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Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique
DIAMAT-06435; No of Pages 5 Diamond & Related Materials xxx (2015) xxx–xxx
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Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique Masami Aono a,⁎, Takanori Takeno b, Toshiyuki Takagi c a b c
Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan Graduate School of Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
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Article history: Received 22 June 2015 Received in revised form 30 July 2015 Accepted 31 July 2015 Available online xxxx Keywords: Amorphous carbon nitride Hybrid deposition RF sputtering Optical property Electrical property
a b s t r a c t Amorphous carbon nitride (a-CNx) thin films were prepared using a hybrid deposition technique (HDT), which is a combination of RF and DC magnetron co-sputtering of a graphite target. We varied the nitrogen gas flow ratio (GFR, N2/N2 + Ar) during deposition to prepare a-CNx films with various nitrogen concentrations. The film properties were characterized using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The optical and electrical properties were also investigated. For 0.3 b GFR b 1, the optical band gap and resistivity increased with increasing nitrogen concentration (x = N/C) and sp3 C–N bonding fraction in the films. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the optical band gap and resistivity of a-CNx films prepared by HDT were narrower and lower, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Amorphous carbon nitride (a-CNx) thin films are a fascinating material with anomalous mechanical, optical, and electrical behaviors. Their properties are strongly dependent on the deposition techniques employed in their fabrication [1–2]. Most a-CNx films prepared by chemical vapor deposition (CVD) have a nitrogen concentration of less than 0.3, and exhibit high hardness, optical transparency, and high resistivity [3–7]. In contrast, a-CNx films prepared by physical vapor deposition (PVD) have a higher nitrogen concentration [8–10]. The attractive features of the high nitrogen content a-CNx films are a low friction coefficient and a high wear resistance [11–13]. In addition, one of the advantages of PVD is unnecessary hydrocarbon gases for a carbon source. Sputtering is the most commonly used method of preparing hydrogen-free a-CNx films. During the past decade, interesting electrical and optical properties have been reported, such as a low dielectric constant [14], high photoconductivity [15], a variable band gap [16], and strong secondary electron emission by heavy ions [17,18]. More recently, photo-induced deformation was also reported [19]. This large spectrum of interesting properties is due to the great variety of bonding states, which consist of a mixture of sp1, sp2, and sp3 configurations. ⁎ Corresponding author. E-mail address: [email protected] (M. Aono).
The hardness of a-CNx films increases with increasing sp3 bonding fraction. Theoretically, carbon nitride, consisting of only sp3 bonding, is well known to be harder than diamond [20]. The sp3 fraction in a-CNx films controls not only the mechanical properties of the films, but also the optical and electrical properties. In fact, the n-type conductivity of a-CNx films in nature is attributed to the substitution of nitrogen into sp2 carbon bonds [21,22]. However, electrical inactivity sites such as pyridine, pyrrole and nitrile are also involved in sp2 bonding configurations [2,23–25]. These various bond types have profoundly different effects on the carrier concentration. Thus, the doping effect of nitrogen in a carbon network is not quite as effective as in a-Si:H with a substitutional phosphorus dopant. The development of carbon-based electronic devices requires good control of both sp2 fraction and nitrogen concentration through the deposition techniques. In attempting to form sputtered a-CNx films with superior electrical properties, however, it is difficult to increase the sp2 fraction of the films through conventional methods. Basically, a-CNx films deposited by DC magnetron sputtering are relatively low nitrogen concentration and high graphitic components. On the other hand, the films prepared by RF magnetron sputtering have high nitrogen concentration and low conductivity. Thus, we represent a deposition method with combination of RF and DC for a-CNx films. This hybrid deposition method was developed to provide CNx coatings with a low friction coefficient [26]. In this study, we investigated the optical and electrical properties of a-CNx films grown using a unique deposition method, which is a combination of RF and DC magnetron co-sputtering of a graphite target.
