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Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD
Solar Energy Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 48 (1997) 321 326
Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD Yuwen Zhao a'*, Zhongming Li a, Saoqi He a, Xianbo Liao b, Shuran Sheng b, Lisheng Deng b, Zhixun Ma b "Beijing Solar Energy Research Institute, Beijing 100083, People's Republic of China bNational Laboratoryfor Surface Physics, Institute of Semiconductors, ChineseAcademy of Sciences, Beijing 100083, People's Republic of China
Abstract Polycrystalline silicon (poly-Si) films ( ~ 10 lam) were grown from dichlorosilane b.y a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 A/s at the substrate temperature (Ts) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn ÷ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm 2, 1 cm2). Keywords. Polycrystalline silicon thin films; Solar cells; Rapid thermal CVD
1. Introduction Rapid thermal C V D or v a p o u r - p h a s e epitaxy (VPE) technology has been widely e m p l o y e d to grow epitaxial monocrystalline silicon layer on single-crystal silicon wafer in microelectronic industry. This m e t h o d can also be used to deposit thick poly-Si films ( ~ 10 gm) on non-silicon substrates or polycrystalline silicon wafers for solar cells. In this aspect, the R T C V D offers advantages over the usual C V D in two ways [1]: one is its wider working t e m p e r a t u r e range from ,-~ 700°C to 1200°C, which provides the potential of direct growth of thicker poly-Si films with big grain sizes in
* Corresponding author. 0927-0248/97/$17.00 ~, 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 1 39-6
322
E Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
a relatively short time (about 10 min) by using a low-cost feedstock gas of dichlorosilane (SiH2Clz), the second is due to its cold wall of the quartz reactor, which depresses the simultaneous deposition of poly-Si films on the reactor wall. In this paper we describe our growth experiments of poly-Si films by using RTCVD, discuss the optoelectronic properties of the films, and also report the preliminary results on preparation of the poly-SI solar cells.
2. Experimental Deposition of Poly-Si films was performed from hydrogen-diluted SiH2C12 in a rapid thermal CVD quartz reactor. The gas source was a mixture of H 2 and SiHzCt2 with a ratio of 150 : 0.4, and the working pressure was ~ 10 torr. The quartz reactor was equipped with a graphite substrate-holder, which could be heated up to 1200°C by a programmable light source. The substrates used in this work were unpolished monocrystalline silicon wafers (for easy to get poly-Si films) and non-silicon substrates (e.g., SiO2 film and fused quartz). The microstructure of the poly-Si layers was analyzed by optical microscope, field emission scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). The optoelectronic properties of the poly-Si layers were investigated by measuring the spread resistance, Hall effect, temperature dependence of the dark- and photoconductivity, and capacitance voltage (C V) characteristics of a Schottky (Au/polySi) configuration. By using this kind of films, a model poly-Si solar cell has been prepared on an unpolished heavily phosphorus-doped silicon substrate with a low resistivity of ~ 5 x 10-3~ cm.
3. Results and discussion 3.1. Growth rate
The variation of the growth rate of the poly-Si films with the substrate temperature Ts is shown in Fig. 1. It is seen that the growth rate changes from about 4.2 to 100 A/s when T~ increases from 780°C to 1030°C. In the lower T~ range ( < 920°C), the growth rate follows a thermal activation relationship with Ts, having an activation energy of 1.7 eV, while in the higher T~ range ( > 920°C), it is not dependent on temperature very much and should be controlled by mass transport. 3.2. Microstructure
Two typical SEM micrographs are shown in Fig. 2a and Fig. 2b, which were taken on the surface of the poly-Si films, one deposited on fused quartz at 780°C and the other on unpolished silicon wafer at 1030~'C, respectively. From the SEM micrographs, the average grain sizes of the poly-Si films were evaluated to be about 0.25 to 3.5 jam, depending on the substrate temperature and material. Some closed moir6-1ike
Y. Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
323
IOO0
~"
0
100
10
1 0.76
0.86
0.96
1/Ts (1000/K) Fig. 1. The growth rate of poly-Si films versus the substrate temperature.
