The effect of encapsulation on the morphology and chemical composition of InAs/GaAs quantum dots grown by molecular beam epitaxy

Abstract

In order to fabricate an effective device structure based on InAs quantum dots (QDs), the QD layers must be encapsulated within a matrix that has a wider band gap. This encapsulation is usually achieved by the overgrowth of GaAs. Coherent strained InAs/GaAs islands, which were previously formed on the (0 0 1) GaAs substrate surface, can then be buried in the semiconductor matrix to form QDs. The capping process has a significant effect on the structure and properties of QDs. In the present study, a range of transmission electron microscopy (TEM) techniques is used to characterise both the microstructure and the chemistry of the QDs with different capping layer thickness. High-resolution in-plane X-ray scattering is also applied to characterise the QD structures. Uncapped InAs/GaAs QDs are found to be multi-faceted. The formation of {1 1 3} facets and {1 1 1} growth steps is clearly visible along the [1 1 ̄ 0] zone axis in high-resolution TEM images. Following the capping process, the QD density is observed to decrease, presumably due to lateral spreading and coalescence. As a result of mass transport and interdiffusion during growth of the first few monolayers of the GaAs capping layers, the in content in the QDs is reduced, with the final In content in a fully buried QD determined to be 65–67%.