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Film-thickness dependence of structural and electrical properties of boron-doped hydrogenated microcrystalline silicon prepared by radiofrequency magnetron sputtering
Journal of Non-Crystalline Solids 356 (2010) 1131–1134
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Film-thickness dependence of structural and electrical properties of boron-doped hydrogenated microcrystalline silicon prepared by radiofrequency magnetron sputtering Akimori Tabata ⁎, Junya Nakano, Koji Mazaki, Kota Fukaya Department of Electrical Engineering and Computer Science, Nagoya University, Chikusa, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2009 Available online 3 May 2010 Keywords: Microcrystalline silicon; Boron-doped; Magnetron sputtering; Dark conductivity; Activation energy
a b s t r a c t Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by radiofrequency magnetron sputtering and the film-thickness dependence of the structural and electrical properties was investigated. The target was a boron-doped silicon wafer on which boron grains were put or not on top of it. At room temperature, the dark conductivities of the μc-Si:H thin films prepared without boron grains were below 10−6 S cm− 1. On the other hand, for the films prepared with boron grains having thickness above 50 nm, the conductivities were higher than 6×10− 1 S cm− 1 and their activation energies were about 0.05 eV. As the film thickness was decreased, the dark conductivity decreased: ∼10− 1 S cm− 1 for the 20 nm film and ∼10− 6 S cm− 1 for the 10 nm film. This decrease was caused by the decrease in the crystalline volume fraction. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (μc-Si:H) has attracted much attention in its application to large-area electronic devices such as solar cells [1] and thin film transistors [2]. In general, μc-Si:H thin films are prepared by plasma-enhanced chemical vapor deposition (PECVD) [3] and hot-wire CVD [4] methods. Magnetron sputtering is also a promising method for the preparation of μc-Si:H thin films, because the crystallinity of the μc-Si:H thin films can be controlled easily [5,6]. Moreover, the use of extremely toxic gases such as silane, diborane and phosphine used in CVD methods can be avoided, thereby lowering production costs. So far, we have clarified relationships between the film properties and preparation parameters such as the hydrogen gas pressure ratio [7], substrate temperature [8], gas pressure [9], target DC bias voltage [10] and substrate DC bias voltage [11]. However, these studies were for undoped μc-Si:H and the preparation of doped μc-Si:H is vital for electronic device applications. For the p-layer (window layer) in Si-based thin-film solar cells, a high conductivity and high transparency are required. The reason for this is that the high conductivity causes a reduction in the series resistance and a high built-in potential, while the high transparency increases the light absorption in the i-layer, resulting in a higher concentration of lightgenerated carriers [12,13]. For the application of μc-Si:H to the p-layer, a very thin (less than 20 nm) layer with high conductivity is required [12–14].
⁎ Corresponding author. Tel.: +81 52 789 3147; fax: +81 52 789 3161. E-mail address: [email protected] (A. Tabata). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.04.011
For preparation of p-type μc-Si:H thin films by magnetron sputtering method, using a highly boron-doped silicon wafer as a target would be a simple way, however, it does not necessarily imply a high conductivity on the resulting films. For this reason, it is considered to be useful to use commercially available boron grain located on top of the silicon wafer target. In the present study, we prepared p-type (boron-doped) μc-Si:H thin films by a radiofrequency (RF) magnetron sputtering method from a boron-doped silicon wafer target with or without boron grains, and investigated the influence of the film thickness on the structure and electrical properties of the resulting μc-Si:H thin films.
2. Experimental details The μc-Si:H thin films were prepared on Corning glass substrates using a RF magnetron sputtering system (SPF-210H, Anelva). The target was a boron-doped silicon wafer (75 mm in diameter, 0.01–0.02 Ω cm) on which four boron grains (about 5 mm in size, 99.5%) were put or not on top of it. The gas flow rates of hydrogen and argon were 30 and 0.5 sccm, respectively, resulting in a hydrogen partial gas pressure ratio of 93%. The total gas pressure and RF power were 2.7 Pa and 150 W, respectively. The substrate temperature, Ts, was 250 or 350 °C. Film thicknesses were measured with a surface profiler (Alpha-Step 500, Tencor Instruments). X-ray diffraction (XRD) patterns and Raman scattering spectra were measured by using an X-ray diffractometer (Rint2000Ultima, Rigaku) and a Raman scattering spectrophotometer (NRS-1000, Jasco) with a semiconductor laser of 532 nm wavelength, respectively. The dark conductivity was measured in a coplanar configuration with the gap length of 0.1 mm and electrode width of
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5 mm. For some films prepared with boron grains, the carrier concentration and the Hall mobility were estimated in a van der Pauw configuration using Hall Effect measurement system (ResiTest8300, Toyo).
