Dielectric Properties of Al/Poly (methyl methacrylate) (PMMA)/p-Si ...

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ScienceDirect Procedia - Social and Behavioral Sciences 195 (2015) 1740 – 1745

World Conference on Technology, Innovation and Entrepreneurship

Dielectric Properties of Al/Poly (methyl methacrylate) (PMMA)/ p-Si Structures at Temperatures Below 300 K G. Arasa*, E. Orhanb, A. B. Selçuka,b, S. Bilge Ocakb, M. Ertu÷rula a

Atatürk University, Graduate School of Natural and Applied Sciences, 25240, Erzurum, Turkey b Gazi University, Graduate School of Natural and Applied Sciences, 06500, Ankara, Turkey a,b Sarayköy Nuclear Research and Training Centre, 06983, Ankara, Turkey

Abstract Al/Poly (methyl methacrylate) (PMMA)/p-Si organic Schottky devices were fabricated on a p-Si semiconductor wafer by spin coating of PMMA solution. Dielectric properties of the Al/ Poly (methyl methacrylate) (PMMA)/p-Si structure have been studied using temperature-dependent admittance-voltage (C/G-V) measurements at temperatures below 300 K temperature at 1MHz. The ү temperature-dependent real and imaginary parts of the dielectric constant (İ , İ") and of the electric modulus (Mү, M") as well as ү the ac electrical conductivity (ıAC,) of structure were obtained using C and G data. Experimental results show that the İ , İ", ıAC, loss tangent (tan į), Mү and M" values were strong functions of the temperature and the applied bias voltage.

© byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Istanbul University. Peer-review under responsibility of Istanbul Univeristy. Keywords: Dielectric properties; electric modulus; Organic compounds

1. Introduction Polymeric materials have attracted much attention in both academic and industrial research fields because of their extensive applications in the last two decades. This is primarily on account of their light weight, good mechanical strength, and optical properties, which make them multifunctional materials (Sun et al., 2011; Di et al., 2013; Park et al., 2013).

* Corresponding author. Tel.: +90 554 918 5476; E-mail address: [email protected]

1877-0428 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Istanbul Univeristy. doi:10.1016/j.sbspro.2015.06.295

