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P-type doping of hydrogenated amorphous silicon films with boron by reactive radio-frequency co-sputtering
Physica B 308–310 (2001) 257–260
P-type doping of hydrogenated amorphous silicon films with boron by reactive radio-frequency co-sputtering Y. Ohmuraa,*, M. Takahashia, M. Suzukia, N. Sakamotoa, T. Megurob a
Department of Electronics and Computer Science, Iwaki Meisei University, 5-5-1, Iino Chuodai, Iwaki, Fukushima 970-8551, Japan b Semiconductors Laboratory, RIKEN, Wako, Japan
Abstract B has been successfully doped into the hydrogenated amorphous Si films without using explosive and/or toxic gases SiH4 or B2H6 by reactive radio-frequency co-sputtering. The target used for co-sputtering was a composite target composed of a B-doped Si wafer and B chips attached on the Si wafer with silver powder bond. The maximum area fraction of B chips used was 0.11. Argon and hydrogen pressures were 5 103 and 5 104 Torr, respectively. Substrates were kept at 2001C or 2501C during sputtering. The maximum B concentration in the film obtained was 2 1019 cm3 from secondary ion mass spectroscopy measurement. Films with resistivity of 104–105 O cm were obtained, which was low for the above acceptor concentration, compared with other group III impurities doping, indicating the high doping efficiency of B. A heterostructure, which was prepared by co-sputtering these B-doped films on an n-type crystalline Si, shows a good rectification characteristic. A small photovoltaic effect is also observed. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogenated amorphous silicon; Sputtering; Doping; Boron
1. Introduction The reactive radio-frequency (RF) co-sputtering has been widely used as a safe and inexpensive method for fabrication of doped hydrogenated amorphous silicon (a-Si : H) or silicon–carbon alloy (a-SiC : H) films without using any toxic or explosive gases. For the dopant used, Al has been investigated most extensively [1–5]. Saito et al. prepared a-SiC : H films doped with Ga [6] and Tl [7] by the co-sputtering method. In these group III acceptor impurities used, however, the concentration in the film reached 10% or more to obtain films with a low resistivity, e.g. 105 O cm, so long as the substrate was kept around 2001C during deposition, where the amorphous Si crystallizes rarely. In some cases, the resistivity has not decreased to 105 O cm, even if the impurity concentration exceeds 10% by far. This may be due to the low doping efficiency of these impurities. Incidentally, little has been reported about B *Corresponding author. Fax: +81-246-29-0577. E-mail address: [email protected] (Y. Ohmura).
doping by this method, although B is the most easily doped to high concentration in the crystalline Si counterpart. In this study, it has been demonstrated that B was successfully doped and low-resistivity a-Si : H films were obtained with relatively low concentration of B by the reactive co-sputtering method. In Section 2, experimental details on the film preparation and the evaluation method are described. Experimental results are shown and discussed in Section 3.
2. Experimental The sputtering targets used in this experiment are undoped Si crystal of 80 mm in diameter, B-doped p-type Si wafers and Si–B composite targets, where several 5 5 1 mm3 and/or 10 10 2 mm3 B chips were attached to p-type Si wafers with silver powder cement. Substrates for sputtered films are optical glasses for UV–Vis optical measurement and electrical measurement, ITO glasses and heavily B-doped p-type Si wafers for electrical measurement and high-resistivity p-type Si
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 8 5 - 2
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wafers for IR measurement. To investigate pn diode characteristics, films were sputtered on n-type Si wafers. The substrates were kept at 2001C or 2501C during deposition for 1 h. The film was prepared in a conventional RF sputtering system operating at 13.56 MHz. The base pressure of the system was of the order of 1 106 Torr. Argon and hydrogen partial pressures used were mostly 5 103 and 5 104 Torr, respectively. A RF power of 2 W/cm2 was used. The deposition ( was obtained. The film thickness was rate of about 2 A/s measured by a stylus monitor (Sloan DEKTAK IIA). The H concentration was obtained from IR absorption, which was measured by a Shimadzu FTIR 4100 spectrophotometer. The B concentration in the film was determined by secondary ion mass spectroscopy (SIMS) using a CAMECA DES II. For electrical measurement, aluminum was evaporated for the electrode. Through the stencil mask, Al dots of 2 mm in diameter were evaporated on films deposited on ITO glasses and p++ Si wafers. The co-planar electrodes were also formed with about 0.2 and 0.5 mm gaps for films on glasses. Electrical properties were determined by a Keithley 2400 SourceMeter.
