Optical transitions involving unconfined energy states in ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs multiple quantum wells

Optical transitions with energies higher than that of the GaAs band gap in highly strained ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs multiple–quantum-well structures have been observed in photoreflectance spectra. In some samples as many as seven such structures were present. We identify them as transitions between the unconfined electron states and the confined heavy-hole states. For energies below the GaAs signal, intense transitions corresponding to such unconfined electron subbands were also observed. The intensity of the transitions involving unconfined electron subbands decreases with increasing well width, but is weakly dependent on the mole fraction x. The transmission coefficients are calculated in order to locate the positions of the unconfined electron subband energies. Good agreement is obtained between the experimental data and the theoretical calculation.