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Electronic properties of silicon thin films prepared by hot-wire chemical vapour deposition
Journal of Non-Crystalline Solids 266±269 (2000) 258±262
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Electronic properties of silicon thin ®lms prepared by hot-wire chemical vapour deposition R. Br uggemann a,*, J.P. Kleider a, C. Longeaud a, D. Mencaraglia a, J. Guillet b, J.E. Bouree b, C. Niikura b a
Laboratoire de G enie Electrique de Paris (CNRS, UMR8507), Sup elec, Universit es Paris VI et Paris XI, Plateau de Moulon, F-91192 Gif-sur-Yvett cedex, France b Laboratoire de Physique des Interfaces et des Couches Minces (CNRS, UMR7647), Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau cedex, France
Abstract A transition from amorphous to microcrystalline silicon occurs in hot-wire chemical vapour deposition silicon ®lms with increasing dilution of silane with hydrogen. This transition is detected for a dilution ratio R [SiH4 ]/[H2 ] between 10% and 9%, where [SiH4 ] and [H2 ] are the silane and hydrogen ¯ow rates, by Raman and optical absorption spectra, and by dark conductivities which are several orders of magnitude larger in microcrystalline as compared to amorphous ®lms. In the microcrystalline ®lms we observe a simultaneous increase of both majority and minority carrier mobilitylifetime products with increasing hydrogen dilution, which is consistent with the measured decrease in sub-gap absorption and defect density deduced from transient photocurrent measurements. This simultaneous increase is in contrast with the general trend observed in amorphous ®lms, where these two quantities vary in opposite ways, and are associated with an improvement of the transport properties of the material. The microcrystalline samples did not show light-induced degradation after prolonged illumination. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Hot-wire chemical vapour deposition (HWCVD) [1±9] is known to be an interesting technique for the deposition of silicon thin ®lms. Depending on the deposition parameters either amorphous or microcrystalline silicon may be prepared. In this paper we measure the electronic properties of such ®lms by means of several com-
* Corresponding author. Tel.: +33-1 69 85 16 42; fax: +33-1 69 41 83 18. E-mail address: [email protected] (R. BruÈggemann).
plementary techniques and concentrate on the eect of dilution of silane in hydrogen.
2. Experiments The ®lms were deposited on glass substrates (Corning 7059) in a high-vacuum HWCVD system. Details about the reactor can be found in Ref. [4]. All ®lms were deposited at a wire and substrate temperature of 1500°C and 300°C, respectively, at a total pressure of 0.1 mbar, and with a substrate-to-wire distance of 3 cm. The total gas ¯ow rate (silane plus hydrogen) was also kept constant at 150 sccm. We concentrate on a series of ®lms prepared with the ratio R, de®ned
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 3 1 - 5
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
as the silane-to-hydrogen ¯ux R [SiH4 ]/[H2 ], ranging between 5% and 11% (i.e., between 0.05 and 0.11). Same ®lm properties were determined using optical spectroscopy, Raman spectroscopy, dark- and steady-state photocurrent (SSPC), transient photocurrent (TPC), constant photocurrent method (CPM), and steady-state photocarrier grating (SSPG). In addition, samples were lightsoaked by water-®ltered white light illumination (500 mW/cm2 ) for more than one day, both at room-temperature and at 80°C. All measurements were performed under atmospheric pressure. 3. Results The Raman spectra in Fig. 1 show the structural dierences of the samples with hydrogen dilution with the crystalline volume fraction, RC , indicated on this ®gure. Fitting the spectra and taking the RC of the areas under the ®t curves of the amorphous (a) and crystalline (x) contributions de®nes RC according to RC x= x a. The absorption coecients of the samples deduced from CPM measurements and optical spectroscopy are shown in Fig. 2. For R 10%, the spectrum is typical of amorphous silicon with the increase at energies >1.3 eV characteristic of the Urbach edge followed by a defect shoulder at smaller energy.
