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Hydrogen at compact sites in hot-wire chemical vapour deposited polycrystalline silicon films
Journal of Non-Crystalline Solids 266±269 (2000) 190±194
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Hydrogen at compact sites in hot-wire chemical vapour deposited polycrystalline silicon ®lms J.K. Rath a,*, R.E.I. Schropp a, W. Beyer b a
Interface Physics, Ornstein Laboratory, Debye Institute, Utrecht University, P.O. Box 80000, NL-3508 TA Utrecht, The Netherlands b Forschungszentrum J ulich, Institut f ur Schicht- und Ionentechnik, D-52425 J ulich, Germany
Abstract Polycrystalline silicon (poly-silicon) ®lms made by hot-wire chemical vapour deposition (HWCVD) in a controlled range of deposition conditions show the Si±H stretching vibration at 2000 cmÿ1 in infrared absorption. This band is in contrast to commonly observed IR spectra in plasma enhanced chemical vapour deposition (PECVD) poly-silicon ®lms in which the Si±H vibration is at 2100 cmÿ1 . Raman spectra of the stretching vibration con®rmed that the 2000 cmÿ1 mode is from the crystalline region (top) of the material. The thickness dependence of the single hydrogen eusion maximum (640°C) reveal a diusion-limited eusion of hydrogen migration similar to the case of the high-temperature hydrogen eusion maximum of amorphous silicon deposited at 25°C. The diusion pro®le of implanted deuterium con®rms the diusive property in a compact material. The diusion is through isolated Si±H sites at a trap depth of 1.5 eV determined by the hydrogen chemical potential. These isolated Si±H bonds, being located at compact sites, have the vibrational mode at 2000 cmÿ1 in their infrared spectra. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Defect passivation in polycrystalline Si (poly-Si) is essential for achieving the largest photosensitivity. In a low-temperature (<600°C) deposition CVD process this sensitivity is achieved in situ during growth and post-deposition hydrogen passivation is not needed. In our case this was achieved by hot-wire chemical vapour deposition (HWCVD). Device-quality poly-Si ®lms without post-deposition hydrogenation have been incorporated in photovoltaic devices and n±i±p solar cell on a stainless steel (SS) substrate had an e-
* Corresponding author. Tel.: +31-30 253 2467; fax: +31-30 254 3165. E-mail address: [email protected] (J.K. Rath).
ciency of 4.41% [1]. Knowledge of hydrogen bonding and transport in the polycrystalline network is essential for understanding the microstructure and grain boundary properties. In amorphous silicon, the incorporated hydrogen concentration, which depends on the growth temperature and the deposition process, determines the distribution of hydrogen at various sites such as isolated sites, voids or even bubbles [2]. At present it is not known if a similar distribution can be expected in poly-Si. Moreover, in poly-Si, hydrogen is assumed to be predominantly accommodated in the grain boundary region rather than in the interior of the crystals owing to the solubility limit of the crystalline lattice [3]. The microscopic structure of the grain boundaries has long-range order imposed by the adjacent crystallites [3]. In this paper we present the hydrogen
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 1 3 - 3
J.K. Rath et al. / Journal of Non-Crystalline Solids 266±269 (2000) 190±194
bonding con®guration in device-quality poly-Si made by HWCVD. Details of the structure and opto-electronic properties reveal [4] that these ®lms in many respects dier from poly-Si or microcrystalline silicon ®lms made by plasma enhanced chemical vapour deposition (PECVD). Most important is the observation of the stretching mode vibration at 2000 cmÿ1 in our HWCVD poly-Si ®lms made in a controlled regime, instead of at 2100 cmÿ1 commonly reported in the literature for microcrystalline ®lms [5]. The purpose of this paper is to make an evaluation of the origin of this absorption mode. 2. Experimental Poly-Si ®lms were deposited on 10 cm ´ 10 cm Corning 7059 glass substrates by HWCVD in one of the chambers of an ultra-high-vacuum multichamber system (PASTA). Details of the hot wireset up and deposition process have been described elsewhere [6]. The thickness of the ®lms was measured with a pro®lometer (Dektak) and with re¯ection/transmission measurements. Fourier transform infrared spectroscopy (FTIR) was performed with a spectrometer (Perkin±Elmer). Raman measurements were performed using an argon ion laser at a wavelength of 514.5 nm and power of 25 mW. Hydrogen eusion experiments were performed with a heating rate of 20 K/min. Deuterium was implanted with an implantation energy of 30 keV. The implantation dose (1 ´ 1016 cmÿ2 ) was chosen to keep the deuterium concentration smaller than the hydrogen content. For secondary-ion mass spectroscopy (SIMS) pro®ling, an oxygen (O 2 ) sputtering beam at normal incidence was used and positive secondary ions were detected. 3. Results By changing the hydrogen dilution in the source gas mixture a range of Si±H stretching vibrations are obtained in HWCVD poly-Si ®lms. For ®lms deposited with small hydrogen dilution (90%), Poly2, the vibration is exclusively at 2000 cmÿ1
191
Fig. 1. Hydrogen eusion as a function of eusion temperature for Poly1 and Poly2 ®lms. Error is 5%.
