There is an urgent need to deposit uniform, high-quality, conformal SiNx thin films at a low-temperature. Conforming to these constraints, we recently developed a plasma enhanced atomic layer deposition (ALD) process with bis(tertiary-butyl-amino)silane (BTBAS) as the silicon precursor. However, deposition of high quality SiNx thin films at reasonable growth rates occurs only when N2 plasma is used as the coreactant; strongly reduced growth rates are observed when other coreactants like NH3 plasma, or N2–H2 plasma are used. Experiments reported in this Letter reveal that NHx- or H- containing plasmas suppress film deposition by terminating reactive surface sites with H and NHx groups and inhibiting precursor adsorption. To understand the role of these surface groups on precursor adsorption, we carried out first-principles calculations of precursor adsorption on the ß-Si3N4(0001) surface with different surface terminations. They show that adsorption of the precursor is strong on surfaces with undercoordinated surface sites. In contrast, on surfaces with H, NH2 groups, or both, steric hindrance leads to weak precursor adsorption. Experimental and first-principles results together show that using an N2 plasma to generate reactive undercoordinated surface sites allows strong adsorption of the silicon precursor and, hence, is key to successful deposition of silicon nitride by ALD.