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Electrical conduction mechanism in plasma polymerized 2-(diethylamino)ethyl methacrylate thin films.

INTRODUCTION

Plasma polymerization is considered to be an excellent method to prepare pinhole free, highly resistive, chemically inert, and thermally stable polymer thin films of varying thickness on various substrates from almost any organic vapor [1]. Plasma polymerized (PP) thin films have noteworthy advantages due to the reduced amounts of materials used, faster processing times, and the ability to modify the surface while preserving the structural properties of the bulk. It is successfully used to manufacture thin film resistors, capacitors, photoelectronic devices, organic light emitting diodes, and transistors which have shown performance comparable to the amorphous silicon transistors, and so forth [2], Lots of applications of plasma polymers such as barrier coatings, dielectric, photo resist, and wave-guiding films for microelectronics and photonics [3, 4], organic solar cells [5], organic memory devices, organic thin film transistor [6], organic circuit [7], organic sensor [8], and N[O.sub.2] sensors [9] are the focus of intense study. The utilization of this technology to assemble electronic devices makes it obligatory to understand the electrical properties of the materials in the thin film form. Investigation of electrical properties of a plasma polymer is one of the most convenient and sensitive methods for its suitability in various electronic devices.

The study of electrical conduction mechanism in polymers is very significant since polymers have inherent properties like light-weight, rust proof, easily processable, and so forth. Hence, constant attempts are being made to understand the mechanism of electrical conduction in PP thin films. There are various electrical conduction mechanisms have been reported by several researchers on different PP organic thin films, such as (i) space charge-limited conduction (SCLC) in PP 1-benzyl-2-methylimidazole (PPBMI) thin films [10], PP N,N,3,5-tctramethylaniline thin films [11], PP polyaniline thin films [12], PP pyrrole (PPPy) [13], (ii) Schottky-type conduction in PP polyfurfural [14], PP eucalyptus oil thin films [15], and so forth, (iii) Schottky and Poole-Frenkel (PF) in PP 2,6,diethylaniline [16] and so forth.

Methacrylates are common monomers which easily form polymers because their double bonds are very reactive. Polymethylmethacrylate (PMMA) is a polymer having carbon-carbon main chains. It is basically classified as insulators due to very small number of charge carriers and their low mobility [17]. It is one of the best polymeric materials broadly used for manufacturing insulating devices [18, 19]. It is seen from the literature survey that PMMA and the products from the derivatives of methylmethacrylate (MMA) have gained a lot of interest among the scientists and technologists for their interesting properties and wide range of applications including sensing material, gate dielectric for organic thin film transistor, dielectric layer for field effect transistor, memory devices, and so forth [20-22]. PP MMA films have been suggested as dry electron-beam resist [23], membranes for gas separators, humidity sensors, and optical devices [24-27], 2-(diethylamino)ethyl methacrylate (DEAEMA) is a derivative of ethyl methacrylate (EMA), which is a methacrylate ester readily polymerizes and rapidly reacts to form a crosslinked polymer [28], It is a monofunctional methacrylate and the attached tertiary amine generates free radicals, which lead polymerization to proceed smoothly without addition of any external reducing agent [29], The presence of amino group imparts cationic properties. The monomers with tertiary amine used to crosslink polymers, giving insoluble network polymers with cationic centers.

Because of possible applications of PMMA and films produced from the derivative of MMA in electrical and electronic, and so forth devices, it is intended to prepare thin films of DEAEMA by plasma polymerization technique. The effect of thickness and heat treatment on the structural and optical characteristics of PP 2-(diethylamino)ethyl methacrylate (PPDEAEMA) have been reported previously [30], Direct current electrical characteristics of the PP DEAEMA (PPDEAEMA) thin films have been investigated, the outcome of which will contribute to the scientific knowledge and applications of this material. So DC electrical conduction mechanism from the dependence of J on V at different temperatures for different thicknesses of PPDEAEMA thin films has been elucidated. The article reports the charge transport phenomenon and the influence of film thickness and temperature on the transport properties of PPDEAEMA thin films.

EXPERIMENTAL DETAILS

The Monomer

The monomer DEAEMA is a derivative of MMA was purchased from Aldrich Chemical Company and was used as an organic precursor. The chemical structure of DEAEMA is shown in Fig. 1. It is a clear light yellow color liquid with molecular formula [C.sub.10][H.sub.19]N[O.sub.2]. Microscope glass slides (25 mm x 76 mm x 1 mm) of Sail Brand, China, purchased from local market, were used as substrate.

Preparation of PP Thin Films

A block diagram of the capacitively coupled plasma polymerization experimental set up is shown in Fig. 2. Chemically cleaned glass slides were used as substrates which were cleaned with acetone and distilled water, before deposition. A rotary pump (Vacuubrand, Germany) was used to evacuate the discharge chamber down to a pressure of about 1.33 Pa. The glass substrates were positioned on the lower electrode.

