Raman spectra of a variety of polymorphous (pm-Si:H) and amorphous silicon
(a-Si:H) samples deposited by plasma enhanced chemical vapor deposition
(PECVD) at different pressures were recorded in the range of 150 cm-1 to 750 cm-1 using a 514 nm excitation source. A comparison of Raman
spectra between a-Si:H and pm-Si:H samples reveals significant differences.
The Transverse Optical (TO) peak in case of pm-Si:H is asymmetric and
shifted to higher wave numbers. In the literature, discrepancies between
predictions of various quantum confinement models and experimental spectra
have typically been attributed to either strain or negligible fraction of
nanocrystallites. We show that a quantum confinement model along with a
Gaussian size distribution is able to accurately predict particle size of
nanocrystallites embedded in the amorphous matrix. The crystallite size and
size distribution obtained by fitting the TO peak is consistent with
high-resolution transmission electron microscopy observations. In our case
typical mean crystallite size obtained is about 3 nm with FWHM of the
distribution varying in the range 0.2–1 nm. A comparison of ratio of heights
in TA and TO peaks of a-Si:H and pm-Si:H material indicates that pm-Si:H as
a material has higher medium range order (MRO). This ratio has been used to
compare the degree of MRO in pm-Si:H samples prepared under different
conditions. Thus, we demonstrate that Raman Spectroscopy along with our
model can be used to obtain the crystallite size distribution and provide a
measure of degree of medium range order.