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The effect of processing parameters on amorphous carbon nitride layer properties
Diamond & Related Materials 13 (2004) 2187 – 2196 www.elsevier.com/locate/diamond
The effect of processing parameters on amorphous carbon nitride layer properties M. Rusop*, T. Soga, T. Jimbo Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Available online 13 August 2004
Abstract Surface morphology, deposition rate (DR), bonding composition, structural, optical and electrical properties of pulsed laser deposited amorphous carbon nitride (a-CNx ) layers using camphoric carbon (C10H16O) target precursor as a function of substrate temperatures (STs), laser fluences (LFs) and target to substrate distances (TSDs) are reported. At fixed LF and TSD, surface roughness, particle density and particle size increase, whereas the DR decreases with higher ST. When the TSD and ST are fixed, surface roughness, particle density, deposition rate and nitrogen (N) content increase, whereas particle size decreases with higher LF. While when the LF and ST are fixed, decreasing TSD results in an increase in the irregular small particle size, particle density, surface roughness, DR and N content. The N content in a-CNx layers is found to increase with higher ST up to 400 8C and decrease thereafter. While the increase of LF and decrease of TSD result in an increase in the N content. We found that the amorphous structure of a-CNx layers and the ratio of sp2 trihedral component to sp3 tetrahedral component are strongly dependent on ST, LF and TSD. The a-CNx layers with high N content have relatively high electrical resistivity. D 2004 Elsevier B.V. All rights reserved. Keywords: a-CNx ; Substrate temperature; Laser fluence; Camphor; PLD
1. Introduction Pulsed laser deposition (PLD) technique used for film preparation has become popular for its simplicity, versatility and capability to generate highly energetic carbon (C) species with large tetrahedral (sp3) fractions that enhance the synthesis of high-quality films with good mechanical and optical properties [1]. In the present study, we have introduced nitrogen (N) gas ambient pressure (NP) at 0.8 torr into the chamber to deposit the Nincorporated carbonaceous films on quartz and silicon (100) substrates by PLD using camphoric C (CC) target [2]. In brief, the CC soot was deposited along the walls of the tube of camphor burning systems, collected and dried in the oven for an hour, after which it will be pressed into targets.
* Corresponding author. Tel.: +81 527355415; fax: +81 527355105. E-mail address: [email protected] (M. Rusop). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.06.024
Amorphous C nitride films (a-CNx , x=N to C ratio) is an interesting material and has been widely investigated in the past decade since Liu and Cohen [3] reported that metastable crystalline h-C3N4 might possess hardness comparable to that of diamond, and also because of the possible applications of this material as magnetic media coatings and as a semiconductor. It is known that deposition parameters affect the formation of crystalline phase and amorphous structural of a-CNx due to incorporation of N and changing in the ratio of tetrahedral bonded carbon (sp3-C) to trihedral bonded carbon (sp2-C) in the carbon films. Therefore, it is necessary to optimize the sp3C and sp2-C in order to achieve high x, which will ensure the stable CUN bonds and avoid graphite-like a-CNx in the films. Since the microstructure of a-CNx films is strongly dependent on the deposition conditions, in this paper, we discuss the different bonding structures and electrical properties that occur in a-CNx films when they are deposited at different deposition conditions. The objective of this study is to develop improved procedures
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for deposition, characterize and improve the properties of a-CNx films.
2. Experimental We have studied the roles of substrate temperature (ST), laser fluence (LF) and target to substrate distance (TSD) on the deposition of a-CNx films. The a-CNx films were deposited on single crystal silicon (Si) (100) and fused quartz (quartz) substrates by PLD using CC target with various of deposition parameters as shown in Table 1. To investigate the roles of ST on the a-CNx films (Method 1), the CC target was ablated by PLD technique (NISSIN 10, XeCl excimer laser, k=308 nm, s=20 ns, repetition rate=2 Hz, shot number=900 shots), which was focused at a 458 angle onto CC target by an ultraviolet-grade plano-convex lens, in 0.8 torr N gas ambient with adjusting ST in the range from 20 to 500 8C. The LF was maintained in a range from 3.4 J/cm2 by adjusting the laser energy and the lens to target distance for adjusting the laser spot size. The substrate was mounted on a metal substrate stage holder parallel to the CC target with fixed TSD at 45 mm. In order to ensure a uniform ablation rate, the CC target was rotated at each 50 shots. The deposition chamber was evacuated to a base pressure at approximately 210 5 torr using a turbomolecular pump, and after that, N gas ambient was allowed until the chamber pressure is allayed at 0.8 torr. For the investigation of the roles of LF (Method 2), the CC target was also ablated using the above procedure and LF was set in the range from 2.8 to 5.2 J/cm2 with fixed TSD at 45 mm and ST at 20 8C. While in order to investigate the roles of TSD (Method 3), the CC target was also ablated using the above procedure and TSD was set in the range from 25 to 45 mm with fixed LF at 3.4 J/cm2 and ST at 20 8C. The reference sample of a-C film, thereafter also referred to as Sample A, has also been prepared with fixed ST at 20 8C, LF at 3.4 J/cm2 and TSD at 45 mm and by using the above procedure without N gas ambient.
3. Results and discussions The thickness of the deposited Sample A and a-CNx films is measured by Alpha-Step 500 profiler. The maximum average deposition rate (DR) was calculated as the quotient of the measured maximum thickness and the Table 1 The PLD deposition method with various deposition parameters Method Sample A Method 1 Method 2 Method 3
Parameter method ST (8C)
LF (J/cm2)
TSD (mm)
NP (torr)
20 20–500 20 20
3.4 3.4 2.8–5.2 3.4
45 45 45 25–45
0 0.8 0.8 0.8
number of pulses applied. It should be noted here that when calculating the DR from the thickness, the changes in the density and roughness of the film have not been taken into account, therefore, the DR might become more and more overestimated as film density decreases and roughness increases with higher ST and LF and lower TSD. The DR as a function of ST, LF and TSD is shown in Fig. 1 (left axis). Experimental results show that at fixed LF, TSD and NP, and with increasing ST (Method 1), the DR decreases from 0.13 nm/pulse at 20 8C to about 0.047 nm/pulse at 500 8C. As can be seen in Fig. 1a, the curve shows a slight decrease in DR at low ST, followed by a sharp drop at high ST. The initial decrease in DR (up to 400 8C) may be due to the adsorption/desorption of the film-forming species physically adsorbed on the ST as well as their direct reflection from the heated surface, and at higher ST (over 400 8C), the release of N-containing volatile molecules, e.g., N2, C2N2 and HCN, starts, which may result in a dramatic drop in DR [4]. The decrease in DR with an increase in ST may also suggest that low DR at high ST can be caused by the decrease in the probability that deposited particles will stick to the substrate surface. At fixed ST, TSD and NP (Method 2), the DR, particle density and surface roughness of a-CNx films are increased, while the particle size decreases with increasing LF. This could be attributed to the increase in the total amount of the C species generated by the laser irradiation at higher LF. At lower LF, the low energy density was not sufficient to ablate a large amount of C species for deposition, which can be confirmed by the reduced DR. Deposition at high LF can enhance the surface mobility and, consequently, may facilitate the film growth. The reason for the increased DR in high LF also can be explained by the more effective momentum transfer of the N atoms and C species during deposition and therefore higher DR. The high energy density of the LF was sufficient to evaporate a large amount of C species for deposition, which can be confirmed by the increased DR as shown in Fig. 1b. The ions in the chamber are more accelerated during deposition by the higher LF. Therefore, at higher LF, the deposition of the film by high ion energy takes place, resulting in high DR. However, as shown in Fig. 1b, our result shows that the increase in the rate of DR reduces at higher LF, indicating saturation. The saturation of the particle and film deposition, in general, is largely due to the saturation in the deposition process [5]. For example, plasma shielding of the target is one of the mechanisms that reduce the ablation rate, and is more often encountered in the PLD using longer wavelength [6]. It should also be noted that the saturation of the particle density at higher LF may be exaggerated somewhat, since the sticking of the particles to the substrates appears to be poorer at elevated LF [7]. It is clear from Fig. 1c that at fixed ST, LF and NP (Method 3), the DR of a-CNx films decreases with increasing TSD. This can satisfactorily be accounted for by the increased number of collisions during deposition [8].