Fig. 1. Raman spectra for B-doped (2 1019 cm3) a-Si : H films prepared at substrate temperatures 2001C and 2501C.
3. Results and discussion In most glow-discharge prepared, B-doped a-Si : H films, the B concentration is expressed in terms of partial pressure ratio B2H6/SiH4 during deposition [8]. In this work, to know the net B concentration directly, SIMS measurement was performed. In Table 1, configurations and the B concentrations for four different targets A, B, C and D are shown. In target A, where the most B chips were attached on the Si wafer, the area fraction reaches about 0.11. It was found that the B concentrations were reasonably constant through the film and were reproducible for each target. Therefore, several data are hereafter shown as a function of B concentrations as in Table 1. Fig. 1 shows micro-Raman spectra determined using Jasco NRS-2000 Raman microspectrometer for films prepared from target A at 2001C and 2501C. No
Table 1 Configurations of composite targets and B concentrations in the films determined by SIMSa Target A B C D
B chip area 11% 8% p++ Si 10%
Si target p++ Si p++ Si p++ Si p Si
B (SIMS) (cm3) 2 1019 3 1018 2 1017 2 1016
a ++ p Si, B-doped 0.005–0.01 O cm Si wafer; p Si, B-doped 5 O cm Si wafer.
Fig. 2. Hydrogen concentrations of monohydride Si–H and dihydride Si–H2 as a function of B concentration.
zone-center LO-TO phonon at 520 cm1, which is a sensitive signal for micro-crystalline Si formation, was observed even for the film prepared at 2501C, while the entire spectra are almost similar to those of the crystalline Si. Fig. 2 shows Si–H and Si–H2 concentrations obtained from IR absorption bands at 2000 and 2100 cm1 for Si–monohydride and Si–dihydride stretching vibrations, respectively, using the method and the proportionality factors between the concentration and the optical densities determined by Langford et al. [9]. It is observed that, below 1019 cm3 of B concentration, Si–H and Si–H2 concentrations do not change significantly from those of undoped films, respectively, for both films prepared at 2001C and 2501C. Above 1019 cm3, however, the hydrogen concentration decreases with B concentration, which is the same behavior as other impurities Al [5] or Ga [6],
Y. Ohmura et al. / Physica B 308–310 (2001) 257–260
Fig. 3. Optical band gap E0 and B1=2 (see the text) obtained from the Tauc plot of the UV–Vis optical data as a function of B concentration.
Fig. 4. Electrical conductivity as a function of temperature for B-doped a-Si : H films prepared at 2501C with and without AM-1 illumination.
although the transition concentration seems to depend on the impurity. In Fig. 3, the optical band gap EO and B factor are plotted against B concentration. Both quantities were obtained from Tauc plot of the optical data [10], hnaðnÞ ¼ Bðhn EO Þ2 ; where a is the absorption coefficient, with a simple assumption that two parabolic, conduction and valence bands are separated by the band gap EO [11]. Both EO and B have often been used as measures of disorder, decreasing with increasing
259
Fig. 5. I2V characteristics for B-doped p-type a-Si : H/n-type epitaxial Si at different temperatures.