Table 1 Mean crystallite sizes and XRD intensity ratio of (220) peak to (111) peak. Without boron grains
With boron grains
Mean crystallite size (111) (220)
11.6 nm 8.3 nm
7.2 nm 9.3 nm
Intensity ratio (220)/(111)
1.36
0.66
3. Results 3.1. Structural properties Fig. 1 shows the XRD patterns of the μc-Si:H thin films prepared at 250 °C with and without boron grains. The film thicknesses of both films were about 1 μm. Table 1 shows the mean crystallite size, estimated from the full width at half maximum of the XRD peaks using Scherrer's formula [15], and the XRD intensity ratio of the (220) peak to the (111) peak. The XRD patterns of the film prepared without boron grain show a (220) preferential orientation. However, the (220) XRD peak intensity decreased by the preparation with boron grains on the silicon wafer target, although the (220) mean crystallite size was almost the same (Table 1). On the other hand, the (111) XRD peak intensity increased and the mean crystallite size decreased. These findings suggest that the incorporation of boron atoms into the film prevents the nucleus formation of (220)-oriented crystallites and enhances random orientation. Fig. 2 shows the Raman scattering spectra of the films prepared with and without boron grains. The spectra were normalized to the signal at 520 cm− 1 corresponding to the peak originating from crystalline silicon. Ts was 250 °C (Fig. 2(a)) and 350 °C (Fig. 2(b)). The film thicknesses were between 21 and 25 nm. For all the Raman scattering spectra, the peak at around 520 cm− 1 due to crystalline silicon was observed. The Raman intensity at 480 cm− 1 due to the amorphous silicon phase of the films prepared with boron grains was higher than that without boron grains. This finding indicates that the preparation with boron grains on the silicon wafer target prevents nucleus formation and the growth of silicon crystallites. Especially for the Ts = 350 °C film, these effects were more pronounced: a peak due to the amorphous phase was predominant for the Ts = 350 °C film prepared with boron grains, although in the preparation without boron grains the spectrum of the Ts = 350 °C film was almost the same as that of the Ts = 250 °C film. Fig. 3 shows the relationship between the crystalline volume fraction and film thickness. The crystalline volume fraction, Xc was calculated from Xc = Ic / (Ic + Ia), where Ic and Ia are the integrated intensities of the Raman peaks due to the crystalline and amorphous silicon phases, respectively. For the thickness above 50 nm, Xc of the
Fig. 1. XRD patterns of the μc-Si:H thin films prepared at 250 °C with and without boron grains. The film thicknesses were about 1 mm.
films prepared with and without boron grains was the same when compared for the same thickness. However, for the thickness below 50 nm, Xc of the films prepared with boron grains was lower than that without boron grains. Moreover, for the films prepared with boron grains, Xc of the Ts = 350 °C films was lower than that of the Ts = 250 °C films, although Xc was the same for the films prepared at Ts = 250 and 350 °C without boron grains.
3.2. Electrical properties Fig. 4 shows the dark conductivity at room temperature of the μcSi:H thin films prepared with and without boron grains as a function of the film thickness. The dark conductivity of the 1 μm films prepared without boron grains was ∼ 10− 6 S cm− 1. This value is the same order
Fig. 2. Raman scattering spectra of the μc-Si:H thin films prepared with and without boron grains. The film thicknesses were 21–25 mm. Ts was 250 (a) and 350 °C (b).
A. Tabata et al. / Journal of Non-Crystalline Solids 356 (2010) 1131–1134
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Fig. 3. Relationship between the crystalline volume fraction and film thickness. The lines are included as visual guides.
Fig. 5. Carrier concentration and Hall mobility of the μc-Si:H thin films prepared with boron grains at 250 °C as a function of film thickness.
as those of undoped μc-Si:H thin films [16]. On the other hand, the dark conductivity of the 1 μm films prepared with boron grains was N100 S cm− 1, which was higher than that prepared without boron grains by more than six orders of magnitude. As the film thickness decreased, the dark conductivity decreased. However it should be noted that for the films with thickness above 50 nm, prepared with boron grains, the conductivities were higher than 6 × 10− 1 S cm− 1. The activation energies were about 0.05 eV, which is close to the ionization energy of the boron impurity in crystalline silicon [17]. For the 20 nm films, the conductivity was about 10− 1 S cm− 1, which is the same order of magnitude as those of the p-layer μc-Si:H in silicon-based thin-film solar cells [12,13,18,19]. However, as the film thickness decreased from 20 to 10 nm, the conductivity decreased dramatically to 10− 6 S cm− 1 by five orders of magnitude, which is almost the same value as those of undoped μc-Si:H thin films [16]. Fig. 5 shows the carrier concentration and the Hall mobility of the μc-Si:H thin films prepared at 250 °C with boron grains as a function of the film thickness. The results could not be obtained for the films of the thickness below 40 nm because of high noise levels. Both the carrier concentration and the Hall mobility decreased with decreasing the film thickness. In materials consisting of amorphous and crystalline phases, the Hall Effect is dominated by the crystalline phase [20]. The
decreases in the carrier concentration and Hall mobility are caused by the decrease in Xc. As mentioned later, for the films of the thickness below 40 nm the value of Xc is around and below percolation threshold value. The high noise levels during the Hall Effect measurements may be caused by a combination of the breakdown of the percolation network and a wrong sign of Hall Effect in amorphous silicon [20]. Further investigation for this is required. Fig. 6 shows the relationship between the dark conductivity and the crystalline volume fraction, Xc. As the Xc decreased, the dark conductivity decreased. Especially, for the Ts = 250 °C films prepared with boron grains a dramatic drop was observed at around Xc = 0.34, which corresponds to the decrease in the film thickness below 30 nm. The conductivities of the Ts = 350 °C films with thicknesses below 40 nm are also shown in Figs. 4 and 6. As shown in Fig. 4, in the preparation without boron grains, the Ts = 350 °C films had the same conductivity as those of the Ts = 250 °C films. However, in the preparation with boron grains, the Ts = 350 °C films had a lower conductivity than the Ts = 250 °C films. On the other hand, as shown in Fig. 6, the dark conductivity of the Ts = 350 °C films prepared with boron grains was higher than that of the Ts = 250 °C films when compared for the same Xc.
