Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.