The PPDEAEMA thin films were deposited on top of glass substrate at room temperature using a cylindrical-type capacitively coupled glow discharge system consisting of two stainless steel parallel plate electrodes of diameter and thickness 0.09 and 0.001 m, respectively positioned at a distance of 0.04 m. By means of a step-up transformer connected to the electrodes, glow discharge plasma was generated around the substrates, with a power of about 40 W at standard line frequency of 50 Hz [1, 10, 30, 31]. During deposition, the pressure of the chamber was maintained approximately at about 13.3 Pa at room temperature.

The PPDEAEMA thin films were deposited for about 40-75 min to obtain a suitable film thicknesses ranging from 100 to 300 nm. The thickness of PPDEAEMA thin films were measured by multiple beam interferrometry technique [30].

In order to explore the kinetic growth of PPDEAEMA plot of thickness versus deposition time at their different plasma power is shown in Fig. 3. The increased number of free radicals increase the polymerization rate up to 60 min but after that due to long duration exposure of the film, the higher rate of surface etching possibly reduce the deposition rate thus thickness of the films. Again at 30 W, the plasma is low hence does not provide film thickness properly and at 50 W, the plasma is too high, which results in slower deposition of the thin films, with the increase in time. But at 40 W, proper polymerization takes place which confers PPDEAEMA thin films of suitable thickness for different measurements.

Direct Current Electrical Measurements

DC electrical measurements were carried out by a conventional method with the sample in a subsidiary vacuum system at a pressure of the order of 1.33 x [10.sup.-4] Pa to avoid any ambient effect. DC voltage (0.1-40 V) was supplied step by step by a stabilized power supply (Agilent 6545A, Agilent Technologies, Malaysia), and the current was measured by a high impedance Keithley electrometer 614 (Keithley Instruments) at different temperatures 298, 323, 348, 373, 398, and 423 K. For DC electrical measurements, A1/PPDEAEMA/A1 sandwiched structure samples of thickness 100, 200, 250, and 300 nm were prepared. The A1 electrodes were deposited using a metal coating unit (Edwards 306, England, UK) at a pressure of about 1.33 x [10.sup.-3] Pa. The lower and upper A1 electrodes were square in shape with an effective cross-sectional thin Film area of [10.sup.-4] [m.sup.2], and the area of the films was 0.015 x 0.015 [m.sup.2]. Silver paste was used to make good electrical and thermal contacts. A chromel-alumel thermocouple was placed very close to the sample and was connected to a digital microvoltmeter (Keithley 197A, Keithley Instruments), in order to measure the sample temperature accurately. A low-frequency impedance analyzer (Model: 4192A, Agilent Technologies Japan, Tokyo, Japan) was used to measure the capacitance of the PPDEAEMA thin films.

RESULTS AND DISCUSSION

Current Density-Voltage Characteristics

The mechanism of conduction in an organic device depends on the traps present in the polymer and the metal-polymer interface. Electrons/holes may also be transported by field-assisted thermal excitation over the lower Coulombic potential barrier [32], The whole J-V characteristics of a polymer are determined by the carrier mobility [mu], the free carrier density [n.sub.o], and the trap density [N.sub.t]. The conduction behavior was studied on the basis of the J-V characteristics of A1/PPDEAEMA/A1 structure of 100, 200, 250, and 300 nm thicknesses recorded at the temperatures of 298, 323, 348, 373, 398, and 423 K within the voltage region from 0.1 to 40 V. The variations of current density with applied voltage at room temperature for different thicknesses are exhibited in Fig. 4.

The representative J-V characteristic curves of a PPDEAEMA thin film of thickness 300 nm at different temperatures are given in Fig. 5. The J-V characteristics presented in Fig. 5 can be expressed as a power law of the form J [varies] [V.sup.n] with two different slopes where n is a power index. The values of slopes are calculated for all the four thicknesses (100, 200, 250, and 300nm) and for all the temperatures (298-423 K) and found 0.9-1.1 in the lower applied voltage region and 2.2-5.2 in the higher voltage region. These observations indicate that the current conduction is ohmic in the low voltage region and is nonohmic in the high-voltage region. The reason behind the nonlinear current-voltage characteristics is the SCLC [33], which is influenced by traps. The defects and impurities can govern the conduction mechanism and also work as trapping centers which get occupied by the injected charge carriers from the (J-V) electrode, hence they become charged and thereby expected to build up a space charge. This buildup of space charge plays the key role in the determination of the SCLC process [34].

