Amorphous Nitrogenated Carbon Films Research Articles

Low temperature critical growth of high quality nitrogen doped graphene on dielectrics by plasma-enhanced chemical vapor deposition.

Nitrogen doping is one of the most promising routes to modulate the electronic characteristic of graphene. Plasma-enhanced chemical vapor deposition (PECVD) enables low-temperature graphene growth. However, PECVD growth of nitrogen doped graphene (NG) usually requires metal-catalysts, and to the best of our knowledge, only amorphous carbon-nitrogen films have been produced on dielectric surfaces by metal-free PECVD. Here, a critical factor for metal-free PECVD growth of NG is reported, which allows high quality NG crystals to be grown directly on dielectrics like SiO2/Si, Al2O3, h-BN, mica at 435 °C without a catalyst. Thus, the processes needed for loading the samples on dielectrics and n-type doping are realized in a simple PECVD, which would be of significance for future graphene electronics due to its compatibility with the current microelectronic processes.

Mechanical Properties, Thermal Properties and Microstructures of Amorphous Carbon-nitrogen Films

We have investigated the relationship among thermal properties and mechanical properties of the amorphous carbon films containing nitrogen prepared by the RFplasma enhanced CVD. Thermal conductivity was measured by the 3 w method and related with In!I0 from the Raman spectroscopy, N/C and density from the RBS/ERDA and hardness from the nanoindentation method. The thermal conduction in the films was discussed in terms of the graphite cluster size La derived from the Inllo and correlated with the mechanical properties and tribological properties.

OPTICAL AND STRUCTURAL PROPERTIES OF NITROGENATED DIAMOND-LIKE CARBON FILMS PREPARED BY r.f. PECVD

Nitrogenated diamond-like carbon films have been deposited on glass and p-type Si (100) substrates by radio frequency (r.f.) plasma-enhanced chemical vapor deposition (PECVD) with a frequency of 13.56 MHz at room temperature using CH 4 as precursor of carbon source and H 2 as a carrier gas. The deposition was performed at a different flow rate of nitrogen from 0 to 12 sccm under a constant r.f. power. The effect of nitrogen incorporation on the bonding states and growth kinetics of the deposited films have been investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy and optical properties by UV spectroscopy measurement. Our experimental results show that the incorporation of nitrogen has a considerable effect on the properties of the deposited films. FTIR spectra show that the nitrogen is bonded to carbon and hydrogen as C=N , C≡N , N–H and C–H bonding configurations in the as-deposited film. The incorporation of nitrogen is found to shift the Raman G peak toward the higher wave number and to increase the Raman I D /I G ratio demonstrating the graphitic character of the hydrogenated amorphous carbon–nitrogen films. Band gap is found to reduce with the increase in nitrogen concentration.

IR characterization of a-C:H:N films sputtered in Ar/CH 4/N 2 plasma

Amorphous nitrogenated carbon films (a-C:H:N) were deposited by rf magnetron sputtering of a graphite target in an Ar/CH 4/N 2 plasma. The films were characterized by infrared spectroscopy. An increase in the intensity of absorption maximum at 3300–3400 and 1620 cm −1 of the IR spectra was observed. For high nitrogen concentrations, a new maximum appears at 2150 cm −1 indicating that the C and N atoms are chemically bonded in the film. The analysis of the broad absorption band observed between 1700 and 1000 cm −1 was made. Deconvolution of this band showed a six band system, indicating a lack of aromatic structures. The content of nitrogen and sp 2 bonded carbon in the films was seen to increase with a growth of the N 2 partial pressure in the deposition gas.

