- Email: [email protected]
Helium effusion, diffusion and precipitation as a probe of microstructure in hydrogenated amorphous silicon
Journal of Non-Crystalline Solids 299–302 (2002) 254–258 www.elsevier.com/locate/jnoncrysol
Helium effusion, diffusion and precipitation as a probe of microstructure in hydrogenated amorphous silicon W. Beyer *, U. Zastrow Institut f€ur Photovoltaik, Forschungszentrum J€ulich, D-52425 J€ulich, Germany
Abstract The effusion of implanted helium was studied for doped and undoped radio frequency plasma-deposited a-Si:H films as well as for Si:H films deposited by other techniques. The results suggest that the observed helium diffusion and precipitation effects give information on the material microstructure. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 66.30.Jt; 68.55.)a; 81.70.Pg
1. Introduction
2. Experimental
Recently it was demonstrated that the effusion of implanted inert gas atoms like Ar or Ne is a useful method for structural characterization of amorphous silicon alloys [1,2]. Since these atoms do not react with other atoms, their out-diffusion can be used to detect openings and voids in the material. While argon and neon atoms have been found not to diffuse significantly in compact hydrogenated amorphous silicon (a-Si:H) at temperatures below crystallization, helium atoms are known to diffuse [3,4]. Here, we report on the structural characterization of a-Si:H and related materials by effusion measurements of implanted helium.
Most a-Si:H films investigated were deposited by standard plasma-enhanced chemical vapor deposition (PECVD). Typical deposition conditions involved a gas pressure of 0.5 mbar, a silane flow of 3 sccm and an rf power (13.56 MHz) of 10 W. Doping was achieved by adding flows of phosphine or diborane. We also investigated microcrystalline Si films (lc-Si:H) deposited by ECR plasma [5], hot wire (HW) a-Si:H [6] (employing a tantalum filament at T ¼ 1800 °C, a silane flow of 8 sccm and a pressure of 3 103 mbar), as well as DC sputtered a-Si:H (with unintentional incorporation of H only) [2]. The crystallinity of the lcSi:H films was verified by Raman measurements. As substrates, crystalline Si (c-Si), sapphire as well as c-Si platelets coated with about 30 nm SiO2 were used. Employing the latter, blistering and bubble formation of a-Si:H [7] was avoided. Typical film thickness was 1 lm and higher. Helium (Heþ ) was implanted at the energy of 30 or 40 keV
*
Corresponding author. Tel.: +49-2461 613 925; fax: +492461 613 735. E-mail address: [email protected] (W. Beyer).
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 0 0 4 - 3
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
255
Effusion spectra of helium and hydrogen of undoped and highly boron- and phosphorusdoped PECVD a-Si:H films deposited at a substrate temperature of 200 °C are shown in Figs. 1(a)–(c) (full lines). The dotted line in Fig. 1(a) refers to H effusion of material not implanted by helium for comparison. The close agreement with the H effusion of implanted material shows that He implantation has little influence on the material structure. In all three types of PECVD a-Si:H, H effusion shows a single effusion peak typical for the presence of compact material [3,9]. The H effusion maximum is found to lie near 600 °C for undoped and P-doped a-Si:H and near 450 °C for B-doped a-Si:H. This difference has been attributed to the dependence of H diffusion on the Fermi level [9].