http://dx.doi.org/10.1016/j.diamond.2015.07.012 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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2. Experimental Hydrogen-free a-CNx films were prepared by the hybrid deposition technique (HDT) proposed previously [27]. Single-crystalline silicon, Si(100), and quartz glass were used as substrate materials. A graphite target with 99.9% purity and a mixed nitrogen (N2) and argon (Ar) gas were used as carbon and nitrogen sources, respectively. The base pressure of the deposition chamber for all depositions was less than 10−3 Pa. A substrate self-bias voltage was induced by an RF plasma and fixed at − 400 V for all depositions. Samples were prepared at various nitrogen gas flow ratios (GFRs), N2/(N2 + Ar). The film thickness was calculated from the refractive index n and the extinction coefficient k obtained using a single-wavelength ellipsometer (Mizojiri DVA-36LD). A semiconductor laser wavelength of 1310 nm was used. In addition to the above measurements, the film thickness was measured using field-emitted scanning electron microscopy (FE-SEM, Hitachi, s-4500). The acceleration voltage was 20 kV. The nitrogen concentration in the films was estimated using X-ray photoelectron spectroscopy (PHI 1600 ESCA System, MgKα X-ray source). Curve fittings were performed using the built-in software, Multipack 6.1. Shirley background subtraction was applied for all spectra, and then least-squares curve fitting was performed with the Gauss–Lorenz function. Raman spectroscopy and infrared (IR) absorption spectroscopy were employed to analyze the carbon bonding structure in the a-CNx. A Raman spectrum was acquired in ambient air in a dark environment, with He–Ne excitation (λ = 632.8 nm) using a micro-Raman system (HORIBA JobinYbon LabRAM HR-800). The spectra were acquired in the range from 800 to 2000 cm−1. It is well known that Raman spectra of a-CNx films can be approximated as two peaks, which are called the G and D peaks [28]. The infrared absorption spectrum was obtained using a Fourier transform infrared spectrometer (Thermo Scientific NICOLET6700) in the range of 400–4000 cm−1, with a resolution of 4 cm−1 and an averaging of 64 scans for samples deposited on a 300 nm crystalline silicon substrate. Optical property investigations were carried out with an ultraviolet and visible light (UV–vis) transmittance spectrometer (JASCO V-570) in the wavelength range from 200 to 2500 nm. The optical gap was defined by the Tauc relationship [29]. The electrical resistivity was measured under vacuum conditions in the dark. The measurement was carried out with a KEITHLEY 2400 source electrometer. Coplanar gold electrodes were formed on the film surface by vacuum evaporation. The applied voltage was fixed at 1 V. 3. Results and discussion
Fig. 2. Variation of the nitrogen concentration of amorphous carbon nitride films as a function of the nitrogen gas flow ratio N2/(Ar + N2). The line is a visual guide.
roughness obtained from atomic force microscope (AFM) images was less than 0.5 nm [26]. The deposition rate increased with increasing gas flow ratio, GFR = N2/(N2 + Ar) [26]. The average film thickness of the present films was approximately 300 nm. The HDT films show a uniformity growth that can be clearly seen in the SEM image of Fig. 1. Films with different nitrogen concentrations were obtained using HDT. Nitrogen concentration, N/C, is plotted against GFR in Fig. 2. In order to study the effects of the CVD process, we compared N/C ratios of amorphous carbon nitride films prepared by different techniques [30–35]. In the low GFR region, the N/C ratios of a-CNx films prepared by HDT increased rapidly with increasing GFR from 0 to 0.16 until the GFR reached approximately 0.15. As the GFR was increased further, the N/C ratio continued to increase, eventually reaching 0.35 at GFR of 1.0. This increasing N/C ratio may have resulted from a balance of two reaction types: bonding between carbon and nitrogen species and dissociation of carbon–nitrogen clusters by Ar ions. This balance seems to be stabilized when the nitrogen content of the feed gas exceeds at 3.0 at.% (GFR ≃ 0.03). The slope of the line passing through the N/C ratios of 0.08 and 1.0 is 0.22. The N/C ratio of HDT films was generally lower than that of carbon nitride films prepared by RF sputtering. One major difference between RF sputtering and other techniques is that a bias voltage is applied to the target. A negative bias could increase the kinetic energy of the incident nitrogen ions, which would enhance the formation of carbon–nitrogen bonds.
The growth films prepared by a hybrid deposition technique, HDT, were found smooth and light brownish transparency. The surface
Fig. 1. Cross-sectional SEM image of a-CNx film prepared at gas flow ratio N2/(Ar + N2) of 0.75.
