Highly efficient photoinduced desorption of N2O and CO from porous silicon

Photoinduced desorption (PID) of N(2)O and CO from porous silicon (PSi) samples is reported. Both adsorbates exhibit unusually large cross sections for PID at 193 nm, up to 10(-15) cm(2), 2-3 orders of magnitude larger than the literature values for similar processes on flat Si. Under this UV irradiation, N(2)O molecules undergo photodissociation (a competing process leading to surface oxidation) with a cross section that is 2 orders of magnitude smaller than photodesorption. In the case of CO desorption is the exclusive photodepletion mechanism. PID efficiency decreases with increasing CO coverage suggesting PID hindrance by interactions among the desorbing CO molecules leading to re-adsorption at higher coverage. The wavelength and fluence dependence measurements exclude the possibility of laser induced thermal desorption for both adsorbates. The proposed mechanism for this phenomenon is desorption induced by hot electron transfer from the substrate to the adsorbate. Enhanced lifetime of transient negative adsorbate due to stabilization by localized holes on PSi nanotips can explain the observed abnormally large PID efficiency on top of porous silicon.