Helium effusion is found to proceed in two stages. A first peak (termed low-temperature (LT) peak) appears near 400 °C and a second one (high temperature (HT) peak) near 700 °C for boron-doped a-Si:H, near 800 °C for undoped a-Si:H and near 900 °C for phosphorus-doped material. The small structure near 700 °C in P-doped a-Si:H is attributed to the crystallization of the material. Doping by both boron and phosphorus is seen to enhance the HT peak while the LT peak is decreased. Such enhanced HT helium effusion is also observed for (undoped) a-Si:H deposited by other deposition techniques as well as for lc-Si:H. This is demonstrated in Fig. 2 showing results for a-Si:H deposited by the HW method (Fig. 2(a)) and by DC sputtering (Fig. 2(b)) as well as for microcrystalline Si:H deposited in an ECR plasma (Fig. 2(c)). In agreement with effusion results of implanted He in crystalline Si [10] we attribute the LT effusion peak to the out-diffusion of He from network sites whereas the HT peak is associated with He precipitated in isolated voids or bubbles, leaving the material in a permeation process or upon void/ bubble rupture. In crystalline Si, the formation of He filled bubbles has been attributed to the implantation process and subsequent annealing [10]. In amorphous Si, however, the wide variation of
Fig. 1. He and H effusion spectra of He implanted PECVD aSi:H deposited at TS ¼ 200 °C. (a) Undoped (UD), (b) doped with 1% B2 H6 , (c) doped with 1% PH3 .
Fig. 2. He and H effusion spectra of He implanted undoped Si:H materials deposited at TS ¼ 200 °C. (a) Hot wire (HW) aSi:H, (b) a-Si:H grown by DC sputtering (SP), (c) lc-Si:H (lc).
(using a mass separator) resulting in helium depth distributions with maxima at depths of approximately 0.3 and 0.4 lm, respectively, estimated by the TRIM routine. Except when noted otherwise, implantation dose was 1016 cm2 . Effusion measurements were performed as described elsewhere [8] employing a heating rate of 20 K/min.
3. Results
256
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
effusion spectra observed suggests that helium precipitates in voids or bubbles present from the deposition process (or formed, e.g., by hydrogen precipitation [11] when the samples are heated) rather than caused by the He implantation process. In agreement, we find the ratio of HT to LT helium effusion essentially independent of the He implantation dose in the range of 1015 –1016 cm2 . If He implantation would cause the formation of voids, an increase of the HT/LT effusion ratio with rising He dose would be expected. In agreement with our assignment of the He effusion peaks, we find for Si:H materials with an interconnected void structure detected by a dominant LT (T 400 °C) hydrogen effusion peak quite generally no (or little) HT helium effusion [4]. Either, these materials do not contain significant concentrations of isolated voids or if isolated voids are present, He penetrating the voids is not trapped. The influence of substrate temperature TS on LT the low temperature effusion maximum TM for PVCVD and HW a-Si:H as well as for lc-Si:H is shown in Fig. 3. Estimated errors for substrate temperature and temperature of LT effusion maximum are indicated. It is seen that for the PECVD a-Si:H films (open symbols) the LT He effusion peak shifts with rising TS to higher temperature with TMLT showing a maximum or plateau
near TS ¼ 400 °C. A slight doping dependence is observed. For our HW samples, the temperature of the major (LT) effusion peak lies in the range of LT the PECVD data as do the TM data of lc-Si:H deposited at TS ¼ 200–250 °C. Furthermore, most HW samples show a minor effusion peak near T ¼ 370 °C. The TMLT values of most of our microcrystalline Si:H films (TS P 300 °C) agree closely with these latter data. Also indicated in Fig. 3 is the He diffusion energy ED derived under the assumption of He evenly distributed in a film thickness of 1 lm and of He effusion limited by diffusion with a constant diffusion prefactor D0 ¼ 103 cm2 s1 [2,4]. Depending on the preparation conditions, the helium diffusion energy is found to range from 0.7 to 1.3 eV. For c-Si, a helium diffusion energy of 1.23 eV has been reported [12]. The substrate temperature dependence of the fraction of high temperature helium effusion, F HT , is shown in Fig. 4 for the same series of samples. This quantity was determined by normalizing the amount of He effusing in the HT peak by the total amount of effused He. Note that due to some uncertainties in determining the onset of HT He
Fig. 3. Temperature TMLT of low-temperature He effusion maximum for undoped (UD, ), phosphorus-doped (P, ) and boron-doped (B, M) PECVD a-Si:H as well as for undoped HW a-Si:H (j) and lc-Si:H () versus substrate temperature TS .
