Diamond-like carbon (DLC) films with 4-400 nm thickness were deposited on silicon substrates using direct ion beam from an RF inductively coupled CH4 - plasma (ICP) source. The dependence of the film electrical and photoelectrical properties on methane flow were examined. Two kinds of trapping centers with different activation energies and capture cross-sections, and very low densities were discovered by the Q-DLTS method. The influence of thermal annealing in air at 100-450°C was investigated. The current leakage and defect concentration were reduced while electrical breakdown field and photoresponse were increased in annealed films. The results obtained have been used for optimization of the technology to prepare thin films with good protective, electrically insulating, and passivating properties.