THE SUBBAND STRUCTURE OF MODULATION-DOPED PSEUDOMORPHIC GAAS/(IN,GA)AS/(AL,GA)AS LAYERS

X-ray diffraction, transmission electron microscopy, Hall effect, Shubnikov-de Haas effect and photoluminescence measurements have been performed on a set of pseudomorphic Al0.25Ga0.75As/In 0.15Ga0.85As/GaAs structures with quantum-well widths from 75 to 450 AA, and undoped spacer thicknesses of 22, 100 and 200 AA. The results have been compared with a self-consistent solution of the coupled Poisson and Schrodinger equations. There is a structural difference between samples with wells of 150 AA or less and those of 300 AA or greater; this is ascribed to partial release of the elastic strain by misfit dislocations in the thicker layers. These dislocations have been observed in the TEM, and a feature consistent with their presence is also observed in the photoluminescence spectra. However, there is no significant decrease in energy gap as the well thickness is increased beyond the critical thickness predicted by the mechanical equilibrium model, indicating that the degree of strain relief increases relatively slowly with thickness. The theoretical model predicts that there is a transition from one to two occupied subbands as the well width is increased, and shows that certain optical transitions may be absent because of the small spatial overlap between the relevant electron and hole wavefunctions. These features have been confirmed experimentally, and there is good agreement between the subband energies and populations, as deduced from electrical and optical measurements, with those predicted theoretically.