Chapter - IV
Influence of Hydrogen, Argon and Helium
Dilution in Synthesis of nc-Si:H Thin Films
by PE-CVD Method
In this chapter, the effect of hydrogen and noble gas (argon and helium) dilution of silane gas on
PE-CVD deposited hydrogenated silicon thin films is studied. In each set of depositions the structural,
optical and electrical conductivity properties of the deposited thin film material have been reported.
Chapter IV
103
4.1: INTRODUCTION
Hydrogenated nanocrystalline silicon (nc-Si:H) has been the subject of scientific and
technological interest in recent years because of its outstanding properties such as higher
electrical conductivity, greater doping efficiency etc. [1]. Also, the optical band gap of nc-Si:H can
easily be tailored by controlling the deposition parameters [2, 3] to absorb sufficient amount of
solar radiation without affecting the series resistance of the device. The material has been
successfully employed in solar cell and yielded initial conversion efficiency of 5.97 % for n-i-p
solar cells with a nanocrystalline i-layer [4]. By integrating nc-Si:H absorber i-layers, Klein et al.
[5] achieved solar cells with an conversion efficiency 7.4 % for p-i-n cell structure and 9.4 % for
n-i-p solar cell structure, respectively.
The nature of feed gas or gas mixture also plays a crucial role in determining the material
properties of the resulting films. The Si:H films deposited by pure silane without any dilution gas
by PE-CVD method are purely amorphous. Addition of hydrogen (H 2 ) in silane (SiH 4 ) is proven to
induce growth of nanocrystallites in the Si:H films. Hydrogen plays an important role in both gas
phase reactions in plasma and reactions at film growing surface. For the synthesis of device
quality nc-Si:H films by PE-CVD method high hydrogen dilution of silane is necessary. However,
there are some serious shortcomings for example, drastic reduction in deposition rate. To
overcome this limitation, inert gas dilution of silane is an alternative way to synthesize device
quality nc-Si:H films by PE-CVD at high deposition rate. The inert gases like helium (He), argon
(Ar), xenon (Xe) can be used as dilution gases. These inert gases do not take part in the
chemical reactions, but relax the strained or weak Si network after impinging on film growing
surface.
In this chapter, we present the influence of dilution of silane by hydrogen, argon and helium
gases on the properties of the nc-Si:H material.
4.2: STUDY OF INFLUENCE OF HYDROGEN DILUTION OF SILANE
In PE-CVD process, the hydrogen dilution of silane is known to promote the growth of good
quality a-Si:H. The beneficial influence of hydrogen dilution of silane is attributed to three
mechanisms [6].
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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1) In the presence of hydrogen atoms the surface passivation of the growing surface is more
complete due to which a more homogenous growth of film can achieve.
2) With increasing H 2 dilution the ratio (SiH 3 )/(SiH 2 ) increases, because hydrogen react with
ambient SiH 4 under the formation of SiH 3 .
3) Atomic H can etch away weak Si-Si bonds in the growing surface, promoting the growth
of a dense and high quality a-Si:H layer.
The atomic H reduces the deposition rate due to reduction of saline partial pressure and etching.
In preparation of nc-Si:H films by PE-CVD method, various process parameters and dilution of
SiH 4 with other gases such as H 2 , Ar or He have a strong influence on the structure and
morphology of the films. Some of the reports indicated that the nc-Si:H films deposited by PECVD do not show any systematic correlation between the process parameters and the resulting
film properties [7] due to the heterogeneity of grown films. On the other hand, some reports [8, 9]
have shown that the crystallite size and height, as well as their density can be controlled by
deposition time, process pressure, RF power and substrate temperature. Furthermore, silicon
solar cells prepared using nc-Si:H have been also reported with marked improvement in near
infrared absorption of the solar spectrum and high stability against the prolonged light illumination
in comparison to its amorphous counterpart [10, 11]. Therefore, more detail and careful
investigations of the synthesis and characterization of PE-CVD grown nc-Si:H films are needed.
With this motivation, an attempt has been made to synthesize undoped nc-Si:H films by
conventional PE-CVD method using hydrogen dilution of silane. We have observed that film
properties are greatly affected by hydrogen dilution of silane.
4.2.1: Experimental
Undoped hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared in a PE-CVD system
by glow discharge decomposition of silane (SiH 4 , Matheson semiconductor grade) and hydrogen
(H 2 ) mixture. The reactor was capacitively coupled, consisting of two parallel electrodes within
the stainless steel chamber, operated at an radio frequency of 13.56 MHz. The lower electrode
was coupled to the plasma generator via an impedance matching network, and the upper
electrode grounded. The feed gases were mixed before they were admitted to the reactor. The
details of the deposition system are discussed in Chapter II. The films were deposited
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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simultaneously on corning #7059 glass to study optical and electrical properties and c-Si wafers
(5-10 Ω-cm, p-type) for structural properties. The hydrogen dilution ratio (defined as ∆ H =
FH2
,
FSiH4
where F SiH4 is silane flow rate and F H2 is hydrogen flow rate) was varied between 100 and 300 in
the step of 50 while the other parameters were kept constant. Other deposition parameters are
listed in Table 4.1.
Table 4.1: Deposition parameters used for the synthesis of nc-Si:H films using H 2
Parameter
Value
RF power
200 Watt
Deposition pressure (P dep )
300 ± 5 mTorr
Substrate temperature (T sub )
200 ± 2 0C
Gas flow rates:
a) Silane (F SiH4 )
0.3 sccm
b) Hydrogen (F H2 )
30-90 sccm
Hydrogen dilution ratio ( ∆ H =
FH 2
)
FSiH 4
100-300
Base pressure (P r )
6-8 x10-7 Torr
Distance between electrodes (d e-s )
40 mm
Substrate temperature (T Sub )
200 0C
4.2.2: Results and discussion
4.2.2 (a): Variation in deposition rate
All the films were deposited for equal deposition time. The average deposition rate (rd ) was then
calculated as thickness divided by the time of deposition. The variation of deposition rate as a
function of hydrogen dilution of silane (∆ H ) is shown in figure 4.1. As seen from the figure, the
deposition rate decreases from 2.5 Å to 1.6 Å as hydrogen dilution of silane increase from 100 to
300. The deposition of a film involves two simultaneous processes; first is growth of film forming
radicals and the second is etching of deposited portion. Deposition rate is, thus, determined from
the competition between deposition and etching process. The decrease in deposition rate with
increase in hydrogen dilution of silane can be attributed to three factors.
