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 w ells for both electrons and holes, resulting in a direct band-gap syst em. When the GaAs domains become small, however, quantum-confinement e ffects may push the Gamma-like conduction-band state localized on GaAs above the X-like conduction-band state of the AlxGa1-x As alloy, lead ing to an indirect band-gap system. Using a pseudopotential band-struc ture method, as well as the conventional one-band effective-mass appro ximation, we investigate the nature of the direct-->indirect (Gamma--> X) transition in GaAs/AlxGa1-xAs quantum films, wires, and dots. In th e case of an isolated GaAs quantum structure embedded in AlAs, we find that the critical size for the onset of the Gamma-->X transition incr eases from similar to 31 A in a two-dimensional film through similar t o 56 Angstrom in a one-dimensional cylindrical wire to similar to 80 A ngstrom in a zero-dimensional spherical dot. The interaction between G aAs 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 spher ical GaAs quantum dots embedded in an AlxGa1-xAs alloy, we show that, as a function of the dot radius and the alloy composition, different a lignments of the band-edge states lead to different regimes of the low est-energy optical transition.