259
For R 6 9% this edge is not detected and the spectrum is close to that of crystalline silicon. Fig. 3 shows that the room-temperature dark conductivity, rd , continuously decreases with decreasing R with an abrupt transition between 9% and 10%, where the decrease in rd is more than two orders of magnitude. Fig. 4 shows the variations of both the electron mobility-lifetime product (ls)n determined from SSPC measurements, and the ambipolar diusion length, Ld , characteristic for the minority-carrier properties and determined from SSPG measurements, as a function of the room-temperature rd . All samples have (ls)n s and Ld s above 10ÿ6 cmÿ2 /V and 90 nm, respectively. Fig. 5(a) shows the transient photocurrent obtained for R: 5%, 7% and 11%. It has been expressed in terms of drift mobility, by dividing the current by the appropriate e N0 A E factor, with the unit electron charge, e, the density of created photocarriers, N0 (assuming a quantum eciency of 1), the cross-sectional area of the current, A, and the electric ®eld, E. The transients are then converted into the density of states (DOS) of Fig. 5(b) after Fourier transformation and use of appropriate equations [10]. Finally, Fig. 6 shows the variation of the photocurrent with the light-soaking time for one sample prepared with R 7%. Obviously, there is no decrease of either the photocurrent or Ld during light-soaking.
Fig. 1. Raman spectroscopy of samples prepared with ®ve ratios of the silane-to-hydrogen gas ¯ux.
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R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
Fig. 2. Absorption coecient of samples for four ratios of the silane-to-hydrogen gas ¯ux, deduced from CPM data which were calibrated with optical transmission measurements. Solid line is part of the spectrum for crystalline silicon.
Fig. 4. Variations of the electron mobility-lifetime product (ls)n (a), and of the ambipolar diusion length Ld (b) as a function of the dark conductivity. (ls)n was determined at a photon ¯ux of 1014 cmÿ2 sÿ1 of red light (k 660 nm), while 1017 cmÿ2 sÿ1 was used for the determination of Ld . Lines are guides to the eye. Fig. 3. Dark conductivity as a function of the silane-to-hydrogen gas ¯ow rate R, indicating the transition from amorphous to microcrystalline between R of 9% and 10%.
4. Discussion A transition from amorphous to microcrystalline samples is observed, from the above results, when R changes from 10% to 9%. This transition is
particularly evident in the Raman spectra (Fig. 1) where the spectrum of the sample at R 10% does not contain the 520 cmÿ1 band due to crystalline material, while this band is observed in the spectra of samples with R 6 9% though there is still a contribution from an amorphous phase which decreases as R decreases. The transition is also observed in the changes in the absorption spectra (Fig. 2) and in the dark conductivities (Fig. 3).
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
261
Fig. 6. Dependence of the photocurrent on the light-soaking time for the sample prepared at a ratio of silane-to-hydrogen gas ¯ux of R 7%. Also indicated are the values of the ambipolar diusion length before and after one day of light-soaking. A line is drawn between the data symbols.
Fig. 5. (a) Time dependence of the transient photoconductivity after pulse excitation at k 590 nm, (b) DOS deduced from this transient measurement for three samples having dierent values of the silane-to-hydrogen gas ¯ow rate R.
