| Summary: | III- V semiconductor materials are the foundation of many modern electronic and optical devices. In particular, Ill-V semiconductor quantum dots (QDs) are successfully utilized in many applications especially where high optical output (laser diodes) is required. With the aim to improve the performance further, the nanostructure and the growth mechanisms of layer and dot structures used for transistors or laser diodes have to be explored. In this work, a JEOL JEM-2010F field-emission gun transmission electron microscope is applied to investigate the crystallography and chemical composition of InAs or InP QDs at near atomic level using conventional transmission election microscopy (TEM), high resolution TEM, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDXS). The bulk of the thesis concerns the study of InAs/GaAs QD samples, which were grown by molecular beam epitaxy (MBE). An analysis of InAs/GaAs QDs deposited on top of InGaAs or GaAsSb buffer layers is presented. It was found that the GaAsSb buffer, compared to the conventional InGaAs buffer, relaxes more of the strain by introducing misfit dislocations at its lower interface and provides a smoother surface for the subsequent growth. Therefore, the QDs above GaAsSb were fully strained without any dislocations, whereas the strain was partially relaxed by dislocations in the QDs above the InGaAs buffer. The effects of thermal annealing on InAs/GaAs QDs covered with an InGaAs strain-reducing layer were also studied. The influence of the annealing temperature on the degree of interdiffusion between In and Ga atoms was analysed by investigating the change in the size of the QDs. A new technique was used to deduce the indium concentration from series of annular dark-field (ADF) STEM measurements at different collection angles. During annealing, the QDs mainly become flatter and wider. Carbon doping of part of the GaAs barriers has been experimentally found to stabilise both the intensity and the energy of optical emission from those InAs/GaAs QDs upon annealing. ADF Z-contrast STEM imaging suggests the out-diffusion of indium from the QDs is compensated by lateral segregation of indium from the InGaAs layers, which is stronger in the C- doped sample compared to the undoped. An InP QD sample grown by metalorganic vapour phase epitaxy (MOVPE) was successfully utilized to generate 200 femtosecond pulses at ~800 nm wavelength with 2 GHz repetition rates. This sample has been investigated and compared with three other InP QD samples grown with different growth temperatures (640 QC or 720°C), growth rates (0.38 or 0.76 nmlsec) or growth interrupts (0, 10 or 20 sec) using STEM and EDXS. The results propose that InP QDs can be formed by MOVPE at relatively low growth temperatures of - 640 QC and QD formation is favoured by slow growth with growth interrupts.
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