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Fig. 1. The deposition rate (left axis) and roughness (right axis) of Sample A, and a-CNx films deposited with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3). (Left axis: A for Sample A, and D for a-CNx films. Right axis: B for Sample A, and R for a-CNx films).
With the decrease of TSD, the DR increased because of the increase of actual density of the primary deposition product of the C atoms, whereas with the increase of TSD, the DR decreased because the reaction products contribute to the film growth [9], and this may also be due to surface etching by activated N species [10]. Our result shows that the relation between LF (Method 2) and TSD (Method 3) to the DR can be considered from the fact that having more influence on the N content in the film (Figs. 2–5), with an increase in the LF (Fig. 1b) and decrease in TSD (Fig. 1c) within this domain, results in an increase in the porosity and roughness of the a-CNx films, leading to an apparent increase in the DR. The average root mean square surface roughness (roughness) estimated over a 11 Am area obtained by AFM measurement, as a function of ST, LF and TSD, is shown in Fig. 2 (right axis). SEM and AFM showed that the surface morphology of a-CNx films was smoother compared with Sample A, which deposited in the same
range of the corresponding parameter. We may attribute this structural transition largely to the chemical sputtering process. In the presence of N, atoms not sitting in stable lattice sites have a high probability to react with N to form volatile molecules, which can desorb. This can more likely occur at edges where the atoms are more loosely bound, and so, protruding clusters will be rounded or completely etched away. Consequently, the films become both denser and smoother. When the LF, TSD and NP are fixed (Method 1), SEM and AFM showed that the surface morphology of a-CNx deposited at 20 8C was very smooth and has only a few feature compared with the Sample A. The particle size and roughness (Fig. 2a) significantly changed with the ST and a few small particles appeared at 200 8C. The particle size and roughness increased with increasing ST. It was found that above 400 8C, the particle size rapidly increased, while the density of the particles and the roughness obviously decreased, which implies that some small particles have
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Fig. 2. The Auger electron spectra of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3).
Fig. 3. The variation of (a) nitrogen (left axis) and oxygen (right axis) content with various ST (Method 1), (b) nitrogen content (left axis) with various LF (Method 2) and (c) nitrogen content (left axis) with various TSD (Method 3). (Left axis: N for nitrogen content. Right axis: O for oxygen content).
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Fig. 4. The FTIR spectra of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3).
merged into a large particle due to the enhanced surface migration of the deposited C species. At fixed ST, TSD and NP (Method 2), the roughness and particle density of a-CNx films increased, whereas the particle size decreased with higher LF (Fig. 2b). These observations agree, to some extent, with the works of Blank et al. [11], which reported that at fixed TSD, the surface roughness and particle density of a-CNx films increased, whereas the particle size decreased with higher LF. When ST, LF and NP are fixed (Method 3), TSD decreased, resulting in an increase in the irregular small particle size, particle density and roughness (Fig. 2c). As the TSD increases, the proportion of the smaller particles decreases, and a few larger particles appeared, indicating a
merge during flight, and adhesion to the substrate of the ejected matter, including the particles and atomic species, is poorer [12]. We considered that the increase of ST and LF and decrease of TSD might accelerate the reactive N ions, N+, which may react with the surface hydrocarbons to form CN+ and (CN)+2, and voids will therefore be left. Hence, the surface becomes rougher. As the ST increases, the increase of mobility attributes for C atoms to migrate to the surface where they follow surface growth process, which results in the formation of more graphitic, sp2-rich material and surface, becomes rougher. The increase of roughness also may be due to the different kinetic energies of C species generated during deposition. The high energy of C species resulting from the
Fig. 5. The FTIR spectra obtained from (a) Sample A, and a-CNx (Method 1), (b) a-C and (c) a-CNx (Method 1) films deposited at various substrate temperatures.
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higher LF irradiation and ST, and lower TSD would considerably enhance the surface mobility of the depositing atoms, which then increase the roughness and decrease the compactness of the a-CNx films. Consistent with other reports [13], the surface roughness increased when more N atoms were incorporated in the films, as have been confirmed by AES, XPS and FTIR measurements, which will be discussed in the next section, indicating some microstructural modifications. We discovered that DR of a-CNx films was decreased and the surface morphology of a-CNx films was smoother compared with Sample A, which deposited in the same range of the corresponding parameter. The decrease of the DR and roughness as well as the densification of the C films when the N is introduced during deposition can be related to the chemical etching [14–16] due to the N incorporation in the a-CNx films. As the N2 is introduced during deposition, an increased desorption of volatile N-rich CN species, such as C2N2, is expected. The upper limit of the achievable maximum achievable N content in a-CNx films is thus apparently limited by extensive chemical sputtering. This phenomenon may also be due to the increased collisions between the ejected species and the ambient gas as the ambient gas pressure increases [16]. It has been reported [17,18] that despite the slight variation of chemical composition, in accordance with the expectations, the films deposited at fixed TSD and ST exhibited a fairly linear DR as a function of LF. It is believed that an almost linear dependence of the DR on the LF is the proof that almost all the energy absorbed by the target contributed to the deposition [18]. We found the DR, particle size, surface roughness and the particle density of a-CNx films to be strongly dependent on the deposition parameters, such as the ST, LF and TSD. Further information on the influence of ST, LF and TSD on the deposition process can be derived from the composition of the films. Compositional chemical analyses of the deposited films were performed by Auger electron spectroscopy (AES). Fig. 2 illustrates the AES spectra of Sample A and a-CNx films, showing that the a-CNx films were mainly composed of C and N. The existence of a small oxygen signal was a result of air exposure during sample transport. The spectra are compared, revealing that the peak increases with the increase of ST (Fig. 2a) and LF (Fig. 2b) and decrease of TSD (Fig. 2c). The intensity of N peak in the low-energy region corresponds to the LF, TSD and ST, indicating that N was effectively doped into the a-C films during deposition and also implying that N content in the films increases with the increase of LF and ST and decrease of TSD as the peak intensity shows the increasing tendency. Although there are still some controversies about the assignments of the individual components of the C 1s and N 1s core level spectra [19–21], the X-ray photoelectron spectroscopy (XPS) analysis is one of the most likely used techniques in the literature to characterize the formation,
