Nitridation is the process in which, during the initial growth of a-SiNx:H layers on Si surfaces, nitrogen is incorporated into the Si lattice near its surface. We show that this nitridation process affects the density of interface states (D-it and fixed charges (Q(f)) at the interface. These parameters determine the effective surface passivation quality of the layers. The nitridation can be tuned independently of the growth of a-SiNx:H layers by using a plasma treatment prior to actual a-SiNx:H layer deposition. It is shown that Q(f) can be varied from 2 x 10(12) to 15 x 10(12) cm(-2) without changing the a-SiNx:H deposition process. It is demonstrated that in our case and processing window, Qf is the determining factor in surface passivation quality in the range of 2 x 10(12) to 8 x 10(12) cm(-2). For higher values of Q(f), D-it, has increased significantly and has become dominant thereby reducing the passivation quality. It is shown that the passivation can be controlled independently of the a-SiNx:H deposition process. In completed solar cells the effect of the controlled Q(f) and D-it is studied. On n-type solar cells, due to increased depletion, increases in Q(f) and D-it resulted in a drop in open-circuit voltage, V-oc of over 20 mV. On p-type solar cells, where the Q(f) results in accumulation, the effect was negligible. (c) 2012 Elsevier B.V. All rights