whereas those made with larger hydrogen dilution (99%), Poly1, it is exclusively at 2100 cmÿ1 [4]. With other deposition conditions [7], both 2000 and 2100 cmÿ1 modes are observed. The 2100 cmÿ1 vibration is attributed to hydrogen on the surfaces of crystalline grains, commonly observed in case of PECVD poly-Si ®lms [5]. However, the presence of the Si±H vibration exclusively at 2000 cmÿ1 in the sample Poly2 has not been explained in the literature. To understand the bonding con®guration, hydrogen eusion experiments were conducted. Fig. 1 shows the hydrogen eusion pro®les of Poly1 and Poly2 ®lms. Poly1 shows a distribution of eusion maxima. The low temperature (LT) contribution is attributed to molecular hydrogen desorption through interconnected voids present in the material [8]. This analysis is supported by the structure of Poly1 ®lm (seen from cross-sectional transmission electron microscopy (XTEM) [1]) which showed a large number of interconnected voids. The Poly2 ®lm shows only one maximum between 550°C and 650°C. This maximum is similar to the eusion of H in a-Si:H. To ascertain the eusion mechanism of such a maximum and determine the Si±H bonding con®guration, eusion experiments were done on samples with various thickness, i.e., 180, 480, 1060, 1470 and 2350 nm. The eusion processes are of two types; (i) surface desorption process of hydrogen into an interconnected network of voids, generally observed as the LT maximum in samples made at a low substrate temperature or in microcrystalline silicon ®lms, (ii) diusion of atomic hydrogen in compact material. The diusion coecient, D, is
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de®ned as D D0 exp ÿED =kT , where D0 is the diusion prefactor and ED is a diusion energy. The temperature, TM , of the eusion maximum is related to D by the expression [9] 2 ln D=ED ln d 2 b=p2 kTM
ln D0 =ED ÿ ED =kTM ;
1
where b is the heating rate (20 K/min) and d is the thickness of the ®lm. From measurement of TM as a function of d or b, the diusion coecient can be evaluated. In our case we ®xed b and varied the thickness. Fig. 2(a) shows the thickness dependence of the eusion maximum of the Poly2 ®lm. The shift of TM with increasing thickness con®rms a diusive process for the hydrogen. This dependence is similar to hydrogen at compact Si±H sites in a-Si:H
Fig. 2. (a) Thickness dependence of the hydrogen eusion temperature maximum, TM . The thin line through the symbols is drawn as guide for the eyes. Data for a-Si:H are from W. 2 Beyer [9]. (b) ln d 2 b=p2 kTM as a function of 1/TM for Poly2 ®lms of curve (a). The function, ln d 2 b=p2 kTM2 A B=TM is ®t to the data with a correlation coecient, R 0:9.