Fig. 4. Dark conductivity at room temperature of the μc-Si:H thin films prepared with and without boron grains as a function of film thickness. The lines are included as visual guides.
Fig. 6. Relationship between the dark conductivity and crystalline volume fraction. The lines are included as visual guides.
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4. Discussion As mentioned in the previous sections, the dark conductivity of the 1 μm film was improved by more than six orders of magnitude by using a silicon wafer with boron grains as a target. This results from the increase in the intake of boron atoms into the film. The silicon wafer used as a target in the present study contains boron atoms with a concentration of about 1019 cm− 3 [21], which is in no manner a low value. However, the dark conductivity of the film prepared from the silicon wafer (without boron grains) was almost the same order as that of undoped μ c-Si:H thin films. Xc of the film prepared without boron grains was almost the same as that with boron grains (Fig. 3), suggesting that the doping efficiency was never lower than that with boron grains. These above findings suggest that the boron atoms in the silicon wafer target did not transfer well into the μc-Si:H thin films and that the boron grains on the silicon wafer target increased the amount of boron atoms incorporated into the film. The setting of boron grains on a silicon wafer target makes it easier to prepare highly conductive p-type μc-Si:H thin films. The decrease in the dark conductivity with decreasing the film thickness results from the decreases in both the carrier concentration and the mobility with the decrease in Xc. Doping efficiency of impurities in amorphous phase is much lower than that in crystalline phase [22]. The decrease in Xc gives rise to the decrease in the amount of activated boron impurities in the films. This is the cause of the decrease in the carrier concentration with the decrease in the film thickness. Similarly, mobility in amorphous phase is lower than that in crystalline phase [22]. The decrease in Xc with the decrease in the film thickness results in the decrease in the mobility. It should be noted that the mobility in the present study decreased with the decrease in the carrier concentration. It is well known that mobility decreases with an increase in carrier concentration, due to the enhancement of impurity scattering of carriers [21,23]. The result obtained in the present study is the opposite of it. This indicates that the mobility in the present study was governed by the film structure rather than by the boron impurity scattering. As mentioned before, the dark conductivity of the Ts = 250 °C films dropped dramatically at around Xc = 0.34. This is closely related to percolation process. The decrease in Xc from percolation threshold value means the breakdown of percolation network, which causes the dramatic drop of the dark conductivity. It has been reported that the percolation threshold value of Xc is around 0.32 from computational analyses for a model assuming that crystalline and amorphous cubes with the same size are distributed at random and that each cubes is connected to its six neighboring cubes [24,25]. Other computational analysis for the packing of spheres has shown that the percolation threshold value is 0.154 [26]. The value obtained in the present study is in good agreement with the former, although the shapes of crystallites and amorphous phase in μs-Si:H are never cube and never of the same size. For the application of boron-doped μc-Si:H thin films to thin-film solar cells, the film thickness less than 20 nm is required [12–14]. However, the conductivity of the films obtained in the present study was very low. Therefore, further challenges are required for improvement of the conductivity for μc-Si:H thin films with thickness of less than 20 nm. One of the approaches is a preparation of thin films with a higher Xc. Elevating Ts from 250 to 350 °C did not affect the structure of the films prepared without boron grains. However, it affected the structure of the films prepared with boron grains. Elevating Ts enhances the migration of boron atoms adsorbed on the film-growing surface, disturbing nucleus formation and crystallite growth. Consequently, Xc decreased, giving rise to the decrease in the dark conductivity (Fig. 4). The elevating Ts from 250 to 350 °C shifted the percolation threshold to a lower value, as shown in Fig. 6. It has been reported that anisotropic