bond types and other useful information on the chemical environment around O, C and N of CN phases that can be obtained [22]. XPS was measured by SSX-100 XPS system of Surface Science Instruments utilizing using an AlKa´ (hv=1486.6 eV) radiation as an X-ray source, under high vacuum conditions of about 10 10 torr. The chemical bonding state in the films were analyzed after the 0.5 keV Ar+ ion etching of film surface for 3 min. The chemical composition of the deposited films can be obtained according to oxygen to C (O/C) and the N to C atomic ratio (N/C). The N/C and O/C content in the films were evaluated using N/C=(A N/1.68)/(A C/1.00) and O/C=(A O/ 2.49)/(A C/1.00) where A N, A O and A C are the areas under the N 1s, O 1s and C 1s core level spectra, and the constants of 1.68, 2.49 and 1.00 are the atomic sensitivity factors of N, O and C, respectively. Fig. 3a shows N/C atomic ratio (N content) of the aCNx films (Method 1) evaluated from XPS measurements as a function of ST. As shown in Fig. 3a, the N/C ratio in the film is about 3.69 atomic percentage (at.%) at 20 8C and increased gradually with an increase in the ST up to about 15.05 at.% 400 8C, but it decreased at ST higher than 400 8C up to about 10.19 at.% at 500 8C. With the increase in ST, the surface migration of C and N species will be enhanced, which will accelerate the chemical reaction between C and N and consequently improve the N content. Though, at ST higher than 400 8C, N content in the film decreases because the stable CUN bonding will remain, while the volatile bonding will be decomposed, resulting in N molecules released from the films [23]. The increase of N content in the film might also be explained by the fact that the lower deposition rate induced by ST corresponded to the lower C/N arriving ratio, therefore, the higher N/C ratio in the a-CNx films. So, the C to N species on the substrate surface is considered to determine the final N/C atomic ratio in the a-CNx films within this range of ST (up to 400 8C). This could be confirmed by comparing with Figs. 3a, 4a, 5a. Furthermore, the increase of ST leads to an increase of C atoms’ mobility. Some weak bonding, such as nitrile and other interstitial species, may become volatile and are detached from the film, which results in the decrease of N/C ratio in the deposited films (over 400 8C). In addition, the high desorption rate of CN and CNH species from the growth surface and the lower incorporation of volatile CUN species at elevated temperatures due to their low temperature of sublimation may also contribute to the low N/C ratio in deposited films [24]. The decrease of N content with increase of ST (over 400 8C) may also be due to the chemical sputtering effect. The sputtering rate of N in a thin film is much higher than that of C [25]. Though our result has shown the preferential sputtering of N, it may be responsible for the reduction of N content incorporated in the a-CNx films deposited at higher ST (over 400 8C). We also suggest that with an
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increase of ST up to 400 8C, the surface migration of C and N species will be enhanced, which will accelerate the chemical reaction between C and N and consequently improve the N content. Though at ST higher than 400 8C, N content in the film decreases because the stable CUN bonding will only remain since the volatile bonding decomposes and N molecules are released from the films [23]. It is well known that for the deposition of DLC films, for temperatures higher than about 200 8C, the surface mobility is sufficient for forming the more energetically favorable graphitic structure over the amorphous phase found at lower temperatures [26]. Therefore, when depositing a-CNx films above that temperature, we can assume that N atoms, and possibly also UCQN molecules, will diffuse on the deposition surface until they either become bound in a low-energy lattice site or react with other particles to form volatile molecules that can desorb. At higher temperatures, the diffusion rate becomes higher, so more reactions are likely to take place. Furthermore, due to surface mobility, we can assume that C diffusion becomes more important at higher temperature, so N from the subsurface region could diffuse toward the surface, and there they recombine or react and eventually desorb. Therefore, these can thus explain the reduced N incorporation with increasing ST over 400 8C. These desorption of N2 will suppress the N incorporation, while the desorption of C-containing species will involve the surface reaction process [27]. Pure carbons are unlikely to be desorbed since that would require physical sputtering, but C2N2 and, to some extent also, other CN species, can be expected to be involved. The formation of these volatile molecules may happen in many ways. It is likely that first UCQN dimmers are formed, either by dissociation of C2N2+ ions at impact at the deposition surface, or by reactions between C and N on the film surface. These dimmers can encounter other UCQN dimmers when diffusing on the surface to form volatile C2N2 molecules. A small residual amount of UCQN in the a-CNx films, in which increases in ST have been confirmed by FTIR, was also present. The formation of C2N2 molecules could result in a lower total N concentration in the a-CNx films deposited at higher ST over 400 8C. This type of a chemical sputtering process would also result in a reduced deposition rate, especially at elevated ST [27]. Fig. 3b and c shows the increase of N/C ratio with the increase of LF (Method 2) and decrease of TSD (Method 3), respectively. This may mean that the LF and TSD might affect the plasma dimension and distribution, which could also influence the arrival of C to N, affect the deposition rate and finally affect the N/C atomic ratio, and agree with Zhao et al. [28] who described the effect of LF on the N/C atomic ratio. The correlation of N content and deposition parameters has also been confirmed by AES and FTIR analyses. This is evidence that the composition and microstructure of
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the a-CNx films can be tuned by the ST, LF and TSD. An overview of XPS spectra of the deposited films indicates the presence of a very small O 1s content, in addition to the C and N constituents. The O 1s intensity decreases constantly with depth during sputter cleaning of the film surface with Ar+ ions, consistent with the presence of a small amount of oxygen resulting from the surface contamination. In addition to C and N, O/C was detected less than 1.5 at.% as a minor contamination (superficial) for Sample A and a-CNx samples deposited at 20 8C (Method 1) with N/ C=3.69 at.%. As shown in Fig. 3a (right axis), it was found that as the ST is increased from 20 through 400 to 500 8C, the oxygen content in the a-CNx films increases from 1.5% through 10.2% and decreases to 9.5%, respectively. The reason behind the increased amount of O/C and N/C in the films with higher ST can be due to the porosity and low density of the a-CNx material [29]. However, it should be noted that the presence of N in the C networks and oxygen due to minor contamination may also increase the electro-negativity of the a-CNx compounds. As a result, with an increase of ST up to 400 8C, the N content in the a-CNx films reached a maximum value, while the deposited films attain more polarity and tend to absorb H2O molecules present in the experimental atmosphere, and therefore increase the amount of oxygen in the a-CNx films. With further increase of ST up to 500 8C, the decreased N content in the a-CNx films caused the decreased electro-negativity and polarity of deposited films. As a result, the oxygen content in the a-CNx films decreased in ST over 400 8C. The bonding states of C, N and hydrogen (H) atoms can be characterized by Fourier transform infrared spectroscopy (FTIR) measurements. The FTIR spectra were measured in the wave number range from 700 to 3200 cm 1. FTIR absorption for CUH stretching vibrations on sp2-C is found in the 2950–3060 cm 1 range, while for sp3-C, it is in the 2850–2945 cm 1 range [30], and the peaks around 2926 and 2956 cm 1 are the most prominent, assigned to UCH2sp3 asymmetric and UCH2sp2 olefinic bonds, respectively, and the peaks around 2855 cm 1 indicate the formation of sp3 bonding [31]. Band 1 at around 700 cm 1 is due to the out-of-plane bending mode for the graphite-like domain [32]. The peak at around the range 1000–1700 cm 1 (band 2) can be assigned to C¯N and CMN bonds and the broad band extending in the range 2800–3000 cm 1 (band 3) corresponds to different CUH configurations [33,34]. Broad absorption band 2 indicates that the films’ structure is predominantly amorphous with sp2-C vibration modes, and is related to Raman active D and G modes [35]. The contributions around 1350 and 1550 cm 1 were initiated from disordered (D band) and graphite-like (G band) CN bonds, respectively [14]. Another contribution at around 1145–1265 cm 1 was due to the symmetric tetrahedral CN bond [36,37]. According to Kaufman et al. [38], these Raman active D and G modes become IR
