High quality surface passivation is important for the reduction of recombination losses in solar cells. In this work, the passivation properties of amorphous hydrogenated silicon nitride for crystalline silicon solar cells were investigated, using electrical characterization, lifetime measurements and spectroscopic ellipsometry. Thin films of varying composition were deposited on p-type monocrystalline silicon wafers by plasma enhanced chemical vapor deposition (PECVD). Highest quality surface passivation was obtained for silicon-rich thin films, where a surface recombination velocity of 30 cm/s was obtained after a heat treatment corresponding to the industrial contact firing process. Electrical characterization of the interface between silicon nitride and silicon was performed by capacitance and conductance measurements. Several challenging aspects related to the interpretation of these measurements were investigated in detail, including charging and discharging, leakage currents, and frequency dependent capacitance.