Capacitance–voltage (C–V) characteristics of aSi/C60heterojunction, i.e., a HF solution (7.3&percent; HF+30&percent;NH4F)treated Si surface interfaced solid crystal ofC60molecules, were measured while applying various ac frequencies. The entireC60thin film and near-interface region of the Si wafer behaved as a depletion region, with theC–Vcurve showing two distinct regions: one above and one below a threshold bias voltage. Below the threshold,C–Vcharacteristics were dependent on applied frequency, which suggests the presence of interface states that only affect capacitance at lower frequencies. This frequency dependence was analyzed by assuming a suitable equivalent circuit, and based on derived curve-fitting circuit parameters the interface states density was accordingly estimated to have a value as low as∼1011/cm2 eV.Such a small density indicates that only a few lattice defects occur within the interface of the HF-treated Si surface andC60crystal. Although no frequency dependence was observed above the threshold, theC–Vcharacteristics were found to be dependent on the width of the depletion region in Si. ©1997 American Institute of Physics.