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active due to the incorporation of N atoms into C network that results in the symmetry breaking of the C network. Band 4 at 1100–1400 cm 1, band 5 at 1500–1700 cm 1 and band 6 at around 2200 cm 1 are due to CUN, C=C and/or C=N and C=N stretching vibration modes, respectively [39–41]. Fig. 4 shows the FTIR transmittance spectra measured in the wavenumber range 1000–3200 cm 1 for Sample A and a-CNx films deposited by Method 1 (Fig. 4a), Method 2 (Fig. 4b) and Method 3 (Fig. 4c). Broad absorption band 2 and band 6 do not appear in Sample A. The absorption peak at around 2350 cm 1, which is observed in a-CNx and Sample A, can be attributed to the CO2 stretching mode arising from oxygen contamination at the film surface [42]. FTIR spectra show that as ST (Method 1) and LF (Method 2) increase and as TSD (Method 3) decreases, the intensity of band 2 and the three characteristic peaks of DLC in the 2800–3000 cm 1 region (band 3) become more and more prominent, suggesting the formation of crystalline structures in the films [43]. The different intensity of band 2 (Fig. 4a) may indicate that there is a different modification of the binding geometry (compared with Fig. 4b and c), which is likely induced by the increase of internal film stress arising from the increase of the trihedral bond density due to the increase of ST. The N content in the films gradually increases with the increase of ST and LF and decrease of TSD, as indicated by the increase in the intensity of bands 2, 4 and 5 of the C and N bonding peaks. These phenomena may suggest that at higher ST and LF, and short distance of TSD, films of high N and hydrogen content (band 3) are achieved in the films. The event may take place during the deposition process; also, adsorption from the atmosphere ambient may contribute to the H content. Since the bonding energy of the CUH configurations is relatively low, collisions between Ccontaining radicals and hydrogenated products present in the CC target may further increase the probability of H incorporation with the increase of ST and LF and decrease of TSD. FTIR shows that the N incorporation and crystalline structure of the films increase with the increase of ST and LF and decrease of TSD. This is evidence that the composition of the films can be tuned by optimizing the ST, LF and TSD. Fig. 5 shows FTIR transmittance spectra of (a) Sample A and a-CNx films (Method 1), measured in the wavenumber range of 600–1900 cm 1, (b) reference sample of a-C films that deposited by the same deposition parameters as Method 1 but without N gas ambient, measured in the wavenumber range of 2750–3050 cm 1 and (c) a-CNx films (Method 1), which were measured in the wavenumber range of 2750– 3050 cm 1. It is clear from the spectra (Fig. 5b and c), that as ST increases, the three characteristic peaks of DLC [43] in the 2850–3000 cm 1 region (band 3) become more and more prominent, suggesting the formation of crystalline structure in the films. It should be noted here that the increasing peaks degree of a-CNx films (Method 1) (Fig. 5c)
is higher than a-C films that deposited in the same range of ST, but without N gas ambient (Fig. 5b). The absence of clear appearance of peaks in the a-C up to 200 8C suggests the amorphous nature of the films. As can be seen in Fig. 5a, the single broad band of a-CNx film deposited at 20 8C, centered at around 1350 cm 1, has gradually shifted to a higher wavenumber and reached about 1550 cm 1 at 500 8C. This red shift indicates that there is a modification of the binding geometry, which is likely induced by the increase of internal film stress arising from the increase of the trihedral bond density due to the increase of ST. In our spectra, bands 1, 2, 3 and 4 (sp3 C N tetrahedral), band 5 (sp2 C N trihedral) and band 6 of aCNx films deposited by Method 1 are weak and as ST is increased, starting from around 200 8C, the broad absorption band 2 becomes narrow, and although small, the intensity of bands 1, 4, 5 and 6 increase gradually up to 400 8C and decrease thereafter. This indicates that the components of the CUN, C=N, C=C and C=N bonds increase with the ST up to 400 8C and decrease thereafter, which is also confirmed by the XPS analyses. The N content in the films gradually increases with the increase of ST up to 400 8C, as indicated by the increase in the intensity of the C and N bonding peak areas. The D band (1350 cm 1) gradually increased with ST up to 400 8C and decreased with higher ST, thereafter. While, the G band (1550 cm 1) seems to be gradually increased with ST up to 400 8C, after which it increased rapidly. The FTIR spectra thus indicated the transformation to the graphite-like sp2 CUN bonds of a-CNx films up to 400 8C, after which the transformation degree increased. The FTIR spectra also indicated up to 400 8C, the tetrahedral sp3 C N bonds in the a-CNx films increased, and decreased with higher ST up to 500 8C. FTIR measurement has shown that the N incorporation and crystalline structure of the films increase with the increase of ST and LF and decrease of TSD. The optical properties and electrical resistivity of the films are investigated by UV–visible spectroscopy in the range of 200–2500 nm and the four-point probe resistance measurements, respectively. Fig. 6 shows the optical gap (E g) and electrical resistivity (q) of Sample A is 0.95 eV and 5.1105 (V cm), respectively. The E g of a-CNx film (Method 1, Fig. 6a) decreased to 0.86 eV and q is increased to 8.51106 (V cm), compared with Sample A. As LF increases, the E g increases up to 0.93 eV at 2.8 J/cm2 and thereafter shows the rate of increase to reduce at higher LF, indicating saturation. While q increases up to 9.01106 (V cm) at 5.2 J/cm2, making these a-CNx films interesting. The E g of a-CNx film (Method 2, Fig. 6b) decreases to 0.93 eV and q is increased to 8.61106 (V cm), compared with Sample A. As TSD decreases, the E g decreases to 0.48 eV, while q increases up to 9.76106 (V cm) at 25 mm. For the a-CNx films deposited by Method 3, the E g decreases to 0.65 eV and q is increased to 8.77106 (V cm) at 50 8C (Fig. 6c). As ST increases, the E g decreases only a little, to 0.44 eV, while q increases up to 1.36107 (V cm) at 400
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Fig. 6. The optical gap (left axis) and resistivity (right axis) of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3). (Left axis: A for Sample A and E for a-CNx films. Right axis: B for Sample A and R for aCNx films).
8C. However, above 400 8C, both E g and q are drastically decreased down to 0.3 eV and 8.45104 (V cm) at 500 8C, respectively. The increases in q with N incorporation may be due to the lattice vibrations leading to the scattering of the charge carriers by the N atoms and more amorphous nature of the C films. Variation of the optical and electrical properties can be related to interstitial doping of N in C films through modifications of CUN bonding configurations by rearranging N atoms upon increase of LF and ST and decrease of TSD. However, with further increase of ST above 400 8C, both E g and q are drastically decreased down to 0.3 eV and 8.45104 (V cm) at 500 8C, respectively. This is probably due to the graphitization of the a-CNx films. Perhaps the doping of N accompanied by an increase of ST above 400 8C increases crystallinity and subsitutional doping of N thereby sharply decreasing resistivity. These phenomena are also supported by the XPS and FTIR spectroscopy measurements.
4. Conclusions Tuning the LF within the 2.8–5.2 J/cm2 domain (Method 1), TSD within the 25–45 mm domain (Method 2) and ST within the 20–500 8C domain (Method 3) results in significant changes in the chemical composition and electrical properties of a-CNx layers deposited by PLD using CC target in ambient 0.8 torr N gas pressure. With higher LF (Method 1), the N content, E g and q are increased. Decreasing TSD (Method 2) results in an increase in the N content and q, whereas E g is decreased. While with higher ST (Method 3), the E g is decreased. N content and q increase with ST up to 400 8C and decrease thereafter. We found that the incorporated N atoms in a-CNx layers were chemically bonded to C, and the amorphous structure of aCNx layers and the ratio of sp2 trihedral component to sp3 tetrahedral component are strongly dependent on the ST, LF and TSD. The a-CNx layers with high N and hydrogen content have high electrical resistivity.
