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Microstructure and electronic properties of pulsed-discharge-deposited amorphous carbon-nitride films
Diamond & Related Materials 14 (2005) 1616 – 1622 www.elsevier.com/locate/diamond
Microstructure and electronic properties of pulsed-discharge-deposited amorphous carbon-nitride films Zongyi Qin1, Peinan Wang*, Hong Shen, Lan Mi, Xuantong Ying State Key Lab for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China Received 30 September 2004; received in revised form 12 April 2005; accepted 9 May 2005 Available online 2 August 2005
Abstract Amorphous carbon-nitride films were grown on the nitridated-diamond substrates by pulsed-discharge of pure nitrogen gas using graphite rods as the discharge electrodes. The deposition conditions were optimized by monitoring the discharge plasma with optical-emission spectroscopy. It is demonstrated that the films were mainly a mixture of nanosized carbon and carbon-nitride with sp2 and sp3 phases. Preliminary results show that the deposited carbon-nitride films exhibit semiconductor behavior and have a cold-cathode-emission property, which make them possible to be superior electronic materials. Improvement in the conductivity and field-emission properties was observed after chemical etching with hydrofluoric acid. The activation energy for electrical conduction of the HF-treated film decreased from 3.42 to 1.19 eV. The similar threshold voltages were obtained for the carbon-nitride films before and after chemical etching, which were 4.0 and 3.5 V/Am, respectively. However, the emission current density after etching increased by one order of magnitude. D 2005 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 61.43.Er; 61.46.+w; 73.22.-f Keywords: Amorphous carbon-nitride; Diamond film; Etching; Field emission
1. Introduction Over the last 10 years, worldwide experimental and theoretical interests have been focused on the synthesis and properties of carbon-nitride materials due in part to the possibility of synthesizing the h-C3N4 phase, which was theoretically predicted to have an exceptional hardness comparable to or greater than that of diamond [1 –10]. Most of the experiments resulted in amorphous carbon-nitride structures, although in a few cases polycrystalline h-C3N4 structures embedded in the amorphous carbon-nitride (aC:N) matrix were observed [5 –7]. Actually, the evidence for the existence of crystalline carbon-nitride can also be truly indexed to different carbon phases, since the varieties T Corresponding author. E-mail addresses: [email protected] (Z. Qin), [email protected] (P. Wang). 1 Current address: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.05.004
of different bonding configurations are close in energy. Theoretical calculations have further predicted the existence of other stable crystalline carbon-nitride structures, such as the hexagonal, the defect cubic zincblende, and the rhombohedral structures [9,10]. In spite of the difficulties to produce crystalline h-C3N4, amorphous carbon-nitride films also exhibit promising mechanical properties such as high hardness and excellent elastic recovery, as well as other tribological properties. In recent years, carbon-based electron field emission has been expected to be a new electron source for the flat panel display, electronic and optoelectronic devices, and so on. It is well known that nitrogen incorporation into amorphous carbon films significantly reduces the stress, electrical resistivity, and optical band gap [11– 13]. Thus, carbonnitride has attracted great engineering interest as a superior electronic material [13 – 15]. Although no success in preparing carbon-nitride films with a single bonding state or a definitive atomic structure has been reported so far, the need for both sp2 and sp3 phases for good field emission
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was in fact emphasized in many recent reports [16 – 18]. In our previous work, we found that the bonding configuration in the substrate was crucial to the structure of the film produced, the sp3 bonded diamond substrate promoted the formation of sp3 bonded carbon-nitrides [6,19]. Therefore, the diamond film was used as substrate in this work to promote the formation of sp3 bonded carbon-nitride. It is worth pointing out that carbon-nitrides as an electronic material was usually required to fabricate dense and conductive films on large-area substrates with high growth rate and at low temperature. It has been shown that the impact of preparation parameters has a large influence on the chemical composites, microstructures and most properties of the synthesized films. Hence, various techniques have been employed in attempts to investigate nitrogen plasma or nitrogen radical effects on the properties and to improve the nitrogen content of the films. In the synthesis of nitrides, atomic nitrogen is believed to play an important role due to its high chemical reactivity. Discharge is one of the most widely used techniques, because nitrogen molecules are highly dissociated and activated in the discharge plasma with a high temperature and high energy density. Various discharge methods provide the possibility of fabricating films of widely varying properties depending on the discharge parameters applied, and even creating new type of carbon-nitride structure [15 –20]. In the present work, pulsed discharge was employed to prepare carbon-nitride films. With its high discharge voltage and current density, N2 could be effectively dissociated by pulsed discharge to produce abundant reactive atomic nitrogen species. By rapid expansion and quenching of the high-temperature-reaction plasma, it is possible to produce materials containing metastable phases, nanometer structures, and multi-component materials [21,22]. The discharge parameters were optimized by monitoring the discharge plasma with optical-emission spectroscopy. The structures of the produced carbon-nitride films were characterized and their electronic properties were studied. The influence of chemical etching on the synthesized carbon-nitride film is also discussed.
