The present invention provides nitrogenous compositions for forming a silicon nitride layer, wherein the nitrogenous composition comprises a hydrazine compound, an amine compound or a mixture thereof. The present invention further provides source compositions for forming a silicon nitride layer, wherein the source composition comprises a nitrogenous composition comprising a hydrazine compound, an amine compound or a mixture thereof, and a silicon source comprising hexachlorodisilane. Methods for forming silicon nitride layers are further provided. The silicon nitride layers provided herein may be formed on a substrate at a low temperature and may further exhibit improved breakdown voltage and an enhanced etch resistance.