disorder. The optical data with the absorption coefficient above about 104 cm1 have been used to avoid the interference effect. In Fig. 3, EO and B1=2 obviously decrease, when B concentration exceeds 1018 cm3, which agrees with the data in a small B fraction region for hydrogenated Si–B alloy films prepared by the glow discharge deposition [8]. It is observed that, for films prepared both at 2001C and 2501C, both quantities at the middle B concentration are slightly higher than those of undoped films. In Fig. 4, the electrical conductivity is plotted as a function of temperature for B-doped a-Si : H films prepared at 2501C with and without AM-1 illumination. Although the order of the conductivity is not necessarily similar to that of B concentration for some films, the film for the target A has the maximum dark conductivity and the activation energy near room temperature is about 0.1 eV, which indicates that the Fermi level is close to the valence band edge. The photoconductivity, although small, is still observed for this film. Finally, the diode data for B-doped a-Si : H/n-type crystalline Si junction will be presented. Fig. 5 shows an I2V characteristic determined at several temperatures for a diode in which a-Si : H film was sputtered using the target A in the Table 1 on the n-type (1 0 0) Si epitaxial wafer through a hole of about 1 cm2 of a stencil mask at 2001C. The electron concentration in the epilayer was 3 1014 cm3. For forward bias Vf p0:4 V, the current I was expressed as IBexpðeVf =nkTÞ with the n value near 2, indicating that the current is dominated by the recombination current in the space-charge region of the junction. For Vf > 0:4 V, however, no diffusion current component with n ¼ 1 is observed, but the current seems to be limited by the high series resistance of the film. It was found that the I2V characteristics are particularly sensitive to the deposition temperature. The forward current is higher and the reverse current is
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much smaller for the diode prepared at 2501C than that prepared at 2001C. The preliminary data showed that the short-circuit current of several tens mA/cm2 was obtained for these diodes under AM-1 illumination.
4. Summary It has been demonstrated that B-doped p-type a-Si : H films were prepared by the reactive RF co-sputtering using composite targets of B chips and Si wafers. No explosive and/or toxic gases, SiH4 or B2H6, were used. The B concentration in the film was directly measured by SIMS and could be controlled by the quantity of B chips and B concentration in the Si wafers in the composite target. The film of the maximum conductivity of about 5 104 S cm1 was obtained for the B concentration of 2 1019 cm3 and the Arrhenius plot near room temperature showed an activation energy of about 0.1 eV, indicating the Fermi level close to the valence band edge. The B-doped a-Si : H/n-type crystalline Si diode structure, which was prepared to confirm the p-type doping, showed a good rectification characteristic and a small photovoltaic effect was observed. These results show that B-doped a-Si : H films in this work contain high concentration of p-type carriers for relatively small B concentration, manifesting the high doping efficiency of sputtered B. Successful doping of B will pave the way for device fabrication by the sputtering
technique, although further optimization of, e.g., deposition condition is still necessary.
Acknowledgements The authors are grateful to Dr. K. Sawada for the Raman scattering measurement.
References [1] M.G. Thompson, D.K. Reinhard, J. Non-Cryst. Solids 37 (1980) 325. [2] M. Dayan, N. Croitoru, Y. Lereah, Phys. Lett. 82 (1981) A306. [3] H. Niu, I. Yoshizawa, T. Shikama, T. Matsuda, M. Takai, Jpn. J. Appl. Phys. 23 (1984) L18. [4] H.S. Fortuna, J.M. Marshall, Philos. Mag. 53 (1986) B383. [5] N. Saito, Y. Tomioka, H. Senda, T. Yamaguchi, K. Kawamura, Philos. Mag. 62 (1990) B527. [6] N. Saito, Y. Inui, T. Yamaguchi, I. Nakaaki, J. Appl. Phys. 83 (1998) 2067. [7] N. Saito, Y. Inui, T. Yamaguchi, I. Nakaaki, Thin Solid Films 281–282 (1996) 302. [8] C.C. Tsai, Phys. Rev. 19 (1979) B2041. [9] A.A. Langford, M.L. Fleet, B.P. Nelson, W.A. Lanford, N. Maley, Phys. Rev. 45 (1992) B13367. [10] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974, p. 159. [11] N.F. Mott, E.A. Davis, Electronic Properties in NonCrystalline Materials, Clarendon, Oxford, 1979, p. 289.
