Quantum-size-induced electronic transitions in quantum dots: Indirect band-gap GaAs

We discuss the physical origin of the previously predicted quantum-size-induced electronic transitions in spherical GaAs quantum dots. By using atomistic pseudopotential calculations for freestanding GaAs dots and for GaAs dots embedded in an AlGaAs matrix, we are able to distinguish two types of direct/indirect transitions: (i) in freestanding GaAs dots, the conduction-band minimum changes from Γ-like to X-like as the radius of the dot is reduced below 1.6 nm, leading to a direct/indirect transition in reciprocal space. (ii) In GaAs dots embedded in AlAs, the conduction-band minimum changes from dot localized to barrier localized as the radius of the dot is reduced below 4.2 nm, corresponding to a direct-to-indirect transition in real space.