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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1) With increase in ∆ H , the SiH 4 density in the gas mixture decreases [12]. As a result, the
concentration of precursors that produces Si:H film decrease. So deposition rate
decreases with increase in hydrogen dilution of silane.
2) Increase in ∆ H increases atomic H in plasma, which leads to excessive etching and
breaking of weak Si-Si bonds [13]. This leads to increase in both the etching rate of aSi:H as well as the etching rate of nc-Si:H. As a result the deposition rate decreases with
increase in hydrogen dilution of silane.
3) Increase in ∆ H increases the hydrogen partial pressure in the deposition chamber, which
increases the gas phase polymerization. This leads to increase in concentration of higher
silicon hydride species [14] with lower sticking coefficients, which further increases the
etching probability. Therefore, the overall deposition rate decreases with increase in
hydrogen dilution of silane.
2.7
Deposition rate (A/s)
2.4
2.1
1.8
1.5
0
50
100
150
200
250
300
350
400
Hydrogen dilution of silane
Figure 4.1: Variation of deposition rate of films deposited by PE-CVD as a function of hydrogen dilution of silane
The low deposition rate values observed at high hydrogen dilution are certainly related to a more
efficient etching process in the deposition-etching competition. The decrease in deposition rate
with increase in hydrogen dilution of nc-Si:H films deposited by PE-CVD has been reported
earlier [15].
4.2.2 (b): Variation in electrical properties
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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The effect of hydrogen dilution of silane (∆ H ) on dark conductivity (σ Dark ) and photoconductivity
(σ Photo ) of nc-Si:H films is shown in figure 4.2. As seen in the figure, σ Dark increase from 10-10
S/cm to 10-5 S/cm when R increases from 100 to 300, whereas the σ Photo remains almost constant
at 10-5 S/cm for whole range of ∆ H studied. The increase of hydrogen dilution of silane increases
the dark conductivity of the film drastically implies a change in transport phenomenon in the film
material. The photosensitivity gain, taken as ratio of photoconductivity to dark conductivity (σ Photo
/σ Dark ) for the film deposited at ∆ H = 100 is ~ 104, which further decrease to ~ 10 for the film
deposited at ∆ H = 300.
Conductivity (S/cm)
10-3
10-4
σPhoto
10-5
σDark
10-6
10-7
10-8
10-9
0
50
100
150
200
250
300
350
400
Hydrogen dilution of silane)
Figure 4.2: Variation of dark conductivity and photoconductivity of nc-Si:H films deposited by PE-CVD as a
function of hydrogen dilution of silane.
This indicates that the films deposited with increasing ∆ H get structurally modified. We attribute
the drastic reduction in the photosensitivity gain to the reduction of amorphous fraction (or
increase in crystalline fraction) and decrease in crystallite size in the film with increasing
hydrogen dilution of silane. The µc-Si:H/nc-Si:H films prepared by different methods or PE-CVD
show high dark conductivity and negligible photosensitivity gain depending upon the crystallite
size and its volume fraction [16]. This inference is further strengthened by the observed variation
in deposition rate with hydrogen dilution of silane (see figure 4.1) since lower deposition rate is
more favourable to the formation of crystallinity in the film structure [17].
4.2.2 (c): Raman spectroscopic analysis
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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Raman scattering is a sensitive tool for studying nc-Si:H material because it gives direct structural
evidence quantitatively related to the nanocrystalline and amorphous component in the material.
Figure 4.3 show the Raman spectra of three nc-Si:H films deposited at various hydrogen dilution
of silane (∆ H ) as depicted in the figure. The corresponding crystalline fraction (X Raman ) and
crystallite size (d Raman ) in the film are also indicated in the figure. Each spectrum shown in figure
4.3 was deconvoluted into three Gaussian peaks with a quadratic base line method mentioned in
the previous section. Figure 4.4 represent a typical deconvoluted Raman spectra for the nc-Si:H
Intensity (Arb. Unit)
film prepared at ∆ H = 200.
∆Η
XRaman
dRaman
100
51 %
8.60 nm
200
76 %
8.10 nm
300
84 %
7.67 nm
400
425
450
475
500
525
Raman shift (cm-1)
550
Figure 4.3: Raman spectra of nc-Si:H films deposited by PE-CVD at various hydrogen dilution of silane
520 cm-1
Intensity (Arb. Units)
501 cm-1
400
480 cm-1
420
440
460
480
Raman shift (cm-1)
500
520
540
Figure 4.4: Typical Raman spectra for a nc-Si:H film prepared at ∆ H = 200, deconvoluted with three Gaussian
peaks and a quadratic base line, with an algorithm based on the Levenberg–Marquardt method [18].
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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As seen from figure 4.3 for the film deposited at ∆ H = 100, the Raman spectra show a broad
shoulder centred near 499 cm-1, associated with the amorphous and defective nanocrystalline
phases [19] and other centred near 520 cm-1 originating from nanocrystalline phase in the film
[20]. For this film, X Raman is ~ 51 % and d Raman is ~ 8.60 nm. It is also observed that the intensity
of peak centred near 499 cm-1 seems to decrease with increasing ∆ H indicating the decrease in
amorphous content in the film. The peak centred near 520 cm-1 shift towards higher wave number
and, its intensity and sharpness increases with increase in ∆ H . It has been reported that the
shifting of TO phonon peak towards higher wave number is related to increase in volume fraction
of crystallite in the films [21] and increase in intensity and sharpness of the peak is related to
decrease in crystallite size in the film. Thus, the film deposited at ∆ H = 300, the Raman spectra
shows nanocrystalline phase with the TO phonon peak centred at 521 cm-1 and a very small
amorphous content in it. For this film, X Raman is ~ 84 % and d Raman is ~ 7.67 nm. Thus, the Raman
scattering study indicates that with increasing ∆ H more Si nanocrystallites were formed in the
amorphous Si matrix of the grown thin films with decreasing crystallite size. The decrease in
crystallite size and the increase in volume fraction of crystallites can be attributed to
enhancement in the nucleation rate with increase in hydrogen dilution of silane [22, 23].
