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Effect of carbon nitride bonding structure on electron field emission
Diamond and Related Materials 9 (2000) 1228–1232 www.elsevier.com/locate/diamond
Effect of carbon nitride bonding structure on electron field emission Y.K. Yap *, S. Kida, Y. Wada, M. Yoshimura, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan
Abstract Predominant formation of tetrahedral CMN bonds was obtained by in situ ion bombardment at 600°C. Tetrahedral carbon nitride (CN ) films emit electrons at lower threshold field compared with RT-prepared samples, that dominated by graphite-like CNN bonds. Likewise, annealing RT-prepared CN films can eliminate the graphite-like CNN bonds while the content of tetrahedral CMN bonds remains. Field emission from annealed films occurred at lower threshold field as well. It is suggested that graphite-like CNN bonds within graphitic basal planes that are aligned nearly parallel to the substrate surface must be suppressed to avoid electron drifting in the direction perpendicular to the applied electric field. Furthermore, randomly distributed sp3 CMN bonds appear to enable conducting path formation preferentially along the direction of the electric field. A model is proposed to explain the improvement in electron field emission. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Carbon; Carbon nitride; Electron field emission; Nitride
1. Introduction There have been many investigations on the development of cold cathode materials for electron field emission. Amorphous materials such as diamond-like carbon (DLC ) and carbon nitride (CN ) are possibilities. Unlike crystallized films, DLC and CN can be synthesized at room temperature (RT ). Thus both materials can be coated on glass and even plastic substrates, which are required for mass-production of large area, low cost flat panel displays. These materials are chemically inert and mechanically robust as coating materials for Si and Mo electron field emitters as well. The synthesis of DLC films and the influence of binding state on field emission have been explained [1]. On the other hand, deposition of CN films is referred to as nitrogen doping of carbon films that enhances the field emission ability of the carbon phase [2]. Most of these films are prepared at RT and have attracted tremendous attention in the past decade, as motivated by the search for the predicted C N compound [3]. 3 4 However, CN films synthesized at RT consist of various binding states. Despite this, CN films were widely tested to be outstanding for electron field emission [1,2,4–6 ]. Owing to their random chemical nature, the influence * Corresponding author. Fax: +81-6-879-77-08. E-mail address: [email protected] ( Y.K. Yap)
of individual CN binding state on electron emission is still relatively unexplored. A better knowledge of the controlling CN bond formation and characterization related emission properties is thus desired. Non-destructive IR spectroscopy has been commonly used to examine CN related bonds. A broad absorption from ~1100 to 1600 cm−1 was usually detected [5–13]. Except for the cynogen-like CNN (nitrile) bond [7] at ~2200 cm−1, the pyridine/graphite-like CNN bond [8,9] at ~1550 cm−1 and the tetrahedral CMN bond [9,10] at ~1212 to 1265 cm−1 were included within the absorption band. Recent study revealed preferential formation of graphite-like CNN bonds, as indicated by both IR absorption and electron energy loss spectroscopy ( EELS ) [11,12]. The nitrile bond was known to be eliminated by annealing at ~550°C [13]. Likewise, annealing CN films at temperatures above ~400°C causes graphitization and the rejection of incorporated nitrogen from the graphite-like CNN bonds [14]. The tetrahedral CMN bonds are least understood in terms of synthesis requirement, physical and chemical natures. Recently, we detected transformation of CN bonds from films deposited with in situ ion bombardment at 600°C [15,16 ]. Further, annihilation of graphite-like CNN bonds was detected by annealing the RT-prepared CN films [17]. Based on these techniques, the effect of CN bonds on electron field emission was investigated in this work.
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2. Experiment All CN samples were prepared by pulsed laser deposition (PLD) in pure N gas or N RF plasma 2 2 (13.56 MHz) at a pressure of 5×10−3 mbar [15–17]. The focused fifth harmonic generation of Nd:YAG lasers (l=213 nm) of 0.8 J/cm2 was used for ablation, as generated by the non-linear optical CsLiB O crystals 6 10 discovered in the laboratory. RF plasma coupled on Mo substrate holder induced negative d.c. bias voltage (V ) on the n++-Si substrate (r#0.002 V cm) and ion b bombardment towards the growth surface. The field emission was measured at RT using typical sphere-toplane configuration with a Mo ball anode (50 mm in diameter), 10 mm away from the sample surface. Such a configuration allowed the examination of a given emission site, but we were unable to estimate the emitting area. Many measurements were repeated by scanning through the film surface. The most reproducible data was selected to represent major emission sites that correlate the structural properties of the film. In contrast, a parallel plate configuration accumulates current from all kinds of emission sites within a given area, which usually indicates lower threshold electric field than that measured by sphere-to-plane configuration [18].
