We have analyzed the role of the bond densities of a-SiNx:H films on the passivation properties at the SiNx:H/Si interface. The films are deposited onto silicon wafers by plasma enhanced chemical vapor deposition using a 13.56 MHz direct plasma system and a SiH4/N2/H2 gas mixture. Fourier transform infrared spectroscopy measurements are performed in order to obtain the bonding concentration of Si–Si, Si–H, Si–N and N–H. The passivation properties are deduced by lifetime measurements using a microwave-detected photoconductance decay technique. Carrier lifetimes of the SiNx:H-passivated silicon wafers of up to 1200 μs correlate to surface recombination velocities, Seff, as low as 4–6 cm/s. This means that the films provide excellent passivation of silicon surfaces, which is necessary for high-efficiency solar cells. The Si–H bond density and the total bond density are considered as measures of the passivation quality. Models for the formation of K+ centers and for the passivation pathways during the plasma deposition are proposed. The addition of a further hydrogen source to the plasma gas (H2) leads to a better defect passivation of Si dangling bonds during the deposition. © 2002 American Institute of Physics.