4.2.2 (d): FTIR spectroscopic analysis
To investigate the silicon-hydrogen bonding configuration and to determine the hydrogen content
in the nc-Si:H films Fourier Transform Infra-Red (FTIR) spectroscopy was used. The FTIR
spectra of three typical nc-Si:H films deposited by PE-CVD at different hydrogen dilution of silane
(∆ H ) is shown in figure 4.5. For clarity the spectra have been offset vertically. As seen from the
spectra the film deposited at ∆ H = 100 have major absorption bands near 620 cm-1 and 1995
cm-1, which correspond to the wagging/stretching modes respectively of vibrations of monohydrogen (Si-H) bonded species [24, 25]. The absorption band at 700-900 cm-1 has been also
observed and can be assigned to the stretching/bending vibrational modes of di-hydride (Si-H 2 )
and (Si-H 2 ) n complexes (isolated or coupled) [26] having relatively lesser intensity. These results
indicate that at low ∆ H the hydrogen predominantly incorporated in the nc-Si:H films in Si-H
bonding configuration. As seen from the FTIR spectra with increase in ∆ H , the absorption of band
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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at 618 cm-1 decrease and at the same time the absorption of band at 700-900 cm-1 increases.
Furthermore, a significant change in the shape and intensity of absorption band at 1900-2250
cm-1 has been observed. With increasing ∆ H the intensity of absorption band at 1995 cm-1
increase and shifted further towards the higher wave number. Thus, the nc-Si:H film deposited at
∆ H = 300 showed a broad shoulder centred near 2100 cm-1. According to the literature the
absorption peak near 2100 cm-1 corresponds to stretching vibrational modes of Si-H 2 and (SiH 2 ) n species [27, 28].
300
Intensity (Arb. Unit)
Si-H/Si-H2/SiH3
200
Si-H2/(Si-H2)n
Si-H
∆H = 100
Si-H
400
600
800
1900
-1
Wave number (cm )
2000
2100
2200
2300
Figure 4.5: The FTIR spectra of three nc-Si:H films deposited by PE-CVD as a function of hydrogen dilution of
silane. The spectra are offset vertically for clarity.
10.0
Hydrogen content (at. %)
9.5
9.0
8.5
8.0
7.5
7.0
6.5
0
50
100
150
200
250
300
350
400
Hydrogen dilution of silane
Figure 4.6: Variation of hydrogen content as a function of hydrogen dilution of silane
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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These results indicates that with increasing R the hydrogen bonding in PE-CVD deposited ncSi:H films shift from Si-H to Si-H 2 bonded species and (Si-H 2 ) n complexes. Although, the
hydrogen bonding shift from Si-H to Si-H 2 bonded species and (Si-H 2 ) n complexes, the bonded
hydrogen content (C H ) in nc-Si:H films decreases with increase in hydrogen dilution of silane
(∆ H ). The variation of C H as a function of ∆ H is shown in figure 4.6. Decrease in C H is observable
from gradual reduction of 630 cm-1 band together with 2000 cm-1 band when hydrogen dilution is
increased. For intrinsic nc-Si:H films it is well documented that the hydrogen content generally
decreases with increase of hydrogen dilution [14, 29, 30].
4.2.2 (e): Low angle X-ray diffraction analysis
The X-ray spectra of nc-Si:H films deposited at different hydrogen dilution of silane (∆ H ) is shown
in figure 4.7. The only features observed for all films are strong peak occurring at 2θ ∼ 280 and
less intense peak occur at 2θ ∼ 450 corresponding to (111) and (220) crystalllographic
orientations. The dominant peak is (111), not (220), in contrast to other work [31]. This result
indicates that the crystallites in the film have preferential orientation in the (111) direction.
(111)
(220)
Intensity (Arbitary Unit)
300
200
∆ H =100
0
10
20
30
40
50
60
2θ
Figure 4.7: Low angle X-ray diffraction pattern of nc-Si:H films deposited at different hydrogen dilution of silane
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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The other noticeable change observed in the diffraction spectra are the broadening and
enhancement of intensity of (111) diffraction peak. The broadening of (111) diffraction peak with
increase in R indicates the decrease in crystallite size of Si nanocrystals in the film because the
crystallite size is inversely proportional to the FWHM of diffraction peak where as the
enhancement of intensity is related to increase in crystallite fraction. These results are consistent
with the Raman results and provides further strong support to the formation of nc-Si:H films.
However, the average crystallite size (d x-ray ) and crystalline fraction (X c ), deduced from low angle
diffraction patterns for the films deposited at different R turn out to be different from those
determined from the spectra (d x-ray = 19.57, 14. 58, 9.92 Å and X c = 57, 70, 78 % for ∆ H = 100,
200 and 300 respectively). The differences in the measured crystallite size and volume fraction
of crystallites using Raman spectroscopy and x-ray diffraction can be due to the different
detection sensitivity of characterization techniques. However, it is important to note that the
crystallite size and the volume fraction of crystallites determined by both techniques at various
hydrogen dilution of silane show same trend.
4.2.2 (f): UV-Visible spectroscopic analysis
The variation of band gap (E g ) as a function of hydrogen dilution of silane (∆ H ) for nc-Si:H films
prepared by PE-CVD is shown in figure 4.8. As seen from the figure, E Tauc of nc-Si:H films
increases from 2.13 eV to 2.25 eV as ∆ H increase from 100 to 300. It is interesting to note that
nc-Si:H thin films has a higher band gap than that of a-Si:H (< 2 eV) over the entire range of
hydrogen dilution of silane studied.
2.30
Band gap (eV)
2.25
2.20
2.15
2.10
Adinath M. Funde
0
50
100
150
200
250
300
Hydrogen dilution of silane
350
400
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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Figure 4.8: Variation of band gap of the films deposited by PE-CVD as a function of hydrogen dilution of silane
The band gap of PE-CVD deposited Si:H is mainly determined by the hydrogen content in the
films [32]. In fact, it increases with increase in hydrogen content in the films [33]. However, in the
present study we observed decrease in hydrogen content with increase in hydrogen dilution of
silane for the nc-Si:H films (figure 4.6). Thus, only the number of Si-H bonds or hydrogen content
in the film cannot account for band gap for PE-CVD deposited nc-Si:H films. The high band gap in
nc-Si:H thin films may be due to the quantum size effect. The decrease in crystallite size with
increase in hydrogen dilution of silane, as revealed from Raman scattering and x-ray diffraction
measurements further support this. The presence of nano-crystals lowers the absorption in the
film and shifts the transmission curve towards higher photon energy. This produces higher optical
band gap, which is estimated by extrapolation of absorption curve on the energy axis. Ali and
Hasegawa [34] have also observed the increase and high value of band gap with increasing
hydrogen flow rate for PE-CVD deposited nc-Si:H films. They have attributed the quantum size
effects for high value of band gap. However, there are still enormous difficulties in making the
quantum effect sufficiently clear and in using such novel nc-Si:H material for opto-electronic
applications due to lack of technology to control the size and position of crystallites.
