A new single chamber HWCVD with vertically mounted substrates and filaments has been designed for depositing device quality a-Si:H films with high deposition rate. Optimization studies on films deposited in this chamber under a variety of deposition conditions yielded uniform films at more than 7Å/sec deposition rate and with very low oxygen content. These films show a photoconductivity gain of more than 105. The working pressure has been kept quite low at 15 mtorr compared to earlier studies. i-layers of a p-i-n single junction solar cells were deposited on the TCO (Asahi-U type) glass in this reactor. The initial p-layer and the final n-layer were deposited in another system with separate chambers for these doped layers thus exposing the p-layer as well as the i-layer to the atmosphere during the transfer. Using this optimized intrinsic layer, a-Si:H based p-i-n solar cell showed a conversion efficiency of 4.7 %.
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Kumbhar, Alka

Soni, S.K.

Wadibhasme, N.A.

English

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P. 942-946 
 2011 SumDU 
(Sumy State University) 
 
 
942 
PACS numbers: 73.50.Pz, 82.80.Gk, 84.60.Jt 
 
DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH 
DEPOSITION RATE DEVICE QUALITY A-Si:H THIN FILMS AND 
SOLAR CELLS 
 
N.A. Wadibhasme, S.K. Soni, Alka Kumbhar, Nagsen Meshram, 
R.O. Dusane 
 
 Indian Institute of Technology Bombay 
 Powai, 400076, Mumbai, India 
 E-mail: rodusane@iitb.ac.in 
 
A new single chamber HWCVD with vertically mounted substrates and filaments has 
been designed for depositing device quality a-Si:H films with high deposition rate. 
Optimization studies on films deposited in this chamber under a variety of deposition 
conditions yielded uniform films at more than 7Å/sec deposition rate and with very low 
oxygen content. These films show a photoconductivity gain of more than 10 5. The 
working pressure has been kept quite low at 15 mtorr compared to earlier studies. i-
layers of a p-i-n single junction solar cells were deposited on the TCO (Asahi-U type) 
glass in this reactor. The initial p-layer and the final n-layer were deposited in another 
system with separate chambers for these doped layers thus exposing the p-layer as well 
as the i-layer to the atmosphere during the transfer. Using this optimized intrinsic 
layer, a-Si:H based p-i-n solar cell showed a conversion efficiency of 4.7 %. 
 
Keywords: HYDROGENATED AMORPHOUS SILICON, HWCVD, FTIR, RAMAN 
SPECTROSCOPY, DARK AND PHOTOCONDUCTIVITY. 
 
(Received 04 February 2011, in final form 02 December 2011) 
 
1. INTRODUCTION 
 
Hydrogenated amorphous silicon (a - Si:H) is found to have important 
electronic properties due to its short range ordered structure and hence it is 
being extensively used in optoelectronic and photovoltaic applications. The 
performance of the device depends on the quality of the a - Si:H intrinsic 
film having minimum bulk defect states. a - Si:H based thin films can be 
deposited by various methods. We have been involved in the Hot Wire 
Chemical Vapor Deposition (HWCVD) of a - Si:H and its various alloys for a 
long time. In order to increase the deposition rate for device quality a - Si:H 
films we have made several attempts till now. It is seen that this method has 
some advantages over conventional PECVD in terms of high deposition rate, 
ion free deposition atmosphere and possibility of large area up-scaling [1, 2]. 
Recently we have designed a single chamber HWCVD system with vertical 
filament-substrate geometry for the deposition of high growth rate device 
quality a - Si:H intrinsic thin films. These films were optimized at different 
deposition pressures keeping all other parameters constant found through 
literature [2]. By using this optimized intrinsic layer a - Si:H based p-i-n 
single junction solar cells were fabricated. Doped n and p-layers were 
deposited in another Hot-Wire CVD cluster tool with horizontal substrate-
filament geometry. After the p-layer deposition in the cluster tool the film 
 
 
 
 DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH… 943 
was taken out to deposit intrinsic layer in the new vertical geometry reactor 
and again put back in cluster tool to deposit the n-layer. However, despite of 
an air break two times, an efficiency of 4.7 % has been achieved which 
clearly indicates device quality of the i-layer. 
 