P-type doping of hydrogenated amorphous silicon films with boron by reactive radio-frequency co-sputtering Y. Ohmuraa,*, M. Takahashia, M. Suzukia, N. Sakamotoa, T. Megurob a
Department of Electronics and Computer Science, Iwaki Meisei University, 5-5-1, Iino Chuodai, Iwaki, Fukushima 970-8551, Japan b Semiconductors Laboratory, RIKEN, Wako, Japan
Abstract B has been successfully doped into the hydrogenated amorphous Si films without using explosive and/or toxic gases SiH4 or B2H6 by reactive radio-frequency co-sputtering. The target used for co-sputtering was a composite target composed of a B-doped Si wafer and B chips attached on the Si wafer with silver powder bond. The maximum area fraction of B chips used was 0.11. Argon and hydrogen pressures were 5 103 and 5 104 Torr, respectively. Substrates were kept at 2001C or 2501C during sputtering. The maximum B concentration in the film obtained was 2 1019 cm3 from secondary ion mass spectroscopy measurement. Films with resistivity of 104–105 O cm were obtained, which was low for the above acceptor concentration, compared with other group III impurities doping, indicating the high doping efficiency of B. A heterostructure, which was prepared by co-sputtering these B-doped films on an n-type crystalline Si, shows a good rectification characteristic. A small photovoltaic effect is also observed. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogenated amorphous silicon; Sputtering; Doping; Boron
1. Introduction The reactive radio-frequency (RF) co-sputtering has been widely used as a safe and inexpensive method for fabrication of doped hydrogenated amorphous silicon (a-Si : H) or silicon–carbon alloy (a-SiC : H) films without using any toxic or explosive gases. For the dopant used, Al has been investigated most extensively [1–5]. Saito et al. prepared a-SiC : H films doped with Ga [6] and Tl [7] by the co-sputtering method. In these group III acceptor impurities used, however, the concentration in the film reached 10% or more to obtain films with a low resistivity, e.g. 105 O cm, so long as the substrate was kept around 2001C during deposition, where the amorphous Si crystallizes rarely. In some cases, the resistivity has not decreased to 105 O cm, even if the impurity concentration exceeds 10% by far. This may be due to the low doping efficiency of these impurities. Incidentally, little has been reported about B *Corresponding author. Fax: +81-246-29-0577. E-mail address: [email protected] (Y. Ohmura).
doping by this method, although B is the most easily doped to high concentration in the crystalline Si counterpart. In this study, it has been demonstrated that B was successfully doped and low-resistivity a-Si : H films were obtained with relatively low concentration of B by the reactive co-sputtering method. In Section 2, experimental details on the film preparation and the evaluation method are described. Experimental results are shown and discussed in Section 3.
2. Experimental The sputtering targets used in this experiment are undoped Si crystal of 80 mm in diameter, B-doped p-type Si wafers and Si–B composite targets, where several 5 5 1 mm3 and/or 10 10 2 mm3 B chips were attached to p-type Si wafers with silver powder cement. Substrates for sputtered films are optical glasses for UV–Vis optical measurement and electrical measurement, ITO glasses and heavily B-doped p-type Si wafers for electrical measurement and high-resistivity p-type Si
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 8 5 - 2
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wafers for IR measurement. To investigate pn diode characteristics, films were sputtered on n-type Si wafers. The substrates were kept at 2001C or 2501C during deposition for 1 h. The film was prepared in a conventional RF sputtering system operating at 13.56 MHz. The base pressure of the system was of the order of 1 106 Torr. Argon and hydrogen partial pressures used were mostly 5 103 and 5 104 Torr, respectively. A RF power of 2 W/cm2 was used. The deposition ( was obtained. The film thickness was rate of about 2 A/s measured by a stylus monitor (Sloan DEKTAK IIA). The H concentration was obtained from IR absorption, which was measured by a Shimadzu FTIR 4100 spectrophotometer. The B concentration in the film was determined by secondary ion mass spectroscopy (SIMS) using a CAMECA DES II. For electrical measurement, aluminum was evaporated for the electrode. Through the stencil mask, Al dots of 2 mm in diameter were evaporated on films deposited on ITO glasses and p++ Si wafers. The co-planar electrodes were also formed with about 0.2 and 0.5 mm gaps for films on glasses. Electrical properties were determined by a Keithley 2400 SourceMeter.
