Hydrogen cyanide (HCN) aqueous solutions were found to possess a great ability of removal of copper contaminants on Si as well as a Si defect passivation effect. Copper contaminants in concentrations of the order of 1013/cm2 could be reduced to below 3 x 109/cm2 taking only 10s even at room temperature when the pH of the HCN aqueous solution was set at 10. The cleaning ability of HCN solutions increases with pH, indicating that cyanide ions were the reaction species with Cu contaminants. Even when HCN solutions were contaminated with 64ppm Cu2+ ions, the cleaning ability was unchanged, indicating avoidance of re-adsorption. Ultraviolet absorption spectra showed that after the cleaning, Cu–cyanide complexes were formed in the HCN solutions. The main Cu species in the HCN solutions were Cu(CN)32- and, due to its stability, re-adsorption did not occur. When SiO2/Si structure was immersed in the HCN aqueous solutions for 24h, the SiO2 thickness was unchanged at all, showing that etching did not occur during the cleaning process. The energy conversion efficiency of pn-junction Si solar cells with a spherical shape was increased by 1.07 to 1.48 times by immersion in the HCN aqueous solutions, and the improvement was attributed to passivation of defect states in Si by cyanide ions.

Silicon Cleaning and Defect Passivation Effects of Hydrogen Cyanide Aqueous Solutions. M.Takahashi, Y.L.Liu, N.Fujiwara, H.Iwasa, H.Kobayashi: Solid State Communications, 2006, 137[5], 263-7