Effect of H2 dilution on Cat-CVD a-SiC:H films
Bibhu P. Swain a, T.K. Gundu Rao b, Mainak Roy c, Jagannath Gupta d, R.O. Dusane a,*
a
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, India
b
Regional Sophisticated Instrumentation Center, IIT Bombay
c
Novel Materials and Structural Chemistry Division, Bhaba Atomic Research Center, Trombay, Mumbai, India
d
TPPED, Bhaba Atomic Research Center, Trombay, Mumbai, India
Abstract
Effect of hydrogen (H2) dilution of the Silane (SiH4), acetylene (C2H2) gas mixture during the deposition of hydrogenated amorphous
silicon carbon alloy (a-SiC:H) films by Cat-CVD process shows that the H2 dilution induced additional carbon incorporation, leading to an
increase of the carbon content in the films from 52% to 70% for the maximum H2 dilution employed. A slight increase in graphitic carbon in
the films deposited with H2 dilution is also observed. A drastic increase in the optical band gap E g from 2.5 eV for zero dilution to 3.5 eV is
observed for a H2 dilution of 10 sccm. Raman spectra for the films deposited with increasing H2 dilution indicate structural changes in the
amorphous network associated with increasing graphitic carbon.
Keywords: Hydrogen dilution; FTIR; Raman spectra; TRPL; XPS
1. Introduction
Hydrogen (H2) dilution of the source gases such as
silane (SiH4) and methane (CH4) or acetylene (C2H2) has
been observed to yield benefits in terms of electronic
properties of hydrogenated amorphous silicon carbon
alloys thin films (a-SiC:H). This advantage of using H2
dilution has been observed in case of the conventional
radio frequency glow discharge (RF g.d.) or the electron
cyclotron resonance CVD (ECRCVD) [1 – 5]. In the case
of the deposition of diamond like carbon (DLC) films by
the HWCVD H2 dilution is a must. In the case of
hydrogenated amorphous silicon (a-Si:H) film deposition
both by conventional PECVD and HWCVD, H2 dilution
has yielded films with better optoelectronic properties and
stability against light induced degradation. The role of
hydrogen is to passivate the dangling bonds at the growing
surface, increase the surface mobility of the ad-atoms and
reduced the film deposition rate. All these effects increase
the densities of the films, reduce the electronic defects and
yield better stability.
However in case of a-SiC:H deposition by the HWCVD
using either SiH4+ CH4 or SiH4+ C2H2 gas mixture, both
the primary dissociation and secondary gas phase reactions
are important to yield a higher incorporation ratio of carbon
into the film. Considering the advantage of using H2
dilution, both in the case of a-Si:H [6 –10] and DLC [11–
15] films deposited by HWCVD, we thought of employing
H2 dilution in the case of a-SiC:H deposition by HWCVD
using SiH4+ C2H2 source gases. As we will show later, the
results indicate that depending on the extent of H2 dilution
different effects take place during the deposition process.
2. Experimental details
a-SiC:H films were deposited by the HWCVD process
using SiH4, C2H2 and H2 gas mixture, We have through our
earlier studies [16 –18] optimized the process conditions to
obtain good quality a-SiC:H films without H2 dilution.
During the present work all these process condition were
kept constant, while the H2 flow rate was varied from 0 to
10 sccm. The other process parameters are given in Table 1.
Films were deposited on c-Si (p-type <100>) and Corning
7059-glass substrate to facilitate different characterizations.
174
Table 1
C-C network
Si-Si network
10 sccm
Deposition parameters
2 sccm
3 sccm
250 -C
1800 -C
0 to 10 sccm
100 mTorr
5 cm
The infrared spectroscopy data was obtained with a Nicolet
Magna 550 Fourier Transform Infrared (FTIR) spectrometer.
The optical band gap was determined from the transmission
data obtained with a UV – Vis spectrophotometer (Jasco V
530). The structural characterization was done by Raman
spectroscopy (Jobin-Yvon/SPEX T64000, 514 nm Ar+,
spectral resolution ~0.45 cm 1) while the chemical composition was determined from X-ray photoelectron spectroscopy (XPS) data (PHI 5700/660 Physical Electronics
Spectrometer). The continuous photoluminescence (PL)
studies were carried out with a PL spectrometer (Jasco V
570, Perkin Elmer Lambda 35) combined with 400 nm filter.