structure with a large number of linked crystalline cubes along one direction (film-growth direction) leads lower values of the percolation threshold in the direction [25]. However, the conductivity measurements in the present study were performed in the lateral direction perpendicular to the growth direction. Therefore, it is probably difficult to explain the shift of the percolation threshold value in the present study using the anisotropic structure. However, this indicates that the formation of the percolation network in the μc-Si:H depends not only on Xc but on other factors such as the shape [26] and size [27] of crystallites. Moreover, voids also should be taken into consideration because μc-Si:H contains many voids [28]. We intend to advance further investigation for them. 5. Conclusion Boron-doped μc-Si:H thin films with high dark conductivity and a low activation energy of the dark conductivity, could be prepared by RF magnetron sputtering using a silicon wafer, with boron grains located on top of it, as a target for the film thickness above 50 nm. However, as the film thickness decreased from 20 to 10 nm, the conductivity decreased dramatically from 10− 1 to 10− 6 S cm− 1, resulting from a decrease in the Xc. In order to obtain highly conductive μc-Si:H thin films with thickness of less than 20 nm, it is important to prepare films with a higher Xc. Acknowledgements We are grateful to the Center for Cooperative Research in Advanced Science and Technology, Nagoya University, for the use of the X-ray diffractometer, to Akasaki Research Center, Nagoya University, for the use of the Hall Effect measurement system and to Mr H. Choshi of the Instrumental Analysis Facility of Nagoya University, who kindly measured the Raman scattering spectra. References [1] J. Poortmans, V. Arkhipov, Thin Film Solar Cells, John Wiley & Sons, 2006, pp. 133–171. [2] S. Wagner, H. Gleskova, I.-C. Cheng, M. Wu, Thin Solid Films 430 (2003) 15. [3] M. Kondo, A. Matsuda, Curr. Opin. Solid State Mater. Sci. 6 (2002) 445. [4] H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. [5] M.F. Cerqueira, M. Andritschly, L. Rebouta, J.A. Ferreira, M.F. da Silva, Vacuum 46 (1995) 1385. [6] Y. Leconte, P. Marie, X. Portier, M. Lejeune, R. Rizk, Thin Solid Films 427 (2003) 252. [7] H. Makihara, A. Tabata, Y. Suzuoki, T. Mizutani, Vacuum 59 (2000) 785. [8] J. Kondo, A. Tabata, T. Kawamura, T. Mizutani, Vacuum 66 (2002) 409. [9] K. Fukaya, A. Tabata, T. Mizutani, Vacuum 74 (2004) 561. [10] K. Fukaya, A. Tabata, T. Mizutani, Thin Solid Films 478 (2005) 132. [11] A. Tabata, K. Fukaya, T. Mizutani, Vacuum 82 (2008) 777. [12] R.E.I. Schroop, M. Zeman, Amorphous and Microcrystalline Silicon Solar Cells, Kluwer Academic Publishers, Boston, 1998. [13] J.K. Rath, R.E.I. Schroop, Sol. Energy Mater. Sol. Cells 53 (1998) 189. [14] T. Sasaki, S. Fujikane, K. Tabuchi, T. Yoshida, T. Hata, H. Sakai, Y. Ichikawa, J. NonCryst. Solids 266–269 (2000) 171. [15] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.John Wiley & Sons, New York, 1954. [16] P. Torres, J. Meier, R. Fluckinger, U. Kroll, J.A.A. Selvan, H. Keppner, A. Shah, S.D. Littelwood, I.E. Kelly, P. Giannoules, Appl. Phys. Lett. 69 (1996) 1373. [17] S.M. Sze, Semiconductor Devices — Physics and Technology, 2nd Ed, John Wiley & Sons, 2001, p. 38. [18] A. Gordijin, J.K. Rath, R.E.I. Schroop, J. Non-Cryst. Solids 338–340 (2004) 110. [19] K. Adhikary, S. Ray, J. Non-Cryst. Solids 353 (2007) 2289. [20] C.E. Nebel, M. Rother, M. Stutzmann, C. Summonte, M. Heintze, Phil. Mag. Lett. 74 (1996) 455. [21] S.M. Sze, Semiconductor Devices — Physics and Technology, 2nd Ed, John Wiley & Sons, 2001, pp. 50–55. [22] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, 1991. [23] P. Luo, Z. Zhou, Y. Li, S. Lin, X. Dou, R. Cui, Microelectron. J. 39 (2008) 12. [24] H. Overhof, M. Otte, M. Schmidtke, U. Bachhausen, R. Carius, J. Non-Cryst. Solids 227–230 (1998) 992. [25] F. Liu, M. Zhu, Q. Wang, Phys. Lett. A 331 (2004) 432. [26] H. Scher, R. Zallen, J. Chem. Phys. 53 (1970) 3759. [27] K. Shimakawa, J. Non-Cryst. Solids 266–269 (2000) 223. [28] P. Roca i Cabarrocas, J. Non-Cryst. Solids 266–269 (2000) 31.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Film-thickness dependence of structural and electrical properties of boron-doped hydrogenated microcrystalline silicon prepared by radiofrequency magnetron sputtering Akimori Tabata ⁎, Junya Nakano, Koji Mazaki, Kota Fukaya Department of Electrical Engineering and Computer Science, Nagoya University, Chikusa, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2009 Available online 3 May 2010 Keywords: Microcrystalline silicon; Boron-doped; Magnetron sputtering; Dark conductivity; Activation energy
