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Structure and physical properties of stable isotopic amorphous carbon films
DIAMAT-06495; No of Pages 5 Diamond & Related Materials xxx (2015) xxx–xxx
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Structure and physical properties of stable isotopic amorphous carbon films Yutaro Suzuki a, Yasuyoshi Kurokawa a, Tsuneo Suzuki b, Kazuhiro Kanda c, Masahito Niibe c, Masayuki Nakano d, Naoto Ohtake a, Hiroki Akasaka a,⁎ a
Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan University of Hyogo, 3-1-2 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1205, Japan d Tokyo National College of Technology, 1220-2 Kunugida, Hachiouji, Tokyo 193-0997, Japan b c
a r t i c l e
i n f o
Article history: Received 14 July 2015 Received in revised form 6 October 2015 Accepted 25 October 2015 Available online xxxx Keywords: Amorphous carbon film Stable isotope Defect density Isotopic effect
a b s t r a c t Hydrogenated amorphous carbon (a-C:H) films containing isotopes of carbon and hydrogen were deposited, and their structure and physical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of carbon films. These isotope films such as deuterated amorphous carbon (a-12C:D) and hydrogenated 13C carbon (a-13C:H) films were deposited under the same deposition condition by the radio frequency plasma CVD method from isotope methane gas. The ratio of sp2/sp3 carbon was same, the deuterium content of a-12C:D was smaller than hydrogen contents of other films. Although the smallest content of hydrogen or deuterium as the termination structure of the carbon network was observed for the a-12C:D film, the density of defects was also smallest in the film. The Tauc gap of the a-12C:D film was approximately 30% larger than the that of the other films. From these results, the deposition process of a-C:D film brings lower defect density compared with the deposition system for a-C:H. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated amorphous carbon (a-C:H) and related films have received considerable attention because of their high hardness and wear resistance [1–3]. Their mechanical applications have already been developed in parts supply industries. On the other hand, some electronic and electrical applications of a-C:H films have been examined [4–8]. Valentini et al. reported amorphous carbon/p-type silicon diodes and their rectifying characteristics [4]. The application of amorphous carbon as a semiconductor in optoelectronic devices such as photovoltaic solar cells has also been attempted. For carbon-based a-C:P/p-Si heterojunction solar cells, the energy conversion efficiency (η) and fill factor (FF) were reported to be 0.84% and 0.49, respectively [5]. As a more complex structure, Chen et al. investigated the photovoltaic characteristics of novel gold/p-type amorphous boron carbon thin film alloy/silicon dioxide/n-type crystalline silicon/aluminum (Au/a-B:C/ SiO2/n-Si/Al) solar cells. They reported value of 3.3% and 0.904 for η and the FF, respectively. These structures almost involved Si layer [6]. Structures containing Si show the rectifying nature of heterojunction structures with amorphous carbon. On these reports, the device had the metal–insulator–semiconductor (MIS) structure. Usually MIS photovoltaic system generates carriers by photovoltaic effect at the part of “Semiconductor”. Hence most of the origin of photo-electromotive force at these devices is probably the Si layer, because the electrical ⁎ Corresponding author. E-mail address: [email protected] (H. Akasaka).
resistance of amorphous carbon and related materials is much higher than Si. The FF for a device consisting of only carbon materials was reported to be 0.26 [9,10]. One of the reasons for these low FF and η is probably the existence of localized states in band gap caused by their high defect density in carbon materials. Under irradiation of light, the semiconductor absorbs these photon energies, and electrons are excited from valence band to conduction band. On the photovoltaic system, these carriers of electron and holes behave as an electrical energy. When there are localized states in the band gap caused by the dangling bond, these carriers are easily trapped by these localized states during the excitation by the photon energy absorption. It is well known that the low defect density should be required for high FF and η. The photovoltaic characteristics including the FF and η are affected by the defect density such as dangling bonds. Some reports indicated that the defect density of a-C:H films is approximately 1018 to 1021 cm−3, which is higher than the defect density of commercially available a-Si of approximately 1015 cm−3 [11–13]. And these defect density distributions such as surface or inside of balk have reported nothing on a-C:H films research. As first step of used for electric applications, a low defect density same level as Si requires for a-C:H films. An a-C:H film consists of carbon and hydrogen atoms, and these atoms have non-radioactive stable isotopes of 13C and deuterium. On diamond researches, some reports indicated that thermal conductivity and band gap can be controlled by the incorporation of 13C [14–16]. These effects are caused by difference of the bonding length between 12 C–12C and 13C–13C. Hence the 13C incorporation into 12C diamond
http://dx.doi.org/10.1016/j.diamond.2015.10.024 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
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Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx
brings grain boundary for phonon scattering, and the change of band gap from the pure 12C diamond. 13C has different characteristics from 12 C at diamond system; thus, the incorporation of 13C into a-C films may lead to more advanced functions. On the other hand, the deuterium incorporation for a-C:H system is considered for an advanced function. Usually, an increase in atomic mass results in a higher bonding energy, for example, the C–D bonding energy is higher than that of C–H owing to the isotope effect. This increase in the bonding energy is expected to reduce damage by high-energy ions during film deposition. In fact, the deuterium termination of amorphous Si has been attempted in the development of solar cell devices [17–19]. These reports indicated that Defects at several place in some crystalline types of silicon material were terminated by deuterium atoms. On a polycrystalline silicon device, the deuterium passivation of grain boundaries by correlated deuterium diffusion and dangling-bond annihilation was revealed in polycrystalline silicon films [17,18]. Hence, dangling bonds at the Si grain boundary could be terminated by deuterium atoms. Furthermore, crystalline Si was also studied [19–21]. The deuterium atoms density on a Si crystal surface is increased by an order of magnitude following monolayer passivation at temperatures between 300 and 700 K [19]. This difference is originated from the vibrational frequencies of H and D on Si crystal surface. This difference in the vibrational frequencies is related to their bonding energies, occurs at the bonding system for carbon. These studies generally indicated that the higher efficiency and stability achieved by the deuterium passivation of dangling bonds on Si were due to the higher vibrational frequency and mass of deuterium atoms than those of hydrogen atoms. That may bring higher bonding strength and long-term stability. In fact in a study of the p-i-n structured amorphous silicon solar cell, replacing hydrogen with deuterium in the intrinsic layer of the cell improved the long-term stability against light exposure [22,23]. The change from H to D in the a-Si solar cell improved stability under light irradiation. These improvements in Si system almost resulted from mass increasing or the isotopic effect. The defect density distribution on the a-C:H films is unknown, but the deuterium passivation for a-C:H films can be expected from the reduction of defects. On the other hand, these have been few reports on the deposition of amorphous isotopic carbon films, and the defect passivation of isotopic carbon films has not been reported. Especially, the isotopic effect on carbon system may be more complicated compared with silicon system, because the bonding structure for silicon is only sp3 hybridized bonding against three types of bonding structure: sp, sp2, and sp3 hybridized bonding can be formed in carbon system [2]. Hence, the isotopic effect on amorphous carbon should be studied for development of electronic applications of amorphous carbon. In this study, amorphous carbon films containing isotopes of carbon were deposited, and their structure and electrical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of carbon films.