These results are similar to those observed in ®lms deposited with dierent reactor geometries (Institut f ur Physikalische Elektronik, Universit at Stuttgart) [9]. The transition is less obvious in the optoelectronic properties. Indeed, the transport properties for both electrons and holes appear less aected by the amorphous or crystalline feature, since all (ls)n are between 10ÿ6 and 10ÿ5 cmÿ2 /V and all Ld s are between 90 and 160 nm, whether the samples are amorphous or microcrystalline. However, examination of Fig. 4(a) and (b) shows that for the amorphous samples the usual asymmetry is observed, namely that Ld decreases and (ls)n increases with rd , e.g., [11]. In contrast, the microcrystalline samples have an unusual symmetry, in that both (ls)n and Ld increase with rd . We suggest
that an increase of the transport properties of both types of carriers is obtained when R is decreased from 9% to 5%. This increase is consistent with the decrease of the absorption coecient observed in Fig. 2 for energies <1.1 eV. Moreover, it is also consistent with the density of states deduced from the TPC experiments (Fig. 5(b)). Indeed, in this ®gure the amorphous sample (R 11%) has a characteristic defect shoulder at the larger energies in addition to the conduction band tail. For the microcrystalline samples we observe a comparable density of shallow states but the defect shoulder is not detected as the dilution ratio R is decreased. This absence indicates an improvement of the material quality for photovoltaics since both types of carriers aect this application. In additional samples produced with R < 5% improvement was not observed. Finally, the result of Fig. 6 shows that no degradation is observed after illumination of the sample deposited at R 7%, as indicated by the nearly constant photocurrent and Ld before and after illumination. It is worth pointing out that no degradation was observed for smaller R, with lightsoaking being performed at either 25° or 80°C.
5. Conclusion By increasing the dilution of silane into hydrogen in the HWCVD deposition chamber, a
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R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
transition from amorphous to microcrystalline material occurs for a dilution ratio R [SiH4 ]/[H2 ] between 10% and 9%. If R is decreased from 9% to 5%, the crystalline fraction of the ®lms increases and the transport properties of both majority and minority carriers increase which we relate to the decrease of both sub-gap absorption and deep defect density. Acknowledgements The authors thank C. K ohler for Raman measurements. One of us (R.B.) gratefully acknowledges the Ministere des Aaires Etrangeres for ®nancial support. This work was also supported by CNRS and ADEME in the framework of ECODEV program. References [1] H. Wiesmann, A.K. Ghosh, T. McMahon, M. Strongin, J. Appl. Phys. 50 (1979) 3752. [2] H. Matsumura, Jpn. J. Appl. Phys. 25 (1986) L949.
[3] J. Cifre, J. Bertomeu, J. Puigdollers, M.C. Polo, J. Andreu, A. Lloret, Appl. Phys. A 59 (1994) 645. [4] A.R. Middya, A. Lloret, J. Perrin, J. Huc, J.L. Moncel, J.Y. Parey, G. Rose, in: E.A. Schi, M. Hack, A. Madan, A. Matsuda, M.J. Powell (Eds), Amorphous Silicon Technology ± 1995, Mater. Res. Soc., Pittsburgh, 1995, p. 119. [5] M. Heintze, R. Zedlitz, H.N. Wanka, M.B. Schubert, J. Appl. Phys. 79 (1996) 2699. [6] H.N. Wanka, R. Zedlitz, M. Heintze, M.B. Schubert, in: W. Freiesleben, W. Palz, H.A. Ossenbrink, P. Helm (Eds.), Proceedings of the 13th European Photovoltaic Solar Energy Conference, H.S. Stephens and Associates, Bedford, 1995, p. 1753. [7] H.N. Wanka, M.B. Schubert, A. Hierzenberger, V. Baumung, in: H.A. Ossenbrink, P. Helm, H. Ehmann (Eds), Proceedings of the 14th European Photovoltaic Solar Energy Conference, H.S. Stephens and Associates, Bedford, 1997, p. 1005. [8] R.E.I. Schropp, K.F. Feenstra, E.C. Molenbroek, H. Meiling, J.K. Rath, Philos. Mag. B 76 (1997) 309. [9] R. Br uggemann, W. Bronner, A. Hierzenberger, M.B. Schubert, I. Zrinscak, in: J.M. Marshall et al. (Eds.), Thin Film Materials and Devices ± Developments in Science and Technology, World Scienti®c, Singapore, 1999, p. 1. [10] C. Main, R. Br uggemann, D.P. Webb, S. Reynolds, Solid State Commun. 83 (1992) 401. [11] J. Kocka, C.E. Nebel, C.-D. Abel, Philos. Mag. B 63 (1991) 221.