3. Results and discussions
Fig. 1 shows the normalized FTIR spectra for a-SiC:H
films deposited with increasing H2 dilution reflecting the IR
signatures corresponding to the various vibrational modes of
the different chemical bonds. From the FTIR spectra we
clearly see the signatures corresponding to the Si – C (780
cm 1), SiC – H3 (980 cm 1), Si – H (2100 cm 1) and C –H
(2900 cm 1) bonds confirming the deposition of a-SiC:H
films. It is interesting to note that the C –H stretch mode is
10 sccm
Absorption (A.U)
8 sccm
8 sccm
Intensity (A.U)
SiH4 flow rate
C2H2 flow rate
Substrate temperature (Ts)
Filament temperature (T F)
H2 flow rate
Pressure
Filament to substrate distance
6 sccm
4 sccm
2 sccm
0 sccm
200
400
600
800
1000
Raman shift
1200
1400
1600
Fig. 2. Raman spectra of a-SiC:H with hydrogen dilution varying from 0
sccm to 10 sccm. Si – Si vibration mode is in between 150 cm 1 and 485
cm 1 while C – C vibration mode is in between 1260 cm 1 and 1600 cm 1.
strongest for the sample deposited with 4 sccm of H2
dilution. Also we see that the Si – H stretching mode
frequency shifts from 2085 cm 1 for no hydrogen dilution
to 2100 cm 1 beyond a dilution of 6 sccm, an effect that is
related to increased C incorporation [19].
Fig. 2 shows the normalized Raman spectra of these
films acquired over a wide spectral range from 100 to 1850
cm 1. Si –Si network related features appear between 125
and 550 cm 1 and C –C related signatures appear from
1300 to 1600 cm 1. We see that the intensity of C – C peak
increases gradually with H2 dilution. From the Raman
spectra of the different films, we can identify two clear
consequences of increasing H2 dilution (1) the network
structure order deteriorates drastically. (2) Vibrational
modes corresponding to excitations of the C – C lattice
increase in intensity.
In Fig. 3 we show the full width at half maximum
(FWHM) of the transverse optic (TO) peak i.e. (TO and the
ratio of the intensities of the transverse acoustic (TA) peak
120
6 sccm
4 sccm
110
0.40
0 sccm
1000
1500
2000
Wavenumber
2500
3000
3500
(cm-1)
ITA/ITO
0.35
100
0.30
0.20
0.15
Fig. 1. FTIR spectra of HWCVD a-SiC:H films deposited with increasing
H2 dilution of the gas phase. C2H2 = 3 sccm and SiH4 = 2 sccm. The H2 flow
rate is indicated in the figure. The signature of Si – H wagging, Si – C
stretching, SiC:H3 wagging C – (CH3) wagging, Si – H stretching and C – H
stretching appear at 640 cm 1, 780 cm 1, 980 cm 1, 1450 cm 1, 2100
cm 1 and 2850 cm 1, respectively.
90
0.25
FWHM (cm-1)
0.45
2 sccm
500
1800
(cm-1)
80
0
2
4
6
8
10
H2 dilution (sccm)
Fig. 3. I TA/I TO ratio and FWHM of TO peak from Raman spectra with
variation of H2 flow rate in the a-SiC:H films. j represents I TA/I TO and ?
represents FWHM of the TO peak of Raman spectra in the a-SiC:H films.
175
3.0
60
55
2.5
0
2
4
6
8
10
50
H2 flow rate (sccm)
Fig. 4. Optical band gap and C content (%) with variation of H2 flow rate in
a-SiC:H films. j represents for band gap (eV) and ? represents carbon
content in the a-SiC:H films.
to that of the (TO) i.e. I TA/I TO as a function of H2 flow rate.
Now these two Raman parameters are correlated to the bond
angle deviation or the short-range order (SRO) and the
intermediate range order (IRO) respectively [20,21]. H2
dilution is known to increase both short range order (SRO)
and intermediate range order (IRO) in a-Si:H [22] and to
some extent in plasma deposited a-SiC:H [23]. Indications
of such an improvement in the SRO is also visible in the
present case for H2 dilution where the (TO decreases initially.
This means that within the low H2 dilution regime the
network order improves to some extent. However for the
higher dilution both the (TO and I TA/I TO increase drastically.
The hydrogen covering of the surface and reduction in the
deposition rate could be responsible for the initial improvement. However as we shall show the subsequent increase in
these parameters for higher dilution could originate from an
increased carbon incorporation.
Fig. 4 gives the carbon concentration and optical band
gap of the films determined by XPS and UV – Vis
spectroscopy respectively. It is seen that the optical band
gap initially increases rapidly to a very high value of 3.5 eV
and then saturates or even decreases to some extent. In order
to explain the drastic increase in the optical band gap of the
films we consider the observed variation in the carbon
content of the film with H2 dilution. We can easily see that
as the hydrogen dilution is increased there is a drastic
increase in the carbon content of the film which could lead
to such a dramatic increase in the optical band gap. Probably
this band gap increase of a-SiC:H is due to C – H bond and
C – C bond which is confirmed by XPS and FTIR. At 4 sccm
H2 dilution we obtained maximum band gap of 3.5 eV
which decreased to about 3.0 eV for the additional hydrogen
dilution of 10 sccm. The latter decrease of the band gap is
not very easy to understand particularly if we consider that
there is no additional carbon that is incorporated for the
higher hydrogen dilution. We believe that the increased
hydrogen dilution now eases the formation of the graphitic
network and so influences the optical properties of the film.
The continuous PL curve obtained at room temperature is
shown in Fig. 5 for different hydrogen dilution. For lower
H2 dilution (2 and 4 sccm) the photoluminescence spectra
are symmetric in nature but after 4 sccm the nature of PL
spectra change significantly and these are more asymmetric.