Spectroscopic analysis of the structure of amorphous nitrogenated carbon films after wear tests

The effects of wear tests on the electronic structure of amorphous nitrogenated carbon (a-C:H:N) films prepared in a radiofrequency plasma-enhanced chemical vapor deposition system from a mixture of methane and nitrogen have been investigated. The tribological properties were investigated with a tribometer in a ball-on-disk configuration. For different N 2 fractions, film structure and chemical composition were examined using conventional Raman and X-ray photoelectron spectroscopy, and also by means of high-lateral-resolution soft X-ray photoemission microscopy using synchrotron radiation. Nitrogen incorporation into the as-deposited amorphous carbon network results in an increase in the number of Nsp 2-C bonded sites with respect to the films grown in a nitrogen-free atmosphere. The wear stability of the films was analyzed as a function of the nitrogen fraction, and a thorough description of the electronic structure was obtained in the as-deposited state and after wear testing. High-lateral-resolution photoemission microscopy reveals for the first time that inhomogeneities within the film after wear testing are correlated to nitrogen incorporation. The study of energy distribution curves and high-lateral-resolution images on the nitrogenated samples shows that a modification of the surface chemistry occurs by mechanical action.

Substrate bias effect on amorphous nitrogenated carbon films deposited by filtered arc deposition

The effect of substrate bias and nitrogen incorporation on the structure and hardness properties of amorphous carbon (a-C) films is characterized in terms of its composition, sp3 bonding fraction, and Raman spectra. The films’ properties were analyzed using Raman spectroscopy, Auger electron spectroscopy (AES), and nanoindentation system (NIS). Experimental results indicate that a-C films were found to possess the highest hardness when deposited at a substrate bias ranging from −50 to −100 V. In addition, the hardness values do not correlate well with the Raman I(D)/I(G) ratio. The hardness of films increases with their sp3 bonds fraction. Furthermore, nitrogen content increases with increasing substrate bias on amorphous nitrogenated carbon (a-C:N) films, and influences the growth of small graphitic crystallines.

Nitrogen doping of tetrahedral amorphous carbon films: Scanning tunneling spectroscopy

Amorphous nitrogenated carbon films with nitrogen atomic concentration between 12% and 29% were deposited using a filtered cathodic vacuum arc and a Kaufman-type ion source. The surface topography of the samples has been investigated by scanning tunneling microscopy in ultrahigh vacuum, showing that the roughness of the film surface decreases with nitrogen concentration. Scanning tunneling spectroscopy is employed to understand the role of nitrogen in the change of the surface microstructure and electronic structure near the Fermi level. The tunneling current (I)–bias voltage (V) curve is flat at low bias regions indicating a finite gap for the sample with low (12%) nitrogen concentration. An increase of tunneling current and its nonlinearity along with the decrease of energy gap occurs in the samples with increase of N concentration. The observed surface density of states [(dI/dV)/(I/V)] has been fitted as a square-root function of bias voltage. An improvement of the quality of these fits in the films with the increase of nitrogen concentration suggests that a depletion of defect density of states near the Fermi level (EF) takes place. These analyses could be attributed to the modification of the structure of amorphous carbon by a large concentration of nitrogen.

Near edge x-ray absorption fine structure of thermally annealed amorphous nitrogenated carbon films

The change of structure of tetrahedral amorphous carbon (ta-C) films after nitrogen incorporation, particularly at a high concentration, was studied by near edge x-ray absorption fine structure (NEXAFS) spectroscopy and it was found to be very close to pyridine. The π* peak at the N K (nitrogen K) edge was decomposed into three components corresponding to different resonances. From a detailed analysis of N K edge by NEXAFS spectroscopy it was revealed that as the nitrogen concentration in the films increases, the σ*/π* intensity ratio decreases, indicating that there is an increase of the amount of C=N relative to the C–N bonds. By thermal annealing at different temperatures, up to 800 °C, the nitrogen concentration in the films is reduced. Intensity as well as the position of the π* peak at the C K edge changed with annealing temperature. At the same time, a decrease of the intensity of the π* peak at the N K edge and a very interesting change of the relative intensities of the three split components of this π* peak have been observed. The possible changes of structure of nitrogenated carbon films by annealing and thermal stability of the films have been thoroughly emphasized.

