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Polycrystalline silicon film deposited by ICP-CVD
Solar Energy Materials & Solar Cells 69 (2001) 139}145
Polycrystalline silicon "lm deposited by ICP-CVD Byeong Yeon Moon , Jae Hyoung Youn, Sung Hwan Won, Jin Jang * School of Architecture, Environment and Life Science, Kyung Woon University, Kumi, Kyungbuk 730-850, South Korea Department of Information Display, Kyung Hee University, Seoul 130-701, South Korea Received 25 September 2000
Abstract We studied the deposition of polycrystalline silicon (poly-Si) using SiH /SiH Cl /H mix tures by inductively coupled plasma chemical vapor deposition. The deposition rate and crystalline quality were improved by increasing RF power. The poly-Si "lm deposited with the [SiH Cl ]/[SiH ] ratio of 2 and the RF power of 1500 W exhibited the deposition rate of 4.2 As /s, the polycrystalline volume fraction of 88%, the Raman FWHM of 7 cm\, and the TEM grain size of &1200 As . The solar cell made of this material exhibited a conversion e$ciency of 3.14%. 2001 Elsevier Science B.V. All rights reserved. Keywords: Poly-Si; SiH Cl ; ICP-CVD
1. Introduction Polycrystalline silicon (poly-Si) thin "lm has attracted much attention recently for application in solar cells. Solid-phase crystallization (SPC) of amorphous silicon is a common method of crystallizing amorphous silicon (a-Si), but its crystallization temperature is too high for use of large-area glass substrates [1]. Recently, liquidphase crystallization of a-Si using excimer laser has been intensively studied, but the thickness for crystallization is less than 100 nm because of a very high absorption
* Corresponding author. Department of Information Display, Kyung Hee University, Seoul 130-701, South Korea. Tel.: #82-2-961-0270; fax: #82-2-968-6924. E-mail address: [email protected] (J. Jang). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 7 - 1
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B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
coe$cient of a-Si for the laser line [2]. The best way of preparing poly-Si, therefore, may be a direct deposition of poly-Si at a temperature low enough to utilize inexpensive large-area glass substrates. Various high-density plasma sources of electron cyclotron resonance (ECR), helicon wave and inductively coupled RF plasma chemical vapor deposition (ICP-CVD) have been developed [3}5]. An ICP-CVD has some advantages in the growth of poly-Si at low temperatures because of its high plasma density of &10 cm\ and a good uniformity without external magnets and less ion bombardment on the growing surface [6]. SiH Cl is added in SiH plasma to enhance the "lm quality of poly-Si and to increase its deposition rate. The addition of Cl species to the plasma enhances etching of Si, and there is a strong chemical interaction between Cl and H atoms back-bonded with silicon atom because of the di!erence in the bond energies for Si}Cl (4.75 eV), Si}Si (2.4 eV) and Si}H (3.4 eV). Therefore, Cl radicals can break weak Si}Si bonds as H radicals do in the growth of microcrystalline silicon and thus promote the growth of crystalline phase [7,8].
2. Experiments The poly-Si "lms were fabricated by an ICP-CVD using the SiH Cl /SiH /H mixtures. The RF power at 13.56 MHz was supplied to produce the plasma. The inductively coupled azimuthal electrical "eld produced from a #at antenna generated the plasma. A spiral type antenna was placed on the top of a quartz plate on a stainless steel reactor. The gases were introduced from the dispersal ring into the reactor chamber. The sum of SiH Cl and SiH #ow rates, H #ow rate and substrate temperature were "xed at 0.6, 6 sccm and 3003C, respectively. The SiH Cl #ow rate was varied between 0 and 0.6 sccm and a RF power between 700 (4.4 W/cm) and 1500 W (9.4 W/cm) was applied to a 6 inductor. To obtain the polycrystalline volume fraction from Raman intensity, the transverse optical phonon (TO) mode spectra was decomposed into three parts: a polycrystalline (520 cm\), an amorphous (480 cm\) and an intermediate (500}510 cm\). The intermediate peak appears to be caused by the small grain nanocrystalline silicon for which the grain size is (10 nm. In this case, the crystalline volume fraction is given by I #I
, " I #I #I
(1)
where, I , I and I are Raman intensities of the amorphous, polycrystalline and
intermediate phases, respectively. For our "lms, the amorphous peak was not found. Thus, the ratio of the polycrystalline volume fraction is de"ned by I . " I #I
(2)
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
141
3. Results and discussion The optimization of the substrate temperature for the deposition of poly-Si using SiH Cl were carried out at the RF power of 700 W. SiH Cl and H #ow rates were "xed at 0.6 and 6 sccm, respectively. Fig. 1 shows the Raman polycrystalline volume fraction and the full-width at half-maximum (FWHM) of the poly-Si "lms deposited as a function of substrate temperature. The Raman polycrystalline volume fraction increases with substrate temperature up to 3003C, and then decreases. The maximum volume fraction and minimum FWHM were 85.1% and 8.9 cm\, respectively, at a substrate temperature of 300. The highest deposition rate was 2.6 As /s at 3003C.
