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Thickness Dependence of AC Electrical Conductivity and Dielectric Behavior of Plasma Polymerized 1,1,3, 3-Tetramethoxypropane Thin Films.

INTRODUCTION

It is very important to study the dielectric properties of thin films because they are usually quite different from those of the bulk and also for their uses in electronic and optical devices. Plasma polymerization (pp) represents a feasible method for preparing organic polymer thin films with desirable dielectric and electronic properties. Thin films produced through glow discharge are known to have free radicals or polar groups independent of the nature of monomers. For good dielectric and electronic properties, pp thin films have been found to be useful as thin film dielectrics, dielectrics in integrated microelectronics, insulating layers for semiconductors, sensors, transistors, organic electric circuitry, organic batteries, light emitting diode, etc. [1, 2], Owing to this reason, organic polymeric thin films are good candidates for the investigation of dielectric and electronic properties. Studies of dielectrics throw light on the molecular structure and relaxation behaviors of the polymers. AC conductivity ([[sigma].sub.ac]) mechanism of materials is very important from the point of their usage in electronic technology. The detail investigation of the [[sigma].sub.ac] and dielectric properties of pp thin films provide information about the conduction process, dielectric constant, relaxation process, etc which are dependent often on thin film thickness, frequency, temperature etc. Because of their importance in the development of electronic technology, enormous efforts have been put forward in search of these kinds of materials. So there are a handful of research publications on various materials appear in the last decade.

Hari Krishna et al. reported the dielectric properties of plasma polymerized polypyrrole and concluded that PPy films synthesized at 50 W were demonstrated conductivity value of 6 X [10.sup.-1] S [m.sup.-1] [3]. Anjana Jain evaluates the effect of grain size, content and other factors under the purview of dielectric and piezoelectric properties of PVDF-PZT composites while evaluating the sensitivity of the material for sensor application [4].

The effect of film thickness on optical properties of the Cr (III) complex having 2-pyridincarbaldehye thiosemicarbazone thin films was investigated by Yakuphanoglu et al. [5). They analyzed the optical constants (refractive index and [epsilon]') of the thin films. The variation of thickness of the thin films causes important changes in refractive index and real part-imaginary parts of the [epsilon]'. The most significant result indicates that thickness of the film can be used to modify the optical band gap and optical constant of the thin films. Liang et al. [6] reported that the e' and conduction current of thin films of polymides on substrates with thicknesses of 80-2,000 nm were measured by using a small electrode system. They explained the dependence of the [epsilon]' on thin film thicknesses by the orientation of polymer chains and observed that the [epsilon]' decreased with film thickness but conduction current increased.

Saravanan et al. [7] found the low dielectric permittivity in the high-frequency range in polyaniline thin films prepared by using a radiofrequency pp technique. The [epsilon]' and [[sigma].sub.ac] of polyaniline thin films prepared by ac pp technique were studied by Mathai et al. [8] and suggested that these films with low d are potential candidates as intermetallic dielectrics in microelectronics. Jae-Sung Lim et al. [9] reported that organic polymer dielectric thin films of styrene and vinyl acetate prepared by the pp method could be applied to functional organic thin film transistor devices as the gate dielectric (a low operation voltage of -10 V and a low threshold voltage of -3 V). Bushra [10] fabricated capacitors of Al-PbSe-Al structure. From the ac conduction studies, it was confirmed that the mechanism responsible for the conduction process was hopping. The thermal activation energy was found to decrease with the increase in film thickness. Jakarta et al. [11] reported the electrical properties of the pp [gamma]-terpinene thin films fabricated using the plasma-enhanced chemical vapor deposition. The optical transparency and high adhesion to a variety of substrates make pp-[gamma]-terpinene thin films a suitable candidate for dielectric applications in electronics. Silvia Loan et al. [12] reported frequencytemperature-dependent conductivity which show that conductivity increased with frequency and also that energy bandgap representation could be suitable for explaining the temperature influence on ac-conductivity. Sibel et al. [13] concluded that PCN films with controllable dielectrical properties at a wide range of frequency and temperature can be used in aerospace and microelectronic applications as functional surface coating materials. Xiong-Yan Zhao et al. [14] show that the dielectric constant of the novel conjugated polycyanurate thin films decreased with increasing frequency, while in contrast, the dielectric loss factor increased with the increasing frequency.