Fig. 1. Raman spectra for B-doped (2 1019 cm3) a-Si : H films prepared at substrate temperatures 2001C and 2501C.
3. Results and discussion In most glow-discharge prepared, B-doped a-Si : H films, the B concentration is expressed in terms of partial pressure ratio B2H6/SiH4 during deposition [8]. In this work, to know the net B concentration directly, SIMS measurement was performed. In Table 1, configurations and the B concentrations for four different targets A, B, C and D are shown. In target A, where the most B chips were attached on the Si wafer, the area fraction reaches about 0.11. It was found that the B concentrations were reasonably constant through the film and were reproducible for each target. Therefore, several data are hereafter shown as a function of B concentrations as in Table 1. Fig. 1 shows micro-Raman spectra determined using Jasco NRS-2000 Raman microspectrometer for films prepared from target A at 2001C and 2501C. No
Table 1 Configurations of composite targets and B concentrations in the films determined by SIMSa Target A B C D
B chip area 11% 8% p++ Si 10%
Si target p++ Si p++ Si p++ Si p Si
B (SIMS) (cm3) 2 1019 3 1018 2 1017 2 1016
a ++ p Si, B-doped 0.005–0.01 O cm Si wafer; p Si, B-doped 5 O cm Si wafer.
Fig. 2. Hydrogen concentrations of monohydride Si–H and dihydride Si–H2 as a function of B concentration.
zone-center LO-TO phonon at 520 cm1, which is a sensitive signal for micro-crystalline Si formation, was observed even for the film prepared at 2501C, while the entire spectra are almost similar to those of the crystalline Si. Fig. 2 shows Si–H and Si–H2 concentrations obtained from IR absorption bands at 2000 and 2100 cm1 for Si–monohydride and Si–dihydride stretching vibrations, respectively, using the method and the proportionality factors between the concentration and the optical densities determined by Langford et al. [9]. It is observed that, below 1019 cm3 of B concentration, Si–H and Si–H2 concentrations do not change significantly from those of undoped films, respectively, for both films prepared at 2001C and 2501C. Above 1019 cm3, however, the hydrogen concentration decreases with B concentration, which is the same behavior as other impurities Al [5] or Ga [6],
Y. Ohmura et al. / Physica B 308–310 (2001) 257–260
Fig. 3. Optical band gap E0 and B1=2 (see the text) obtained from the Tauc plot of the UV–Vis optical data as a function of B concentration.
Fig. 4. Electrical conductivity as a function of temperature for B-doped a-Si : H films prepared at 2501C with and without AM-1 illumination.
although the transition concentration seems to depend on the impurity. In Fig. 3, the optical band gap EO and B factor are plotted against B concentration. Both quantities were obtained from Tauc plot of the optical data [10], hnaðnÞ ¼ Bðhn EO Þ2 ; where a is the absorption coefficient, with a simple assumption that two parabolic, conduction and valence bands are separated by the band gap EO [11]. Both EO and B have often been used as measures of disorder, decreasing with increasing
259
Fig. 5. I2V characteristics for B-doped p-type a-Si : H/n-type epitaxial Si at different temperatures.