2. EXPERIMENTAL DETAILS 
 
Intrinsic a - Si:H films were deposited by HWCVD process in single separate 
chamber on glass substrate (Corning 1737) for conductivity and Raman 
spectra analysis and p-type 100  crystalline silicon wafer for FTIR spectra 
analysis. Tantalum (Ta) filament with two wire arrangement is used as hot 
wire and kept constant at 1600 C temperature. The substrate temperature 
was maintained at 200 C. Pure silane (SiH4) is used as a precursor gas with 
constant flow rate of 20 SCCM. The deposition pressure was varied from 
15 mTorr to 25 during the deposition. The filament to substrate distance was 
kept at 6.5 cm with vertical substrate-filament geometry. 
 Film thickness was determined by Dektak II surface profilometer, from 
which the deposition rate was determined. Fourier transform infrared (FTIR) 
spectroscopy was used to measure bonding configuration in the a-Si:H films. 
The microstructure factor (R*) and the hydrogen content was evaluated as per 
the accepted procedure [3]. Raman spectroscopy was performed to evaluate 
the structural properties in the a-Si:H films [4]. UV-Vis spectrophotometry 
was used for the evaluation of the band gap of the films. Dark and 
photoconductivity measurements of the films and I-V characteristic of the 
solar cells were measured with the help of Keithley 2400 Source meter and 
xenon arc lamp at 100 mW/cm2 intensity. The power of xenon arc lamp was 
set to 100 mW/cm2 by an AM 1.5 calibrated solar cell. 
 
 
 
 
a 
 
b 
 
Fig. 1 – Photograph of single chamber HWCVD designed at our laboratory (a), Inside 
view of the chamber showing vertical substrate-filament assembly (b) 
 
 
 
 
944 N.A. WADIBHASME, S.K. SONI, ALKA KUMBHAR, ET AL.  
3. RESULTS AND DISCUSSION 
 
All intrinsic a-Si:H films deposited by HWCVD are found to be amorphous 
in nature as revealed from Raman spectra analysis. It has been observed 
that there is slight increase in the deposition rate from 7.2 Å/sec to 10 Å/sec 
with increase in deposition pressure from 15 mtorr to 25 mtorr [5]. The best 
a-Si:H film was obtained at lower chamber pressure with photoconductivity 
gain ( ph/ d) of 8.4  10
4. The good quality of the film can be attributed to 
increased mean free path of dissociated radicals at lower pressure and higher 
surface mobility of the ad-atoms on the substrate. This leads to better Si - Si 
and Si - H bonding configuration with minimum bulk defect states. The dark 
and photoconductivity values with different deposition rates are shown in 
the Table 1. 
 
Table 1 – Deposition rate and photoconductivity gain at different deposition 
pressure  
 
Sample P (mtorr) Rd 
(Å/sec) 
d (
-1cm-1) ph (
–1cm–1) Photo- 
sensitivity 
01 15 7.2 4.3  10-10 3.6  10–5 8.4  104 
02 20 8.2 2.7  10-8 2.3  10–4 8.8  103 
03 25 10 9.5  10-10 3.1  10–6 3  103 
 
 FTIR spectra of a - Si:H films deposited at different deposition pressure 
reflects that the film deposited at 15 mtorr pressure shows better device 
quality properties compared to films deposited at higher pressure. The peak 
at 1100 cm  1 indicates that there is still small amount of oxygen presence 
in the form of Si-O bond in the film. However this could be due to post 
oxidation of the film. A microstructure factor (R*) was determined through 
FTIR spectra analysis and was found to be 0.4 and hydrogen content of 
about 8 % was observed. 
 
 
 
Fig. 2 – FTIR spectra of films deposited by HWCVD at different deposition pressure 
 
 
 
 DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH… 945 
 The Raman spectra of the a-Si:H films are shown in Fig. 3. Si-Si TO like 
peak for all the films appears near 480 cm  1, clearly indicating that all films 
are amorphous in nature. The ITA/ITO peak ratio is found to be 0.2 and bond 
angle deviation ( ) is 6.7, as determined from the FWHM of the TO-like peak. 
 
