Nitrogen doping of tetrahedral amorphous carbon films: Scanning tunneling spectroscopy

Amorphous nitrogenated carbon films with nitrogen atomic concentration between 12% and 29% were deposited using a filtered cathodic vacuum arc and a Kaufman-type ion source. The surface topography of the samples has been investigated by scanning tunneling microscopy in ultrahigh vacuum, showing that the roughness of the film surface decreases with nitrogen concentration. Scanning tunneling spectroscopy is employed to understand the role of nitrogen in the change of the surface microstructure and electronic structure near the Fermi level. The tunneling current (I)–bias voltage (V) curve is flat at low bias regions indicating a finite gap for the sample with low (12%) nitrogen concentration. An increase of tunneling current and its nonlinearity along with the decrease of energy gap occurs in the samples with increase of N concentration. The observed surface density of states [(dI/dV)/(I/V)] has been fitted as a square-root function of bias voltage. An improvement of the quality of these fits in the films with the increase of nitrogen concentration suggests that a depletion of defect density of states near the Fermi level (EF) takes place. These analyses could be attributed to the modification of the structure of amorphous carbon by a large concentration of nitrogen.