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/lp/ou_press/soft-x-ray-emission-spectroscopy-study-of-characteristic-bonding-dtDiA3R5qk
- Publisher
- Oxford University Press
- Copyright
- © The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
- ISSN
- 0022-0744
- eISSN
- 1477-9986
- DOI
- 10.1093/jmicro/dfy024
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- See Article on Publisher Site
Abstract
Abstract Soft X-ray emission spectroscopy based on electron microscopy was applied to investigate bonding electron states of amorphous carbon nitride (a-CNx) films with different nitrogen contents of x. Carbon K-emission spectrum showed characteristic intensity distribution of not only sp2 bonding but also sp3 bonding. The a-CNx film with lager x, which has a larger macroscopic electric resistivity, shows a larger content of the carbon sp3: C–C bonding signal. Furthermore, the dependence of spectral intensity distribution on x suggests the presence of sp2: C–N and sp3: C–N bonding. Those results show that the relation between macroscopic electrical resistivity of a-CNx film and its nitrogen content is because of the decrease of sp2: C–C bonding and the formation of sp2: C–N and sp3: C–C and C–N bonding conformation induced by an introduction of nitrogen atoms. Spatial variation of a signal ratio of sp3/sp2 was visualized and was confirmed as a relation between sp3 boding amount and nitrogen content x. soft X-ray emission spectroscopy, amorphous carbon-nitride, C K-emission, sp3 and sp2 bonding, chemical shift Introduction Amorphous carbon nitride (a-CNx) has been one of the prospective coating materials because of its mechanical properties of high hardness, high wear resistance and low friction coefficient [1–3]. In recent years, the electrical and optical properties of a-CNx films, such as photoconductivity, low dielectric constant, high electrical breakdown field and photo-induced deformation, have been reported [4–9]. Light-emitting properties in visible light energy region are reported for a-CNx and hydrogenated a-CNx films [10–12]. Furthermore, a possibility of white light-emitting devices, red–green–blue emissions, for hydrogenated films and a controllability of optical bandgap energy by nitrogen content x has been reported [13]. Because of those electrical properties, a-CNx film becomes one of promising materials for future low-cost electronic and optoelectronic devices. Those electrical and optical characteristics depend on the content of nitrogen x, which can be controlled by the temperature of a substrate in deposition process under nitrogen gas pressure. This may suggest that the deposition temperature may change not only the content of nitrogen, x, but also the chemical bonding states of a-CNx film. Those characteristic electrical and optical properties of a-CNx films presumably depend on chemical state, local bonding states, of nitrogen atoms incorporated in an amorphous carbon network. Souto et al. [14] reported an photoelectron spectroscopy study of a-CNx with x = 0.1–0.43. N-1s core-level spectra showed two peaks, suggesting that two valence states of N atoms are existing in a-CNx. From comparisons between experimental valence-band photoelectron spectra and theoretical calculations, the two peak structures were assigned to bonding states of N atoms of sp2 bonding in aromatic rings and sp3 bonding in sp3–C network. Ripalda et al. [15] reported the correlation analysis of x-ray absorption and x-ray photoemission spectroscopies of N-1s core level. It was concluded that chemical states of N atoms bonded to sp3 C is faint and dominated by sp2 bonding in aromatic rings. Rodil and Muhl [16] reported from electron energy-loss spectroscopy (EELS) and near-edge X-ray absorption spectroscopy experiments of N-1s and C-1s core levels that a-CNx films at high nitrogen content are preferentially π-bonded. Tamura et al. [17] examined a-CNx films by X-ray photoemission spectroscopy. N-1s core-level spectra were assigned to be dominantly composed of sp2: C–N and sp3: C–N bondings. C-1s core-level intensity profiles were decomposed into dominant C–C bonding with a smaller sp2: C–N and a minor sp3: C–N bonding components. The C–C component was placed almost at the value of sp2 C bonding and was not divided into sp2 and sp3. Those