a b s t r a c t Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by radiofrequency magnetron sputtering and the film-thickness dependence of the structural and electrical properties was investigated. The target was a boron-doped silicon wafer on which boron grains were put or not on top of it. At room temperature, the dark conductivities of the μc-Si:H thin films prepared without boron grains were below 10−6 S cm− 1. On the other hand, for the films prepared with boron grains having thickness above 50 nm, the conductivities were higher than 6×10− 1 S cm− 1 and their activation energies were about 0.05 eV. As the film thickness was decreased, the dark conductivity decreased: ∼10− 1 S cm− 1 for the 20 nm film and ∼10− 6 S cm− 1 for the 10 nm film. This decrease was caused by the decrease in the crystalline volume fraction. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (μc-Si:H) has attracted much attention in its application to large-area electronic devices such as solar cells [1] and thin film transistors [2]. In general, μc-Si:H thin films are prepared by plasma-enhanced chemical vapor deposition (PECVD) [3] and hot-wire CVD [4] methods. Magnetron sputtering is also a promising method for the preparation of μc-Si:H thin films, because the crystallinity of the μc-Si:H thin films can be controlled easily [5,6]. Moreover, the use of extremely toxic gases such as silane, diborane and phosphine used in CVD methods can be avoided, thereby lowering production costs. So far, we have clarified relationships between the film properties and preparation parameters such as the hydrogen gas pressure ratio [7], substrate temperature [8], gas pressure [9], target DC bias voltage [10] and substrate DC bias voltage [11]. However, these studies were for undoped μc-Si:H and the preparation of doped μc-Si:H is vital for electronic device applications. For the p-layer (window layer) in Si-based thin-film solar cells, a high conductivity and high transparency are required. The reason for this is that the high conductivity causes a reduction in the series resistance and a high built-in potential, while the high transparency increases the light absorption in the i-layer, resulting in a higher concentration of lightgenerated carriers [12,13]. For the application of μc-Si:H to the p-layer, a very thin (less than 20 nm) layer with high conductivity is required [12–14].
⁎ Corresponding author. Tel.: +81 52 789 3147; fax: +81 52 789 3161. E-mail address: [email protected] (A. Tabata). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.04.011
For preparation of p-type μc-Si:H thin films by magnetron sputtering method, using a highly boron-doped silicon wafer as a target would be a simple way, however, it does not necessarily imply a high conductivity on the resulting films. For this reason, it is considered to be useful to use commercially available boron grain located on top of the silicon wafer target. In the present study, we prepared p-type (boron-doped) μc-Si:H thin films by a radiofrequency (RF) magnetron sputtering method from a boron-doped silicon wafer target with or without boron grains, and investigated the influence of the film thickness on the structure and electrical properties of the resulting μc-Si:H thin films.
2. Experimental details The μc-Si:H thin films were prepared on Corning glass substrates using a RF magnetron sputtering system (SPF-210H, Anelva). The target was a boron-doped silicon wafer (75 mm in diameter, 0.01–0.02 Ω cm) on which four boron grains (about 5 mm in size, 99.5%) were put or not on top of it. The gas flow rates of hydrogen and argon were 30 and 0.5 sccm, respectively, resulting in a hydrogen partial gas pressure ratio of 93%. The total gas pressure and RF power were 2.7 Pa and 150 W, respectively. The substrate temperature, Ts, was 250 or 350 °C. Film thicknesses were measured with a surface profiler (Alpha-Step 500, Tencor Instruments). X-ray diffraction (XRD) patterns and Raman scattering spectra were measured by using an X-ray diffractometer (Rint2000Ultima, Rigaku) and a Raman scattering spectrophotometer (NRS-1000, Jasco) with a semiconductor laser of 532 nm wavelength, respectively. The dark conductivity was measured in a coplanar configuration with the gap length of 0.1 mm and electrode width of
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5 mm. For some films prepared with boron grains, the carrier concentration and the Hall mobility were estimated in a van der Pauw configuration using Hall Effect measurement system (ResiTest8300, Toyo).
