As I supposed the carrier lifetime studies did not fare well. Any slightest modification affects the results. The results do not coincide in the series. Any defects present in the wafers or other grains distribution significantly change the lifetime. Also each mixture etch the silicon wafer in the different way - different surface morphology and thickness of eatched layer. Too many variables force to simplify the experiment. In the next step monocrystalline silicon will be used.