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Influence of atomic bonds on electrical property of boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition
Diamond & Related Materials 19 (2010) 1441–1445
Contents lists available at ScienceDirect
Diamond & Related Materials 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 / d i a m o n d
Influence of atomic bonds on electrical property of boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition Hiroshi Sota, Chiharu Kimura ⁎, Hidemitsu Aoki, Takashi Sugino Department of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2009 Received in revised form 7 June 2010 Accepted 17 June 2010 Available online 23 June 2010 Keywords: Boron carbon nitride Plasma-assisted CVD
a b s t r a c t Boron carbon nitride (BCN) films are synthesized with various growth conditions by remote plasma-assisted chemical vapor deposition method. The chemical bonding in the BCN film is modified by the growth condition. Optical and electrical properties are investigated for BCN films with various chemical bonding. Electrical characterization is carried out for the BCN films which have the same bandgap energy and different C composition ratio and have the same C composition ratio and different bandgap energy. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Boron carbon nitride (BCN) has been expected as one of the promising materials applicable to the light emitting device because the bandgap energy can be changed in the wide range [1]. Moreover, we have demonstrated that a dielectric constant lower than 2 can be achieved for the BCN film, suggesting that the BCN film can be applied to an insulating film for the multilevel wiring in Si LSI devices [2]. We have also shown that the BCN has a small electron affinity, and have studied field emission characteristics of the BCN film [3–8]. These electronic applications to a low dielectric constant (low-k) film and a field emitter with a low threshold electric field have been proposed using the BCN film synthesized by remote plasma-assisted chemical vapor deposition (RPCVD). The BCN film consists of amorphous region and nano-sized crystal grains [9]. Atomic bond structure of the amorphous material can be changed by the growth condition [10]. The bandgap energy of the BCN decreases with the increasing carbon composition ratio in comparison with that of BN [1]. On the other hand, we have recently found that the bandgap energy of the BCN can be decreased with increasing N=C and C=C bonds even for the BCN film with a low C composition ratio [10]. Transmission electron microscopy (TEM) observation of the BN and BCN (C 26%) films are performed in our previous work [10]. An amorphous region in the BCN film increases with increasing C composition ratio compared with the polycrystalline BN film though the BN film has several percents of C component by unintentional incorporation of the residual C atoms. This means that the BCN film becomes a defective film with increasing C composition ratio. In order to attempt further applications of BCN film to electronic and optoelectronic devices such ⁎ Corresponding author. Tel.: +81 6 6877 5111. E-mail address: [email protected] (C. Kimura). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.06.021
as transparent thin film transistor and light emitting device, it is important to understand both electrical and optical properties for the BCN films with different bond structures. In this paper electrical properties are investigated for the amorphous BCN films with different composition ratio of the constituent atoms and atomic bonds. BCN samples are prepared under different growth conditions by RPCVD. The electrical conductivities of the BCN films are demonstrated, and the relationship between the electrical conductivity and the bond structure is discussed. 2. Experimental procedure In order to investigate relationship between electrical properties and atomic bonds, amorphous BCN films were prepared by RPCVD [3]. p-Type Si wafer and quartz plate were used as the substrate. After the substrate was heated, N2 plasma was produced at the turn coil installed around the horizontal quartz reactor away from the substrate pedestal. Radio frequency power (13.56 MHz) was supplied to the coil. Borontrichloride (BCl3) was transported close to the substrate with H2 gas. Methane (CH4) was introduced into N2 plasma. The growth conditions used in the present experiment are summarized in Table 1. A change of the growth condition makes it possible to modify the property of the BCN film [10]. Ultraviolet–visible light absorption measurement was carried out and the optical bandgap was estimated from Tauc plot, (αhν)1/2 −hν, in which the absorption coefficient α was obtained by the optical absorption measurement. Fourier transform infrared absorption (FTIR) measurement was also performed to investigate atomic bonds in the BCN film. X-ray photoelectron spectroscopy (XPS) measurement was used to examine the composition ratio of the constituent atoms. XPS measurements were carried out on the BCN surface etched with Ar ions every 20 s. It was confirmed that no variation in the XPS signal intensity was observed after Ar ion etching for 100 s. Therefore, the composition ratio of