Table 1 Deposition condition. Film type
a-12C:H
Nature of gas Precursor Pressure [Pa] Flow rate [cm3/min]
Ar (99.999%) Ar (99.999%) Ar (99.999%) 12 13 12 CH4 (99.999%) CH4 (99%) CD4 (99%) 13 13 13 Ar: 20, methane: 3 p-Si(100): 0.1 Ω cm, n-Si(100): 1.0 Ωcm, #7059 glass, quartz
Substrate
a-13C:H
a-12C:D
spectroscopy (NRS-1000 JASCO) using a Nd:YVO4 laser (532 nm) with an aperture size of ϕ = 200 μm. The elemental compositions of the film was determined by a Rutherford backscattering (RBS), and elastic recoil detection analysis (ERDA) system which was performed under 1 MeV He+ irradiation using an electrostatic accelerator (NT-1700HS: Nissin High Voltage Co.). Moreover, the sp2 content in the deposited films was determined by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. This measurement was performed at the BL09A beamline of New SUBARU at the University of Hyogo. NEXAFS carbon K-edge spectra were obtained in the energy range of 275– 320 eV with a resolution of 0.5 eV (FWHM) in the total electron yield mode. The density of paramagnetic defects was evaluated using electron spin resonance (ESR) measurements in the X-band (9.44 GHz) at room temperature, using a resonance spectrometer (JES-FA100 JEOL). The g-value is usually 2.002316 for unpaired electrons. Hence, the measurement magnetic field was from 328.5 to 343.5 mT. The spin concentration was determined by comparison with the 1-Oxyl2,2,6,6-tetramethyl4-hydroxypiperidine (TEMPOL) spin standard. To estimate the optical properties, the optical band gaps for the obtained films were estimated by ultraviolet–visible (UV–Vis) absorption spectroscopy, where UV–Vis spectra were obtained using a UV–Vis spectrophotometer (V-630 JASCO). After measurement of spectra of transmittance and reflectance, absorption spectra were obtained from these spectra. 3. Results and discussion The film thickness of all three types of film was approximately 370 nm at 60 min deposition duration. Although the deposition conditions of these films were the same except for the type of methane, the film structures and properties were different. Figure 1 shows XRR patterns of the films. Without any sample preparation or damage, XRR can be used to obtain information about density, roughness and crosssectional layering for any amorphous carbon films on flat substrate. Hence XRR technique has been frequently used for the determination of film density and thickness on amorphous carbon films [24,25]. These three XRR patterns also indicated a similar film thickness for the three films owing to their similar fringe patterns. By simulation fitting,
2. Experimental a-C:H films were deposited onto quartz glass, Si(100) (0.1 Ω cm), and #7059 optical glass substrates by radio frequency (RF) plasma chemical vapor deposition. Electrodes were set parallel in the vacuum chamber to generate a capacitively coupled plasma. In the a-C:H deposition, methane(CH4, 99.999%), deuterated methane(CD4, 99%) or 13C methane(13CH4, 99%) gas was used as the source material. Before deposition, the substrate surface was irradiated with Ar ions for 30 min at a flow rate of 20 cm3/min to remove the adsorbed substances and the oxidation layer on Si. Then, one of the methane gases at a flow rate of 3 cm3/min and Ar gas at a flow rate of 20 cm3/min were introduced into the chamber, and a 13.56 MHz RF power of 20 W was applied to the substrate at a pressure of 13 Pa. All deposition conditions were fixed except for the source gas, as shown in Table 1. The film density was evaluated by the X-ray reflectivity (XRR). The XRR pattern was obtained using a CuKα X-ray diffractometer (X'Pert PRO MRD Phillips). The structure of the films was analyzed by Raman
Fig. 1. XRR patterns of a-12C:H, a-12C:D, and a-13C:H films.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx
Fig. 2. Raman spectra of the a-C:H films.
