Optical and electronic properties of amorphous diamond

The optical and electronic properties of highly tetrahedral amorphous diamond-like carbon (amorphous diamond, a-D) films were investigated. The structure of the films grown on silicon and glass substrates, under similar deposition conditions using a compact filtered cathodic vacuum arc system, are compared using electron energy loss spectroscopy (EELS). Results from hydrogenation of the films are also reported. The hydrogenated films show two prominent IR absorption peaks centered at 2920 and 2840 cm-1, which are assigned to the stretch mode of the C-H bond in the sp3 configuration on the C-H3 and C-H sites respectively. The high loss EELS spectra show no reduction in the high sp3 content in the hydrogenated films. UV and visible transmission spectra of a-D thin films are also presented. The optical band gap of 2.0-2.2 eV for the a-D films is found to be consistent with the electronic bandgap. The relationship between the intrinsic compressive stress in the films and the refractive index is also presented. The space charge limited current flow is analyzed and coupled with the optical absorption data to give an estimate of 1018 cm-3 eV-1 for the valence band edge density of states.

Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films

A balanced planar r.f. powered magnetron sputter source has been used to deposit carbon nitride films from a graphite target under various conditions. Sample temperature, bias voltage and nitrogen content in the gas mixture were varied. The effects of oxygen, methane and ammonia on the film growth were also studied. Special attention was paid to the effects of the deposition parameters on the structure of the films, in particular the hybridisation of the carbon and nitrogen bonding. The chemical bonding of the carbon and nitrogen atoms was studied by electron energy loss spectroscopy (EELS). The chemical composition was evaluated by Rutherford back-scattering. The intensity of transitions to π antibonding orbitals, as revealed by EELS, was found to increase with the nitrogen content in the films. Ion bombardment of the films during growth and the addition of oxygen or hydrogen-rich gases further increased the proportion of π bonds of both the carbon and nitrogen atoms. It is suggested that the increase in the transitions to μ antibond orbitals is to be explained by increased sp2 or possibly sp hybridisation of the carbon and nitrogen. Also, the effect of annealing on the bonding of nitrogen rich films after deposition was tested. The changes caused by nitrogen and deposition conditions are consistent with previous reports on the formation of paracyanogen structures.

Density, sp3 content and internal layering of DLC films by X-ray reflectivity and electron energy loss spectroscopy

A variety of hydrogenated and non-hydrogenated amorphous carbon thin films have been characterized by means of grazing-incidence X-ray reflectivity (XRR) to give information about their density, thickness, surface roughness and layering. We used XRR to validate the density of ta-C, ta-C:H and a-C:H films derived from the valence plasmon in electron energy loss spectroscopy measurements, up to 3.26 and 2.39 g/cm3 for ta-C and ta-C:H, respectively. By comparing XRR and electron energy loss spectroscopy (EELS) data, we have been able for the first time to fit a common electron effective mass of m*/me = 0.87 for all amorphous carbons and diamond, validating the `quasi-free' electron approach to density from valence plasmon energy. While hydrogenated films are found to be substantially uniform in density across the film, ta-C films grown by the filtered cathodic vacuum arc (FCVA) show a multilayer structure. However, ta-C films grown with an S-bend filter show a high uniformity and only a slight dependence on the substrate bias of both sp3 and layering.