Methods and related systems for filling a gap feature comprised in a substrate are disclosed. The methods comprise a step of providing a substrate comprising one or more gap features into a reaction chamber. The one or more gap features comprise an upper part comprising an upper surface and a lower part comprising a lower surface. The methods further comprise a step of subjecting the substrate to a plasma treatment. Thus, the upper surface is inhibited while leaving the lower surface substantially unaffected. Then, the methods comprise a step of selectively depositing a silicon-containing material on the lower surface.