A method of filling a gap on a substrate includes providing flows of silicon-containing processing gas oxidizing processing gas, and phosphorous-containing processing gas to a chamber housing the substrate and depositing a first portion of a P-doped silicon oxide film as a substantially conformal layer in the gap by causing a reaction among the processing gases and varying over time a ratio of the gases. The temperature of the substrate is maintained below about 500.degree. C. throughout deposition of the conformal layer. The method also includes depositing a second portion of the P-doped silicon oxide film as a bulk layer by maintaining the ratio of the gases substantially constant throughout deposition of the bulk layer. The temperature of the substrate is maintained below about 500.degree. C. throughout deposition of the bulk layer.