densities of 1.70, 1.54, and 1.75 g/cm3 were obtained for the a-12C:H, a-12C:D, and a-13C:H films, respectively. Although the density for the a-12C:H and a-13C:H films were almost same, the density of the a-12C:D film was smaller than that of others. The Raman spectra of the three types of a-C:H films are shown in Fig. 2. In all the spectra, two broad peaks appear, namely, the G (graphitic)-band at approximately 1560 cm−1 and the D (disorder)-band at approximately 1350 cm−1. G-band peaks originate from the E2g resonance oscillation in sp2 bonded carbon included in graphitic structures. And Tuinstra et al. indicated that the G-band represents the order of a graphitic structure [26]. The D-band can be assigned to first-order scattering from a zone boundary phonon activated by disorder on the basis of the finite crystallite size [26]. Briefly, the D-band represents the boundary of the graphite structure. To analyze data on the order of the graphitic cluster size, the integrated peak ratio, ID/IG, was calculated from the Raman spectra. A change in ID/IG suggests a change in the continuous order of the graphitic cluster size. These Raman spectra were fitted by Gaussian line shapes. The values of ID/IG were 0.57, 0.37, and 0.70 for the a-12C:H, a-12C:D, and a-13C:H films, respectively. The order of the graphitic cluster size of a-12C:H is larger than that of a-13C:H, and the graphitic cluster size of a-12C:D is larger than that of the other films. As mentioned later, there were no differences in sp2/sp3 ratio about these films. Hence, the reason for the low density in a-12C:D film compared with that of the other films is the not graphite layer. The low density may be caused by the inclusion of polymer-like components or increasing of vacancy volumes. Deuterium atom terminations bring larger atomic vacancy compared with H termination because of their bonding length differences. The Raman spectra for the a-C:D film exhibited a gradient baseline, which indicates contained a uniaxially oriented
3
Fig. 4. NEXAFS spectra of the a-C:H films.
conjugating polymer. The G-peak position of the a-13C:H film clearly indicated a lower wave number than that of the films consisting of 12C. The shift of the related peak for carbon such as diamond has been reported as an isotopic effect [27,28]. The G-peak center position of the a-13C:H and a-12C:H films are 1474 and 1526 cm−1, respectively. The increasing of atomic mass brings the resonance frequency shift in Raman scattering phenomenon [29]. From the atomic mass difference and G-peak position of a-12C:H, G-peak position of a-13C:H was calculated and obtained to 1466 cm− 1. This value indicated good agreement with the measurement value of 1474 cm−1 with error of 0.5%. Hence, this peak shift did not indicate a structural difference compared with the films consisting of 12C. To determine the composition and sp2/sp3 ratio of these films, RBS/ ERDA and NEXAFS spectra were measured, as shown in Figs. 3 and 4, respectively. From the RBS/ERDA spectra, the hydrogen contents of the a-12C:H and a-13C:H films were 34 at.%. The deuterium content of the a-12C:D film was 24 at.%. The difference of the contents was 10% between hydrogen and deuterium. These RBS/ERDA results indicated the difference between the carbon bonding network termination structures formed by hydrogen and deuterium atoms, respectively. From the NEXAFS spectra in Fig. 4, the broad band observed in the range of 288–310 eV is assignable to the result of overlapping C 1s → σ* transitions at the sp3, sp2, and sp1 sites of the a-C:H and related films [30–33]. The absolute sp2 content was determined by comparison with the spectrum of highly oriented poly-graphite. The sp2/(sp2 + sp3) ratio for all the obtained films was approximately 0.4. These film structures indicated from the results of NEXAFS and RBS/ERDA were similar because the deposition conditions were the same except for the source materials.
Fig. 3. RBS (left) and ERDA (right) spectra of the a-C:H films.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
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Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx Table 3 Results of the data by all measurements.
True density [g/cm−3] E04 [eV] ETauc [eV] H or D content [%] sp2/(sp2 + sp3) [−] Density of unpaired electron [×1021 cm−3]
a-12C:H
a-13C:H
a-12C:D
1.70 1.90 1.17 34 0.42 1.4
1.75 1.94 1.20 34 0.42 1.4
1.54 2.31 1.56 25 0.41 1.0
network is longer than other films. Usually, these bonding networks in amorphous carbon system are terminated by termination atoms such as D and H, or dangling bond. And unpaired electron densities in a-C:D film is also indicated the smallest dangling bond density. Hence, deposition process of a-C:D film have the calming effect of calming of dang ling bond. Fig. 5. ESR spectra of the a-C:H films.