From the literature review, it is seen that pp process is an important physical process of depositing organic thin films with properties different from their conventionally prepared counterpart. In this respect pp thin films of organic materials warrant detail investigation of their ac electrical properties. Variation of [epsilon]' and <rac of the plasma polymerized 1, 1,3, 3-tetramethoxypropane (PPTMP) thin films of a particular thickness with frequency and temperature have already been reported [15]. The ac electrical conduction mechanism in the PPTMP thin films is observed to be dominated by hopping of carriers between the localized states. The trend of [epsilon]' is to decrease slowly with increasing frequency. The [epsilon]' is weakly dependent on temperature. The dielectric loss tangent, tan o decreases slightly with a broad minimum around 103 Hz and again increases with increasing frequency.

In this article, an attempt is made to explore the frequency and thickness dependence of dielectric properties as well as [[sigma].sub.ac] of the PPTMP thin films. The relation between the thin film thickness and [[sigma].sub.ac], [epsilon]', and tan <5 of PPTMP thin films have been analyzed and discussed using various existing theories.

EXPERIMENTAL DETAILS

Material

The monomer 1, 1, 3, 3-tetramethoxypropane ([C.sub.7][H.sub.16][O.sub.4]) (Sigma Chemical Company, St. Louis, MO) and microscope glass slides (Sail Brand, China) were purchased from local market. Its molecular weight is 164.2 g [mol.sup.-1], and density 0.997 g [cm.sup.-3]. It is a clear colorless, inflammable liquid with slight pungent smell. Its chemical structure has been reported earlier [15].

Growth of the PPTMP Thin Film

PPTMP thin films were prepared using a capacitively coupled glow discharge system [16]. The PPTMP thin films were deposited for about 40-60 min to obtain film thicknesses of 100, 150, and 200 nm. The PPTMP thin films were deposited on the top of the glass substrate at room temperature using a cylindrical-type capacitively coupled glow discharge system made up of two stainless steel parallel plate electrodes of diameter and thickness 0.09 and 0.001 m, respectively positioned at a distance of 0.035 m. Glow discharge plasma was generated around the substrates, with a power of about 40 W at standard line frequency of 50 Hz. All through the period of deposition, the pressure of the chamber was maintained approximately at about 13.3 Pa at room temperature. Multiple-Beam Interferometry technique was used for the measurement of thickness of the thin films [16].

AC Electrical Measurements

Aluminum (A1)/PPTMP/A1 sandwich samples were prepared for ac electrical measurements. Al electrodes were deposited using an Edward coating unit (model-E-396A, Edward, UK). The ac measurement was performed in the frequency range from 30 to [10.sup.6] Hz at room temperature (298 K), by a low frequency Impedance analyzer, (Agilent 4192A, 5 Hz to 13 MHz, Agilent Technologies, Japan). All measurements were performed in a vacuum of about 1.33 Pa [15].

RESULTS AND DISCUSSION

Effect of Thin Film Thickness on AC Conductivity of PPTMP

The frequency dependence of [[sigma].sub.ac] of PPTMP thin films of 100, 150, and 200 nm thicknesses at room temperature is presented in Fig. 1. The [[sigma].sub.ac] was calculated by using the formula [[sigma].sub.ac] = [G.sub.p]d/S, where [G.sub.p] is the conductance, d is the thickness of the sample and S is the cross sectional area of the measuring electrode. It is also observed that the [[sigma].sub.ac] increases as frequency increases in all samples with a higher slope in the mid frequency region. The dependence of [[sigma].sub.ac] on frequency can be described by the equation [[sigma].sub.ac]([omega]) = A[[omega].sup.n] [15], where [omega] is the angular frequency, "A" is a constant independent of frequency, "n" is the index which is used to understand the type of conduction mechanism in noncrystalline materials.