Fig. 3. The component proportions in the N1s XPS spectra of hybrid sputtered a-CNx films: N–C (N1), N_C (N2), and N–O (N3) bonding configurations as a function of the nitrogen gas flow ratio.
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
M. Aono et al. / Diamond & Related Materials xxx (2015) xxx–xxx
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Fig. 6. Variation of optical band gap Eo of a-CNx films as a function of nitrogen gas flow ratio. The line is only a guide for the eyes. Fig. 4. FTIR spectra of hybrid sputtered a-CNx films for various gas flow ratios.
N1s core level peaks of XPS spectra for various CNx films were decomposed according to the methods employed in previous reports [36–38]: the lower peak (N1) and higher peak (N2) located at 398 ± 0.5 eV and 400 ± 0.5 eV are due to N atoms bonded to sp3-hybridized C atoms and to sp2-hybridized C atoms, respectively. The expression of N1 indicates that N atoms in the coating can bond to two carbon neighbors with σ and σ + π bonds to form –C_N–C–. Such a configuration is also possible within aromatic rings. The other peak, N2, indicates N atoms bonded to three carbon neighbors with three σ bonds. In this study, we also assigned an additional peak
Fig. 5. Variation of the G-peak position, FWHM of the G-peak, and the I(D)/ I(G) ratio, as deduced from the Raman spectra as a function of the nitrogen gas flow ratio N2/(Ar + N2).
(N3) located at 402 ± 0.5 eV to N–O bonding, because the XPS measurements were performed in ex-situ conditions under which surface contamination must be considered. We carried out XPS analysis several days to a week or more after the film deposition. Fig. 3 shows the relative peak area ratio plotted against GFR. We could not perform fittings for the lower GFR specimens because these spectra were noisy due to their low intensity. The N3 peak area ratio was less than 10% and remained constant because of the surface contamination. As shown in Fig. 3, the relative peak area of N1 increased to approximately 60% with increasing GFR. The opposite behavior was observed for N2. With increasing GFR, the sp3 bonding structure becomes dominant. Furthermore, the sp3 fraction is linearly related to GFR. It seems that high energy N+ ions are produced at a constant rate in the RF plasma. With increasing GFR, more high energy N+ ions are generated in the plasma, which further promotes the accelerating the decomposition of additional N2 molecules. On FTIR spectra of carbon nitride, remarkably absorption bands appeared at approximately 1500 cm−1 and 2200 cm−1. The absorption band at 1500 cm−1 can be assigned to C–N and C–C bonding structures [2]. Details of this absorption band will be discussed in the Raman analysis section. The absorption band at 2200 cm−1 indicates the presence of –C`N, − N`C, and –C_N– bonding structures [2]. These bonding states act as a network termination of carbon nitride. Fig. 4 shows FTIR
Fig. 7. Optical band gap of various amorphous carbon nitride films as a function of the nitrogen content N/C. The preparation methods: hybrid deposition technique using graphite and a mixture gas of N2 and Ar, DC magnetron sputtering of graphite and a mixture gas of N2 and Ar [41], DC magnetron sputtering of graphite and pure N2 [42], RF magnetron sputtering using graphite and a mixture gas of N2 and Ar [43], and RF magnetron sputtering of graphite and N2 gas [15,44,45].
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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Fig. 8. Various configurations of carbon nitride available for the activation of N donor electrons.The dots represent unpaired electrons.