4. Summary To estimate the defect density in these films, ESR spectra were obtained, as shown in Fig. 5. A peak in the absorption was observed in all the spectra at 332–333 mT. The density of paramagnetic defects and the peak-to-peak width of the films obtained from the ESR spectra are shown in Table 2. The peak-to-peak width of the a-13C:H film is approximately twice than that of the films consisting of 12C, which can be attributed to the interaction between the nuclear spin of 13C and the electron spin. The density of paramagnetic defects in the a-12C:D film is less than that in the a-12C:H and a-13C:H films. The difference between the elemental changes in hydrogen or deuterium is 30%. Although the film with the smallest hydrogen or deuterium content as the termination structure of the carbon network was the a-12C:D film, the density of paramagnetic defects was also smaller than that of the other films. These results mean that the effect of deuterium on reducing the paramagnetic defect density is greater than that of hydrogen atoms in an a-C:H system (Table 3). If such defect reduction affects the electronic properties of films, it can be considered that the optical band gap may be expanded because the localization of states by the defects was reduced. Optical absorption spectra were measured, and the Tauc plots obtained for the deposited films are shown in Fig. 6. We found differences in the spectral shape of the optical absorption coefficient for each film. The Tauc gap (ETauc), which is used as the band gap of amorphous semiconductors [22] can be obtained by extrapolating the linear area of a Tauc plot to the horizontal axis, and values of 1.17, 1.20, and 1.56 eV were obtained for the a-12C:H, a-13C:H, and a-12C:D films, respectively. Although these films had same film thickness, the values of the optical band gap, E04, indicated also larger difference in Fig. 6(b) between the a-C:D film and the a-C:H films. ETauc for the a-12C:D film is about 30% larger than that of for other films. The density of electronic defects of the a-12C:D film was lower than that of the a-12C:H and a-13C:H films, and there are expected to be more defect levels in the band structure of the a-12C:H film than in that of the a-12C:D film. That may be one of the reasons for the larger ETauc for the a-12C:D film. And sp2 cluster size is largest in the obtained samples. It is well known that the band gap in amorphous carbon materials is affected by sp2/sp3 ratio [34]. However, our samples indicated sp2/sp3 ratios are same. And the largest sp2 cluster size and the smallest atomic concentration which forms termination structure indicated bonding
To investigate the isotopic effects in an attempt to reduce the defect density of carbon films, amorphous carbon films containing isotopes of carbon were deposited, and their structures and electronic properties were evaluated. The film composition and structure of a-12C:D showed large differences from those of other films. Although the smallest content of hydrogen or deuterium as the termination structure of the carbon network was observed for the a-12C:D film, the density of defects was also smallest in the film. The Tauc gap of the a-12C:D film was approximately 30% larger than the that of the other films. On comparison
Table 2 Results of ESR measurements.
Density of unpaired electron [10 Peak-to-peak width ΔHpp [mT]
21
cm
−3
]
a-12C:H
a-13C:H
a-12C:D
1.4 0.34
1.4 0.63
1.0 0.34
Fig. 6. Optical absorption spectra and Tauc plots of the a-C:H films. (a) Tauc plots. (b) Optical absorption spectr.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx
of the deposition system for a-C:H, the deposition processes of a-C:D film have the calming effect of calming of dang ling bond. Prime novelty statement In this study, hydrogenated amorphous carbon (a-C:H) films containing isotopes of carbon and hydrogen were deposited, and their structure and electrical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of amorphous carbon films. This report is first report that deuterium is more efficient for decreasing of defect density compared with the hydrogen in an amorphous carbon system. Acknowledgments This work was supported by “Planting Seeds for Research” program in Tokyo Institute of Technology. And some parts of this work were supported by JSPS KAKENHI (grant Number 15K18038). References [1] S. Yamamoto, A. Kawana, H. Ichimura, C. Masuda, Relationship between tribological properties and sp3/sp2 structure of nitrogenated diamond-like carbon deposited by plasma CVD, Surf. Coat. Technol. 210 (2012) 1–9. [2] S. Yamamoto, A. Kawana, C. Masuda, Tribological behavior of diamond-like carbon produced by rf-PCVD based on energetic evaluation, Surf. Coat. Technol. 236 (2013) 457–464. [3] T. Michler, M. Grischke, I. Traus, K. Bewilogua, H. Dimigen, Mechanical properties of DLC films prepared by bipolar pulsed DC PACVD, Diam. Relat. Mater. 7 (1998) 1333–1337. [4] L. Valentini, V. Salerni, I. Armentano, J.M. Kenny, L. Lozzi, S. Santucci, Effects of fluorine incorporation on the properties of amorphous carbon/p-type crystalline silicon heterojunction diodes, J. Non-Cryst. Solids 321 (2003) 175–182. [5] M. Rusop, T. Soga, T. Jimbo, Photovoltaic characteristics of phosphorus-doped amorphous carbon films grown by r.f. plasma-enhanced CVD, Sol. Energy Mater. Sol. Cells 90 (2006) 3214–3222. [6] T.S. Chen, Y.C. Hsueh, S.E. Chiou, S. Tsong Shiue, The effect of the native silicon dioxide interfacial layer on photovoltaic characteristics of gold/p-type amorphous boron carbon thin film alloy/silicon dioxide/n-type silicon/aluminum solar cells, Sol. Energy Mater. Sol. Cells 137 (2015) 185–192. [7] C. Ronning, E. Dreher, J.U. Thiele, P. Oelhafen, H. Hofsass, Electronic and atomic structure of undoped and doped ta-C films, Diam. Relat. Mater. 6 (1997) 830–834. [8] M. Guerino, M. Massi, H.S. Maciel, C. Otani, R.D. Mansano, P. Verdonck, J. Libardi, The influence of nitrogen on the dielectric constant and surface hardness in diamondlike carbon (DLC) films, Diam. Relat. Mater. 13 (2004) 316–319. [9] H. Akasaka, T. Imai, N. Ohtake, Fabrication and properties of a-C:H/boron-doped diamond structure, New Diamond Front. Carbon Technol. 17 (2007) 301–308. [10] H. Akasaka, N. Ohtake, Electric properties of a-C:H/a-C:N:H/aluminum structure, Diam. Relat. Mater. 17 (2008) 673–675. [11] Rusli, G.A.J. Amaratunga, S.R.P. Silva, Photoluminescence in amorphous carbon thin films and its relation to the microscopic properties, Thin Solid Films 270 (1995) 160–164. [12] N.M.J. Conway, A.C. Ferrari, A.J. Flewitt, J. Robertson, W.I. Milne, A. Tagliaferro, W. Beyer, Defect and disorder reduction by annealing in hydrogenated tetrahedral amorphous carbon, Diam. Relat. Mater. 9 (2000) 765–770.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Structure and physical properties of stable isotopic amorphous carbon films Yutaro Suzuki a, Yasuyoshi Kurokawa a, Tsuneo Suzuki b, Kazuhiro Kanda c, Masahito Niibe c, Masayuki Nakano d, Naoto Ohtake a, Hiroki Akasaka a,⁎ a
Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan University of Hyogo, 3-1-2 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1205, Japan d Tokyo National College of Technology, 1220-2 Kunugida, Hachiouji, Tokyo 193-0997, Japan b c
a r t i c l e
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Article history: Received 14 July 2015 Received in revised form 6 October 2015 Accepted 25 October 2015 Available online xxxx Keywords: Amorphous carbon film Stable isotope Defect density Isotopic effect