Bonding regimes of nitrogen in amorphous carbon

Abstract Nitrogen can have numerous effects on diamond-like carbon: it can dope, it can form the hypothetical superhard compound C 3 N 4 , or it can create fullerene-like bonding structures. We studied amorphous carbon nitrogen films deposited by a filtered cathodic vacuum arc as a function of nitrogen content, ion energy and deposition temperature. The incorporation of nitrogen from 10 −2 to 10 at% was measured by secondary ion mass spectrometry and elastic recoil detection analysis and was found to vary slightly sublinearly with N 2 partial pressure during deposition. In the doping regime from 0 to about 0.4% N, the conductivity changes while the sp 3 content and optical gap remain constant. From 0.4 to ∼10% N, existing sp 2 sites condense into clusters and reduce the band gap. Nitrogen contents over 10% change the bonding from mainly sp 3 to mainly sp 2 . Ion energies between 20 and 250 eV do not greatly modify this behaviour. Deposition at higher temperatures causes a sudden loss of sp 3 bonding above about 150°C. Raman spectroscopy and optical gap data show that existing sp 2 sites begin to cluster below this temperature, and the clustering continues above this temperature. This transition is found to vary only weakly with nitrogen addition, for N contents below 10%.

Growth, microstructure and electronic properties of amorphous carbon nitride films investigated by plasma diagnostics

Amorphous nitrogenated carbon films are prepared in a dual electron cyclotron resonance-radio frequency plasma from a mixture of methane and nitrogen gas. A marked variation of electronic properties and microstructure of the films as a function of nitrogen concentration is observed from Fourier transform infrared (FTIR) spectra, electron energy loss spectra, optical absorption spectra, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and electrical conductivity. From the variation of intensity of different positive ions and neutral radicals, using quadrupole mass spectroscopy and optical emission spectroscopy, the growth rate, structure and properties of the films are investigated. The density of methyl (CH3) radicals and the film growth rate are found to decrease with the increase of nitrogen concentration. A correlation between the C/N atomic ratio in the films and CH/CN and also CH/N ratio in the plasma is noticed. Also, the CH radical intensity in the plasma and the amount of CH bonds in the films, observed from FTIR spectra, vary in a similar fashion as a function of nitrogen concentration. A model describing film growth and nitrogen incorporation in the films is proposed.

Effects of thermal annealing on the microstructure and mechanical properties of carbon–nitrogen films deposited by radio frequency-magnetron sputtering

Amorphous carbon–nitrogen films deposited by radio frequency-magnetron sputtering were annealed in vacuum for 30 min at temperatures between 300 and 700 °C, without any kind of sequential annealing. The annealing-induced modifications on the chemical composition of the films were followed by ion beam analysis (IBA). Raman scattering and infrared absorption spectroscopies were used to determine the microstructure modifications, while atomic force microscopy (AFM) was used to investigate the surface morphology. The internal stress of the films was obtained by measuring the bending of the substrate and the nanoindentation technique was used to measure the film hardness. Besides the nitrogen loss, determined by IBA analyses, Raman results suggested an increase in the size or in the number of the graphitic domains with the annealing temperature. AFM shows a decrease of the surface roughness as a function of the annealing temperature. The density, the hardness, and the internal stress of the films present a similar dependence on the annealing temperature, i.e., they increase with the temperature of the thermal treatment. The thermal treatment induces a structural modification on the carbon–nitrogen films changing from a soft paracyanogen-like material to a harder and more graphitic one.

Studies on structural properties of a-CN:H films prepared in electron cyclotron resonance plasma

Abstract Amorphous nitrogenated carbon films were prepared from a mixture of acetylene and nitrogen in an electron cyclotron resonance plasma and were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and spectroscopic ellipsometry. In Raman spectra the ratio I D / I G increases by a factor of five when adding 5.5% of nitrogen in the carbon films, but unlike the previous reports, no significant shift of the G peak was noticed. Ellipsometry experiments, according to electron energy loss spectroscopy results, do not show any significant change in the π plasmon peak position but a reduction of its area was observed for the nitrogenated samples which may suggest that there was no improvement of the graphitic structure after nitrogen introduction. On the other hand, the displacement of the maximum of the N 1s peak indicates a modification in the electronic structure of the film as a function of the nitrogen concentration.