3. Results and discussion 3.1. Effect from the predominant formation of tetrahedral CMN bonds Predominant formation of tetrahedral CMN bonds at 600°C was indicated by Fourier transform infrared ( FTIR) and Raman spectroscopy [15,16 ]. In Fig. 1, IR absorption of (a) RT-prepared CN films is centered at ~1550 cm−1, indicating predominant graphite-like CNN bonds. Likewise, IR absorption of samples prepared at (b) 600°C initially occurred at ~1550 cm−1 and ~1200 cm−1, but transformed later into one centered at ~1265 cm−1. Absorption at ~1550 cm−1 was suppressed, while absorption at ~1200 to 1265 cm−1 was enhanced with the magnitude of the bias voltage (V ). Predominant formation of tetrahedral CMN bonds b happened at V =−120 to −150 V. The mechanism b involved was discussed elsewhere [15,17]. In short, ion bombardment suppresses the size of graphitic structures and increases the content of sp3 bonds that connect the sp2 components. This was verified by the red-shift of G band Raman frequency and the increase of I /I ratio D G [15,16,19]. Preferential doping of nitrogen atoms occurs at the sp3 sites, forming tetrahedral CMN bonds. The related CN bonds were also correlated with X-ray photoelectron spectroscopy ( XPS) and high-resolution electron microscopy (HREM ) [20]. There was no absorption at 2200 cm−1 detected from our samples
Fig. 1. IR spectra of various CN films deposited at both RT (a) and 600°C (b) at different substrate bias voltage (V ). b
deposited at both RT and 600°C, indicating insignificant content of nitrile bonds. Measurement of electron emission was carried out. First, CN film prepared at (a) RT with V =−120 V b was compared with samples deposited at 600°C with (b) zero V and (c) V =−120 V. The current–voltage (I– b b V ) relations are shown in Fig. 2. Sample (a) dominated with graphite-like CNN bonds containing 28 at% nitrogen. Sample (b) consists of mixed CN bonds while sample (c) has predominant tetrahedral CMN bonds containing ~4 and 20 at% nitrogen, respectively. As in Fig. 2, samples (a) and (b) start emitting at ~830 V while sample (c) emits electrons at ~380 V. All samples emit electrons in Fowler–Nordheim (F–N )-like character, as indicated in the inset to Fig. 2. It is interesting that sample (c) emits electrons at lower threshold voltage than sample (a), although both were deposited at V =−120 V. The film thicknesses b were 60 and 56 nm for samples (a) and (c), respectively. Furthermore, both samples are ultra-smooth in surface morphology with a roughness of <0.6 nm r.m.s., as measured by atomic force microscopy (AFM ). Thus, it is possible to relate the lower threshold voltage of sample (c) to the predominant tetrahedral CMN bonds. The superior performance of sample (c) over sample (b) is due to its higher donor density, especially for those donors bonded in a tetrahedral structure. Control samples of amorphous carbon (a-C ) films deposited in Ar gas at ambient pressure 5×10−3 mbar at RT and 600°C (at zero V ) were tested as well. These b a-C films are merely zero in optical gap (<0.2 eV ) and
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Fig. 3. Typical IR spectra of two CN films deposited at both RT (a) before and (b) after annealing. Absorption at positions (i), (ii) and (iii) is induced by graphite-like, tetrahedral and aliphatic-like CN bonds, respectively. Fig. 2. Current–voltage relation of three CN films prepared at (a) RT with V =−120 V, (b) 600°C with V =0 V and (c) 600°C with b b V =−120 V. The related Fowler–Nordheim (F–N ) plots are indicated b in the inset.