2. Experiment Carbon-nitride was synthesized on a diamond surface by a pulsed discharge of high-purity N2 using graphite rods as the electrodes. The pulsed-discharge circuit was switched by a spark gap, and discharge parameters could be easily adjusted over a wide range by changing the circuit parameters and the conditions of the spark gap (gap distance and gas pressure). Additional details concerning the pulsed-discharge system have been described in our previous work [21,23]. Compared with other discharge methods, pulsed discharge can dissociate N2 molecules much more efficiently. We have success-
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fully synthesized nanosized nitrides using the method of pulsed discharge [21,23]. In the present work, the chamber was evacuated to below 0.13 Pa and then filled with flowing N2. The pressure of N2 was controlled by the flow rate of the nitrogen gas. The discharge plasma was monitored by optical-emission spectroscopy. The discharge condition was optimized by spectral diagnostics as described in our previous work [21,23]. Nitrogen gas was fully dissociated when the nitrogen pressure was above 3 kPa and the discharge voltage was 5 kV, which were chosen as the discharge parameters for the film deposition in this work. All the films were deposited at room temperature on the nitridated-diamond substrate while no deliberate external bias voltage was applied to the substrate. It is believed that the nitridated-diamond substrate, which contains the hC3N4 phase, favors the formation of the h-C3N4 phase during the film growth [24] and improves the adhesion to the substrate. Initially, a hot filament chemical vapor deposited (CVD) diamond film on a silicon substrate with a diameter of 20 mm was nitridated by a pulsed discharge of pure nitrogen gas at 9 kPa with tungsten electrodes for 7 h at a repetition rate of 10 Hz. Tungsten electrodes were then replaced by graphite electrodes and the N2 pressure in the chamber was reduced to 3 kPa for deposition of carbon-nitride films. The substrate was placed parallel to the discharge plasma column and about 10 mm from the center of the column. The thickness of the deposited film was estimated to be approximately 300 nm for the deposition time of 1 h as measured by ellipsometry. Chemical etching of carbon-nitride film was carried out for 3 min with 2% hydrofluoric acid and then rinsed by distilled water and dried by pure nitrogen gas. The estimated thickness of the deposited film decreased from 300 to about 200 nm after etching. The deposited film was slightly brownish. The substrate with carbon-nitride film deposited was cut into small pieces. Some pieces were examined via atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman analyses, while others were used for the measurement of physical properties, such as conductivity, field emission, and etc. The chemical composition and bonding states of the films before and after chemical etching in the hydrofluoric acid were determined by XPS in a VG ESCALAB 220i-XL spectrometer by using Mg Ka1,2 (1253.6 eV) excitation. Surface morphology of the films was investigated by AFM (Digital Instruments, Dimension 3100), which was used in its tapping mode configuration, in air and at room temperature. The measurement of the Raman spectra of the films was carried out in the backscattering mode with a RM3000 (Renishaw) spectrometer (excitation 633 nm). The temperature dependence of the electrical conductivity of the deposited film was measured by means of a fourpoint probe technique in a liquid-nitrogen cryostat. The field-emission measurement in a parallel-plate configuration
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was performed in a vacuum of 10 5 Pa by using the deposited film as a cathode and collecting the emitted electrons on an indium tin oxide coated glass anode with an effective collection diameter of 5 mm and the spacing distance between the anode and the cathode of 50 Am. The voltage between the anode and the cathode was varied while simultaneously recording the current using combined source/measure electrometers.
3. Results and discussion Fig. 1 shows the emission spectra of the nitrogen discharge plasma at a pressure of 3 kPa with a pair of tungsten (a) and graphite (b) electrodes, respectively. For tungsten electrodes (Fig. 1(a)), nearly all the strong lines in the emission spectra corresponded to atomic nitrogen ions, and the molecular lines were too weak to be observed. The nitrogen molecules were highly dissociated due to the high temperature (3 103 K) and high electron density (1016 cm 3) in the pulsed-discharge plasma [23]. With graphite electrodes, very intensive CN bands and carbon peaks were observed in the emission spectrum while N+ peaks almost disappeared as shown in Fig. 1(b), indicting that a strong reaction between atomic nitrogen and sputtered carbon species occurred in the plasma. Subsequently, the deposition of carbon-nitride films was
Fig. 2. AFM images for the deposited films before (a) and after (b) chemical etching.
Fig. 1. Optical-emission spectra from the plasma in the pulsed discharge of N2 using tungsten rods (a) and graphite rods (b) as electrodes, respectively. The various molecular and ionic lines are labeled.