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The effect of processing parameters on amorphous carbon nitride layer properties M. Rusop*, T. Soga, T. Jimbo Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Available online 13 August 2004
Abstract Surface morphology, deposition rate (DR), bonding composition, structural, optical and electrical properties of pulsed laser deposited amorphous carbon nitride (a-CNx ) layers using camphoric carbon (C10H16O) target precursor as a function of substrate temperatures (STs), laser fluences (LFs) and target to substrate distances (TSDs) are reported. At fixed LF and TSD, surface roughness, particle density and particle size increase, whereas the DR decreases with higher ST. When the TSD and ST are fixed, surface roughness, particle density, deposition rate and nitrogen (N) content increase, whereas particle size decreases with higher LF. While when the LF and ST are fixed, decreasing TSD results in an increase in the irregular small particle size, particle density, surface roughness, DR and N content. The N content in a-CNx layers is found to increase with higher ST up to 400 8C and decrease thereafter. While the increase of LF and decrease of TSD result in an increase in the N content. We found that the amorphous structure of a-CNx layers and the ratio of sp2 trihedral component to sp3 tetrahedral component are strongly dependent on ST, LF and TSD. The a-CNx layers with high N content have relatively high electrical resistivity. D 2004 Elsevier B.V. All rights reserved. Keywords: a-CNx ; Substrate temperature; Laser fluence; Camphor; PLD
1. Introduction Pulsed laser deposition (PLD) technique used for film preparation has become popular for its simplicity, versatility and capability to generate highly energetic carbon (C) species with large tetrahedral (sp3) fractions that enhance the synthesis of high-quality films with good mechanical and optical properties [1]. In the present study, we have introduced nitrogen (N) gas ambient pressure (NP) at 0.8 torr into the chamber to deposit the Nincorporated carbonaceous films on quartz and silicon (100) substrates by PLD using camphoric C (CC) target [2]. In brief, the CC soot was deposited along the walls of the tube of camphor burning systems, collected and dried in the oven for an hour, after which it will be pressed into targets.
* Corresponding author. Tel.: +81 527355415; fax: +81 527355105. E-mail address: [email protected] (M. Rusop). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.06.024
Amorphous C nitride films (a-CNx , x=N to C ratio) is an interesting material and has been widely investigated in the past decade since Liu and Cohen [3] reported that metastable crystalline h-C3N4 might possess hardness comparable to that of diamond, and also because of the possible applications of this material as magnetic media coatings and as a semiconductor. It is known that deposition parameters affect the formation of crystalline phase and amorphous structural of a-CNx due to incorporation of N and changing in the ratio of tetrahedral bonded carbon (sp3-C) to trihedral bonded carbon (sp2-C) in the carbon films. Therefore, it is necessary to optimize the sp3C and sp2-C in order to achieve high x, which will ensure the stable CUN bonds and avoid graphite-like a-CNx in the films. Since the microstructure of a-CNx films is strongly dependent on the deposition conditions, in this paper, we discuss the different bonding structures and electrical properties that occur in a-CNx films when they are deposited at different deposition conditions. The objective of this study is to develop improved procedures
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for deposition, characterize and improve the properties of a-CNx films.
2. Experimental We have studied the roles of substrate temperature (ST), laser fluence (LF) and target to substrate distance (TSD) on the deposition of a-CNx films. The a-CNx films were deposited on single crystal silicon (Si) (100) and fused quartz (quartz) substrates by PLD using CC target with various of deposition parameters as shown in Table 1. To investigate the roles of ST on the a-CNx films (Method 1), the CC target was ablated by PLD technique (NISSIN 10, XeCl excimer laser, k=308 nm, s=20 ns, repetition rate=2 Hz, shot number=900 shots), which was focused at a 458 angle onto CC target by an ultraviolet-grade plano-convex lens, in 0.8 torr N gas ambient with adjusting ST in the range from 20 to 500 8C. The LF was maintained in a range from 3.4 J/cm2 by adjusting the laser energy and the lens to target distance for adjusting the laser spot size. The substrate was mounted on a metal substrate stage holder parallel to the CC target with fixed TSD at 45 mm. In order to ensure a uniform ablation rate, the CC target was rotated at each 50 shots. The deposition chamber was evacuated to a base pressure at approximately 210 5 torr using a turbomolecular pump, and after that, N gas ambient was allowed until the chamber pressure is allayed at 0.8 torr. For the investigation of the roles of LF (Method 2), the CC target was also ablated using the above procedure and LF was set in the range from 2.8 to 5.2 J/cm2 with fixed TSD at 45 mm and ST at 20 8C. While in order to investigate the roles of TSD (Method 3), the CC target was also ablated using the above procedure and TSD was set in the range from 25 to 45 mm with fixed LF at 3.4 J/cm2 and ST at 20 8C. The reference sample of a-C film, thereafter also referred to as Sample A, has also been prepared with fixed ST at 20 8C, LF at 3.4 J/cm2 and TSD at 45 mm and by using the above procedure without N gas ambient.
3. Results and discussions The thickness of the deposited Sample A and a-CNx films is measured by Alpha-Step 500 profiler. The maximum average deposition rate (DR) was calculated as the quotient of the measured maximum thickness and the Table 1 The PLD deposition method with various deposition parameters Method Sample A Method 1 Method 2 Method 3
Parameter method ST (8C)
LF (J/cm2)
TSD (mm)
NP (torr)
20 20–500 20 20
3.4 3.4 2.8–5.2 3.4
45 45 45 25–45
0 0.8 0.8 0.8
number of pulses applied. It should be noted here that when calculating the DR from the thickness, the changes in the density and roughness of the film have not been taken into account, therefore, the DR might become more and more overestimated as film density decreases and roughness increases with higher ST and LF and lower TSD. The DR as a function of ST, LF and TSD is shown in Fig. 1 (left axis). Experimental results show that at fixed LF, TSD and NP, and with increasing ST (Method 1), the DR decreases from 0.13 nm/pulse at 20 8C to about 0.047 nm/pulse at 500 8C. As can be seen in Fig. 1a, the curve shows a slight decrease in DR at low ST, followed by a sharp drop at high ST. The initial decrease in DR (up to 400 8C) may be due to the adsorption/desorption of the film-forming species physically adsorbed on the ST as well as their direct reflection from the heated surface, and at higher ST (over 400 8C), the release of N-containing volatile molecules, e.g., N2, C2N2 and HCN, starts, which may result in a dramatic drop in DR [4]. The decrease in DR with an increase in ST may also suggest that low DR at high ST can be caused by the decrease in the probability that deposited particles will stick to the substrate surface. At fixed ST, TSD and NP (Method 2), the DR, particle density and surface roughness of a-CNx films are increased, while the particle size decreases with increasing LF. This could be attributed to the increase in the total amount of the C species generated by the laser irradiation at higher LF. At lower LF, the low energy density was not sufficient to ablate a large amount of C species for deposition, which can be confirmed by the reduced DR. Deposition at high LF can enhance the surface mobility and, consequently, may facilitate the film growth. The reason for the increased DR in high LF also can be explained by the more effective momentum transfer of the N atoms and C species during deposition and therefore higher DR. The high energy density of the LF was sufficient to evaporate a large amount of C species for deposition, which can be confirmed by the increased DR as shown in Fig. 1b. The ions in the chamber are more accelerated during deposition by the higher LF. Therefore, at higher LF, the deposition of the film by high ion energy takes place, resulting in high DR. However, as shown in Fig. 1b, our result shows that the increase in the rate of DR reduces at higher LF, indicating saturation. The saturation of the particle and film deposition, in general, is largely due to the saturation in the deposition process [5]. For example, plasma shielding of the target is one of the mechanisms that reduce the ablation rate, and is more often encountered in the PLD using longer wavelength [6]. It should also be noted that the saturation of the particle density at higher LF may be exaggerated somewhat, since the sticking of the particles to the substrates appears to be poorer at elevated LF [7]. It is clear from Fig. 1c that at fixed ST, LF and NP (Method 3), the DR of a-CNx films decreases with increasing TSD. This can satisfactorily be accounted for by the increased number of collisions during deposition [8].