 
 
Fig. 3 – Raman spectra of a - Si:H films deposited by HWCVD at different deposition 
pressure 
 
 Further, a single junction p-i-n a - Si:H silicon solar cell with structure, 
glass/SnO2:F (Asahi-U type TCO)/p-a-Si:H/i-a-Si:H/n- c-Si:H was fabricated 
on 2  3 cm2 area substrate with optimized parameters of intrinsic a - Si:H 
film. Al contacts of size 3 mm in diameter were deposited on top of the n-
layer by thermal evaporation technique to complete the device. The overall 
thickness of the cell was 380 nm with n and p layer contributing about 
40 nm in the configuration. No special light trapping 
 
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
0
2
4
6
8
10
12
C
u
rr
e
n
t 
D
e
n
s
it
y
 (
m
A
/c
m
2
)
Voltage (V)
 
 
Fig. 4 – I - V characteristic of fabricated single junction a - Si:H solar cell 
VOC  738 mV,  
JSC  11 mA/cm
2 
FF = 0.57 
  4.7 % 
Area  7cm2 
 
 
 
 
 
 
946 N.A. WADIBHASME, S.K. SONI, ALKA KUMBHAR, ET AL.  
scheme was employed and commercial substrates were used with no further 
texturing [6]. We achieved an efficiency of 4.7 % (FF  57 %, Voc  738 mV 
and Jsc  11 mA) under AM 1.5 (100 mW / cm
2) illumination. I-V plot of 
solar cell is shown in the Fig. 4. 
 
4. CONCLUSION 
 
Intrinsic a-Si:H layers were deposited by hot-wire CVD technique in the 
newly developed reactor where the geometry is vertical, at different chamber 
pressures keeping other parameters constant. Device quality film has been 
found at lower deposition pressure of about 15 mtorr with deposition rate of 
7.2 Å/sec and photoconductivity gain of about ~ 105. Further, a-Si:H single 
junction solar cell has been fabricated using this optimized intrinsic layer as 
absorber with air break during device fabrication. The conversion efficiency 
of 4.7 % was achieved under AM 1.5 illumination without applying any 
anti-reflection coating and light trapping schemes. 
 
REFERENCES 
1. A.H. Mahan, Sol. Energ. Mat. Sol. C 78, 299 (2003). 
2. S.K. Soni, A. Pathak, R.O. Dusane, Sol. Energ. Mat. Sol. C 94, 1512 (2010). 
3. C. Godet, P. Morin, P. Roca, I. Cabarrocas, J. Non-Cryst. Solids 198-200, 449 (1996). 
4. S.T. Kshirsagar, R.O. Dusane, V.G. Bhide, Phys. Rev. B 40, 8026 (1989). 
5. S. Klein, F. Finger, R. Carius, J. Appl. Phys. 98, 24905 (2005). 
6. W.P. Kumar, M. Kupich, W. Bock, R.O. Dusane, B. Schroeder, J. Non-Cryst. 
Solids 352, 1855 (2006). 


Designing single chamber hwcvd system for high deposition rate device quality A-Si:h thin films and solar cells

