Theoretical study of luminescence degradation by oxidation in Si(001) and Si(111) ultrathin films: Gap states induced by oxidation

A possible mechanism for degradation of light emission by oxidation in Si nanostructures is proposed, on the basis of electronic state calculations by the extended H\"uckel-type nonorthogonal tight-binding method for possible oxygen configurations on the well-defined surfaces of Si(001) and Si(111) ultrathin films. The on-top or top-bridge site configuration of oxygen on Si(001) ultrathin films presents electronic states of a Si dangling-bond character across the fundamental band gap, suggesting the occurrence of nonradiative recombination centers and thus the luminescence degradation. In H-terminated Si(111) ultrathin films, the combination of the surface Si-H bond states and the states due to the Si-Si backbond to which oxygen attaches causes gap states, in contrast to the case of H-terminated Si(001) ultrathin films where the similar oxygen configuration does not produce any gap states. Finally, it is found that the luminescence degradation by oxidation observed in porous Si can be explained by a combination of hydrogen desorption and the following creation of the on-top or top-bridge site configuration of oxygen on the Si surfaces.