Direct observation of above-barrier quasibound states in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/AlAs/GaAs quantum wells

We report the direct observation of numerous above-barrier quasibound states of potential barriers, formed by a thick layer of GaAs sandwiched between ultrathin AlAs interface layers in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs strained quantum wells. Interband transitions between the conduction and the valence subband up to n=26 are clearly observed in the room-temperature photoreflectance spectra. Fairly good agreement is obtained between the peak positions and the calculated interband transitions of the quasibound states found in the GaAs barriers. © 1996 The American Physical Society.