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P. 942-946  2011 SumDU (Sumy State University)   942 PACS numbers: 73.50.Pz, 82.80.Gk, 84.60.Jt  DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH DEPOSITION RATE DEVICE QUALITY A-Si:H THIN FILMS AND SOLAR CELLS  N.A. Wadibhasme, S.K. Soni, Alka Kumbhar, Nagsen Meshram, R.O. Dusane    Indian Institute of Technology Bombay   Powai, 400076, Mumbai, India   E-mail: rodusane@iitb.ac.in  A new single chamber HWCVD with vertically mounted substrates and filaments has been  designed  for  depositing  device  quality  a-Si:H  films  with  high  deposition  rate. Optimization studies on films deposited in this chamber under a variety of deposition conditions yielded uniform films at more than 7Å/sec deposition rate and with very low oxygen  content.  These  films  show  a  photoconductivity  gain  of  more  than  10 5.  The working pressure has been kept quite low at 15 mtorr compared to earlier studies. i-layers of a p-i-n single junction solar cells were deposited on the TCO (Asahi-U type) glass in this reactor. The initial p-layer and the final n-layer were deposited in another system with separate chambers for these doped layers thus exposing the p-layer as well as  the  i-layer  to  the  atmosphere  during  the  transfer.  Using  this  optimized  intrinsic layer, a-Si:H based p-i-n solar cell showed a conversion efficiency of 4.7 %.  Keywords: HYDROGENATED AMORPHOUS SILICON, HWCVD, FTIR, RAMAN SPECTROSCOPY, DARK AND PHOTOCONDUCTIVITY.  (Received 04 February 2011, in final form 02 December 2011)  1.  INTRODUCTION  Hydrogenated  amorphous  silicon  (a - Si:H)  is  found  to  have  important electronic properties due to its short range ordered structure and hence it is being extensively used in optoelectronic and photovoltaic applications. The performance of the device depends on the quality of the a - Si:H intrinsic film having minimum bulk defect states. a - Si:H based thin films can be deposited  by  various  methods.  We  have  been  involved  in  the  Hot  Wire Chemical Vapor Deposition (HWCVD) of a - Si:H and its various alloys for a long time. In order to increase the deposition rate for device quality a - Si:H films we have made several attempts till now. It is seen that this method has some advantages over conventional PECVD in terms of high deposition rate, ion free deposition atmosphere and possibility of large area up-scaling [1, 2]. Recently we have designed a single chamber HWCVD system with vertical filament-substrate geometry for the deposition of high growth rate device quality a - Si:H intrinsic thin films. These films were optimized at different deposition  pressures  keeping  all  other  parameters  constant  found  through literature  [2].  By  using  this  optimized  intrinsic  layer  a - Si:H  based  p-i-n single  junction  solar  cells  were  fabricated.  Doped  n  and  p-layers  were deposited in another Hot-Wire CVD cluster tool with horizontal substrate-filament geometry. After the p-layer deposition in the cluster tool the film      DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH…  943 was taken out to deposit intrinsic layer in the new vertical geometry reactor and again put back in cluster tool to deposit the n-layer. However, despite of an  air  break  two  times,  an  efficiency  of  4.7 %  has  been  achieved  which clearly indicates device quality of the i-layer.  2.  EXPERIMENTAL DETAILS  Intrinsic a - Si:H films were deposited by HWCVD process in single separate chamber  on  glass  substrate  (Corning  1737)  for  conductivity  and  Raman spectra analysis and p-type  100  crystalline silicon wafer for FTIR spectra analysis. Tantalum (Ta) filament with two wire arrangement is used as hot wire  and  kept  constant  at  1600  C temperature.  The substrate temperature was maintained at 200  C. Pure silane (SiH4) is used as a precursor gas with constant  flow  rate  of  20  SCCM.  The  deposition  pressure  was  varied  from 15 mTorr to 25 during the deposition. The filament to substrate distance was kept at 6.5 cm with vertical substrate-filament geometry.   Film  thickness  was  determined  by  Dektak  II  surface  profilometer,  from which the deposition rate was determined. Fourier transform infrared (FTIR) spectroscopy was used to measure bonding configuration in the a-Si:H films. The microstructure factor (R*) and the hydrogen content was evaluated as per the  accepted procedure  [3].  Raman spectroscopy  was  performed to evaluate the structural properties in the a-Si:H films [4]. UV-Vis spectrophotometry was  used  for  the  evaluation  of  the  band  gap  of  the  films.  Dark  and photoconductivity  measurements  of  the  films  and  I-V  characteristic  of  the solar cells were measured with the help of Keithley 2400 Source meter and xenon arc lamp at 100 mW/cm2 intensity. The power of xenon arc lamp was set to 100 mW/cm2 by an AM 1.5 calibrated solar cell.     a  b  Fig. 1 – Photograph of single chamber HWCVD designed at our laboratory (a), Inside view of the chamber showing vertical substrate-filament assembly (b)     944  N.A. WADIBHASME, S.K. SONI, ALKA KUMBHAR, ET AL.   3.  