QUANTUM-CONFINEMENT-INDUCED GAMMA-]U-CHI TRANSITION IN GAAS/ALGAAS QUANTUM FILMS, WIRES, AND DOTS

Large GaAs domains embedded in an AlxGa1-xAs matrix act as potential wells for both electrons and holes, resulting in a direct band-gap system. When the GaAs domains become small, however, quantum-confinement effects may push the Gamma-like conduction-band state localized on GaAs above the X-like conduction-band state of the AlxGa1-x As alloy, leading to an indirect band-gap system. Using a pseudopotential band-structure method, as well as the conventional one-band effective-mass approximation, we investigate the nature of the direct-->indirect (Gamma-->X) transition in GaAs/AlxGa1-xAs quantum films, wires, and dots. In the case of an isolated GaAs quantum structure embedded in AlAs, we find that the critical size for the onset of the Gamma-->X transition increases from similar to 31 A in a two-dimensional film through similar to 56 Angstrom in a one-dimensional cylindrical wire to similar to 80 Angstrom in a zero-dimensional spherical dot. The interaction between GaAs quantum structures tends to reduce the critical size for the Gamma-->X transition. We further study the effect of the alloy composition on the Gamma-->X transition, finding that the critical size decreases when the Ga concentration of the alloy increases. In the case of spherical GaAs quantum dots embedded in an AlxGa1-xAs alloy, we show that, as a function of the dot radius and the alloy composition, different alignments of the band-edge states lead to different regimes of the lowest-energy optical transition.