A thin layer of a silicon-carbon-containing film is deposited on a substrate by generating hydrogen radicals from hydrogen gas supplied to a radicals generation chamber, supplying the hydrogen radicals to a substrate processing chamber separate from the substrate processing chamber via a multiport gas distributor, and reacting the hydrogen radicals therein with an organosilicon reactant introduced into the substrate processing chamber concurrently. The hydrogen radicals are allowed to relax into a ground state in a radicals relaxation zone within the substrate processing chamber before reacting with the organosilicon reactant.