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Table 1 The growth condition of BCN samples. Growth pressure (Torr) Substrate temperature (°C) Source gas flow rate
RE power (W)
H2 (SCCM) BCl3 (SCCM) CH4 (SCCM) N2 (SCCM)
0.2, 1.0 400, 650 1.0 0.2–0.8 0.1–1.0 1.0, 2.5 40, 80
Table 2 The growth conditions, composition ratios of B, C and N, and optical bandgap energy of #1, #2 and #3 BCN samples. Sample
#1
#2
#3
Substrate temperature (°C)
400
650
650
1.0 0.4 0.6 2.5 41.8 13.2 34.3 10.7 4.5
1.0 0.8 0.3 1.0 41.5 25.4 23.7 9.4 4.5
1.0 0.8 0.1 1.0 43.5 14.4 26.9 15.2 5.5
Source gas flow rate
Composition rate
the constituent atoms in the film can be estimated by the XPS signal intensity after Ar ion etching for 40 s. And atomic bonds in the BCN film were also investigated by XPS measurement. Deconvolution of XPS spectra from B1s, N1s and C1s core levels was performed, and XPS signal intensity of each atomic bond was compared. Ni electrodes of 1 mm in diameter were formed on the BCN film grown on the p-type Si substrate. Ag epoxy was used for the electrode of the rear face of the Si substrate. Current versus voltage (I–V) characteristics were measured at various temperatures. 3. Results and discussion The optical bandgap was evaluated for BCN films prepared under various growth conditions. Fig. 1 shows the optical bandgap plotted as a function of the C composition ratio. The bandgap indicated by the squares decreases with increasing C composition ratio for the BCN film grown with increasing CH4 from 0.1 to 0.3 SCCM at 650 °C. BCl3 and N2 were fixed to be 0.8 and 1.0 SCCM, respectively. In the case of changing CH4 from 0.6 to 1.0 SCCM at 400 °C, on the other hand, incorporation of C atom into the BCN film is much suppressed as shown by the triangles. However, a reduction in the bandgap occurs for the BCN films with low C composition ratio. Further reduction in the bandgap is shown by the circles. BCN films of this group were grown increasing BCl3 from 0.2 to 0.4 SCCM at 400 °C. CH4 and N2 were fixed to be 0.6 and 2.5 SCCM. Three BCN samples (#1, #2 and #3) were chosen to characterize film structure and electrical property. Samples #1 and #2 have the same bandgap energy and the different C composition ratio, and samples #1 and #3 have the same C composition ratio and the different bandgap energy. The growth condition, composition ratio of the constituent atoms and optical bandgap of samples #1, #2 and #3 are summarized in Table 2. Fig. 2 shows the FTIR spectra of #1, #2 and #3 BCN samples. It has been known that the absorption bands at 800 and 1380 cm−1 are due to
Fig. 1. Optical bandgap energy plotted as a function of C composition ratio.
Bandgap energy (eV)
H2 (SCCM) BCl3 (SCCM) CH4 (SCCM) N2 (SCCM) B (%) C (%) N (%) O (%)
the B–N–B bending mode and the B–N stretching mode of h-BN, respectively [11]. There have also been reports that absorption bands due to B–C, C–N, C=N and C=C bonds are at 1170, 1250, 1550 and 1750 cm−1, respectively [12–19]. The spectra of samples #2 and #3 are almost the same in the wavenumber region higher than 1380 cm−1. The absorption band at 1550 cm−1 due to the C=N bond becomes dominant for sample #1 compared with that of the #2 and #3 samples. Moreover, the intensity of the absorption band at 1750 cm−1 of sample #1 is larger than that of the #2 and #3. In comparison with sample #1, absorption due to the B–C and the C–N bonds at 1170 and 1250 cm−1, respectively, increases for samples #2 and #3. A variation in the atomic bonds of these BCN samples was investigated by deconvolution of XPS spectra from B1s, C1s and N1s core levels. The binding energy and the full width at half maximum of XPS signal of each atomic bond such as B–N, N–C, B–C etc. used for deconvolution are indicated in the previous paper [10]. Deconvolution of XPS spectra were also carried out for samples #1, #2 and #3. In the FTIR spectra, C=C, C=N, C–N and B–C bonds were compared between samples #1, #2 and #3. A variation in these bonds was investigated using XPS spectra as shown in Fig. 3. The XPS signal intensity of each atomic bond obtained is normalized to the peak intensity of the B1s, N1s or C1s spectrum. Not area intensity but peak intensity of the XPS spectrum for the normalization is used here. A variation in the atomic bonds shown by the XPS spectra supports well a behavior of the FTIR
Fig. 2. FTIR spectra of #1, #2 and #3 BCN samples.