The curves in Fig. 5 show a thermally activated conduction in the entire temperature region. It is observed in Fig. 5 that the current increases gradually with applied voltage at lower voltage region. Conversely, the rate of increase is faster with applied voltage at higher voltages. This strong dependence of temperature may be due to the enhanced molecular motion and increased generation of carriers at higher temperatures in PPDEAEMA thin films. This type of strong temperature dependence was also reported for PMMA by Shukla and Gaur [34, 35].

At the higher voltage region, the dependence of J on thickness and voltage recommend that the conduction of current may be due to SCLC, Schottky, or PF conduction mechanisms in PPDDEAEMA thin films. The dependence of J on film thickness, d, for the thin films of different thicknesses at a constant voltage (30 V) and at room temperature is plotted in Fig. 6, which would provide a clear idea about the specific conduction mechanism. The J depends on film thickness as

J [alpha] [d.sup.-k], in the nonohmic region of applied voltage, where k represents the distribution of traps in the thin films. A slope of k < 3 recommends the possibility of Schottky or PF mechanism, whereas k [greater than or equal to] 3 recommends the possibility of SCLC mechanism [33, 35]. Shukla and Gaur [34] reported that as a polar, amorphous material PMMA provide a large number of trapping centers and trapping of charge carriers in these trapping sites which would result in the buildup of a space charge. Thus as a derivative of MMA, PPDEAEMA may also have the same characteristics.

The slope of the J-d plot of Fig. 6 is 4.2. This value of k is greater than that mentioned in previous paragraph for the Schottky and PF conduction mechanisms. This value is in good conformity to the condition for SCLC mechanism [33, 35]. Accordingly, it can be recommended that SCLC is the most probable mechanism of charge transport in PPDEAEMA thin films. The SCLC [33] mechanism is influenced by traps. The defects and impurities can govern the conduction mechanism and also work as trapping centers, which get occupied by the injected charge carriers from the electrode, become charged and build up a space charge which plays key role in the determination of the SCLC process [34].

The classical Mott-Gurney relation for the current density of SCLC is [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where [mu] is the mobility of charge carriers, [[epsilon]' is the dielectric constant of the material, and [[epsilon].sub.0] is the permittivity of free space [36]. Figure 4 shows low-current densities at low voltages, which may be due to the capture of injected charges in traps. The number of injected carriers increases with the increase of applied voltage gradually and filling the limited traps. According to Lampert [37], if sufficient charge is injected into the polymer film, all the traps will be filled reaches the trap-filled limit (TFL), and consequently, the current becomes SCLC. Further, injection of carriers from the metal electrode enters into the conduction band and contribute to the current. So the current is increasing sharply in the SCLC region.

The [V.sub.TFL] is the applied voltage at which all traps are filled. This is given by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where [N.sub.t] is the total concentration of traps [38]. Above a certain transition voltage [V.sub.tr], it is seen that J turn out to be nonohmic and the system then switches to the SCLC. The methodology to extract [V.sub.tr] is to draw two tangents one at lower voltage region and another at higher voltage region. Then [V.sub.tr] is the voltage where the two slopes intersects. The free carrier density [n.sub.0] can be calculated from the equation [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] where [V.sub.tr] is the voltage at which the transition from ohmic to SCLC occurs [33]. The variation of [V.sub.tr] (between ohmic and SCLC) with thickness d is shown in Fig. 7, which represents that [V.sub.tr] is higher for higher thickness.

In order to find out both [n.sub.0] and [N.sub.t], the capacitance ([C.sub.p]) of the PPDEAEMA thin films was measured at 5 kHz and the value of dielectric constant was calculated using the formula [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] where [[epsilon].sub.0] is the permittivity of free space and [epsilon]' is the dielectric constant of the medium, A is the surface area of each of the plates/electrodes, d is the thickness of the dielectric, and [C.sub.p] is the capacitance. The value of dielectric constant lies between 4.1 and 16.1 for different thicknesses at room temperature. The [C.sub.p] versus d curve is shown in Fig. 8.

From Fig. 4, the carrier mobility in the SCLC region was calculated and is depicted in Table 1. The calculated values of [n.sub.0] and [N.sub.t] are also presented in Table 1, where 35 V was used as [V.sub.TFL]. The low mobility arises because of the movement of carriers by a hopping process from the center to center involving a finite energy jump for each transition in an amorphous material. From Table 1, it is perceived that the trap density increases with the increase in thickness. The total density of traps may arise from (i) the surface and (ii) the volume or bulk. When the thickness of the PPDEAEMA films increases the interfacial strain may be reduced due to increase in the volume of the samples. Moreover, some fragmentation of the monomer takes place due to plasma interaction. The presence of C=C bonds [30], unsaturated bonds of carbon and nitrogen, carboxyl groups, and so forth, are also responsible for the SCLC type of conduction in the material. According to Shukla and Gaur [34], as a polar, amorphous material PMMA provide a large number of trapping centers and trapping of charge carriers in these trapping sites would result in the buildup of a space charge. Thus as a derivative of MMA, DEAEMA may have the same characteristics. Thus PPDEAEMA may possess similar buildup of trapping centers. The free carrier density is almost constant for all the thickness of PPDEAEMA thin films. Therefore, as trap density decreases with the increase in thickness, carrier mobility also decreases with the same.