RESULTS AND DISCUSSION  All intrinsic a-Si:H films deposited by HWCVD are found to be amorphous in  nature  as  revealed  from  Raman  spectra  analysis.  It  has  been  observed that there is slight increase in the deposition rate from 7.2 Å/sec to 10 Å/sec with increase in deposition pressure from 15 mtorr to 25 mtorr [5]. The best a-Si:H film was obtained at lower chamber pressure with photoconductivity gain ( ph/ d) of 8.4   104. The good quality of the film can be attributed to increased mean free path of dissociated radicals at lower pressure and higher surface mobility of the ad-atoms on the substrate. This leads to better Si - Si and Si - H bonding configuration with minimum bulk defect states. The dark and photoconductivity values with different deposition rates are shown in the Table 1.  Table 1 – Deposition rate and photoconductivity gain at different deposition pressure   Sample  P (mtorr)  Rd (Å/sec) d ( -1cm-1)  ph ( –1cm–1)  Photo- sensitivity 01  15  7.2  4.3   10-10  3.6   10–5  8.4   104 02  20  8.2  2.7   10-8  2.3   10–4  8.8   103 03  25  10  9.5   10-10  3.1   10–6  3   103    FTIR spectra of a - Si:H films deposited at different deposition pressure reflects  that  the  film  deposited  at  15 mtorr  pressure  shows  better  device quality properties compared to films deposited at higher pressure. The peak at 1100 cm  1 indicates that there is still small amount of oxygen presence in the form of Si-O bond in the film. However this could be due to post oxidation of the film. A microstructure factor (R*) was determined through FTIR  spectra  analysis  and  was  found  to  be  0.4  and  hydrogen  content  of about 8 % was observed.    Fig. 2 – FTIR spectra of films deposited by HWCVD at different deposition pressure      DESIGNING SINGLE CHAMBER HWCVD SYSTEM FOR HIGH…  945   The Raman spectra of the a-Si:H films are shown in Fig. 3. Si-Si TO like peak for all the films appears near 480 cm  1, clearly indicating that all films are amorphous in nature. The ITA/ITO peak ratio is found to be 0.2 and bond angle deviation ( ) is 6.7, as determined from the FWHM of the TO-like peak.    Fig. 3 – Raman spectra of a - Si:H films deposited by HWCVD at different deposition pressure    Further, a single junction p-i-n a - Si:H silicon solar cell with structure, glass/SnO2:F (Asahi-U  type  TCO)/p-a-Si:H/i-a-Si:H/n- c-Si:H  was  fabricated on 2   3 cm2 area substrate with optimized parameters of intrinsic a - Si:H film. Al contacts of size 3 mm in diameter were deposited on top of the n-layer by thermal evaporation technique to complete the device. The overall thickness of the cell  was 380 nm with  n and  p layer contributing about 40 nm in the configuration. No special light trapping  0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8024681012Current Density (mA/cm2)Voltage (V)  Fig. 4 – I - V characteristic of fabricated single junction a - Si:H solar cell VOC   738 mV,  JSC   11 mA/cm2 FF = 0.57    4.7 % Area   7cm2       946  N.A. WADIBHASME, S.K. SONI, ALKA KUMBHAR, ET AL.   scheme was employed and commercial substrates were used with no further texturing [6]. We achieved an efficiency of 4.7 % (FF   57 %, Voc   738 mV and  Jsc   11 mA)  under  AM  1.5  (100 mW / cm2)  illumination.  I-V  plot  of solar cell is shown in the Fig. 4.  4.  CONCLUSION  Intrinsic  a-Si:H  layers  were  deposited  by  hot-wire  CVD  technique  in  the newly developed reactor where the geometry is vertical, at different chamber pressures keeping other parameters constant. Device quality film has been found at lower deposition pressure of about 15 mtorr with deposition rate of 7.2 Å/sec and photoconductivity gain of about ~ 105. Further, a-Si:H single junction solar cell has been fabricated using this optimized intrinsic layer as absorber with air break during device fabrication. The conversion efficiency of  4.7 %  was  achieved  under  AM 1.5  illumination  without  applying  any anti-reflection coating and light trapping schemes.  REFERENCES 1.  A.H. Mahan, Sol. Energ. Mat. Sol. C 78, 299 (2003). 2.  S.K. Soni, A. Pathak, R.O. Dusane, Sol. Energ. Mat. Sol. C 94, 1512 (2010). 3.  C. Godet, P. Morin, P. Roca, I. Cabarrocas, J. Non-Cryst. Solids 198-200, 449 (1996). 4.  S.T. Kshirsagar, R.O. Dusane, V.G. Bhide, Phys. Rev. B 40, 8026 (1989). 5.  S. Klein, F. Finger, R. Carius, J. Appl. Phys. 98, 24905 (2005). 6.  W.P. Kumar,  M. Kupich,  W. Bock,  R.O. Dusane,  B. Schroeder,  J.  Non-Cryst. Solids 352, 1855 (2006). 

Solids 198-200,

Solids 352,

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Designing single chamber hwcvd system for high deposition rate device quality A-Si:h thin films and solar cells

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