Transistors structures comprising a semiconductor features and dielectric material comprising silicon and oxygen in gaps or spaces between the features. The dielectric material may fill the gaps from bottom-up with an atomic layer deposition (ALD) process that includes a silicon deposition phase, and an oxidation phase augmented by N2:NH3 plasma activated nitrogen species. Being plasma activated, the nitrogen species have short mean free paths, and therefore preferentially passivate surfaces with low aspect ratios. This aspect-ratio dependent passivation may increase an energy barrier to surface reactions with a silicon precursor, resulting in a concomitant differential in deposition rate. With N2:NH3 plasma passivation, deposited dielectric material may have a nitrogen concentration that varies by at least order of magnitude as a function of the aspect ratio of the filled gaps.