low in resistance (<32 V cm). DLC films can be prepared in vacuum at much higher laser density (>5 J/cm2). No consistent electron emission was detected from all these a-C films within the tested voltage range. It is possible that graphitic basal planes of these a-C films are aligned in a direction nearly parallel to the substrate surface. Since resistance across the planes is 3000 times higher than that along the planes, electrons do not flow in the films preferentially in the direction of applied electric field, thus avoiding effective electron transport across these a-C films. Thus electron emission from CN films in Fig. 2 was related to the CN bonds, as discussed earlier. 3.2. Effect from the suppression of graphite-like CNN bonds Annihilation of graphite-like CNN bonds occurred by annealing the RT-prepared samples. The content of tetrahedral CMN bonds remained after annealing. In Fig. 3, absorption at positions (i), (ii) and (iii) is seen, representing graphite-like, tetrahedral and aliphatic-like [9,20] CN bonds, respectively. As shown, absorption at position (i) reduced after annealing, while absorption at position (ii) was unvaried. Absorption at position (iii) appeared after annealing, indicating rupture of some graphite-like CNN bonds into the sp2-bonded
aliphatic-like structure. A detailed description was given elsewhere [17]. Four samples were compared for their field emission characters with (a) and (b) synthesized at RT with V =0 and −120 V, respectively. Samples (c) and (d ) b were deposited together with sample (a) and (b), respectively, but subjected to annealing after deposition. Thus samples (a) and (c) have identical film thickness of 70 nm, while samples (b) and (d ) were 60 nm thick. The nitrogen contents of annealed samples (c) and (d ) were ~7 and 15 at%, respectively; about half the content before annealing. The surface morphologies (about 0.7 nm r.m.s.) of annealed films were not changed after annealing, as confirmed by AFM. Annealing was performed at 600°C for 1 h. Like the a-C films discussed in a previous section, all CN samples (a) to (d ) have merely zero optical gap and low resistance (<30 V cm). The related I–V characteristics are given in Fig. 4. The annealed samples (c) and (d ) indicate lower emission threshold voltage than their counterparts (a) and (b), respectively. Since the contents of tetrahedral CMN bonds, optical gap and resistance were merely unvaried after annealing between samples (a) and (c) as well as samples (b) and (d ), the enhanced performance of annealed samples is related to the annihilation of graphite-like CNN bonds. Sample (d ) emits electrons from a threshold voltage of ~346 V, 2.4 times lower than that of sample (b). An identical improvement was discussed in Fig. 2, which did not involve aliphatic-like CN bonds. Thus, the contribution of aliphatic-like CN bonds in the present case might not be significant. Anyway, sample
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Fig. 4. Current–voltage relation of four CN films prepared at (a) RT with V =0 V and (b) RT with V =−120 V. Samples (c) and (d ) were b b deposited together with sample (a) and (b), respectively, but subjected to annealing after deposition. The related F–N plots are indicated in the inset.
(d ) in this case has slightly lower nitrogen content than sample (c) in Fig. 2, but performed better than the latter. Thus, future experiments are needed to clarify quantitatively the influence of aliphatic-like CN bonds. The better performance of sample (d ) than sample (c) is due to the higher content of donors, especially those bonded in a tetrahedral structure.
density scattered within the film. The shortest conduction path occurs preferentially in the direction of the applied electric field. Thus, electrons reach the film surface with sufficient energy to tunnel through the surface potential barrier at relatively low electric field. This is illustrated schematically in Fig. 5a. Note that
3.3. Relation between CN bonds and electron field emission In summary, tetrahedral CN films initiate electron emission at lower threshold electric field. A higher content of tetrahedral CMN bonds induced field emission at lower threshold field. Likewise, suppression of graphite-like CNN bonds indicated a similar phenomenon. A possible explanation is given. First, predominant tetrahedral CMN bonds provide distributed donor sites in all directions. As electric field is applied, an ionized path will be formed across the film from the cathode (substrate) towards the film surface. A higher content of tetrahedral CMN bonds signifies a higher donor
Fig. 5. Schematic representation of conducting paths formed by (a) predominant tetrahedral CMN bonds and (b) mixed graphite-like CNN and tetrahedral CMN bonds.