4.2.2 (g): Before concluding.....
The structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H)
thin films have been systematically investigated as a function of hydrogen dilution of silane. The
Raman and low angle x-ray diffraction spectroscopy measurement results indicate that the
crystallite size in the films tends to decrease and at same time the volume fraction of crystallites
increases with increase in hydrogen dilution of silane. The FTIR studies indicate that at low
hydrogen dilution of silane the hydrogen predominantly incorporated in the nc-Si:H films in the
mono-hydrogen (Si-H) bonding configuration. However, with increasing hydrogen dilution of
silane the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (Si-H) to di-hydrogen (SiH 2 ) and (Si-H 2 ) n complexes. The hydrogen content in the nc-Si:H films was found to be less than
10 at. % and it decreases with increase in hydrogen dilution of silane. On the other hand, the
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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Tauc band gap remains as high as 2 eV or much higher over the entire range of hydrogen dilution
of silane studied. The high value of band gap may be due to the quantum size effect. For
optimized deposition conditions, we obtained nc-Si:H films with crystallite size ∼ 7.67 nm having
good degree of crystallinity (∼ 84 % ) and high band gap (2.25 eV) with a low hydrogen content
(6.5 at.%). However, for these optimized deposition conditions, the deposition rate was quite
small (1.6 Å/s).
4.3: STUDY OF INFLUENCE OF ARGON DILUTION OF SILANE
In preparation of nc-Si:H films by PE-CVD method, the dilution of the source gas, silane (SiH 4 )
with argon (Ar) gas have a strong influence on the structure and morphology of the films. Some
groups have previously studied the effect of Ar flow rate on nc-Si:H thin films by PE-CVD method.
Knights et al. [35] first studied the Si:H samples deposited from Ar diluted SiH 4 . They have
reported that due to polymerization within SiH 4 plasma the opto-electronic properties of the
samples were not good. It has been reported that addition of Ar in SiH 4 plasma introduces rapid
crystallization of a-Si:H network. However, extremely high Ar dilution adversely affects the
nanocrystallization process and induces the growth of columnar structures [36]. Das et al. [37]
suggested that Ar acts not only as a passive diluent gas but also plays an important role in the
growth of the amorphous or microcrystalline network. Wang et al. [38] have successfully grown aSi:H and μc-Si:H films with smooth morphology and stable structure by PE-CVD from high Ar and
H 2 diluted SiH 4 and high rf power. Ray et al. [39] have studied the structural properties of Si:H
films deposited by PE-CVD using silane-argon mixtures over a large range of power density.
They have observed a gradual transition from an amorphous to a microcrystalline material.
Recently, Chen et al. [40] have reported that the hydrogen annealing incorporated Ar gas is an
effective method to promote the performances of nc-Si:H thin films prepared by layer-by-layer
technology. Thus, up till now it is not clear the effect of Ar dilution of silane on nc-Si:H film
properties prepared by PE-CVD. Hence, more detail and careful investigations on the synthesis
and characterization of PE-CVD grown nc-Si:H films from Ar dilution of silane are essential. With
this motivation an attempt has been made to investigate the electrical, optical and structural
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
115
properties of nc-Si:H films deposited by conventional PE-CVD method as a function of Ar flow
rate.
4.3.1: Experimental
Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared in a PE-CVD system
(ANELVA Corporation, Japan) by glow discharge decomposition of (SiH 4 + H 2 + Ar) gas mixture.
The details of the deposition system are discussed in Chapter II. The films were deposited
simultaneously on corning #7059 glass to study optical and electrical properties and c-Si wafers
for structural properties. The flow rates of SiH 4 and H 2 were kept constant at 0.3 sccm and 90
sccm, respectively, and Ar flow rate was varied accurately between 5 and 50 sccm using mass
flow controller (Tylan General, USA). Other deposition parameters are listed in Table 4.2.
Table 4.2: Deposition parameters used for the synthesis of nc-Si:H films using Ar
Parameter
Value
RF power
200 Watt
Deposition pressure (P dep )
300 ± 5 mTorr
Substrate temperature (T sub )
200 ± 2 0C
Gas flow rates:
a) Silane (F SiH4 )
0.3 sccm
b) Hydrogen (F H2 )
90 sccm
c) Argon (F Ar )
0-50 sccm
Base pressure (P r )
6-8 x10-7 Torr
Distance between electrodes (d e-s )
40 mm
Substrate temperature (T Sub )
200 0C
Deposition time (t)
40 Min
4.3.2: Results and discussion
4.3.2 (a): Variation in deposition rate
Figure 4.9 shows the variation of deposition rate as a function of argon flow rate. As seen from
the figure, the deposition rate increases monotonically with increase in argon flow rate. It is
interesting to note that with increase in hydrogen dilution of silane the deposition rate
monotonically decreases (see figure 4.1) while with increase in argon flow rate/argon dilution of
silane the deposition rate increases.
3.0
2.5
ition rate (A/s)
Adinath M. Funde
2.0
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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Figure 4.9: Variation of deposition rate of nc-Si:H films prepared at various Ar flow rates
The increase in deposition rate with increase in argon flow rate may be attributed to the two
dynamic processes,
1) With increase in Ar flow rate, the bombardment of growing film surface by excited argon
molecules, Ar* and as well as Ar+ ions in SiH 4 plasma increases. The amount of energy
transferred to the growing matrix by these species will depend upon the equilibrium
concentrations of Ar* and Ar+ in the plasma and their interaction with the growth zone. The
reaction rate of Ar+ ions with SiH 4 molecules is lower than excited argon molecules Ar*
[41]. In fact bombardment of growing surface by Ar* in SiH 4 plasma plays an important
role since its lifetime is order of seconds (> 1.3 s) [42]. Moreover, Ar* will play a vital role
in the process of dissociation of SiH 4 [43]. These Ar* molecules reacts with SiH 4
molecules to form SiH 3 , SiH 2 etc. [44], which further contribute to the film growth. As a
result, the deposition rate increases with increase in Ar flow rate.
2) With increase in Ar flow rate the H 2 coverage of the growth surface is reduced. This
reduces the etching rate of the growing surface. Consequently, the deposition rate
increases with increasing Ar flow rate.