Fig. 1. Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited using a H /SiH Cl mixture. (H "6 sccm/SiH Cl "0.6 sccm).
Fig. 2. The deposition rate of the poly-Si "lms fabricated by using the SiH Cl /SiH mixtures.
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Under the existence of hydrogen bonded with silicon, Cl in a strong back-bond with Si was predominantly released from the "lms in the propagation of Si-network. When substrate temperature is less than 3003C, it is hard to eliminate chlorine remaining in the "lms even under the existence of an excessive amount of atomic hydrogen. Therefore, the deposition temperature was "xed at 3003C to have a good quality poly-Si. Fig. 2 shows the deposition rate of the poly-Si "lms as a function of RF power using SiH Cl and SiH mixtures. Deposition rate increased with RF power due to the increased dissociation rate of the feeding gas. The deposition rate reached a maximum when the #ow rate of ratio of [SiH C ] to [SiH ] was 2 keeping the total #ow rate at 0.6 sccm. The maximum deposition rate was 4.2 As /s at the RF power of 1500 W.
Fig. 3. Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited as a function of RF power. (SiH Cl "0.4 sccm/SiH "0.2 sccm).
Fig. 4. XRD spectra of the poly-Si "lms deposited at 1500 W as a variation of SiH Cl #ow rate.
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
143
Fig. 3 shows the Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited as a function of RF power. The #ow rates of SiH Cl and SiH were "xed at 0.4 and 0.2 sccm, respectively. The RF power was varied from 700 to 1500 W. The polycrystalline volume fraction increased and FWHM decreased with increase of RF power up to 1000 W, and beam nearly saturated. The Raman peak position of the polycrystalline phase was &518 cm\, while the peak position of nanocrystalline (intermediate) phase, I , increased with increasing RF power. The grain size of poly-Si
"lm is related to the FWHM and peak position of Raman intensity. Therefore, the crystallinity seems to have improved by increasing RF power. Fig. 4 shows the X-ray di!raction (XRD) spectra of the poly-Si "lms as a function of SiH Cl #ow rate. The sum of SiH Cl and SiH #ow rates was "xed at 0.6 sccm. The XRD intensity exhibits (1 1 1), (2 2 0) and (3 1 1) crystalline peaks, and (1 1 1) peak is dominant for all the poly-Si "lms. The highest XRD peak intensity is shown for the
Fig. 5. TEM bright-"eld images and their SAD (selected area di!raction) patterns of the poly-Si "lms deposited at various SiH Cl #ow rates. [SiH "0.6 sccm (a); SiH Cl "0.6 sccm (b); SiH "0.4 sccm/SiH Cl "0.2 sccm (c); SiH "0.2 sccm/SiH Cl "0.4 sccm (d)].
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Fig. 6. I}V characteristics of a poly-Si solar cell fabricated by ICP}CVD.
poly-Si "lm deposited at SiH Cl "0.4 sccm. This trend of the XRD peak intensity was the same as that of Raman crystallinity. From the Raman and XRD intensities the optimum #ow rate of SiH Cl seems to be 0.4 sccm, under this condition the Raman polycrystalline and nanocrystalline peaks are 519.5 and 516 cm\, respectively. The polycrystalline volume fraction and FWHM for the "lm are 88% and 7 cm\, respectively. Fig. 5 shows transmission electron microscopy (TEM) bright-"eld images and their selected area di!raction (SAD) patterns for the poly-Si "lms deposited at various SiH Cl #ow rates. The sum of SiH Cl and SiH #ow rates was 0.6 sccm. The poly-Si deposited using SiH , has a smaller grain size than that using SiH Cl . The grain size becomes larger when both SiH Cl and SiH are used. When the #ow rate of SiH Cl is 0.4 sccm, the largest grains of &1200 As was obtained with clear grain boundary, indicating a good crystalline quality. The SAD pattern of this sample also shows spotted rings rather than line rings because of its good crystalline quality. Fig. 6 shows current}voltage characteristics of a solar cell using the poly-Si by ICP-CVD. This fabrication process for the cells is as follows: on MoW/Glass substrates, polycrystalline n-, i-, and p-layers were deposited at a temperature of 3003C. The solar cell exhibit an e$ciency of 3.14% (V "0.23 V, J "19.7 mA/cm, FF"0.54) at 79 mW/cm. The poly-Si quality can be improved signi"cantly by adding SiH Cl into SiH plasma. But an optimum #ow rate ratio of SiH Cl to SiH seems to be 2. The Cl radicals appear to promote the growth of crystalline phase when they reside on the growth surface.