Fig. 2. Typical SEM images of poly-Si fihns deposited on fused quartz at 780°C (a) and on unpolished Si at 1030' C (b), respectively.
cracks with typical sizes of 100-200 ~tm were also observed on the surface of the poly-Si films which were deposited on fused quartz. Fig. 3 illustrates the SEM micrograph of a part of the cracks found on the surface of the poly-Si film deposited on fused quartz at 920°C. The appearance of the cracks could be attributed to the internal stress existing in the poly-Si layers deposited on non-silicon substrates, and this might be a serious problem for their photovoltaic applications. The XRD spectra of the poly-Si films exhibit three sharp poly-crystal diffraction peaks, corresponding to Si (1 1 1), (2 2 0) and (4 0 0) directions. There is no amorphous envelope observed in the XRD spectra.
E Zhao et al./Solar Energy Materials and Solar (_'ella' 48 (1997) 321 326
324
Fig. 3. SEM image illustrates a part of the closed moire-like cracks observed on the surface of poly-Si films deposited on fused quartz at 920 C.
3.3. Optoelectronic properties Fig. 4 shows the measured dark conductivity ad of the poly-Si films as a function of the temperature (T). For the samples deposited on fused quartz substrate at 780'~C and 1030°C, the activation energies were deduced to be about 0.50 and 0.52 eV, respectively. These values indicate that the Fermi level Ev of the poly-Si films grown in this T~ range lies close to the middle of the energy band gap of crystalline silicon, and the densities of the donor-like defect states or impurities in the poly-Si films are rather low, in contrast with the case found in hydrogenated gc- or poly-Si films prepared by plasma enhanced CVD at lower Ts ( "-~ 200-300°C), where the densities of the donorlike defect states were reported to be ~ 10~/cm 3 [2]. The capacitance-voltage (C-V) characteristics of a Schottky structure of Au/polySi, deposited on unpolished Si substrate at 1030°C, are illustrated In Fig. 5. It is found that the linear relationship between I / C 2 and V for an ideal pn junction seems to be also held for the Au/poly-Si junction [3], 1/C 2 =
2(Vb -- V )/(EE0qN0),
(1)
where Vb and N0 stand for the built-in potential and the density of donor-like states in the poly-Si film, q is the electron charge, ~ and e0 is the dielectric constant in Poly-Si and in vacuum, respectively. According to this relationship, Vb and Nd in the poly-Si layer near to the Schottky junction could be deduced. From the high-frequency characteristics (at 1 MHz) in this figure, Vb and N0 is evaluated to be equal to ~ 0.6 V and ~ 2.9 x 10~% m - 3 respectively. Another feature seen in this figure is the frequency dependence of the C V curves, which may be correlated to the distribution of the gap state densities and needs more study. From Hall-effect measurements, the electron mobility (I-re)and carrier concentration (n) of the poly-Si films were determined. For unpolished silicon substrate the values I,to and n of the poly-Si films are about 120-450 cm2/Vs and 3-6 x 101S/cm 3, respectively, while for fused quartz substrate the lae is much lower, about 1-80 c m 2 / V s .
E Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
325
10 6
E
o
105
v
.> -I "10 1-
o L)
104.
Q
\ ~-
1030C
~780C
103 2.5
3
3.5
l I T (IO00/K) Fig. 4. Temperature dependence of the dark conductivity of the poly-Si films deposited on fused quartz at 720'C and 1030"C.
8
--- 1 M H z ..e- I O O K H z
7
~
-~-10KHz -~-IKHz
6
2-5 E o 4 3 ~- 2
0
-0.6
I
-0.4
-0.2
0
Voltage (V) Fig. 5. C-V characteristics of a Schottky structure of Au/poly-Si on unpolished Si substrate at 1030cC.
These values of n are consistent with the results of the s p r e a d resistance m e a s u r e ments. T h e p h o t o c o n d u c t i v i t y of the p o l y Si film d e p o s i t e d on a fused q u a r t z s u b s t r a t e at 1030°C is ~ 2.7 x 10 - 5 S/cm a n d stable for p r o l o n g e d e x p o s u r e to AM1.5 illumination of 100 m W / c m 2.