The power law mentioned above is a modified version of the Austin and Mott model [17] that describes the [[sigma].sub.ac] in the materials with inhomogeneity caused by the lack of long-range order. It is based on phonon-assisted hopping of charge carriers through tunneling from a localized site to another one. The power law discussed above has been applied to explore the conduction mechanism (hopping) in plasma polymerized aniline [7] and evaporated zinc phthalocyanine thin films [18].

The values of the exponent "n" for PPTMP thin films of various thicknesses are found to be 0.76-1.04 below [10.sup.4] Hz and 1.24-1.60 above [10.sup.4] Hz which are presented in Table 1. The "n" values depicted in Table 1 suggest that Debye type of loss mechanism is operative in the frequency region below [10.sup.4] Hz and other mechanisms may be operative in the high frequency region [15]. The observed values of "n" are thickness dependent. It is also seen that "n" values are dependent on frequency and increase with the increase in frequency.

It is reported [15] that PPTMP thin films show very low activation energy in the low frequency region at low temperature, which is mainly caused by hopping of charge carriers between localized sites. This very low activation of the carriers and strong dependence of conductivity on frequency are indicative of a hopping conduction mechanism in PPTMP thin films.

The [[sigma].sub.ac] increases with the increase of thin film thickness which may be due to the decrease of energy band gap with the increase in thin film thickness [19]. In plasma polymerization, dangling bonds and radicals arise during deposition process of thin films under glow discharge. The deposition time of thin films in this process also affects the structure of the thin films. The changes in the energy band gap and hence conductivity may be due to the structural modifications of PPTMP [19] during plasma polymerization.

Effect of Thin Film Thickness on Dielectric Constant of PPTMP

Frequency dependence of [epsilon]' of PPTMP thin films at room temperature is shown in Fig. 2. The value of [epsilon]' was calculated by using the formula, [epsilon]'= [C.sub.p]d/[[epsilon].sub.0]S [15], where [[epsilon].sub.0] is the permittivity of free space, [C.sub.p] is the parallel capacitance. Film thickness is an important parameter affecting the dielectric properties of the material particularly in thin films. It is observed that the [epsilon]' values are about 2.8, 3.8, and 6.8 for PPTMP thin films of thick-nesses 100, 150, and 200 nm, respectively in the frequency region of [10.sup.2] to 5 X [10.sup.3] Hz. The increase of [epsilon]' with increasing thickness of PPTMP thin films may be explained as follows. Because of the complex nature of the deposition process of thin films in glow discharge as deposition of thin film progresses, the dangling bonds may arise in the bulk of PPTMP. The deposition time of the thin film in plasma atmosphere may also be one of the factors affecting the PPTMP bulk structure. In this situation, the amount of dangling bonds may not be uniform throughout the PPTMP thin films, which may result in an increase of the [epsilon]' with thin film thickness.

The characteristic dependence of the [epsilon]' of PPTMP on frequency can be explained with interfacial polarization and dipolar polarization in the lower and higher frequency regions, respectively. It is reported that there are some charges present in the plasma polymerized materials which can migrate some distance through the dielectric as an electric field is applied. When such carriers are impeded in their motion, space charge and microscopic field distortions may occur. Because the charge carriers migrate under the influence of an electric field, they may get blocked at the electrode dielectric interface, which leads to interfacial polarization [20]. Usually, interfacial polarization is found in a sandwich type of configuration. Space charge accumulation at the structural interfaces of an inhomogeneous dielectric material causes interfacial polarization [7]. It is also reported that plasma deposited film has a high concentration of radicals which cause an increase in the polarization in the low frequency region that correspond to the high dielectric constant in the low frequency region [20].

Interfacial space charge layer may be responsible for the thickness dependent [epsilon]' of the PPTMP thin film in the lower frequency region. The local field in the thin film originating from the space charge layers at the interfaces, located at both the top and the bottom interface of PPTMP thin film and the electrode, may also be a cause of this thickness dependence of [epsilon]' [21, 22], The [epsilon]' decreases above 5 x [10.sup.3] Hz since dipole rotation cannot keep up at this frequency. This can be attributed to structural rearrangement of the molecule [23]. In the low frequency range the increase in [epsilon]' with the decrease in frequency is characteristic of electrical conduction, either by impurity electron and hole conduction, or ionic conduction. In either case, conduction in such a high-resistivity film could lead to injection of space charge coupled with material polarization [23].