4. Conclusion We fabricated poly-Si "lms and solar cell by ICP-CVD using SiH Cl /SiH /H mixtures. The poly-Si exhibited Raman polycrystalline volume fraction of 88%,
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
145
FWHM of 7 cm\ and TEM grain size of &1200 As , when the RF power and the mixing ratio of [SiH Cl ]/[SiH ] were 1500 W and 2, respectively. We conclude that the poly-Si quality can be improved signi"cantly by adding SiH Cl into SiH plasma. The solar cell using this material exhibited a conversion e$ciency of 3.14%.
Acknowledgements This work was supported by G-7 project of Korea.
References [1] R.B. Iverson, R. Reif, J. Appl. Phys. 62 (1987) 1675. [2] T. Sameshima, M. Hara, S. Usui, Jpn. J. Appl. Phys. 28 (1989) 1789. [3] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, Wiley Interscience, New York, 1994 Chapter 12. [4] Y. Ra, S.G. Bradely, C.H. Chen, J. Vac. Sci. Technol. A 12 (4) (1994) 1328. [5] T. Fukasawa, A. Nakamura, H. Shindo, Y. Horiike, Jpn. J. Appl. Phys. 33 (1994) 2139. [6] S. Ashida, M.R. Shim, M.A. Lieberman, J. Vac. Sci. Technol. A 14 (2) (1996) 391. [7] I. Shimizu, J. Hanna, H. Shirai, Mat. Res. Soc. Symp. Proc. 164 (1990) 195. [8] CRC Handbook of Chemistry and Physics, 59th Edition, CRC Press, FL, 1978 p. F224.
Polycrystalline silicon "lm deposited by ICP-CVD Byeong Yeon Moon , Jae Hyoung Youn, Sung Hwan Won, Jin Jang * School of Architecture, Environment and Life Science, Kyung Woon University, Kumi, Kyungbuk 730-850, South Korea Department of Information Display, Kyung Hee University, Seoul 130-701, South Korea Received 25 September 2000
Abstract We studied the deposition of polycrystalline silicon (poly-Si) using SiH /SiH Cl /H mix tures by inductively coupled plasma chemical vapor deposition. The deposition rate and crystalline quality were improved by increasing RF power. The poly-Si "lm deposited with the [SiH Cl ]/[SiH ] ratio of 2 and the RF power of 1500 W exhibited the deposition rate of 4.2 As /s, the polycrystalline volume fraction of 88%, the Raman FWHM of 7 cm\, and the TEM grain size of &1200 As . The solar cell made of this material exhibited a conversion e$ciency of 3.14%. 2001 Elsevier Science B.V. All rights reserved. Keywords: Poly-Si; SiH Cl ; ICP-CVD
1. Introduction Polycrystalline silicon (poly-Si) thin "lm has attracted much attention recently for application in solar cells. Solid-phase crystallization (SPC) of amorphous silicon is a common method of crystallizing amorphous silicon (a-Si), but its crystallization temperature is too high for use of large-area glass substrates [1]. Recently, liquidphase crystallization of a-Si using excimer laser has been intensively studied, but the thickness for crystallization is less than 100 nm because of a very high absorption
* Corresponding author. Department of Information Display, Kyung Hee University, Seoul 130-701, South Korea. Tel.: #82-2-961-0270; fax: #82-2-968-6924. E-mail address: [email protected] (J. Jang). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 7 - 1
140
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
coe$cient of a-Si for the laser line [2]. The best way of preparing poly-Si, therefore, may be a direct deposition of poly-Si at a temperature low enough to utilize inexpensive large-area glass substrates. Various high-density plasma sources of electron cyclotron resonance (ECR), helicon wave and inductively coupled RF plasma chemical vapor deposition (ICP-CVD) have been developed [3}5]. An ICP-CVD has some advantages in the growth of poly-Si at low temperatures because of its high plasma density of &10 cm\ and a good uniformity without external magnets and less ion bombardment on the growing surface [6]. SiH Cl is added in SiH plasma to enhance the "lm quality of poly-Si and to increase its deposition rate. The addition of Cl species to the plasma enhances etching of Si, and there is a strong chemical interaction between Cl and H atoms back-bonded with silicon atom because of the di!erence in the bond energies for Si}Cl (4.75 eV), Si}Si (2.4 eV) and Si}H (3.4 eV). Therefore, Cl radicals can break weak Si}Si bonds as H radicals do in the growth of microcrystalline silicon and thus promote the growth of crystalline phase [7,8].
