Grazing-angle x-ray reflectivity ͑XRR͒ is described as an efficient, nondestructive, parameter-free means to measure the mass density of various types of amorphous carbon films down to the nanometer thickness range. It is shown how XRR can also detect layering if it is present in the films, in which case the reflectivity profile must be modeled to derive the density. The mass density can also be derived from the valence electron density via the plasmon energy, which is measured by electron energy-loss spectroscopy ͑EELS͒. We formally define an interband effective electron mass m*, which accounts for the finite band gap. Comparison of XRR and EELS densities allows us to fit an average m*ϭ0.87m for carbon systems, m being the free-electron mass. We show that, within the Drude-Lorentz model of the optical spectrum, m*ϭ͓1Ϫn(0) Ϫ2 ͔m, where n(0) is the refractive index at zero optical frequency. The fraction of sp 2 bonding is derived from the carbon K-edge EELS spectrum, and it is shown how a choice of ''magic'' incidence and collection angles in the scanning transmission electron microscope can give sp 2 fraction values that are independent of sample orientation or anisotropy. We thus give a general relationship between mass density and sp 3 content for carbon films.

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The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon ͑a-C:H͒ films has been characterized in terms of its composition, sp 3 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.

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