Electronic states in near-surface quantum wells

We have calculated the energies and the spatial distributions of the electr onic bound states of AlGaAs/GaAs near-surface quantum wells. We have employ ed a semi-empirical sp(3)s* tight-binding Hamiltonian including spin-orbit coupling, a Green function technique and the slab approximation. We have fo und blue shifts for the transition energies associated with the main optica l transitions when the thickness of the AlGaAs top barrier decreases. The s ign and the magnitude of these shifts agree quite well with the experimenta l data obtained recently. Both types of surface termination, cation and ani on, are considered. In the case of As termination we have found a red shift for the transition energy of the first excited optical transition when the top barrier thickness decreases. An interpretation of these results in ter ms of the spatial distributions and orbital components of the electronic bo und states is discussed. (C) 1998 Elsevier Science B.V. All rights reserved .