The increase in deposition rate with increase in argon flow rate or silane to argon flow ratio for μcSi:H/nc-Si:H films deposited by PE-CVD has been reported earlier [15, 41].
4.3.2 (b): Raman spectroscopic analysis
Raman spectra of nc-Si:H films normalized to thickness deposited by PE-CVD at various Ar flow
rate (F Ar ) is shown in figure 4.10. The corresponding crystalline fraction (X Raman ) and crystallite
size (d Raman ) are also indicated in the figure. The film deposited at F Ar = 0 sccm has a shoulder
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
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centered ~ 502 cm-1, associated with the amorphous and defective part of crystalline phase [19]
and other centered ~ 520 cm-1 attributed to silicon crystallites of different sizes in the film [45]. For
this film, X Raman is ~ 84 % and d Raman is ~ 7.7 nm. For the film deposited at F Ar = 15 sccm, the
peak corresponding to amorphous and defective nanocrystalline phases disappears and X Raman
in the film enhances (~ 94 %).
Furthermore, with increasing F Ar the intensity of the band
centered ~ 520 cm-1 increased and shifted to higher wave number. The shift of the peak towards
higher wave number is related to increase in X Raman while increase in intensity and sharpness are
related to the decrease in d Raman in the film [21].
521 cm-1
dRaman
Intensity (Arbitary Unit)
FAr
XRaman
50
7.2 nm
98 %
25
7.2 nm
96 %
15
7.7 nm
94 %
520 cm-1
502 cm-1
7.7 nm
0
400
425
450
475
84 %
500
525
550
575
600
Raman Shift (cm )
-1
Figure 4.10: Raman spectra of some nc-Si:H films deposited at various argon flow rate
Thus, the film deposited at F Ar = 50 sccm have X c ~ 98 % and d Raman ~ 7.2 nm. These results
indicate that addition of Ar into the SiH 4 -H 2 plasma endorses the growth of crystallinity in the film,
which is very important to promote the nc-Si:H films for commercial applications. The
disappearance of 502 cm-1 peak also supports this inference. The increase in crystallinity with
increase in F Ar can be attributed to increase in density of Ar* as well as by Ar+ ions. In SiH 4 -H 2
plasma containing Ar, Ar* and Ar+ play an active role in dissociation of SiH 4 and of H 2 . Hydrogen
radical density can increase via reactions [44, 46]
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
118
SiH 4 + Ar*
SiH 2 + 2H + Ar and
H 2 + Ar+
ArH+ + H
These hydrogen radicals break weak Si-Si bonds involved in Si-H network and replaced by new
film precursors to form a rigid and strong Si-Si bond in crystalline mode [47]. Furthermore, the
energy released by de-excitation of Ar* leads to structural relaxation and reorientation [37] which
thereby promotes the crystallization. The appearance of the band ~ 508 cm-1 in FTIR spectra,
corresponds to the Si-Si absorption [48], further supports this.
4.3.2 (c): FTIR spectroscopic analysis
Figure 4.11 depicts the FTIR spectra of the nc-Si:H films, normalized to thickness deposited at
different Ar flow rates (F Ar ). The FTIR spectra of the film prepared without Ar are also included in
the figure for comparison. For clarity the spectra are shifted vertically.
2113 cm-1
618 cm-1
50
700-900 cm-1
Intensity (Arb. Unit)
503 cm-1
25
2074 cm-1
Si-Si
2041 cm-1
Si-H2/(Si-H2)n
Si-H
0
FAr (sccm)
Si-H
400
600
15
800
1900
-1
2000
2100
2200
2300
Wave number (cm )
Figure 4.11: FTIR spectra of the nc-Si:H films (normalized to thickness) deposited at different Ar flow rates
The film deposited at F Ar = 0 sccm have major absorption bands ~ 618 cm-1 and ~ 2000 cm-1,
which correspond to wagging and stretching modes respectively of vibrations of mono-hydride
(Si-H) bonded species [24]. The absorption band in between 700 and 900 cm-1 has been also
observed and can be assigned to stretching/bending vibrational modes of di-hydride (Si-H 2 ) and
(Si-H 2 ) n complexes (isolated or coupled) [26] having comparatively lesser intensity. Thus, Si-H
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
119
bonding configuration is dominant for the film deposited at F Ar = 0 sccm. For nc-Si:H films
prepared at various F Ar , the Si-H wagging band is seen to decrease in absorption. Moreover,
significant changes in shape and intensity of absorption band ~ 2000 cm-1 have been observed.
With increasing F Ar the intensity of absorption band ~ 2000 cm-1 increased and shifted further
towards the higher wave number. Thus, the nc-Si:H film deposited at F Ar = 50 sccm show an
additional broad shoulder centered ~ 2110 cm-1. According to the literature the absorption peak ~
2110 cm-1 corresponds to stretching vibrational modes of Si-H 2 and (Si-H 2 ) n species [40, 45].
These results clearly indicate that with addition of Ar in SiH 4 -H 2 plasma the predominant
hydrogen bonding in the films changed from Si-H group to Si-H 2 and (Si-H 2 ) n complexes.
10
Hydrogen content (at. %)
9
8
Zero point
7
6
5
4
-10
0
10
20
30
40
50
60
Argon flow rate (sccm)
Figure 4.12: Variation in hydrogen content as a function of Ar flow rate for nc-Si:H films
Figure 4.12 shows the variation in hydrogen content (C H ) as a function of Ar flow rate (F Ar ). The
zero point indicates the value of hydrogen content for the films deposited with no Ar addition. As
can be seen from the figure, hydrogen content for the film deposited without Ar in SiH 4 -H 2
plasma is ∼ 10 at. % while it decreases from 6.98 at % to 4.52 at. % as Ar flow rate increases
from 15 sccm to 50 sccm. The decrease in hydrogen content may be attributed to increase in
volume fraction of crystallites with increase in Ar flow rate.
4.3.2 (d): Variation of band gap
The effect of addition of Ar in SiH 4 -H 2 plasma on the band gap (E g ) is depicted in figure 4.13.
