Effect of the oxidation process on the luminescence of HF-treated porous silicon

Porous silicon (PS) films were prepared by lateral anodization of crystalline silicon in HF based solutions at different current densities. The oxidation mechanism of the HF-treated PS films has been monitored by means of photoluminescence (PL) and Fourier transform infra-red vibrational studies. HF treatment of PS films for a short duration of ∼10 s resulted in the increment of polysilane/hydride species on the surface of PS rather than any change in the dimensions of the silicon-crystallites. It has been found that HF-treated PS surfaces are relatively stable against oxidation as compared to untreated PS films. Upon oxidation of the HF-treated PS films, the PL intensity initially increases as a result of reduction in crystallite size to exhibit quantum size effects and then decreases owing to loss of luminescing structures due to over-oxidation of the Si-columns. We infer that the surface passivation either by hydrogen or oxygen is one of the requisite conditions for obtaining strong PL efficiency in PS. It seems that more than one emission mechanisms are responsible to explain the luminescence properties of PS.