carried out with the expansion of the pulsed plasma. A marked similarity has been reported between the content of carbon-nitride in the film and the fraction of CN in the plasma [18]. It is well known that the microstructure and the property of the deposited film depend strongly on the preparation parameters, such as applied voltage, substrate temperature, and gas pressure. However, no attempt such as heating the substrate, applying a bias voltage or shielding the large particles to the substrate was made in our experiment. Thus, an amorphous structure with a certain amount of grains was observed on the deposited film surface. The AFM exami-
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nation of the deposited film, as demonstrated in Fig. 2, indicates that the film surface was relatively rough. The roughness of 248 nm (RMS) over an area of 100 100 Am2 is believed to result from the large particles formed on the film surface during deposition as well as the original rough surface of the polycrystalline micro-diamond substrate, because diamond grows only in faceted grains, not in a smooth layer [25]. Chemical etching was found to remove the majority of loosely bonded carbon particles and some amorphous materials [16]. One can observe the difference of the surface morphology before and after etching from the AFM images (Fig. 2). After etching, the grain size became smaller and more uniform, which would enhance the field emission effectively. The XPS measurements have been performed in order to obtain further information about chemical composites and bonding states [7,26 –28]. As shown in Fig. 3, widescan spectra reveal that the deposited film before chemical etching was composed primarily of carbon and nitrogen, with a small amount of oxygen contamination. After chemical etching, the oxygen peak vanished and the nitrogen-to-carbon ratio, which was obtained by integrating the core-level peak area of N (1s) and C (1s), corrected by the atomic sensitivity factors [(ASF) C: 0.296 and N: 0.477], increased from 16% to 22%. It is evident that nitrogen was incorporated into the films, while the oxygen was the surface contaminant due to the exposure of the samples to air prior to analysis [29]. Raman spectroscopy is a commonly used technique to analyze carbon-related materials. As shown in Fig. 4, the Raman spectra of our synthesized film exhibit a broad asymmetric band between 1000 and 1700 cm 1, further confirming that various components were contained in the film. The Raman spectra before and after chemical etching can be deconvoluted into four bands. In addition to the commonly observed D band at 1360 cm 1 and G band at 1570 cm 1 for amorphous carbon, there is strong evidence of the diamond band at 1332 cm 1 and a well-identified peak near 1507 cm 1 between the D and G bands.
Fig. 3. Wide-scan XPS spectra before (a) and after (b) chemical etching.
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Fig. 4. Deconvoluted Raman spectra of carbon-nitride films before (a) and after (b) chemical etching.
The typical G band associates with the optically allowed E 2g zone-center mode of crystalline graphite, representing a relatively high content of sp2 carbon while the D band associates with the disorder-allowed zone-edge modes of graphite, representing mainly sp2 — with a small amount of sp3-carbon [8,30 – 32]. The need for both sp2 and sp3 phases for good field emission was emphasized in some recent reports [16,31]. Either sp2- or sp3-carbon atoms can be bonded to nitrogen to form carbon-nitride in the films. The graphite structure in the film could be understood from the existence of carbon species in the discharge plasma, as shown in Fig. 1(b). The sputtered carbon species was not fully converted to the CN radicals, but deposited on the substrate. The intermediate band at 1507 cm 1 was unambiguously originated from the incorporation of nitrogen into the carbon network and was designated as the nitrogen band [30]. The diamond peak at 1332 cm 1 might result from the diamond substrate since the deposited film is very thin, or from the nanosized diamond formed during the deposition. In fact, there can be observed a weak band at around 1150 cm 1, which is regarded as an evidence of nano-diamond character [31]. As can easily be seen in Fig. 4, the intensities of the diamond and nitrogen bands increased after chemical etching. The increase of the diamond signal might be due to the reduction of the film thickness by removing some
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Fig. 5. Temperature dependences of electrical conductivity of carbon-nitride films before and after chemical etching.
surface particles or the increase of diamond content near the diamond substrate. The increase of carbon-nitride intensity can be attributed to the increase of nitrogen content near the diamond surface, which is consistent with the wide-scan XPS analysis mentioned above. Generally, the average size of the graphite microcrystallites can be estimated according to the I(D) / I(G) ratio [32,33]. As described in Refs. [32] and [33], due to the two competing phenomena, there is a critical ratio R crit = I(D) / I(G) å 2.2, corresponding to a critical average size of about 2 nm. If the average size is smaller than the critical size of 2 nm, it increases with the increasing I(D) / I(G) ratio. In contrast, the average size decreases with the increasing I(D) / I(G) ratio if it is larger than 2 nm [32,33]. In our Raman spectra, as shown in Fig. 4, a reduction in G bandwidth and a decrease of I(D) / I(G) ratio was observed after chemical etching, indicating a decrease in the average size of the graphite microcrystallites [34], which matches the case that the average size was less than the critical size. Hence, the average sizes of those crystallites in the films before and after chemical etching can be estimated to be approximately 1.5 and 1.2 nm, respectively. However, large uncertainties are associated with this estimation and there exists a relatively wide size distribution in the pulseddischarge deposition as demonstrated in our previous work [35]. The relationship between electrical conductivity and temperature can provide important information concerning the nature of the charge-transport-related phenomena. Fig. 5 shows the logarithm of the conductivity versus temperature (T 1/4) before and after chemical etching. The ln(r) shows an almost linear dependence on T 1/4, revealing that the carrier transport in the film was dominated by variable-range hopping conduction with a semiconducting character in the presence of local disorders [36,37]. The conduction by tunneling or phonon-assisted hopping between localized states, rather than through propagation in extended states, is expected throughout a broad range of temperatures for the deposited films. Charge carriers are
likely to be localized throughout the entire range of the sp2 clusters and the dangling bond states within the band gaps associated with sp2 and sp3 sites. This model is supported by the above characterization results of the films. The resistivity of the film after etching decreased from 23 to 0.55 Vcm at 300 K in this work. The increase of electrical conductivity after chemical etching can be attributed to the increase of N content as well as the better contact with measuring electrode due to the removal of surface contaminants and those loosely bonded carbon particles. Nitrogen in amorphous carbon films usually acts as an n-type shallow donor below the conduction band of amorphous carbon and to alter the joint density of states in the film [38]. Nitrogen does not cause more clustering of sp2 sites, but more crosslinking, and thus increased disorders, even though not necessarily through a significant increase of sp3 sites. This is in agreement with our Raman results, which show the increase of nitrogen content but the similar estimated average size of the graphite nanocrystallites before and after chemical etching. In our deposited films, a relatively low resistivity of about 0.55 Vcm indicates that the films presumably contain a fraction of sp3 sites in an amorphous sp2 matrix, which was suggested as an optimized microstructure for field emission [39]. The activation energy of the film after chemical etching decreased from 3.42 to 1.19 eV, which can be derived from the slope of the fitted line in Fig. 5. Fig. 6 shows the field-emission current density versus electric field of the deposited films before and after chemical etching. It is evident from Fig. 6 that after etching, a remarkable enhancement in the current density and a reduction of field threshold were observed. The field threshold occurred at about 4.0 and 3.5 V/Am, respectively, which is defined as the field to give an emission current density of 10 7 A/cm2. At the same electric field above the thresholds, the emission current density after chemical etching increased by almost one order of magnitude. The degradation of the current density was less than 10% when measured again 1 h later.