spectra of the films. The peak intensities can be assumed to represent the relative amount of each bond type present in the sample. The network termination formed in higher GFR because the probability of collisions between carbon and nitrogen increased. On the other hand, no H–O related bonds were observed, which supports our prediction that the N–O peak in the XPS spectra was caused by surface contamination in air. The obtained Raman spectra were very similar to those typical of amorphous carbon films [28], indicating the presence of an amorphous carbon network in the films. The obtained Raman spectra were profilefitted using a Lorenz function for the D-peak (centered at 1344 ± 10 cm−1) and the Breit–Wignar–Fano function for the G-peak (centered at 1538 ± 5 cm−1). In Fig. 5, the G-peak position, the full-width at halfmaximum (FWHM) of the G-peak, and the intensity ratio of the D-peak to the G-peak (I(D)/I(G)) are plotted as a function of GFR. The G-peak position shifted to higher wavenumbers with increasing GFR. This trend was accompanied by an increase in nitrogen content. The FWHM of the G-peak decreased from 237.5 to 206.1 cm−1 with increasing GFR to 0.4, and increased slowly with further increases in GFR. The I(D)/I(G) ratio followed the opposite trend. In amorphous carbon films, the early model proposed by Robertson [39] suggests that the shift of the G-peak position toward higher wavenumbers indicates that the bond length of the sp2C atoms became shorter. The ratio of D-peak to G-peak intensities I(D)/I(G) reflects the average sp2 bond cluster size. That is, an increase of the I(D)/I(G) ratio indicates that sp2 cluster size increases. The variation of features such as I(D)/I(G) and FWHM with deposition conditions has been associated with the mechanical properties of the present films [40]. The optical and electrical properties are also affected by the sp2C structure. Fig. 6 shows the optical band gap Eo of the HDT films as a function of GFR. One notable result is that the Eo values were much lower than those of various sputtered a-CNx films [15,41–45], as shown in Fig. 7. Almost all optical band gaps of sputtered CNx films prepared using pure N2 gas are over 1 eV. A small Eo of a-CNx suggests that the films have a low resistivity. In fact, compared to RF sputtered films with a similar nitrogen concentration [46], the conductivities of the HDT films were two or three orders of magnitude lower. The small band gap and high electrical conductivity may have been caused by a higher graphitic component content in the HDT films than in other carbon nitride films. As shown in Fig. 6 Eo of the HDT films increased with increasing GFR, which means that an expansion of Eo was induced by increases in the nitrogen concentration and sp3 bonding fraction. Moreover, the relatively short graphitic network presented in the films prepared at low GFR according to the Raman results. On the other hand, the electrical conductivity σ at room temperature increased from 0.19 to 4.2 Ω–1 cm−1, depending on the nitrogen concentration. The temperature dependent
σ of the HDT films exhibited typical semiconducting behavior. They obey the Arrhenius law with activation energy Ea of 18–63 meV. The increase in conductivity with increasing GFR can be explained by an increase in –C`N, − N`C, and –C_N– bonding structures, which were observed in the FTIR spectra, along with a rising sp3 bonding fraction. Silva et al. [47] reported that only three binding modes in nine possible configurations of nitrogen in carbon nitride contribute to the electrical conductivity (Fig. 8). Unpaired electrons are present and available for doping. In our case, the configurations cannot be clearly observed in the FTIR spectra due to the overlapping C–C and C–N modes in the narrow region from 1300 to 1700 cm− 1. On the other hand, it was obvious that –C`N, −N`C, and –C = N– (two nitrogen electrons participating in σ bonds, one in a π bond, leaving one lone pair) bonding structures, which act as network terminators, increased with increasing GFR. That is, an increase in nitrogen concentration interrupts an expanding the conductive path. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the high conductivity of the HDT films may also be caused by both an extensive graphitic network and a relatively high concentration of desirable donor-providing configurations. The high resistivity of carbon nitride remains an obstacle to the development of electrical and opto-electrical device applications. The present results indicate the possibility of obtaining the desired configurations for semiconducting behavior in amorphous carbon nitride using the hybrid deposition technique. 4. Conclusions Amorphous carbon nitride thin films were synthesized using a hybrid sputtering system at different gas flow ratios (GFRs) of nitrogen in the sputtering gas. As the nitrogen content in the sputtering gas increased, the nitrogen concentration in the films increased, reaching 0.35. The sp3 C–N bonding fraction was increasingly favored as the collision frequency of the sputtered species increased with increasing nitrogen gas in the system. The carbon network became fragmentally when the nitrogen feed gas constituted more than half of the sputtering gas. Changes in the electrical and optical properties of the deposited films were also investigated. With increasing GFR, the sp3 bonding configurations became dominant and the termination structures, which were primarily triple bonds, increased in the films. Therefore, the band gap expanded and the conductivity decreased. Prime novelty statement Amorphous carbon nitride (a-CNx) films were prepared by using the hybrid deposition technique (HDT), which is a combination of RF and
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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DC magnetron co-sputtering of a graphite target. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the optical band gap and resistivity of a-CNx films prepared by HDT were narrower and lower, respectively.
