Investigation of the dielectric relaxation in plasma polymerized 1, 1, 3, 3- tetramethoxypropane thin film

Abstract

Transparent and light yellow color plasma polymerized 1, 1,3, 3-tetrame1hoxypropane (PPTMP) thin films of different thicknesses were prepared through glow discharge of 1, 1,3, 3-tetramethoxypropane (TMP) under various depositIOn conditions using a capacitively coupled glow discharge reactor. The surface morphology of the PPTMP thin films has been studied by scanning electron spectroscopy and is observed to be smooth, uniform and pmhole free, The compOSItional analysis was performed by a Energy- Dispersive spectrometer, The carbon (C), hydrogen (H) and oxygen (0) contents in PPT.MP thin films were determined by Elemental Analysis. The empirical fonmlla of the PPTMP thin films is fOLmdto be C6i,H4 .•.,0,tj. The'infrared (lR) spectroscopic mvestigation reveals that prTMP thin film may be formed with certain amount of conjugation. Due to heat treatment aliphatic conjugated C~C hond forms in PPTMP and oxygen is incorporated in the strucluTe as (=0 and (-O-C. The optical properties of as-deposited and heat-treated PPTMP thin films were investigated hy ultraviolet-visible (lJV-VIS) spectroscopy, From the tN-VIS absorption spectra, ab80rption co-efficient, allowed direct transition, Eq<l,allowed indirect transition, £4" energy gaps, and Tauc parameter, B, were delennined. 1nc red shin in the maximum ahsmption wavelength for all the PPTMP thin mms was observed as compared with the mOnOmer maximum absorption wavelength. The direct transitJon energy gaps (Egd ) are about 2,69 -3.15 eV for as-deposited Imaged and 3.22-3.46 eV for 200 days aged PPIMP. The indirect transition energy gaps (Eq;) arc found to be about 0,92-1,53, 1.75-2.1, 0,99- 1,7 and 0,70"0.84 eV for as-deposited unaged, 200 days aged, heat treated for 1 hour at 523 and 673 K respectively, The calculated value of Tal!c parameter B ranges from 80 to 122 cm-1i2(eV)-li2 lor as depo5ited PPIMP and 44 to 151 cm-l,~ (eV~l" for heat treated PPTMP thin films. The a1tematmg cnrrent (ae) eondl!ction mechanism iu the a~-deposited PPIMP thin films is observed to he dominated by hoppmg of eaniers behvecn the loeal1zed states. The ae conductivity oj" the films increases with the increase in fi:eqneney upto about 10'Hz but ',ho""& almost no variation with temperature, . The 'n' value>; are calculated from ac conductivity and arc found to be about 0.55- 1,04 up to 104 Hz and 1.14-1.63 abovel0' Hz which suggest that the Debye type of conductJon mechanism is operative ill the frequency region below 10' H? and other mechanisms may be operative in the high frequency region. The estimated activation energy is very low (0.02-0,07 eV) for PPTMP thin films. This very low aetiv3tion ()r the carriers lilld strong dependence of conductivity On fTequency arc mdicativc of a hopping conduction mechanism. The dielectric const3nt decreases rapidly with frequency above 104 Hz. The general trend of dielectric constant is to decrease slightly with increasing tempcrature in the lower fTequency region. But in the h]gh frequency region (>1OlHz), it starts to decrease rapidly "11th the increase in Irequency. It is observed that dielectric const3nt increases with increasing thickness of the PPTMP films which may because of the complex nmure of the dcposllion process of thin films in glow discharge. Thc dependence of the dielectric Joss tangent, tano, with frequency at different temperatures (298 to 423 K) for as deposited PPTMP Ihm films of different thicknesses shows small relaxation peak m the very low Irequeucy region «102Hz) and then bno decreases slightly with a broad minimum in between 102 and 103 Hz and again increases with increasing frequency. The f"",",values are plotted as a function of lIT and the dE values associated with the relaxations are found to be 0.08-0.49 eV, The Cole - Cole curves are plolled as 8 ' versus c" and the values of D are found to be 0.93- 0.98, whieh are smaller th3n the value (unity) of the Debye lllodel with a single relaxation, indicatmg the presence of distnbution of rc13xation time in these materials.