2. Experiments The poly-Si "lms were fabricated by an ICP-CVD using the SiH Cl /SiH /H mixtures. The RF power at 13.56 MHz was supplied to produce the plasma. The inductively coupled azimuthal electrical "eld produced from a #at antenna generated the plasma. A spiral type antenna was placed on the top of a quartz plate on a stainless steel reactor. The gases were introduced from the dispersal ring into the reactor chamber. The sum of SiH Cl and SiH #ow rates, H #ow rate and substrate temperature were "xed at 0.6, 6 sccm and 3003C, respectively. The SiH Cl #ow rate was varied between 0 and 0.6 sccm and a RF power between 700 (4.4 W/cm) and 1500 W (9.4 W/cm) was applied to a 6 inductor. To obtain the polycrystalline volume fraction from Raman intensity, the transverse optical phonon (TO) mode spectra was decomposed into three parts: a polycrystalline (520 cm\), an amorphous (480 cm\) and an intermediate (500}510 cm\). The intermediate peak appears to be caused by the small grain nanocrystalline silicon for which the grain size is (10 nm. In this case, the crystalline volume fraction is given by I #I
, " I #I #I
(1)
where, I , I and I are Raman intensities of the amorphous, polycrystalline and
intermediate phases, respectively. For our "lms, the amorphous peak was not found. Thus, the ratio of the polycrystalline volume fraction is de"ned by I . " I #I
(2)
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
141
3. Results and discussion The optimization of the substrate temperature for the deposition of poly-Si using SiH Cl were carried out at the RF power of 700 W. SiH Cl and H #ow rates were "xed at 0.6 and 6 sccm, respectively. Fig. 1 shows the Raman polycrystalline volume fraction and the full-width at half-maximum (FWHM) of the poly-Si "lms deposited as a function of substrate temperature. The Raman polycrystalline volume fraction increases with substrate temperature up to 3003C, and then decreases. The maximum volume fraction and minimum FWHM were 85.1% and 8.9 cm\, respectively, at a substrate temperature of 300. The highest deposition rate was 2.6 As /s at 3003C.
Fig. 1. Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited using a H /SiH Cl mixture. (H "6 sccm/SiH Cl "0.6 sccm).
Fig. 2. The deposition rate of the poly-Si "lms fabricated by using the SiH Cl /SiH mixtures.
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B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
Under the existence of hydrogen bonded with silicon, Cl in a strong back-bond with Si was predominantly released from the "lms in the propagation of Si-network. When substrate temperature is less than 3003C, it is hard to eliminate chlorine remaining in the "lms even under the existence of an excessive amount of atomic hydrogen. Therefore, the deposition temperature was "xed at 3003C to have a good quality poly-Si. Fig. 2 shows the deposition rate of the poly-Si "lms as a function of RF power using SiH Cl and SiH mixtures. Deposition rate increased with RF power due to the increased dissociation rate of the feeding gas. The deposition rate reached a maximum when the #ow rate of ratio of [SiH C ] to [SiH ] was 2 keeping the total #ow rate at 0.6 sccm. The maximum deposition rate was 4.2 As /s at the RF power of 1500 W.
Fig. 3. Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited as a function of RF power. (SiH Cl "0.4 sccm/SiH "0.2 sccm).
Fig. 4. XRD spectra of the poly-Si "lms deposited at 1500 W as a variation of SiH Cl #ow rate.
