UV-induced formation of oxygen-derived dangling bonds on hydroxyl-terminated SiC.

A combined theoretical and multi-technique experimental study was employed to comprehensively determine the electronic structure of 6H-SiC(0001) surfaces upon hydroxyl and oxygen termination. We demonstrate the UV-induced formation of single-coordinated oxygen radicals in on-top (OT) sites above the atoms of the uppermost silicon layer of the substrate on initially hydroxyl-terminated SiC. A suchlike configuration of oxygen radicals represents an unprecedented adsorbate-derived system of unpaired electrons, bearing analogy to silicon and carbon dangling bonds on clean, unreconstructed SiC surfaces. We evidence the presence of adsorbate-derived surface states within the fundamental band gap for both hydroxyl- and oxygen-terminated SiC. For hydroxyl termination, a hydrogen-induced unoccupied surface state is revealed consistently by inverse photoemission spectroscopy (IPES) and density-functional theory calculations employing self-interaction-corrected pseudopotentials (DFT-SIC). The formation of oxygen dangling bonds is accompanied by the occurrence of an occupied surface state derived from px- and py-orbitals associated with the unpaired electrons as proven by both ultraviolet photoemission spectroscopy (UPS) and DFT-SIC.