A method for forming a dielectric film in a trench on a substrate by plasma-enhanced atomic layer deposition (PEALD) performs one or more process cycles, each process cycle including: (i) feeding a silicon-containing precursor in a pulse; (ii) supplying a hydrogen-containing reactant gas at a flow rate of more than about 30 sccm but less than about 800 sccm in the absence of nitrogen-containing gas; (iii) supplying a noble gas to the reaction space; and (iv) applying RF power in the presence of the reactant gas and the noble gas and in the absence of any precursor in the reaction space, to form a monolayer constituting a dielectric film on a substrate at a growth rate of less than one atomic layer thickness per cycle.