Low-Temperature Deposition of nc-SiOx:H below

CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 046802
Low-Temperature Deposition of nc-SiO𝑥 :H below 400∘C Using Magnetron
Sputtering *
LI Yun(李云), YIN Chen-Chen(尹辰辰), JI Yun(季云), SHI Zhen-Liang(史振亮), JIN Cong-Hui(靳聪慧),
YU Wei(于威)** , LI Xiao-Wei(李晓苇)**
Hebei Key Laboratory of Optic-Electronic Information Materials, College of Physics Science and Technology,
Hebei University, Baoding 071002
(Received 12 October 2014)
Silicon oxide films containing nanocrystalline silicon (nc-SiO𝑥 :H) are deposited by co-sputtering technology at
low temperatures (<400∘C) that are much lower than the typical growth temperature of nc-Si in SiO2 . The
microstructures and bonding properties are characterized by Raman and FTIR. It is proven that an optimum
range of substrate temperatures for the deposition of nc-SiO𝑥 :H films is 200–400∘C, in which the ratio of transition
crystalline silicon decreases, the crystalline fraction is higher, and the hydrogen content is lower. The underlying
mechanism is explained by a competitive process between nc-Si Wolmer–Weber growth and oxidation reaction,
both of which achieve a balance in the range of 200–400∘C. We further implement this technique in the fabrication
of multilayered nc-SiO𝑥 :H/a-SiO𝑥 :H films, which exhibit controllable nc-Si sizes with high crystallization quality.
PACS: 68.55.−a, 61.82.Rx
DOI: 10.1088/0256-307X/32/4/046802
Silicon oxide films containing nanocrystalline silicon (nc-SiO𝑥 :H) exhibit great potential applications in
the fields of full-color display and photovoltaic devices
due to their unique optoelectronic properties.[1−3] Furthermore, they also attract significant interest in thin
film solar cells due to their wavelength-tunable absorption of the sunlight through the controlling of ncSi sizes.[4] Normally, precipitation of nc-Si in thin film
requires thermal annealing of a Si-rich oxide layer at
a temperature of around 1100∘C or above, due to the
lower diffusion coefficient of silicon atoms in SiO2 .[5,6]
However, such a high annealing temperature would
bring about dopant redistribution, uneven strain distribution and high defect density in thin films. Moreover, the annealing temperature is too high to directly
deposit nc-SiO𝑥 :H films on organic material or glass
substrates, thus the processes are incompatible with
solar cell thin film technology. Therefore, it is of great
significance to prepare nc-SiO𝑥 :H films at a low temperature.
Recently, low temperature deposition technology
of nc-SiO𝑥 :H thin films has been widely applied as
a p-, n- or intrinsic layer in the silicon thin film solar cells.[7−10] However, the growth mechanism of ncSiO𝑥 :H films usually involves a gas phase and a surface oxidation reaction, which is significantly different from that of nc-Si:H films. Furthermore, the ncSi particle growth is also affected by the oxidation
reaction,[11] and the accelerated hydrogen atoms and
ions can increase the spinodal decomposition rate of
SiO𝑥 in the film.[12] Therefore, the growth mechanism
of nc-SiO𝑥 :H films at a low temperature should be
discussed in detail to control their structures.
In this Letter, single-layer nc-SiO𝑥 :H films were
real-time deposited at a low temperature on quartz
and p-type c-Si(100) wafer substrates by an rf
(13.56 MHz) magnetron co-sputtering system. The
power densities of Si and SiO2 targets were 0.9 W/cm2
and 0.8 W/cm2 , respectively. The flow rates of Ar and
H2 were 8 sccm and 32 sccm respectively. The working pressure was maintained at 4 Pa and the deposition
time was 60 min. The substrate temperatures were set
at 90, 150, 200, 250, 300, 350, 400 and 500∘C, respectively. The microstructures and bonding properties
of the films were investigated by Raman and FTIR
spectra. As the substrate temperature changed, the
microstructure of the films could be adjusted through
the competition between nc-Si growth and the oxidation process, which could suggest an optimum range
of deposition temperature for the nc-SiO𝑥 :H films. In
addition, we implemented this low temperature technique to fabricate the nc-SiO𝑥 :H/a-SiO𝑥 :H multilayer
structure with size-controlled nc-Si particles.