Studies on structural properties of amorphous nitrogenated carbon films from electron energy loss, ellipsometry, Auger electron spectroscopy, and electron-spin resonance

We report the variation of the structure and the electronic properties of amorphous nitrogenated carbon films (a-CH:Nx) prepared in dual electron cyclotron resonance-radio-frequency plasma from different mixtures of methane and nitrogen. Electron energy-loss spectroscopy, Auger electron spectroscopy, spectroscopic ellipsometry, and electron-spin-resonance spectroscopy are used to characterize the films. Unlike previous reports, addition of a low percentage (2.3%) of nitrogen in the films induces a strong change in their structure. The variation of electronic properties is rather small for a high concentration of nitrogen. From these experimental studies it seems that the efficiency of nitrogen doping depends on the nitrogen concentration. Modification of structure of the carbon network by nondoping and doping configurations of carbon nitrogen bonds is also emphasized. Our analyses establish an inter-relationship between the structure and electronic properties of nitrogenated carbon films, which helps to understand the structural change occurring in the carbon films with the incorporation of a low amount of nitrogen.

Deposition of amorphous nitrogenated carbon films using an alternate plasma deposition method in a dual electron cyclotron resonance-radio frequency plasma

In this article, we present a new method for the synthesis of amorphous nitrogenated carbon films using an alternate CH4 plasma deposition/subsequent N2 plasma treatment, so-called alternate plasma deposition (APD), in a dual electron cyclotron resonance (ECR)-rf plasma. The nitrogen to carbon ratio in the film surface deposited by this method, estimated by x-ray photoelectron spectroscopy (XPS), is considerably higher, by about a factor of 2, than that obtained in the film deposited from a conventional CH4/N2 plasma. From infrared spectroscopy it is shown that the APD film has the more enhanced disordered phase, which is accompanied by an increase of the amount of sp2 hybridized carbon-nitrogen bonds, as shown by XPS spectra.

Synthesis, composition, surface roughness and mechanical properties of thin nitrogenated carbon films

In this study, we explored the properties of amorphous nitrogenated carbon films grown in a standard magnetron sputter-deposition system without a magnetic trap. Silicon wafers and hard disk substrates were used. To simplify process control, we operated the graphite target at constant voltage and used 3% nitrogen in argon from a premixed feed. Under these conditions, most nitrogenated carbon films contain about 6–8 a/o of nitrogen. Films with the highest hardness values (28 GPa) and lowest root-mean-square surface roughness (0.2 nm on hard disk substrates) were obtained at low sputtering pressure (5 mTorr) and large substrate pulse bias (−300 V). These observations demonstrate the importance of ion bombardment in controlling film morphology and properties. The harder films appear to have better resistance against scratch damage, as is demonstrated by the nano-scratch tests using a two-dimensional nanoindentor.

Spectroscopic determination of the structure of amorphous nitrogenated carbon films

Studies on structure and electronic properties of amorphous nitrogenated carbon films prepared in dual electron cyclotron resonance–radio frequency plasma from a mixture of methane and nitrogen are presently reported. These films are characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), electrical conductivity measurement, and optical absorption spectroscopy. Symmetry breaking of aromatic rings are at a very small amount of nitrogen incorporation is understood from FTIR spectra. The relative contribution of C=N and C–N bonds is found to change with the variation of the nitrogen content in the samples, which shows a similar trend with the shift of the G peak to a higher wave number and the increase of the ID/IG ratio. From decomposition of XPS C 1s and N 1s peaks a three-phase model of CN bonds is proposed. UPS valence band spectra obtained by using a Helium II source, are decomposed into p-π, p-σ, 2s bands and a mixture of s-p band. The intensity of p-π band increases as a function of nitrogen concentration, confirming the increase of sp2 bonds in the samples. An enhancement of the room temperature electrical conductivity and a decrease of the optical gap are observed with the addition of nitrogen in the films. The effect of nitrogen doping in carbon films is also emphasized. Our analyses establish an interrelationship between the microstructure and electronic structure of nitrogenated carbon films, which helps to understand the change in electronic properties of the carbon films due to a low amount of nitrogen incorporation.

