Download Journal of Non-Crystalline Solids 114 (1989) 813-815

Journal of Non-Crystalline Solids 114 (1989) 813-815
North-Holland
813
RAMAN SCATTERING FROM MICROCRYSTALLINE Si FILMS: CONSIDERATIONS OF COMPOSITE
STRUCTURES WITH DIFFERENT OPTICAL ABSORPTION PROPERTIES
R.J. NEMANICH, E.C. BUEHLER, Y.M. LEGRICE, R.E. SHRODER,* G.N. PARSONS, C. WANG, G. LUCOVSKY
Department of Physics, and Department of Materials Science and Engineering
North Carolina State University
Raleigh, NC 27695-8202 USA
and
J.B. BOYCE
Xerox Palo Alto Research Center
Palo Alto, CA 94304 USA
Raman scattering measurements are used to characterize the amorphous and crystalline components of microcrystalline Si
films. A model is described which addresses the properties of Raman scattering from composites of materials of different
optical absorption. The analysis shows that the observed spectra is dependent on both the percentage of the components and
on the domain size of the more highly absorbing domains. Samples of microcrystalline silicon prepared by exeimer laser
exposure of hydrogenated a-Si and by magnetmn sputtering were measured, and the results were analyzed in terms of the
model. The experimental results reflect the length scales of the domains and vibrational excitations.
INTRODUCrlON
Microcrystalline Si films are often produced by
variations of the conditions for the the deposition of
hydrogenated amorphous Si (a-Si:H). The films exhibit a
structure which is a combination of amorphous Si and
crystalline Si regions. The films often exhibit a higher
electron mobility than a-Si:H, and are considered for thin
film transistor applications. This study describes the
application of Raman spectroscopy to characterize the films.
Raman spectroscopy is particularly useful for
identifying whether a film exhibits amorphous or crystalline
structure. The Raman spectrum of crystalline Si is
dominated by a sharp feature at 520 cm- 1 while the spectrum
of a-Si:H displays features which resemble the broadened
density of vibrational states of Si. There have been several
detailed analysis of the evolution of the Raman spectra as a
function of crystalline size. 1-3 For the particular case of Si,
as the crystalline domain size decreases, the Ram_an peak
broadens and shifts to lower frequency.2, 3
While Raman scattering measurements are often used
to characterize the films, the optical absorption properties of
the amorphous and crystalline regions are different, and
these effects will influence the analysis. In this study,
Raman measurements of a series of amorphous and
microcrystalline Si films are presented. The spectra are
decomposed into amorphous and crystalline components.
The effects of optical absorption and crystal size are
discussed.
D
r~
7
r~
z
I
100
i
300
I
!
500
FREQUENCY SHIFT (CM-I)
FIGURE 1
The Ram.an spectra of excimer laser crystallized films (B-D)
compared to that of amorphous Si (A).
* Current address: Rockwell Int., Rocketdyne Division, Canoga Park, CA 91303
0022-3093/89/$03.50 © Elsevier Science Publishers B.V.
(North-Holland)
j
814
R.J. Nemanich et al./ Raman scattering from microcrystalline Si films
b.
Z
r~
z
z
I
100
'
300
'
500
100
FREQUENCY SHIFT (CM-I)
FIGURE 2
The computer reconstructed Raman spectra of film B from
Fig. 1.
RESULTS
Series of microcrystalline samples were prepared by
excimer laser exposure of a-Si:H and by magnetron
sputtering with high H concentrations. The Raman spectra
of the series of films prepared by excimer laser exposure are
shown in Fig. 1. The Raman spectra were excited with
-150 mW of 514.5nm radiation from an Ar ion laser, and
the scattered light was dispersed with a triple grating
monochromator. Essentially similar spectra were obtained
from the magnetron sputtered films. The spectrum A is of
amorphous Si network vibrations while D represents
predominantly crystalline regions with small domain sizes.
Spectra B and C display spectral components which are
attributed to both crystalline and amorphous regions in the
sample. The goal of this study is to define the limits of
using Raman spectroscopy to determine the relative amounts
of these components.
To model the crystalline and amorphous components
of the samples, the spectra A and D were added together to
duplicate the results shown in spectra B and C. The results
are shown in Fig. 2 and 3. The general aspects of the
spectra are well described by this procedure. There is a
small discrepancy at ~500era-1 which could not be fit in
either spectra. This aspect is discussed later. The results
I
300
I
I
500
FREQUENCY SHIFT (CM-1)
FIGURE 3
The computer reconstructed Raman spectra of film C from
Fig. 1.
indicate that spectrum B can be described by 0.95 of A and
0.05 of D, and spectrum C was described by 0.78 of A and
0.22 of D. The following discussion describes the limitation
of using this analysis to represent the relative concentrations
of the amorphous and crystalline components.