materials which is also diusion limited [9]. However, the dependence show that hydrogen diusion in Poly2 diers from that for device-quality a-Si:H ®lms made at 300°C, as the thickness dependence of TM resemble the high temperature (HT) maximum in the hydrogen eusion of a-Si:H ®lm (CH 40%) made at substrate temperature TS 25°C [9]. From Fig. 2(b), the slope of the function, ln d 2 =TM2 A B=TM , ®tted to the data gives an activation energy, ED , of 2.36 0.05 eV. This energy diers from ED 1:5 eV observed for standard amorphous silicon. However, this diusion energy agrees with that of the HT maximum (2.3 eV) within error in the data [9] of low TS aSi:H. This suggests that a polymeric type a-Si:H may be present in the poly-Si through which hydrogen diuses. However the absence of any LT maximum in the eusion pro®le of the Poly2 sample indicates that the hydrogen environments are not similar to low TS a-Si:H samples. To investigate these dierences, diusion measurements of implanted deuterium in poly-Si were made. The diusion pro®les of D implanted material was ®tted to a complementary error function (erfc) and from the relation D D0 exp ÿED =kT , we obtained D0 20 cm2 /s and ED 2.06 0.05 eV. The diusion coecient, D, at 400°C is (6.30 0.1 ´ 10ÿ15 cm2 /s. These coecients again are correlated well with the HT peak of low TS deposited ®lms : D0 100 cm2 /s and ED 2.3 eV [9]. On the other hand, they dier from the standard a-Si:H which has typically D0 3 ´ 10ÿ2 cm2 / s and ED 1.6 eV [9]. Using the D0 s and ED s of poly-Si ®lms obtained from SIMS, we calculate the TM using Eq. (1). We obtained TM 630°C which agrees well with that obtained from the eusion experiment at 640°C. Another interesting observation is that the diusion is not time dependent. The time dependence of diusion, D D tÿa has been attributed to loss of hydrogen which moves into voids during diusion [10]. Absence of the time dependence of diusion in Poly2 material essentially proves that very few voids are present in the material. From the above result (from the diusion properties being similar to a-Si:H material) one may infer that some amorphous region in the ®lm is involved in the 2000 cmÿ1 mode. However, the
J.K. Rath et al. / Journal of Non-Crystalline Solids 266±269 (2000) 190±194
Fig. 3. Raman spectra of Poly2 and Poly1 ®lm deposited on c-Si. Error is 10%.
XTEM picture of the Poly2 made on c-Si wafer (the samples used in FTIR and eusion experiments) showed that the interface does not contain any detectable amorphous region [11]. The poly-Si at the top is a densely packed crystalline region, con®rmed by the Raman spectrum (TO band) at 520 cmÿ1 and Hall measurements of mobility with an activation energy of 0.012 eV [7]. We used Raman measurements to detect the stretching modes. As the beam penetration depth is only 100 nm we can be certain that we are detecting only the top crystalline region. Fig. 3 shows the Raman spectrum of the samples. We observed that the Poly2 has only one band at 2000 cmÿ1 whereas the Poly1 sample has a band at 2100 cmÿ1 , consistent with the IR result. These data give de®nite proof that the 2000 cmÿ1 mode in Poly2 is indeed due to Si±H bonds in the crystalline region and not to Si±H bonds in amorphous phase.