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
143
Fig. 3 shows the Raman polycrystalline volume fraction and FWHM of the poly-Si "lms deposited as a function of RF power. The #ow rates of SiH Cl and SiH were "xed at 0.4 and 0.2 sccm, respectively. The RF power was varied from 700 to 1500 W. The polycrystalline volume fraction increased and FWHM decreased with increase of RF power up to 1000 W, and beam nearly saturated. The Raman peak position of the polycrystalline phase was &518 cm\, while the peak position of nanocrystalline (intermediate) phase, I , increased with increasing RF power. The grain size of poly-Si
"lm is related to the FWHM and peak position of Raman intensity. Therefore, the crystallinity seems to have improved by increasing RF power. Fig. 4 shows the X-ray di!raction (XRD) spectra of the poly-Si "lms as a function of SiH Cl #ow rate. The sum of SiH Cl and SiH #ow rates was "xed at 0.6 sccm. The XRD intensity exhibits (1 1 1), (2 2 0) and (3 1 1) crystalline peaks, and (1 1 1) peak is dominant for all the poly-Si "lms. The highest XRD peak intensity is shown for the
Fig. 5. TEM bright-"eld images and their SAD (selected area di!raction) patterns of the poly-Si "lms deposited at various SiH Cl #ow rates. [SiH "0.6 sccm (a); SiH Cl "0.6 sccm (b); SiH "0.4 sccm/SiH Cl "0.2 sccm (c); SiH "0.2 sccm/SiH Cl "0.4 sccm (d)].
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Fig. 6. I}V characteristics of a poly-Si solar cell fabricated by ICP}CVD.
poly-Si "lm deposited at SiH Cl "0.4 sccm. This trend of the XRD peak intensity was the same as that of Raman crystallinity. From the Raman and XRD intensities the optimum #ow rate of SiH Cl seems to be 0.4 sccm, under this condition the Raman polycrystalline and nanocrystalline peaks are 519.5 and 516 cm\, respectively. The polycrystalline volume fraction and FWHM for the "lm are 88% and 7 cm\, respectively. Fig. 5 shows transmission electron microscopy (TEM) bright-"eld images and their selected area di!raction (SAD) patterns for the poly-Si "lms deposited at various SiH Cl #ow rates. The sum of SiH Cl and SiH #ow rates was 0.6 sccm. The poly-Si deposited using SiH , has a smaller grain size than that using SiH Cl . The grain size becomes larger when both SiH Cl and SiH are used. When the #ow rate of SiH Cl is 0.4 sccm, the largest grains of &1200 As was obtained with clear grain boundary, indicating a good crystalline quality. The SAD pattern of this sample also shows spotted rings rather than line rings because of its good crystalline quality. Fig. 6 shows current}voltage characteristics of a solar cell using the poly-Si by ICP-CVD. This fabrication process for the cells is as follows: on MoW/Glass substrates, polycrystalline n-, i-, and p-layers were deposited at a temperature of 3003C. The solar cell exhibit an e$ciency of 3.14% (V "0.23 V, J "19.7 mA/cm, FF"0.54) at 79 mW/cm. The poly-Si quality can be improved signi"cantly by adding SiH Cl into SiH plasma. But an optimum #ow rate ratio of SiH Cl to SiH seems to be 2. The Cl radicals appear to promote the growth of crystalline phase when they reside on the growth surface.
4. Conclusion We fabricated poly-Si "lms and solar cell by ICP-CVD using SiH Cl /SiH /H mixtures. The poly-Si exhibited Raman polycrystalline volume fraction of 88%,
B. Yeon Moon et al. / Solar Energy Materials & Solar Cells 69 (2001) 139}145
145
FWHM of 7 cm\ and TEM grain size of &1200 As , when the RF power and the mixing ratio of [SiH Cl ]/[SiH ] were 1500 W and 2, respectively. We conclude that the poly-Si quality can be improved signi"cantly by adding SiH Cl into SiH plasma. The solar cell using this material exhibited a conversion e$ciency of 3.14%.
Acknowledgements This work was supported by G-7 project of Korea.
References [1] R.B. Iverson, R. Reif, J. Appl. Phys. 62 (1987) 1675. [2] T. Sameshima, M. Hara, S. Usui, Jpn. J. Appl. Phys. 28 (1989) 1789. [3] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, Wiley Interscience, New York, 1994 Chapter 12. [4] Y. Ra, S.G. Bradely, C.H. Chen, J. Vac. Sci. Technol. A 12 (4) (1994) 1328. [5] T. Fukasawa, A. Nakamura, H. Shindo, Y. Horiike, Jpn. J. Appl. Phys. 33 (1994) 2139. [6] S. Ashida, M.R. Shim, M.A. Lieberman, J. Vac. Sci. Technol. A 14 (2) (1996) 391. [7] I. Shimizu, J. Hanna, H. Shirai, Mat. Res. Soc. Symp. Proc. 164 (1990) 195. [8] CRC Handbook of Chemistry and Physics, 59th Edition, CRC Press, FL, 1978 p. F224.