GAMMA-X MIXING IN GAAS/ALXGA1-XAS COUPLED DOUBLE QUANTUM-WELLS UNDER HYDROSTATIC-PRESSURE

We have investigated the energies of the electronic states of GaAs/AlxGa1-xAs strongly coupled double quantum wells and uncoupled multiple quantum wells as functions of hydrostatic pressure up to 35 kbar. The energies of the quantum-well states at 4 K were determined at each pressure by photoluminescence excitation spectra. The pressure coefficients of the energies of the allowed transitions between the valence-band and conduction-band quantized states of wide (200 angstrom) uncoupled wells were all equal to the pressure coefficient of the bulk GaAs band gap. For a strongly coupled double quantum well consisting of two 72-angstrom wells separated by an 18-angstrom barrier, the energies of the allowed transitions all showed a decrease in their pressure coefficients beginning near 20 kbar. These results are interpreted in terms of a drop in the conduction-band quantum-well confinement energy, due to GAMMA-X mixing, as the X valleys of the barrier materials are brought nearly equal to the energies of the confined electron states by pressure. An envelope-function-approximation model which includes GAMMA-X mixing at the interfaces is compared quantitatively with these results and found to be consistent for a certain range of the phenomenological mixing strength of the model.