Fig. 4. High temperature fraction FHT of He effusion for samples of Fig. 3 versus substrate temperature TS . (ðÞ undoped, () P-doped, (M) B-doped PECVD a-Si:H; (j) HW aSi:H; () lc-Si:H).
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
effusion, the data of F HT should be regarded as preliminary. Moreover, values of F HT 6 102 should be considered as an upper limit only, because of the detection limit of the He effusion experiment. While the PECVD samples (open symbols) show a broad maximum centered near TS ¼ 250 °C and rather small values below TS ¼ 100 °C and above TS ¼ 400 °C, both HW and lc-Si:H films show rather high values throughout the substrate temperature range investigated.
4. Discussion 4.1. Microstructure analysis Since helium does not form bonds with silicon, the He diffusion coefficient is expected to depend on sizes of network openings and (interconnected) voids and/or on elastic constants. Assuming a doorway diffusion process [2,13], the observed inLT crease in TM and the He diffusion energy ED of plasma grown and HW a-Si:H with rising substrate temperature could be due either to an increase of the (local) shear modulus, i.e., of the stiffness of the network, or to a decrease of the doorway radius which is a measure of the network openings. Since plasma-grown a-Si:H is known to densify with rising TS (and decreasing hydrogen content) presumably the latter parameter predominates. The observation of a second LT He effusion peak at T 370 °C in HW material which coincides with the LT He effusion peak of our l c-Si:H films suggests the presence of structural heterogeneity. We note that we obtain for Heimplanted c-Si also an effusion peak near T ¼ 370–390 °C. When isolated voids are present, helium is expected to precipitate in these voids during the LT He effusion process and, apparently, can effuse from these voids only at T P 600 °C in a permeation process. Indeed, using the permeation and diffusion data of He in crystalline silicon published by Van Wieringen and Warmoltz [12] we can relate a HT He effusion peak near 700 °C to an average void size of about 3 nm while a peak near 800 °C would be due to a void size of about 20 nm. These estimated void sizes, however, refer to the crys-
257
tallized material. Differences in the position of the HT He effusion peaks (see Figs. 1 and 2), which were also observed for films deposited on different substrate materials, thus can be attributed to different crystallization processes leading to differently sized voids or bubbles in the crystallized material. Our results suggest for plasma-grown aSi:H an enhanced concentration of isolated voids when films are deposited at 100 °C 6 TS 6 400 °C (see Fig. 4). Doping enhances this void concentration further. The nature of these effects is not yet understood. We note, however, that hydrogen seems to be involved, since for all three PECVD a-Si:H series investigated the concentration of precipitated helium decreases strongly when the hydrogen concentration decreases below a value of about 5 at.%. The nature of the rather high He precipitation in HW a-Si:H and in lc-Si:H also is not well understood so far. Yet we note that lc-Si:H films are expected to have a pronounced microstructure due to the presence of grain boundaries. 4.2. Impact on hydrogen incorporation and diffusion When precipitation of He in voids is observed, precipitation of molecular hydrogen must also be expected provided the surface recombination process for formation of H2 is fast. This would imply that e.g., in highly boron and phosphorus-doped a-Si:H deposited near 200 °C as well as in our HW a-Si:H and lc-Si:H materials, after some H diffusion a fraction of hydrogen should be present in molecular form, precipitated in voids. Indeed, such effects have been suspected earlier, in particular for H diffusion in microcrystalline Si [5]. If the readsorption of H2 on void surfaces in amorphous Si:H is fast, the H diffusion process would still be limited by the motion of atomic H through compact Si:H separating the voids, i.e., the actual H diffusion process might not be affected significantly.
5. Conclusions Our results suggest that effusion of implanted He from a-Si:H and lc-Si:H is a useful technique
258
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
for microstructure characterization. The observed diffusion and precipitation effects can be used to describe the density of the material as well as the concentration of isolated voids. Our results suggest for materials deposited near T ¼ 200 °C the presence of rather low concentrations of isolated voids for undoped plasmagrown a-Si:H while doped plasma-grown a-Si:H, undoped a-Si:H deposited by HW or sputtering methods as well as lc-Si:H showed considerably higher values.