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such a preferential conducting path appears as dotted areas on the film surface which induced local field enhancement. The graphite-like CNN bonds degrade electron emission. Owing to the much higher conductivity within basal planes of graphite-like structures than that across the planes, the donor path within the basal planes is more likely to exhibit effective electron flow. Consistent with our discussion on a-C films at the end of Section 3.1, it is possible that graphite-like CNN bonds in our samples are preferentially grown within the graphitic planes along the direction of the substrate surface. For CN films dominated by graphite-like CNN bonds, velocity components perpendicular to the electric field are increased. Thus, electron flow across the film drifts away from the field direction and is transported through a longer conducting path. Electrons lose their potential energies while drifting through the films, and thus require a higher supply of electrical energy to escape into the vacuum. The schematic representation of such a condition is shown in Fig. 5b. This explained reduction of the threshold field occurs when graphitelike CNN bonds are eliminated by annealing.
4. Conclusions CN films with predominant tetrahedral CMN bonds were observed to emit electrons at lower threshold electric field. A higher density of tetrahedral CMN bonds reduced the threshold electric field further for electron emission. Likewise, elimination of the graphitelike CNN bonds formed on graphitic planes, aligned nearly parallel to the substrate surface, can enhance electron emission. A model based on the structural arrangement of CN binding states is proposed to explain the related mechanism. Graphitic structures are known as a preferential conducting path for electron transport, but the related basal planes must be aligned in the direction of the applied electric field.
Acknowledgement The authors acknowledge Akimitsu Hatta of Kochi University of Technology for the measurement of the I– V relation discussed in this work.
References [1] B.S. Satyanarayana, A. Hart, W.I. Milne, J. Robertson, Appl. Phys. Lett. 71 (1997) 1430. [2] G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 (1996) 2529. [3] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. [4] S.R.P. Silva, G.A.J. Amaratunga, J.R. Barnes, Appl. Phys. Lett. 71 (1997) 1477. [5] E.J. Chi, J.Y. Shim, H.K. Baik, S.M. Lee, Appl. Phys. Lett. 71 (1997) 324. [6 ] E.J. Chi, J.Y. Shim, D.J. Choi, H.K. Baik, J. Vac. Sci. Technol. B 16 (1998) 1219. [7] J.J. Cuomo, P.A. Leart, D. Yu, W. Reuter, M. Frish, J. Vac. Sci. Technol. 16 (1979) 299. [8] J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 (1989) 13 053. [9] Y. Taki, T. Kitagawa, O. Takai, Thin Solid Films 304 (1997) 183. [10] M.R. Wixom, J. Am. Ceram. Soc. 73 (1990) 1973. [11] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Raffererty, L.M. Brown, J. Schwan, D.F. Franceschini, G. Mariotto, J. Appl. Phys. 81 (1997) 2626. [12] J. Hu, P. Yang, C.M. Lieber, Phys. Rev. B 57 (1998) R3185. [13] Z.J. Zhang, S. Fan, J. Huang, C.M. Lieber, Appl. Phys. Lett. 68 (1996) 2639. [14] D.G. McCulloch, A.R. Merchant, Thin Solid Films 290/291 (1996) 99. [15] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915. [16 ] Y.K. Yap, Y. Mori, S. Kida, T. Aoyama, T. Sasaki, J. Cryst. Growth 198/199 (1999) 1028. [17] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Diamond Relat. Mater. 8 (1999) 614. [18] A. Hart, B.S. Satyanarayana, W.I. Milne, J. Robertson, Appl. Phys. Lett. 74 (1999) 1594. [19] Y.K. Yap, Ph.D. Thesis, Osaka University, 1999. [20] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 37 (1998) L746.