4. Discussion In a polycrystalline ®lm Si±H bonds can reside in three con®gurations: (i) dispersed in the interior of crystals, (ii) at grain boundaries, on the surfaces of crystal columns and (iii) in amorphous regions. From this work, the third case can be excluded in our case for the Poly2 material on c-Si substrate. However, it is dicult to understand why the diffusion constants are similar to low TS a-Si:H. We explain the diusion by assuming that in the eusion experiments on low TS a-Si:H, the HT eusion maximum is obtained after the eusion of the
193
LT maximum during which most of the hydrogen has eused out of the system. The HT eusion maximum would thus be same as in the case of an a-Si:H sample containing smaller hydrogen content. In fact an increase of ED and D0 has been observed for samples made at high (>500°C) substrate temperature [10]. For such an a-Si:H sample containing only 1% hydrogen, D0 and ED values of 120 cm2 /s and 2.1 eV, respectively, have been measured which agree with the D0 and ED for our poly-Si sample and the HT peak of the low TS aSi:H sample. We suggest that the low initial hydrogen content in our poly-Si ®lms (8.3 ´ 1020 cmÿ3 ) is a possible reason for these diusion constants. This diusion property can be explained by a model proposed for compact a-Si:H ®lms [12]. In the hydrogen density of states the hydrogen chemical potential, lH , de®nes the energy above which the shallow un®lled traps are located and below which are the deep traps. The trap depth, Etr ÿ lH , where Etr is the energy of hydrogen transport, can be obtained from the relation, D DH0 exp ÿ Etr ÿ lH =kT , where DH0 is the microscopic diusion coecient. Assuming DH0 10ÿ3 cm2 /s [10], we obtained the trap depth 1.5 eV (for our Poly2 sample) which is greater than the trap depth for standard amorphous silicon (10% hydrogen) and agrees with the trap depth for the a-Si:H containing similar low hydrogen content (1%) [10]. The activation energy of our Poly2 ®lm can be explained by a statisical shift of the chemical potential given by the relation [10] where ED Etr ÿ lH T Etr ÿ lH 0 ÿ cT , Etr ÿ lH (0). The simultaneous increase of D0 and ED is due to Meyer±Neldel type of eect [10]. This diusion energy is a con®rmation of the compact structure of the ®lm. These Si±H bonds with greater binding energies, being located at compact sites, have a vibrational mode at 2000 cmÿ1 . Though, it is at present not possible to ascertain whether such sites are in the inter-columnar region or inside the crystals, the similarities of the diusion coecient to amorphous ®lms indicates that strained Si±Si bonds are involved in the migration. The inter-columnar regions are sites in which the probability of being occupied by H is greater than in other possible sites. The proximity of crystal columns facilitates strained bonds between the
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columns, creating a compact network. The hydrogen in isolated Si±H bonds in this region migrates through these strained bonds. 5. Conclusion Poly-silicon produced by HWCVD in a narrow regime of growth contains hydrogen at compact sites. The IR stretching mode vibration at 2000 cmÿ1 is attributed to these isolated Si±H sites. As in the case of amorphous silicon, the hydrogen migration in our HWCVD poly-silicon is diusion limited. A trap depth of 1.5 eV is obtained from a hydrogen density of states model. Statistical shift of the hydrogen chemical potential and Meyer± Neldel eects have been proposed to explain diffusion pre-factor and activation energy. The diffusion is time independent. All these results con®rm that very few voids, bubbles or hydrogen molecules are present in our samples, which is therefore compact in structure. Acknowledgements The authors thank Karine van der Werf of Utrecht University for sample deposition and F.D. Tichelaar of TU Delft for XTEM analysis. This research was partially funded by NOVEM (Netherlands Organisation for Energy and Environment).