Fig. 6. Current density ( J) versus applied electric field (E) for carbonnitride films before and after chemical etching.
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It is well known that emission occurs at localized sites rather than uniformly over the surface. It has been found that carbons with mixed sp3 and sp2 sites, such as nano-diamond and nano-graphitic films, can emit at lower fields and have a higher emission-site density than single-phase films [31]. When an external electric field is applied, the geometrical factors of nanoparticles would cause a redistribution of electrons and field concentration, increasing the electric field strength locally around the areas with higher curvature. Therefore, although the magnitude of the applied external field should not be large enough to induce field emission, its local value in the regions of highest curvature of the nanoparticle could be concentrated just enough to exceed threshold field [16]. Although the mechanism of electron emission from amorphous carbon-nitride films was still under argument, the enhancement of field emission was proved to be associated to the increase of the grain boundary density, sp2-bonded components and defects incorporated in the film [40]. The AFM image of the film after etching in this work exhibits smaller grain size, thereby, the higher grain boundary density. Nitrogen addition can elevate the Fermi level, and consequently, the energy barrier that the electrons must tunnel through is reduced, resulting in emission enhancement [32,40]. Besides, the increase of N content increases the concentration of donor centers, leading to a higher field across the film at a given applied voltage [43]. On the other hand, a thinner film is more favorable for the field emission because the film can be more fully depleted, thus an electron can gain enough energy with respect to the conduction band at low thickness for them to be able to surmount the emission barrier to vacuum. Also, a thinner film is beneficial in that it reduces the scattering of electrons and increases the probability that an electron will be emitted through the emission barrier of the a-C:N/ vacuum interface [41,42]. Hence, the decrease of the
Fig. 7. Fowler – Nordheim plot of carbon-nitride films before and after chemical etching.
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thickness from 300 to 200 nm by chemical etching also enhanced the field emission. The Fowler – Nordheim (F – N) theory is the most commonly used model for cold electron emissions. Fig. 7 demonstrates the F– N plots of the films before and after etching, respectively. The linear F – N characteristic clearly reveals that the films performed a cold field-emission process, in which the electron emission into vacuum is based on the tunneling mechanism. The effective work functions of the films before and after chemical etching, calculated from the slopes of the F –N plots with the dimensionless field enhancement factor of 1, were found to be about 0.050 and 0.046 eV, respectively. As can be seen in Fig. 7, the F – N plot before etching exhibits a downward bending, similar to those demonstrated in some other papers [15,40,41,44]. This downward bending can be understood by considering the resonance-tunneling model [15,44]. In this model, adsorbates lying on the surface of the film produce surface states, which create a resonant tunneling condition for electrons, greatly increasing the local tunneling current at adsorbates and consequently the overall tunneling probability, leading to a steeper slope. However, at a high-field region (low 1/V region), the adsorbate states are completely removed while the resonant is removed well, leading to a decrease in F –N slope [15,44]. For the film after chemical etching, since a majority of surface adsorbates were removed, a better linearity is exhibited in the F – N plot as shown in Fig. 7.
4. Conclusions In conclusion, amorphous carbon-nitride films were grown on the nitridated-diamond surface by pulsed-discharge deposition. The deposited amorphous carbon-nitride films exhibit a good semiconductor behavior and a coldcathode-emission process, which make it possible for these carbon-nitride films to be an excellent electron emitter material. These superior properties are believed to result from this film_s unique microstructures, which mainly consist of sp2- and sp3-carbon-nitrides and graphite nanocrystallites. It well satisfied the demand of an ideal material for a cold-cathode emitter, in which an optimized microstructure should consist of nanosized sp2 clusters as the conductor and n-type doped wide-gap sp3 sites as the field emitter. The chemical etching greatly influenced the electrical and field-emission properties of the carbon-nitride films. It is found that after chemical etching, the activation energy of the films decreased from 3.42 to 1.19 eV, the field-emission threshold decreased from 4 to 3.5 V/Am and the emission current density increased by one order of magnitude. Further work should be focused on the detailed investigation on the growth process for the preparation of high-quality carbon-nitride films and the influence of the growth process on the electronic properties.
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Acknowledgments Financial support from the National Natural Science Foundation of China (No. 10075012 and 60178031) is gratefully acknowledged. Zongyi Qin would like to thank the financial support of National Postdoctoral Council of China.