a b s t r a c t Hydrogenated amorphous carbon (a-C:H) films containing isotopes of carbon and hydrogen were deposited, and their structure and physical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of carbon films. These isotope films such as deuterated amorphous carbon (a-12C:D) and hydrogenated 13C carbon (a-13C:H) films were deposited under the same deposition condition by the radio frequency plasma CVD method from isotope methane gas. The ratio of sp2/sp3 carbon was same, the deuterium content of a-12C:D was smaller than hydrogen contents of other films. Although the smallest content of hydrogen or deuterium as the termination structure of the carbon network was observed for the a-12C:D film, the density of defects was also smallest in the film. The Tauc gap of the a-12C:D film was approximately 30% larger than the that of the other films. From these results, the deposition process of a-C:D film brings lower defect density compared with the deposition system for a-C:H. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated amorphous carbon (a-C:H) and related films have received considerable attention because of their high hardness and wear resistance [1–3]. Their mechanical applications have already been developed in parts supply industries. On the other hand, some electronic and electrical applications of a-C:H films have been examined [4–8]. Valentini et al. reported amorphous carbon/p-type silicon diodes and their rectifying characteristics [4]. The application of amorphous carbon as a semiconductor in optoelectronic devices such as photovoltaic solar cells has also been attempted. For carbon-based a-C:P/p-Si heterojunction solar cells, the energy conversion efficiency (η) and fill factor (FF) were reported to be 0.84% and 0.49, respectively [5]. As a more complex structure, Chen et al. investigated the photovoltaic characteristics of novel gold/p-type amorphous boron carbon thin film alloy/silicon dioxide/n-type crystalline silicon/aluminum (Au/a-B:C/ SiO2/n-Si/Al) solar cells. They reported value of 3.3% and 0.904 for η and the FF, respectively. These structures almost involved Si layer [6]. Structures containing Si show the rectifying nature of heterojunction structures with amorphous carbon. On these reports, the device had the metal–insulator–semiconductor (MIS) structure. Usually MIS photovoltaic system generates carriers by photovoltaic effect at the part of “Semiconductor”. Hence most of the origin of photo-electromotive force at these devices is probably the Si layer, because the electrical ⁎ Corresponding author. E-mail address: [email protected] (H. Akasaka).
resistance of amorphous carbon and related materials is much higher than Si. The FF for a device consisting of only carbon materials was reported to be 0.26 [9,10]. One of the reasons for these low FF and η is probably the existence of localized states in band gap caused by their high defect density in carbon materials. Under irradiation of light, the semiconductor absorbs these photon energies, and electrons are excited from valence band to conduction band. On the photovoltaic system, these carriers of electron and holes behave as an electrical energy. When there are localized states in the band gap caused by the dangling bond, these carriers are easily trapped by these localized states during the excitation by the photon energy absorption. It is well known that the low defect density should be required for high FF and η. The photovoltaic characteristics including the FF and η are affected by the defect density such as dangling bonds. Some reports indicated that the defect density of a-C:H films is approximately 1018 to 1021 cm−3, which is higher than the defect density of commercially available a-Si of approximately 1015 cm−3 [11–13]. And these defect density distributions such as surface or inside of balk have reported nothing on a-C:H films research. As first step of used for electric applications, a low defect density same level as Si requires for a-C:H films. An a-C:H film consists of carbon and hydrogen atoms, and these atoms have non-radioactive stable isotopes of 13C and deuterium. On diamond researches, some reports indicated that thermal conductivity and band gap can be controlled by the incorporation of 13C [14–16]. These effects are caused by difference of the bonding length between 12 C–12C and 13C–13C. Hence the 13C incorporation into 12C diamond
http://dx.doi.org/10.1016/j.diamond.2015.10.024 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
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brings grain boundary for phonon scattering, and the change of band gap from the pure 12C diamond. 13C has different characteristics from 12 C at diamond system; thus, the incorporation of 13C into a-C films may lead to more advanced functions. On the other hand, the deuterium incorporation for a-C:H system is considered for an advanced function. Usually, an increase in atomic mass results in a higher bonding energy, for example, the C–D bonding energy is higher than that of C–H owing to the isotope effect. This increase in the bonding energy is expected to reduce damage by high-energy ions during film deposition. In fact, the deuterium termination of amorphous Si has been attempted in the development of solar cell devices [17–19]. These reports indicated that Defects at several place in some crystalline types of silicon material were terminated by deuterium atoms. On a polycrystalline silicon device, the deuterium passivation of grain boundaries by correlated deuterium diffusion and dangling-bond annihilation was revealed in polycrystalline silicon films [17,18]. Hence, dangling bonds at the Si grain boundary could be terminated by deuterium atoms. Furthermore, crystalline Si was also studied [19–21]. The deuterium atoms density on a Si crystal surface is increased by an order of magnitude following monolayer passivation at temperatures between 300 and 700 K [19]. This difference is originated from the vibrational frequencies of H and D on Si crystal surface. This difference in the vibrational frequencies is related to their bonding energies, occurs at the bonding system for carbon. These studies generally indicated that the higher efficiency and stability achieved by the deuterium passivation of dangling bonds on Si were due to the higher vibrational frequency and mass of deuterium atoms than those of hydrogen atoms. That may bring higher bonding strength and long-term stability. In fact in a study of the p-i-n structured amorphous silicon solar cell, replacing hydrogen with deuterium in the intrinsic layer of the cell improved the long-term stability against light exposure [22,23]. The change from H to D in the a-Si solar cell improved stability under light irradiation. These improvements in Si system almost resulted from mass increasing or the isotopic effect. The defect density distribution on the a-C:H films is unknown, but the deuterium passivation for a-C:H films can be expected from the reduction of defects. On the other hand, these have been few reports on the deposition of amorphous isotopic carbon films, and the defect passivation of isotopic carbon films has not been reported. Especially, the isotopic effect on carbon system may be more complicated compared with silicon system, because the bonding structure for silicon is only sp3 hybridized bonding against three types of bonding structure: sp, sp2, and sp3 hybridized bonding can be formed in carbon system [2]. Hence, the isotopic effect on amorphous carbon should be studied for development of electronic applications of amorphous carbon. In this study, amorphous carbon films containing isotopes of carbon were deposited, and their structure and electrical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of carbon films.
