D. F. Franceschini

Nitrogen modification of hydrogenated amorphous carbon films

The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition, sp3 bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. % N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner.

Nitrogenated amorphous carbon as a semiconductor

Abstract At present, hydrogenated amorphous carbon (a-C:H) is a poor electronic material primarily due to the excessive density of defect states in the band gap which act as trapping centres. The ability of nitrogen to improve the semiconducting properties of a-C:H is examined. A reduction in the activation energy for electronic conduction in nitrogenated a-C:H (a-C:H:N) films and the approximately constant optical band gap with increasing N content suggest that N influences the bulk electronic properties of a-C:H. Electron spin resonance shows a reduction of the density of gap states in a-C:H:N with increasing N content. Electron energy loss spectroscopy shows the films to be predominantly sp2 bonded with band edge properties which change significantly as a function of the N content. The C:H:N contents of the films were determined by elastic recoil detection analysis and Rutherford backscattering.

Hard amorphous hydrogenated carbon-nitrogen films obtained by PECVD in methane-ammonia atmospheres

Abstract Hard a-C(N):H films were deposited onto Si(100) substrates by r.f. self-bias glow discharge in CH 4 NH 3 atmospheres (NH 3 content varying from 0% to 12.5%). The chemical composition of the films was determined by nuclear techniques, and the film structure was monitored by IR and Raman spectroscopies. The nitrogen incorporation into the films was found to be more than four times greater than that previously obtained with N 2 gas as the nitrogen source at the same partial pressure. Nitrogen incorporation up to levels of 11 at.% resulted in a 50% decrease in the internal compressive stress. Raman spectra showed that nitrogen incorporation increased the size or number of graphitic domains in the film, while exhibiting smaller changes in the I D / I G ratio. IR spectra showed the same trend as observed in N 2 -derived films, with an increasing presence of nitrogen-containing network terminating groups at the expense of C H groups.

Influence of precursor gases on the structure of plasma deposited amorphous hydrogenated carbon–nitrogen films

The atomic structure of amorphous hydrogenated carbon–nitrogen films was studied by electron energy loss spectroscopy (EELS). The films were deposited onto Si(100) substrates by rf plasma decomposition of CH4–NH3 and CH4–N2 mixtures, with substrates placed on the powered electrode of a diode glow‐discharge system. The sp2 fraction of C and N atoms as a function of the nitrogen content in the films was obtained by EELS analysis. An increase of the carbon sp2 fraction with increasing fraction of NH3 and N2 feed gases was observed. The variation in the atomic structure of the a‐C(N):H thin films is correlated to the internal compressive stress.

Growth kinetics and relationship between structure and mechanical properties of a-C(N):H films deposited in acetylene–nitrogen atmospheres

Amorphous hydrogenated carbon–nitrogen films, a-C(N):H, were deposited by plasma enhanced chemical vapor deposition using acetylene–nitrogen mixtures. Film composition and density were determined by means of ion beam techniques being the film microstructure studied by infrared and Raman spectroscopies. Films were obtained with nitrogen content up to 22 at. %. As for films obtained using other gas mixtures, the deposition rate showed a strong decrease upon nitrogen incorporation, although with a smaller rate. The film growth kinetic is discussed and some specific features of acetylene–nitrogen precursor gas mixtures are pointed out. A remarkable decrease on the C atom sp3 fraction was inferred for nitrogen contents higher than 10 at. %, and was correlated to the film density behavior. The mechanical hardness and internal stress were relatively insensitive to low nitrogen incorporation, with a systematic decreasing behavior for nitrogen contents above 10 at. %.

Carbon nitride thin films prepared by reactive sputtering: Elemental composition and structural characterization

Amorphous carbon nitride thin films (a-CNx) have been deposited onto Si (100) substrates by using a rf diode sputtering system. The films were deposited in reactive nitrogen-argon atmospheres. The partial pressure of nitrogen ranged from 0% to 100% at two different deposition pressures (Pd=2 Pa and Pd=8 Pa). The film composition was determined by ion beam techniques: Rutherford backscattering spectrometry and nuclear reaction analysis. The relative amount of carbon and nitrogen in the films is nearly independent of the nitrogen partial pressure in the reactive plasma. The maximum nitrogen content is 48 at. %. The structural characterization was performed by means of Raman and infrared spectroscopies. Raman spectra revealed the presence of the D and G bands, typical of disordered carbon based materials, and a third band, at about 680 cm−1, also attributed to film disorder. Infrared spectroscopy results showed the D and G Raman bands, IR allowed due to the nitrogen incorporation in the carbon network, the presence of a band at 2220 cm−1 due to C≡N bonds, and a broadband at 3300 cm−1 that can be attributed to the O–H stretching of water molecules absorbed in the films’ voids. A linear correlation between the density and the internal stress of the films was also determined and the maximum values of the film density and the mechanical internal stress were measured for films deposited at 50% of nitrogen partial pressure (Pd=8 Pa).Amorphous carbon nitride thin films (a-CNx) have been deposited onto Si (100) substrates by using a rf diode sputtering system. The films were deposited in reactive nitrogen-argon atmospheres. The partial pressure of nitrogen ranged from 0% to 100% at two different deposition pressures (Pd=2 Pa and Pd=8 Pa). The film composition was determined by ion beam techniques: Rutherford backscattering spectrometry and nuclear reaction analysis. The relative amount of carbon and nitrogen in the films is nearly independent of the nitrogen partial pressure in the reactive plasma. The maximum nitrogen content is 48 at. %. The structural characterization was performed by means of Raman and infrared spectroscopies. Raman spectra revealed the presence of the D and G bands, typical of disordered carbon based materials, and a third band, at about 680 cm−1, also attributed to film disorder. Infrared spectroscopy results showed the D and G Raman bands, IR allowed due to the nitrogen incorporation in the carbon network, the p...