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Microstructure and electronic properties of pulsed-discharge-deposited amorphous carbon-nitride films Zongyi Qin1, Peinan Wang*, Hong Shen, Lan Mi, Xuantong Ying State Key Lab for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China Received 30 September 2004; received in revised form 12 April 2005; accepted 9 May 2005 Available online 2 August 2005
Abstract Amorphous carbon-nitride films were grown on the nitridated-diamond substrates by pulsed-discharge of pure nitrogen gas using graphite rods as the discharge electrodes. The deposition conditions were optimized by monitoring the discharge plasma with optical-emission spectroscopy. It is demonstrated that the films were mainly a mixture of nanosized carbon and carbon-nitride with sp2 and sp3 phases. Preliminary results show that the deposited carbon-nitride films exhibit semiconductor behavior and have a cold-cathode-emission property, which make them possible to be superior electronic materials. Improvement in the conductivity and field-emission properties was observed after chemical etching with hydrofluoric acid. The activation energy for electrical conduction of the HF-treated film decreased from 3.42 to 1.19 eV. The similar threshold voltages were obtained for the carbon-nitride films before and after chemical etching, which were 4.0 and 3.5 V/Am, respectively. However, the emission current density after etching increased by one order of magnitude. D 2005 Elsevier B.V. All rights reserved. PACS: 52.77.Dq; 61.43.Er; 61.46.+w; 73.22.-f Keywords: Amorphous carbon-nitride; Diamond film; Etching; Field emission
1. Introduction Over the last 10 years, worldwide experimental and theoretical interests have been focused on the synthesis and properties of carbon-nitride materials due in part to the possibility of synthesizing the h-C3N4 phase, which was theoretically predicted to have an exceptional hardness comparable to or greater than that of diamond [1 –10]. Most of the experiments resulted in amorphous carbon-nitride structures, although in a few cases polycrystalline h-C3N4 structures embedded in the amorphous carbon-nitride (aC:N) matrix were observed [5 –7]. Actually, the evidence for the existence of crystalline carbon-nitride can also be truly indexed to different carbon phases, since the varieties T Corresponding author. E-mail addresses: [email protected] (Z. Qin), [email protected] (P. Wang). 1 Current address: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.05.004
of different bonding configurations are close in energy. Theoretical calculations have further predicted the existence of other stable crystalline carbon-nitride structures, such as the hexagonal, the defect cubic zincblende, and the rhombohedral structures [9,10]. In spite of the difficulties to produce crystalline h-C3N4, amorphous carbon-nitride films also exhibit promising mechanical properties such as high hardness and excellent elastic recovery, as well as other tribological properties. In recent years, carbon-based electron field emission has been expected to be a new electron source for the flat panel display, electronic and optoelectronic devices, and so on. It is well known that nitrogen incorporation into amorphous carbon films significantly reduces the stress, electrical resistivity, and optical band gap [11– 13]. Thus, carbonnitride has attracted great engineering interest as a superior electronic material [13 – 15]. Although no success in preparing carbon-nitride films with a single bonding state or a definitive atomic structure has been reported so far, the need for both sp2 and sp3 phases for good field emission
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was in fact emphasized in many recent reports [16 – 18]. In our previous work, we found that the bonding configuration in the substrate was crucial to the structure of the film produced, the sp3 bonded diamond substrate promoted the formation of sp3 bonded carbon-nitrides [6,19]. Therefore, the diamond film was used as substrate in this work to promote the formation of sp3 bonded carbon-nitride. It is worth pointing out that carbon-nitrides as an electronic material was usually required to fabricate dense and conductive films on large-area substrates with high growth rate and at low temperature. It has been shown that the impact of preparation parameters has a large influence on the chemical composites, microstructures and most properties of the synthesized films. Hence, various techniques have been employed in attempts to investigate nitrogen plasma or nitrogen radical effects on the properties and to improve the nitrogen content of the films. In the synthesis of nitrides, atomic nitrogen is believed to play an important role due to its high chemical reactivity. Discharge is one of the most widely used techniques, because nitrogen molecules are highly dissociated and activated in the discharge plasma with a high temperature and high energy density. Various discharge methods provide the possibility of fabricating films of widely varying properties depending on the discharge parameters applied, and even creating new type of carbon-nitride structure [15 –20]. In the present work, pulsed discharge was employed to prepare carbon-nitride films. With its high discharge voltage and current density, N2 could be effectively dissociated by pulsed discharge to produce abundant reactive atomic nitrogen species. By rapid expansion and quenching of the high-temperature-reaction plasma, it is possible to produce materials containing metastable phases, nanometer structures, and multi-component materials [21,22]. The discharge parameters were optimized by monitoring the discharge plasma with optical-emission spectroscopy. The structures of the produced carbon-nitride films were characterized and their electronic properties were studied. The influence of chemical etching on the synthesized carbon-nitride film is also discussed.