The zero point indicates the value band gap for the films deposited with no Ar addition. As can be
seen from the figure, no significant change has been observed in the band gap with increasing Ar
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
120
flow rate. The value of band gap was found > 2 eV over the entire range of argon flow rate
studied. This result indicates that nc-Si:H films have higher band gap than that of a-Si:H. We think
that decrease in crystallite size (as revealed by Raman spectroscopy) with increase in Ar flow
rate may be responsible for the higher band gap in nc-Si:H films. The decrease in crystallite size
could increase the average Si-Si distance. This lowers the absorption in the film and shifts the
transmission curve towards higher photon energy. This produces higher band gap, which is
estimated by extrapolation of absorption curve on the energy axis.
2.45
Band Gap (eV)
2.40
2.35
Zero point
2.30
2.25
2.20
-10
0
10
20
30
40
50
60
Argon flow rate (sccm)
Figure 4.13: Variation in band as a function of Ar flow rate for nc-Si:H films
However, more detailed studies are still required for the understanding of higher band gap of PECVD deposited nc-Si:H films.
4.3.2 (e): Variation in dark conductivity
Variation of dark conductivity (σ Dark ) and photoconductivity (σ Photo ) for nc-Si:H films deposited by
PE-CVD at various Ar flow rates (F Ar ) is shown in figure 4.14. The zero point indicates the values
of conductivities for the films deposited with no Ar addition.
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
121
Figure 4.14: Variation of dark conductivity and photoconductivity for nc-Si:H films deposited by PE-CVD at
various Ar flow rates
As seen from the figure values of dark conductivity are in the range 10-5-10-7 S/cm whereas
values of photoconductivity are found in the range 10-4-10-7 S/cm over the entire range of Ar flow
rate studied. In fact both conductivities decreases with increase in Ar flow rate. The decrease in
dark conductivity can be attributed to the change in hydrogen bonding configuration in the film
from mono-hydrogen (Si-H) to di-hydrogen (Si-H 2 ) and poly-hydrogen (Si-H 2 ) n complexes with
increasing Ar flow rate as revealed from the FTIR spectroscopy. These values are consistent with
those reported previously for nc-Si:H films grown under different plasma conditions [49, 50]. The
shift of hydrogen bonding from Si-H to Si-H 2 and (Si-H 2 ) n complexes increases the number of
dangling bonds and hence trapping density of charge carrier in the film. As a result, dark
conductivity decreases with increasing Ar flow rate. Thus, the film deposited at no argon flow rate
shows a small photosensitivity which can attribute to the presence of small amorphous
component in it.
4.3.2 (f): Before concluding.....
Undoped nc-Si:H films have been prepared by conventional PE-CVD at 200 0C on glass
substrates. The film properties have been systematically studied as a function of Ar flow rate.
Raman spectroscopy studies showed that addition of Ar into SiH 4 -H 2 plasma increases the
growth of crystallinity in the films whereas the FTIR spectroscopic analysis showed that the
hydrogen bonding in nc-Si:H films shifts from Si-H to Si-H 2 and (Si-H 2 ) n complexes with
increasing Ar flow rate. The band gap of nc-Si:H films is found higher than that of a-Si:H films.
The nc-Si:H films with dark conductivity 1.3x10-7 S/cm having deposition rate as high as 2.5 Å/s
with crystalline fraction 98 % have been obtained.
4.4: STUDY OF INFLUENCE OF HELIUM DILUTION OF SILANE
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
122
Aiming the nanocrystalline nature of the film material deposited in PE-CVD, which has shown
greater stability of the material against the light induced degradation, high H 2 dilution is one of the
well studied parameters facilitating the growth of nanocrystalline network. However, high
hydrogen dilution retards the film deposition rate. Several other approaches also have been
attempted to increase the deposition rate by using high density plasma sources e. g. microwave,
very high frequency, and electron–cyclotron resonance plasmas, with their individual merits and
demerits [51]. Noble gases (e. g. He, Ar, Xe) dilution to SiH 4 in conventional 13.56 MHz PE-CVD
system is another way of producing high density plasma to increase the deposition rate and to
induce crystallinity in the resulting films [52]. This has been anticipated and observed because of
the different ionization potential, collision cross sections, ion energy of the dilution gas compared
to silane. However, it has been observed that high Ar dilution leads to columnar growth and
mostly produce defective network [53, 54], while Xe dilution maintains an amorphous nature of
the network even at a very high RF power applied to the plasma [55]. Some reports in the
literatures quote that though He dilution increases the growth rate, it does not produce
nanocrystalline silicon network under certain experimental conditions [56]. On the other hand
Bhattacharya et al. [51] also studied the effect of helium dilution of silane without hydrogen in
synthesis of nc-Si:H thin films. They varied the electrical power applied to the electrodes in the
reactor from 40-120 W. They have reported that to get higher deposition and induce crystallinity
in the material using helium dilution, application of higher power is necessary. In an independent
study, Gutierrez et al. [57] also had prepared a-Si:H thin films by PE-CVD using helium dilution
and reported a noticeable improvement of optoelectronic properties. It was observed from FTIR
spectroscopic analysis that by helium the absorption peak shifts from 2000 cm-1 to 2100 cm-1.
The bandgap as high as 2.13 eV and conductivity of 2.3 x 10-3 S/cm was reported in the same
studies. In another studies, Saadane et al. [58] have shown that the use of helium dilution leads
to higher deposition rate and also leads to a lower hydrogen content in the a-Si:H films as
compared to hydrogen dilution. They have also observed that higher deposition rate in the films
from the He-dilution films does not result in degradation of the film’s electronic and transport
properties, which are strongly improved with respect to those of standard amorphous silicon.
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
123
With the intention of development of nanocrystalline silicon network at high growth rate from
purely helium diluted silane plasma, present study of helium dilution was carried out in two steps:
Firstly, the effect of helium dilution of silane without hydrogen dilution and secondly the effect of
helium dilution in addition to hydrogen dilution at higher deposition pressure.
4.4.1: Experimental
Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared in a PE-CVD system
(ANELVA Corporation, Japan) by glow discharge decomposition of (SiH 4 + He) gas mixture. The
details of the deposition system are discussed in Chapter II. The films were deposited
simultaneously on corning #7059 glass to study optical and electrical properties and c-Si wafers
for structural properties. In the present study, the flow rate of silane was kept constant at 0.5
sccm, whereas helium flow rate was varied between 15 sccm and 75. Other deposition
parameters are listed in Table 4.3.