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Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique Masami Aono a,⁎, Takanori Takeno b, Toshiyuki Takagi c a b c
Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan Graduate School of Engineering, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
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Article history: Received 22 June 2015 Received in revised form 30 July 2015 Accepted 31 July 2015 Available online xxxx Keywords: Amorphous carbon nitride Hybrid deposition RF sputtering Optical property Electrical property
a b s t r a c t Amorphous carbon nitride (a-CNx) thin films were prepared using a hybrid deposition technique (HDT), which is a combination of RF and DC magnetron co-sputtering of a graphite target. We varied the nitrogen gas flow ratio (GFR, N2/N2 + Ar) during deposition to prepare a-CNx films with various nitrogen concentrations. The film properties were characterized using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The optical and electrical properties were also investigated. For 0.3 b GFR b 1, the optical band gap and resistivity increased with increasing nitrogen concentration (x = N/C) and sp3 C–N bonding fraction in the films. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the optical band gap and resistivity of a-CNx films prepared by HDT were narrower and lower, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Amorphous carbon nitride (a-CNx) thin films are a fascinating material with anomalous mechanical, optical, and electrical behaviors. Their properties are strongly dependent on the deposition techniques employed in their fabrication [1–2]. Most a-CNx films prepared by chemical vapor deposition (CVD) have a nitrogen concentration of less than 0.3, and exhibit high hardness, optical transparency, and high resistivity [3–7]. In contrast, a-CNx films prepared by physical vapor deposition (PVD) have a higher nitrogen concentration [8–10]. The attractive features of the high nitrogen content a-CNx films are a low friction coefficient and a high wear resistance [11–13]. In addition, one of the advantages of PVD is unnecessary hydrocarbon gases for a carbon source. Sputtering is the most commonly used method of preparing hydrogen-free a-CNx films. During the past decade, interesting electrical and optical properties have been reported, such as a low dielectric constant [14], high photoconductivity [15], a variable band gap [16], and strong secondary electron emission by heavy ions [17,18]. More recently, photo-induced deformation was also reported [19]. This large spectrum of interesting properties is due to the great variety of bonding states, which consist of a mixture of sp1, sp2, and sp3 configurations. ⁎ Corresponding author. E-mail address: [email protected] (M. Aono).
The hardness of a-CNx films increases with increasing sp3 bonding fraction. Theoretically, carbon nitride, consisting of only sp3 bonding, is well known to be harder than diamond [20]. The sp3 fraction in a-CNx films controls not only the mechanical properties of the films, but also the optical and electrical properties. In fact, the n-type conductivity of a-CNx films in nature is attributed to the substitution of nitrogen into sp2 carbon bonds [21,22]. However, electrical inactivity sites such as pyridine, pyrrole and nitrile are also involved in sp2 bonding configurations [2,23–25]. These various bond types have profoundly different effects on the carrier concentration. Thus, the doping effect of nitrogen in a carbon network is not quite as effective as in a-Si:H with a substitutional phosphorus dopant. The development of carbon-based electronic devices requires good control of both sp2 fraction and nitrogen concentration through the deposition techniques. In attempting to form sputtered a-CNx films with superior electrical properties, however, it is difficult to increase the sp2 fraction of the films through conventional methods. Basically, a-CNx films deposited by DC magnetron sputtering are relatively low nitrogen concentration and high graphitic components. On the other hand, the films prepared by RF magnetron sputtering have high nitrogen concentration and low conductivity. Thus, we represent a deposition method with combination of RF and DC for a-CNx films. This hybrid deposition method was developed to provide CNx coatings with a low friction coefficient [26]. In this study, we investigated the optical and electrical properties of a-CNx films grown using a unique deposition method, which is a combination of RF and DC magnetron co-sputtering of a graphite target.
