PHYSICAL CRITERIA FOR THE DIRECT-TO-INDIRECT GAP CROSSOVER IN ALXGA1-XAS ALLOYS

AlxGa1-xAs alloys undergo a direct-to-indirect gap transition when the Al concentration x is increased, or when hydrostatic pressure P is applied to a direct-gap sample of fixed composition. It is usually assumed that either hydrostatic pressure or alloying produce a GAMMA-X conduction band crossing responsible for the change in the nature of the gap. A critical discussion regarding this widely adopted criterion for the direct-to-indirect gap transition is presented. Alloying and pressure effects in the electronic properties of AlxGa1-xAs are determined through the small crystal approach with a 64-site basic cluster. Finite-size effects are discussed. The photoluminescence emission intensity is calculated as a function of x and P, and physically reliable criteria for the transition from the direct-to-indirect gap regimes based on these results are proposed. A critical concentration consistent with experimental results associated to this change of regime is obtained from our data.