Scanning tunnelling microscopy imaging and spectroscopy of p-type degenerate 4H-SiC

In this work we present scanning tunnelling microscopy (STM) imaging and spectroscopy of a highly p-doped wide bandgap semiconducting 4H-SiC surface. Whereas n- and p-doped 6H-SiC or n-doped 4H-SiC surfaces can be relatively easily imaged with the STM, the p-doped 4H-SiC cannot be imaged due to the absence of any surface conductivity. This is very surprising given the presence of a p-doped, degenerate epitaxial layer. The behaviour can be explained by the formation of a Schottky barrier either between the tip and the surface or between the surface and the sample holder, depending on the polarity of the applied voltage. We found that prolonged and repeated exposures of the SiC surface to a Si atomic flux followed by thermal annealing are required before the surface conductivity is sufficient to allow STM images to be recorded. The result is the deposition of overlayers of Si, with structures similar to Si(111) 7 × 7, Si(113) 3 × 2, and Si(110) 16 × 2 rather than the expected stable SiC 3 × 3 reconstruction. We have further demonstrated the ability of scanning tunnelling spectroscopy to distinguish between the Si and the SiC phases based on the difference in their bandgaps.

Source:IOPscience

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