http://dx.doi.org/10.1016/j.diamond.2015.07.012 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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2. Experimental Hydrogen-free a-CNx films were prepared by the hybrid deposition technique (HDT) proposed previously [27]. Single-crystalline silicon, Si(100), and quartz glass were used as substrate materials. A graphite target with 99.9% purity and a mixed nitrogen (N2) and argon (Ar) gas were used as carbon and nitrogen sources, respectively. The base pressure of the deposition chamber for all depositions was less than 10−3 Pa. A substrate self-bias voltage was induced by an RF plasma and fixed at − 400 V for all depositions. Samples were prepared at various nitrogen gas flow ratios (GFRs), N2/(N2 + Ar). The film thickness was calculated from the refractive index n and the extinction coefficient k obtained using a single-wavelength ellipsometer (Mizojiri DVA-36LD). A semiconductor laser wavelength of 1310 nm was used. In addition to the above measurements, the film thickness was measured using field-emitted scanning electron microscopy (FE-SEM, Hitachi, s-4500). The acceleration voltage was 20 kV. The nitrogen concentration in the films was estimated using X-ray photoelectron spectroscopy (PHI 1600 ESCA System, MgKα X-ray source). Curve fittings were performed using the built-in software, Multipack 6.1. Shirley background subtraction was applied for all spectra, and then least-squares curve fitting was performed with the Gauss–Lorenz function. Raman spectroscopy and infrared (IR) absorption spectroscopy were employed to analyze the carbon bonding structure in the a-CNx. A Raman spectrum was acquired in ambient air in a dark environment, with He–Ne excitation (λ = 632.8 nm) using a micro-Raman system (HORIBA JobinYbon LabRAM HR-800). The spectra were acquired in the range from 800 to 2000 cm−1. It is well known that Raman spectra of a-CNx films can be approximated as two peaks, which are called the G and D peaks [28]. The infrared absorption spectrum was obtained using a Fourier transform infrared spectrometer (Thermo Scientific NICOLET6700) in the range of 400–4000 cm−1, with a resolution of 4 cm−1 and an averaging of 64 scans for samples deposited on a 300 nm crystalline silicon substrate. Optical property investigations were carried out with an ultraviolet and visible light (UV–vis) transmittance spectrometer (JASCO V-570) in the wavelength range from 200 to 2500 nm. The optical gap was defined by the Tauc relationship [29]. The electrical resistivity was measured under vacuum conditions in the dark. The measurement was carried out with a KEITHLEY 2400 source electrometer. Coplanar gold electrodes were formed on the film surface by vacuum evaporation. The applied voltage was fixed at 1 V. 3. Results and discussion
Fig. 2. Variation of the nitrogen concentration of amorphous carbon nitride films as a function of the nitrogen gas flow ratio N2/(Ar + N2). The line is a visual guide.
roughness obtained from atomic force microscope (AFM) images was less than 0.5 nm [26]. The deposition rate increased with increasing gas flow ratio, GFR = N2/(N2 + Ar) [26]. The average film thickness of the present films was approximately 300 nm. The HDT films show a uniformity growth that can be clearly seen in the SEM image of Fig. 1. Films with different nitrogen concentrations were obtained using HDT. Nitrogen concentration, N/C, is plotted against GFR in Fig. 2. In order to study the effects of the CVD process, we compared N/C ratios of amorphous carbon nitride films prepared by different techniques [30–35]. In the low GFR region, the N/C ratios of a-CNx films prepared by HDT increased rapidly with increasing GFR from 0 to 0.16 until the GFR reached approximately 0.15. As the GFR was increased further, the N/C ratio continued to increase, eventually reaching 0.35 at GFR of 1.0. This increasing N/C ratio may have resulted from a balance of two reaction types: bonding between carbon and nitrogen species and dissociation of carbon–nitrogen clusters by Ar ions. This balance seems to be stabilized when the nitrogen content of the feed gas exceeds at 3.0 at.% (GFR ≃ 0.03). The slope of the line passing through the N/C ratios of 0.08 and 1.0 is 0.22. The N/C ratio of HDT films was generally lower than that of carbon nitride films prepared by RF sputtering. One major difference between RF sputtering and other techniques is that a bias voltage is applied to the target. A negative bias could increase the kinetic energy of the incident nitrogen ions, which would enhance the formation of carbon–nitrogen bonds.
The growth films prepared by a hybrid deposition technique, HDT, were found smooth and light brownish transparency. The surface
Fig. 1. Cross-sectional SEM image of a-CNx film prepared at gas flow ratio N2/(Ar + N2) of 0.75.
Fig. 3. The component proportions in the N1s XPS spectra of hybrid sputtered a-CNx films: N–C (N1), N_C (N2), and N–O (N3) bonding configurations as a function of the nitrogen gas flow ratio.
