Cottom, JP; (2017) The characterisation of performance limiting defects in 4H-SiC devices using density functional theory. Doctoral thesis , UCL (University College London).

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Abstract

This thesis is focused on the atomistic modelling of defects both within silicon carbide (SiC) and at the interface between SiC and silicon dioxide (SiO2). These defects are discussed and compared to available experimental data to allow for the identification of performance limiting defects in the current and next generation of 4H-SiC metal oxide semiconductor (MOS) devices. The results presented throughout this thesis are calculated in the 4H-SiC polytype, which is the most relevant polytype for electronic applications, and as such the devices of interest. These simple bulk models were developed and adapted to produce model 4H-SiC / SiO2 interface systems. All the models used were tested and calibrated against the available experimental and theoretical data to ensure they were representative of the device regions of interest. Through the application of density functional theory (DFT) and the models outlined above, defects, both intrinsic and extrinsic, were calculated, allowing for a comparison with electrically detected magnetic resonance (EDMR) measurements to be made. EDMR and to a lesser extent EPR provide a powerful tool for defect identification, providing a magnetic, symmetric and atomistic picture of a defect from a single technique. This allowed the dominant recombination defect in N-implanted pn-junctions to be identified as the neutral NCVSi, giving atomistic meaning to the unidentified signal from N-implanted devices. The effect of the anneal on forming this and other defects is described, allowing the accumulation and observation of the NCVSi to be understood. Without this mechanistic understanding, it was impossible to explain how the NCVSi is able to persist into the fully processed devices when thermodynamically more stable defects did not. Before these calculations could be conducted, an interface model that was appropriate for the devices of interest was required. This was achieved through the comparison of electron energy loss spectroscopy (EELS) and DFT calculations, allowing an interface that is abrupt, but stepped, to be described. Using this model, the combination of EDMR and DFT calculations was then applied to the problem of defect identification at the SiC / SiO2 interface. This approach allowed the PbC (and dual-PbC) to be identified as the dominant interface defect in the current generation of devices. These results provide an atomistic meaning to the experimentally observed signals, allowing the defects linked to the suboptimal device performance to be identified. This makes it possible to envisage a design paradigm to based upon the knowledge of the target defect, and the processes by which they form, guiding and enhancing the synthetic approach. Ultimately, this approach has the potential to allow SiC (and any other material it is applied to) to reach its full technological potential.

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