References [1] J.K. Rath, F.D. Tichelaar, R.E.I. Schropp, Solid State Phenomena 67&68 (1999) 465. [2] S. Acco, D.L. Williamson, W.G.J.H.M. van Sark, W.C. Sinke, W.F. van der Weg, A. Polman, S. Roorda, Phys. Rev. B 58 (1998) 12853. [3] D. Wei, G.N. Patey, Phys. Rev. A 46 (1992) 7783. [4] J.K. Rath, F.D. Tichelaar, H. Meiling, R.E.I. Schropp, in: R. Schropp, H. Branz, M. Hack, I. Shimizu, S. Wagner (Eds.), Amorphous and Microcrystalline Silicon Technology, vol. 507, 1998, p. 879. [5] J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A. Anna Selvan, N. Pellaton Vaucher, Ch. Hoh, D. Fischer, H. Keppner, A. Shah, K.-D. Ufert, P. Giannoules, J. Koeler, in: M. Hack, E.A. Schi, S. Wagner, R. Schropp, A. Matsuda (Eds.), Amorphous Silicon Technology, vol. 420, 1996, p. 879. [6] R.E.I. Schropp, K.F. Feenstra, E.C. Molenbroek, H. Meiling, J.K. Rath, Philos. Mag. B 76 (1997) 309. [7] J.K. Rath, H. Meiling, R.E.I. Schropp, Jpn. J. Appl. Phys. 36 (1997) 5436. [8] F. Finger, K. Prasad, S. Dubail, A. Shah, X.-M. Tang, J. Weber, W. Beyer, Mater. Res. Soc. Symp. Proc. 219 (1991) 383. [9] W. Beyer, in: D. Adler, H. Fritzsche (Eds.), TetrahedrallyBonded Amorphous Semiconductors, Plenum, New York, 1985, p. 129. [10] W. Beyer, H. Wagner, in: E.A. Schi, M. Hack, A. Madan, M. Powell, A. Matsuda (Eds.), Amorphous Silicon Technology, vol. 336, 1994, p. 323. [11] J.K. Rath, A.J.M.M. van Zutphen, H. Meiling, R.E.I. Schropp, in: S. Wagner, M. Hack, E.A. Schi, R. Schropp, I. Shimizu (Eds.), Amorphous and Microcrystalline Silicon Technology, vol. 467, 1997, p. 445. [12] W.B. Jackson, C.C. Tsai, Phys. Rev. B 45 (1992) 6564.
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Hydrogen at compact sites in hot-wire chemical vapour deposited polycrystalline silicon ®lms J.K. Rath a,*, R.E.I. Schropp a, W. Beyer b a
Interface Physics, Ornstein Laboratory, Debye Institute, Utrecht University, P.O. Box 80000, NL-3508 TA Utrecht, The Netherlands b Forschungszentrum J ulich, Institut f ur Schicht- und Ionentechnik, D-52425 J ulich, Germany
Abstract Polycrystalline silicon (poly-silicon) ®lms made by hot-wire chemical vapour deposition (HWCVD) in a controlled range of deposition conditions show the Si±H stretching vibration at 2000 cmÿ1 in infrared absorption. This band is in contrast to commonly observed IR spectra in plasma enhanced chemical vapour deposition (PECVD) poly-silicon ®lms in which the Si±H vibration is at 2100 cmÿ1 . Raman spectra of the stretching vibration con®rmed that the 2000 cmÿ1 mode is from the crystalline region (top) of the material. The thickness dependence of the single hydrogen eusion maximum (640°C) reveal a diusion-limited eusion of hydrogen migration similar to the case of the high-temperature hydrogen eusion maximum of amorphous silicon deposited at 25°C. The diusion pro®le of implanted deuterium con®rms the diusive property in a compact material. The diusion is through isolated Si±H sites at a trap depth of 1.5 eV determined by the hydrogen chemical potential. These isolated Si±H bonds, being located at compact sites, have the vibrational mode at 2000 cmÿ1 in their infrared spectra. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Defect passivation in polycrystalline Si (poly-Si) is essential for achieving the largest photosensitivity. In a low-temperature (<600°C) deposition CVD process this sensitivity is achieved in situ during growth and post-deposition hydrogen passivation is not needed. In our case this was achieved by hot-wire chemical vapour deposition (HWCVD). Device-quality poly-Si ®lms without post-deposition hydrogenation have been incorporated in photovoltaic devices and n±i±p solar cell on a stainless steel (SS) substrate had an e-
* Corresponding author. Tel.: +31-30 253 2467; fax: +31-30 254 3165. E-mail address: [email protected] (J.K. Rath).