disorder. The optical data with the absorption coefficient above about 104 cm1 have been used to avoid the interference effect. In Fig. 3, EO and B1=2 obviously decrease, when B concentration exceeds 1018 cm3, which agrees with the data in a small B fraction region for hydrogenated Si–B alloy films prepared by the glow discharge deposition [8]. It is observed that, for films prepared both at 2001C and 2501C, both quantities at the middle B concentration are slightly higher than those of undoped films. In Fig. 4, the electrical conductivity is plotted as a function of temperature for B-doped a-Si : H films prepared at 2501C with and without AM-1 illumination. Although the order of the conductivity is not necessarily similar to that of B concentration for some films, the film for the target A has the maximum dark conductivity and the activation energy near room temperature is about 0.1 eV, which indicates that the Fermi level is close to the valence band edge. The photoconductivity, although small, is still observed for this film. Finally, the diode data for B-doped a-Si : H/n-type crystalline Si junction will be presented. Fig. 5 shows an I2V characteristic determined at several temperatures for a diode in which a-Si : H film was sputtered using the target A in the Table 1 on the n-type (1 0 0) Si epitaxial wafer through a hole of about 1 cm2 of a stencil mask at 2001C. The electron concentration in the epilayer was 3 1014 cm3. For forward bias Vf p0:4 V, the current I was expressed as IBexpðeVf =nkTÞ with the n value near 2, indicating that the current is dominated by the recombination current in the space-charge region of the junction. For Vf > 0:4 V, however, no diffusion current component with n ¼ 1 is observed, but the current seems to be limited by the high series resistance of the film. It was found that the I2V characteristics are particularly sensitive to the deposition temperature. The forward current is higher and the reverse current is
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much smaller for the diode prepared at 2501C than that prepared at 2001C. The preliminary data showed that the short-circuit current of several tens mA/cm2 was obtained for these diodes under AM-1 illumination.
4. Summary It has been demonstrated that B-doped p-type a-Si : H films were prepared by the reactive RF co-sputtering using composite targets of B chips and Si wafers. No explosive and/or toxic gases, SiH4 or B2H6, were used. The B concentration in the film was directly measured by SIMS and could be controlled by the quantity of B chips and B concentration in the Si wafers in the composite target. The film of the maximum conductivity of about 5 104 S cm1 was obtained for the B concentration of 2 1019 cm3 and the Arrhenius plot near room temperature showed an activation energy of about 0.1 eV, indicating the Fermi level close to the valence band edge. The B-doped a-Si : H/n-type crystalline Si diode structure, which was prepared to confirm the p-type doping, showed a good rectification characteristic and a small photovoltaic effect was observed. These results show that B-doped a-Si : H films in this work contain high concentration of p-type carriers for relatively small B concentration, manifesting the high doping efficiency of sputtered B. Successful doping of B will pave the way for device fabrication by the sputtering
technique, although further optimization of, e.g., deposition condition is still necessary.
Acknowledgements The authors are grateful to Dr. K. Sawada for the Raman scattering measurement.
References [1] M.G. Thompson, D.K. Reinhard, J. Non-Cryst. Solids 37 (1980) 325. [2] M. Dayan, N. Croitoru, Y. Lereah, Phys. Lett. 82 (1981) A306. [3] H. Niu, I. Yoshizawa, T. Shikama, T. Matsuda, M. Takai, Jpn. J. Appl. Phys. 23 (1984) L18. [4] H.S. Fortuna, J.M. Marshall, Philos. Mag. 53 (1986) B383. [5] N. Saito, Y. Tomioka, H. Senda, T. Yamaguchi, K. Kawamura, Philos. Mag. 62 (1990) B527. [6] N. Saito, Y. Inui, T. Yamaguchi, I. Nakaaki, J. Appl. Phys. 83 (1998) 2067. [7] N. Saito, Y. Inui, T. Yamaguchi, I. Nakaaki, Thin Solid Films 281–282 (1996) 302. [8] C.C. Tsai, Phys. Rev. 19 (1979) B2041. [9] A.A. Langford, M.L. Fleet, B.P. Nelson, W.A. Lanford, N. Maley, Phys. Rev. 45 (1992) B13367. [10] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974, p. 159. [11] N.F. Mott, E.A. Davis, Electronic Properties in NonCrystalline Materials, Clarendon, Oxford, 1979, p. 289.