Figure 1 gives the deposition rates of the ncSiO𝑥 :H films at different substrate temperatures. The
results reflect that the migration rate of Si and O
atoms on the growth surface increases due to the rise
in temperature, thus making the film dense. At the
same time, the rate of the desorption reaction increases on the growth surface, which also leads to the
decrease of the deposition rates of the films.
Figure 2 presents the Raman spectra of the sin-
* Supported by the Key Basic Research Project of Hebei Province under Grant No 12963930D, the Natural Science Foundation
of Hebei Province under Grant No F2013201250, and the Science and Technology Research Projects of the Educational Department
of Hebei Province under Grant No ZH2012030.
** Corresponding author. Email: [email protected]; [email protected]
© 2015 Chinese Physical Society and IOP Publishing Ltd
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CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 046802
gle nc-SiO𝑥 :H layer on quartz substrates at different
temperatures. The experimental results show that all
spectra are comprised of amorphous and crystalline
components scattering, which suggests that nc-SiO𝑥 :H
films are successfully prepared at low temperature by
magnetron sputtering. We analyze the TO mode of
the Raman spectra by a Gaussian–Lorenz function
and give the fitted curves with three peaks (inset in
Fig. 2): 𝐼a (480 cm−1 ) for the amorphous fraction, 𝐼b
(510 cm−1 ) for the transition crystalline (small nc-Si
particles) or grain boundaries,[13,14] and 𝐼c (520 cm−1 )
for the crystalline fraction. The fraction 𝑋𝑖 can be calculated from 𝑋𝑖 = 𝐼𝑖 /(𝐼a + 𝐼b + 𝐼c ) (𝑖=a, b, c), where
𝐼𝑖 is the integrated intensity of every peak. The calculated results of 𝑋𝑖 are shown in Fig. 3.
silicon ratio increase with the high temperature while
the crystalline ratio decreases.
The average size of nc-Si particles (𝐿) has been estimated from the Raman spectra using the following
equation[15]
1
[𝜔𝐿 − 𝜔0 ]2 + (Γ0 /2)2 ∼
· exp(−𝜋 2 ),
=
3𝐿
(1)
where 𝜔𝐿 is the frequency of the crystalline-like mode
for nc-Si particles size 𝐿. The values of 𝜔0 and Γ0 are
520 cm−1 and 3.5 cm−1 , respectively, for crystalline
silicon. The crystalline fraction 𝐹c is calculated by
the equation 𝐹c = (𝐼c + 𝐼b )/(𝐼a + 𝐼b + 𝐼c ).
On c-Si(100)
On quartz
10
(nm/min)
(arb. units)
0.6
9
a
b
c
0.4
II
I
0.2
8
100
7
200
300
(
100
200
300
T (C)
400
500
Fig. 1. Deposition rates from the single nc-SiO𝑥 :H layers
grown at different substrate temperatures.
III
400
C)
500
Fig. 3. Plots of amorphous (𝑋a ), transition crystalline
(𝑋b ), and crystalline (𝑋c ) in the nc-SiO𝑥 :H films versus
the substrate temperature.
14
nc-Si
10
100
550
-1
(cm
TA
)
LO
LA
200
300
400
500
CC
CC
CC
CC
90
150
200
250
300
350
400
500
8
40
9
0.44
8
7
6
C)
100 200 300 400 500
(
20
0.40
6
0
20
30
2
4
600
100
200
300
(
-1
(cm
60
(nm)
500
XRD
450
(nm)
TO
(arb. units)
0.48
c
c
12
a-Si
Raman
(arb. units)
unc-Si
)
C)
40
(deg)
400
50
60
0.36
500
Fig. 2. Raman spectra from the single nc-SiO𝑥 :H layers. The inset presents the fitted curves of the TO mode
and deconvolution by a Gaussian–Lorentz function for the
sample at 90∘C.
Fig. 4. Evolution of nc-Si particle size (𝐿) and crystalline
fraction (𝐹c ) as a function of the substrate temperature.
The inset presents the average size of nc-Si particles calculated by XRD data.
