Hydrogen content and density in nanocrystalline carbon films of a predominant diamond character

Abstract

Nanocrystalline carbon films possessing a prevailing diamond or graphite character, depending on substrate temperature, can be deposited from a methane hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While at a temperature of ∼880 °C, following the formation of a thin precursor graphitic film, diamond nucleation occurs and a nanodiamond film grows, at higher and lower deposition temperatures the films maintain their graphitic character. In this study the hydrogen content, density and nanocrystalline phase composition of films deposited at various temperatures are investigated. We aim to elucidate the role of hydrogen in nanocrystalline films with a predominant diamond character. Secondary ion mass spectroscopy revealed a considerable increase of the hydrogen concentration in the films that accompanies the growth of nanodiamond. It correlates with near edge x-ray adsorption spectroscopy measurements, that showed an appearance of spectroscopic features associated with the diamond structure, and with a substantial increase of the film density detected by x-ray reflectivity. Electron energy loss spectroscopy showed that nanocrystalline diamond films can be deposited from a CH4/H2 mixture with hydrogen concentration in the 80%–95% range. For a deposition temperature of 880 °C, the highest diamond character of the films was found for a hydrogen concentration of 91% of H2. The deposition temperature plays an important role in diamond formation, strongly influencing the content of adsorbed hydrogen with an optimum at 880 °C. It is suggested that diamond nucleation and growth of the nanodiamond phase is driven by densification of the deposited graphitic films which results in high local compressive stresses. Nanodiamond formation is accompanied by an increase of hydrogen concentration in the films. It is suggested that hydrogen retention is critical for stabilization of nanodiamond crystallites. At lower deposition temperatures an excess of hydrogen in the deposited layers helps to prevent the densification of the films and accumulation of microstresses and consequently the films maintains its graphitic character. At higher temperatures the hydrogen content in the films is relatively low and the film maintains its graphitic character.

Hydrogen in nano‐diamond films: experimental and computational studies

“…The hydrogen concentration depth profile of the dc GD film re- flects the evolution of its structure from an initial low density, low hydrogen concentration graphitic film through a medium density, hydrogen containing film until a dense, hydrogen containing matrix evolves, in which ~5 nm sized nano-diamond crystallites precipitate. Our previous measurements of absolute hydrogen concentration within these films by means of ERDA revealed hydrogen concentration of 15-20 at% [12] which confirms the present SIMS measurements.…”

“…In our previous works [9,10] we showed that the growth of the dc GD CVD film advances through the following stages: (1) graphitic film with its basal plane perpendicular to the substrate, a low H concentration and low (~2.2 g/cm 3 ) density, (2) an increase of the density followed by incorporation of hydrogen, (3) the formation of a dense (~3 g/cm 3 ), hydrogen rich layer in which diamond nucleates and grows. The hydrogen concentration in the nano-diamond film deposited by dc GD CVD increases from 3.3 × 10 21 at the nanographitic precursor region to 1.45 × 10 22 atoms/cm 3 in the ~5 nm sized nano-diamond crystallites region [12]. The hydrogen concentration depth profile of the dc GD film re- flects the evolution of its structure from an initial low density, low hydrogen concentration graphitic film through a medium density, hydrogen containing film until a dense, hydrogen containing matrix evolves, in which ~5 nm sized nano-diamond crystallites precipitate.…”

Hydrogen concentration and bonding configuration in polycrystalline diamond films: From micro-to nanometric grain size

“…28 The hydrogen concentration depth profile of the dc GD film reflects the evolution of its structure from an initial low density, low hydrogen concentration graphitic film through a medium density, hydrogen containing film until a dense, hydrogen containing matrix evolves, in which ϳ5 nm sized nanodiamond crystallites precipitate. Our previous measurements of absolute hydrogen concentration within these films by means of ERDA revealed hydrogen concentration of 15-20 at.…”

“…21 While hydrogen incorporation and effects in hydrogenated amorphous carbon films, which may contain up to 45 at % hydrogen, 22 are relatively well studied, [22][23][24][25][26][27] only little information is available on hydrogen distribution, concentration, and location in diamond films. 21,[28][29][30][31] Fourier transform infrared absorption and elastic recoil detection analysis ͑ERDA͒ were used to evaluate the location of hydrogen in diamond. It has been found that the hydrogen in CVD diamond is incorporated in both grain boundaries [19][20][21]30 ͓on diamond surfaces and in nondiamond ͑graphitic and amorphous carbon͒ regions͔ and also is trapped in grain defects.…”

Correlation between diamond grain size and hydrogen incorporation in nanocrystalline diamond thin films

“…The well-known negative electron affinity and high conductivity of diamond surfaces are properties of fully hydrogenated diamond surfaces [5][6][7]. Hydrogen is said to be distributed nonhomogeneously in diamond films and it is likely to be found on the surface, at grain boundaries and dislocations or simply as lattice defects [8][9][10][11].…”