Electrical and optical characterization of plasma polymerized n-butyl methacrylate thin films deposited on glass substrate

Abstract

Plasma polymerized n-butyl mehacrylate (PPnBMA) thin films of varying thicknesses were prepared at an optimized condition at room temperature by an AC plasma polymerization (PP) system using a capacitively coupled parallel plate reactor. The flat and defect free nature of thin films were confirmed by field emission scanning electron microscopy and atomic force microscopy images. A comparison between the micrographs of as-deposited and heat treated (at 473 K for 1 hour) PPnBMA thin films exhibited that no considerable variation in the surface morphology is detected due to heat treatment. With declining plasma power average roughness and root mean square (rms) roughness increases whereas with increasing deposition time the rms roughness of the PPnBMA thin films increases. From the energy dispersive X-ray spectra it is observed that the mass% of C increases accordingly with increament of thickness whereas mass% of O decreases. A comparative analysis of the elemental mass % of as-deposited and heat treated (at 473 K for 1 hour) PPnBMA thin films indicates the increament of mass% of C in heat treated PPnBMA compared to those of as-deposited PPnBMA which may be as a result of structural rearrangement during heat treatment. X-ray diffraction pattern confirms the amorphous nature of the PPnBMA thin films. Comparison of the fourier transform infrared (FTIR) spectra of nBMA with that of as-deposited and heat treated (at 473 K for 1 hour) PPnBMA demonstrated that structure of as-deposited PPnBMA is different to some extent from that of nBMA due to PP and further structural modification is observed owing to conjugation and/or cross-linking. The differential thermal analysis and thermogravimetric analysis show that PPnBMA was thermally stable up to about 476 and 500 K in air and N2 environments, respectively. Allowed direct transition energy gaps (Egd) were found to be 3.73 to 3.84 eV and 3.68 to 3.78 eV, respectively for as-deposited and heat treated PPnBMA thin films whereas the indirect transition energy gap (Egi) values were found to be 3.26 to 3.43 eV and 3.28 to 3.40 eV, respectively for as-deposited and heat treated PPnBMA thin films of thicknesses 150, 215, 320 and 450 nm. Values of Egd as well as Egi increase with the increase of film thicknesses. Extinction coefficient, Urbach energy, steepness parameter, refractive index and optical conductivity were also determined for these thin films. The current density – voltage and current density – film thickness characteristics of fabricated thin films of different thicknesses sandwiched between aluminum thin film electrodes indicate that the dominant conduction mechanism in the high-voltage region is space charge limited conduction in these films. The estimated carrier mobility, trap density and free carrier density decrease with the increase of thickness of the prepared thin films. The activation energy (ΔE) associated in low and high temperature regions were calculated for as- deposited and heat treated PPnBMA thin films of various thicknesses. For applied voltage of 10 V, at low and high temperature regions the ΔE are found to be around 0.21 - 0.36 eV and 0.43 – 0.78 eV for as -deposited whereas 0.17 – 0.47 eV and 0.35 – 0.88 eV for heat treated PPnBMA thin films of different thicknesses, respectively. On the other hand ΔE values in the low and high temperature region are found to be 0.14 - 0.39 and 0.47 - 0.64 eV for as deposited and 0.16 - 0.30 eV and 0.45 - 0.78 eV for heat treated PPnBMA thin films for an applied voltage 50 V. The low ΔE in the low temperature region indicate that the thermally activated hopping conduction is operative in this material. Frequency dependant AC electrical conductivity indicates Debye type conduction mechanism in low frequency and that in the high frequency other than Debye type. With increasing film thickness the peaks of tanδ shifts a little towards the lower frequency region indicating an increase of relaxation time. The ΔE value for dielectric relaxation is found to be 0.05 eV. Cole- Cole plot exhibits little distorted semicircle which also indicating nearly Debye type relaxation process in these films.