H. Sota et al. / Diamond & Related Materials 19 (2010) 1441–1445
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Fig. 3. XPS signal intensity of atomic bond in #1, #2 and #3 BCN samples.
spectra qualitatively though it is difficult to perform a quantitative estimation here. Fig. 4(a) shows I–V characteristics of sample #1 measured at temperatures from 300 to 500 K. The horizontal axis indicates the polarity of the voltage applied to the Ni electrode on the BCN film. No significant variation in the negatively biased I–V characteristics is observed. On the other hand, an increase of the current occurs with increasing temperature in the positively biased I–V characteristics. The positively biased I–V characteristics are replotted logarithmically in Fig. 4(b). Ohmic characteristics are obtained in the low bias region, while the current increases with square of the bias voltage in the high bias region. This means that the electrical conduction in the high bias region is possibly dominated by the space charge limited current [20]. I–V characteristics of samples #2 and #3 were similar to those of sample #1. The electrical conductivity at 300 K were estimated to be 7.1 × 10−11, 1.0 × 10−11 and 5.8 × 10−13 S/cm for samples #1, #2 and #3, respectively, from ohmic region. Arrhenius plots of the electrical conductivity are indicated for samples #1, #2 and #3 in Fig. 5(a), (b) and (c), respectively. Arrhenius plot of sample #1 consists of three straight lines with different slopes. The activation energies, Ea1, Ea2 and Ea3, are estimated to be 0.01, 0.23
and 0.58 eV from the slope of the straight lines. The activation energies are estimated to be 0.25 and 0.55 eV for sample #2 and to be 0.58 eV for sample #3. The electrical conductivity σ is related to the reciprocal temperature as follows: σ = ðn1 eμÞ exp ð−Ea1 = kT Þ + ðn2 eμÞ exp ð−Ea2 = kT Þ + ðn3 eμÞ exp ð−Ea3 = kT Þ where n is the density of the energy level, e is electron charge magnitude, μ is carrier mobility, Ea is activation energy, T is temperature and k is Boltzmann's constant. In order to evaluate a variation in the density of the energy level from which conduction carriers are supplied, it is assumed that μ is 0.1 cm2 V−1 s−1 and does not depend on the temperature in the present measurement range. Though the values of Ea2 and Ea3 are slightly different between samples 1 and #2, these are regarded as the same energy levels here and 0.25 and 0.58 eV are used as the values of Ea2 and Ea3, respectively. The curve fitting of the electrical conductivity is carried out as shown in Fig. 6 using the following values of the densities. The densities n of sample #1 are estimated to be 2.5 × 109, 1.0 × 1014 and 1.1 × 1018 cm−3 for the energy levels of 0.01,
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Fig. 4. (a) I–V characteristics of #1 BCN sample at various temperatures. (b) Logarithmic plot of the I–V characteristics of sample #1.
0.25 and 0.58 eV, respectively. The densities of sample #2 are estimated to be 6.6× 1012 and 7.5 × 1017 cm−3 for the energy levels of 0.25 and 0.58 eV. The density of sample #3 is estimated to be 6.1 × 1017 cm−3 for the energy level of 0.58 eV. Fig. 7 depicts the relationship between the electrical conductivity and XPS signal intensity of the atomic bonds of the three samples. A remarkable variation in the atomic bonds of N=C, C=C, B–C and C–N is observed for the three samples. Sample #1 is featured by an increase of N=C and C=C bonds. Though the C composition ratio of the #1 sample is as low as 13%, the bandgap energy decreases to 4.5 eV in comparison with that of the BN film (6.0 eV) [3]. This is possibly due to contribution of π electrons of the N=C and C=C bonds. It is reported that π electrons of the sp2 sites in the amorphous carbon have a strong influence on the optical bandgap [21]. Moreover, a decrease of the optical bandgap is observed in the CN film in which the state density of π bonding of the sp2 clusters is increased by enhancing N incorporation [22]. A decrease of the bandgap energy occurs possibly as a result of distribution of the π electrons near the band edge. The electrical conductivity of sample #1 decreases to 7.1 × 10−11 S/cm, and this is possibly related to generation of the
Fig. 5. Arrhenius plots of conductivity of (a) samples #1, (b) #2 and (c) #3.
