Formation, Structure, and Properties of InAs/GaAs Quantum Dots.

Abstract

Semiconductor quantum dots (QDs) have shown significant promise for a wide range of optoelectronic devices, including solar cells, photodetectors, and lasers. Typically, QDs are fabricated by the misfit-driven Stranski-Krastanov (SK) mode, which results in elliptical-shaped QDs with sizes and densities limited by the lattice misfit. Recently, the nucleation of metal droplets and their conversion to QDs, often termed droplet epitaxy (DE), has attracted much attention because it allows QD fabrication without lattice misfit. However, the mechanisms for the conversion of In islands to InAs QDs and the origins of misfit dislocation (MD) displacement are still unclear. Thus, further examination on the formation mechanism and microstructures of DE QDs is essential. Here, we report on the structure and properties of InAs/GaAs QDs formed by DE and SK approaches. These results suggest that DE is promising for tuning QD sizes and densities, as well as tailoring carrier confinement in the vicinity of QDs. Using a finite-element Schrodinger-Poisson model that considers experimentally measured QD and wetting layer (WL) shapes, sizes, and spacings, we examined the influence of InAs/GaAs SK QDs on the solar cell external quantum efficiency (EQE). A comparison between the computed and measured EQEs reveals a broadening of sub-bandgap EQE induced by QD size distribution, and a weak EQE contribution from the WLs. To further enhance the control of QD size, density, and microstructure, we investigate alternative QD fabrication approaches via annealing In islands under an As flux. We revealed the influence of As surface coverage on the QD formation mechanisms. On c(4x4) GaAs surfaces, QD formation follows DE. For the As capped surfaces, QDs nucleate by solid phase epitaxy during annealing of an amorphous film. Furthermore, we revealed the origin of interlayer formation and MD displacement in the vicinity of InAs/GaAs QDs. For SK QDs, MDs nucleate at the QD/buffer interface. For DE QDs with low In exposure, an InGaAs interlayer at the QD/buffer interface results in MD vertical displacement. For DE QDs with high In exposure, the formation of an InAs interlayer at the island/buffer interface leads to MDs at the QD/buffer interface.