The disclosure relates to methods of filling gaps in semiconductor substrates. A method of filling a gap is disclosed. The method including providing a substrate having a gap in a reaction chamber, providing a first precursor including silicon and carbon into the reaction chamber in a vapor phase, wherein the first precursor includes at least one unsaturated carbon-carbon bond and at least one atom selected from oxygen and nitrogen. The method further includes providing a first plasma into the reaction chamber to polymerize the first precursor for forming a gap filling material, thereby at least partially filling the gap with the gap filling material. In some embodiments, the at least one unsaturated bond is a double bond.