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Fig. 1. The deposition rate (left axis) and roughness (right axis) of Sample A, and a-CNx films deposited with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3). (Left axis: A for Sample A, and D for a-CNx films. Right axis: B for Sample A, and R for a-CNx films).
With the decrease of TSD, the DR increased because of the increase of actual density of the primary deposition product of the C atoms, whereas with the increase of TSD, the DR decreased because the reaction products contribute to the film growth [9], and this may also be due to surface etching by activated N species [10]. Our result shows that the relation between LF (Method 2) and TSD (Method 3) to the DR can be considered from the fact that having more influence on the N content in the film (Figs. 2–5), with an increase in the LF (Fig. 1b) and decrease in TSD (Fig. 1c) within this domain, results in an increase in the porosity and roughness of the a-CNx films, leading to an apparent increase in the DR. The average root mean square surface roughness (roughness) estimated over a 11 Am area obtained by AFM measurement, as a function of ST, LF and TSD, is shown in Fig. 2 (right axis). SEM and AFM showed that the surface morphology of a-CNx films was smoother compared with Sample A, which deposited in the same
range of the corresponding parameter. We may attribute this structural transition largely to the chemical sputtering process. In the presence of N, atoms not sitting in stable lattice sites have a high probability to react with N to form volatile molecules, which can desorb. This can more likely occur at edges where the atoms are more loosely bound, and so, protruding clusters will be rounded or completely etched away. Consequently, the films become both denser and smoother. When the LF, TSD and NP are fixed (Method 1), SEM and AFM showed that the surface morphology of a-CNx deposited at 20 8C was very smooth and has only a few feature compared with the Sample A. The particle size and roughness (Fig. 2a) significantly changed with the ST and a few small particles appeared at 200 8C. The particle size and roughness increased with increasing ST. It was found that above 400 8C, the particle size rapidly increased, while the density of the particles and the roughness obviously decreased, which implies that some small particles have
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Fig. 2. The Auger electron spectra of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3).
Fig. 3. The variation of (a) nitrogen (left axis) and oxygen (right axis) content with various ST (Method 1), (b) nitrogen content (left axis) with various LF (Method 2) and (c) nitrogen content (left axis) with various TSD (Method 3). (Left axis: N for nitrogen content. Right axis: O for oxygen content).
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Fig. 4. The FTIR spectra of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3).
merged into a large particle due to the enhanced surface migration of the deposited C species. At fixed ST, TSD and NP (Method 2), the roughness and particle density of a-CNx films increased, whereas the particle size decreased with higher LF (Fig. 2b). These observations agree, to some extent, with the works of Blank et al. [11], which reported that at fixed TSD, the surface roughness and particle density of a-CNx films increased, whereas the particle size decreased with higher LF. When ST, LF and NP are fixed (Method 3), TSD decreased, resulting in an increase in the irregular small particle size, particle density and roughness (Fig. 2c). As the TSD increases, the proportion of the smaller particles decreases, and a few larger particles appeared, indicating a
merge during flight, and adhesion to the substrate of the ejected matter, including the particles and atomic species, is poorer [12]. We considered that the increase of ST and LF and decrease of TSD might accelerate the reactive N ions, N+, which may react with the surface hydrocarbons to form CN+ and (CN)+2, and voids will therefore be left. Hence, the surface becomes rougher. As the ST increases, the increase of mobility attributes for C atoms to migrate to the surface where they follow surface growth process, which results in the formation of more graphitic, sp2-rich material and surface, becomes rougher. The increase of roughness also may be due to the different kinetic energies of C species generated during deposition. The high energy of C species resulting from the
Fig. 5. The FTIR spectra obtained from (a) Sample A, and a-CNx (Method 1), (b) a-C and (c) a-CNx (Method 1) films deposited at various substrate temperatures.
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higher LF irradiation and ST, and lower TSD would considerably enhance the surface mobility of the depositing atoms, which then increase the roughness and decrease the compactness of the a-CNx films. Consistent with other reports [13], the surface roughness increased when more N atoms were incorporated in the films, as have been confirmed by AES, XPS and FTIR measurements, which will be discussed in the next section, indicating some microstructural modifications. We discovered that DR of a-CNx films was decreased and the surface morphology of a-CNx films was smoother compared with Sample A, which deposited in the same range of the corresponding parameter. The decrease of the DR and roughness as well as the densification of the C films when the N is introduced during deposition can be related to the chemical etching [14–16] due to the N incorporation in the a-CNx films. As the N2 is introduced during deposition, an increased desorption of volatile N-rich CN species, such as C2N2, is expected. The upper limit of the achievable maximum achievable N content in a-CNx films is thus apparently limited by extensive chemical sputtering. This phenomenon may also be due to the increased collisions between the ejected species and the ambient gas as the ambient gas pressure increases [16]. It has been reported [17,18] that despite the slight variation of chemical composition, in accordance with the expectations, the films deposited at fixed TSD and ST exhibited a fairly linear DR as a function of LF. It is believed that an almost linear dependence of the DR on the LF is the proof that almost all the energy absorbed by the target contributed to the deposition [18]. We found the DR, particle size, surface roughness and the particle density of a-CNx films to be strongly dependent on the deposition parameters, such as the ST, LF and TSD. Further information on the influence of ST, LF and TSD on the deposition process can be derived from the composition of the films. Compositional chemical analyses of the deposited films were performed by Auger electron spectroscopy (AES). Fig. 2 illustrates the AES spectra of Sample A and a-CNx films, showing that the a-CNx films were mainly composed of C and N. The existence of a small oxygen signal was a result of air exposure during sample transport. The spectra are compared, revealing that the peak increases with the increase of ST (Fig. 2a) and LF (Fig. 2b) and decrease of TSD (Fig. 2c). The intensity of N peak in the low-energy region corresponds to the LF, TSD and ST, indicating that N was effectively doped into the a-C films during deposition and also implying that N content in the films increases with the increase of LF and ST and decrease of TSD as the peak intensity shows the increasing tendency. Although there are still some controversies about the assignments of the individual components of the C 1s and N 1s core level spectra [19–21], the X-ray photoelectron spectroscopy (XPS) analysis is one of the most likely used techniques in the literature to characterize the formation,