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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Fig. 6. Variation of optical band gap Eo of a-CNx films as a function of nitrogen gas flow ratio. The line is only a guide for the eyes. Fig. 4. FTIR spectra of hybrid sputtered a-CNx films for various gas flow ratios.
N1s core level peaks of XPS spectra for various CNx films were decomposed according to the methods employed in previous reports [36–38]: the lower peak (N1) and higher peak (N2) located at 398 ± 0.5 eV and 400 ± 0.5 eV are due to N atoms bonded to sp3-hybridized C atoms and to sp2-hybridized C atoms, respectively. The expression of N1 indicates that N atoms in the coating can bond to two carbon neighbors with σ and σ + π bonds to form –C_N–C–. Such a configuration is also possible within aromatic rings. The other peak, N2, indicates N atoms bonded to three carbon neighbors with three σ bonds. In this study, we also assigned an additional peak
Fig. 5. Variation of the G-peak position, FWHM of the G-peak, and the I(D)/ I(G) ratio, as deduced from the Raman spectra as a function of the nitrogen gas flow ratio N2/(Ar + N2).
(N3) located at 402 ± 0.5 eV to N–O bonding, because the XPS measurements were performed in ex-situ conditions under which surface contamination must be considered. We carried out XPS analysis several days to a week or more after the film deposition. Fig. 3 shows the relative peak area ratio plotted against GFR. We could not perform fittings for the lower GFR specimens because these spectra were noisy due to their low intensity. The N3 peak area ratio was less than 10% and remained constant because of the surface contamination. As shown in Fig. 3, the relative peak area of N1 increased to approximately 60% with increasing GFR. The opposite behavior was observed for N2. With increasing GFR, the sp3 bonding structure becomes dominant. Furthermore, the sp3 fraction is linearly related to GFR. It seems that high energy N+ ions are produced at a constant rate in the RF plasma. With increasing GFR, more high energy N+ ions are generated in the plasma, which further promotes the accelerating the decomposition of additional N2 molecules. On FTIR spectra of carbon nitride, remarkably absorption bands appeared at approximately 1500 cm−1 and 2200 cm−1. The absorption band at 1500 cm−1 can be assigned to C–N and C–C bonding structures [2]. Details of this absorption band will be discussed in the Raman analysis section. The absorption band at 2200 cm−1 indicates the presence of –C`N, − N`C, and –C_N– bonding structures [2]. These bonding states act as a network termination of carbon nitride. Fig. 4 shows FTIR
Fig. 7. Optical band gap of various amorphous carbon nitride films as a function of the nitrogen content N/C. The preparation methods: hybrid deposition technique using graphite and a mixture gas of N2 and Ar, DC magnetron sputtering of graphite and a mixture gas of N2 and Ar [41], DC magnetron sputtering of graphite and pure N2 [42], RF magnetron sputtering using graphite and a mixture gas of N2 and Ar [43], and RF magnetron sputtering of graphite and N2 gas [15,44,45].
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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Fig. 8. Various configurations of carbon nitride available for the activation of N donor electrons.The dots represent unpaired electrons.
