Fixed charge density in silicon nitride films on crystalline silicon surfaces under illumination

A novel method is applied to determine the fixed positive charge density Q_f in plasma-enhanced chemical vapor deposited silicon nitride (SiN_x) films on crystalline silicon surfaces. In this method, both surfaces of the SiN_x-passivated silicon wafers are charged using a corona chamber and the deposited charge density is measured by means of a Kelvin probe. Subsequently, the carrier lifetime is measured and Q_f is determined from the dependence of the measured carrier lifetime on the corona charge density. This measurement technique allows us for the first time to determine the crucial parameter Q_f under illumination. In contrast to previous studies, a very high Q_f of about 2.3 × 10¹² cm^-2 is found, which is in good correspondence with CV measurements carried out in the dark. Finally, we show that the injection level dependence of the surface recombination velocity is mainly due to recombination in the space charge region at the Si/SiN_x interface.