C. De Martino

Deposition and characterization of amorphous carbon nitride thin films

Abstract In this paper, a set of amorphous carbon nitride (a-C:N) films with different nitrogen contents were analysed. The most relevant deposition parameter is the nitrogen partial pressure, and the role of nitrogen is to reduce the disordered sp 2 phase and the sp 3 sp 2 ratio. The origin of the electron spin resonance active centres is identified to be the disordered sp 2 phase.

Improvement of mechanical properties of a-C:H by silicon addition

Abstract The structure of a hard a-C:Si:H deposited by RF reactive sputtering from a graphite target in an argon-silane atmosphere is analysed. It is shown that Si and C atoms are intermixed and form a rigid network. The details of the deposition process are discussed and the crucial role of argon evidenced. Argon atoms dehydrogenate silane to form SiH groups, which have a high sticking coefficient and a low surface mobility. These groups become centres of nucleation. The energy of argon ions impinging on the growing surface is important in determining a good intermixing of carbon and silicon and the removal of voids.

Determination of the sp3sp2 ratio in a-C:H films by infrared spectrometry analysis

Abstract In this paper, a method for the determination of the sp 3 sp 2 ratio for carbon atoms in a-C:H films is presented. First, the infrared analysis of the films is performed and the sp 3 sp 2 ratio for the hydrogenated carbon atoms is determined. From this value, and making different assumptions for the different types of a-C:H (graphite-, diamond- or polymer-like), the overall sp 3 sp 2 ratio is determined. The results obtained are in good agreement with NMR data for the types of a-C:H considered.

Characteristics of a-C:H:Si films deposited by r.f. sputtering under various deposition conditions

In this paper we report the results of an extensive investigation of a-C:H:Si alloys deposited by an r.f. diode sputtering system, operated with a graphite target and an ArSiH4 atmosphere. This allowed us to study the influence of the inclusion of silicon in a-C:H. It is shown that, according to the deposition conditions, various sets of properties can be realized. In particular, small amounts of silicon added at a low substrate temperature give diamond-like characteristics, while silicon added at a high substrate temperature causes polymer-like behaviour. An increase in silane in the gas phase enables the deposition of C:H:Si with tetrahedral structure. The sets of deposition conditions giving different types of material were identified.

Temperature dependence analysis of the electron paramagnetic resonance signal and electrical conductivity in a-C and a-C:H

Abstract In this paper, a study of the electron paramagnetic resonance (EPR) signal and of the electrical conductivity for sputtered amorphous carbon (a-C) and graphite-like amorphous hydrogenated carbon (a-C:H) films is reported. In particular, the comparison of the temperature dependence of conductivity and of the EPR resonance signal line width shows that different mechanisms are responsible for conduction in the two materials. Conduction is by overlapping states in a-C down to 60 K and by hopping in a-C:H up to 400 K. Further differences are observed in the spin-lattice relaxation times in the whole temperature range from 10 to 500 K where the values in a-C:H were found longer by an order of magnitude as compared with those in a-C.

Mechanical and physical properties of amorphous carbon-based alloys

Abstract Silane and nitrogen were added in the gas phase in a technique suitable for the deposition of a-C and a-C:H films, even in the absence of an external bias. It has been shown that nitrogen dehydrogenates the films and weakens CH bonds, while a small amount of silane is sufficient to achieve high values (up to 40 GPa) of hardness, as measured by nanoindentation of thick films. This result has to do with the kinetics of deposition and not the alloying itself. Preliminary results concerning a new alloy (a-C:H:Si:N) are discussed too.

Comparison of bulk and surface structure in hydrogenated amorphous carbon films

Abstract Hydrogenated amorphous carbon (a-C:H) films having different characteristics (diamond like, graphite like and polymer like) are analysed. By comparing the results obtained with a surface-sensitive technique (high resolution electron energy loss spectroscopy) and a bulk-sensitive technique (IR spectrometry) knowledge of the hydrogen and sp3-sp2 depth distributions is achieved. For instance, it is shown that diamond-like films have a hydrogen enrichment near the surface, while the sp3 to sp2 ratio for protonated sites does not change with depth. By means of spectral ellipsometry, the real and imaginary parts of the dielectric constant are measured. Their shapes for the three types of a-C:H are discussed on the basis of the sp2 cluster model. For instance, it is shown that, in the bulk of graphite-like a-C:H, sp2 sites are grouped in a limited range of almost undistorted cluster sizes.

High gap sputtered DLC layers

Abstract Adherent and transparent unhydrogenated DLC layers were deposited by means of r.f. and d.c. magnetron sputtering at low deposition temperatures. The optical gap value increases from 0.7 to 2.2 eV when deposition pressure is increased from 10 −2 to 10 −1 Torr. In high gap materials, large amounts of sp 3 hybridized carbon atoms are present. When Al contacts are evaporated in coplanar configuration on these high gap materials, rectifying and photoelectrical effects are observed.

Characterization of the structure of carbon material through the sp3/sp2 bonding ratio measurements

Hydrogenated amorphous carbon (a-C:H) films have been deposited by sputter assisted plasma chemical vapor deposition (CVD). The relative concentration of sp 3 and sp 2 hybridized carbon in samples is determined by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies and by a new method through the complex dielectric constant deduced from optical transmittance and reflectance. The results are compared and discussed.