Table 1 Mean crystallite sizes and XRD intensity ratio of (220) peak to (111) peak. Without boron grains
With boron grains
Mean crystallite size (111) (220)
11.6 nm 8.3 nm
7.2 nm 9.3 nm
Intensity ratio (220)/(111)
1.36
0.66
3. Results 3.1. Structural properties Fig. 1 shows the XRD patterns of the μc-Si:H thin films prepared at 250 °C with and without boron grains. The film thicknesses of both films were about 1 μm. Table 1 shows the mean crystallite size, estimated from the full width at half maximum of the XRD peaks using Scherrer's formula [15], and the XRD intensity ratio of the (220) peak to the (111) peak. The XRD patterns of the film prepared without boron grain show a (220) preferential orientation. However, the (220) XRD peak intensity decreased by the preparation with boron grains on the silicon wafer target, although the (220) mean crystallite size was almost the same (Table 1). On the other hand, the (111) XRD peak intensity increased and the mean crystallite size decreased. These findings suggest that the incorporation of boron atoms into the film prevents the nucleus formation of (220)-oriented crystallites and enhances random orientation. Fig. 2 shows the Raman scattering spectra of the films prepared with and without boron grains. The spectra were normalized to the signal at 520 cm− 1 corresponding to the peak originating from crystalline silicon. Ts was 250 °C (Fig. 2(a)) and 350 °C (Fig. 2(b)). The film thicknesses were between 21 and 25 nm. For all the Raman scattering spectra, the peak at around 520 cm− 1 due to crystalline silicon was observed. The Raman intensity at 480 cm− 1 due to the amorphous silicon phase of the films prepared with boron grains was higher than that without boron grains. This finding indicates that the preparation with boron grains on the silicon wafer target prevents nucleus formation and the growth of silicon crystallites. Especially for the Ts = 350 °C film, these effects were more pronounced: a peak due to the amorphous phase was predominant for the Ts = 350 °C film prepared with boron grains, although in the preparation without boron grains the spectrum of the Ts = 350 °C film was almost the same as that of the Ts = 250 °C film. Fig. 3 shows the relationship between the crystalline volume fraction and film thickness. The crystalline volume fraction, Xc was calculated from Xc = Ic / (Ic + Ia), where Ic and Ia are the integrated intensities of the Raman peaks due to the crystalline and amorphous silicon phases, respectively. For the thickness above 50 nm, Xc of the
Fig. 1. XRD patterns of the μc-Si:H thin films prepared at 250 °C with and without boron grains. The film thicknesses were about 1 mm.
films prepared with and without boron grains was the same when compared for the same thickness. However, for the thickness below 50 nm, Xc of the films prepared with boron grains was lower than that without boron grains. Moreover, for the films prepared with boron grains, Xc of the Ts = 350 °C films was lower than that of the Ts = 250 °C films, although Xc was the same for the films prepared at Ts = 250 and 350 °C without boron grains.
3.2. Electrical properties Fig. 4 shows the dark conductivity at room temperature of the μcSi:H thin films prepared with and without boron grains as a function of the film thickness. The dark conductivity of the 1 μm films prepared without boron grains was ∼ 10− 6 S cm− 1. This value is the same order
Fig. 2. Raman scattering spectra of the μc-Si:H thin films prepared with and without boron grains. The film thicknesses were 21–25 mm. Ts was 250 (a) and 350 °C (b).
A. Tabata et al. / Journal of Non-Crystalline Solids 356 (2010) 1131–1134
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Fig. 3. Relationship between the crystalline volume fraction and film thickness. The lines are included as visual guides.
Fig. 5. Carrier concentration and Hall mobility of the μc-Si:H thin films prepared with boron grains at 250 °C as a function of film thickness.
as those of undoped μc-Si:H thin films [16]. On the other hand, the dark conductivity of the 1 μm films prepared with boron grains was N100 S cm− 1, which was higher than that prepared without boron grains by more than six orders of magnitude. As the film thickness decreased, the dark conductivity decreased. However it should be noted that for the films with thickness above 50 nm, prepared with boron grains, the conductivities were higher than 6 × 10− 1 S cm− 1. The activation energies were about 0.05 eV, which is close to the ionization energy of the boron impurity in crystalline silicon [17]. For the 20 nm films, the conductivity was about 10− 1 S cm− 1, which is the same order of magnitude as those of the p-layer μc-Si:H in silicon-based thin-film solar cells [12,13,18,19]. However, as the film thickness decreased from 20 to 10 nm, the conductivity decreased dramatically to 10− 6 S cm− 1 by five orders of magnitude, which is almost the same value as those of undoped μc-Si:H thin films [16]. Fig. 5 shows the carrier concentration and the Hall mobility of the μc-Si:H thin films prepared at 250 °C with boron grains as a function of the film thickness. The results could not be obtained for the films of the thickness below 40 nm because of high noise levels. Both the carrier concentration and the Hall mobility decreased with decreasing the film thickness. In materials consisting of amorphous and crystalline phases, the Hall Effect is dominated by the crystalline phase [20]. The
decreases in the carrier concentration and Hall mobility are caused by the decrease in Xc. As mentioned later, for the films of the thickness below 40 nm the value of Xc is around and below percolation threshold value. The high noise levels during the Hall Effect measurements may be caused by a combination of the breakdown of the percolation network and a wrong sign of Hall Effect in amorphous silicon [20]. Further investigation for this is required. Fig. 6 shows the relationship between the dark conductivity and the crystalline volume fraction, Xc. As the Xc decreased, the dark conductivity decreased. Especially, for the Ts = 250 °C films prepared with boron grains a dramatic drop was observed at around Xc = 0.34, which corresponds to the decrease in the film thickness below 30 nm. The conductivities of the Ts = 350 °C films with thicknesses below 40 nm are also shown in Figs. 4 and 6. As shown in Fig. 4, in the preparation without boron grains, the Ts = 350 °C films had the same conductivity as those of the Ts = 250 °C films. However, in the preparation with boron grains, the Ts = 350 °C films had a lower conductivity than the Ts = 250 °C films. On the other hand, as shown in Fig. 6, the dark conductivity of the Ts = 350 °C films prepared with boron grains was higher than that of the Ts = 250 °C films when compared for the same Xc.
