Bernadette Marcus

Analysis of the structure of multi‐component carbon films by resonant Raman scattering

Diamond and carbonaceous films grown in a microwave assisted plasma reactor have been characterized by x‐ray diffraction, scanning electron microscopy, and Raman spectroscopy. This study is mainly focused on the identification of the different carbonaceous compounds which can coexist with the diamond depending on the synthesis parameters. Selective etching reactions and the excitation wavelength dependence of Raman cross sections of the different carbon species reveal that the broad and poorly structured Raman spectra in the 1100–1700 cm−1 region contain six components that arise from three different carbonaceous species: (i) one species is graphitic and is identified from the components at about 1350 and 1590 cm−1 which are resonantly enhanced as compared to the other components when the excitation energy is lowered from 3 to 2 eV. (ii) The second species is associated with the two broad bands at 1350 and 1550 cm−1 and is attributed to amorphous diamond‐like carbon (a‐C:H); both bands are resonantly enha...

In-situ Raman investigation of diamond films during growth and etching processes

Abstract Diamond films were prepared in a microwave plasma-assisted chemical vapour deposition (CVD) reactor. The deposition parameters were deliberately optimized to obtain mixtures of diamond and different carbonaceous compounds. We present the results of an in-situ Raman study of the growth of such films and of their etching under different atmospheres. At least six Raman bands in the range 1100–1600 cm−1 were observed; their relative intensity variation with the elaboration parameters, with the excitation wavelength and with the etching conditions led to the conclusion that at least three different carbonaceous forms are present in CVD diamond films. During the growth process, one first obtains the diamond phase along with a diamond-like compound; the disordered graphitic form appears only in a second stage.

Analysis of the fine structure of the Raman line and of X-ray reflection profiles for textured CVD diamond films

Abstract Textured diamond films prepared by CVD were studied by X-ray measurements and Raman spectroscopy in the high dispersive mode in order to get structural information. A 〈100〉 fibre texture is observed for the films prepared with a 2% methane concentration and a low substrate temperature. For all the 2% CH4 samples the diffraction spectra reveal an important broadening of the 111 reflection and two additional bands at about 0.207 and 0.217 nm attributed to stacking faults along the 〈111〉 direction. Domains of hexagonal symmetry can then locally be formed with the defect ordering. The recording of a 1322 cm−1 Raman line generally attributed to the lonsdaleite or hexagonal diamond polytypes supports this assumption. This 〈100〉 fibre texture is recorded for samples obtained with a high methane concentration (2%) related to a fast growth rate, which may explain these stacking faults along the 〈111〉 direction.

Micro-Raman scattering from undoped and phosphorous-doped (111) homoepitaxial diamond films: Stress imaging of cracks

We report postgrowth micro-Raman stress imaging of cracks in (111) homoepitaxial diamond films. Undoped and phosphorous-doped diamond thin films grown by microwave plasma-enhanced chemical-vapor deposition on Ib (111)-oriented diamond substrates have been studied by confocal micro-Raman spectroscopy. For comparison purposes, a film grown on a (100) Ib substrate was also examined. Thanks to the confocal optics, the Raman signal arising from the epilayer could be discriminated from that arising from the substrate. As was already reported, the (111) films exhibited substantial tensile stress, indicated by a downshift in the Raman peak and by spontaneous cracking in films grown thicker than 5μm. The sixfold symmetry of the cracks supported that the films were homoepitaxial. A high compressive stress was also detected at the substrate near surface, and a partial stress relaxation was observed to occur in the vicinity of the cracks. Possible origins of the high tensile stress observed in the (111) homoepitaxial...

In situ Raman spectroscopy during diamond growth in a microwave plasma reactor

An experimental set‐up designed for in situ Raman analysis during the growth of diamond films in a microwave plasma reactor is described. A gated multichannel detection synchronized with a pulsed YAG laser is used to discriminate the Raman signals from the plasma emission. The in situ detection of a diamond film during its growth on a single crystal of alumina substrate is presented. The detectivity of the method has been estimated to be about a few tens of μg/cm2 for an acquisition time of 800 s. Peak shifts are interpreted in terms of temperature and stress dependences. It is shown that the diamond in the first stages of deposition is free of stress, then when grains come into contact compressive stresses are observed, when the film thickness reaches about 1 μm stresses are relaxed.

