Study of the optical properties and DC electrical carrier transport on thin films of plasma polymerized methyl acrylate

Abstract

Plasma polymerization is a distinctive technique for direct polymeric thin films deposition from different organic monomers. The organic compound methyl acrylate (MA) has been chosen as monomer to deposit plasma polymerized methyl acrylate (PPMA) thin films of different thicknesses. The polymerization is carried out by using a capacitively coupled glow discharge reactor by optimizing the plasma parameters. The thermal, morphological, structural and optical properties of PPMA thin films are ascertained by using different characterization techniques. The DC electrical properties of PPMA thin films are also investigated. DTA/ TGA analysis suggests that PPMA is thermally stable up to about 534 K in air and 543 K in N2 environments. The weight loss in N2 environment is lower than air environment. FESEM micrographs show that structures of the films are observed to have clusters/agglomeration structure separated by grain boundaries. The EDX analysis indicates the presence of prominent percentage of carbon and oxygen in PPMA thin films. From the ATR-FTIR analyses, it is found that the PPMA thin films deposited by plasma polymerization technique have partially changed chemical structure owing to reorganization of MA structure due to plasma polymerization. From the UV-Vis analyses, it is observed that the values of direct energy band gap, (Eg(d)) varies from 3.77 to 3.83 eV and that of the indirect energy band gap, (Eg(i)) varies from 3.40 to 3.48 eV for the PPMA thin films. The increase of Eg(i) values with the increasing thicknesses is due to the increase in fragmentation/cross-linking in the bulk of the material due to the impact of plasma on the surface of the thin films with plasma duration. The values of the Urbach energy, Eu varies from 0.29 to 0.35 eV. So the Eu increases with the variation of thickness due the increase of disorder in the PPMA thin films. The increase of extinction co-efficient, k with the increase of hν indicates the probability of electron transfer across the mobility gap rises with hν. The J-V characteristics revealed that the dependence of J on V is Ohmic is the lower voltage (0.1~10 V) region and in higher voltage (10~75 V) region it is non-Ohmic. J–d and Vtr-d curves confirmed that the conduction mechanism is space charge limited conduction (SCLC) type. The activation energy for Ohmic region is observed higher than non-Ohmic region in the lower temperature region whereas the activation energy for Ohmic region is observed lower than non-Ohmic region in the higher temperature region. These findings may facilitate using PPMA in manufacturing optical and electronic devices.