326
Y. Zhao el al./Solur Energy Materials and Solar Cells 48 f1997) 321 326
Voltage (v) 0
0.1
0.2
0.3
0.4
0.5
o 4-"
E o <
g
-5
Voc=O.46V Jsc=14.3mA/cm2 FF=0.691 EFF=4.54% Area=l.0cm2
r"
a ¢.
-10
0
-15 - Fig. 6. AM 1.5 illuminated .l 1 characteristics of the poly-Si solar cell.
3.4. Poly-Si solar cells The first model of poly-Si solar cells has been prepared with the configuration of metal grid/p + poly-Si/n-poly-Si/n ~ + unpolished c-Si/metal contact. The p+ layer was formed by boron diffusion, with a ,junction deepness of - 1 pm. The AM 1.5 illuminated J V curve of the poly-Si solar cell is demonstrated in Fig. 6. The conversion efficiency of the solar cell on an area ofl cm 2 without AR coating is 4.54% (AM 1.5, 100 mW/cm2), with J~, - 14.3 mA/cm 2, V,,,, = 0.460 V and FF = 0.69. The design of cell and film technology was not optimized. We convince that the cell performance will be improved with optimized design and technology in the near future.
4. Summary Poly-Si films ( ~ 10 pro) were grown flom dichlorosilane by a rapid thermal CVD, with a growth rate up to 100 A,/s at 103OC. A model poly-Si solar cell without AR coating has been prepared, with the conversion efficiency of 4.54%.
References [ t ] R. Monna, A. Slaoui, A. Lachiq, .I. Kopp. J.C. Muller, Book of Abstracts of European PVSEC-13, Po6B.3, Nice, France, October 23 27. 1995. [2] P.G. Lecornber, G. Willeke, W.E. Spear, J. Non-cryst. Solids 59&60 (1983) 791. [-3] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.
ELSEVIER
Solar Energy Materials and Solar Cells 48 (1997) 321 326
Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD Yuwen Zhao a'*, Zhongming Li a, Saoqi He a, Xianbo Liao b, Shuran Sheng b, Lisheng Deng b, Zhixun Ma b "Beijing Solar Energy Research Institute, Beijing 100083, People's Republic of China bNational Laboratoryfor Surface Physics, Institute of Semiconductors, ChineseAcademy of Sciences, Beijing 100083, People's Republic of China
Abstract Polycrystalline silicon (poly-Si) films ( ~ 10 lam) were grown from dichlorosilane b.y a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 A/s at the substrate temperature (Ts) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn ÷ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm 2, 1 cm2). Keywords. Polycrystalline silicon thin films; Solar cells; Rapid thermal CVD
1. Introduction Rapid thermal C V D or v a p o u r - p h a s e epitaxy (VPE) technology has been widely e m p l o y e d to grow epitaxial monocrystalline silicon layer on single-crystal silicon wafer in microelectronic industry. This m e t h o d can also be used to deposit thick poly-Si films ( ~ 10 gm) on non-silicon substrates or polycrystalline silicon wafers for solar cells. In this aspect, the R T C V D offers advantages over the usual C V D in two ways [1]: one is its wider working t e m p e r a t u r e range from ,-~ 700°C to 1200°C, which provides the potential of direct growth of thicker poly-Si films with big grain sizes in
* Corresponding author. 0927-0248/97/$17.00 ~, 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 1 39-6
322
E Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
a relatively short time (about 10 min) by using a low-cost feedstock gas of dichlorosilane (SiH2Clz), the second is due to its cold wall of the quartz reactor, which depresses the simultaneous deposition of poly-Si films on the reactor wall. In this paper we describe our growth experiments of poly-Si films by using RTCVD, discuss the optoelectronic properties of the films, and also report the preliminary results on preparation of the poly-SI solar cells.
