Determination of In-distribution in InGaAs quantum dots

Abstract

We investigated InAs/GaAs quantum dot (QD) layers by quantitative transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. To achieve the highest possible In-concentration in the QDs and maximum QD sizes, InAs QD layers with a nominal thickness of 2.6 ML were grown by molecular-beam epitaxy on GaAs (001) with a low deposition rate of 0.006 monolayers (ML)/sec at 500 °C. To reduce the strong In-segregation, which is observed in InAs/GaAs heterostructures during the growth of the GaAs cap layer, the InAs QD layers were capped by InGaAs with a nominal thickness of 6 nm and finally by a GaAs cap layer with 30 nm thickness. The influence of different In-concentrations x in the InxGa1−x As cap layer (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) on the structural and optical properties was studied. The Indistribution in the QD layers was determined with the composition evaluation by lattice fringe analysis technique (CELFA) [1] which is based on the chemical sensitivity of the (002) reflection for the sphalerite structure. The measured In-concentration in the QDs is artificially lowered due to the averaging effect of a TEM sample with finite thickness and the three-dimensional nature of the QDs embedded in a matrix with lower Inconcentration. A post-processing procedure was developed and applied to the raw CELFA data to determine the real In-concentration in the QDs.