Table 4.3: Deposition parameters used for the synthesis of nc-Si:H films using He
Parameter
Value
RF power
200 Watt
Deposition pressure (P dep )
300 ± 5 mTorr
Substrate temperature (T sub )
200 ± 2 0C
Gas flow rates:
a) Silane (F SiH4 )
0.5 sccm
b) Hydrogen (F H2 )
------
c) Helium (F He )
15-75 sccm
Base pressure (P r )
6-8 x10-7 Torr
Distance between electrodes (d e-s )
20 mm
Deposition time (t)
40 Min
Films were deposited for desired amount of time and then allowed to cool to room temperature in
vacuum. Then films were taken out for the characterization.
4.4.2: Results and discussion
4.4.2(a): Raman spectroscopy and low angle x-ray diffraction analysis
Raman spectra of the films deposited at various values of helium flow rate (F He ) at a constant
silane flow rate of 0.5 sccm are depicted in the figure 4.15. The Raman spectrum of c-Si is also
included in the figure for the comparison.
FHe
. units)
Adinath M. Funde
75 sccm
50 sccm
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
124
Figure 4.15: Raman spectra of samples deposited at different helium flow rates.
As seen from the figure, the films deposited at F He = 15 sccm is completely amorphous with
appearance of a broad shoulder ~ 480 cm-1. On increasing the helium flow rate from 25 sccm to
75 sccm in steps of 25 sccm, a systematic evolution of intensity of the peak near 520 cm-1,
attributed to crystalline counterparts has been observed suggesting the structural changes in the
deposited material. The detail analysis of each spectrum is shown in Table 4.4. It is observed that
the crystalline volume fraction is increased from 18.3 % to 40.6 %, whereas, the crystallite size
remained below 8 nm as estimated from Raman measurements.
Table 4.4: Analysis of the Raman spectroscopic data of helium dilution in pure silane plasma
F He (sccm)
Peak parameters
Position (cm-1)
Area
FWHM (cm-1)
15
480.00
47.61
60.66
25
480.00
48.31
62.95
502.40
05.51
27.95
519.30
05.28
13.81
480.00
48.39
64.07
501.70
4.84
23.59
518.10
09.30
14.92
480.00
25.82
64.05
50
75
Adinath M. Funde
X Raman (%)
d Raman (nm)
-----
-----
18.3
7.43
19.4
5.53
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
c-Si
125
501.50
7.29
29.15
518.60
10.36
15.28
520.70
4.60
02.94
40.6
6.15
100.0
-----
The low angle x-ray diffraction (XRD) pattern for the films deposited at various values of helium
flow rate (F He ) at a constant silane flow rate of 0.5 sccm are depicted in the figure 4.16. The XRD
patterns were recorded at 10 angle of incidence. The XRD patterns shows the complimentary
results to those obtained from Raman spectroscopic analysis. A broad hump at ~ 250 is observed
in each spectrum, which is due to the corning glass substrate. At helium flow rate of 15 sccm, no
peak is observed except the broad hump due to substrate, confirming the amorphous form of the
deposited material.
(111)
Intensity (arb. units)
FHe (sccm)
75
(220)
(311)
50
25
15
10
20
30
40
50
60
Diffraction angle (2θ)
Figure 4.16: Low angle XRD patterns of the films deposited at helium flow rates depicted near the spectra.
For the films deposited with increasing F He from 25 to 75 sccm, the spectra shows appearance of
XRD peaks at ~ 28.40 due to (111) crystallographic orientation. Apart from this a small peaks at ~
47.50 and ~ 56.10 are also seen with close observation of spectra deposited at F He = 75 sccm.
The XRD patterns only provide the qualitative supporting results to those obtained from Raman
spectroscopy since it was not possible to extract any quantitative data.
4.4.2(b): UV-VIS-NIR spectroscopic analysis
The optical absorption coefficient (α) spectra of the films deposited with different helium flow rates
(F He ) have been plotted in figure 4.17.
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
126
Absorption coefficient, α (cm -1)
1.8x105
15 sccm
25 sccm
50 sccm
75 sccm
1.6x105
1.4x105
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
2.0x104
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
Photon energy (eV)
Figure 4.17: Absorption coefficient as a function of photon energy for the films deposited at different He flow rates
As observable from the spectra, the film prepared with F He = 15 sccm has shown the highest
optical absorption; whereas, on increasing the F He the intensity of optical absorption coefficient
spectra gradually decreases. This kind of observation was also noted by other researchers [59].
The bandgap values of the material were extracted using Tauc’s plots. The plot of band gap
versus helium flow rate is shown in figure 4.18(a). The bandgap values of the material increases
from ~1.76 eV to 1.82 eV when helium flow rate increased from 15 sccm to 50 sccm and remains
constant with further increase in the helium flow rate to 75 sccm as shown in the plot.
1.84
3.6
3.4
Refractive index
Band gap (eV)
1.82
1.80
1.78
1.76
1.74
10
Curve Fitting Data for HHe = 75 sccm sample
Equation: y = a + b*x^c R^2 = 0.96115
a:
2.97389 ± 0.01509
b:
138002170.10149 ± 12407911.20795
c:
-3 ± 0
3.5
3.3
3.2
3.1
3.0
2.9
20
30
40
50
60
Helium flow rate (sccm)
70
80
400
800
1200
1600
2000
2400
Wavelength (nm)
Figure 4.18: Tauc’s band-gap of the films versus helium flow rate and Variation of refractive index with
wavelength for sample deposited at F He = 75 sccm estimated from UV-VIS-NIR spectroscopy
Table 4.5: Static refractive index values of helium dilution in pure silane plasma set
Helium flow rate (sccm)
Adinath M. Funde
Static refractive index
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
127
15
3.17
25
3.08
50
3.01
75
2.97
The static refractive index values of the material deposited were also extracted by fitting the
numerical values of the refractive index. Table 4.5 shows the values of static refractive index for
all values of helium dilution. The values of static refractive index decreases from 3.17 to 2.97 for
the films deposited with helium flow rate of 15 sccm to 75 sccm.
4.4.2(c): FTIR spectroscopic analysis
Figure 4.19 shows the FTIR spectra of the films deposited at different helium flow rates (F He ).
The results of FTIR spectroscopy of nc-Si:H films revealed peculiar hydrogen bonding
configuration. It can been observed from the figure that the stretching mode at ~ 2000 cm-1, which
is associated with the mono-hydrogen (Si-H) bonds shift towards 2100 cm-1 which is associated
with di-hydrogen (Si-H 2 ) and poly-hydrogen (Si-H 2 ) n complexes. This shift in absorption band to
2100
cm-1 is attributed to saturation/passivation of multiple dangling bonds at the grain
boundaries [60]. Bonded hydrogen content in the films was calculated from wagging mode of 630
cm-1. The values of the bonded hydrogen content in the films are shown in the figure 4.20, which
decrease from ~ 18 at. % to ~ 10 at. %. The film growth rate is also included in figure 4.20. The
growth rate increases from 1.5 Å/s to 2 Å/s in the range of F He studied.