ciency of 4.41% [1]. Knowledge of hydrogen bonding and transport in the polycrystalline network is essential for understanding the microstructure and grain boundary properties. In amorphous silicon, the incorporated hydrogen concentration, which depends on the growth temperature and the deposition process, determines the distribution of hydrogen at various sites such as isolated sites, voids or even bubbles [2]. At present it is not known if a similar distribution can be expected in poly-Si. Moreover, in poly-Si, hydrogen is assumed to be predominantly accommodated in the grain boundary region rather than in the interior of the crystals owing to the solubility limit of the crystalline lattice [3]. The microscopic structure of the grain boundaries has long-range order imposed by the adjacent crystallites [3]. In this paper we present the hydrogen
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 1 3 - 3
J.K. Rath et al. / Journal of Non-Crystalline Solids 266±269 (2000) 190±194
bonding con®guration in device-quality poly-Si made by HWCVD. Details of the structure and opto-electronic properties reveal [4] that these ®lms in many respects dier from poly-Si or microcrystalline silicon ®lms made by plasma enhanced chemical vapour deposition (PECVD). Most important is the observation of the stretching mode vibration at 2000 cmÿ1 in our HWCVD poly-Si ®lms made in a controlled regime, instead of at 2100 cmÿ1 commonly reported in the literature for microcrystalline ®lms [5]. The purpose of this paper is to make an evaluation of the origin of this absorption mode. 2. Experimental Poly-Si ®lms were deposited on 10 cm ´ 10 cm Corning 7059 glass substrates by HWCVD in one of the chambers of an ultra-high-vacuum multichamber system (PASTA). Details of the hot wireset up and deposition process have been described elsewhere [6]. The thickness of the ®lms was measured with a pro®lometer (Dektak) and with re¯ection/transmission measurements. Fourier transform infrared spectroscopy (FTIR) was performed with a spectrometer (Perkin±Elmer). Raman measurements were performed using an argon ion laser at a wavelength of 514.5 nm and power of 25 mW. Hydrogen eusion experiments were performed with a heating rate of 20 K/min. Deuterium was implanted with an implantation energy of 30 keV. The implantation dose (1 ´ 1016 cmÿ2 ) was chosen to keep the deuterium concentration smaller than the hydrogen content. For secondary-ion mass spectroscopy (SIMS) pro®ling, an oxygen (O 2 ) sputtering beam at normal incidence was used and positive secondary ions were detected. 3. Results By changing the hydrogen dilution in the source gas mixture a range of Si±H stretching vibrations are obtained in HWCVD poly-Si ®lms. For ®lms deposited with small hydrogen dilution (90%), Poly2, the vibration is exclusively at 2000 cmÿ1
191
Fig. 1. Hydrogen eusion as a function of eusion temperature for Poly1 and Poly2 ®lms. Error is 5%.
whereas those made with larger hydrogen dilution (99%), Poly1, it is exclusively at 2100 cmÿ1 [4]. With other deposition conditions [7], both 2000 and 2100 cmÿ1 modes are observed. The 2100 cmÿ1 vibration is attributed to hydrogen on the surfaces of crystalline grains, commonly observed in case of PECVD poly-Si ®lms [5]. However, the presence of the Si±H vibration exclusively at 2000 cmÿ1 in the sample Poly2 has not been explained in the literature. To understand the bonding con®guration, hydrogen eusion experiments were conducted. Fig. 1 shows the hydrogen eusion pro®les of Poly1 and Poly2 ®lms. Poly1 shows a distribution of eusion maxima. The low temperature (LT) contribution is attributed to molecular hydrogen desorption through interconnected voids present in the material [8]. This analysis is supported by the structure of Poly1 ®lm (seen from cross-sectional transmission electron microscopy (XTEM) [1]) which showed a large number of interconnected voids. The Poly2 ®lm shows only one maximum between 550°C and 650°C. This maximum is similar to the eusion of H in a-Si:H. To ascertain the eusion mechanism of such a maximum and determine the Si±H bonding con®guration, eusion experiments were done on samples with various thickness, i.e., 180, 480, 1060, 1470 and 2350 nm. The eusion processes are of two types; (i) surface desorption process of hydrogen into an interconnected network of voids, generally observed as the LT maximum in samples made at a low substrate temperature or in microcrystalline silicon ®lms, (ii) diusion of atomic hydrogen in compact material. The diusion coecient, D, is
192
J.K. Rath et al. / Journal of Non-Crystalline Solids 266±269 (2000) 190±194
de®ned as D D0 exp ÿED =kT , where D0 is the diusion prefactor and ED is a diusion energy. The temperature, TM , of the eusion maximum is related to D by the expression [9] 2 ln D=ED ln d 2 b=p2 kTM
ln D0 =ED ÿ ED =kTM ;
1
where b is the heating rate (20 K/min) and d is the thickness of the ®lm. From measurement of TM as a function of d or b, the diusion coecient can be evaluated. In our case we ®xed b and varied the thickness. Fig. 2(a) shows the thickness dependence of the eusion maximum of the Poly2 ®lm. The shift of TM with increasing thickness con®rms a diusive process for the hydrogen. This dependence is similar to hydrogen at compact Si±H sites in a-Si:H
Fig. 2. (a) Thickness dependence of the hydrogen eusion temperature maximum, TM . The thin line through the symbols is drawn as guide for the eyes. Data for a-Si:H are from W. 2 Beyer [9]. (b) ln d 2 b=p2 kTM as a function of 1/TM for Poly2 ®lms of curve (a). The function, ln d 2 b=p2 kTM2 A B=TM is ®t to the data with a correlation coecient, R 0:9.