Hard a-C(N):H films obtained from plasma decomposition of methylamine-containing mixtures

Abstract Hard amorphous hydrogenated carbon-nitrogen films were obtained by PECVD of pure methylamine and methylamine-acetylene mixtures. The chemical composition of the films was determined by nuclear techniques, and their structure examined by infrared and Raman spectroscopies. The film obtained from pure methylamine have a N C atomic ratio of 0.64, and a Raman spectrum that is characteristic of nanocrystalline graphitic carbon films. The films obtained from acetylene-methylamine mixtures show the same dependence of the structure details on the nitrogen content as already observed in a-C(N):H films deposited using other gas mixtures, i.e. a progressive increase in sp2 character of the carbon atoms when the nitrogen content in the film increases.

Amorphous hydrogenated carbon nitride films obtained by plasma-enhanced chemical vapour deposition

Abstract Amorphous hydrogenated carbon nitride (a-CN x :H) films grown by plasma-enhanced chemical vapour deposition using methane-nitrogen mixtures as precursor gas were characterized by a combination of several techniques: MeV ion beam techniques (Rutherford backscattering, nuclear reaction analysis and elastic recoil detection analysis), secondary ion mass spectrometry, Raman spectroscopy and positron annihilation spectroscopy (Doppler broadening method). The Vickers hardness and the internal stress of the films were also measured. Finally, the thermal stability of the films was investigated. Our results indicate that the incorporation of nitrogen into the amorphous carbon network increases the void density and the number (or size) of graphitic domains. These microstructural modifications and the decrease in the average coordination number may account for the observed reduction of the internal stress of the films. For annealing temperatures higher than 300 °C we observe hydrogen and nitrogen losses as well as the progressive graphitization of the films.

Study of nitrogen implanted amorphous hydrogenated carbon thin films by variable-energy positron annihilation spectroscopy

Hard amorphous hydrogenated carbon (a-C:H) films deposited by self-bias glow discharge were implanted at room temperature with 70 keV nitrogen ions at fluences between 2.0 and 9.0×1016 N/cm2. The implanted samples were analyzed by positron Doppler broadening annihilation spectroscopy to determine the voids distribution. For samples implanted with 2.0×1016 N/cm2 the defect distribution is broader than the vacancies depth profile predicted by Monte Carlo simulation. For higher fluences we observed a reduction of the defect density. These results are discussed in terms of a competition between two processes: ion induced defects and structural modifications induced in the films due to ion implantation.

Amorphous carbon films deposited by direct current-magnetron sputtering: Void distribution investigated by gas effusion and small angle x-ray scattering experiments

Amorphous carbon films were deposited by direct current-magnetron sputtering onto p-doped (100) silicon crystals and onto ultrapure aluminum foils at different argon pressures, ranging from 0.17 to 1.4 Pa. The film density was determined by the combination of the areal density, obtained from ion beam analysis, and the film thickness measured by a stylus profilometer. Film density decreased when the argon pressure used during deposition was increased. Gas effusion measurements indicated that the films deposited at low pressures are more compact than the films deposited at higher pressures. In the case of the latter, C2Hn effusion at temperatures as low as 250 °C indicated that they have an open structure that allows the evolution of large molecules. Small angle x-ray scattering results revealed an increase of the void density with increasing plasma pressure. Guinier plots show that these voids have a broad distribution of sizes, ranged from 7 to 26 A, which is nearly independent of the plasma pressure. A direct correlation between film density and the open volume fraction in the films was found. These different film microstructures could be explained by the existence of different bombardment regimes during film growth: films deposited at lower plasma pressures are hard and dense, while soft films grown at higher pressures have an open microstructure.Amorphous carbon films were deposited by direct current-magnetron sputtering onto p-doped (100) silicon crystals and onto ultrapure aluminum foils at different argon pressures, ranging from 0.17 to 1.4 Pa. The film density was determined by the combination of the areal density, obtained from ion beam analysis, and the film thickness measured by a stylus profilometer. Film density decreased when the argon pressure used during deposition was increased. Gas effusion measurements indicated that the films deposited at low pressures are more compact than the films deposited at higher pressures. In the case of the latter, C2Hn effusion at temperatures as low as 250 °C indicated that they have an open structure that allows the evolution of large molecules. Small angle x-ray scattering results revealed an increase of the void density with increasing plasma pressure. Guinier plots show that these voids have a broad distribution of sizes, ranged from 7 to 26 A, which is nearly independent of the plasma pressure. A d...