A method and system for forming material within a gap on a surface of a substrate are disclosed. An exemplary method includes forming a material layer on a surface of the substrate within a first reaction chamber, exposing the material layer to a halogen reactant in a second reaction chamber to thereby form a flowable layer comprising a halogen within the gap, and optionally exposing the flowable layer to a converting reactant in a third reaction chamber to form a converted material within the gap. Exemplary methods can further include a step of heat treating the flowable layer or the converted material. Exemplary systems can perform the method.