Effect of Cd2+ Cations on AC Conductivity of Stearic Acid Metal-Insulator-Semiconductor Diode

Abstract

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 MIS structure was fabricated using 40-layers Langmuir-Blodgett (LB) films of stearic acid (SA) on hydrophobic n-type silicon (n-Si) substrates. Cadmium sulphide (CdS) nanoparticles were embedded inside the SA matrix by exposure to H2S gas for a period of 12 hours. The frequency dependence of capacitance (C-f), conductance (G-f) and tan𝛿 characteristics of the Al/SA/Si structures were investigated in dark at room temperature. The capacitance for LB film of stearic acid with Cd2+ ions showed almost no frequency dependence. However, the device containing CdS nanoparticles exhibited strong frequency dependencies at frequencies greater than 100 kHz. The variation in the capacitance of the device as a function of modulating frequency was due to the polarization of the dielectric medium in which positive and negative charges were displaced with respect to their equilibrium positions. The conductance measurement (G-f) shown similar result which translated this to the increase in relaxation time. Futhermore, the dielectric loss (tan𝛿) measurement of SA+CdS device showed the increase in power losses as a function of frequency for both devices. The series resistance is believed to be dominant in the higher frequency range since the LB film resistance (𝑅𝐿𝐵) decreases with frequency.