Nanoscale Mapping of Sub-Gap Electroluminescence from Step-Bunched, Oxidized 4H-SiC Surfaces

Scanning tunneling luminescence microscopy (STLM) and scanning tunneling spectroscopy (STS) are used to study step-bunched, oxidized 4H-SiC surfaces prepared using a silicon melt process. The step-bunched surface consists of atomically smooth terraces parallel to [0001] crystal planes, and rougher risers containing nanoscale steps formed by the termination of these planes. The striking topography of this surface is well resolved with large tip biases around −8 V and set currents less than 1 nA. Hysteresis in the STS spectra is preferentially observed on risers, suggesting that they contain a higher density of surface charge traps than the terraces where hysteresis is more frequently absent. Similarly, intense sub-gap light emission centered around 2.4 eV is observed mainly on the risers albeit only with larger tunneling currents equal to or greater than 10 nA. The surface-oxide-related origin of this emission is reinforced by comparing tunneling electroluminescence spectra on the 4H- and 6H-polytypes, and by the observation of a drastic reduction in emitted intensity after removal of the oxide in buffered HF. These results demonstrate the capability of STLM for the observation of surface impurities and defects responsible for sub-gap light emission with spatial resolutions approaching the length scale of the defects themselves.