www.elsevier.com/locate/jnoncrysol
Electronic properties of silicon thin ®lms prepared by hot-wire chemical vapour deposition R. Br uggemann a,*, J.P. Kleider a, C. Longeaud a, D. Mencaraglia a, J. Guillet b, J.E. Bouree b, C. Niikura b a
Laboratoire de G enie Electrique de Paris (CNRS, UMR8507), Sup elec, Universit es Paris VI et Paris XI, Plateau de Moulon, F-91192 Gif-sur-Yvett cedex, France b Laboratoire de Physique des Interfaces et des Couches Minces (CNRS, UMR7647), Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau cedex, France
Abstract A transition from amorphous to microcrystalline silicon occurs in hot-wire chemical vapour deposition silicon ®lms with increasing dilution of silane with hydrogen. This transition is detected for a dilution ratio R [SiH4 ]/[H2 ] between 10% and 9%, where [SiH4 ] and [H2 ] are the silane and hydrogen ¯ow rates, by Raman and optical absorption spectra, and by dark conductivities which are several orders of magnitude larger in microcrystalline as compared to amorphous ®lms. In the microcrystalline ®lms we observe a simultaneous increase of both majority and minority carrier mobilitylifetime products with increasing hydrogen dilution, which is consistent with the measured decrease in sub-gap absorption and defect density deduced from transient photocurrent measurements. This simultaneous increase is in contrast with the general trend observed in amorphous ®lms, where these two quantities vary in opposite ways, and are associated with an improvement of the transport properties of the material. The microcrystalline samples did not show light-induced degradation after prolonged illumination. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Hot-wire chemical vapour deposition (HWCVD) [1±9] is known to be an interesting technique for the deposition of silicon thin ®lms. Depending on the deposition parameters either amorphous or microcrystalline silicon may be prepared. In this paper we measure the electronic properties of such ®lms by means of several com-
* Corresponding author. Tel.: +33-1 69 85 16 42; fax: +33-1 69 41 83 18. E-mail address: [email protected] (R. BruÈggemann).
plementary techniques and concentrate on the eect of dilution of silane in hydrogen.
2. Experiments The ®lms were deposited on glass substrates (Corning 7059) in a high-vacuum HWCVD system. Details about the reactor can be found in Ref. [4]. All ®lms were deposited at a wire and substrate temperature of 1500°C and 300°C, respectively, at a total pressure of 0.1 mbar, and with a substrate-to-wire distance of 3 cm. The total gas ¯ow rate (silane plus hydrogen) was also kept constant at 150 sccm. We concentrate on a series of ®lms prepared with the ratio R, de®ned
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 3 1 - 5
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
as the silane-to-hydrogen ¯ux R [SiH4 ]/[H2 ], ranging between 5% and 11% (i.e., between 0.05 and 0.11). Same ®lm properties were determined using optical spectroscopy, Raman spectroscopy, dark- and steady-state photocurrent (SSPC), transient photocurrent (TPC), constant photocurrent method (CPM), and steady-state photocarrier grating (SSPG). In addition, samples were lightsoaked by water-®ltered white light illumination (500 mW/cm2 ) for more than one day, both at room-temperature and at 80°C. All measurements were performed under atmospheric pressure. 3. Results The Raman spectra in Fig. 1 show the structural dierences of the samples with hydrogen dilution with the crystalline volume fraction, RC , indicated on this ®gure. Fitting the spectra and taking the RC of the areas under the ®t curves of the amorphous (a) and crystalline (x) contributions de®nes RC according to RC x= x a. The absorption coecients of the samples deduced from CPM measurements and optical spectroscopy are shown in Fig. 2. For R 10%, the spectrum is typical of amorphous silicon with the increase at energies >1.3 eV characteristic of the Urbach edge followed by a defect shoulder at smaller energy.
