A method for forming a single-phase multi-element film on a substrate in a reaction zone by PEALD repeating a single deposition cycle. The single deposition cycle includes: adsorbing a precursor on the substrate in the absence of reactant and plasma; decomposing the precursor adsorbed on the substrate by an inert gas plasma; and reacting the decomposed precursor with a reactant gas plasma in the presence of the inert gas plasma. The multi-element film contains silicon and at least two non-metal elements constituting a matrix of the film, the precursor contains silicon and optionally at least one non-metal element to be incorporated in the matrix, and the reactant gas contains at least one non-metal element to be incorporated in the matrix.