materials which is also diusion limited [9]. However, the dependence show that hydrogen diusion in Poly2 diers from that for device-quality a-Si:H ®lms made at 300°C, as the thickness dependence of TM resemble the high temperature (HT) maximum in the hydrogen eusion of a-Si:H ®lm (CH 40%) made at substrate temperature TS 25°C [9]. From Fig. 2(b), the slope of the function, ln d 2 =TM2 A B=TM , ®tted to the data gives an activation energy, ED , of 2.36 0.05 eV. This energy diers from ED 1:5 eV observed for standard amorphous silicon. However, this diusion energy agrees with that of the HT maximum (2.3 eV) within error in the data [9] of low TS aSi:H. This suggests that a polymeric type a-Si:H may be present in the poly-Si through which hydrogen diuses. However the absence of any LT maximum in the eusion pro®le of the Poly2 sample indicates that the hydrogen environments are not similar to low TS a-Si:H samples. To investigate these dierences, diusion measurements of implanted deuterium in poly-Si were made. The diusion pro®les of D implanted material was ®tted to a complementary error function (erfc) and from the relation D D0 exp ÿED =kT , we obtained D0 20 cm2 /s and ED 2.06 0.05 eV. The diusion coecient, D, at 400°C is (6.30 0.1 ´ 10ÿ15 cm2 /s. These coecients again are correlated well with the HT peak of low TS deposited ®lms : D0 100 cm2 /s and ED 2.3 eV [9]. On the other hand, they dier from the standard a-Si:H which has typically D0 3 ´ 10ÿ2 cm2 / s and ED 1.6 eV [9]. Using the D0 s and ED s of poly-Si ®lms obtained from SIMS, we calculate the TM using Eq. (1). We obtained TM 630°C which agrees well with that obtained from the eusion experiment at 640°C. Another interesting observation is that the diusion is not time dependent. The time dependence of diusion, D D tÿa has been attributed to loss of hydrogen which moves into voids during diusion [10]. Absence of the time dependence of diusion in Poly2 material essentially proves that very few voids are present in the material. From the above result (from the diusion properties being similar to a-Si:H material) one may infer that some amorphous region in the ®lm is involved in the 2000 cmÿ1 mode. However, the
J.K. Rath et al. / Journal of Non-Crystalline Solids 266±269 (2000) 190±194
Fig. 3. Raman spectra of Poly2 and Poly1 ®lm deposited on c-Si. Error is 10%.
XTEM picture of the Poly2 made on c-Si wafer (the samples used in FTIR and eusion experiments) showed that the interface does not contain any detectable amorphous region [11]. The poly-Si at the top is a densely packed crystalline region, con®rmed by the Raman spectrum (TO band) at 520 cmÿ1 and Hall measurements of mobility with an activation energy of 0.012 eV [7]. We used Raman measurements to detect the stretching modes. As the beam penetration depth is only 100 nm we can be certain that we are detecting only the top crystalline region. Fig. 3 shows the Raman spectrum of the samples. We observed that the Poly2 has only one band at 2000 cmÿ1 whereas the Poly1 sample has a band at 2100 cmÿ1 , consistent with the IR result. These data give de®nite proof that the 2000 cmÿ1 mode in Poly2 is indeed due to Si±H bonds in the crystalline region and not to Si±H bonds in amorphous phase.