Effect of carbon nitride bonding structure on electron field emission Y.K. Yap *, S. Kida, Y. Wada, M. Yoshimura, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan
Abstract Predominant formation of tetrahedral CMN bonds was obtained by in situ ion bombardment at 600°C. Tetrahedral carbon nitride (CN ) films emit electrons at lower threshold field compared with RT-prepared samples, that dominated by graphite-like CNN bonds. Likewise, annealing RT-prepared CN films can eliminate the graphite-like CNN bonds while the content of tetrahedral CMN bonds remains. Field emission from annealed films occurred at lower threshold field as well. It is suggested that graphite-like CNN bonds within graphitic basal planes that are aligned nearly parallel to the substrate surface must be suppressed to avoid electron drifting in the direction perpendicular to the applied electric field. Furthermore, randomly distributed sp3 CMN bonds appear to enable conducting path formation preferentially along the direction of the electric field. A model is proposed to explain the improvement in electron field emission. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Carbon; Carbon nitride; Electron field emission; Nitride
1. Introduction There have been many investigations on the development of cold cathode materials for electron field emission. Amorphous materials such as diamond-like carbon (DLC ) and carbon nitride (CN ) are possibilities. Unlike crystallized films, DLC and CN can be synthesized at room temperature (RT ). Thus both materials can be coated on glass and even plastic substrates, which are required for mass-production of large area, low cost flat panel displays. These materials are chemically inert and mechanically robust as coating materials for Si and Mo electron field emitters as well. The synthesis of DLC films and the influence of binding state on field emission have been explained [1]. On the other hand, deposition of CN films is referred to as nitrogen doping of carbon films that enhances the field emission ability of the carbon phase [2]. Most of these films are prepared at RT and have attracted tremendous attention in the past decade, as motivated by the search for the predicted C N compound [3]. 3 4 However, CN films synthesized at RT consist of various binding states. Despite this, CN films were widely tested to be outstanding for electron field emission [1,2,4–6 ]. Owing to their random chemical nature, the influence * Corresponding author. Fax: +81-6-879-77-08. E-mail address: [email protected] ( Y.K. Yap)
of individual CN binding state on electron emission is still relatively unexplored. A better knowledge of the controlling CN bond formation and characterization related emission properties is thus desired. Non-destructive IR spectroscopy has been commonly used to examine CN related bonds. A broad absorption from ~1100 to 1600 cm−1 was usually detected [5–13]. Except for the cynogen-like CNN (nitrile) bond [7] at ~2200 cm−1, the pyridine/graphite-like CNN bond [8,9] at ~1550 cm−1 and the tetrahedral CMN bond [9,10] at ~1212 to 1265 cm−1 were included within the absorption band. Recent study revealed preferential formation of graphite-like CNN bonds, as indicated by both IR absorption and electron energy loss spectroscopy ( EELS ) [11,12]. The nitrile bond was known to be eliminated by annealing at ~550°C [13]. Likewise, annealing CN films at temperatures above ~400°C causes graphitization and the rejection of incorporated nitrogen from the graphite-like CNN bonds [14]. The tetrahedral CMN bonds are least understood in terms of synthesis requirement, physical and chemical natures. Recently, we detected transformation of CN bonds from films deposited with in situ ion bombardment at 600°C [15,16 ]. Further, annihilation of graphite-like CNN bonds was detected by annealing the RT-prepared CN films [17]. Based on these techniques, the effect of CN bonds on electron field emission was investigated in this work.
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2. Experiment All CN samples were prepared by pulsed laser deposition (PLD) in pure N gas or N RF plasma 2 2 (13.56 MHz) at a pressure of 5×10−3 mbar [15–17]. The focused fifth harmonic generation of Nd:YAG lasers (l=213 nm) of 0.8 J/cm2 was used for ablation, as generated by the non-linear optical CsLiB O crystals 6 10 discovered in the laboratory. RF plasma coupled on Mo substrate holder induced negative d.c. bias voltage (V ) on the n++-Si substrate (r#0.002 V cm) and ion b bombardment towards the growth surface. The field emission was measured at RT using typical sphere-toplane configuration with a Mo ball anode (50 mm in diameter), 10 mm away from the sample surface. Such a configuration allowed the examination of a given emission site, but we were unable to estimate the emitting area. Many measurements were repeated by scanning through the film surface. The most reproducible data was selected to represent major emission sites that correlate the structural properties of the film. In contrast, a parallel plate configuration accumulates current from all kinds of emission sites within a given area, which usually indicates lower threshold electric field than that measured by sphere-to-plane configuration [18].