bond types and other useful information on the chemical environment around O, C and N of CN phases that can be obtained [22]. XPS was measured by SSX-100 XPS system of Surface Science Instruments utilizing using an AlKa´ (hv=1486.6 eV) radiation as an X-ray source, under high vacuum conditions of about 10 10 torr. The chemical bonding state in the films were analyzed after the 0.5 keV Ar+ ion etching of film surface for 3 min. The chemical composition of the deposited films can be obtained according to oxygen to C (O/C) and the N to C atomic ratio (N/C). The N/C and O/C content in the films were evaluated using N/C=(A N/1.68)/(A C/1.00) and O/C=(A O/ 2.49)/(A C/1.00) where A N, A O and A C are the areas under the N 1s, O 1s and C 1s core level spectra, and the constants of 1.68, 2.49 and 1.00 are the atomic sensitivity factors of N, O and C, respectively. Fig. 3a shows N/C atomic ratio (N content) of the aCNx films (Method 1) evaluated from XPS measurements as a function of ST. As shown in Fig. 3a, the N/C ratio in the film is about 3.69 atomic percentage (at.%) at 20 8C and increased gradually with an increase in the ST up to about 15.05 at.% 400 8C, but it decreased at ST higher than 400 8C up to about 10.19 at.% at 500 8C. With the increase in ST, the surface migration of C and N species will be enhanced, which will accelerate the chemical reaction between C and N and consequently improve the N content. Though, at ST higher than 400 8C, N content in the film decreases because the stable CUN bonding will remain, while the volatile bonding will be decomposed, resulting in N molecules released from the films [23]. The increase of N content in the film might also be explained by the fact that the lower deposition rate induced by ST corresponded to the lower C/N arriving ratio, therefore, the higher N/C ratio in the a-CNx films. So, the C to N species on the substrate surface is considered to determine the final N/C atomic ratio in the a-CNx films within this range of ST (up to 400 8C). This could be confirmed by comparing with Figs. 3a, 4a, 5a. Furthermore, the increase of ST leads to an increase of C atoms’ mobility. Some weak bonding, such as nitrile and other interstitial species, may become volatile and are detached from the film, which results in the decrease of N/C ratio in the deposited films (over 400 8C). In addition, the high desorption rate of CN and CNH species from the growth surface and the lower incorporation of volatile CUN species at elevated temperatures due to their low temperature of sublimation may also contribute to the low N/C ratio in deposited films [24]. The decrease of N content with increase of ST (over 400 8C) may also be due to the chemical sputtering effect. The sputtering rate of N in a thin film is much higher than that of C [25]. Though our result has shown the preferential sputtering of N, it may be responsible for the reduction of N content incorporated in the a-CNx films deposited at higher ST (over 400 8C). We also suggest that with an
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increase of ST up to 400 8C, the surface migration of C and N species will be enhanced, which will accelerate the chemical reaction between C and N and consequently improve the N content. Though at ST higher than 400 8C, N content in the film decreases because the stable CUN bonding will only remain since the volatile bonding decomposes and N molecules are released from the films [23]. It is well known that for the deposition of DLC films, for temperatures higher than about 200 8C, the surface mobility is sufficient for forming the more energetically favorable graphitic structure over the amorphous phase found at lower temperatures [26]. Therefore, when depositing a-CNx films above that temperature, we can assume that N atoms, and possibly also UCQN molecules, will diffuse on the deposition surface until they either become bound in a low-energy lattice site or react with other particles to form volatile molecules that can desorb. At higher temperatures, the diffusion rate becomes higher, so more reactions are likely to take place. Furthermore, due to surface mobility, we can assume that C diffusion becomes more important at higher temperature, so N from the subsurface region could diffuse toward the surface, and there they recombine or react and eventually desorb. Therefore, these can thus explain the reduced N incorporation with increasing ST over 400 8C. These desorption of N2 will suppress the N incorporation, while the desorption of C-containing species will involve the surface reaction process [27]. Pure carbons are unlikely to be desorbed since that would require physical sputtering, but C2N2 and, to some extent also, other CN species, can be expected to be involved. The formation of these volatile molecules may happen in many ways. It is likely that first UCQN dimmers are formed, either by dissociation of C2N2+ ions at impact at the deposition surface, or by reactions between C and N on the film surface. These dimmers can encounter other UCQN dimmers when diffusing on the surface to form volatile C2N2 molecules. A small residual amount of UCQN in the a-CNx films, in which increases in ST have been confirmed by FTIR, was also present. The formation of C2N2 molecules could result in a lower total N concentration in the a-CNx films deposited at higher ST over 400 8C. This type of a chemical sputtering process would also result in a reduced deposition rate, especially at elevated ST [27]. Fig. 3b and c shows the increase of N/C ratio with the increase of LF (Method 2) and decrease of TSD (Method 3), respectively. This may mean that the LF and TSD might affect the plasma dimension and distribution, which could also influence the arrival of C to N, affect the deposition rate and finally affect the N/C atomic ratio, and agree with Zhao et al. [28] who described the effect of LF on the N/C atomic ratio. The correlation of N content and deposition parameters has also been confirmed by AES and FTIR analyses. This is evidence that the composition and microstructure of
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the a-CNx films can be tuned by the ST, LF and TSD. An overview of XPS spectra of the deposited films indicates the presence of a very small O 1s content, in addition to the C and N constituents. The O 1s intensity decreases constantly with depth during sputter cleaning of the film surface with Ar+ ions, consistent with the presence of a small amount of oxygen resulting from the surface contamination. In addition to C and N, O/C was detected less than 1.5 at.% as a minor contamination (superficial) for Sample A and a-CNx samples deposited at 20 8C (Method 1) with N/ C=3.69 at.%. As shown in Fig. 3a (right axis), it was found that as the ST is increased from 20 through 400 to 500 8C, the oxygen content in the a-CNx films increases from 1.5% through 10.2% and decreases to 9.5%, respectively. The reason behind the increased amount of O/C and N/C in the films with higher ST can be due to the porosity and low density of the a-CNx material [29]. However, it should be noted that the presence of N in the C networks and oxygen due to minor contamination may also increase the electro-negativity of the a-CNx compounds. As a result, with an increase of ST up to 400 8C, the N content in the a-CNx films reached a maximum value, while the deposited films attain more polarity and tend to absorb H2O molecules present in the experimental atmosphere, and therefore increase the amount of oxygen in the a-CNx films. With further increase of ST up to 500 8C, the decreased N content in the a-CNx films caused the decreased electro-negativity and polarity of deposited films. As a result, the oxygen content in the a-CNx films decreased in ST over 400 8C. The bonding states of C, N and hydrogen (H) atoms can be characterized by Fourier transform infrared spectroscopy (FTIR) measurements. The FTIR spectra were measured in the wave number range from 700 to 3200 cm 1. FTIR absorption for CUH stretching vibrations on sp2-C is found in the 2950–3060 cm 1 range, while for sp3-C, it is in the 2850–2945 cm 1 range [30], and the peaks around 2926 and 2956 cm 1 are the most prominent, assigned to UCH2sp3 asymmetric and UCH2sp2 olefinic bonds, respectively, and the peaks around 2855 cm 1 indicate the formation of sp3 bonding [31]. Band 1 at around 700 cm 1 is due to the out-of-plane bending mode for the graphite-like domain [32]. The peak at around the range 1000–1700 cm 1 (band 2) can be assigned to C¯N and CMN bonds and the broad band extending in the range 2800–3000 cm 1 (band 3) corresponds to different CUH configurations [33,34]. Broad absorption band 2 indicates that the films’ structure is predominantly amorphous with sp2-C vibration modes, and is related to Raman active D and G modes [35]. The contributions around 1350 and 1550 cm 1 were initiated from disordered (D band) and graphite-like (G band) CN bonds, respectively [14]. Another contribution at around 1145–1265 cm 1 was due to the symmetric tetrahedral CN bond [36,37]. According to Kaufman et al. [38], these Raman active D and G modes become IR
