Fabrication and Characterization of Polyaniline Based Schottky Diode

Transcription

1 Fabrication and Characterization of Polyaniline Based Schottky Diode Asgar Hajibadali, Majid Baghaei Nejhad Faculty of Electrical and Computer engineering Hakim Sabzevari University Sabzevar, Iran Gholamali Farzi Department of Material and Polymer engineering Hakim Sabzevari University Sabzevar, Iran Hashem Hojjati Rad Department of Information technology and Communication Payame noor University of Tehran Tehran, Iran Abstract In this work polyaniline was chemically synthesized. Schottky diode was fabricated based on polyaniline as p- type semiconductor, gold as Ohmic contact and aluminum as Schottky contact. Coating of metals was carried out with physically vapour deposition method and coating of polymer was done with solvent casting method. Current-voltage (I-V) characteristics of diode were measured and current transport mechanism of diode was studied. I-V characteristics showed that Schottky diode based on pure polyaniline follows the thermionic emission mechanism. The electronic parameters of Schottky diode such as ideality factor, barrier height and reverse saturation current have been determined. I. INTRODUCTION Electrical conductivity of conjugated polymers can be varied over the wide range from insulator to semiconductor and to conductor through p- doping or n- doping. In the recent decades lots of research works have been carried out on the use of conducting polymers such as polyaniline (PANI) [1], polypyrrole [2], polythiophen [3], and etc. as active materials in electronic devices. Several devices have been made from this polymers, such as field effect transistor (FETs) [4], [5], Schottky diodes [6]-[8], light emitting diodes (LEDs) [9], [10], photovoltaic cells [11]. Among the conducting polymers, polyaniline has received great attention due to its good thermal and environmental stability, unique properties in conducting form and its ability to tune the electronic properties by careful choice of chemical and electrochemical preparative routes. Rectifying metal-semiconductor Schottky junction diodes compared with p-n junction diodes have benefits such as higher switching speeds, lower noise level, lower forward resistance and intrinsic suitability for low-voltage, highcurrent applications [12]. Several research groups have illustrated fabrication of Schottky diodes based on polyaniline using vacuum deposition [13], solvent casting [14]-[16], electro polymerization [17], [18], and pellets [19]. In this work, polyaniline was chemically synthesized, Schottky diode with Al-PANI-Au structure was fabricated, current-voltage (I-V) characteristics of diode were measured, current transport mechanism was studied and electronic parameters of fabricated Schottky diode were determined. II. EXPERIMENTAL DETAILS A. Materials Aniline was supplied from Merck and was distilled prior to synthesis. Hydrochloric acid (HCl), Ammonium persulfate (APS), and N-methyl-2-pyrrolidinone (NMP), purchased from Merck and were used as received. B. Instrumentation Gold and aluminum were physically vapour deposited with PVD-EMS-160. Gold was coated on test glass by thermalresistance evaporation technique and aluminum was coated on polyaniline film by electron beam gun evaporation technique.

2 A Philips, XL30 scanning electron microscope (SEM) was employed for imaging the sample. Sample for the SEM were prepared by tearing the polymer pellet. The sample was placed in a gold coater to assist in easier electron mapping of the surface and to reduce charging of the sample. Once the gold coating was complete, sample was viewed on the SEM. The instrument was operated at an accelerating potential of 5 kv. Current-voltage (I-V) measurements were carried out with Protek DF 1731SB3A, DC power supply and Keithley 6485 Pico ammeter, by applying a ramped voltage and measuring the resultant diode currents. C. Polyaniline Synthesis Polyaniline was chemically synthesized by a previously reported method [20]. Aniline: ammonium persulfate (APS) molar ratio was 1:1. In a typical procedure APS was dissolved in distilled water, and then the solution was added drop wise to a solution of aniline dissolved in HCl aqueous solution, while the reaction mixture was stirred for 12 h. The greenish precipitate was collected and washed repeatedly with HCl solution to remove residual oxidant and monomer. Then the remained polymer on the filter paper was washed several times with distilled water and dried to get the polyaniline. The SEM microstructural analysis for the polyaniline obtained in this work was shown in Fig. 1. D. Fabrication of Schottky Diode Polyaniline solution was prepared at room temperature by adding 0.05 g of polyaniline powder in 10 ml N- methyl-2- pyrrolidinone (NMP), sonication was continued for 1 hour and then solution was stirred for 5 hours. Polyaniline has been known to be a p-type Semiconductor [13]. For the formation of ohmic contact at the junction, the work function of the metal must be greater than the work function of the p-type polyaniline and for the formation of a rectifying barrier at the junction, the work function of the metal must be smaller than that of the p-type polyaniline. Thus the gold was chosen as the ohmic contact of the diode because of its relatively high work function (Φ =5.1eV) and aluminium was chosen as the Schottky contact metal because of its relatively low work function (Φ =3.9eV) in compare with p-type polyaniline (Φ = ev) [21]. For fabrication of Schottky diode, a thin layer of gold was first coated on a circular test glass by physically vapour deposition technique with thermal-resistance evaporation procedure. Gold coating on circular test glass has been shown in Fig. 2(a). Then a thin film layer of prepared polyaniline solution was coated on gold with solvent casting method and let it to dry. Coating of polyaniline film on gold layer has been shown in Fig. 2(b). Finally, a thin layer of aluminium was deposited on the surface of polymer film by electron beam evaporation technique under ~10-5 Torr pressure. Figure 1. SEM micrograph of the polyaniline film Figure 2. (a) Gold coated on circular glass. (b) Polyaniline coated on gold layer Schottky contact to the front side of the polymer film was formed using a shadow mask to form square shape with approximately length of 3 mm. A schematic representation of this device structure was shown in Fig. 3. III. RESULTS AND DISCUSSION Current vs. voltage (I-V) measurement of fabricated diode was performed at room temperature. The currents were measured from zero bias voltage to positive side up to 5 V. Similar procedure was followed in the negative bias region. Schematic setup for I-V measurement was shown in Fig. 3. I- V curve of the Al-PANI-Au Schottky diode were shown in Fig. 4. It is known that an asymmetric flow of electrons through a system is called rectifier of electrical current. The curve shown in Fig. 4 is asymmetric and non-linear indicating a rectifying behaviour of the PANI-Al hetero junctions. Figure 3. Schematic of PANI Schottky diode structure