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shallow level due to π electrons of the N=C and C=C bonds. In contrast to this, the electrical conductivity also decreases with increasing C composition ratio as shown by sample #2 in which the B–C and C–N bonds increase. 4. Conclusion Optical and electrical characteristics are investigated for BCN films with various atomic bond structures, which are deposited by changing the growth conditions by RPCVD method. The optical bandgap decreases with increasing B–C bond due to increasing C composition ratio and with increasing C=C and C=N bonds. A significant increase of the electrical conductivity from 5.8 × 10−13 to 7.1 × 10−11 S/cm occurs for the BCN film with C composition ratio of 13%. This is due to π electrons of C=N and C=C bonds. References
Fig. 6. Electrical conductivity estimated using the activation energies obtained by the Arrhenius plot.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 7. Relationship between XPS signal intensity and electrical conductivity of samples #1, #2 and #3.
T. Yuki, S. Umeda, T. Sugino, Diamond Relat. Mater. 13 (2004) 1130. S. Umeda, T. Yuki, T. Sugiyama, T. Sugino, Diamond Relat. Mater. 13 (2004) 1135. T. Sugino, K. Tanioka, S. Kawasaki, J. Shirafuji, Jpn. J. Appl. Phys. 36 (1997) 463. T. Sugino, S. Kawasaki, K. Tanioka, J. Shirafuji, Appl. Phys. Lett. 71 (1997) 2704. T. Sugino, Y. Etou, S. Tagawa, M.N. Gamo, T. Ando, J. Vac. Sci. Technol. B 18 (2000) 1089. T. Sugino, C. Kimura, T. Yamamoto, Appl. Phys. Lett. 80 (2002) 3602. C. Kimura, K. Okada, S. Funakawa, S. Sakata, T. Sugino, Diamond Relat. Mater. 14 (2005) 719. K. Okada, C. Kimura, T. Sugino, Diamond Relat. Mater. 15 (2006) 1000. T. Sugino, H. Hieda, Diamond Relat. Mater. 9 (2000) 1233. C. Kimura, H. Sota, H. Aoki, T. Sugino, Diamond Relat. Mater. 18 (2009) 478. F. Qian, V. Nagabushnam, R.K. Singh, Appl. Phys. Lett. 63 (1993) 317. S.V. Deshpande, E. Gulari, S.J. Harris, A.M. Weiner, Appl. Phys. Lett. 65 (1994) 1757. E. Pascual, M. Martnez, J. Esteve, A. Lousa, Diamond Relat. Mater. 8 (1999) 402. S. Ulrich, T. Theel, J. Schwan, H. Ehrhardt, Surf. Coat. Technol. 97 (1–3) (1997) 45. J. Yue, W. Cheng, X. Zhang, D. He, G. Chen, Thin Solid Films 375 (2000) 247. M.L. Kosinova, N.I. Fainer, Y.M. Rumyantsev, A.N. Golubenko, F.A. Kuznetsov, J. Phys. IV France 9 (1999) 915. Y. Wada, Y.K. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Diamond Relat. Mater. 9 (2000) 620. J. Hu, P. Yang, C.M. Lieber, Phys. Rev. B 57 (1998) R3185. N. Nakayama, Y. Tsuchiya, S. Tamada, K. Kosuge, Jpn. J. Appl. Phys. 32 (1993) L1465. M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, 1970. J. Robertson, E.P. O'Reilly, Phys. Rev. B 35 (1987) 2946. J.R. Shi, Y.J. Xu, J. Zhang, Thin Solid Films 483 (2005) 169.