It indicates that there is predominantly band to band (could
be across the tail states) transitions for films deposited with
lower hydrogen dilution. In higher hydrogen flow case, the
single peak becomes asymmetric and finally splits into two.
It can be explained with the help of the modulated band gap
model where sp2 and sp3 bonds inside the a-SiC:H play a
role [24]. The PL peak position shifts towards higher energy
while the FWHM of the peak increases. This essentially
H2 dilution
A
Intensity (A.U)
0
2
4
6
8
10
peak position (eV)
3.0
2.8
2.2
2.0
2.4
2.6
2.8
3.0
0.4
B
2.6
0.3
2.4
0.2
2.2
2.0
0
2
4
6
8
FWHM (eV)
Band gap (eV)
65
Carbon content (%)
70
3.5
10
H2 flow rate (sccm)
Fig. 5. A. Photoluminescence intensity as a function of energy (eV) of H2 diluted a-SiC:H films with different H2 dilution. B. Variation of the peak position and
the FWHM of the PL peak as a function of H2 dilution.
176
indicates a broader distribution of radiative states with
additional features.
4. Conclusion
H2 dilution of source gases in the HWCVD of the aSiC:H films leads to significant increase in the C content of
the films. Though the band gap increases on one hand, this
deteriorates the structural and electronic properties. However the results indicate beneficial effects could be obtained
if H2 dilution would be used in the case of low C2H2
fraction in the gas phase.
Acknowledgement
This work was carried out with financial support from
BRNS, Department of Atomic Energy, Government of
India.
References
[1] Z. Hu, X. Liao, H. Diao, G. Kong, X. Zeng, Y. Xu, J. Cryst. Growth
264 (2004) 7.
[2] S.F. Yoon, Y.J. Liu, J. Ahn, W.I. Milne, Mater. Sci. Eng., B, SolidState Mater. Adv. Technol. 39 (1996) 188.
[3] M. Fathallah, R. Gharbi, G. Crovini, F. Demichelis, F. Giorgis, C.F.
Pirri, E. Tresso, P. Rava, J. Non-Cryst. Solids 198 – 200 (1996) 490.
[4] J. Daey Ouwens, R.E.I. Schropp, W.F. van der Weg, Solid State
Commun. 92 (1994) 853.
[5] S.S. Camargo Jr., W. Beyer, J. Non-Cryst. Solids 114 (1989) 807.
[6] P. Chaudhuri, D. Das, P.P. Ray, N. Duttagupta, D. Roy, C. Longeaud,
J. Non-Cryst. Solids 338 – 340 (2004) 236.
[7] J. Xu, J. Mei, X. Huang, W. Li, Z. Li, X. Li, K. Chen, J. Non-Cryst.
Solids 338 – 340 (2004) 481.
[8] S. Guha, J. Yang, A. Banerjee, B. Yan, K. Lord, Sol. Energy Mater.
Sol. Cells 78 (2003) 329.
[9] P. Chaudhuri, R. Meaudre, C. Longeaud, J. Non-Cryst. Solids
338 – 340 (2004) 690.
[10] S. Guha, J. Yang, A. Banerjee, B. Yan, Kenneth Lord, Sol. Energy
Mater. Sol. Cells 78 (2003) 329.
[11] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129.
[12] M. Malhotra, T. Som, V.N. Kulkarni, S. Kumar, Vacuum 47 (1996)
1265.
[13] C.J. Tang, A.J. Neves, L. Rino, A.J.S. Fernandes, Diamond Relat.
Mater. 13 (2004) 958.
[14] S.L. Sung, X.J. Guo, K.P. Huang, F.R. Chen, H.C. Shih, Thin Solid
Films 315 (1998) 345.
[15] S.B. Kim, J.F. Wager, D.C. Morton, Thin Solid Films 189 (1990) 45.
[16] B.P. Swain, S.B. Patil, A. Kumbhar, R.O. Dusane, Thin Solid Films
430 (2003) 186.
[17] S.B. Patil, A.A. Kumbhar, S. Saraswat, R.O. Dusane, Thin Solid Films
430 (2003) 257.
[18] A. Kumbhar, S.B. Patil, Sanjay Kumar, R. Lal, R.O. Dusane, Thin
Solid Films 395 (2004) 244.
[19] M. Shinohara, Y. Kimura, D. Shoji, M. Niwano, Appl. Surf. Sci.
175 – 176 (2001) 591.
[20] A.P. Sokolov, A.P. Shebanin, O.A. Golikova, M.M. Mezdrogina,
J. Non-Cryst. Solids 137&138 (1991) 99.
[21] A.P. Sokolov, A.P. Shebanin, Sov. Phys., Semicond. 24 (1990) 720.
[22] R. Tsu, P. Menna, A.H. Mahan, Sol. Cells 21 (1987) 189.
[23] R. Tsu, J.G. Hern~dez, F.H. Pollak, Solid State Commun. 54 (1985)
447.
[24] C. Palsule, S. Gangopadhyay, Phys. Rev., B 48 (1993) 10804.