3. Results and discussion 3.1. Effect from the predominant formation of tetrahedral CMN bonds Predominant formation of tetrahedral CMN bonds at 600°C was indicated by Fourier transform infrared ( FTIR) and Raman spectroscopy [15,16 ]. In Fig. 1, IR absorption of (a) RT-prepared CN films is centered at ~1550 cm−1, indicating predominant graphite-like CNN bonds. Likewise, IR absorption of samples prepared at (b) 600°C initially occurred at ~1550 cm−1 and ~1200 cm−1, but transformed later into one centered at ~1265 cm−1. Absorption at ~1550 cm−1 was suppressed, while absorption at ~1200 to 1265 cm−1 was enhanced with the magnitude of the bias voltage (V ). Predominant formation of tetrahedral CMN bonds b happened at V =−120 to −150 V. The mechanism b involved was discussed elsewhere [15,17]. In short, ion bombardment suppresses the size of graphitic structures and increases the content of sp3 bonds that connect the sp2 components. This was verified by the red-shift of G band Raman frequency and the increase of I /I ratio D G [15,16,19]. Preferential doping of nitrogen atoms occurs at the sp3 sites, forming tetrahedral CMN bonds. The related CN bonds were also correlated with X-ray photoelectron spectroscopy ( XPS) and high-resolution electron microscopy (HREM ) [20]. There was no absorption at 2200 cm−1 detected from our samples
Fig. 1. IR spectra of various CN films deposited at both RT (a) and 600°C (b) at different substrate bias voltage (V ). b
deposited at both RT and 600°C, indicating insignificant content of nitrile bonds. Measurement of electron emission was carried out. First, CN film prepared at (a) RT with V =−120 V b was compared with samples deposited at 600°C with (b) zero V and (c) V =−120 V. The current–voltage (I– b b V ) relations are shown in Fig. 2. Sample (a) dominated with graphite-like CNN bonds containing 28 at% nitrogen. Sample (b) consists of mixed CN bonds while sample (c) has predominant tetrahedral CMN bonds containing ~4 and 20 at% nitrogen, respectively. As in Fig. 2, samples (a) and (b) start emitting at ~830 V while sample (c) emits electrons at ~380 V. All samples emit electrons in Fowler–Nordheim (F–N )-like character, as indicated in the inset to Fig. 2. It is interesting that sample (c) emits electrons at lower threshold voltage than sample (a), although both were deposited at V =−120 V. The film thicknesses b were 60 and 56 nm for samples (a) and (c), respectively. Furthermore, both samples are ultra-smooth in surface morphology with a roughness of <0.6 nm r.m.s., as measured by atomic force microscopy (AFM ). Thus, it is possible to relate the lower threshold voltage of sample (c) to the predominant tetrahedral CMN bonds. The superior performance of sample (c) over sample (b) is due to its higher donor density, especially for those donors bonded in a tetrahedral structure. Control samples of amorphous carbon (a-C ) films deposited in Ar gas at ambient pressure 5×10−3 mbar at RT and 600°C (at zero V ) were tested as well. These b a-C films are merely zero in optical gap (<0.2 eV ) and
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Fig. 3. Typical IR spectra of two CN films deposited at both RT (a) before and (b) after annealing. Absorption at positions (i), (ii) and (iii) is induced by graphite-like, tetrahedral and aliphatic-like CN bonds, respectively. Fig. 2. Current–voltage relation of three CN films prepared at (a) RT with V =−120 V, (b) 600°C with V =0 V and (c) 600°C with b b V =−120 V. The related Fowler–Nordheim (F–N ) plots are indicated b in the inset.
low in resistance (<32 V cm). DLC films can be prepared in vacuum at much higher laser density (>5 J/cm2). No consistent electron emission was detected from all these a-C films within the tested voltage range. It is possible that graphitic basal planes of these a-C films are aligned in a direction nearly parallel to the substrate surface. Since resistance across the planes is 3000 times higher than that along the planes, electrons do not flow in the films preferentially in the direction of applied electric field, thus avoiding effective electron transport across these a-C films. Thus electron emission from CN films in Fig. 2 was related to the CN bonds, as discussed earlier. 3.2. Effect from the suppression of graphite-like CNN bonds Annihilation of graphite-like CNN bonds occurred by annealing the RT-prepared samples. The content of tetrahedral CMN bonds remained after annealing. In Fig. 3, absorption at positions (i), (ii) and (iii) is seen, representing graphite-like, tetrahedral and aliphatic-like [9,20] CN bonds, respectively. As shown, absorption at position (i) reduced after annealing, while absorption at position (ii) was unvaried. Absorption at position (iii) appeared after annealing, indicating rupture of some graphite-like CNN bonds into the sp2-bonded