Contents lists available at ScienceDirect
Diamond & Related Materials 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 / d i a m o n d
Influence of atomic bonds on electrical property of boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition Hiroshi Sota, Chiharu Kimura ⁎, Hidemitsu Aoki, Takashi Sugino Department of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2009 Received in revised form 7 June 2010 Accepted 17 June 2010 Available online 23 June 2010 Keywords: Boron carbon nitride Plasma-assisted CVD
a b s t r a c t Boron carbon nitride (BCN) films are synthesized with various growth conditions by remote plasma-assisted chemical vapor deposition method. The chemical bonding in the BCN film is modified by the growth condition. Optical and electrical properties are investigated for BCN films with various chemical bonding. Electrical characterization is carried out for the BCN films which have the same bandgap energy and different C composition ratio and have the same C composition ratio and different bandgap energy. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Boron carbon nitride (BCN) has been expected as one of the promising materials applicable to the light emitting device because the bandgap energy can be changed in the wide range [1]. Moreover, we have demonstrated that a dielectric constant lower than 2 can be achieved for the BCN film, suggesting that the BCN film can be applied to an insulating film for the multilevel wiring in Si LSI devices [2]. We have also shown that the BCN has a small electron affinity, and have studied field emission characteristics of the BCN film [3–8]. These electronic applications to a low dielectric constant (low-k) film and a field emitter with a low threshold electric field have been proposed using the BCN film synthesized by remote plasma-assisted chemical vapor deposition (RPCVD). The BCN film consists of amorphous region and nano-sized crystal grains [9]. Atomic bond structure of the amorphous material can be changed by the growth condition [10]. The bandgap energy of the BCN decreases with the increasing carbon composition ratio in comparison with that of BN [1]. On the other hand, we have recently found that the bandgap energy of the BCN can be decreased with increasing N=C and C=C bonds even for the BCN film with a low C composition ratio [10]. Transmission electron microscopy (TEM) observation of the BN and BCN (C 26%) films are performed in our previous work [10]. An amorphous region in the BCN film increases with increasing C composition ratio compared with the polycrystalline BN film though the BN film has several percents of C component by unintentional incorporation of the residual C atoms. This means that the BCN film becomes a defective film with increasing C composition ratio. In order to attempt further applications of BCN film to electronic and optoelectronic devices such ⁎ Corresponding author. Tel.: +81 6 6877 5111. E-mail address: [email protected] (C. Kimura). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.06.021
as transparent thin film transistor and light emitting device, it is important to understand both electrical and optical properties for the BCN films with different bond structures. In this paper electrical properties are investigated for the amorphous BCN films with different composition ratio of the constituent atoms and atomic bonds. BCN samples are prepared under different growth conditions by RPCVD. The electrical conductivities of the BCN films are demonstrated, and the relationship between the electrical conductivity and the bond structure is discussed. 2. Experimental procedure In order to investigate relationship between electrical properties and atomic bonds, amorphous BCN films were prepared by RPCVD [3]. p-Type Si wafer and quartz plate were used as the substrate. After the substrate was heated, N2 plasma was produced at the turn coil installed around the horizontal quartz reactor away from the substrate pedestal. Radio frequency power (13.56 MHz) was supplied to the coil. Borontrichloride (BCl3) was transported close to the substrate with H2 gas. Methane (CH4) was introduced into N2 plasma. The growth conditions used in the present experiment are summarized in Table 1. A change of the growth condition makes it possible to modify the property of the BCN film [10]. Ultraviolet–visible light absorption measurement was carried out and the optical bandgap was estimated from Tauc plot, (αhν)1/2 −hν, in which the absorption coefficient α was obtained by the optical absorption measurement. Fourier transform infrared absorption (FTIR) measurement was also performed to investigate atomic bonds in the BCN film. X-ray photoelectron spectroscopy (XPS) measurement was used to examine the composition ratio of the constituent atoms. XPS measurements were carried out on the BCN surface etched with Ar ions every 20 s. It was confirmed that no variation in the XPS signal intensity was observed after Ar ion etching for 100 s. Therefore, the composition ratio of