G. Aras et al. / Procedia - Social and Behavioral Sciences 195 (2015) 1740 – 1745

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The behaviors of the dielectric characteristics of metal-polymer semiconductor (MPS) structures are similar to those of metal/ insulator/ semiconductor (MIS) structures, and many in the field think that they will shape the next generation of cheap and disposable electronic inventions. The electrical and the dielectric properties of the metalsemiconductor (MS) structure can be modified by using metal-doped (Co, Ni, Zn) polymer materials. Pure polymers are well known to have poor electrical conductivities, but their conductivities can be improved by blending and doping them with some metals that can induce modifications in the molecular structure and, hence, in the microstructural properties of the polymer. Poly (methyl methacrylate) (PMMA) is one of the promising polymeric materials and there are numerous papers for its application as a gate dielectric in organic thin film transistors (OTFTs) (Puigdollers et al., 2004; Uemura et al., 2003). PMMA’s thermal and mechanical stability, together with its high electrical resistivity (>2×1015 cm), suitable dielectric constant and thin film processability on large areas by spin coating make it an ideal candidate as a dielectric layer in organic electronics. Optical, electrical and micro-gravimetric properties of PMMA thin films are used to investigate the chemical sensing capability. This is a thermoplastic volatile organic compound material with good tensile strength and hardness, high rigidity, transparency, good insulation properties, excellent planarity and thermal stability dependent on tacticity (Matsuguchi, & Uno, 2006;Vassiltsova et al., 2007). PMMA’s disadvantages such as brittleness and low chemical resistance can be eliminated by chemical or physical modifications. The performance and the quality of MS, MIS, and MPS structures depend on various parameters, such as the interfacial layer’s thickness and its homogeneity, the shape and height of the barrier, the density distribution of interface traps at the M/S interface, the series resistance of the device, the concentration of acceptor or donor atoms, and the samples temperature. Traditionally, SiO2 is used an interfacial layer at the M/S interface in these devices, but it can’t completely passive the active dangling bonds at the semiconductor’s surface. The electrical and the dielectric characteristics of the MS, MIS, and MPS with an interfacial insulator or polymeric layer are generally determined by the quality of the interface, the distribution of interface states at the PMMA/Si interface, the homogeneity of the interfacial layer (PMMA), the barrier homogeneity at the M/S interface, the substrate temperature and series resistance of the structures. In addition, the C-V and the G/Ȧ-V characteristics depend considerably on the temperature, especially at low temperatures. When these measurements are carried out only at room temperature, they cannot provide sufficient information on the electrical and the dielectric properties. On the other hand, when they are carried out over a wide temperature range, especially at temperatures below room temperature, they provide more information about the conduction mechanism in these devices. In the present paper, novel Al/PMMA/p-Si structures with small area were studied by C-V and G/Ȧ-V measurements at temperatures below room temperatures at 1 MHz. The variation of dielectric constant (İ‫)މ‬, dielectric loss (İ"), loss tangent (tanį), ac electrical conductivity (ıAC) and real and imaginary part of electric modulus (M‫މ‬ and M") with temperature and voltage were investigated as a function of frequency and voltage. 2. Methodology 2.1. Experimental In this work, the samples were prepared on a p-type Si (111) wafer which had 280 ȝm thickness and 10 ohm-cm resistivity. Before processing the wafer, cleaning procedures were applied. Firstly, it was dipped into acetone for 10 min at 50oC. Then, it was washed by deionized water and released into methanol for 2 min. Again, the wafer was washed by deionized water and inserted into NOH4:H2O:H2O2 solution for 15 min at 70oC. It was dipped into deionized water to remove the solution on the wafer surface. In order to take away free oxygen on the surface, the wafer was bathed in 2% hydrofluoric acid (HF) solution for 2 min. Finally, deionized water was used to complete the cleaning procedure. After this chemical cleaning procedure, the wafer was put into a vacuum chamber where Al (99.999%) was evaporated on the unpolished surface as ohmic contact with a thickness of 1240 Å. Then, the wafer was annealed at 500oC in vacuum for 10 min to dope aluminum into the back surface of the wafer. Again, the back

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surface of the wafer was coated by Al 1240 Å to complete ohmic contact. The PMMA (950 PMMAC2 MicroChem) was diluted in chlorobenzene (PMMA:chlorobenzene = 1:7). The PMMA layer was constructed by the spin coating technique. The film was deposited by spin coating at 5000 rpm for 45 s on the polished surface of the wafer and was heated on a hot plate at 180oC for 60 s to drive off the solvents. After spin coating the wafer, 1280- Å -thick Al circular rectifying contacts, 1.3 mm in diameter, were deposited by evaporation at 2x10-6 torr. G-V and C–V measurements were taken in the temperature range of 118 í 298 K to determine the dielectric characteristics of the Schottky diode. The schematic representation of the device is shown in Fig. 1. The thickness of the oxide layer was 10 nm which was evaluated from an ellipsometer.

Fig. 1. Schematic representational cross sectional views of Al/PMMA/p-Si Schottky diode for electrical characterization.

2.2. Analyses and Results In the generalized model of the metal–semiconductor contact a thin dielectric interface layer and surface states must be taken into account. In that case, applied voltage is distributed between dielectric layers, depletion layer and recharged surface states. The metal–semiconductor contact, being the capacitor, characterizes by dielectric losses also. Shift of phases between action factor and reaction of contact can be caused by the inertia charge carriers, which are transferred through depletion layer, recharging of surface states. Under Maxwell–Wagner’s theory interlayer polarization is the accumulation of a charge on the interface of layers, having various values of the conductivity. Such polarization results in change of the intensity of an electric field in layers. An interlayer polarization is characterized by a relaxation. The time of a relaxation is defined by layer parameters such as conductivity, dielectric permeability and thickness. The capacitance, dielectric constant, and electric modulus are important parameters in the selection of materials ү for device application. The temperature-dependent, İ , İ", tanį, ıAC, Mү and M" were evaluated from the C and the G/Ȧ data for the MPS structure in the temperature range of 118 í 298 K at 1 MHz. By using the measured values of ү C and G/Ȧ at -2 and 3 V, we determined the, İ , İ", tanį and ıAC values of the MPS structure by using the following expressions (Yüksel et al., 2008) : (1) H c (2) H cc