Table 1 Deposition condition. Film type
a-12C:H
Nature of gas Precursor Pressure [Pa] Flow rate [cm3/min]
Ar (99.999%) Ar (99.999%) Ar (99.999%) 12 13 12 CH4 (99.999%) CH4 (99%) CD4 (99%) 13 13 13 Ar: 20, methane: 3 p-Si(100): 0.1 Ω cm, n-Si(100): 1.0 Ωcm, #7059 glass, quartz
Substrate
a-13C:H
a-12C:D
spectroscopy (NRS-1000 JASCO) using a Nd:YVO4 laser (532 nm) with an aperture size of ϕ = 200 μm. The elemental compositions of the film was determined by a Rutherford backscattering (RBS), and elastic recoil detection analysis (ERDA) system which was performed under 1 MeV He+ irradiation using an electrostatic accelerator (NT-1700HS: Nissin High Voltage Co.). Moreover, the sp2 content in the deposited films was determined by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. This measurement was performed at the BL09A beamline of New SUBARU at the University of Hyogo. NEXAFS carbon K-edge spectra were obtained in the energy range of 275– 320 eV with a resolution of 0.5 eV (FWHM) in the total electron yield mode. The density of paramagnetic defects was evaluated using electron spin resonance (ESR) measurements in the X-band (9.44 GHz) at room temperature, using a resonance spectrometer (JES-FA100 JEOL). The g-value is usually 2.002316 for unpaired electrons. Hence, the measurement magnetic field was from 328.5 to 343.5 mT. The spin concentration was determined by comparison with the 1-Oxyl2,2,6,6-tetramethyl4-hydroxypiperidine (TEMPOL) spin standard. To estimate the optical properties, the optical band gaps for the obtained films were estimated by ultraviolet–visible (UV–Vis) absorption spectroscopy, where UV–Vis spectra were obtained using a UV–Vis spectrophotometer (V-630 JASCO). After measurement of spectra of transmittance and reflectance, absorption spectra were obtained from these spectra. 3. Results and discussion The film thickness of all three types of film was approximately 370 nm at 60 min deposition duration. Although the deposition conditions of these films were the same except for the type of methane, the film structures and properties were different. Figure 1 shows XRR patterns of the films. Without any sample preparation or damage, XRR can be used to obtain information about density, roughness and crosssectional layering for any amorphous carbon films on flat substrate. Hence XRR technique has been frequently used for the determination of film density and thickness on amorphous carbon films [24,25]. These three XRR patterns also indicated a similar film thickness for the three films owing to their similar fringe patterns. By simulation fitting,
2. Experimental a-C:H films were deposited onto quartz glass, Si(100) (0.1 Ω cm), and #7059 optical glass substrates by radio frequency (RF) plasma chemical vapor deposition. Electrodes were set parallel in the vacuum chamber to generate a capacitively coupled plasma. In the a-C:H deposition, methane(CH4, 99.999%), deuterated methane(CD4, 99%) or 13C methane(13CH4, 99%) gas was used as the source material. Before deposition, the substrate surface was irradiated with Ar ions for 30 min at a flow rate of 20 cm3/min to remove the adsorbed substances and the oxidation layer on Si. Then, one of the methane gases at a flow rate of 3 cm3/min and Ar gas at a flow rate of 20 cm3/min were introduced into the chamber, and a 13.56 MHz RF power of 20 W was applied to the substrate at a pressure of 13 Pa. All deposition conditions were fixed except for the source gas, as shown in Table 1. The film density was evaluated by the X-ray reflectivity (XRR). The XRR pattern was obtained using a CuKα X-ray diffractometer (X'Pert PRO MRD Phillips). The structure of the films was analyzed by Raman
Fig. 1. XRR patterns of a-12C:H, a-12C:D, and a-13C:H films.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx
Fig. 2. Raman spectra of the a-C:H films.