DISCUSSION
The optical absorption differences of the amorphous
and microcrystalline regions and spectral changes can effect
the straightforward analysis described above. The aspects of
optical absorption were identified in recent studies of Raman
characterization of CVD diamond films and diamondgraphite powder composites.4 In these studies it was noted
that the Raman intensity from the highly absorbing regions
of the sample depended on the crystalline domain size. The
ratio of the Raman intensities of the amorphous and
crystalline regions can be given by
Ia
Aa Na Va
l~t
A ~ Np. Vgt
(I)
R.J. Nemanich et al. / Raman scattering from microcrystalline Si films
where I, A, N and V are the Raman intensity, Raman crosssection, atomic density, and illuminated volume respectively,
and the subscripts a and Ix refer to the amorphous and
microcrystalline components. The illuminated volume will
depend on the fraction of amorphous or microcrystalline
regions and on the absorption constant and size of the
domains. For the case of microcrystalline Si we can assume
that a - 1 of the crystalline regions is larger than the domain
size, thus it follows that the crystalline regions are fully
illuminated. In contrast, the a of the amorphous regions is
- 1 0 times greater than that of crystalline Si and cannot be
neglected. Eq 1. thus becomes
Ia
AaNaPa
[ 1]
ii - A: N~ (-T~_pa)L~a~a/j
(2)
Where I is the size of the domains, and Pa is the fraction of
a-Si. This equation is valid for c~-a~<l. The dependence of
the observed Raman intensities for several different domain
sizes are shown in Fig. 4. For the calculation the ratio of the
Raman cross-sections was determined from the peak
intensities of the materials, c¢ for a-Si of 2 xl05 cm-1, and
the densities were assumed to be the same. From the figure
it is clear that the observed Raman ratio cannot be directly
related to the amorphous/microcrystalline ratio unless the
domain size is known. Below ~500A domain size, the
model will break down, and in the limit of very small
domains, the term, 1/Ctal will go to 1. (In that limit, the
straightforward analysis will apply.)
The second aspect of the analysis that must be
considered is that the spectral shape and scattering efficiency
of the amorphous and crystalline regions can vary as a
function of the crystalline domain size. It has been
demonsWated that the linewidth and peak frequency changes
as a function of the crystalline domain size for Si.2,3 These
changes are most important in the range of 30 to 500A
domain sizes. It has been estimated that vibrational modes in
amorphous materials are localized to regions -50 A, thus the
Raman spectra of the amorphous regions should not change
unless the domain size approaches this limit.
The most evident spectral change is the broadening
of the 520cm -1 Raman feature for microcrystalline domains.
Inspection of the spectra shown in Fig. 1 indicates that the
crystalline Si feature has a different linewidth for the
different samples. The sample with the smallest amorphous
component exhibits the narrowest linewidth. This aspect can
account to large part for the deviation in the reconstructed
spectra shown in Fig. 2 and 3.
815
0.3"
~A
0.2"
3000
0.l"
0.0
0.0
•
,
.
0.2
FRACTION
,
0.4
OF
.
,
0.6
AMORPHOUS
.
,
0.8
1.0
SILICON
FIGURE 4
The theoretical ratio of the Raman signal from the
am.orphous and crystalline regions vs. fraction of amorphous
regmn. The calculations are for different domain sizes.
CONCLUDING REMARKS
The results described here show that the Raman
spectra of microcrystalline Si films can be described as a
combination of the spectra of a-Si and crystalline Si with
small domains. Because of the large difference in the optical
absorption of the crystalline and amorphous domains, the
relative fraction of the two regions cannot be deduced unless
the domain size has been determined. In addition, it has
been established that the lineshape of the spectra of
microcrystalline regions changes, and this leads to deviations
in the fitting procedure. In principle, these changes can also
be accounted for by fitting the lineshape of the crystalline
component. One aspect that is yet to be determined is the
dependence of the absolute intensity vs. crystalline domain
size.
ACKNOWI .EDGEMENTS
We acknowledge Shannon Wells for her assistance
on the data analysis. This work was partially supported by
the North Carolina Board of Science and Technology.
REFERENCES
1. R.J. Nemanich, S.A. Solin, and R.M. Martin, Phys.
Rev. B23 (1981) 6348.
2. Z. Iqbal and S. Veprek, J. Phys. C15 (1982) 377.
3. P.M. Fauchet and I.H. Campbell, CRC Critical Reviews
in Solid State and Materials Sciences 14 (1988) $79.
4. R.E. Shroder, R.J. Nemanich, and J.T. Glass, (to be
published).