Controllable-unbalanced de magnetron sputtering of a B4C target in mixed Ar-N-2 discharges has been used to deposit BN:C thin films with carbon concentrations in the range of 5-21 at, % on Si(001) substrates. The variation of the nitrogen gas consumption with nitrogen partial pressure was used to determine the sorption capacity of the sputtering source and was then correlated to the film discharge plasma density near the substrate in a wide range. Hence, the ion flux J(i) of primary Ar+ and N-2(+) ions accelerated to the substrate by an applied negative substrate bias could be varied while keeping the deposition flux J(n) (the sum of film building species, B, C, and N atoms) near constant. BN:C films were grown at large ion-to-neutral flux ratios 3 less than or equal to J(i)/J(n) less than or equal to 24, ion energies E(i) less than or equal to 500 eV, and substrate temperatures 150 less than or equal to T-s less than or equal to 350 degrees C. The phase and elemental composition of as-deposited BN:C films were characterized by Fourier transform infrared spectroscopy and wavelength dispersive x-ray spectroscopy, respectively. Deposition of cubic phase c-BN:C containing 5-7 at. % of C is demonstrated under conditions of low energy (110 eV) ion bombardment, a high ion-to-atom arrival rate ration (J(i)/J(n) similar to 24), and low growth temperatures (similar to 150 degrees C).