Fig. 4. Dark conductivity at room temperature of the μc-Si:H thin films prepared with and without boron grains as a function of film thickness. The lines are included as visual guides.
Fig. 6. Relationship between the dark conductivity and crystalline volume fraction. The lines are included as visual guides.
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4. Discussion As mentioned in the previous sections, the dark conductivity of the 1 μm film was improved by more than six orders of magnitude by using a silicon wafer with boron grains as a target. This results from the increase in the intake of boron atoms into the film. The silicon wafer used as a target in the present study contains boron atoms with a concentration of about 1019 cm− 3 [21], which is in no manner a low value. However, the dark conductivity of the film prepared from the silicon wafer (without boron grains) was almost the same order as that of undoped μ c-Si:H thin films. Xc of the film prepared without boron grains was almost the same as that with boron grains (Fig. 3), suggesting that the doping efficiency was never lower than that with boron grains. These above findings suggest that the boron atoms in the silicon wafer target did not transfer well into the μc-Si:H thin films and that the boron grains on the silicon wafer target increased the amount of boron atoms incorporated into the film. The setting of boron grains on a silicon wafer target makes it easier to prepare highly conductive p-type μc-Si:H thin films. The decrease in the dark conductivity with decreasing the film thickness results from the decreases in both the carrier concentration and the mobility with the decrease in Xc. Doping efficiency of impurities in amorphous phase is much lower than that in crystalline phase [22]. The decrease in Xc gives rise to the decrease in the amount of activated boron impurities in the films. This is the cause of the decrease in the carrier concentration with the decrease in the film thickness. Similarly, mobility in amorphous phase is lower than that in crystalline phase [22]. The decrease in Xc with the decrease in the film thickness results in the decrease in the mobility. It should be noted that the mobility in the present study decreased with the decrease in the carrier concentration. It is well known that mobility decreases with an increase in carrier concentration, due to the enhancement of impurity scattering of carriers [21,23]. The result obtained in the present study is the opposite of it. This indicates that the mobility in the present study was governed by the film structure rather than by the boron impurity scattering. As mentioned before, the dark conductivity of the Ts = 250 °C films dropped dramatically at around Xc = 0.34. This is closely related to percolation process. The decrease in Xc from percolation threshold value means the breakdown of percolation network, which causes the dramatic drop of the dark conductivity. It has been reported that the percolation threshold value of Xc is around 0.32 from computational analyses for a model assuming that crystalline and amorphous cubes with the same size are distributed at random and that each cubes is connected to its six neighboring cubes [24,25]. Other computational analysis for the packing of spheres has shown that the percolation threshold value is 0.154 [26]. The value obtained in the present study is in good agreement with the former, although the shapes of crystallites and amorphous phase in μs-Si:H are never cube and never of the same size. For the application of boron-doped μc-Si:H thin films to thin-film solar cells, the film thickness less than 20 nm is required [12–14]. However, the conductivity of the films obtained in the present study was very low. Therefore, further challenges are required for improvement of the conductivity for μc-Si:H thin films with thickness of less than 20 nm. One of the approaches is a preparation of thin films with a higher Xc. Elevating Ts from 250 to 350 °C did not affect the structure of the films prepared without boron grains. However, it affected the structure of the films prepared with boron grains. Elevating Ts enhances the migration of boron atoms adsorbed on the film-growing surface, disturbing nucleus formation and crystallite growth. Consequently, Xc decreased, giving rise to the decrease in the dark conductivity (Fig. 4). The elevating Ts from 250 to 350 °C shifted the percolation threshold to a lower value, as shown in Fig. 6. It has been reported that anisotropic