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Table 1 The growth condition of BCN samples. Growth pressure (Torr) Substrate temperature (°C) Source gas flow rate
RE power (W)
H2 (SCCM) BCl3 (SCCM) CH4 (SCCM) N2 (SCCM)
0.2, 1.0 400, 650 1.0 0.2–0.8 0.1–1.0 1.0, 2.5 40, 80
Table 2 The growth conditions, composition ratios of B, C and N, and optical bandgap energy of #1, #2 and #3 BCN samples. Sample
#1
#2
#3
Substrate temperature (°C)
400
650
650
1.0 0.4 0.6 2.5 41.8 13.2 34.3 10.7 4.5
1.0 0.8 0.3 1.0 41.5 25.4 23.7 9.4 4.5
1.0 0.8 0.1 1.0 43.5 14.4 26.9 15.2 5.5
Source gas flow rate
Composition rate
the constituent atoms in the film can be estimated by the XPS signal intensity after Ar ion etching for 40 s. And atomic bonds in the BCN film were also investigated by XPS measurement. Deconvolution of XPS spectra from B1s, N1s and C1s core levels was performed, and XPS signal intensity of each atomic bond was compared. Ni electrodes of 1 mm in diameter were formed on the BCN film grown on the p-type Si substrate. Ag epoxy was used for the electrode of the rear face of the Si substrate. Current versus voltage (I–V) characteristics were measured at various temperatures. 3. Results and discussion The optical bandgap was evaluated for BCN films prepared under various growth conditions. Fig. 1 shows the optical bandgap plotted as a function of the C composition ratio. The bandgap indicated by the squares decreases with increasing C composition ratio for the BCN film grown with increasing CH4 from 0.1 to 0.3 SCCM at 650 °C. BCl3 and N2 were fixed to be 0.8 and 1.0 SCCM, respectively. In the case of changing CH4 from 0.6 to 1.0 SCCM at 400 °C, on the other hand, incorporation of C atom into the BCN film is much suppressed as shown by the triangles. However, a reduction in the bandgap occurs for the BCN films with low C composition ratio. Further reduction in the bandgap is shown by the circles. BCN films of this group were grown increasing BCl3 from 0.2 to 0.4 SCCM at 400 °C. CH4 and N2 were fixed to be 0.6 and 2.5 SCCM. Three BCN samples (#1, #2 and #3) were chosen to characterize film structure and electrical property. Samples #1 and #2 have the same bandgap energy and the different C composition ratio, and samples #1 and #3 have the same C composition ratio and the different bandgap energy. The growth condition, composition ratio of the constituent atoms and optical bandgap of samples #1, #2 and #3 are summarized in Table 2. Fig. 2 shows the FTIR spectra of #1, #2 and #3 BCN samples. It has been known that the absorption bands at 800 and 1380 cm−1 are due to
Fig. 1. Optical bandgap energy plotted as a function of C composition ratio.
Bandgap energy (eV)
H2 (SCCM) BCl3 (SCCM) CH4 (SCCM) N2 (SCCM) B (%) C (%) N (%) O (%)
the B–N–B bending mode and the B–N stretching mode of h-BN, respectively [11]. There have also been reports that absorption bands due to B–C, C–N, C=N and C=C bonds are at 1170, 1250, 1550 and 1750 cm−1, respectively [12–19]. The spectra of samples #2 and #3 are almost the same in the wavenumber region higher than 1380 cm−1. The absorption band at 1550 cm−1 due to the C=N bond becomes dominant for sample #1 compared with that of the #2 and #3 samples. Moreover, the intensity of the absorption band at 1750 cm−1 of sample #1 is larger than that of the #2 and #3. In comparison with sample #1, absorption due to the B–C and the C–N bonds at 1170 and 1250 cm−1, respectively, increases for samples #2 and #3. A variation in the atomic bonds of these BCN samples was investigated by deconvolution of XPS spectra from B1s, C1s and N1s core levels. The binding energy and the full width at half maximum of XPS signal of each atomic bond such as B–N, N–C, B–C etc. used for deconvolution are indicated in the previous paper [10]. Deconvolution of XPS spectra were also carried out for samples #1, #2 and #3. In the FTIR spectra, C=C, C=N, C–N and B–C bonds were compared between samples #1, #2 and #3. A variation in these bonds was investigated using XPS spectra as shown in Fig. 3. The XPS signal intensity of each atomic bond obtained is normalized to the peak intensity of the B1s, N1s or C1s spectrum. Not area intensity but peak intensity of the XPS spectrum for the normalization is used here. A variation in the atomic bonds shown by the XPS spectra supports well a behavior of the FTIR
Fig. 2. FTIR spectra of #1, #2 and #3 BCN samples.
H. Sota et al. / Diamond & Related Materials 19 (2010) 1441–1445
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Fig. 3. XPS signal intensity of atomic bond in #1, #2 and #3 BCN samples.
