Characterisation of point defects in SiC by microscopic optical spectroscopy

Defects have a dramatic effect on the properties of semiconductors. In SiC, intrinsic defects can be introduced during growth or device-processing steps such as implantation. In this work electron irradiation has been used for the controlled generation of defects in SiC. The irradiated material has been annealed and subsequent low temperature photoluminescence (LTPL) measurements have been performed. A key element in this work has been the ability to perform both the irradiation and characterisation on a microscopic scale. These results have allowed a variety of new optical centres to be discovered, and have also significantly enhanced the pool of knowledge about other defect centres. Utilising low voltage irradiations has enabled the electron irradiation voltage displacement thresholds for Carbon and Silicon displacements to be investigated. In 4H-SiC the electron irradiation voltage displacement thresholds were found to be 88kV for C displacement and 225kV for Si displacement. A large number of previously unreported luminescence features have been measured in 4H, 6H and 15R-SiC material. The criteria used for comparison are the voltage threshold, annealing characteristics, spatial distribution with respect to the irradiated region, and the characteristics of associated local modes and vibronic structures. Compelling evidence has been found to support the assignment of centres in 4H and 6H-SiC to a C-C dumbbell split interstitial defect. Two high energy local modes at 133meV and 180meV are associated with these centres. In 13C enriched 6H-SiC material the 180meV local mode splits into three components whilst the 133meV local mode splits into two components. This splitting is interpreted as being caused by isotopic substitutions between the components of the C-C dumbbell. The high energy local mode corresponds to the bonding between the two constituent atoms of the dumbbell whilst the low energy local mode is associated with the bonding between either a C13 or 12C atom with the nearest Si atom. A new centre that is common to 4H, 6H and 15R-SiC has been investigated. The dominant component of this centre is measured at 673nm, 764nm and 766nm in 4H, 6H and 15R-SiC respectively and are collectively known as the B1 centre. The centres exhibit an intermediate irradiation voltage threshold value that is approximately mid-way between the threshold associated with the generation of isolated C and Si vacancies. It is likely that a C-Si dumbbell split interstitial complex would be generated at these voltages and this is consistent with the two-fold splitting exhibited by all components of the B1 centre in 13C 6H-SiC material. Equivalent features to the alphabet lines in 4H-SiC and the neutral silicon vacancy (VSi0) related features in 4H and 6H-SiC have been investigated in 4H, 6H and 15R-SiC polytypes. The 'equivalent' alphabet lines exhibit virtually identical characteristics in all polytypes that are consistent with the proposed isoelectronic natures of the defect centres. Considerable spectral variety was found in the energy range associated with the VSi0 luminescence with the intermittent appearance of additional features attributed to regions with a high density of stacking faults. It has been proposed that remnant electric Fields, perturbations to the Fermi level and intrinsic stress are responsible for the spatial variations in the energy and intensity exhibited by some centres. (author)