Low-Absorbing and Thermally Stable Industrial Silicon Nitride Films With Very Low Surface Recombination

Abstract

Amorphous silicon nitride has become the state-of-the-art antireflection coating for silicon solar cells. Optimization of silicon nitride films requires consideration of both the film's optical and electrical properties. It is commonly assumed that silicon-rich silicon nitride films (films with high refractive index) provide better surface passivation, compared to that obtained by films with lower indices. However, silicon-rich films are usually very absorptive in the short (and even medium) wavelength range. Development of low absorption silicon nitride films, that provide good surface passivation, is therefore highly valuable. In this study we compare nine different industrial silicon nitride films, all with similarly low refractive index of 2.09 ± 0.01 measured at 633 nm. We demonstrate that these films exhibit very different electrical, chemical, and optical properties despite their similar refractive index values and correlate these differences with the specific deposition conditions. As a result of this investigation, we have developed industrial thermally stable low-absorbing silicon nitride films that provide excellent surface passivation, with surface saturation current density of 7 fA/cm2 on both n- and p-type wafers. We demonstrate that the developed low absorption films provide surface passivation with equal quality to that obtained by industrial silicon-rich silicon nitride films.