4. Discussion In a polycrystalline ®lm Si±H bonds can reside in three con®gurations: (i) dispersed in the interior of crystals, (ii) at grain boundaries, on the surfaces of crystal columns and (iii) in amorphous regions. From this work, the third case can be excluded in our case for the Poly2 material on c-Si substrate. However, it is dicult to understand why the diffusion constants are similar to low TS a-Si:H. We explain the diusion by assuming that in the eusion experiments on low TS a-Si:H, the HT eusion maximum is obtained after the eusion of the
193
LT maximum during which most of the hydrogen has eused out of the system. The HT eusion maximum would thus be same as in the case of an a-Si:H sample containing smaller hydrogen content. In fact an increase of ED and D0 has been observed for samples made at high (>500°C) substrate temperature [10]. For such an a-Si:H sample containing only 1% hydrogen, D0 and ED values of 120 cm2 /s and 2.1 eV, respectively, have been measured which agree with the D0 and ED for our poly-Si sample and the HT peak of the low TS aSi:H sample. We suggest that the low initial hydrogen content in our poly-Si ®lms (8.3 ´ 1020 cmÿ3 ) is a possible reason for these diusion constants. This diusion property can be explained by a model proposed for compact a-Si:H ®lms [12]. In the hydrogen density of states the hydrogen chemical potential, lH , de®nes the energy above which the shallow un®lled traps are located and below which are the deep traps. The trap depth, Etr ÿ lH , where Etr is the energy of hydrogen transport, can be obtained from the relation, D DH0 exp ÿ Etr ÿ lH =kT , where DH0 is the microscopic diusion coecient. Assuming DH0 10ÿ3 cm2 /s [10], we obtained the trap depth 1.5 eV (for our Poly2 sample) which is greater than the trap depth for standard amorphous silicon (10% hydrogen) and agrees with the trap depth for the a-Si:H containing similar low hydrogen content (1%) [10]. The activation energy of our Poly2 ®lm can be explained by a statisical shift of the chemical potential given by the relation [10] where ED Etr ÿ lH T Etr ÿ lH 0 ÿ cT , Etr ÿ lH (0). The simultaneous increase of D0 and ED is due to Meyer±Neldel type of eect [10]. This diusion energy is a con®rmation of the compact structure of the ®lm. These Si±H bonds with greater binding energies, being located at compact sites, have a vibrational mode at 2000 cmÿ1 . Though, it is at present not possible to ascertain whether such sites are in the inter-columnar region or inside the crystals, the similarities of the diusion coecient to amorphous ®lms indicates that strained Si±Si bonds are involved in the migration. The inter-columnar regions are sites in which the probability of being occupied by H is greater than in other possible sites. The proximity of crystal columns facilitates strained bonds between the
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columns, creating a compact network. The hydrogen in isolated Si±H bonds in this region migrates through these strained bonds. 5. Conclusion Poly-silicon produced by HWCVD in a narrow regime of growth contains hydrogen at compact sites. The IR stretching mode vibration at 2000 cmÿ1 is attributed to these isolated Si±H sites. As in the case of amorphous silicon, the hydrogen migration in our HWCVD poly-silicon is diusion limited. A trap depth of 1.5 eV is obtained from a hydrogen density of states model. Statistical shift of the hydrogen chemical potential and Meyer± Neldel eects have been proposed to explain diffusion pre-factor and activation energy. The diffusion is time independent. All these results con®rm that very few voids, bubbles or hydrogen molecules are present in our samples, which is therefore compact in structure. Acknowledgements The authors thank Karine van der Werf of Utrecht University for sample deposition and F.D. Tichelaar of TU Delft for XTEM analysis. This research was partially funded by NOVEM (Netherlands Organisation for Energy and Environment).
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