Analytical Transmission Electron Microscopy of InAs/GaAs quantum dots and GaInNAs/GaAs quantum wells

Abstract

Structure and chemical composition of InAs/GaAs quantum dots and GaInNAs/GaAs quantum wells were investigated using analytical transmission electron microscopy. These material systems are of importance for fabricating optoelectronic devices capable of operating in the 1.3 – 1.55 μm wavelength range. The samples were grown by molecular beam epitaxy. The optical properties of the quantum dots and quantum wells are largely determined by their structure and composition. Properties such as the shape, size and composition of the InAs/GaAs quantum dots and the morphology and composition of the GaInNAs/GaAs quantum wells were studied using high angle annular dark field imaging and electron energy-loss spectroscopy. In the case of the InAs/GaAs quantum dots, in particular, the effects of the GaAs spacer layer thickness in bilayer quantum dots and overgrowing the quantum dots with GaInAs instead of GaAs on the properties of the dots were investigated. The results indicated that the GaAs spacer layer can have a significant influence on the shape, size and nucleation position of the quantum dots in the second layer. In addition to this, it was demonstrated that overgrowing the quantum dots with GaInAs instead of GaAs introduces smaller changes to the structure and composition of the dots. In case of the quantum wells, the influence of the growth temperature and In composition on their morphology and composition were examined. Higher growth temperatures and In contents were shown to lead to the degradation of the 2D morphology and formation of inhomogeneities in the chemical composition of the quantum wells. When correlated with the optical performance of these structures, these results can provide important information about their structure – property relationships. Finally, the possibility of utilising electron energy-loss spectroscopy in monochromated transmission electron microscopy instruments to measure and map the band gap of semiconductor structures was studied and the results discussed.