259
For R 6 9% this edge is not detected and the spectrum is close to that of crystalline silicon. Fig. 3 shows that the room-temperature dark conductivity, rd , continuously decreases with decreasing R with an abrupt transition between 9% and 10%, where the decrease in rd is more than two orders of magnitude. Fig. 4 shows the variations of both the electron mobility-lifetime product (ls)n determined from SSPC measurements, and the ambipolar diusion length, Ld , characteristic for the minority-carrier properties and determined from SSPG measurements, as a function of the room-temperature rd . All samples have (ls)n s and Ld s above 10ÿ6 cmÿ2 /V and 90 nm, respectively. Fig. 5(a) shows the transient photocurrent obtained for R: 5%, 7% and 11%. It has been expressed in terms of drift mobility, by dividing the current by the appropriate e N0 A E factor, with the unit electron charge, e, the density of created photocarriers, N0 (assuming a quantum eciency of 1), the cross-sectional area of the current, A, and the electric ®eld, E. The transients are then converted into the density of states (DOS) of Fig. 5(b) after Fourier transformation and use of appropriate equations [10]. Finally, Fig. 6 shows the variation of the photocurrent with the light-soaking time for one sample prepared with R 7%. Obviously, there is no decrease of either the photocurrent or Ld during light-soaking.
Fig. 1. Raman spectroscopy of samples prepared with ®ve ratios of the silane-to-hydrogen gas ¯ux.
260
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
Fig. 2. Absorption coecient of samples for four ratios of the silane-to-hydrogen gas ¯ux, deduced from CPM data which were calibrated with optical transmission measurements. Solid line is part of the spectrum for crystalline silicon.
Fig. 4. Variations of the electron mobility-lifetime product (ls)n (a), and of the ambipolar diusion length Ld (b) as a function of the dark conductivity. (ls)n was determined at a photon ¯ux of 1014 cmÿ2 sÿ1 of red light (k 660 nm), while 1017 cmÿ2 sÿ1 was used for the determination of Ld . Lines are guides to the eye. Fig. 3. Dark conductivity as a function of the silane-to-hydrogen gas ¯ow rate R, indicating the transition from amorphous to microcrystalline between R of 9% and 10%.
4. Discussion A transition from amorphous to microcrystalline samples is observed, from the above results, when R changes from 10% to 9%. This transition is
particularly evident in the Raman spectra (Fig. 1) where the spectrum of the sample at R 10% does not contain the 520 cmÿ1 band due to crystalline material, while this band is observed in the spectra of samples with R 6 9% though there is still a contribution from an amorphous phase which decreases as R decreases. The transition is also observed in the changes in the absorption spectra (Fig. 2) and in the dark conductivities (Fig. 3).
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
261
Fig. 6. Dependence of the photocurrent on the light-soaking time for the sample prepared at a ratio of silane-to-hydrogen gas ¯ux of R 7%. Also indicated are the values of the ambipolar diusion length before and after one day of light-soaking. A line is drawn between the data symbols.
Fig. 5. (a) Time dependence of the transient photoconductivity after pulse excitation at k 590 nm, (b) DOS deduced from this transient measurement for three samples having dierent values of the silane-to-hydrogen gas ¯ow rate R.
