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polycarbazole contact
Synthetic Metals 207 (2015) 96–101
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
An organic Schottky diode (OSD) based on a-silicon/polycarbazole contact Aditi Srivastavaa , P. Chakrabartib,* ,1 a b
Department of Electronics and Communication Engineering, National Institute of Technology Allahabad, Allahabad 211004, India Department of Electronics Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2015 Received in revised form 15 May 2015 Accepted 23 May 2015 Available online xxx
In this paper a novel organic Schottky diode (OSD) based on polycarbazole is proposed. The article presents fabrication and experimental characterization of an amorphous silicon/polycarbazole organic Schottky diode, particularly with configuration a-Si/p-PCz/ITO, wherein in the said novel Schottky diode amorphous silicon (a-Si) contact has been used in place of conventional metal contact on the semiconducting polymer to form a rectifying contact. The characterization of polymer film was done in terms of morphology, absorbance versus wavelength, bandgap and thickness. I–V measurement was performed by Semiconductor Device Analyzer. The electrical parameters such as barrier height, ideality factor, rectification ratio and reverse saturation current as extracted from I–V characteristics of the device were found to be much better than those reported earlier for Schottky diodes based on polycarbazole. The proposed organic Schottky diode (OSD) exhibits a very low dark current (1014 A) and is therefore expected to have great commercial applications. ã 2015 Published by Elsevier B.V.
Keywords: Electrochemical deposition Organic semiconductors Schottky diode Vacuum evaporation coating
1. Introduction Schottky diode is a metal-semiconductor contact that exhibits rectification property like p–n junction diode. A Schottky diode is a majority carrier device and, therefore, has a fast switching speed. A primitive form of this device in the form of cat’s-whisker was reported to be used in early days of radio detection [1,2]. In principle, both p-type and n-type semiconductors can be used for making Schottky diodes using a variety of semiconducting materials including organic semiconductors. Metals like Pd, Pt, Au, Ti, Al, W, Cr, molybdenum and a few metal silicides are also used for making Schottky contact with semiconductors. A variety of Schottky diode configurations have been studied, fabricated and tested in the past [3–5]. A conducting polymer, polyanthranilic acid (PANA) was synthesized and used for fabrication of contacts with configurations of (Al, Ti, Sn metal)/PANA/indium tin oxide-coated glass [6]. The I–V characteristics of the proposed configuration exhibited a rectifying contact for the case of Al and Ti metal contacts but an ohmic contact for the case of Sn metal contact. The bandgap of PANA was estimated to be 3.8 eV. The fabrication and characterization of an organic Schottky diode with configuration
* Corresponding author. E-mail address: [email protected] (P. Chakrabarti). 1 The author is currently with the Motilal Nehru National Institute of Technology Allahabad, Allahabad 211004, India. http://dx.doi.org/10.1016/j.synthmet.2015.05.024 0379-6779/ ã 2015 Published by Elsevier B.V.
ITO/PCz/Al has also been reported [7]. The value of ideality factor, reverse saturation current density and barrier height were found to be 1.9, 5.32 1010 A/cm2 and 0.85 eV respectively. An Au/ polypyrrole/Al (or In) Schottky barrier diode in the form of sandwich configuration was fabricated and characterized [8]. The values of ideality factor for these configurations are 1.2 and 2.1 and the reverse saturation current densities are 3.2 1010 and 8.7 109 A/cm2 for the junctions with Al and In, respectively. The respective barrier potentials are reported to be 0.97 and 0.89 eV. Organic semiconducting polymers like polyaniline, polythiophene, polyacetylene, polypyrrole, polyindole, polycarbazole etc. have been gaining increasing interest in the fabrication of junction devices, optoelectronics and energy storage devices [9–11]. Low-cost manufacturing methods and compatibility with flexible substrates are two of the exciting features of organic electronics. In the present work an organic Schottky diode (OSD) based on electrochemically deposited p-type polycarbazole semiconductor and e-beam deposited amorphous Si contact is proposed, fabricated and characterized [12]. 2. Experimental procedure The two processes i.e., polymerization of carbazole monomer and electrochemical deposition of polymer on the working electrode, i.e., ITO coated glass were performed simultaneously by using electrochemical workstation (Metrohm Autolab B.V.). The
A. Srivastava, P. Chakrabarti / Synthetic Metals 207 (2015) 96–101
Fig. 1. Schematic diagram of a-Si/PCz/ITO device.