Si-H/Si-H2/Si-H3
75
Intensity (Arb. Units)
2161 cm-1
882 cm-1
Si-H2/(Si-H2)n
400
25
630 cm-1
Si-H
Si-H2
2019 cm-1
Adinath M. Funde
50
600
800
1000
1800
15
FHe(sccm)
2000
2200
2400 (November 2010)
Ph. D. Thesis,
Pune University
-1
Wave number (cm )
Chapter IV
128
Figure 4.19: Fourier transform infrared spectra of films deposited at various helium flow rates
The degree of ionization of SiH 4 radicals increases strongly and the ionic composition changed
with noble gas dilution, in general. The higher deposition rate in the case of helium dilution has
been attributed to the enhancement of the secondary electron emission coefficient induced by He
ions [58, 61]. Formation of nanocrystals in the amorphous network requires a certain amount of
energy called Gibb’s free energy for crystallization. With helium as a diluent gas a large amount
of energy happens to transfer to the growth zone by the bombardment of ionized He+ (24 eV) and
metastable He∗ (20 eV) from the plasma. The metastable He∗ plays a vital role in the process of
dissociation of SiH 4 . In addition to the regular process of electron impact dissociation of SiH 4 ,
metastable state energy transfer from the noble gases would lead directly to increased
dissociation and, thereby, contribute to the high density plasma and result in an increased
deposition rate when SiH 4 is diluted with noble gases [62]. Some of the He∗ react with SiH 4
molecules to form radicals such as SiH 3 and SiH 2 . The formation energy of He∗ is less than that
of ionized He+, so that the effect of the bombardment of He∗ is more important than that of He+.
Ionized He+ may have sufficient energy to cause sputtering from the growing surface, which
would result in a rough surface. The growth of successive layers on such a surface will result in a
void rich film.
2.2
20
18
Growth rate (Å/s)
16
1.8
14
12
1.6
10
1.4
10
20
30
40
50
60
Helium flow rate (sccm)
70
Hydrogen content (at.%)
2.0
8
80
Figure 4.20: Effect of helium flow on film growth rate and hydrogen content in the films
On the other hand, excited atoms of He∗ come into contact with the growing film surface only at
thermal velocities which are much less than the impingement velocity of He+ ions. The amount of
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
129
energy transferred to lattice will be the energy released on de-excitation of He∗ to its ground state.
In comparison with energy transferred by He+ this process is much softer. First energy released
by He∗ breaks up weak Si-Si bonds. A portion of this energy may also be utilized in releasing
loosely bonded hydrogen from SiH 2 sites. In the next step, the dangling bonds resulting from the
breaking of weak Si-Si bonds, form strong bonds with silicon or are terminated by hydrogen.
Energy of de-excitation of He∗ atoms will relax the strain at the boundary of nc-Si nucleation
centres and amorphous matrix by replacing weak Si–Si bonds with stronger as reported by
Bhattacharya et.al. [59]. Thus, nano crystallization is initiated in the network at helium flow rate 25
sccm and above, aided by helium in plasma with its atomic, ionic and metastable species.
4.4.2(d): Dark conductivity, photoconductivity and photosensitivity
Figure 4.21 represents dark conductivity (σDark ) and photoconductivity (σ Photo ) of the samples
estimated from the two probe method in co-planar geometry at different helium flow rates (F He ).
The dark conductivity increases from 10-9 S/cm to 10-5 S/cm when F He increases from 15 sccm to
75 sccm, whereas the photoconductivity remains constant around 10-5 S/cm. Thus, the
photosensitivity gain (defined as (σ Photo /σ Dark ) decreases from 105 to 101, clearly indicating the
changes in the material properties. This change can be directly related to structural changes as
revealed by the Raman and Low-angle XRD analysis.
Figure 4.21: Influence of helium flow rate on dark and photo-conductivity of the samples
4.3.2.(e): Before concluding.....
Helium dilution is an alternative to the conventional hydrogen dilution to synthesize hydrogenated
nanocrystalline silicon for application in solar cell. With increase in the helium dilution of silane the
Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
130
deposition rate increases. Raman spectroscopic analysis observed that the crystalline volume
fraction increases with increase in helium dilution of silane whereas the crystallite size remained
below 8 nm over the entire range of helium dilution of silane studied. The bonded hydrogen
content in the films remained below 18 at.% and shown decreasing trend with increase in helium
flow rate. With increase in helium dilution of silane, deposition rate increases and maximum
deposition rate was observed up to 2 Å/s. Measurement of dark conductivity and
photoconductivity clearly indicates structural changes occur in the film with increase in helium
dilution of silane.
4.5: CONCLUSIONS
The structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H)
thin films have been systematically investigated as a function of hydrogen (H 2 ), argon (Ar) and
helium (He) dilution of silane. The deposition rate increases with increase in argon and helium
dilution of silane whereas it decreases with increase in hydrogen dilution of silane. The Raman
and low angle x-ray diffraction spectroscopy measurement results indicate that for all dilutions of
silane, the volume fraction of crystallites in the film increases whereas the crystallite size shows
decreasing trend except for the films deposited at helium dilution of silane. However, the
crystallite size is found less than 8 nm over the entire range of helium dilution studied. The FTIR
studies indicate that for all dilutions the hydrogen mainly incorporated in the nc-Si:H films in dihydrogen (Si-H 2 ) and poly-hydrogen (Si-H 2 ) n complexes. The hydrogen content in the nc-Si:H
films was found to be less than 10 at. % for hydrogen and argon diluted whereas it is much higher
(18 at. %) for helium diluted films. The bandgap values of the material increases from ~1.76 eV to
1.82 eV when helium flow rate increased from 15 sccm to 50 sccm and remains constant with
further increase in the helium flow rate to 75 sccm as shown in the plot. The Tauc band gap
remains as high as 2 eV or much higher for hydrogen and argon diluted films but it remains as
high as 1.82 eV for helium diluted films.

Adinath M. Funde
Ph. D. Thesis, Pune University (November 2010)
Chapter IV
131
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Ph. D. Thesis, Pune University (November 2010)