Temperature Dependence of Current Density

The temperature dependence of the J can be expressed by the well-known Arrhenius law, J = [J.sub.0] exp(-[DELTA]E/kT), where [J.sub.0] is a constant, [DELTA]E is the thermal activation energy of electrical conduction, and k is the Boltzmann constant. Figure 9 illustrates the dependence of J on inverse absolute temperature, 1/T, for a typical PPDEAEMA thin film of thickness 300 nm.

In this figure, there are two curves, corresponding to the temperature dependence in the ohmic (V = 2 V) and in the SCLC (V = 30 V) regions. The increase in J with temperature may be due to the increased movement of the ions and/or electrons. It is seen that both the curves can be characterized by two different slopes in the low- and high-temperature regions. The curves have varying slopes at low temperature but become almost linear in the high-temperature region, corresponding to well-defined activation energy. The activation energies associated with two temperature regions were determined from the slopes of J - 1/T plot for samples of thicknesses 100, 200, 250, and 300 nm and found about 0.005 to 0.016 eV. The variation of current density with temperature in the ohmic and nonohmic regions for PPDEAEMA thin films of thickness 300 nm is presented in Fig. 9 as a representative one. When the charge carriers are localized, there is no free motion of charge carriers and the conduction proceeds via the phonon-assisted hopping of charge carriers between localized sites. Since the localized states have quantized energies extending over a certain range, activation energy is required for each hop.

These small values of the activation energies suggest the existence of the shallow traps levels in PPDEAEMA thin films. The low-activation energies indicate that the thermally activated hopping conduction is operative in this material. It possibly occur in PPDEAEMA thin films since the existence of ester functional group, with the carbonyl oxygen atom being a basic site, leads to DEAEMA (like PMMA [39]) exhibiting electron-donor ability. So the conduction process takes place by electron hopping between donor atoms and empty sites situated in the energy band gap.

CONCLUSIONS

The current density-voltage characteristics show two regions--one ohmic in the lower voltage region and another SCLC in the higher voltage region in A1/PPDEAEMA/A1 structure sample in the experimental thicknesses and temperatures. Therefore, the DC conduction mechanism in PPDEAEMA thin films is SCLC. The carrier mobility was found about 6.80 x [10.sup.-19] to 2.38 x [10.sup.-18] [m.sup.-2] [V.sup.-1] [s.sup.-1]. The free carrier density and the trap density are calculated to be about 1.78 x 1[0.sub.23] to 2.04 x [10.sup.23] [m.sup.-23] and 6.93 x [10.sup.23] to 15.9 x [10.sup.23] [m.sup.-3], respectively, for different thicknesses. The activation energies were found very low, which reveal the thermally activated hopping conduction in this material because of the presence of shallow trap levels. These findings may facilitate using PPDEAEMA in manufacturing electronic devices.

ACKNOWLEDGMENTS

Tamanna Afroze is thankful to the authority of Ahsanullah University of Science and Technology, Dhaka for permitting her to continue research work. She is also grateful to the Govt, of the People's Republic of Bangladesh for providing her financial assistance through Prime Minister's Scholarship for Ph. D work.

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Tamanna Afroze, (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 to: T. Afroze; e-mail: tamanna_af@yahoo.com

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

DOI 10.1002/pen.24161

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

TABLE 1. Carrier mobility, [mu], free carrier density, [n.sub.0], and
trap density, [N.sub.t] for PPDEAEMA thin films of different
thicknesses at room temperature.

                                             [mu] ([m.sup.2]
Film             [V.sub.tr]   [epsilon]'       [V.sup.-1]
Thickness (nm)      (V)        at 5 kHz        [s.sup.-1])

100                  8           4.1       2.38 X [10.sup.-18]
200                  10           9        2.25 X [10.sup.-18]
250                  12          12.5      2.07 x [10.sup.-18]
300                  16          16.1      6.80 X [10.sup.-19]

                     [n.sub.0]
Film                ([m.sup.-3])          [N.sub.t]
Thickness (nm)      ([m.sup.-3])         ([m.sup.-3])

100              2.04 X [10.sup.23]   15.9 X [10.sup.23]
200              1.40 X [10.sup.23]   8.72 X [10.sup.23]
250              1.49 X [10.sup.23]   7.75 X [10.sup.23]
300              1.78 X [10.sup.23]   6.93 X [10.sup.23]
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Author:Afroze, Tamanna; Bhuiyan, A.H.
Publication:Polymer Engineering and Science
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Date:Dec 1, 2015
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