The silicon vacancy in SiC

Sammanfattning: SiC has for decades been considered to be a very promising semiconductor, but several problems related to the material quality, did earlier prevent an exploitation of the supreme electrical characteristics of the material. Now most of these problems have been overcome, high quality material can be grown and research around this material is going through an intense phase of development.The photoluminescence (PL) technique is simple, non-destructive and requires no sample preparation. If a PL signal can be associated with a fundamental defect it certainly is an important piece of information. A fingerprint signal obtainable with such a simple technique is very useful in routine characterisation of the material. In our ambition to reveal the origin of the PL signals from SiC, we have used a technique called optically detected magnetic resonance (ODMR). It is efficient in supplying microscopic information about defects and have enabled us to assign a deep PL band to the silicon vacancy in 3C, 4H and 6H SiC. To be more specific, we argue that the deep PL band originates from internal transitions of the silicon vacancy in its neutral charge state.SiC is the classical example of polytypism i. e. it appears in many different but closely related crystal structures. A substitutional atom or a vacancy can therefore occupy sites, with different arrangements of their neighbours. If a defect is sensitive to its immediate neighbourhood, the signals from the different sites can usually be separated and new possibilities opens up for a materials scientist. A certain defect system can then be investigated and compared between the polytypes and much more about how the local environment of the defect affects its properties can be learned, than otherwise would have been possible.Here we have been able to investigate the signals from the silicon vacancy in 3C, 4H and 6H SiC and by comparing the characteristics of the signals from the different sites, we have been able to correlate the individual signals to the corresponding sites in the 4H and 6H crystals (in the 3C crystal there is just one site). There are some pronounced differences in the signals from the different sites. Only the PL lines from the sites with a cubic environment are split by applied stress while the ODMR signals from the sites with a hexagonal environment have a stronger relation to the symmetry axis of the crystal.Substitutional nitrogen is a shallow donor in SiC. Our magnetic resonance measurements show that the loosely bound electron of the nitrogen atom is transferred to various deep defects. This direct transfer of electrons from a shallow defect to deeper ones opens up new efficient recombination channels. This type of recombination channels is not included in standard recombination models.