spectra qualitatively though it is difficult to perform a quantitative estimation here. Fig. 4(a) shows I–V characteristics of sample #1 measured at temperatures from 300 to 500 K. The horizontal axis indicates the polarity of the voltage applied to the Ni electrode on the BCN film. No significant variation in the negatively biased I–V characteristics is observed. On the other hand, an increase of the current occurs with increasing temperature in the positively biased I–V characteristics. The positively biased I–V characteristics are replotted logarithmically in Fig. 4(b). Ohmic characteristics are obtained in the low bias region, while the current increases with square of the bias voltage in the high bias region. This means that the electrical conduction in the high bias region is possibly dominated by the space charge limited current [20]. I–V characteristics of samples #2 and #3 were similar to those of sample #1. The electrical conductivity at 300 K were estimated to be 7.1 × 10−11, 1.0 × 10−11 and 5.8 × 10−13 S/cm for samples #1, #2 and #3, respectively, from ohmic region. Arrhenius plots of the electrical conductivity are indicated for samples #1, #2 and #3 in Fig. 5(a), (b) and (c), respectively. Arrhenius plot of sample #1 consists of three straight lines with different slopes. The activation energies, Ea1, Ea2 and Ea3, are estimated to be 0.01, 0.23
and 0.58 eV from the slope of the straight lines. The activation energies are estimated to be 0.25 and 0.55 eV for sample #2 and to be 0.58 eV for sample #3. The electrical conductivity σ is related to the reciprocal temperature as follows: σ = ðn1 eμÞ exp ð−Ea1 = kT Þ + ðn2 eμÞ exp ð−Ea2 = kT Þ + ðn3 eμÞ exp ð−Ea3 = kT Þ where n is the density of the energy level, e is electron charge magnitude, μ is carrier mobility, Ea is activation energy, T is temperature and k is Boltzmann's constant. In order to evaluate a variation in the density of the energy level from which conduction carriers are supplied, it is assumed that μ is 0.1 cm2 V−1 s−1 and does not depend on the temperature in the present measurement range. Though the values of Ea2 and Ea3 are slightly different between samples 1 and #2, these are regarded as the same energy levels here and 0.25 and 0.58 eV are used as the values of Ea2 and Ea3, respectively. The curve fitting of the electrical conductivity is carried out as shown in Fig. 6 using the following values of the densities. The densities n of sample #1 are estimated to be 2.5 × 109, 1.0 × 1014 and 1.1 × 1018 cm−3 for the energy levels of 0.01,
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Fig. 4. (a) I–V characteristics of #1 BCN sample at various temperatures. (b) Logarithmic plot of the I–V characteristics of sample #1.
0.25 and 0.58 eV, respectively. The densities of sample #2 are estimated to be 6.6× 1012 and 7.5 × 1017 cm−3 for the energy levels of 0.25 and 0.58 eV. The density of sample #3 is estimated to be 6.1 × 1017 cm−3 for the energy level of 0.58 eV. Fig. 7 depicts the relationship between the electrical conductivity and XPS signal intensity of the atomic bonds of the three samples. A remarkable variation in the atomic bonds of N=C, C=C, B–C and C–N is observed for the three samples. Sample #1 is featured by an increase of N=C and C=C bonds. Though the C composition ratio of the #1 sample is as low as 13%, the bandgap energy decreases to 4.5 eV in comparison with that of the BN film (6.0 eV) [3]. This is possibly due to contribution of π electrons of the N=C and C=C bonds. It is reported that π electrons of the sp2 sites in the amorphous carbon have a strong influence on the optical bandgap [21]. Moreover, a decrease of the optical bandgap is observed in the CN film in which the state density of π bonding of the sp2 clusters is increased by enhancing N incorporation [22]. A decrease of the bandgap energy occurs possibly as a result of distribution of the π electrons near the band edge. The electrical conductivity of sample #1 decreases to 7.1 × 10−11 S/cm, and this is possibly related to generation of the
Fig. 5. Arrhenius plots of conductivity of (a) samples #1, (b) #2 and (c) #3.
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shallow level due to π electrons of the N=C and C=C bonds. In contrast to this, the electrical conductivity also decreases with increasing C composition ratio as shown by sample #2 in which the B–C and C–N bonds increase. 4. Conclusion Optical and electrical characteristics are investigated for BCN films with various atomic bond structures, which are deposited by changing the growth conditions by RPCVD method. The optical bandgap decreases with increasing B–C bond due to increasing C composition ratio and with increasing C=C and C=N bonds. A significant increase of the electrical conductivity from 5.8 × 10−13 to 7.1 × 10−11 S/cm occurs for the BCN film with C composition ratio of 13%. This is due to π electrons of C=N and C=C bonds. References
Fig. 6. Electrical conductivity estimated using the activation energies obtained by the Arrhenius plot.
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Fig. 7. Relationship between XPS signal intensity and electrical conductivity of samples #1, #2 and #3.
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