The curves (in Fig. 3) can be divided into three
zones: in region I (𝑡 < 200∘C), with the increase of
temperature, the crystalline and transition crystalline
silicon ratios increase gradually, meanwhile the amorphous silicon ratio decreases; when the temperature
comes to region II (200∘C < 𝑡 < 400∘C), the crystalline
ratio continues to increase, while the transition crystalline silicon ratio decreases; in region III (𝑡 > 400∘C),
the transition crystalline silicon ratio and amorphous
Figure 4 gives 𝐿 and 𝐹c versus the substrate temperatures. Here 𝐹c keeps increasing at the low temperature stage until the substrate temperature comes
up to 300∘C. After that, the oxidation reaction inhibits the growth of nc-Si, leading to the decrease of
𝐹c . Different from 𝐹c , the grain size of the nc-Si keeps
increasing with the temperature. The inset presents
the average size of nc-Si calculated by XRD data.[12]
Although the results may not be quite the same due
to different calculation methods, the change rule of
046802-2
CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 046802
O
15
10
I
II
III
Si-H
100 200 300 400 500
(
C)
C
C
C
300C
250C
200C
150C
90C
500
400
350
500
750
1000
1250 2000
(cm
3000
𝑋Si−rich = (𝐼980 + 𝐼1012 )/𝐼,
𝑋O−rich = (𝐼1034 + 𝐼1076 )/𝐼,
(𝐼 = 𝐼980 + 𝐼1012 + 𝐼1034 + 𝐼1076 ).
Based on the Raman analysis, the curve can also be divided into three zones: in region I (𝑡 < 200∘C), along
with the rise of the substrate temperature, 𝑋Si−rich
increases due to the higher growth rate of nc-Si; in
region II (200∘C < 𝑡 < 400∘C), the 𝑋O−rich gradually
increases, and the value of 𝑋Si−rich /𝑋O−rich is approximately equal to one, which reflects that the oxidation
reaction rate is gradually strengthened, and it is equal
to the growth rate of nc-Si; in region III, 𝑋Si−rich increases again due to oxidation of the nc-Si particles
boundary. A higher substrate temperature leads to
the release hydrogen reaction rate increasing, thus the
promoting effect of active hydrogen is reduced, while
the hydrogen etching effect is to enhance.
4000
-1
)
Si -rich/
To explain the competition mechanism, Fig. 5
shows the FTIR spectra of nc-SiO𝑥 :H films on p-type
c-Si(100) substrates at different temperatures.
We identify all the absorption peaks corresponding
to the Si–H, Si–O vibration modes in the spectra,[16,17]
and obtain the hydrogen content (𝐶H ) by the Si–H
absorption peak near 630 cm−1 and the oxygen content (𝐶O ) by the Si–O absorption peak near 900–
1300 cm−1 .[18] The inset of Fig. 5 shows that 𝐶O increases gradually with the rising substrate temperature, indicating that the oxidation reaction is enhanced during the film deposition, while 𝐶H is presented in three stages, and it is lower in the optimum
temperature range (200–400∘C) from the Raman data.
We process the Si–O stretching mode absorption
peak around 900–1300 cm−1 by multi-peak Gaussian
fitting. We rule out the peaks around 1100 cm−1 and
1150 cm−1 related to the surface oxidation of the cSi(100) substrate and the SiO4 reverse vibration. The
rest of the four absorption peaks around 980 cm−1 ,
1012 cm−1 , 1034 cm−1 , 1076 cm−1 should correspond
to HSi(Si2 O), HSi(SiO2 ), HSi(O3 ), and SiO4 [17] , respectively. We divide them into two groups, the Si-rich
Si–O vibration includes two peaks near 980 cm−1 and
1012 cm−1 , and the O-rich Si–O vibration includes two
peaks near 1034 cm−1 and 1076 cm−1 , and then we obtain the relative integral area ratio (𝑋Si−rich /𝑋O−rich )
O -rich
Fig. 5. FTIR spectra of the film samples. The inset
presents oxygen and hydrogen contents (𝐶O , 𝐶H ) of the
films at different substrate temperatures.
1.6
1.4
1.2
(arb. units)
)
H
20
changing with the substrate temperature (Fig. 6),
(arb. units)
25
(at
Si-H
Si-O
Si-H
(arb. units)
Si-O
Si-O
average particle size with the substrate temperature
is similar.
The Raman results indicate an optimum deposition temperature range (200–400∘C) for the low temperature real time growth of nc-SiO𝑥 :H, in which the
ratio of transition crystalline silicon decreases, and the
nc-Si particle size becomes uniform. Moreover, the
crystalline fraction of the film is higher. During the
deposition process, the film growth mechanism can
be explained by a competition between nc-Si Volmer–
Weber growth and oxidation reaction.