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active due to the incorporation of N atoms into C network that results in the symmetry breaking of the C network. Band 4 at 1100–1400 cm 1, band 5 at 1500–1700 cm 1 and band 6 at around 2200 cm 1 are due to CUN, C=C and/or C=N and C=N stretching vibration modes, respectively [39–41]. Fig. 4 shows the FTIR transmittance spectra measured in the wavenumber range 1000–3200 cm 1 for Sample A and a-CNx films deposited by Method 1 (Fig. 4a), Method 2 (Fig. 4b) and Method 3 (Fig. 4c). Broad absorption band 2 and band 6 do not appear in Sample A. The absorption peak at around 2350 cm 1, which is observed in a-CNx and Sample A, can be attributed to the CO2 stretching mode arising from oxygen contamination at the film surface [42]. FTIR spectra show that as ST (Method 1) and LF (Method 2) increase and as TSD (Method 3) decreases, the intensity of band 2 and the three characteristic peaks of DLC in the 2800–3000 cm 1 region (band 3) become more and more prominent, suggesting the formation of crystalline structures in the films [43]. The different intensity of band 2 (Fig. 4a) may indicate that there is a different modification of the binding geometry (compared with Fig. 4b and c), which is likely induced by the increase of internal film stress arising from the increase of the trihedral bond density due to the increase of ST. The N content in the films gradually increases with the increase of ST and LF and decrease of TSD, as indicated by the increase in the intensity of bands 2, 4 and 5 of the C and N bonding peaks. These phenomena may suggest that at higher ST and LF, and short distance of TSD, films of high N and hydrogen content (band 3) are achieved in the films. The event may take place during the deposition process; also, adsorption from the atmosphere ambient may contribute to the H content. Since the bonding energy of the CUH configurations is relatively low, collisions between Ccontaining radicals and hydrogenated products present in the CC target may further increase the probability of H incorporation with the increase of ST and LF and decrease of TSD. FTIR shows that the N incorporation and crystalline structure of the films increase with the increase of ST and LF and decrease of TSD. This is evidence that the composition of the films can be tuned by optimizing the ST, LF and TSD. Fig. 5 shows FTIR transmittance spectra of (a) Sample A and a-CNx films (Method 1), measured in the wavenumber range of 600–1900 cm 1, (b) reference sample of a-C films that deposited by the same deposition parameters as Method 1 but without N gas ambient, measured in the wavenumber range of 2750–3050 cm 1 and (c) a-CNx films (Method 1), which were measured in the wavenumber range of 2750– 3050 cm 1. It is clear from the spectra (Fig. 5b and c), that as ST increases, the three characteristic peaks of DLC [43] in the 2850–3000 cm 1 region (band 3) become more and more prominent, suggesting the formation of crystalline structure in the films. It should be noted here that the increasing peaks degree of a-CNx films (Method 1) (Fig. 5c)
is higher than a-C films that deposited in the same range of ST, but without N gas ambient (Fig. 5b). The absence of clear appearance of peaks in the a-C up to 200 8C suggests the amorphous nature of the films. As can be seen in Fig. 5a, the single broad band of a-CNx film deposited at 20 8C, centered at around 1350 cm 1, has gradually shifted to a higher wavenumber and reached about 1550 cm 1 at 500 8C. This red shift indicates that there is a modification of the binding geometry, which is likely induced by the increase of internal film stress arising from the increase of the trihedral bond density due to the increase of ST. In our spectra, bands 1, 2, 3 and 4 (sp3 C N tetrahedral), band 5 (sp2 C N trihedral) and band 6 of aCNx films deposited by Method 1 are weak and as ST is increased, starting from around 200 8C, the broad absorption band 2 becomes narrow, and although small, the intensity of bands 1, 4, 5 and 6 increase gradually up to 400 8C and decrease thereafter. This indicates that the components of the CUN, C=N, C=C and C=N bonds increase with the ST up to 400 8C and decrease thereafter, which is also confirmed by the XPS analyses. The N content in the films gradually increases with the increase of ST up to 400 8C, as indicated by the increase in the intensity of the C and N bonding peak areas. The D band (1350 cm 1) gradually increased with ST up to 400 8C and decreased with higher ST, thereafter. While, the G band (1550 cm 1) seems to be gradually increased with ST up to 400 8C, after which it increased rapidly. The FTIR spectra thus indicated the transformation to the graphite-like sp2 CUN bonds of a-CNx films up to 400 8C, after which the transformation degree increased. The FTIR spectra also indicated up to 400 8C, the tetrahedral sp3 C N bonds in the a-CNx films increased, and decreased with higher ST up to 500 8C. FTIR measurement has shown that the N incorporation and crystalline structure of the films increase with the increase of ST and LF and decrease of TSD. The optical properties and electrical resistivity of the films are investigated by UV–visible spectroscopy in the range of 200–2500 nm and the four-point probe resistance measurements, respectively. Fig. 6 shows the optical gap (E g) and electrical resistivity (q) of Sample A is 0.95 eV and 5.1105 (V cm), respectively. The E g of a-CNx film (Method 1, Fig. 6a) decreased to 0.86 eV and q is increased to 8.51106 (V cm), compared with Sample A. As LF increases, the E g increases up to 0.93 eV at 2.8 J/cm2 and thereafter shows the rate of increase to reduce at higher LF, indicating saturation. While q increases up to 9.01106 (V cm) at 5.2 J/cm2, making these a-CNx films interesting. The E g of a-CNx film (Method 2, Fig. 6b) decreases to 0.93 eV and q is increased to 8.61106 (V cm), compared with Sample A. As TSD decreases, the E g decreases to 0.48 eV, while q increases up to 9.76106 (V cm) at 25 mm. For the a-CNx films deposited by Method 3, the E g decreases to 0.65 eV and q is increased to 8.77106 (V cm) at 50 8C (Fig. 6c). As ST increases, the E g decreases only a little, to 0.44 eV, while q increases up to 1.36107 (V cm) at 400
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Fig. 6. The optical gap (left axis) and resistivity (right axis) of Sample A, and a-CNx films with various: (a) substrate temperatures (Method 1), (b) laser fluences (Method 2) and (c) target to substrate distances (Method 3). (Left axis: A for Sample A and E for a-CNx films. Right axis: B for Sample A and R for aCNx films).
8C. However, above 400 8C, both E g and q are drastically decreased down to 0.3 eV and 8.45104 (V cm) at 500 8C, respectively. The increases in q with N incorporation may be due to the lattice vibrations leading to the scattering of the charge carriers by the N atoms and more amorphous nature of the C films. Variation of the optical and electrical properties can be related to interstitial doping of N in C films through modifications of CUN bonding configurations by rearranging N atoms upon increase of LF and ST and decrease of TSD. However, with further increase of ST above 400 8C, both E g and q are drastically decreased down to 0.3 eV and 8.45104 (V cm) at 500 8C, respectively. This is probably due to the graphitization of the a-CNx films. Perhaps the doping of N accompanied by an increase of ST above 400 8C increases crystallinity and subsitutional doping of N thereby sharply decreasing resistivity. These phenomena are also supported by the XPS and FTIR spectroscopy measurements.
4. Conclusions Tuning the LF within the 2.8–5.2 J/cm2 domain (Method 1), TSD within the 25–45 mm domain (Method 2) and ST within the 20–500 8C domain (Method 3) results in significant changes in the chemical composition and electrical properties of a-CNx layers deposited by PLD using CC target in ambient 0.8 torr N gas pressure. With higher LF (Method 1), the N content, E g and q are increased. Decreasing TSD (Method 2) results in an increase in the N content and q, whereas E g is decreased. While with higher ST (Method 3), the E g is decreased. N content and q increase with ST up to 400 8C and decrease thereafter. We found that the incorporated N atoms in a-CNx layers were chemically bonded to C, and the amorphous structure of aCNx layers and the ratio of sp2 trihedral component to sp3 tetrahedral component are strongly dependent on the ST, LF and TSD. The a-CNx layers with high N and hydrogen content have high electrical resistivity.
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