Raman spectra in a broad frequency region of p− type porous silicon

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Throughout the 50–1050 cm−1 range, in addition to the first order Raman peak of optical phonon, a weak structure in the vicinity of 147 cm−1 and two intense peaks (bands) in the vicinities of 632 and 956 cm−1 have been observed. The structure at 147 cm−1 is believed to be mainly due to scattering by the transverse acoustic single phonon mode; the intense peak at 632 cm−1 is thought to be mainly due to the transverse acoustic and optical combination phonon modes and the intense peak at 956 cm−1 to the transverse optical overtone double phonon mode of the p− type porous silicon (PS) layer. A comparison of spectral features indicates that, in PS Raman spectra, there is no substantial contribution due to the a-Si component and for PS the crystalline characteristic still remains to a great extent. Contrary to what would be expected using the conventional and simplified microcrystal model, Raman frequency downshifts of the aforementioned structures near 632 and 956 cm−1 were clearly observed, which indicates that, unlike the case of the first order Raman spectrum of optical phonon, the microcrystal model is too simple to give an interpretation of PS second order Raman spectra.