Investigation of the optical and electrical properties of plasma polymerized vinylene carbonate thin films

Abstract

Plasma polymerized vinylene carbonate (PPYC) thin films of dilTerent thicknesses were deposited on to glass substratcs by low pressure glow discharge plasma from vinylene carbonate monomer at room temperature. Surface morphology and chemical structure of as deposited and heat treated rrye were analyzed by Scanning elcctron microscopy and Fourin transform infrared (FTIR) spectroscopy, respectively, The surfaccs of the PPYC thin films are observed to be smooth and pinhole free and no significant change is observed due to heat treatment up to 573 K. The analyses of the chemical structurc of PPye by FTIR spectroscopy indicate that PPVC chemical structure diffcrs frolll ve because of structural rearrangements due to glow discharge and change in chemical structure owing to hcat treatment. Ultraviolet-visible spectroscopic studies have been carried out on thc as dcposited and heat treated rrye thin films of different thicknesses. '1he cffcct of heat treatment on the optical properties 01" prye thin films is significant. The direct band gap energies are found to be in the mnge of3.96 to 4.15 eY for as deposited and that . - " are in (he range of 3,15 to 4.00 eY lor the ppye thin films heat treatcd at 473 and'~~, , 573 K. The band gap of the ppye thin film decreascs gradually with increasing heat treatment temperaturc uscd, The current density-voltage characteristics indicate Ohmic conduction in thc low voltag'c r~gion and non-Ohmic conduction in the high voltage region. Theoretically calculated and experimental results of Schottky(~,) and roole-Frenkel coefficients and (~pr) show that the most probable conduction mcchanism in ppye thin films is Schottky type. Strong temperaturc depcndcnce of current dcnsity shows consistency with Schottky type conduction mechanism. The AC conductivity, cr,,, increa<;esas frequency increases with a lower slope in the high frequency (abovc lxl04 Hz) region at all temperatures. The (J" is more dependent on tcmpcraltlre in the low frequency region than that in the high frequency region. The values of "n" in the high frequency region are in accordance with the theory of Debye typc loss mcchanism and ()lher mechanisms may be operative in the low frequency region indicating asymmetry and broooness of the dielectric dispcrsion. m • , ", The dielectric constant (€') ~ecreases slo\vly with increasing frequency up to 10kHz and then decreases very rapidly in the high frequency rcgion. The values of E' lie hetween 13 and I for the samples of different thicknesses. The variation of e' and dielectric loss factor with frequency and temperature which is related with multicomponcnt contribution of polarizabiJity of thc polar material~, as more or less all thin polymcr films deposited by plasma polymerization technique, contain a , certain quantity of polar groups irrespective of monomer used.