorption profile of a single modulation-doped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs heterojunction

The interband optical properties of a single modulation-doped ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs heterojunction are calculated. The calculation includes the absorption from the extended valence-band states into the first five conduction-subband states which are confined by the band offset at the heterojunction and the space-charge potential. Modulation of the absorption is calculated under the assumption that the modulation mechanism is the variation of the two-dimensional electron concentration. The predominant effect of the modulation on the absorption arises from the modulation of the Burstein-Moss edge in the bottom subband. Results of the calculation are compared to experimental electroreflectance data and are found to be in agreement.