Abstract
The modification of the optical properties of quantum well structures by post growth thermal diffusion techniques is important for the fabrication and integration of quantum well devices for optoelectronic applications. This thesis is concerned with the characterisation of as-grown and thermally interdiffused quantum well structures using room temperature photoreflectance. In particular, all the interband transitions (symmetry 'allowed' and 'forbidden') in the subband of four AlGaAs/GaAs single quantum well structures were determined using photoreflectance. The identification of the transitions was complicated by the presence of Franz-Keldysh oscillations in all photoreflectance spectra which were associated with an interface built-in electric field in the as-grown structures. The source and magnitude of the field on either side of the interface was determined after the sequential etching and photoreflectance analysis of these structures. From the spectra of the etched structures, optical interference effects were found which enabled the layer thicknesses in these structures to be determined. High temperature rapid thermal annealing of these structures resulted in interdiffusion across the well barrier interfaces of the quantum well structures which modified the subband structure, and therefore the optical properties, of the quantum well. This annealing was found to produce limited interdiffusion by correlating the relative 'blue shifts' of the interband transitions with those of a theoretical model. Good agreement with the model was obtained, which allowed temperature dependent interdiffusion co-efficients and an activation energy to be determined. The effects of oxygen ion implantation followed by annealing was also studied and found to significantly enhance the extent of the interdiffusion. In addition, interband transitions of heavy-hole and light-hole character were distinguished from polarisation studies while temporal studies resulted in the identification of unintentional impurities and the determination of photocarrier trap times in the as-grown and implanted structures. The work presented here demonstrates the versatility of photoreflectance for the room temperature characterisation of as-grown and thermally processed quantum well structures for optoelectronic devices, prior to fabrication.