Optical and structural properties of GaAsBi and InGaBiAs nanostructured semiconductors grown by Molecular Beam Epitaxy

This thesis investigates the optical and structural properties of GaAs1-xBix/GaAs SQWs and InGaBiAs QDs grown on the conventional (100) and non-conventional (311)B GaAs substrates by Molecular Beam Epitaxy (MBE). The effect of the substrate orientation and the Bi content (x) on the optical properties of GaAs1-xBix/GaAs SQWs grown by MBE has been investigated. The Photoluminescence (PL) spectra show an enhancement of PL intensity and reduction of the PL linewidth as Bi content increases from 1% to 3% due to a reduction of the density of nonradiative defects. The optical quality of GaAs1-xBix QWs grown on (311)B is found to be inferior to that of (100) QWs. This could be explained by enhanced clustering of Bi in (311)B and carrier localization at Bi pairs and clusters. The temperature dependence of the PL spectra provided two different types of defects. The first type is related to lattice disorder and composition fluctuations and the other related to Bi clusters. The effects of furnace annealing (FA) and rapid thermal annealing (RTA) on the structural and optical properties of GaAs1-xBix/GaAs SQWs grown on (001) and (311)B substrates MBE were also investigated. From High-Resolution X-ray Diffraction (HR-XRD) measurements, the Bi composition was found to be similar for both as-grown samples. Transmission Electron Microscopy (TEM) images show the existence of Bi-segregation in the samples; however, the segregation is higher in the QWs grown on the (001) GaAs substrates after annealing. It was found that RTA is a more effective method to reduce possible defects and improve the PL intensity than FA. The optimum annealing temperature (TA) was determined to be 300°C for 2 mins for all studied samples regardless of the substrate orientations. The structural and optical properties of a set of GaAs1-xBix/GaAs SQWs having the same nominal Bi content (samples grown under similar Bi flux) and different nominal QW thicknesses (12-24 nm) were studied. The HR-XRD results showed that different Bi concentrations for different QWs which is due to the non-uniformity of Bi incorporation. The temperature dependence of the PL peak energy showed a non-monotonous S-shape behaviour at high temperature which is a signature and a characteristic of a system where carrier localization is present. PL and HR-XRD findings indicate good optical and structural properties for the sample with 5.1% Bi and 9 nm QW thickness as compared to the other samples. Self-assembled In0.5Ga0.5As QDs grown at various growth temperatures (TG = 510, 482, 450 oC) with and without exposure to bismuth flux have been investigated. The PL intensity was found to improve by a factor of ~ 2.1 when the QDs were exposed to a Bi flux which acts as a surfactant. In addition, a reduction of the PL peak energy was observed when the In0.5Ga0.5As QDs were grown by using Bi as a surfactant, which suggests that Bi affects the size of the QDs. Therefore, Bi surfactant can be used to control the morphology of QDs and enhance their optical properties. A temperature of 510oC was found to be optimal in terms of optical efficiency. The use of Bi as a surfactant could have a great potential for applications in photonic and optoelectronic devices in the future