3 The SCLC mechanism depends on the traps at the interfacial region between metal and polymer [12]. The current density for SCLC mechanism is given by the following equation, [( 8 V ) ( d )] V J = εε μ. (5) 0 9 Figure 4. I-V Characteristics of Al - PANI- Au Schottky diode The nonlinear current-voltage (I-V) characteristics of the metal-polymer junction could be due to the thermionic emission, space charge limited conduction (SCLC) or Poole- Frenkel emission [7]. According to the thermionic emission model, I-V relationship for the Schottky barrier devices is given as [8], [12], ( ev nkt) J = J0 exp. (1) Where e is the electric charge, n is the ideality factor, k is the Boltzmann constant and T is absolute temperature and J 0 = I 0 /A is the reverse saturation current density, in which A is the effective area and I 0 is the reverse saturation current of diode. From J 0 ; the barrier height (φ b ) can be calculated using the Richardson equation [8], [12], and [22], Where, µ is the carrier mobility. In thermionic emission mechanism the plot of log (I) vs. V is linear, in Poole-Frenkel emission; the plot of log (I/V) vs. V 1/2 should be straight line. However, when the plot of log (I) vs. log (V) is linear, system will follow the SCLC mechanism. In this case, a slope of 2 is expected. In the polyaniline based Schottky diode, the nonlinear log (I) vs. log (V) curve (Fig. 5) eliminates the possibility of SCLC process; also log (I/V) vs. V 1/2 plot is nonlinear (Fig. 6) that eliminates the possibility of Poole-Frenkel process. But log (I) vs. V curve (Fig. 7) can be fitted to a straight line. These results suggest that the thermionic emission theory can be applied to evaluate junction parameters for Au-PANI-Al Schottky diode. Using the plots of log (I) vs. V (bias voltage) were given in Fig. 7, Experimentally, J 0 obtain by extrapolating the linear part of log (J) vs. V and taking the intercept with J-axis. This extrapolated value of current density at zero voltage is the saturation current J 0. The ideality factor can be obtained from (3) [24]. The parameters J 0, φ b and n were calculated and given in Table I. ( eφ kt ) J = A 2 0 T exp b. (2) Where A* is Richardson constant (120 A/cm 2 K 2 for free electron).the value of ideality factor (n) can be obtained from following equation, e = kt. (3) n d ln J dv The Poole- Frenkel emission depends on the material bulk barriers (Acceptor and donor sites, traps, or electrons in the valence band) [23]. The current density for Poole-Frenkel emission can be expressed by the following equation, Figure 5. log (I) - log (V) plot of diode 2 [ nkt ] 1 ( J / V ) ( V d ) J / V = 0 0 exp β. (4) Where d is the thickness of the coated polymer and β= (e 3 /πεε 0 ) 1/2, in which ε is the dielectric constant of the polymer and ε 0 is the free-space permittivity.