densities of 1.70, 1.54, and 1.75 g/cm3 were obtained for the a-12C:H, a-12C:D, and a-13C:H films, respectively. Although the density for the a-12C:H and a-13C:H films were almost same, the density of the a-12C:D film was smaller than that of others. The Raman spectra of the three types of a-C:H films are shown in Fig. 2. In all the spectra, two broad peaks appear, namely, the G (graphitic)-band at approximately 1560 cm−1 and the D (disorder)-band at approximately 1350 cm−1. G-band peaks originate from the E2g resonance oscillation in sp2 bonded carbon included in graphitic structures. And Tuinstra et al. indicated that the G-band represents the order of a graphitic structure [26]. The D-band can be assigned to first-order scattering from a zone boundary phonon activated by disorder on the basis of the finite crystallite size [26]. Briefly, the D-band represents the boundary of the graphite structure. To analyze data on the order of the graphitic cluster size, the integrated peak ratio, ID/IG, was calculated from the Raman spectra. A change in ID/IG suggests a change in the continuous order of the graphitic cluster size. These Raman spectra were fitted by Gaussian line shapes. The values of ID/IG were 0.57, 0.37, and 0.70 for the a-12C:H, a-12C:D, and a-13C:H films, respectively. The order of the graphitic cluster size of a-12C:H is larger than that of a-13C:H, and the graphitic cluster size of a-12C:D is larger than that of the other films. As mentioned later, there were no differences in sp2/sp3 ratio about these films. Hence, the reason for the low density in a-12C:D film compared with that of the other films is the not graphite layer. The low density may be caused by the inclusion of polymer-like components or increasing of vacancy volumes. Deuterium atom terminations bring larger atomic vacancy compared with H termination because of their bonding length differences. The Raman spectra for the a-C:D film exhibited a gradient baseline, which indicates contained a uniaxially oriented
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Fig. 4. NEXAFS spectra of the a-C:H films.
conjugating polymer. The G-peak position of the a-13C:H film clearly indicated a lower wave number than that of the films consisting of 12C. The shift of the related peak for carbon such as diamond has been reported as an isotopic effect [27,28]. The G-peak center position of the a-13C:H and a-12C:H films are 1474 and 1526 cm−1, respectively. The increasing of atomic mass brings the resonance frequency shift in Raman scattering phenomenon [29]. From the atomic mass difference and G-peak position of a-12C:H, G-peak position of a-13C:H was calculated and obtained to 1466 cm− 1. This value indicated good agreement with the measurement value of 1474 cm−1 with error of 0.5%. Hence, this peak shift did not indicate a structural difference compared with the films consisting of 12C. To determine the composition and sp2/sp3 ratio of these films, RBS/ ERDA and NEXAFS spectra were measured, as shown in Figs. 3 and 4, respectively. From the RBS/ERDA spectra, the hydrogen contents of the a-12C:H and a-13C:H films were 34 at.%. The deuterium content of the a-12C:D film was 24 at.%. The difference of the contents was 10% between hydrogen and deuterium. These RBS/ERDA results indicated the difference between the carbon bonding network termination structures formed by hydrogen and deuterium atoms, respectively. From the NEXAFS spectra in Fig. 4, the broad band observed in the range of 288–310 eV is assignable to the result of overlapping C 1s → σ* transitions at the sp3, sp2, and sp1 sites of the a-C:H and related films [30–33]. The absolute sp2 content was determined by comparison with the spectrum of highly oriented poly-graphite. The sp2/(sp2 + sp3) ratio for all the obtained films was approximately 0.4. These film structures indicated from the results of NEXAFS and RBS/ERDA were similar because the deposition conditions were the same except for the source materials.
Fig. 3. RBS (left) and ERDA (right) spectra of the a-C:H films.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
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Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx Table 3 Results of the data by all measurements.
True density [g/cm−3] E04 [eV] ETauc [eV] H or D content [%] sp2/(sp2 + sp3) [−] Density of unpaired electron [×1021 cm−3]
a-12C:H
a-13C:H
a-12C:D
1.70 1.90 1.17 34 0.42 1.4
1.75 1.94 1.20 34 0.42 1.4
1.54 2.31 1.56 25 0.41 1.0
network is longer than other films. Usually, these bonding networks in amorphous carbon system are terminated by termination atoms such as D and H, or dangling bond. And unpaired electron densities in a-C:D film is also indicated the smallest dangling bond density. Hence, deposition process of a-C:D film have the calming effect of calming of dang ling bond. Fig. 5. ESR spectra of the a-C:H films.
4. Summary To estimate the defect density in these films, ESR spectra were obtained, as shown in Fig. 5. A peak in the absorption was observed in all the spectra at 332–333 mT. The density of paramagnetic defects and the peak-to-peak width of the films obtained from the ESR spectra are shown in Table 2. The peak-to-peak width of the a-13C:H film is approximately twice than that of the films consisting of 12C, which can be attributed to the interaction between the nuclear spin of 13C and the electron spin. The density of paramagnetic defects in the a-12C:D film is less than that in the a-12C:H and a-13C:H films. The difference between the elemental changes in hydrogen or deuterium is 30%. Although the film with the smallest hydrogen or deuterium content as the termination structure of the carbon network was the a-12C:D film, the density of paramagnetic defects was also smaller than that of the other films. These results mean that the effect of deuterium on reducing the paramagnetic defect density is greater than that of hydrogen atoms in an a-C:H system (Table 3). If such defect reduction affects the electronic properties of films, it can be considered that the optical band gap may be expanded because the localization of states by the defects was reduced. Optical absorption spectra were measured, and the Tauc plots obtained for the deposited films are shown in Fig. 6. We found differences in the spectral shape of the optical absorption coefficient for each film. The Tauc gap (ETauc), which is used as the band gap of amorphous semiconductors [22] can be obtained by extrapolating the linear area of a Tauc plot to the horizontal axis, and values of 1.17, 1.20, and 1.56 eV were obtained for the a-12C:H, a-13C:H, and a-12C:D films, respectively. Although these films had same film thickness, the values of the optical band gap, E04, indicated also larger difference in Fig. 6(b) between the a-C:D film and the a-C:H films. ETauc for the a-12C:D film is about 30% larger than that of for other films. The density of electronic defects of the a-12C:D film was lower than that of the a-12C:H and a-13C:H films, and there are expected to be more defect levels in the band structure of the a-12C:H film than in that of the a-12C:D film. That may be one of the reasons for the larger ETauc for the a-12C:D film. And sp2 cluster size is largest in the obtained samples. It is well known that the band gap in amorphous carbon materials is affected by sp2/sp3 ratio [34]. However, our samples indicated sp2/sp3 ratios are same. And the largest sp2 cluster size and the smallest atomic concentration which forms termination structure indicated bonding
To investigate the isotopic effects in an attempt to reduce the defect density of carbon films, amorphous carbon films containing isotopes of carbon were deposited, and their structures and electronic properties were evaluated. The film composition and structure of a-12C:D showed large differences from those of other films. Although the smallest content of hydrogen or deuterium as the termination structure of the carbon network was observed for the a-12C:D film, the density of defects was also smallest in the film. The Tauc gap of the a-12C:D film was approximately 30% larger than the that of the other films. On comparison
Table 2 Results of ESR measurements.
