By incorporating iodine gas during the films’ deposition, amorphous diamondlike carbon (a:DLC) films were doped anda:I–DLC films were obtained. Optical measurements showed that iodine affects the optical band gap, decrease it from 1.1 to 0.78 eV. Iodine doping decreased the resistivity by four orders of magnitude, from 108to 104&OHgr; cm. Iodine doped film was found to have a largerpdoping thana:DLC films. TheI–Vcharacteristics of devices made of doped and undoped films grown on silicon substrates, were studied. ac measurements of these devices have shown that thepdopeda:DLC films behave as semiconductors, and give a rectification withn‐type silicon. ©1996 American Institute of Physics.