investigations were focused on a chemical bonding state of incorporated nitrogen atoms into amorphous carbon network. There seems to be a basic understanding that nitrogen atoms in a-CNx are mainly bonded to sp2 conformation, which may be a dominant bonding in amorphous carbon films. Thus, the role of sp3 bonding has not been lighted and concluded. It may originate from preparation conditions of a-CNx films, and also, experimental methods for evaluation are not unified. For improving physical properties and the controllability of functions of a-CNx films, it is important to clarify the relation between those physical properties and chemical bonding states, not only sp2 but also sp3, via nitrogen content. In this report, a-CNx films with different nitrogen content x prepared by reactive magnetron sputtering [9] were examined by soft X-ray emission spectroscopy (SXES). SXES spectra of carbon K-emission, which reflect partial density of states of valence bands (bonding electrons) with p-symmetry, are compared with those of amorphous carbon, graphite and diamond, and a change of intensity profile with nitrogen content was discussed. Experimental procedures Specimen preparation Amorphous carbon nitride films were prepared by reactive RF magnetron sputtering on Si(100) substrates as already reported [9]. A purity of reactive nitrogen gas was 99.999%. A graphite plate with a purity of 99.99% was used as the sputtering target. The background pressure was below 5 × 10−5 Torr. Three a-CNx films with different substrate temperatures of 473 K, 573 K and 773 K were prepared. The thicknesses of those films are ~1 μm. The nitrogen content of each film was evaluated by X-ray photoelectron spectroscopy (XPS; PHI ESCA1600) by using Mg-Kα (1253.6 eV) radiation under an ultrahigh vacuum pressure of 10−11 Torr. The substrate temperature for deposition process, nitrogen content, and electrical resistivity of each film are shown in Table 1. Resistivity was measured at 60°C for a-CNx films deposited on SiO2 substrate [17]. Table 1. The substrate temperature for deposition process, nitrogen content, and electrical resistivity of a-CNx films examined Sample CNx Substrate temperature (K) Nitrogen content x Resistivity (Ω m) 1 473 0.52 6.4 × 107 2 673 0.41 2.3 × 105 3 773 0.40 3.4 × 102 Sample CNx Substrate temperature (K) Nitrogen content x Resistivity (Ω m) 1 473 0.52 6.4 × 107 2 673 0.41 2.3 × 105 3 773 0.40 3.4 × 102 Table 1. The substrate temperature for deposition process, nitrogen content, and electrical resistivity of a-CNx films examined Sample CNx Substrate temperature (K) Nitrogen content x Resistivity (Ω m) 1 473 0.52 6.4 × 107 2 673 0.41 2.3 × 105 3 773 0.40 3.4 × 102 Sample CNx Substrate temperature (K) Nitrogen content x Resistivity (Ω m) 1 473 0.52 6.4 × 107 2 673 0.41 2.3 × 105 3 773 0.40 3.4 × 102 Specimens for transmission EELS measurements were prepared by scratching the a-CNx films on Si wafer and the fragments were placed on a carbon-supported grid for transmission electron microscopy experiments. An electron diffraction pattern obtained from a fragment showed a broad ring intensity, which confirms that there is no long-range periodic order in the material. Pristine a-CNx films formed on Si wafer were used for SXES experiments. The reference material of amorphous carbon was prepared using the same method of a-CNx without nitrogen, x = 0. Graphite of single crystalline fragments and diamond powder particles were used for comparison. Spectroscopy experiments A high-energy resolution EELS transmission electron microscope (TEM) equipped with a two-stage Wien-filter monochromator was used for obtaining valence electron excitation spectra [18]. EELS spectra obtained at an accelerating voltage of 60 kV showed a change in its intensity distribution at around a few eV energy region, depending on acquisition times. This apparently showed that the electronic state of a-CNx around the bandgap is easily damaged by high-energy electron beam irradiation. Thus, all discussions in the report below are done with the results of SXES experiments conducted at an accelerating voltage of 5 kV. At this voltage, there has been no change noticed in the spectral intensity distribution within acquisition times within some minutes. This presents the importance of