The effect of H2SiF6 on the surface morphology of textured multi-crystalline silicon

The surface structure of multi-crystalline silicon (mc-Si) etched in HF/HNO₃ with various added amounts of hexafluorosilicic acid (H₂SiF₆) as influencing parameter was studied. The experiments simulate the accumulation of the reaction product H₂SiF₆ in an HNO₃-rich and in an HF-rich solution which are used in the photovoltaic industry for the recycling of solar cells and modules. The resulting texture or surface quality, which largely determines the efficiency of solar cells, was investigated by measurements of roughness and reflectance, whereas both parameters increase with enhancing content of H₂SiF₆. The produced etch pits on the surface could be identified by scanning electron microscopy (SEM) and focused ion beam (FIB) technology. Etching without H₂SiF₆ is isotropic and produces favoured oval etch pits. With increasing H₂SiF₆ concentration the etch pits become angled and have a V-shaped profile. Irregular etching of the crystal orientations of the mc-Si occurs showing the anisotropic etch characteristic of H₂SiF₆. A model for the mechanism of influence was derived from etch-time experiments and FIB micrographs. We found out that both an increasing etch rate due to the additional acid and the anisotropic etch character contribute to an inhomogeneous surface, and therefore to the deterioration of the reflectance behaviour. Three different regions of surface quality depending on the H₂SiF₆ concentration were established. This dependence is applicable as a reference for the controlling of industrial etch baths.