C Cd p C0 H 0 A d pG

AH 0Z H cc (3) tan G Hc (4)

V AC

2SfH 0 H c tan G


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where G is the measured conductance, Ȧ=2ʌf is the angular frequency, İo is the permittivity of free space charge, A is the sample surface area, dp is the interfacial layer thickness. tanį is the dielectric loss tangent and ıAC is the ү AC electrical conductivity. The voltage dependences of the, , İ , İ", tanį and ıAC values of the MPS structure at temperatures below 300 K are presented in Figs. 2 (a), (b), (c) and (d), respectively.

Fig. 2. Voltage dependence of the: (a) İү; b) İ"; (c) tanį ; (d) ıAC for Al/PMMA/p-Si (MPS) Structure at below 300 K. ү

As can be seen in these figures, the values of the, İ , İ", tanį and ıAC values are strong functions of the ү temperature and the applied bias voltage. As can be seen from these figures, İ and İ", increase as the temperature is increased (Pissis & Kyritsis, 1997; Maurya et al., 2005). These results show that this MPS Schottky diode possess ү better dielectric properties at temperatures lesser than room temperature. The variation of the, İ , İ", and tanį with temperature is a general trend in ionic solids. It may be due to space charge polarization caused by impurities or interstitials in the materials. Moreover in narrow band semiconductors, the charge carriers are not free to move but are trapped causing a polarization. By increasing temperature, the number of charge carriers increases exponentially ү and thus produces further space charge polarization and hence leads to a rapid increase in the dielectric constant, İ . Of course, both types of charge carriers n and p contribute to the polarization. Fig2. (d) shows the voltage dependence of the AC electrical conductivity calculated at 1 MHz. It is clear that the conductivity increases with temperature. It is suggested that the process of dielectric polarization in MIS Schottky diode takes place through a mechanism similar to the conduction process. According to (Migahed et al., 2004) the increase of the electrical conductivity at low temperature is attributed to the impurities, which reside at the grain boundaries. These impurities lie below the bottom of the conduction band and thus it has small activation energy. This means that the contribution to the conduction mechanism comes from the grain boundaries while it mainly results from the grains for higher temperature.

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The dielectric properties of materials can be expressed in various ways, using different representations. For example, a comparison of the complex dielectric permittivity İ* and electric modulus M* representation allows us to distinguish the local dielectric relaxation (e.g. dipolar orientation) from long-range electrical conductivity. The complex impedance (Z*) and complex electric modulus (M*) formalisms were discussed by various authors with regard to the analysis of dielectric materials (Jiwei et al., 2001). Analysis of the complex permittivity (İ*) data in the Z* formalism (Z* = 1/Y* = 1/iwC0 İ") is commonly used to separate the bulk and the surface phenomena and to determine the bulk dc conductivity of the material. The electrical modulus corresponds to the relaxation of the electric field in the material when the electric displacement remains constant. Therefore, the electric modulus represents the real dielectric relaxation process (Moon et al., 2000). Furthermore, the electric modulus formalism is also a useful tool to study the electrical transport mechanisms such as carriers hopping rate and conductivity relaxation phenomena. The complex impedance or the complex permittivity (İ* = 1/ M*data are transformed into the M* formalism using the following relation

M*

iwC 0 Z *

(5)

or

M

*

1

H*

'

Hc H cc M jM cc j 2 2 2 H c H cc H c H cc2 '

(6)

The real component Mү and the imaginary component M" are calculated from H c and H cc Figs. 3(a) and 3(b) depict the real (Mү) and the imaginary (M") parts of the complex electric modulus (M*) versus voltage for the samples at 1MHz. Clearly, the value of M‫ މ‬decreases with increasing temperature. On the other hand, the value of M" decreases with increasing temperature. All of these obtained results confirm that the changes in the electric and the dielectric parameters are very large. In the other word, at low temperatures, the conduction mechanism is very different from that at high temperatures. Therefore, in our opinion, this study is valuable became it was carried out at lower temperatures below room temperature.

