Direct X-ray Studies of Epitaxial Semiconductor Quantum Dots.

Abstract

Quantum dots have sparked a remarkable amount of interest in device development and the understanding of fundamental laws of nature. The peculiar properties of quantum dots arise from the confinement of charge carriers in three dimensions resulting in discrete energy states. Using the coherent Bragg Rod Analysis x-ray phase retrieval technique, electron density maps obtained close to the x-ray absorption edges of the constituent elements are compared to directly determine the morphology and the atomic structure and composition of the systems studied. Results on ultrathin layers of nominal GaAs on InGaAs show how an interplay between surface coarsening and chemical intermixing lead to a relaxation of strain from the nominal 3.7% tensile misfit strain. The strain is found to increase continuously from the interface, where most of the strain is relieved due to Indium incorporation into the GaAs film, to a maximum at the top of the film of 0.7%(Tdeposition(GaAs)=480 C) and 1.0%(Tdeposition(GaAs)=520 C). The structure of uncapped epitaxial InAs quantum dots grown using the Stranski-Krastanow method on GaAs(001) reveal that the dots contain significant amounts of Ga with the Ga concentration decreasing from 50% at the base of the dots to 0% at the top of the dots. A contraction of the out-of-plane lattice constant at the dot-substrate interface to about 3.5 A is observed. The out-of-plane lattice spacing in the dot region is found to be GaAs-like. It is inferred from the folded structure that the atomic planes are curved to partially relax strain with the most relaxation occurring at the top of the dots. The nominal InSb dots grown on GaAs(001) using the droplet heteroepitaxy method are found to contain very little Indium resulting in dots that have a GaAs core with an outer GaSb shell. A vertical stacking shift is observed in the dots relative to the substrate structure. The dot structure is shown to extend about 2 nm below the substrate surface. The advantage of the characterization technique developed here is that it provides a direct quantitative non-invasive determination of the three-dimensional structure and composition of epitaxial systems with atomic-scale resolution.