6
3
4
2
0
6
4
5
2
1
1000
1100
1200
-1
(cm
1.0
III
II
I
0.8
Si -rich/
100
1300
)
200
C)
300
(
O-rich
400
500
Fig. 6.
Integral area ratio of rich silicon Si–O and
rich oxygen Si–O (𝑋Si−rich /𝑋O−rich ) versus the substrate
temperature. The inset presents the fitted result of the absorption peak around 900–1300 cm−1 by the multi-peak
Gaussian function.
Raman and FTIR analysis results reveal that the
optimum range of substrate temperature for the deposition of nc-SiO𝑥 :H films is 200–400∘C, in which
the transition crystalline silicon ratio is decreased, the
crystalline fraction is higher, and the hydrogen content
is lower. The growth of nc-SiO𝑥 :H at low temperatures can be interpreted as a competition between
nc-Si Volmer–Weber growth and oxidation reaction.
In the low temperature range (𝑡 < 200∘C), the active
hydrogen promotes the growth of nc-Si, however, the
thermal activation could neither guarantee the effective crystallization of nc-Si, nor is enough to promote
Si atoms segregated from the SiO𝑥 area. As a result,
the nc-Si particle size is smaller and the amorphous
ratio is higher in the films which also contain more
transition crystalline silicon particles. In the optimum
substrate temperature range (200–400∘C), the migration rate of the active Si and O atoms increases with
the substrate temperature. The growth rate of nc-Si is
046802-3
CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 046802
accelerated by the higher thermal activation; the average size of nc-Si particles increases while the amorphous ratio decreases. At the same time, the oxidation
reaction rate increases, which is gradually equal to the
growth rate of nc-Si. Thus the transition crystalline
silicon ratio decreases. There are two possible reasons
for this. On the one hand, given in the Raman data,
𝑋b decreases due to the increase of temperature. It
can be interpreted that the Osterwald mature rate increases, and smaller nc-Si particles are more likely to
spread to larger nc-Si particles nearby, therefore, the
ratio of transition crystalline silicon decreases significantly, while the average size of nc-Si particles continues to increase with the substrate temperature. On
the other hand, the gradually enhanced oxidation reaction inhibits the growth of nc-Si, in the form of the
decrease of transition crystalline silicon and crystalline
fraction. In a higher temperature range (𝑡 > 400∘C),
increasing release hydrogen reaction makes the surface
diffusion rate of active Si and O atoms reduce, while
the etching effect of active hydrogen is to enhance so
that the hydrogen content increases in the film.
(b)
(a)
100 nm
10 nm
Fig. 7. The multilayer film sample section (a) lower magnification TEM image and (b) HRTEM image, selected
layers of SiNCs (marked by black circles).
To better reveal the feasibility and superiority of
this low temperature deposition technology, we implemented it in the fabrication of nc-Si in the multilayer
superlattice structure, which recently has been proved
to be effective in controlling crystallite sizes.[19] We
have prepared nc-SiO𝑥 :H/a-SiO𝑥 :H multilayer films at
300∘C. The nc-Si size was effectively controlled in the
nc-SiO𝑥 :H layers by inserting a-SiO𝑥 :H buffer layer
about 5 nm. Multilayer film section TEM images are
shown in Fig. 7.
The experimental results reveal that the sample
shows a complete periodic structure, and the nc-Si
particle sizes are uniform in the quantum dot layer.
The low temperature deposition technology is partic-
ularly fitted to the fabrication of nc-Si in the multilayer structure which exhibits controllable nc-Si sizes
with a high crystallization quality.
In conclusion, we have obtained the nc-SiO𝑥 :H thin
films at low temperature below 400∘C using magnetron
co-sputtering deposition technology. Based on the
analysis of the microstructure and bonding characteristics of the samples with different substrate temperatures, the growth mechanism of the film is attributed
to a competition between nc-Si growth and oxidation
reaction processes. The present experimental findings
suggest that real-time low temperature deposition of
nc-SiO𝑥 :H is a promising technique for the fabrication
of nanoscale devices on a low cost glass substrate, and
the experimental results pave the way for the development of the quantum dot thin-film solar cells.
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