Heterojunction diodes with hydrogenated amorphous carbon (a-C:H) and nitrogen doped amorphous carbon (a-C:H:N) films on p-type silicon were prepared by means of plasma enhanced chemical vapor deposition. The electronic and structural properties of the films are analyzed as a function of nitrogen doping as well as thermal treatment after deposition.. X-ray photoelectron spectroscopy valence band spectra reveal that the electronic structure of the prepared a-C:H:N films depends on thermal annealing. The nature of the heterojunction is confirmed by the rectifying current-voltage characteristic of the carbonaceous deposit/p-Si junction with a heterojunction structure showing a behavior dependent on the amount of both nitrogen concentration and thermal annealing. In particular, the photovoltaic effect is observed only from annealed a-C:H:N heterojunction structures. Raman spectroscopy performed on heterojunction diodes after thermal treatment indicates that this behavior is most likely due to an extended graphitization. (C) 2003 American Vacuum Society.