spectroscopy method at a low accelerating voltage for defective materials as amorphous materials. SXES spectra of a-CNx films were obtained by using a commercial SXES system (JEOL SS-98000 SXES) attached to an electron probe microanalyzer (EPMA) of JXA-9800F. A varied-line-spacing grating, JS200N, with an average groove density of 1200 lines/mm, was used in these experiments. Because the acceptable energy range of this spectrometer is limited, the second-order spectrum of C K-emission, C–K(2) and third-order N K-emission spectrum, N–K(3), were measured. Acquisition time, probe current, and accelerating voltage were 2 min, 40 nA and 5 kV, respectively. For spectral mapping, acquisition time was reduced to 30 s at one point to shorten the total acquisition time. Examined volume depth for amorphous carbon at 5 kV evaluated by Reed’s equation is ~0.6 μm in diameter [19]. The energy of the spectrum was determined by a calibration curve, which was derived by using energies of higher-order spectrum lines of C–K, B–K emissions, whose spectra were obtained under the same experimental setup for a-CNx films. Those emission energies were adapted from the study of Bearden [20]. Results and discussion Figure 1 shows a second-order carbon K-emission spectrum, C–K(2), of a-CNx film with x = 0.52. The C–K(2) spectral intensity appears at the half of C–K(1) emission energy, which is the real X-ray energy emitted from the specimen of ~280 eV. A peak intensity around 132 eV corresponds to third-order nitrogen K-emission, N–K(3), whose true X-ray energy is ~395 eV. C–K emission spectra of amorphous carbon (a-C), graphite and diamond obtained at the same experimental condition with those of a-CNx are also shown for comparison. Each spectrum intensity was normalized by its intensity maximum. Those intensity distributions correspond to a partial density of states of the valence band with p-symmetry (p-DOS) of each material. The spectrum of a-CNx shows three structures labeled as A, B and C. The top of the valence band indicated by a vertical arrow is ~143.8 eV. The energy is apparently larger than those of a-C, graphite and diamond, as indicated by vertical lines. It is seen that the structures of A and C are also seen for a-C and graphite almost at the same energy positions. Those structures A and C correspond to sp2 bonding of σ and π states, respectively. The spectrum of a-CNx has an additional structure B, whose energy position corresponds to the peak of diamond formed by sp3–σ bonding. The presence of sp3–σ bonding is also noticed by a structure at 136 eV, which is characteristic for diamond. Those data suggest that C–C bonding of sp2 and sp3 types exist in a-CNx. As C-1s level of sp2: C–N is shifted by 1.5 eV in larger binding energy side than that of C–C bonding [17], emission energy of sp2: C–N may exist at the larger energy sides of those of carbon by ~0.8 eV in C–K(2) spectrum. The shifted structure corresponding to sp2–σ: C–N is difficult to assign in intensity profile. However, the higher energy of the spectrum onset than that of a-C by 0.8 eV can be an indication of sp2–π: C–N. This suggests a presence of sp2: C–N in the a-CNx film. The C-1s binding energies of sp3: C–N and sp3: C–C (diamond) are +3.1 eV [17] and +0.8 eV [21] larger than that of sp2: C–C bonding, respectively. Then, the intensity of sp3: C–N is expected in the higher-energy side by (3.1–0.8)/2 to 1.2 eV than sp3: C–C intensity, structure B, in C–K(2), but not clear in the experimental spectrum of Fig. 1. Fig. 1. View largeDownload slide Second-order carbon K-emission spectrum, C–K(2), of an a-CNx film with x = 0.52. The C–K(2) spectral intensity appears at the half of C–K(1) emission energy. Peak intensity around 132 eV corresponds to third-order nitrogen K-emission, N–K(3). C–K emission spectra of amorphous carbon (a-C), graphite and diamond are also shown for comparison. Those intensity distributions correspond to a partial density of states of the valence band with p-symmetry (p-DOS) of each material. The spectrum of a-CNx apparently shows sp2: C–C and sp3: C–C character. Fig. 1. View largeDownload slide Second-order carbon K-emission spectrum, C–K(2), of an a-CNx film with x = 0.52. The C–K(2) spectral intensity appears at the half of C–K(1) emission energy. Peak intensity around 