Hybrid coupled quantum dot (QD) structures have a high absorption coefficient along with the minimum cumulative strain in the heterostructure compared to that in the homogeneously coupled heterostructure of only Stranski-Krastanov (SK) QDs. Here, we are introducing a theoretical analysis of the hybrid heterostructure consisting of six submonolayer (SML) stacks above SK QDs with a various capping layer combinations. Sample A (InGaAs-InGaAs) has both SK and SML capping layers of InGaAs. Similarly, Sample B (InGaAsInAlGaAs), sample C (InAlGaAs-InGaAs), and sample D (InAlGaAs-InAlGaAs) have variations in the capping composition of SK and SML dots. The barrier thickness between SML stacks and SK dots is taken to be 7.5nm, and the capping layer thickness of the SK dot is 3nm. The number of SML stacks and barrier thickness has been optimized from our previous experimental work. Hydrostatic and biaxial strains of four samples are analyzed and compared. It has been found that sample D shows the lowest magnitude of hydrostatic strain in both SML and SK dots, suggesting better carrier confinement in both QDs. Moreover, Sample D has the highest biaxial strain in the SK dot indicating the maximum splitting of the valence band which leads to a lower band gap in the sample. Thus, after optimizing all the performance parameters, we found that Sample D could be the potential candidate for optoelectronic device applications.
It is known that heterogeneously coupled dot structure consisting of sub monolayer (SML) and Stranski Krastanov quantum dots (QDs) has less cumulative strain compared to that in the homogeneously coupled SK dots structure which leads to better carrier confinement in the heterogeneously coupled structure. Here we have theoretically analysed the two heterogeneously coupled dot structures having SML series deposited over SK QDs (Samples A, B, C) and SK QDs over the SML stacks (samples D, E, F). Samples A and D have 1 nm, samples B and E have 2 nm, and samples C and F have 3 nm of InGaAs capping layer thickness over the InAs QD. The optimized structure obtained from previous experimental study consists of six SML stacks with barrier thickness of 7.5 nm between SML and SK QDs. The simulated peaks were validated with experimental data for reliability. Our motivation is to compare hydrostatic and biaxial strains and to find the better structure for long wavelength detection along with high-temperature operation conditions. The result shows that magnitude of hydrostatic strain decreases with the capping layer thickness in both systems indicating carrier confinement of samples C and F are better than the others. Therefore, they can be operated at a slightly higher temperature compared to the other samples. Furthermore, in both systems, biaxial strain in the dot has a positive correlation with the capping layer thickness, showing maximum valence band splitting in samples C and F, thus having lower band gap which makes them a better choice for longer wavelength detection.
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