aliphatic-like structure. A detailed description was given elsewhere [17]. Four samples were compared for their field emission characters with (a) and (b) synthesized at RT with V =0 and −120 V, respectively. Samples (c) and (d ) b were deposited together with sample (a) and (b), respectively, but subjected to annealing after deposition. Thus samples (a) and (c) have identical film thickness of 70 nm, while samples (b) and (d ) were 60 nm thick. The nitrogen contents of annealed samples (c) and (d ) were ~7 and 15 at%, respectively; about half the content before annealing. The surface morphologies (about 0.7 nm r.m.s.) of annealed films were not changed after annealing, as confirmed by AFM. Annealing was performed at 600°C for 1 h. Like the a-C films discussed in a previous section, all CN samples (a) to (d ) have merely zero optical gap and low resistance (<30 V cm). The related I–V characteristics are given in Fig. 4. The annealed samples (c) and (d ) indicate lower emission threshold voltage than their counterparts (a) and (b), respectively. Since the contents of tetrahedral CMN bonds, optical gap and resistance were merely unvaried after annealing between samples (a) and (c) as well as samples (b) and (d ), the enhanced performance of annealed samples is related to the annihilation of graphite-like CNN bonds. Sample (d ) emits electrons from a threshold voltage of ~346 V, 2.4 times lower than that of sample (b). An identical improvement was discussed in Fig. 2, which did not involve aliphatic-like CN bonds. Thus, the contribution of aliphatic-like CN bonds in the present case might not be significant. Anyway, sample
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Fig. 4. Current–voltage relation of four CN films prepared at (a) RT with V =0 V and (b) RT with V =−120 V. Samples (c) and (d ) were b b deposited together with sample (a) and (b), respectively, but subjected to annealing after deposition. The related F–N plots are indicated in the inset.
(d ) in this case has slightly lower nitrogen content than sample (c) in Fig. 2, but performed better than the latter. Thus, future experiments are needed to clarify quantitatively the influence of aliphatic-like CN bonds. The better performance of sample (d ) than sample (c) is due to the higher content of donors, especially those bonded in a tetrahedral structure.
density scattered within the film. The shortest conduction path occurs preferentially in the direction of the applied electric field. Thus, electrons reach the film surface with sufficient energy to tunnel through the surface potential barrier at relatively low electric field. This is illustrated schematically in Fig. 5a. Note that
3.3. Relation between CN bonds and electron field emission In summary, tetrahedral CN films initiate electron emission at lower threshold electric field. A higher content of tetrahedral CMN bonds induced field emission at lower threshold field. Likewise, suppression of graphite-like CNN bonds indicated a similar phenomenon. A possible explanation is given. First, predominant tetrahedral CMN bonds provide distributed donor sites in all directions. As electric field is applied, an ionized path will be formed across the film from the cathode (substrate) towards the film surface. A higher content of tetrahedral CMN bonds signifies a higher donor
Fig. 5. Schematic representation of conducting paths formed by (a) predominant tetrahedral CMN bonds and (b) mixed graphite-like CNN and tetrahedral CMN bonds.
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such a preferential conducting path appears as dotted areas on the film surface which induced local field enhancement. The graphite-like CNN bonds degrade electron emission. Owing to the much higher conductivity within basal planes of graphite-like structures than that across the planes, the donor path within the basal planes is more likely to exhibit effective electron flow. Consistent with our discussion on a-C films at the end of Section 3.1, it is possible that graphite-like CNN bonds in our samples are preferentially grown within the graphitic planes along the direction of the substrate surface. For CN films dominated by graphite-like CNN bonds, velocity components perpendicular to the electric field are increased. Thus, electron flow across the film drifts away from the field direction and is transported through a longer conducting path. Electrons lose their potential energies while drifting through the films, and thus require a higher supply of electrical energy to escape into the vacuum. The schematic representation of such a condition is shown in Fig. 5b. This explained reduction of the threshold field occurs when graphitelike CNN bonds are eliminated by annealing.
4. Conclusions CN films with predominant tetrahedral CMN bonds were observed to emit electrons at lower threshold electric field. A higher density of tetrahedral CMN bonds reduced the threshold electric field further for electron emission. Likewise, elimination of the graphitelike CNN bonds formed on graphitic planes, aligned nearly parallel to the substrate surface, can enhance electron emission. A model based on the structural arrangement of CN binding states is proposed to explain the related mechanism. Graphitic structures are known as a preferential conducting path for electron transport, but the related basal planes must be aligned in the direction of the applied electric field.
Acknowledgement The authors acknowledge Akimitsu Hatta of Kochi University of Technology for the measurement of the I– V relation discussed in this work.
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