2. Experiment Carbon-nitride was synthesized on a diamond surface by a pulsed discharge of high-purity N2 using graphite rods as the electrodes. The pulsed-discharge circuit was switched by a spark gap, and discharge parameters could be easily adjusted over a wide range by changing the circuit parameters and the conditions of the spark gap (gap distance and gas pressure). Additional details concerning the pulsed-discharge system have been described in our previous work [21,23]. Compared with other discharge methods, pulsed discharge can dissociate N2 molecules much more efficiently. We have success-
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fully synthesized nanosized nitrides using the method of pulsed discharge [21,23]. In the present work, the chamber was evacuated to below 0.13 Pa and then filled with flowing N2. The pressure of N2 was controlled by the flow rate of the nitrogen gas. The discharge plasma was monitored by optical-emission spectroscopy. The discharge condition was optimized by spectral diagnostics as described in our previous work [21,23]. Nitrogen gas was fully dissociated when the nitrogen pressure was above 3 kPa and the discharge voltage was 5 kV, which were chosen as the discharge parameters for the film deposition in this work. All the films were deposited at room temperature on the nitridated-diamond substrate while no deliberate external bias voltage was applied to the substrate. It is believed that the nitridated-diamond substrate, which contains the hC3N4 phase, favors the formation of the h-C3N4 phase during the film growth [24] and improves the adhesion to the substrate. Initially, a hot filament chemical vapor deposited (CVD) diamond film on a silicon substrate with a diameter of 20 mm was nitridated by a pulsed discharge of pure nitrogen gas at 9 kPa with tungsten electrodes for 7 h at a repetition rate of 10 Hz. Tungsten electrodes were then replaced by graphite electrodes and the N2 pressure in the chamber was reduced to 3 kPa for deposition of carbon-nitride films. The substrate was placed parallel to the discharge plasma column and about 10 mm from the center of the column. The thickness of the deposited film was estimated to be approximately 300 nm for the deposition time of 1 h as measured by ellipsometry. Chemical etching of carbon-nitride film was carried out for 3 min with 2% hydrofluoric acid and then rinsed by distilled water and dried by pure nitrogen gas. The estimated thickness of the deposited film decreased from 300 to about 200 nm after etching. The deposited film was slightly brownish. The substrate with carbon-nitride film deposited was cut into small pieces. Some pieces were examined via atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman analyses, while others were used for the measurement of physical properties, such as conductivity, field emission, and etc. The chemical composition and bonding states of the films before and after chemical etching in the hydrofluoric acid were determined by XPS in a VG ESCALAB 220i-XL spectrometer by using Mg Ka1,2 (1253.6 eV) excitation. Surface morphology of the films was investigated by AFM (Digital Instruments, Dimension 3100), which was used in its tapping mode configuration, in air and at room temperature. The measurement of the Raman spectra of the films was carried out in the backscattering mode with a RM3000 (Renishaw) spectrometer (excitation 633 nm). The temperature dependence of the electrical conductivity of the deposited film was measured by means of a fourpoint probe technique in a liquid-nitrogen cryostat. The field-emission measurement in a parallel-plate configuration
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was performed in a vacuum of 10 5 Pa by using the deposited film as a cathode and collecting the emitted electrons on an indium tin oxide coated glass anode with an effective collection diameter of 5 mm and the spacing distance between the anode and the cathode of 50 Am. The voltage between the anode and the cathode was varied while simultaneously recording the current using combined source/measure electrometers.
3. Results and discussion Fig. 1 shows the emission spectra of the nitrogen discharge plasma at a pressure of 3 kPa with a pair of tungsten (a) and graphite (b) electrodes, respectively. For tungsten electrodes (Fig. 1(a)), nearly all the strong lines in the emission spectra corresponded to atomic nitrogen ions, and the molecular lines were too weak to be observed. The nitrogen molecules were highly dissociated due to the high temperature (3 103 K) and high electron density (1016 cm 3) in the pulsed-discharge plasma [23]. With graphite electrodes, very intensive CN bands and carbon peaks were observed in the emission spectrum while N+ peaks almost disappeared as shown in Fig. 1(b), indicting that a strong reaction between atomic nitrogen and sputtered carbon species occurred in the plasma. Subsequently, the deposition of carbon-nitride films was
Fig. 2. AFM images for the deposited films before (a) and after (b) chemical etching.
Fig. 1. Optical-emission spectra from the plasma in the pulsed discharge of N2 using tungsten rods (a) and graphite rods (b) as electrodes, respectively. The various molecular and ionic lines are labeled.