Fig. 3. Voltage dependence of the (a) Mү; (b) M" for Al/PMMA/p-Si (MPS) structure at below 300 K.


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3. Conclusion We have fabricated and determined the dielectric characteristics of the Al/PMMA/p-Si MPS Schottky structures formed by spin coating of the PMMA. The dielectric properties of the Al/PMMA/p-Si (MPS) structure have been studied at temperatures in the range of 118K-298K at 1 MHz. The experimental results indicated the dielectric properties of the MPS structure were quite sensitive to the temperature and the applied bias voltage. The values of ү dielectric constant (İ ) and dielectric loss (İ") decrease with decreasing temperature. AC electrical conductivity (ıAC) decreases with decreasing temperature. The real component Mү and the imaginary component M" of the electrical modulus decreases with increasing temperature. The behavior of dielectric properties especially depends on temperature. Acknowledgements This work is supported by Gazi University BAP office with the research project numbers 41/2012-02 and 41/201201. References Pissis, P., & Kyritsis, A. (1997). Electrical conductivity studies in hydrogels. Solid State Ionics, 97, 105. Moon, K.S., Choi, H.D., Lee, A.K., Cho, K.Y., Yoon, H.G., Suh, K.S. (2000). Dielectric properties of epoxy-dielectrics-carbon black composite for phantom materials at radio frequencies. J. Appl. Poly. Sci., 77, 1294. Jiwei, Z., Xi, Y., Mingzhong, W., Liangying, Z. (2001). Preparation and microwave characterization of PbTiO3 ceramic and powder. J. Phys. D, 34, 1413. Uemura, S., Yoshida, M., Hoshino, S., Kodzasa, T., Kamata, T. (2003). Investigation for surface modification of polymer as an insulator layer of organic FET. Thin Solid Films, 378, 438–439. Migahed, M.D., Ishra, M., Fahmy, T., Barakat, A. (2004). Electric modulus and AC conductivity studies in conducting PPy composite films at low temperature. J. Phys. Chem. Solids, 65, 1121. Puigdollers, J., Voss, C., Orpella, R., Quidant, MartÕn, I., Vetter, M., Alcubilla, R. (2004). Electronic characterization of Al/PMMA[poly(methyl methacrylate)]/p-Si and Al/CEP(cyanoethyl pullulan)/p-Si structures. Org. Electron., 5, 67. Maurya, D., Kumar, J., Shripal, J. (2005). Dielectric-spectroscopic and a.c. conductivity studies on layered Na2-XKXTi3O7 (X=0.2, 0.3, 0.4) ceramics. Phys. Chem. Solids, 66, 1614. Matsuguchi, M., & Uno,T. (2006). Molecular imprinting strategy for solvent molecules and its application for QCM-based VOC vapor sensing. Sens. Actuators B Chem., 113, 94. Vassiltsova, O.V., Zhao, Z., Petrukhina, M. A., Carpenter, M. A. (2007). Surface-functionalized CdSe quantum dots for the detection of hydrocarbons. Sens. Actuators B Chem., 123, 522. Yüksel, O. F., Ocak, S.B., Selcuk, A.B. (2008). High frequency characteristics of tin oxide thin films on Si.Vacuum, 82, 1183. Sun, J., Zhang, B., Katz, H.E. (2011). Materials for printable, transparent, and low-voltage transistors. Adv Funct Mater., 21(1), 29. Di, C.A., Zhang, F., Zhu, D. (2013). Multi-functional integration of organic field-effect transistors (OFETs): Advances and perspectives. Adv. Mater., 25(3), 313. Park, H.M., Yun, D. Y., Kim S. W., Kim, T. W. (2013). Electrical bistability of organic bistable devices based on colloidal CuInS2 quantum dots embedded in a Poly (N-vinylcarbazole) layer. Jpn. J. Appl. Phys., 52.