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The KCN aqueous solution is used as the cyanide treatment for the reduction of defect states in amorphous silicon (a-Si:H) thin films. The concentration of the KCN aqueous solution and immersing time are probed to find the optimum condition to improve a-Si:H thin film. After cyanide treatment, the combination between silicon atoms and cyanide ions is confirmed by observing the binding energy of N 1s through x-ray photoelectron spectroscopy (XPS) measurement in 395 eV ~ 406 eV. In order to determine the penetrating depth of cyanide ions into a-Si:H thin film, the XPS measurement is also applied to detect the N 1s signal after etching processes; the N 1s signal can be detected from surface to the depth of 15 nm. Through the measurement of low temperature cathodoluminescence (CL), higher concentration KCN treatment makes stronger radiation intensity indicates that the reduction of defect states in a-Si:H thin films by the cyanide ions. The ratio between photo-current and dark-current is maximum when a-Si:H thin film is immersed in 0.4 M KCN aqueous solution in two minutes.