Above-barrier states in InxGa1-xAs/GaAs multiple quantum wells with a thin cap layer

The effective-mass approximation in a transfer-matrix formalism is used to investigate above-barrier states in strained InxGa1-xAs/GaAs multiple quantum wells (MQW's). A condition for finding above-barrier states, semiconfined by a finite cap layer, is formulated. In the derivation of the transfer matrices, boundary conditions that include the discontinuity of the lattice constant in the growth direction are used. In a series of InxGa1-xAs/GaAs MQW's (4-6 periods, x similar or equal to 0.1, with the topmost barrier used as a cap) the energies of the light-hole and heavy-hole excitonic peaks, involving both above-barrier and confined states, are observed by photoluminescence excitation spectroscopy (PLE) and polarized PLE. The experimental values;are in very good agreement with the calculated ones, for transitions involving above-barrier as well as confined states, supporting the validity of our calculations.