Effect of Thicknesses on Dielectric Loss Tangent

The dependence of the dielectric loss tangent, tan [delta], on frequency at room temperature of PPTMP thin films of different thicknesses is shown in Fig. 3. The tan [delta] was calculated by using the formula tan [delta] = [G.sub.p]/2[pi]f[C.sub.p], where f is the linear frequency in Hz.

It is observed that a small relaxation peak is appeared at the very low frequency region (at around ~ [10.sup.2]Hz) due to interfacial polarization [15]. Then tan o decreases slightly with a broad minimum at around [10.sup.3] Hz and then again increases with frequency. But PPTMP thin film of 200 nm thick shows a broad maximum above [10.sup.4] Hz. Though loss peak of the 200-nm-thick film is seen around 5 x [10.sup.4] Hz, the loss peak for the films of lower thicknesses may arise above the present experimental frequency range.

The [epsilon]'and tan [delta] are important properties since these help to decide the suitability of a material for a certain application. Dielectric relaxation is studied to gather knowledge about the dielectric losses in the materials used in practically important areas of dielectric and electronic applications.

The dielectric loss tangent tan [delta] = [epsilon]"/[epsilon]', where [epsilon]" is usually referred to as the loss factor and tan [delta] is the dissipation factor. The Cole-Cole [24] plot of [epsilon]" versus [epsilon]' for the PPTMP thin films of different thicknesses are shown in Fig. 4. It is evident that as the film thickness increases the plot approaches more likely to a semicircle with larger diameter. The Cole-Cole plots are semicircles when a single relaxation time is dominant. The semicircular nature of the Cole-Cole plot also supports that the Debye type mechanism is operative in PPTMP.

CONCLUSIONS

The [[sigma].sub.ac] of the PPTMP thin films increases with thicknesses as well as frequency. The ac electrical conduction mechanism in PPTMP thin films is hopping. The value of [epsilon]' also increases with thin film thickness and decreases with frequency. It is observed in PPTMP that there are interfacial polarization in the lower frequencies and dipolar polarization in the higher frequencies. Two dielectric loss peaks corresponding to interfacial and dipolar polarizations in the lower and higher frequency regions are observed. The Cole-Cole plot support the Debye type relaxation in PPTMP thin films.

ACKNOWLEDGMENTS

T. Afroze sincerely thanks the authority of Ahsanullah University of Science and Technology for permitting her to perform this research.

REFERENCES

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Tamanna Afroze (iD), (1) A.H. Bhuiyan (2)

(1) Department of Arts and Sciences, Ahsanullah University of Science and Technology, Dhaka 1208, Bangladesh

(2) Department of Physics, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh

Correspondence lo: T. Afroze; e-mail: tamanna_af@yahoo.com

Contract grant sponsor: Bangladesh University of Engineering and Technology (BUET), Dhaka.

DOI 10.1002/pen.24712

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. AC conductivity [[sigma].sub.ac] as a function of frequency of the PPTMP thin films of different thicknesses at room temperature (298 K).

Caption: FIG. 2. Dependence of dielectric constant, [epsilon]', on frequency for PPTMP thin films of different thicknesses.

Caption: FIG. 3. Dependence of dielectric loss tangent, tan[delta], on frequency, for PPTMP thin films of different thicknesses at room temperature (298 K).

Caption: FIG. 4. Cole-Cole plot, for PPTMP thin films of different thicknesses at room temperature (298 K).
TABLE 1. Values of "n" of PPTMP thin films of various thicknesses at
298 K.

                           Values of "n" in the frequency range

                             [10.sup.3]-      [10.sup.4]-
Thickness of PPTMP (nm)      [10.sup.4] Hz    [10.sup.5] Hz

100                             0.76             1.50
150                             1.04             1.24
                              [10.sup.2]-[10.sup.4] Hz
200                             1.60
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Author:Afroze, Tamanna; Bhuiyan, A.H.
Publication:Polymer Engineering and Science
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Date:Aug 1, 2018
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