spectra of the films. The peak intensities can be assumed to represent the relative amount of each bond type present in the sample. The network termination formed in higher GFR because the probability of collisions between carbon and nitrogen increased. On the other hand, no H–O related bonds were observed, which supports our prediction that the N–O peak in the XPS spectra was caused by surface contamination in air. The obtained Raman spectra were very similar to those typical of amorphous carbon films [28], indicating the presence of an amorphous carbon network in the films. The obtained Raman spectra were profilefitted using a Lorenz function for the D-peak (centered at 1344 ± 10 cm−1) and the Breit–Wignar–Fano function for the G-peak (centered at 1538 ± 5 cm−1). In Fig. 5, the G-peak position, the full-width at halfmaximum (FWHM) of the G-peak, and the intensity ratio of the D-peak to the G-peak (I(D)/I(G)) are plotted as a function of GFR. The G-peak position shifted to higher wavenumbers with increasing GFR. This trend was accompanied by an increase in nitrogen content. The FWHM of the G-peak decreased from 237.5 to 206.1 cm−1 with increasing GFR to 0.4, and increased slowly with further increases in GFR. The I(D)/I(G) ratio followed the opposite trend. In amorphous carbon films, the early model proposed by Robertson [39] suggests that the shift of the G-peak position toward higher wavenumbers indicates that the bond length of the sp2C atoms became shorter. The ratio of D-peak to G-peak intensities I(D)/I(G) reflects the average sp2 bond cluster size. That is, an increase of the I(D)/I(G) ratio indicates that sp2 cluster size increases. The variation of features such as I(D)/I(G) and FWHM with deposition conditions has been associated with the mechanical properties of the present films [40]. The optical and electrical properties are also affected by the sp2C structure. Fig. 6 shows the optical band gap Eo of the HDT films as a function of GFR. One notable result is that the Eo values were much lower than those of various sputtered a-CNx films [15,41–45], as shown in Fig. 7. Almost all optical band gaps of sputtered CNx films prepared using pure N2 gas are over 1 eV. A small Eo of a-CNx suggests that the films have a low resistivity. In fact, compared to RF sputtered films with a similar nitrogen concentration [46], the conductivities of the HDT films were two or three orders of magnitude lower. The small band gap and high electrical conductivity may have been caused by a higher graphitic component content in the HDT films than in other carbon nitride films. As shown in Fig. 6 Eo of the HDT films increased with increasing GFR, which means that an expansion of Eo was induced by increases in the nitrogen concentration and sp3 bonding fraction. Moreover, the relatively short graphitic network presented in the films prepared at low GFR according to the Raman results. On the other hand, the electrical conductivity σ at room temperature increased from 0.19 to 4.2 Ω–1 cm−1, depending on the nitrogen concentration. The temperature dependent
σ of the HDT films exhibited typical semiconducting behavior. They obey the Arrhenius law with activation energy Ea of 18–63 meV. The increase in conductivity with increasing GFR can be explained by an increase in –C`N, − N`C, and –C_N– bonding structures, which were observed in the FTIR spectra, along with a rising sp3 bonding fraction. Silva et al. [47] reported that only three binding modes in nine possible configurations of nitrogen in carbon nitride contribute to the electrical conductivity (Fig. 8). Unpaired electrons are present and available for doping. In our case, the configurations cannot be clearly observed in the FTIR spectra due to the overlapping C–C and C–N modes in the narrow region from 1300 to 1700 cm− 1. On the other hand, it was obvious that –C`N, −N`C, and –C = N– (two nitrogen electrons participating in σ bonds, one in a π bond, leaving one lone pair) bonding structures, which act as network terminators, increased with increasing GFR. That is, an increase in nitrogen concentration interrupts an expanding the conductive path. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the high conductivity of the HDT films may also be caused by both an extensive graphitic network and a relatively high concentration of desirable donor-providing configurations. The high resistivity of carbon nitride remains an obstacle to the development of electrical and opto-electrical device applications. The present results indicate the possibility of obtaining the desired configurations for semiconducting behavior in amorphous carbon nitride using the hybrid deposition technique. 4. Conclusions Amorphous carbon nitride thin films were synthesized using a hybrid sputtering system at different gas flow ratios (GFRs) of nitrogen in the sputtering gas. As the nitrogen content in the sputtering gas increased, the nitrogen concentration in the films increased, reaching 0.35. The sp3 C–N bonding fraction was increasingly favored as the collision frequency of the sputtered species increased with increasing nitrogen gas in the system. The carbon network became fragmentally when the nitrogen feed gas constituted more than half of the sputtering gas. Changes in the electrical and optical properties of the deposited films were also investigated. With increasing GFR, the sp3 bonding configurations became dominant and the termination structures, which were primarily triple bonds, increased in the films. Therefore, the band gap expanded and the conductivity decreased. Prime novelty statement Amorphous carbon nitride (a-CNx) films were prepared by using the hybrid deposition technique (HDT), which is a combination of RF and
Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012
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DC magnetron co-sputtering of a graphite target. Compared to carbon nitride films with a comparable nitrogen content prepared by RF and DC sputtering, the optical band gap and resistivity of a-CNx films prepared by HDT were narrower and lower, respectively.
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Please cite this article as: M. Aono, et al., Structural, electrical, and optical properties of amorphous carbon nitride films prepared using a hybrid deposition technique, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.07.012