Study of silicon-silicon nitride interface properties on flat and textured surfaces by deep level transient spectroscopy

Silicon nitride (SiN_x) films deposited by direct plasma-enhanced chemical vapor deposition (PECVD) are widely used in silicon solar cell fabrication as passivation layers, yielding very low surface recombination velocities on crystalline Si (c-Si) material. So far, there have been some reports on deep-level transient spectroscopy (DLTS) of as-deposited SiN_x layers on Si, but the impact of rapid thermal anneal (RTA) processing step and the textured surface has not been investigated yet. In this paper, low frequency direct PECVD Si-SiN_x interface properties with and without plasma NH₃ pre-treatment, with and without RTA on both flat (100) and (111) orientations and textured n-type silicon samples have been investigated with DLTS. Lifetime and Fourier Transform (FT)-DLTS measurements were performed and analyzed using a QSSPC tool and a Boonton C-V bridge operating at 1 MHz, respectively. Capacitance-voltage (C-V) and current-voltage (I-V) measurements were also performed at temperatures varying between 77 and 320 K. A DLT-spectrum can be obtained at fixed t_w by varying the temperature from 77 K to room temperature or at a fixed T, by sweeping the sampling period t_w. Both methods have been used on all samples in this study. It is shown that three different kinds of defect states are identified at the Si-SiN_x interface. With (100) flat surface, samples with plasma NH₃ pre-treatment plus RTA show the lowest DLTS signals which suggests the lowest overall interface states density. With (111) flat surface, plasma NH₃ pre-treatment and RTA do not show any improvement. With the textured surface, the RTA step improves the surface passivation quality further but no obvious impact is found with plasma NH₃ pre-treatment. Energy-dependent electron capture cross sections were also measured by small-pulse DLTS. The capture cross sections depend strongly on the energy level an- - d decrease towards the band edges.