Theoretical study of luminescence enhancement in oxidized Si(001) ultrathin films

The electronic structure for the oxidized Si(001) ultrathin films is calculated as a function of Si film thickness by the extended H\"uckel-type nonorthogonal tight-binding method and compared to that for the hydrogen-terminated one. It is found that the molecular oxygen configuration on the film surface makes the direct band gap less sensitive to the Si film thickness than the indirect one, and intensifies the oscillator strength by an order of magnitude compared to the H-terminated films. The configuration of the H-terminated Si(001) film with oxygen at the backbond sites makes the oscillator strength three orders of magnitude higher than the H-terminated case, suggesting luminescence as strong as for the H-terminated zero-dimensional Si crystallites. The presence of Si dihydrides on the surface backbonded to oxygen has been found to be critical for the very high values in oscillator strength.