2. Experimental Deposition of Poly-Si films was performed from hydrogen-diluted SiH2C12 in a rapid thermal CVD quartz reactor. The gas source was a mixture of H 2 and SiHzCt2 with a ratio of 150 : 0.4, and the working pressure was ~ 10 torr. The quartz reactor was equipped with a graphite substrate-holder, which could be heated up to 1200°C by a programmable light source. The substrates used in this work were unpolished monocrystalline silicon wafers (for easy to get poly-Si films) and non-silicon substrates (e.g., SiO2 film and fused quartz). The microstructure of the poly-Si layers was analyzed by optical microscope, field emission scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). The optoelectronic properties of the poly-Si layers were investigated by measuring the spread resistance, Hall effect, temperature dependence of the dark- and photoconductivity, and capacitance voltage (C V) characteristics of a Schottky (Au/polySi) configuration. By using this kind of films, a model poly-Si solar cell has been prepared on an unpolished heavily phosphorus-doped silicon substrate with a low resistivity of ~ 5 x 10-3~ cm.
3. Results and discussion 3.1. Growth rate
The variation of the growth rate of the poly-Si films with the substrate temperature Ts is shown in Fig. 1. It is seen that the growth rate changes from about 4.2 to 100 A/s when T~ increases from 780°C to 1030°C. In the lower T~ range ( < 920°C), the growth rate follows a thermal activation relationship with Ts, having an activation energy of 1.7 eV, while in the higher T~ range ( > 920°C), it is not dependent on temperature very much and should be controlled by mass transport. 3.2. Microstructure
Two typical SEM micrographs are shown in Fig. 2a and Fig. 2b, which were taken on the surface of the poly-Si films, one deposited on fused quartz at 780°C and the other on unpolished silicon wafer at 1030~'C, respectively. From the SEM micrographs, the average grain sizes of the poly-Si films were evaluated to be about 0.25 to 3.5 jam, depending on the substrate temperature and material. Some closed moir6-1ike
Y. Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
323
IOO0
~"
0
100
10
1 0.76
0.86
0.96
1/Ts (1000/K) Fig. 1. The growth rate of poly-Si films versus the substrate temperature.
Fig. 2. Typical SEM images of poly-Si fihns deposited on fused quartz at 780°C (a) and on unpolished Si at 1030' C (b), respectively.
cracks with typical sizes of 100-200 ~tm were also observed on the surface of the poly-Si films which were deposited on fused quartz. Fig. 3 illustrates the SEM micrograph of a part of the cracks found on the surface of the poly-Si film deposited on fused quartz at 920°C. The appearance of the cracks could be attributed to the internal stress existing in the poly-Si layers deposited on non-silicon substrates, and this might be a serious problem for their photovoltaic applications. The XRD spectra of the poly-Si films exhibit three sharp poly-crystal diffraction peaks, corresponding to Si (1 1 1), (2 2 0) and (4 0 0) directions. There is no amorphous envelope observed in the XRD spectra.
E Zhao et al./Solar Energy Materials and Solar (_'ella' 48 (1997) 321 326
324
Fig. 3. SEM image illustrates a part of the closed moire-like cracks observed on the surface of poly-Si films deposited on fused quartz at 920 C.
