Cyanide treatment which simply involves immersion of semiconductors in cyanide solutions can passivate interface states as well as surface states. When Si surfaces are treated with KCN solutions, a surface photovoltage greatly increases, and the surface recombination velocity is calculated to be decreased from similar to3000 cm/s to less than 200 cm/s. When the cyanide treatment is applied to ultrathin SiO2/single-crystalline Si structure, interface states are passivated. The passivation of the SiO2/Si interface states increases the energy conversion efficiency of MOS solar cells to 16.2% and decreases the leakage current density for MOS diodes to 1/3-1/8. When the cyanide treatment is performed on polycrystalline (poly-) Si, defect states in Si up to at least 0.5 pm depth from the surface are passivated, resulting in a vast increase in the energy conversion efficiency of solar cells and a decrease in the dark current density of MOS diodes to 1/100-1/15 that without cyanide treatment. The defect passivation is attributed to the formation of Si-CN bonds from defect states. Si-CN bonds are found not to be ruptured by heat treatment at 800 degreesC and AM 1.5 100 mW/cm(2) irradiation for more than 1000 h. Density functional calculations show that the thermal and irradiation stability results from strong Si-CN bonds with the bond energy of 4.5 eV. When the cyanide treatment is performed on oxide/GaAs(1 0 0) structure, the interface state density decreases to similar to50%. The cyanide treatment can also passivate defect states in Cu2O films, resulting in increases in the carrier density and the band-to-band photoluminescence intensity. (C) 2004, Elsevier B.V. All rights reserved.