Above-barrier states in GaAs-AlGaAs superlattices studied by photoconductivity and photoreflectance

We report that the quasibound states at the above-barrier region in AlGaAs- GaAs superlattices can be clearly observed at room temperature by photocond uctivity as well as photoreflectance measurements. We provide concrete evid ence to confirm that free-carrier confinement at barrier layer does exist. It is also found that the barrier-width dependence of the above-barrier tra nsition energies can be described quite well by the modified Messiah's calc ulation. However, the simple calculation using the constructive interferenc e condition can only explain the transitions at lower energies, and fails w ith increasing transition energy. (C) 1999 American Institute of Physics. [ S0021-8979(99)05015-X].