carried out with the expansion of the pulsed plasma. A marked similarity has been reported between the content of carbon-nitride in the film and the fraction of CN in the plasma [18]. It is well known that the microstructure and the property of the deposited film depend strongly on the preparation parameters, such as applied voltage, substrate temperature, and gas pressure. However, no attempt such as heating the substrate, applying a bias voltage or shielding the large particles to the substrate was made in our experiment. Thus, an amorphous structure with a certain amount of grains was observed on the deposited film surface. The AFM exami-
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nation of the deposited film, as demonstrated in Fig. 2, indicates that the film surface was relatively rough. The roughness of 248 nm (RMS) over an area of 100 100 Am2 is believed to result from the large particles formed on the film surface during deposition as well as the original rough surface of the polycrystalline micro-diamond substrate, because diamond grows only in faceted grains, not in a smooth layer [25]. Chemical etching was found to remove the majority of loosely bonded carbon particles and some amorphous materials [16]. One can observe the difference of the surface morphology before and after etching from the AFM images (Fig. 2). After etching, the grain size became smaller and more uniform, which would enhance the field emission effectively. The XPS measurements have been performed in order to obtain further information about chemical composites and bonding states [7,26 –28]. As shown in Fig. 3, widescan spectra reveal that the deposited film before chemical etching was composed primarily of carbon and nitrogen, with a small amount of oxygen contamination. After chemical etching, the oxygen peak vanished and the nitrogen-to-carbon ratio, which was obtained by integrating the core-level peak area of N (1s) and C (1s), corrected by the atomic sensitivity factors [(ASF) C: 0.296 and N: 0.477], increased from 16% to 22%. It is evident that nitrogen was incorporated into the films, while the oxygen was the surface contaminant due to the exposure of the samples to air prior to analysis [29]. Raman spectroscopy is a commonly used technique to analyze carbon-related materials. As shown in Fig. 4, the Raman spectra of our synthesized film exhibit a broad asymmetric band between 1000 and 1700 cm 1, further confirming that various components were contained in the film. The Raman spectra before and after chemical etching can be deconvoluted into four bands. In addition to the commonly observed D band at 1360 cm 1 and G band at 1570 cm 1 for amorphous carbon, there is strong evidence of the diamond band at 1332 cm 1 and a well-identified peak near 1507 cm 1 between the D and G bands.
Fig. 3. Wide-scan XPS spectra before (a) and after (b) chemical etching.
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Fig. 4. Deconvoluted Raman spectra of carbon-nitride films before (a) and after (b) chemical etching.
The typical G band associates with the optically allowed E 2g zone-center mode of crystalline graphite, representing a relatively high content of sp2 carbon while the D band associates with the disorder-allowed zone-edge modes of graphite, representing mainly sp2 — with a small amount of sp3-carbon [8,30 – 32]. The need for both sp2 and sp3 phases for good field emission was emphasized in some recent reports [16,31]. Either sp2- or sp3-carbon atoms can be bonded to nitrogen to form carbon-nitride in the films. The graphite structure in the film could be understood from the existence of carbon species in the discharge plasma, as shown in Fig. 1(b). The sputtered carbon species was not fully converted to the CN radicals, but deposited on the substrate. The intermediate band at 1507 cm 1 was unambiguously originated from the incorporation of nitrogen into the carbon network and was designated as the nitrogen band [30]. The diamond peak at 1332 cm 1 might result from the diamond substrate since the deposited film is very thin, or from the nanosized diamond formed during the deposition. In fact, there can be observed a weak band at around 1150 cm 1, which is regarded as an evidence of nano-diamond character [31]. As can easily be seen in Fig. 4, the intensities of the diamond and nitrogen bands increased after chemical etching. The increase of the diamond signal might be due to the reduction of the film thickness by removing some
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Fig. 5. Temperature dependences of electrical conductivity of carbon-nitride films before and after chemical etching.
surface particles or the increase of diamond content near the diamond substrate. The increase of carbon-nitride intensity can be attributed to the increase of nitrogen content near the diamond surface, which is consistent with the wide-scan XPS analysis mentioned above. Generally, the average size of the graphite microcrystallites can be estimated according to the I(D) / I(G) ratio [32,33]. As described in Refs. [32] and [33], due to the two competing phenomena, there is a critical ratio R crit = I(D) / I(G) å 2.2, corresponding to a critical average size of about 2 nm. If the average size is smaller than the critical size of 2 nm, it increases with the increasing I(D) / I(G) ratio. In contrast, the average size decreases with the increasing I(D) / I(G) ratio if it is larger than 2 nm [32,33]. In our Raman spectra, as shown in Fig. 4, a reduction in G bandwidth and a decrease of I(D) / I(G) ratio was observed after chemical etching, indicating a decrease in the average size of the graphite microcrystallites [34], which matches the case that the average size was less than the critical size. Hence, the average sizes of those crystallites in the films before and after chemical etching can be estimated to be approximately 1.5 and 1.2 nm, respectively. However, large uncertainties are associated with this estimation and there exists a relatively wide size distribution in the pulseddischarge deposition as demonstrated in our previous work [35]. The relationship between electrical conductivity and temperature can provide important information concerning the nature of the charge-transport-related phenomena. Fig. 5 shows the logarithm of the conductivity versus temperature (T 1/4) before and after chemical etching. The ln(r) shows an almost linear dependence on T 1/4, revealing that the carrier transport in the film was dominated by variable-range hopping conduction with a semiconducting character in the presence of local disorders [36,37]. The conduction by tunneling or phonon-assisted hopping between localized states, rather than through propagation in extended states, is expected throughout a broad range of temperatures for the deposited films. Charge carriers are
likely to be localized throughout the entire range of the sp2 clusters and the dangling bond states within the band gaps associated with sp2 and sp3 sites. This model is supported by the above characterization results of the films. The resistivity of the film after etching decreased from 23 to 0.55 Vcm at 300 K in this work. The increase of electrical conductivity after chemical etching can be attributed to the increase of N content as well as the better contact with measuring electrode due to the removal of surface contaminants and those loosely bonded carbon particles. Nitrogen in amorphous carbon films usually acts as an n-type shallow donor below the conduction band of amorphous carbon and to alter the joint density of states in the film [38]. Nitrogen does not cause more clustering of sp2 sites, but more crosslinking, and thus increased disorders, even though not necessarily through a significant increase of sp3 sites. This is in agreement with our Raman results, which show the increase of nitrogen content but the similar estimated average size of the graphite nanocrystallites before and after chemical etching. In our deposited films, a relatively low resistivity of about 0.55 Vcm indicates that the films presumably contain a fraction of sp3 sites in an amorphous sp2 matrix, which was suggested as an optimized microstructure for field emission [39]. The activation energy of the film after chemical etching decreased from 3.42 to 1.19 eV, which can be derived from the slope of the fitted line in Fig. 5. Fig. 6 shows the field-emission current density versus electric field of the deposited films before and after chemical etching. It is evident from Fig. 6 that after etching, a remarkable enhancement in the current density and a reduction of field threshold were observed. The field threshold occurred at about 4.0 and 3.5 V/Am, respectively, which is defined as the field to give an emission current density of 10 7 A/cm2. At the same electric field above the thresholds, the emission current density after chemical etching increased by almost one order of magnitude. The degradation of the current density was less than 10% when measured again 1 h later.
