Effect of Cd2+ ions on AC conductivity of stearic acid metal-insulator-semiconductor diode

We report on observations of the effect of Cd2+ ions on the AC electrical properties of stearic acid (SA) thin films in metal-insulator-semiconductor (MIS) diode-like structure. The insulating thin film was fabricated using 40-layers Langmuir-Blodgett (LB) films of SA containing Cd2+ ions on hydrophobic n-type silicon (n-Si) substrate. Frequency- dependent capacitance (C-f) and conductance (G/ω-f) characteristics of the Al/SA+Cd2+/Si MIS structure were investigated in dark at room temperature with frequency ranges from 80 kHz to 1 MHz. The capacitance for LB film of stearic acid with Cd2+ ions showed almost no frequency dependence which translated into uniform dielectric value. However, the film AC conductivity shown an exponential frequency-dependent behavior at a signal frequency higher than 100 kHz. The metal-insulator contact resistance is believed to be dominant in the high-frequency range.