Physical criteria for the direct-to-indirect gap crossover in AlxGa1−xAs alloys

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Abstract
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 Γ–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.