3.3. Optoelectronic properties Fig. 4 shows the measured dark conductivity ad of the poly-Si films as a function of the temperature (T). For the samples deposited on fused quartz substrate at 780'~C and 1030°C, the activation energies were deduced to be about 0.50 and 0.52 eV, respectively. These values indicate that the Fermi level Ev of the poly-Si films grown in this T~ range lies close to the middle of the energy band gap of crystalline silicon, and the densities of the donor-like defect states or impurities in the poly-Si films are rather low, in contrast with the case found in hydrogenated gc- or poly-Si films prepared by plasma enhanced CVD at lower Ts ( "-~ 200-300°C), where the densities of the donorlike defect states were reported to be ~ 10~/cm 3 [2]. The capacitance-voltage (C-V) characteristics of a Schottky structure of Au/polySi, deposited on unpolished Si substrate at 1030°C, are illustrated In Fig. 5. It is found that the linear relationship between I / C 2 and V for an ideal pn junction seems to be also held for the Au/poly-Si junction [3], 1/C 2 =
2(Vb -- V )/(EE0qN0),
(1)
where Vb and N0 stand for the built-in potential and the density of donor-like states in the poly-Si film, q is the electron charge, ~ and e0 is the dielectric constant in Poly-Si and in vacuum, respectively. According to this relationship, Vb and Nd in the poly-Si layer near to the Schottky junction could be deduced. From the high-frequency characteristics (at 1 MHz) in this figure, Vb and N0 is evaluated to be equal to ~ 0.6 V and ~ 2.9 x 10~% m - 3 respectively. Another feature seen in this figure is the frequency dependence of the C V curves, which may be correlated to the distribution of the gap state densities and needs more study. From Hall-effect measurements, the electron mobility (I-re)and carrier concentration (n) of the poly-Si films were determined. For unpolished silicon substrate the values I,to and n of the poly-Si films are about 120-450 cm2/Vs and 3-6 x 101S/cm 3, respectively, while for fused quartz substrate the lae is much lower, about 1-80 c m 2 / V s .
E Zhao et al./Solar Energy Materials and Solar Cells 48 (1997) 321 326
325
10 6
E
o
105
v
.> -I "10 1-
o L)
104.
Q
\ ~-
1030C
~780C
103 2.5
3
3.5
l I T (IO00/K) Fig. 4. Temperature dependence of the dark conductivity of the poly-Si films deposited on fused quartz at 720'C and 1030"C.
8
--- 1 M H z ..e- I O O K H z
7
~
-~-10KHz -~-IKHz
6
2-5 E o 4 3 ~- 2
0
-0.6
I
-0.4
-0.2
0
Voltage (V) Fig. 5. C-V characteristics of a Schottky structure of Au/poly-Si on unpolished Si substrate at 1030cC.
These values of n are consistent with the results of the s p r e a d resistance m e a s u r e ments. T h e p h o t o c o n d u c t i v i t y of the p o l y Si film d e p o s i t e d on a fused q u a r t z s u b s t r a t e at 1030°C is ~ 2.7 x 10 - 5 S/cm a n d stable for p r o l o n g e d e x p o s u r e to AM1.5 illumination of 100 m W / c m 2.
326
Y. Zhao el al./Solur Energy Materials and Solar Cells 48 f1997) 321 326
Voltage (v) 0
0.1
0.2
0.3
0.4
0.5
o 4-"
E o <
g
-5
Voc=O.46V Jsc=14.3mA/cm2 FF=0.691 EFF=4.54% Area=l.0cm2
r"
a ¢.
-10
0
-15 - Fig. 6. AM 1.5 illuminated .l 1 characteristics of the poly-Si solar cell.
3.4. Poly-Si solar cells The first model of poly-Si solar cells has been prepared with the configuration of metal grid/p + poly-Si/n-poly-Si/n ~ + unpolished c-Si/metal contact. The p+ layer was formed by boron diffusion, with a ,junction deepness of - 1 pm. The AM 1.5 illuminated J V curve of the poly-Si solar cell is demonstrated in Fig. 6. The conversion efficiency of the solar cell on an area ofl cm 2 without AR coating is 4.54% (AM 1.5, 100 mW/cm2), with J~, - 14.3 mA/cm 2, V,,,, = 0.460 V and FF = 0.69. The design of cell and film technology was not optimized. We convince that the cell performance will be improved with optimized design and technology in the near future.
4. Summary Poly-Si films ( ~ 10 pro) were grown flom dichlorosilane by a rapid thermal CVD, with a growth rate up to 100 A,/s at 103OC. A model poly-Si solar cell without AR coating has been prepared, with the conversion efficiency of 4.54%.
References [ t ] R. Monna, A. Slaoui, A. Lachiq, .I. Kopp. J.C. Muller, Book of Abstracts of European PVSEC-13, Po6B.3, Nice, France, October 23 27. 1995. [2] P.G. Lecornber, G. Willeke, W.E. Spear, J. Non-cryst. Solids 59&60 (1983) 791. [-3] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.