Fig. 6. Current density ( J) versus applied electric field (E) for carbonnitride films before and after chemical etching.
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It is well known that emission occurs at localized sites rather than uniformly over the surface. It has been found that carbons with mixed sp3 and sp2 sites, such as nano-diamond and nano-graphitic films, can emit at lower fields and have a higher emission-site density than single-phase films [31]. When an external electric field is applied, the geometrical factors of nanoparticles would cause a redistribution of electrons and field concentration, increasing the electric field strength locally around the areas with higher curvature. Therefore, although the magnitude of the applied external field should not be large enough to induce field emission, its local value in the regions of highest curvature of the nanoparticle could be concentrated just enough to exceed threshold field [16]. Although the mechanism of electron emission from amorphous carbon-nitride films was still under argument, the enhancement of field emission was proved to be associated to the increase of the grain boundary density, sp2-bonded components and defects incorporated in the film [40]. The AFM image of the film after etching in this work exhibits smaller grain size, thereby, the higher grain boundary density. Nitrogen addition can elevate the Fermi level, and consequently, the energy barrier that the electrons must tunnel through is reduced, resulting in emission enhancement [32,40]. Besides, the increase of N content increases the concentration of donor centers, leading to a higher field across the film at a given applied voltage [43]. On the other hand, a thinner film is more favorable for the field emission because the film can be more fully depleted, thus an electron can gain enough energy with respect to the conduction band at low thickness for them to be able to surmount the emission barrier to vacuum. Also, a thinner film is beneficial in that it reduces the scattering of electrons and increases the probability that an electron will be emitted through the emission barrier of the a-C:N/ vacuum interface [41,42]. Hence, the decrease of the
Fig. 7. Fowler – Nordheim plot of carbon-nitride films before and after chemical etching.
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thickness from 300 to 200 nm by chemical etching also enhanced the field emission. The Fowler – Nordheim (F – N) theory is the most commonly used model for cold electron emissions. Fig. 7 demonstrates the F– N plots of the films before and after etching, respectively. The linear F – N characteristic clearly reveals that the films performed a cold field-emission process, in which the electron emission into vacuum is based on the tunneling mechanism. The effective work functions of the films before and after chemical etching, calculated from the slopes of the F –N plots with the dimensionless field enhancement factor of 1, were found to be about 0.050 and 0.046 eV, respectively. As can be seen in Fig. 7, the F – N plot before etching exhibits a downward bending, similar to those demonstrated in some other papers [15,40,41,44]. This downward bending can be understood by considering the resonance-tunneling model [15,44]. In this model, adsorbates lying on the surface of the film produce surface states, which create a resonant tunneling condition for electrons, greatly increasing the local tunneling current at adsorbates and consequently the overall tunneling probability, leading to a steeper slope. However, at a high-field region (low 1/V region), the adsorbate states are completely removed while the resonant is removed well, leading to a decrease in F –N slope [15,44]. For the film after chemical etching, since a majority of surface adsorbates were removed, a better linearity is exhibited in the F – N plot as shown in Fig. 7.
4. Conclusions In conclusion, amorphous carbon-nitride films were grown on the nitridated-diamond surface by pulsed-discharge deposition. The deposited amorphous carbon-nitride films exhibit a good semiconductor behavior and a coldcathode-emission process, which make it possible for these carbon-nitride films to be an excellent electron emitter material. These superior properties are believed to result from this film_s unique microstructures, which mainly consist of sp2- and sp3-carbon-nitrides and graphite nanocrystallites. It well satisfied the demand of an ideal material for a cold-cathode emitter, in which an optimized microstructure should consist of nanosized sp2 clusters as the conductor and n-type doped wide-gap sp3 sites as the field emitter. The chemical etching greatly influenced the electrical and field-emission properties of the carbon-nitride films. It is found that after chemical etching, the activation energy of the films decreased from 3.42 to 1.19 eV, the field-emission threshold decreased from 4 to 3.5 V/Am and the emission current density increased by one order of magnitude. Further work should be focused on the detailed investigation on the growth process for the preparation of high-quality carbon-nitride films and the influence of the growth process on the electronic properties.
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Acknowledgments Financial support from the National Natural Science Foundation of China (No. 10075012 and 60178031) is gratefully acknowledged. Zongyi Qin would like to thank the financial support of National Postdoctoral Council of China.
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