Density of unpaired electron [10 Peak-to-peak width ΔHpp [mT]
21
cm
−3
]
a-12C:H
a-13C:H
a-12C:D
1.4 0.34
1.4 0.63
1.0 0.34
Fig. 6. Optical absorption spectra and Tauc plots of the a-C:H films. (a) Tauc plots. (b) Optical absorption spectr.
Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024
Y. Suzuki et al. / Diamond & Related Materials xxx (2015) xxx–xxx
of the deposition system for a-C:H, the deposition processes of a-C:D film have the calming effect of calming of dang ling bond. Prime novelty statement In this study, hydrogenated amorphous carbon (a-C:H) films containing isotopes of carbon and hydrogen were deposited, and their structure and electrical properties were evaluated to investigate the isotopic effects in an attempt to reduce the defect density of amorphous carbon films. This report is first report that deuterium is more efficient for decreasing of defect density compared with the hydrogen in an amorphous carbon system. Acknowledgments This work was supported by “Planting Seeds for Research” program in Tokyo Institute of Technology. And some parts of this work were supported by JSPS KAKENHI (grant Number 15K18038). References [1] S. Yamamoto, A. Kawana, H. Ichimura, C. Masuda, Relationship between tribological properties and sp3/sp2 structure of nitrogenated diamond-like carbon deposited by plasma CVD, Surf. Coat. Technol. 210 (2012) 1–9. [2] S. Yamamoto, A. Kawana, C. Masuda, Tribological behavior of diamond-like carbon produced by rf-PCVD based on energetic evaluation, Surf. Coat. Technol. 236 (2013) 457–464. [3] T. Michler, M. Grischke, I. Traus, K. Bewilogua, H. Dimigen, Mechanical properties of DLC films prepared by bipolar pulsed DC PACVD, Diam. Relat. Mater. 7 (1998) 1333–1337. [4] L. Valentini, V. Salerni, I. Armentano, J.M. Kenny, L. Lozzi, S. Santucci, Effects of fluorine incorporation on the properties of amorphous carbon/p-type crystalline silicon heterojunction diodes, J. Non-Cryst. Solids 321 (2003) 175–182. [5] M. Rusop, T. Soga, T. Jimbo, Photovoltaic characteristics of phosphorus-doped amorphous carbon films grown by r.f. plasma-enhanced CVD, Sol. Energy Mater. Sol. Cells 90 (2006) 3214–3222. [6] T.S. Chen, Y.C. Hsueh, S.E. Chiou, S. Tsong Shiue, The effect of the native silicon dioxide interfacial layer on photovoltaic characteristics of gold/p-type amorphous boron carbon thin film alloy/silicon dioxide/n-type silicon/aluminum solar cells, Sol. Energy Mater. Sol. Cells 137 (2015) 185–192. [7] C. Ronning, E. Dreher, J.U. Thiele, P. Oelhafen, H. Hofsass, Electronic and atomic structure of undoped and doped ta-C films, Diam. Relat. Mater. 6 (1997) 830–834. [8] M. Guerino, M. Massi, H.S. Maciel, C. Otani, R.D. Mansano, P. Verdonck, J. Libardi, The influence of nitrogen on the dielectric constant and surface hardness in diamondlike carbon (DLC) films, Diam. Relat. Mater. 13 (2004) 316–319. [9] H. Akasaka, T. Imai, N. Ohtake, Fabrication and properties of a-C:H/boron-doped diamond structure, New Diamond Front. Carbon Technol. 17 (2007) 301–308. [10] H. Akasaka, N. Ohtake, Electric properties of a-C:H/a-C:N:H/aluminum structure, Diam. Relat. Mater. 17 (2008) 673–675. [11] Rusli, G.A.J. Amaratunga, S.R.P. Silva, Photoluminescence in amorphous carbon thin films and its relation to the microscopic properties, Thin Solid Films 270 (1995) 160–164. [12] N.M.J. Conway, A.C. Ferrari, A.J. Flewitt, J. Robertson, W.I. Milne, A. Tagliaferro, W. Beyer, Defect and disorder reduction by annealing in hydrogenated tetrahedral amorphous carbon, Diam. Relat. Mater. 9 (2000) 765–770.
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Please cite this article as: Y. Suzuki, et al., Structure and physical properties of stable isotopic amorphous carbon films, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.10.024