Acknowledgements The authors thank M. Gebauer and A. Dahmen for the ion implantations, R. Carius for the Raman measurements, R. Saleh for sample preparation and W. Hilgers, D. Lennartz and F. Pennartz for technical assistance. Financial support by BMBF/BMWi is gratefully acknowledged.
References [1] W. Beyer, J. Non-Cryst. Solids 266–269 (2000) 845. [2] W. Beyer, S.S. Camargo Jr., R. Saleh, Mater. Res. Soc. Symp. Proc. 609 (2000) A23.4.1. [3] W. Beyer, in: D. Adler, H. Fritzsche (Eds.), Tetrahedrally Bonded Amorphous Semiconductors, Plenum, New York, 1985, p. 129. [4] W. Beyer, Mater. Res. Soc. Symp. Proc. 664 (2001) A9.2.1. [5] W. Beyer, P. Hapke, U. Zastrow, Mater. Res. Soc. Symp. Proc. 467 (1997) 343. [6] L. Zanzig, Ber. Forschungszentrums J€ ulich 3087 (1995). [7] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Phys. Status Solidi 100 (1980) 43. [8] W. Beyer, J. Herion, H. Wagner, U. Zastrow, Philos. Mag. B 63 (1991) 269. [9] W. Beyer, Mater. Res. Soc. Symp. Proc. 664 (2001) A13.1.1. [10] G.F. Cerofolini, F. Corni, S. Frabboni, C. Nobili, G. Ottaviani, R. Tonini, Mater. Sci. Eng. R 27 (2000) 1. [11] S. Acco, D.L. Williamson, P.A. Stolk, F.W. Saris, M.J. van den Boogaard, W.C. Sinke, W.F. van der Weg, S. Roorda, P.C. Zalm, Phys. Rev. B 53 (1996) 4415. [12] A. Van Wieringen, N. Warmoltz, Physica 22 (1956) 849. [13] O.L. Anderson, D.A. Stuart, J. Am. Ceram. Soc. 37 (1954) 573.
Helium effusion, diffusion and precipitation as a probe of microstructure in hydrogenated amorphous silicon W. Beyer *, U. Zastrow Institut f€ur Photovoltaik, Forschungszentrum J€ulich, D-52425 J€ulich, Germany
Abstract The effusion of implanted helium was studied for doped and undoped radio frequency plasma-deposited a-Si:H films as well as for Si:H films deposited by other techniques. The results suggest that the observed helium diffusion and precipitation effects give information on the material microstructure. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 66.30.Jt; 68.55.)a; 81.70.Pg
1. Introduction
2. Experimental
Recently it was demonstrated that the effusion of implanted inert gas atoms like Ar or Ne is a useful method for structural characterization of amorphous silicon alloys [1,2]. Since these atoms do not react with other atoms, their out-diffusion can be used to detect openings and voids in the material. While argon and neon atoms have been found not to diffuse significantly in compact hydrogenated amorphous silicon (a-Si:H) at temperatures below crystallization, helium atoms are known to diffuse [3,4]. Here, we report on the structural characterization of a-Si:H and related materials by effusion measurements of implanted helium.
Most a-Si:H films investigated were deposited by standard plasma-enhanced chemical vapor deposition (PECVD). Typical deposition conditions involved a gas pressure of 0.5 mbar, a silane flow of 3 sccm and an rf power (13.56 MHz) of 10 W. Doping was achieved by adding flows of phosphine or diborane. We also investigated microcrystalline Si films (lc-Si:H) deposited by ECR plasma [5], hot wire (HW) a-Si:H [6] (employing a tantalum filament at T ¼ 1800 °C, a silane flow of 8 sccm and a pressure of 3 103 mbar), as well as DC sputtered a-Si:H (with unintentional incorporation of H only) [2]. The crystallinity of the lcSi:H films was verified by Raman measurements. As substrates, crystalline Si (c-Si), sapphire as well as c-Si platelets coated with about 30 nm SiO2 were used. Employing the latter, blistering and bubble formation of a-Si:H [7] was avoided. Typical film thickness was 1 lm and higher. Helium (Heþ ) was implanted at the energy of 30 or 40 keV
*
Corresponding author. Tel.: +49-2461 613 925; fax: +492461 613 735. E-mail address: [email protected] (W. Beyer).
