It is observed that Si-H stretching mode vibrational spectra in porous silicon and in hydrogenated amorphous silicon: (i) is broad with a full width at half maxima of approx= 20-40 cm~(-1); (ii) has a long low energy tail; and (iii) has a peak value shifted by 100 cm~(-1) from 2000 cm~(-1) (crystalline silicon case) to 2100 cm~(-1). We propose a simple phenomenological model to account for the above features. The model invokes a narrow distribution in the Si-H bond distances. The first feature namely the broadening can be satisfactorily explained. although our model yields a low energy tail, it is not as pronounced as the experimental tail. Our model can be shown to be consistent with the third feature, namely the peak shift.
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