structure with a large number of linked crystalline cubes along one direction (film-growth direction) leads lower values of the percolation threshold in the direction [25]. However, the conductivity measurements in the present study were performed in the lateral direction perpendicular to the growth direction. Therefore, it is probably difficult to explain the shift of the percolation threshold value in the present study using the anisotropic structure. However, this indicates that the formation of the percolation network in the μc-Si:H depends not only on Xc but on other factors such as the shape [26] and size [27] of crystallites. Moreover, voids also should be taken into consideration because μc-Si:H contains many voids [28]. We intend to advance further investigation for them. 5. Conclusion Boron-doped μc-Si:H thin films with high dark conductivity and a low activation energy of the dark conductivity, could be prepared by RF magnetron sputtering using a silicon wafer, with boron grains located on top of it, as a target for the film thickness above 50 nm. However, as the film thickness decreased from 20 to 10 nm, the conductivity decreased dramatically from 10− 1 to 10− 6 S cm− 1, resulting from a decrease in the Xc. In order to obtain highly conductive μc-Si:H thin films with thickness of less than 20 nm, it is important to prepare films with a higher Xc. Acknowledgements We are grateful to the Center for Cooperative Research in Advanced Science and Technology, Nagoya University, for the use of the X-ray diffractometer, to Akasaki Research Center, Nagoya University, for the use of the Hall Effect measurement system and to Mr H. Choshi of the Instrumental Analysis Facility of Nagoya University, who kindly measured the Raman scattering spectra. References [1] J. Poortmans, V. Arkhipov, Thin Film Solar Cells, John Wiley & Sons, 2006, pp. 133–171. [2] S. Wagner, H. Gleskova, I.-C. Cheng, M. Wu, Thin Solid Films 430 (2003) 15. [3] M. Kondo, A. Matsuda, Curr. Opin. Solid State Mater. Sci. 6 (2002) 445. [4] H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175. [5] M.F. Cerqueira, M. Andritschly, L. Rebouta, J.A. Ferreira, M.F. da Silva, Vacuum 46 (1995) 1385. [6] Y. Leconte, P. Marie, X. Portier, M. Lejeune, R. Rizk, Thin Solid Films 427 (2003) 252. [7] H. Makihara, A. Tabata, Y. Suzuoki, T. Mizutani, Vacuum 59 (2000) 785. [8] J. Kondo, A. Tabata, T. Kawamura, T. Mizutani, Vacuum 66 (2002) 409. [9] K. Fukaya, A. Tabata, T. Mizutani, Vacuum 74 (2004) 561. [10] K. Fukaya, A. Tabata, T. Mizutani, Thin Solid Films 478 (2005) 132. [11] A. Tabata, K. Fukaya, T. Mizutani, Vacuum 82 (2008) 777. [12] R.E.I. Schroop, M. Zeman, Amorphous and Microcrystalline Silicon Solar Cells, Kluwer Academic Publishers, Boston, 1998. [13] J.K. Rath, R.E.I. Schroop, Sol. Energy Mater. Sol. Cells 53 (1998) 189. [14] T. Sasaki, S. Fujikane, K. Tabuchi, T. Yoshida, T. Hata, H. Sakai, Y. Ichikawa, J. NonCryst. Solids 266–269 (2000) 171. [15] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.John Wiley & Sons, New York, 1954. [16] P. Torres, J. Meier, R. Fluckinger, U. Kroll, J.A.A. Selvan, H. Keppner, A. Shah, S.D. Littelwood, I.E. Kelly, P. Giannoules, Appl. Phys. Lett. 69 (1996) 1373. [17] S.M. Sze, Semiconductor Devices — Physics and Technology, 2nd Ed, John Wiley & Sons, 2001, p. 38. [18] A. Gordijin, J.K. Rath, R.E.I. Schroop, J. Non-Cryst. Solids 338–340 (2004) 110. [19] K. Adhikary, S. Ray, J. Non-Cryst. Solids 353 (2007) 2289. [20] C.E. Nebel, M. Rother, M. Stutzmann, C. Summonte, M. Heintze, Phil. Mag. Lett. 74 (1996) 455. [21] S.M. Sze, Semiconductor Devices — Physics and Technology, 2nd Ed, John Wiley & Sons, 2001, pp. 50–55. [22] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, 1991. [23] P. Luo, Z. Zhou, Y. Li, S. Lin, X. Dou, R. Cui, Microelectron. J. 39 (2008) 12. [24] H. Overhof, M. Otte, M. Schmidtke, U. Bachhausen, R. Carius, J. Non-Cryst. Solids 227–230 (1998) 992. [25] F. Liu, M. Zhu, Q. Wang, Phys. Lett. A 331 (2004) 432. [26] H. Scher, R. Zallen, J. Chem. Phys. 53 (1970) 3759. [27] K. Shimakawa, J. Non-Cryst. Solids 266–269 (2000) 223. [28] P. Roca i Cabarrocas, J. Non-Cryst. Solids 266–269 (2000) 31.