Influence of the surface termination of boron-doped diamond electrodes on oxygen reduction in basic medium

This paper reports on the influence of the surface termination (H-, HO-, NH2-) of boron-doped diamond (BDD) electrodes on the oxygen reduction reaction (ORR) in alkaline solution. The aminated BDD surface displays a higher oxygen reduction current density and a positive shift in the oxygen reduction potential compared to H- and HO-terminated BDD surfaces. The behavior is most likely due to a preferential adsorption of oxygen species on the NH2 termination. The ORR mechanism on the H-, HO-, and NH2-terminated BDD surfaces was investigated for the first time using electrochemiluminescence (ECL) of luminol. The results indicate that the ORR proceeds via a two-electron process on H-terminated diamond, while a four-electron pathway is observed for oxidized and aminated BDD electrodes. We further show that the ECL approach can be easily extended to investigate the ORR mechanism on different electrodes and composite materials. The influence on the surface termination on the reduction of oxygen on gold-nanoparticle-modified BDD electrodes has been studied.

Early stages of diamond growth by chemical‐vapor deposition monitored both by electron spectroscopies and microstructural probes

We performed a complete study of the nucleation and growth kinetics of chemical‐vapor‐deposition (CVD) diamond on Si(100). The diamond film was grown using the microwave‐assisted MWACVD method and the substrate was preliminary pretreated by ultrasonic agitation with 300 μm diamond grains, which provides a high nucleation density at saturation (≳108 cm−2). The evolution of the diamond particles coverage was investigated by two independent ways. The size distribution, mean size of the individual diamond particles, the surface coverage, and the nucleation density were monitored by scanning electron microscopy, including analysis of the pictures, and the overall carbon coverage was recorded by x‐ray photoemission spectroscopy. Results agree to predict a break point in the growth law: Initially the kinetics obey a law in t1/3, whereas after about 30–45 min the behavior becomes linear. This is interpreted as a change of the rate limiting step of the growth which is governed by the surface coverage of diamond pa...

Role of the nucleation step in the growth rate of diamond films

Abstract We have studied separately the influence of the substrate temperature and of the methane concentration in the gas phase either on the nucleation step or on the whole growth process. This study was achieved using a separate substrate heater which allows us to decouple the substrate temperature and the plasma parameters. The nucleation density is shown to change abruptly in a narrow temperature range before stabilizing, while the overall growth rate increases monotically when the substrate temperature is raised. For lower substrate temperatures, the growing process is limited by the nucleation step. Depending on the methane concentration in the gas phase, the nucleation density is found to increase up to 1% CH4 before becoming constant with a value close to 8 × 108cm−2 for higher methane concentrations. The dependence of the nucleation density on the substrate temperature can be explained by changes in the atom mobilities and sticking coefficients in one hand, and by the formation of amorphous carbon and carbide phase on the other hand.

Raman characterization of diamond and carbon films grown by remote microwave plasma enhanced CVD

Abstract We present the Raman characterization of carbon films prepared on (100) silicon substrates by remote microwave plasma enhanced CVD in a gaseous mixture of CH4 in H2. The set of samples was obtained by varying the methane concentration from 0.5 to 5.0% for different substrate temperatures ranging from 600 °C to 1000 °C. The following conclusions were obtained: 1. (i) For a given methane concentration, the substrate temperature range leading to diamond formation is very narrow. 2. (ii) The average temperature of diamond formation decreases when the methane concentration increases.

CONFOCAL RAMAN IMAGING FOR THE ANALYSIS OF CVD DIAMOND FILMS

Abstract Raman imaging has been used to investigate the microstructure of some (100)-textured diamond films. Results have shown that different crystals within a film can give rise to different Raman line positions, intensities and line widths, with the result that the overall diamond line is the sum of all the individual contributions from all the different crystals. The images presented herein first show considerable variation in the distribution of amorphous carbon and defects producing the luminescence background. These defects were mostly detected within the grain boundaries, confirming most of the previous studies. These examples also emphasize the amount of variability that may be detected in the line shape of the Raman diamond line. In particular, line splitting was observed for all the samples examined, and in some particular cases was the most dominant feature that was observed. Such a line splitting has to be related to strain fields that exist within the crystals. However, it was impossible to correlate line shift or line splitting to the presence of defects such as amorphous carbon or point defects giving rise to the luminescence background.