Photoluminescence and Raman spectroscopy study on oxidized free-standing porous Si

Photoluminescence and Raman scattering spectra were used to study three typ es of free-standing porous Si: as-grown samples with various porosities ref erred to as APS samples, these samples oxidized in air at 200 degreesC fur 200 h as OPS samples, and these samples aged further in air at room tempera ture for 20 months as AOPS samples. The PL peak energies of OPS and AOPS sa mples have shifted in small energy ranges centered at about 1.61 and 1.59 e V, respectively. Each Raman spectrum was fitted with an amorphous and a cry stalline component. The crystalline component was used to determine the siz es of nanometer Si particles (NSPs), which were in ranges of 2.5-2.2 and 3. 3-2.3 nm in OPS and AOPS samples, respectively. The reason why OPS or AOPS samples with different NSP sizes have almost the same FL peak wavelength is discussed with a novel multiple mechanism model.