132 eV corresponds to third-order nitrogen K-emission, N–K(3). C–K emission spectra of amorphous carbon (a-C), graphite and diamond are also shown for comparison. Those intensity distributions correspond to a partial density of states of the valence band with p-symmetry (p-DOS) of each material. The spectrum of a-CNx apparently shows sp2: C–C and sp3: C–C character. To investigate the relation between characteristics of C–K spectrum of a-CNx and the content of nitrogen atoms, C–K(2) emission spectra of a-CNx films listed in Table 1 were measured as shown in Fig. 2. Each spectrum intensity is normalized by the height of peak A as shown in Fig. 1. It is seen that a-CNx film with a lager x shows a larger N–K(3) intensity because of the larger content of N. However, the change of N–K emission intensity seems larger than that of x evaluated by XPS. This discrepancy may be because XPS is a surface-sensitive method and presents SXES measurement that reflects a bulk nature. When the value x = 0.52 of sample 1 is true, x values of samples 2 and 3 are evaluated, respectively, to be 0.35 and 0.26, by referring to the height of N–K (3) intensity. It should be noticed that a-CNx film with a larger x shows a larger intensity in peak B, showing an increase of sp3: C–C with the nitrogen increase. It is also supported by an increase of a shoulder intensity at 136 eV, which is characteristic for diamond as shown in Fig. 1. Furthermore, energy positions of the top of the spectrum intensity, as indicated by vertical short lines, increase with the increase of nitrogen content. This energy shift can be because of the increase of sp2–π: C–N intensity, which is expected in this energy as discussed above. The N–K intensity dominantly changes in its peak intensity. There is also a little shift of the peak position to the lower-energy side with an increase of x. Fig. 2. View largeDownload slide C–K(2) and N–K(3) spectra of three a-CNx films listed in Table 1. The top of the spectrum intensity position in each spectrum is indicated by a short line. It is seen that N–K(3), structure B, and the top of the spectrum intensity systematically depend on nitrogen content x. Fig. 2. View largeDownload slide C–K(2) and N–K(3) spectra of three a-CNx films listed in Table 1. The top of the spectrum intensity position in each spectrum is indicated by a short line. It is seen that N–K(3), structure B, and the top of the spectrum intensity systematically depend on nitrogen content x. To clarify the change in the intensity distribution of C–K emission of a-CNx with x, each spectrum was normalized by the integrated intensity form of 134–146 eV as shown in Fig. 3a. Figure 3b shows a change in intensity distribution with an increase of x. It is clearly seen that (i) a decrease of sp2–σ: C–C bonding at 138.5 eV (structure A in Fig. 1), (ii) an increase of sp3: C–C bonding at 140 eV (structure B in Fig. 1), and (iii) an increase in intensity at the top of the valence bands at 142.5 eV are related to an increase of N–K(3) intensity at ~131.5 eV. The increase of (iii) can assigned to the formation of sp2–π: C–N bonding as discussed above. It should be also noted that a decrease of sp2–π: C–C intensity at ~141 eV (structure C in Fig. 1) is almost the same for CN0.41 and CN0.52, nevertheless sp2–σ: C–C intensity of CN0.52 is much decreased than that of CN0.41. This suggests that some spectral intensity overlapped at ~141 eV for a larger nitrogen content x. As this energy is almost equal to that of sp3: C–N signal as discussed in Fig. 1, the non-decrease of sp2–σ: C–C intensity for CN0.52 may be due to an overlap of sp3: C–N signal. It is consistent with the presence of sp3: C–N bonding already reported [14,17]. Much more variety of C–N bonding are discussed in Souto et al. [14], but cannot be assigned as an apparent structure in this experiment. As the N-1s binding energy of N atoms bonded to sp3–C is smaller than N atoms bonded to sp2–C [17], an increase of sp3: C–N component can cause a decrease of N–K emission energy. This is consistent with the peak shift of N–K intensity with the increase of x as pointed out in Fig. 2. Fig. 3. View largeDownload slide (a) Emission spectra of a-CNx films (x = 0.52, 0.41, 0.40). Each spectrum was normalized as the integrated intensity form 134 eV