Methods, materials, and systems are described for forming conformal dielectric layers containing silicon and nitrogen (e.g., a silicon-nitrogen-hydrogen (Si—N—H) film) from a carbon-free silicon-and-nitrogen precursor and radical-nitrogen precursor. The carbon-free silicon-and-nitrogen precursor is predominantly excited by contact with the radical-nitrogen precursor. Because the silicon-and-nitrogen film is formed without carbon, the conversion of the film into hardened silicon oxide is done with less pore formation and less volume shrinkage. The deposited silicon-and-nitrogen-containing film may be wholly or partially converted to silicon oxide which allows the optical properties of the conformal dielectric layer to be selectable. The deposition of a thin silicon-and-nitrogen-containing film may be performed at low temperature to form a liner layer in a substrate trench. The low temperature liner layer has been found to improve the wetting properties and allows flowable films to more completely fill the trench.