Indirect band gaps in quantum dots made from direct-gap bulk materials

Abstract

The conditions under which the band gaps of free standing and embedded semiconductor quantum dots are direct or indirect are discussed. Semiconductor quantum dots are classified into three categories; (i) free standing dots, (ii) dots embedded in a direct gap matrix, and (iii) dots embedded in an indirect gap matrix. For each category, qualitative predictions are first discussed, followed by the results of both recent experiments and state of the art pseudopotential calculations. We show that: • Free standing dots of InP, InAs, and CdSe will remain direct for all sizes, while dots made of GaAs and InSb will turn indirect below a critical size. • Dots embedded within a direct gap matrix material will either stay direct (InAs/GaAs at zero pressure) or will become indirect at a critical size (InSb/ InP). • Dots embedded within an indirect gap matrix material will exhibit a transition to indirect gap for sufficiently small dots (GaAs/AlAs and InP/ GaP quantum well) or will be always indirect (InP/GaP dots, InAs/GaAs above 43 kbar pressure and GeSi/Si dots). In indirect nanostructures, charge separation can occur with electrons and holes localized on different materials (flat InP/GaP quantum well) or with electrons and holes localized in different layers of the same material (concentric cylindrical GaAs/AlAs layers).

Strain-induced fundamental optical transition in (In,Ga)As/GaP quantum dots

“…The reduced quantum confinement effect on the X Z -like state as compared to the X XY -like state may be attributed to the smoother confinement potential profile in the GaP matrix and the stronger X-effective mass in GaP than in GaAs. 30 Such strain-induced interface localized states have also been predicted in InP/GaP QD [37][38][39] and SiGe/Si QD. [40][41][42] Nevertheless, it differs from a true type-II band alignment such as GaSb/GaAs QD 43 in which the electron wave function is delocalized in the whole GaAs barrier when the Coulomb interaction is not taken into account.…”

Time‐resolved and single dot spectroscopy of type II InP/GaInP quantum dots

“…Our assumption is that the InP/GaInP quantum dots show a type II band alignment. Several theoretical studies [6][7][8] have calculated that in this material system the electron is confined in the quantum dot and the hole sits outside. With this model we also can explain the behaviour of our two dot types.…”

Relaxation and recombination in InAs quantum dots

“…As pointed out by Williamson et al [54] for the InP/GaP quantum dot system the local strain field in the GaP caused by the InP quantum dots can lead to localized states associated with the GaP X minima. The depth of the localisation is influenced by the size of the quantum dots and also the degree of lattice mismatch between the quantum dots and the barrier X X Γ materials.…”