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 0 0 4 - 3
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
255
Effusion spectra of helium and hydrogen of undoped and highly boron- and phosphorusdoped PECVD a-Si:H films deposited at a substrate temperature of 200 °C are shown in Figs. 1(a)–(c) (full lines). The dotted line in Fig. 1(a) refers to H effusion of material not implanted by helium for comparison. The close agreement with the H effusion of implanted material shows that He implantation has little influence on the material structure. In all three types of PECVD a-Si:H, H effusion shows a single effusion peak typical for the presence of compact material [3,9]. The H effusion maximum is found to lie near 600 °C for undoped and P-doped a-Si:H and near 450 °C for B-doped a-Si:H. This difference has been attributed to the dependence of H diffusion on the Fermi level [9].
Helium effusion is found to proceed in two stages. A first peak (termed low-temperature (LT) peak) appears near 400 °C and a second one (high temperature (HT) peak) near 700 °C for boron-doped a-Si:H, near 800 °C for undoped a-Si:H and near 900 °C for phosphorus-doped material. The small structure near 700 °C in P-doped a-Si:H is attributed to the crystallization of the material. Doping by both boron and phosphorus is seen to enhance the HT peak while the LT peak is decreased. Such enhanced HT helium effusion is also observed for (undoped) a-Si:H deposited by other deposition techniques as well as for lc-Si:H. This is demonstrated in Fig. 2 showing results for a-Si:H deposited by the HW method (Fig. 2(a)) and by DC sputtering (Fig. 2(b)) as well as for microcrystalline Si:H deposited in an ECR plasma (Fig. 2(c)). In agreement with effusion results of implanted He in crystalline Si [10] we attribute the LT effusion peak to the out-diffusion of He from network sites whereas the HT peak is associated with He precipitated in isolated voids or bubbles, leaving the material in a permeation process or upon void/ bubble rupture. In crystalline Si, the formation of He filled bubbles has been attributed to the implantation process and subsequent annealing [10]. In amorphous Si, however, the wide variation of
Fig. 1. He and H effusion spectra of He implanted PECVD aSi:H deposited at TS ¼ 200 °C. (a) Undoped (UD), (b) doped with 1% B2 H6 , (c) doped with 1% PH3 .
Fig. 2. He and H effusion spectra of He implanted undoped Si:H materials deposited at TS ¼ 200 °C. (a) Hot wire (HW) aSi:H, (b) a-Si:H grown by DC sputtering (SP), (c) lc-Si:H (lc).
(using a mass separator) resulting in helium depth distributions with maxima at depths of approximately 0.3 and 0.4 lm, respectively, estimated by the TRIM routine. Except when noted otherwise, implantation dose was 1016 cm2 . Effusion measurements were performed as described elsewhere [8] employing a heating rate of 20 K/min.
