Importance of defect photoionization in silicon-rich SiNx dielectrics for high PID resistance

We demonstrate the presence of significant defect-induced photoconductance in silicon-rich SiN_x films that creates high resistance to potential induced degradation, PID, of silicon photovoltaic modules. This photoconductance phenomenon is extrinsic in nature. It originates from the photoionization of deep defect levels in SiN_x, rather than from the much higher energy band-to-band carrier generation involved in intrinsic photoconductance. The defect photoconductance is not present in the IR spectral range. Its onset appears in the visible red light range indicating deep levels with an energy separation from the SiN_x bands edges of about 1.8eV. The photoconductance is significantly less in stoichiometric Si₃N₄ films while appearing in films deposited under silicon-rich conditions and especially in films with a refractive index above n=2.05. The deep level photoionization rate (and the magnitude of the photoconductance) increases in films with larger n and indicates a higher concentration of defects related to excess silicon in the SiN_x. The present study was performed using semiconductor industry proven corona charging CV metrology that provides noncontact quantitative characterization of dielectric and interface charges, surface and interface potentials, and dielectric leakage currents. Measurements were carried out on sister wafers to those in a modular PID susceptibility study.