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.