Optical transitions and electrical properties in Plasma polymerized thin films of 2-(Diethylamino)ethyl; methacrylate

Abstract

Plasma polymerized (PP) thin films of different thicknesses were prepared through glow discharge of 2-(diethylamino)ethyl methacrylate (DEAEMA) using a capacitively coupled reactor at room temperature. The scanning electron micrographs of as deposited, heat treated and iodine doped PPDEAEMA thin films are found uniform and pinhole free. The energy dispersive x-ray analyses reveal the presence of C, N, O in as deposited, heat treated PPDEAEMA thin films and the presence of iodine along with C, N, O in iodine doped samples. The thermogravimetric analysis and differential thermal analysis indicate that heat treated PPDEAEMA is more stable in comparison to as deposited samples. The Fourier transform infrared spectroscopic investigations indicate that the material is a simple carbonyl compound and the chemical structure of DEAEMA retains to some extent in the PPDEAEMA. The spectral analysis also indicates the presence of CH and CH2 groups in both monomer DEAEMA and as deposited PPDEAEMA and the presence of CH3 only in DEAEMA. The presence of C=O, and C=C are very prominent in all the spectra of as deposited, heat treated and iodine doped PPDEAEMA. The absorption coefficient, allowed direct transition, Eqd, allowed indirect transition, Eqi, energy gaps; Urbach energy, extinction coefficient and steepness parameter of as deposited, heat treated (at 373, 473 and 573 K for an hour) and iodine doped PPDEAEMA thin films were determined. The variation of energy gap with thickness and heat temperature are also discussed. The Eqd and Eqi decrease with the increase in film thickness for as deposited PPDEAEMA thin films of different thicknesses. The Eqd and Eqi values decrease as the heat treatment temperature increases. The Eqd decreases due to iodine doping. Current density–voltage (J-V) characteristics were studied over the temperature range from 298 to 423 K for PPDEAEMA thin films of thicknesses 100, 200, 250 and 300 nm in aluminum/ PPDEAEMA /aluminum sandwich configuration. J-V curves reveal that in the low voltage region, the conduction current obeys Ohm’s law while in the high voltage region the behavior attributed to be space charge limited conduction in PPDEAEMA thin films. The carrier mobility, the free carrier density and the trap density are found to be about 9.48 × 10 -19 to 2.78 × 10-18 m-2 V-1s-1; 1.78 × 1023 to 2.10 × 1022 m-3 and 6.88 × 1023 to 1.58 × 1024 m-3 respectively for different thicknesses and temperatures at room temperature. The activation energies were estimated to be about 0.12 to 0.20 eV and 0.16 to 0.28 eV for 2 V and 30 V of PPDEAEMA thin films of different thicknesses. These small values of the activation energies suggest the existence of the shallow traps levels in PPDEAEMA thin films. The conductivity of the iodine doped PPDEAEMA thin films increases as compared to as deposited PPDEAEMA thin films. AC conductivity increases sharply as the frequency increases but at very high frequency (near about 105 Hz) it becomes almost stable and it increases a little with the increase of temperature. It is found that the activation energy of PPDEAEMA thin films is very low about 0.02 eV. Dielectric constant decreases slightly upto 104 Hz with the increase of frequency and above this frequency it start to decrease rapidly. Dielectric constant decreases slightly as the temperature increases from 298 to 348 K and above this temperature the decrease is quite higher. Dielectric loss increases with increasing frequency with loss peaks found at higher frequency which is shifted towards lower frequency as the thickness of the films increases. Dielectric loss increases as the temperature increases from 298 to 398 K. From the Cole Cole plot the values of β are found 0.77-0.91 which are smaller than the value (unity) of the Debye model with a single relaxation, indicating the presence of distribution of relaxation time in PPDEAEMA thin films. Both the ac conductivity and dielectric constant increases due to iodine doping.