These results are similar to those observed in ®lms deposited with dierent reactor geometries (Institut f ur Physikalische Elektronik, Universit at Stuttgart) [9]. The transition is less obvious in the optoelectronic properties. Indeed, the transport properties for both electrons and holes appear less aected by the amorphous or crystalline feature, since all (ls)n are between 10ÿ6 and 10ÿ5 cmÿ2 /V and all Ld s are between 90 and 160 nm, whether the samples are amorphous or microcrystalline. However, examination of Fig. 4(a) and (b) shows that for the amorphous samples the usual asymmetry is observed, namely that Ld decreases and (ls)n increases with rd , e.g., [11]. In contrast, the microcrystalline samples have an unusual symmetry, in that both (ls)n and Ld increase with rd . We suggest
that an increase of the transport properties of both types of carriers is obtained when R is decreased from 9% to 5%. This increase is consistent with the decrease of the absorption coecient observed in Fig. 2 for energies <1.1 eV. Moreover, it is also consistent with the density of states deduced from the TPC experiments (Fig. 5(b)). Indeed, in this ®gure the amorphous sample (R 11%) has a characteristic defect shoulder at the larger energies in addition to the conduction band tail. For the microcrystalline samples we observe a comparable density of shallow states but the defect shoulder is not detected as the dilution ratio R is decreased. This absence indicates an improvement of the material quality for photovoltaics since both types of carriers aect this application. In additional samples produced with R < 5% improvement was not observed. Finally, the result of Fig. 6 shows that no degradation is observed after illumination of the sample deposited at R 7%, as indicated by the nearly constant photocurrent and Ld before and after illumination. It is worth pointing out that no degradation was observed for smaller R, with lightsoaking being performed at either 25° or 80°C.
5. Conclusion By increasing the dilution of silane into hydrogen in the HWCVD deposition chamber, a
262
R. Br uggemann et al. / Journal of Non-Crystalline Solids 266±269 (2000) 258±262
transition from amorphous to microcrystalline material occurs for a dilution ratio R [SiH4 ]/[H2 ] between 10% and 9%. If R is decreased from 9% to 5%, the crystalline fraction of the ®lms increases and the transport properties of both majority and minority carriers increase which we relate to the decrease of both sub-gap absorption and deep defect density. Acknowledgements The authors thank C. K ohler for Raman measurements. One of us (R.B.) gratefully acknowledges the Ministere des Aaires Etrangeres for ®nancial support. This work was also supported by CNRS and ADEME in the framework of ECODEV program. References [1] H. Wiesmann, A.K. Ghosh, T. McMahon, M. Strongin, J. Appl. Phys. 50 (1979) 3752. [2] H. Matsumura, Jpn. J. Appl. Phys. 25 (1986) L949.
[3] J. Cifre, J. Bertomeu, J. Puigdollers, M.C. Polo, J. Andreu, A. Lloret, Appl. Phys. A 59 (1994) 645. [4] A.R. Middya, A. Lloret, J. Perrin, J. Huc, J.L. Moncel, J.Y. Parey, G. Rose, in: E.A. Schi, M. Hack, A. Madan, A. Matsuda, M.J. Powell (Eds), Amorphous Silicon Technology ± 1995, Mater. Res. Soc., Pittsburgh, 1995, p. 119. [5] M. Heintze, R. Zedlitz, H.N. Wanka, M.B. Schubert, J. Appl. Phys. 79 (1996) 2699. [6] H.N. Wanka, R. Zedlitz, M. Heintze, M.B. Schubert, in: W. Freiesleben, W. Palz, H.A. Ossenbrink, P. Helm (Eds.), Proceedings of the 13th European Photovoltaic Solar Energy Conference, H.S. Stephens and Associates, Bedford, 1995, p. 1753. [7] H.N. Wanka, M.B. Schubert, A. Hierzenberger, V. Baumung, in: H.A. Ossenbrink, P. Helm, H. Ehmann (Eds), Proceedings of the 14th European Photovoltaic Solar Energy Conference, H.S. Stephens and Associates, Bedford, 1997, p. 1005. [8] R.E.I. Schropp, K.F. Feenstra, E.C. Molenbroek, H. Meiling, J.K. Rath, Philos. Mag. B 76 (1997) 309. [9] R. Br uggemann, W. Bronner, A. Hierzenberger, M.B. Schubert, I. Zrinscak, in: J.M. Marshall et al. (Eds.), Thin Film Materials and Devices ± Developments in Science and Technology, World Scienti®c, Singapore, 1999, p. 1. [10] C. Main, R. Br uggemann, D.P. Webb, S. Reynolds, Solid State Commun. 83 (1992) 401. [11] J. Kocka, C.E. Nebel, C.-D. Abel, Philos. Mag. B 63 (1991) 221.