3. Results
256
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
effusion spectra observed suggests that helium precipitates in voids or bubbles present from the deposition process (or formed, e.g., by hydrogen precipitation [11] when the samples are heated) rather than caused by the He implantation process. In agreement, we find the ratio of HT to LT helium effusion essentially independent of the He implantation dose in the range of 1015 –1016 cm2 . If He implantation would cause the formation of voids, an increase of the HT/LT effusion ratio with rising He dose would be expected. In agreement with our assignment of the He effusion peaks, we find for Si:H materials with an interconnected void structure detected by a dominant LT (T 400 °C) hydrogen effusion peak quite generally no (or little) HT helium effusion [4]. Either, these materials do not contain significant concentrations of isolated voids or if isolated voids are present, He penetrating the voids is not trapped. The influence of substrate temperature TS on LT the low temperature effusion maximum TM for PVCVD and HW a-Si:H as well as for lc-Si:H is shown in Fig. 3. Estimated errors for substrate temperature and temperature of LT effusion maximum are indicated. It is seen that for the PECVD a-Si:H films (open symbols) the LT He effusion peak shifts with rising TS to higher temperature with TMLT showing a maximum or plateau
near TS ¼ 400 °C. A slight doping dependence is observed. For our HW samples, the temperature of the major (LT) effusion peak lies in the range of LT the PECVD data as do the TM data of lc-Si:H deposited at TS ¼ 200–250 °C. Furthermore, most HW samples show a minor effusion peak near T ¼ 370 °C. The TMLT values of most of our microcrystalline Si:H films (TS P 300 °C) agree closely with these latter data. Also indicated in Fig. 3 is the He diffusion energy ED derived under the assumption of He evenly distributed in a film thickness of 1 lm and of He effusion limited by diffusion with a constant diffusion prefactor D0 ¼ 103 cm2 s1 [2,4]. Depending on the preparation conditions, the helium diffusion energy is found to range from 0.7 to 1.3 eV. For c-Si, a helium diffusion energy of 1.23 eV has been reported [12]. The substrate temperature dependence of the fraction of high temperature helium effusion, F HT , is shown in Fig. 4 for the same series of samples. This quantity was determined by normalizing the amount of He effusing in the HT peak by the total amount of effused He. Note that due to some uncertainties in determining the onset of HT He
Fig. 3. Temperature TMLT of low-temperature He effusion maximum for undoped (UD, ), phosphorus-doped (P, ) and boron-doped (B, M) PECVD a-Si:H as well as for undoped HW a-Si:H (j) and lc-Si:H () versus substrate temperature TS .
Fig. 4. High temperature fraction FHT of He effusion for samples of Fig. 3 versus substrate temperature TS . (ðÞ undoped, () P-doped, (M) B-doped PECVD a-Si:H; (j) HW aSi:H; () lc-Si:H).
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
effusion, the data of F HT should be regarded as preliminary. Moreover, values of F HT 6 102 should be considered as an upper limit only, because of the detection limit of the He effusion experiment. While the PECVD samples (open symbols) show a broad maximum centered near TS ¼ 250 °C and rather small values below TS ¼ 100 °C and above TS ¼ 400 °C, both HW and lc-Si:H films show rather high values throughout the substrate temperature range investigated.
4. Discussion 4.1. Microstructure analysis Since helium does not form bonds with silicon, the He diffusion coefficient is expected to depend on sizes of network openings and (interconnected) voids and/or on elastic constants. Assuming a doorway diffusion process [2,13], the observed inLT crease in TM and the He diffusion energy ED of plasma grown and HW a-Si:H with rising substrate temperature could be due either to an increase of the (local) shear modulus, i.e., of the stiffness of the network, or to a decrease of the doorway radius which is a measure of the network openings. Since plasma-grown a-Si:H is known to densify with rising TS (and decreasing hydrogen content) presumably the latter parameter predominates. The observation of a second LT He effusion peak at T 370 °C in HW material which coincides with the LT He effusion peak of our l c-Si:H films suggests the presence of structural heterogeneity. We note that we obtain for Heimplanted c-Si also an effusion peak near T ¼ 370–390 °C. When isolated voids are present, helium is expected to precipitate in these voids during the LT He effusion process and, apparently, can effuse from these voids only at T P 600 °C in a permeation process. Indeed, using the permeation and diffusion data of He in crystalline silicon published by Van Wieringen and Warmoltz [12] we can relate a HT He effusion peak near 700 °C to an average void size of about 3 nm while a peak near 800 °C would be due to a void size of about 20 nm. These estimated void sizes, however, refer to the crys-
