Abstract
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 NH 3 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 NH 3 pre-treatment plus RTA show the lowest DLTS signals which suggests the lowest overall interface states density. With (111) flat surface, plasma NH 3 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 NH 3 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 and decrease towards the band edges.