to 146 eV is combined. Change in intensity distribution due to x is clearly seen. (b) Changes in intensity distribution with an increase of x. 1: [a-CN0.41] – [a–CN0.40] and 2: [a–CN0.52] – [a–CN0.40]. Fig. 3. View largeDownload slide (a) Emission spectra of a-CNx films (x = 0.52, 0.41, 0.40). Each spectrum was normalized as the integrated intensity form 134 eV to 146 eV is combined. Change in intensity distribution due to x is clearly seen. (b) Changes in intensity distribution with an increase of x. 1: [a-CN0.41] – [a–CN0.40] and 2: [a–CN0.52] – [a–CN0.40]. The formation of sp2: C–N and sp3: C–N bonding in a-CNx with an increase of nitrogen content x is reasonable. On the other hand, it is interesting that the experimental result indicates that sp3: C–C bonding seems to be induced by the increase of nitrogen content. It can be an origin of a higher electrical resistivity of a-CNx film with a larger x. Each electrical resistivity of Table 1 shows a macroscopically averaged nature of each a-CNx film. Thus, it is important to evaluate the variation of sp3: C–C bonding signal in the material. Thus, spectrum mapping measurements were done. Figure 4a shows three regions of IN, I2 and I3 to integrate spectrum intensity written in the spectrum of a-CN0.52 shown in Fig. 3a. Intensities of IN, I2, and I3 are indications of N, sp2–C and sp3–C amounts, respectively. Figure 4b shows a spatial distribution of I3/I2 obtained for a-CN0.52 film. The value for the region ranged between 0.89 and 0.94, and the average value was 0.91. Circled areas with solid and dotted lines, respectively, indicate lager and smaller I3/I2 value regions, which correspond to larger and smaller sp3–C component regions. Figure 4c shows the distribution of IN/I2 in the same area of Fig. 4b. The value ranged between 0.72 and 0.80 with an average value of 0.77. Circled areas, which are located at the same areas as in Fig. 4b, by solid and dotted lines indicate lager and smaller IN/I2 value regions, respectively. It can be seen that a tendency from a comparison between Fig. 4b and c that larger N content regions have a larger sp3–C component. Then, it is visually confirmed that sp3: C–C bonding seems to be induced by the increase of nitrogen content. Fig. 4. View largeDownload slide (a) The three regions of IN, I2 and I3 integrate spectrum intensity written in the spectrum of a-CN0.52 as shown in Fig. 3a. (b) Spatial distribution of I3/I2 obtained for a-CN0.52 film. The value for the region ranged between 0.89 and 0.94, and with an average value of 0.91. (c) Distribution of IN/I2 of the same area. The value was ranged between 0.72 and 0.80 with an average value of 0.77. A tendency from a comparison between Fig. 4b and (c) that a larger N content region has a larger sp3–C component can be seen. Fig. 4. View largeDownload slide (a) The three regions of IN, I2 and I3 integrate spectrum intensity written in the spectrum of a-CN0.52 as shown in Fig. 3a. (b) Spatial distribution of I3/I2 obtained for a-CN0.52 film. The value for the region ranged between 0.89 and 0.94, and with an average value of 0.91. (c) Distribution of IN/I2 of the same area. The value was ranged between 0.72 and 0.80 with an average value of 0.77. A tendency from a comparison between Fig. 4b and (c) that a larger N content region has a larger sp3–C component can be seen. Conclusion Chemical bonding states of amorphous CNx film is successfully analyzed by using an EPMA-SXES instrument. This experimental result showed sp3: C–C bonding is induced by the introduction of nitrogen, which is a key issue for a macroscopic relation between resistivity and nitrogen content of a-CNx film. Because amorphous materials do not have sharp interfaces like domain boundaries in crystalline materials, the bonding state in amorphous material can be directly related to the macroscopic properties. This experiment also shows the importance of spectroscopy method with a low-energy electron beam, which realizes a low or less irradiation damage for materials examined. This SEM/EPMA-SXES also has a possibility of chemical analysis of soft materials based on electron microscopy with an integration of a cryogenic specimen stage. 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Microscopy – Oxford University Press
Published: Aug 1, 2018