257
tallized material. Differences in the position of the HT He effusion peaks (see Figs. 1 and 2), which were also observed for films deposited on different substrate materials, thus can be attributed to different crystallization processes leading to differently sized voids or bubbles in the crystallized material. Our results suggest for plasma-grown aSi:H an enhanced concentration of isolated voids when films are deposited at 100 °C 6 TS 6 400 °C (see Fig. 4). Doping enhances this void concentration further. The nature of these effects is not yet understood. We note, however, that hydrogen seems to be involved, since for all three PECVD a-Si:H series investigated the concentration of precipitated helium decreases strongly when the hydrogen concentration decreases below a value of about 5 at.%. The nature of the rather high He precipitation in HW a-Si:H and in lc-Si:H also is not well understood so far. Yet we note that lc-Si:H films are expected to have a pronounced microstructure due to the presence of grain boundaries. 4.2. Impact on hydrogen incorporation and diffusion When precipitation of He in voids is observed, precipitation of molecular hydrogen must also be expected provided the surface recombination process for formation of H2 is fast. This would imply that e.g., in highly boron and phosphorus-doped a-Si:H deposited near 200 °C as well as in our HW a-Si:H and lc-Si:H materials, after some H diffusion a fraction of hydrogen should be present in molecular form, precipitated in voids. Indeed, such effects have been suspected earlier, in particular for H diffusion in microcrystalline Si [5]. If the readsorption of H2 on void surfaces in amorphous Si:H is fast, the H diffusion process would still be limited by the motion of atomic H through compact Si:H separating the voids, i.e., the actual H diffusion process might not be affected significantly.
5. Conclusions Our results suggest that effusion of implanted He from a-Si:H and lc-Si:H is a useful technique
258
W. Beyer, U. Zastrow / Journal of Non-Crystalline Solids 299–302 (2002) 254–258
for microstructure characterization. The observed diffusion and precipitation effects can be used to describe the density of the material as well as the concentration of isolated voids. Our results suggest for materials deposited near T ¼ 200 °C the presence of rather low concentrations of isolated voids for undoped plasmagrown a-Si:H while doped plasma-grown a-Si:H, undoped a-Si:H deposited by HW or sputtering methods as well as lc-Si:H showed considerably higher values.
Acknowledgements The authors thank M. Gebauer and A. Dahmen for the ion implantations, R. Carius for the Raman measurements, R. Saleh for sample preparation and W. Hilgers, D. Lennartz and F. Pennartz for technical assistance. Financial support by BMBF/BMWi is gratefully acknowledged.
References [1] W. Beyer, J. Non-Cryst. Solids 266–269 (2000) 845. [2] W. Beyer, S.S. Camargo Jr., R. Saleh, Mater. Res. Soc. Symp. Proc. 609 (2000) A23.4.1. [3] W. Beyer, in: D. Adler, H. Fritzsche (Eds.), Tetrahedrally Bonded Amorphous Semiconductors, Plenum, New York, 1985, p. 129. [4] W. Beyer, Mater. Res. Soc. Symp. Proc. 664 (2001) A9.2.1. [5] W. Beyer, P. Hapke, U. Zastrow, Mater. Res. Soc. Symp. Proc. 467 (1997) 343. [6] L. Zanzig, Ber. Forschungszentrums J€ ulich 3087 (1995). [7] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Phys. Status Solidi 100 (1980) 43. [8] W. Beyer, J. Herion, H. Wagner, U. Zastrow, Philos. Mag. B 63 (1991) 269. [9] W. Beyer, Mater. Res. Soc. Symp. Proc. 664 (2001) A13.1.1. [10] G.F. Cerofolini, F. Corni, S. Frabboni, C. Nobili, G. Ottaviani, R. Tonini, Mater. Sci. Eng. R 27 (2000) 1. [11] S. Acco, D.L. Williamson, P.A. Stolk, F.W. Saris, M.J. van den Boogaard, W.C. Sinke, W.F. van der Weg, S. Roorda, P.C. Zalm, Phys. Rev. B 53 (1996) 4415. [12] A. Van Wieringen, N. Warmoltz, Physica 22 (1956) 849. [13] O.L. Anderson, D.A. Stuart, J. Am. Ceram. Soc. 37 (1954) 573.