4 REFERENCES Figure 6. log (I/V) - V 1/2 plot of diode Figure 7. log (I) - V plot of diode TABLE I. ELECTRONIC PARAMETERS OF SCHOTTKY DIODE WITH AL - PANI - AU STRUCTURE Parameters Al-PANI-Au J DIODE 0 φ b n (µa/cm 2 ) (ev) value IV. CONCLUSIONS Polyaniline was chemically synthesized. Schottky diode was fabricated using Al as Schottky contact, Au as Ohmic contact, and Polyaniline as semiconductor. I-V characteristics for current transport mechanism were studied. It was observed that Schottky diode based on polyaniline follows the thermionic emission mechanism. The electronic parameters of Schottky diode such as ideality factor, barrier height, and reverse saturation current were calculated. According to these results, Polyaniline has the potential to be used in organic electronic devices. Further experimental works and modelling will be needed to optimize the electronic parameters and current transport mechanisms in polyaniline based device. [1] E. W. Paul, A. J. Ricco, and M. S. Wrighton, Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyaniline-based microelectronic devices, J. Phys. Chem, vol. 89, pp , [2] S. Miyauchi, A. Fueki, Y. Kushihi, H. Abiko, and Y. Sorimachi, Schottky barrier formation between polypyrrole and indium, Synth. Met., vol. 18, pp , [3] T. A. Abdalla, W. Mammo, and B. Workalemahu, Electronic and Photovoltaic Properties of a Single Layer Poly [3-(2, 5 diheptyloxyphenyl)-2, 2'-bithiophene Devices, Synth. Met., vol. 144, pp , [4] W. Lee, D. Kim, Y. Jang, J. Cho, M. Hwang, Y. Park, Y. Kim, J. Han, and K. Cho, Solution-processable pentacene microcrystal arrays for high performance organic field-effect transistors, Appl. Phys. Lett., vol. 90, no. 13, pp , [5] S. Chung, S. O. Kim, S. K. Kwon, C. Lee, and Y. Hong, All-Inkjet- Printed Organic Thin-Film Transistor Inverter on Flexible Plastic Substrate, IEEE ELECTRON DEVICE LETTERS, Vol. 32, No. 8, [6] K. S. Kang, Y. Chen, H. K. Lim, K. Y. Cho, K. J. Han, and J. Kim, Performance enhancement of polymer Schottky diode by doping pentacene, Thin Solid Films, vol. 517, pp , [7] V. Saxena, and K. S. V. Santhanam, Junction Properties of Schottky Diode with Chemically Prepared Copolymer Having Hexylthiophene and Cyclohexylthiophene units, Cur. Appl. Phys., vol. 3, pp , [8] R. K. Gupta, and R. A. Singh, Schottky Diode Based on Composite Organic Semiconductors, Mater. Sci. in Semicond. Proc., vol. 7, pp , 2004 [9] T. Sekitani, H. Nakajima, H. Maeda, T. Fukushima, T. Aida, K. Hata, and T. Someya, Stretchable active-matrix organic light-emitting diode display using printable elastic conductors, Nat. Mater., vol. 8, pp , [10] S. F. Tedde, J. Kern, T. Sterzl, J. Frst, P. Lugli, and O. Hayden, Fully spray coated organic photodiodes, Nano Lett., vol. 9, pp , [11] J. H. Schon, Ch. Kloc, E. Bucher and B. Batlogg, Effcient organic photovoltaic diodes based on doped pentacene, Nature,Vol. 403, Jan [12] S. M. Sze semiconductor devices, physics and technology, 2nd ed., New York: John Wiley & sons, 2002, pp [13] S. C. K. Misra, M. K. Ram, S. S. Pandey, B. D. Malhotra, and S.Chandra, Vacuum,deposited metal/polyaniline Schottky device, Appl. Phys. Lett., vol. 61, pp. 1219, [14] C. Li, Y. Wang, M. Wan, and S. Li, Rectifying effect of soluble polyaniline films, Synth. Metals, vol. 39, pp. 91, [15] S. A. Chen, Y. Fang, and H. T. Lee, Polyacrylic acid-doped polyaniline as p-type semiconductor in Schottky barrier electronic device, Synth. Metals, vol. 57, pp. 4082, [16] S. A. Chen and Y. Fang, Polyaniline schottky barrier: effect of doping on rectification and photovoltaic characteristics, Synth. Metals, vol. 60, pp. 215, [17] S. S. Pandey, S. C. K. Misra, B. D. Malhotra, and S.Chandra, Some recent studies on metal/polyaniline schottky devices, J. Appl. Polym. Sci., vol. 44, pp. 911, [18] Y. Renkuan, Y. Hong, Z. Zheng, Z. Youdou, and W. Yongan, Polyaniline / silicon heterojunctions, Synth. Metals, vol. 41, pp. 731, [19] S. S. Pandey, M. K. Ram, V. R. Srivastava, and B. D. Malhotra, Electrical properties of metal (indium)/polyaniline Schottky devices, J. Appl. Polym. Sci., vol. 65, pp. 2745, [20] P. J. Saikia, and P. C. Sarmah, Investigation of Polyaniline Thin Film and Schottky Junction with Aluminium for Electrical and Optical Characterization, Materials Sciences and Application, vol. 2, pp , 2011.

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