Determination of the structure of amorphous nitrogenated carbon films by combined Raman and x-ray photoemission spectroscopy

Amorphous nitrogenated carbon (a-CN:H) films are prepared from a mixture of methane and nitrogen in an electron cyclotron resonance plasma at a pressure of 2 mTorr and applying a substrate bias of −300 V. Based on the characterization done by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on the films containing different amounts of nitrogen, a similarity in variation between the binding energy of C 1s peak and position of Raman active G peak is noticed. XPS C 1s and N 1s are deconvoluted into four components to separate the contribution of CC, C–N, C=N, and C≡N bonds. The change of intensity ratio of Raman active D and G peaks shows a marked correlation with the intensity ratio of single and double bonds between carbon and nitrogen. Combined XPS and Raman spectroscopy throw some light on the variation in structure of carbon films as a function of nitrogen concentration.

Raman spectroscopy of annealed carbon nitride films deposited by RF-magnetron sputtering

Abstract Amorphous carbon-nitrogen films deposited by RF-magnetron sputtering onto Si substrates were annealed in vacuum in the temperature range of 300–800 C during 30 min. with no sequential treatment. The films were analyzed by Raman spectroscopy, used as a probe of microstructural modifications induced by thermal treatment. The main features observed in the Raman spectra are two broad bands at ∼1360 cm−1 (D-band) and ∼ 1575 cm−1 (G-band), characteristic of amorphous carbon materials. The ratio between their intensities ( I D I G ) is found to increases with the annealing temperature. Additionally, the spectra present other two weak bands, one at ∼700 cm−1 (I700) and another at 2230 cm−1 (I2230). Normalized against the G-band, their intensities present opposite temperature dependence: I700 decreases by a factor of three, while I2230 increases by the same factor in the temperature range studied here. A strong reduction of the luminescence background with the annealing temperature is observed. These results indicate a substantial reduction of the dangling bonds accompanied by an increase of the number of CN bonds and of the size, or the number, of the graphitic domains.

On the Effects of the Thermal Treatment on the Structural, Mechanical and Tribological Properties of Amorphous Carbon-Nitrogen Films Deposited by Magnetron Sputtering

ABSTRACTAmorphous carbon-nitrogen films, a-CNx, deposited by rf-magnetron sputtering in N2 atmosphere were annealed in vacuum at temperatures between 300 and 700 °C. The annealing time was 30 minutes. The modifications on the film microstructure were monitored by infrared spectroscopy (IR), while the composition and the atomic density were determined by Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA). The internal stress was determined by measuring the film-induced bending of the substrate and the hardness was measured by nanoindentation. Atomic force microscopy (AFM) provided the friction coefficient and the surface roughness. The ratio between nitrogen and carbon atomic concentration decreases for temperatures higher than 500 °C, whereas the film density increases with the annealing temperature: 40 % in the temperature range here studied. The behavior of the D and G Raman bands, IR active due to the nitrogen incorporation in the carbon network, suggests a progressive increase of the size of the graphite-like domains. The hardness of the as-deposited a-CNx film is around 2 GPa. However, both hardness and internal stress increase by a factor of three in samples annealed at 700 °C, while the surface roughness and the friction coefficient decrease by a factor of about two.

Structure and mechanical properties of hard amorphous carbon-nitrogen films obtained by plasma decomposition of methane-ammonia mixtures

Hard amorphous hydrogenated carbon-nitrogen films (a-C(N):H) were deposited by rf plasma decomposition of methane-ammonia mixtures onto silicon substrates. The incorporation of nitrogen increases with the partial pressure of ammonia. On the other hand, the deposition rate decreases. The samples were characterized by a a combination of analytical techniques: MeV ion beam techniques (RBS,NRA and ERDA), SIMS, AFM, EELS, infrared absorption and Raman spectroscopy. The Vickers hardness and the internal stress of the films were also determined. Infrared results indicate that the nitrogen is incorporated into the amorphous network mainly as terminating groups at the expense of C-H radicals. EELS and Raman results show that the incorporation of nitrogen induces a progressive graphitization of the films. The incorporation of up to 11 at% nitrogen reduces the internal stress of the films by a factor of two without significant modification of the film hardness.