reaction solution was prepared by dissolving 60 mM carbazole monomer, 0.1 M tetrabutylammonium perchlorate (TBAP) in dichloromethane. Experiment was performed potentiostatically at 1.3 V (employing chronoamperometry technique) in an electrochemical cell having three electrodes. The electrode, where the electrochemical reaction should take place (in our case ITO coated glass), is called the working electrode (WE). Pt electrode, which is closing the circuit, is called the counter electrode (CE). Ag/AgCl used to measure the voltage between the electrolyte and WE, is called the reference electrode (RE). Amorphous-silicon dots were deposited on PCz/ITO sample using Vacuum Coating unit (model 12A4-D HINDVAC, India) by electron beam vacuum evaporation method. These a-Si dots have been deposited with the help of a mask having 1 mm diameter (area 0.785 mm2) circular pattern in it. The thickness of a-Si was kept in the range of 150–200 nm with
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the help of the built-in thickness monitor. In this device polycarbazole film acts as an anode and a-Si dots acts as a cathode. The schematic diagram of the device has been shown in Fig. 1. The morphological studies of polycarbazole film were carried out by SEM (Zeiss, Model EVO MA 15) and atomic force microscopy (Agilent Technologies N9445A). Ellipsometer (J.A. Wollam, VB-400) has been used to perform transmittance and thickness measurement. With the measured value of transmittance we have calculated the bandgap of the semiconducting polymer. The amorphous nature of Si was confirmed by XRD (Rigaku Smart Lab 3KW) studies at ambient conditions. The XRD image does not show the existence of any peak in the entire range of measurement. The I–V characteristics have been obtained by using Semiconductor Device Analyzer (Agilent Technologies B 1500A). The other electronic parameters such as barrier height, ideality factor, reverse saturation current and rectification ratio were extracted from the measured I–V characteristics. 3. Results and discussion SEM and AFM analyses have been performed to know the surface morphology and roughness profile of the electrochemically polymerized polycarbazole film on ITO coated glass substrate. The SEM analysis has been performed at 11,000 magnification. Fig. 2(a) shows surface morphology of a typical bare ITO coated glass plate. The image shows a smooth surface of indium tin oxide. Fig. 2(b) shows the surface morphology of electrochemically
Fig. 2. (a) Surface morphology of bare ITO, (b) surface morphology of electrochemically deposited polycarbazole film on ITO.
Fig. 3. 2-D and 3-D AFM images of electrochemically polymerized PCz film.
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Fig. 4. XRD studies of a-Si film.
deposited polycarbazole film on ITO coated glass substrate which confirms the deposition of polycarbazole film on ITO. It can be seen from the SEM image that PCz film exhibits porous and fibrous structures that help to enhance the sensing reaction because the film has a large surface area. The film can be used to develop a variety of devices such as photodetectors, gas sensors and solar cells. The polycarbazole fibers form a porous mesh that can serve as a flexible semiconducting film which can be deposited on flexible plastic substrate. It can be seen from the SEM image that electrochemically polymerized film is a little rough. So AFM analysis has been performed to measure the roughness profile of electrochemically polymerized polycarbazole film under contact mode (8 8 mm2 scan size) as shown in Fig. 3. Uniformly organized globular structure can be seen in the AFM images, which are due to polycarbazole chains bundles. From the AFM analysis, mean height of the roughness profile elements, arithmatic mean deviation of the roughness profile and root-mean square (RMS) deviation of the roughness profile are found to be 0.213 mm, 0.066 mm and 0.078 mm respectively. The XRD pattern of e-beam deposited silicon contact is shown in Fig. 4. It can be easily seen from the figure that in place of a clear peak, a broad hump is observed in the XRD pattern. This confirms the amorphous nature of the deposited silicon contact. The purpose of using amorphous silicon as a contact is that it melts at a temperature around 1487 K [13] which is very high as compared to melting points of contact metals such as Al, Ag and tin. So thermal stability of amorphous silicon is better than that of metals. Furthermore, it retains the material properties of silicon. a-Si layers can be made thinner than c-Si, which may
Fig. 6. Bandgap estimation of polymerized PCz film.
produce savings on silicon material cost. Since a-Si is full of defects naturally, any other defects such as impurities, do not affect the overall material characteristics too drastically. It has been demonstrated by others that the electron beam evaporated Si films are amorphous in nature and have suitable conductivity for molecular junction fabrication [14]. Fig. 5 shows the absorbance versus wavelength plot of polymer film. The bandgap of polycarbazole film was obtained from the (ahy)2 versus hy plot by extrapolating the linear portion of the plot to intersect the hy axis (X-axis) as shown in Fig. 6. Here hy is the energy of absorbed light, a is absorbance coefficient which is obtained from the measured value of transmittance using the ellipsometer. Estimated bandgap of the polymer was found to be 2.7 eV. The estimated thickness of the polycarbazole film was around 200 nm as obtained by ellipsometric measurement. Rectifying behavior of the Schottky diode can be described by thermionic emission-diffusion theory and/or field emission theory in the case of heavily doped semiconductor [5]. Since the organic semiconducting polymer behaves as a lightly doped p-type material [6], the electrical characteristics of ITO/PCz/a-Si junction have been analyzed by assuming the standard emission-diffusion theory. According to this theory, the I–V relationship is expressed as: qV I ¼ I0 exp 1 (1) hkT where I is the forward current, I0 is reverse saturation current, h is ideality factor, q is the electronic charge, V is the applied voltage, T is the absolute temperature and k is the Boltzmann constant. Further, Schottky barrier height related to reverse saturation current I0 as: qfB I0 ¼ AA T 2 exp (2) kT where A* is the effective Richardson constant. The value of A* for polycarbazole thin film is 1.2 A cm2 K2 [15], and A is the area of a single a-Si dot.Under the assumption (qV/hkT) 1, Eq. (1) can be approximated as lnðIÞ ¼ lnðI0 Þ þ
Fig. 5. Absorbance versus wavelength plot of PCz film.
q
hkT
V
(3)
The I–V measurement was performed by using Semiconductor Device Analyzer. The I–V characteristic of the prepared ITO/PCz/a-
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Fig. 7. Energy band-diagram of the proposed device (a) before contact of a-Si and PCz, (b) after contact of a-Si and PCz.
Si device shows nonlinear, asymmetric and rectifying behavior. This is due to the difference between the work function of PCz (fos = 5.08 eV) and a-Si (fa-Si = 4.5 eV) and their interfacial properties. For the formation of a rectifying contact, the work function of a-Si must be smaller than that of a p-type semiconductor i.e., polycarbzole. Energy band-diagram of the proposed device is shown in Fig. 7. In order to explore the effect of a-Si on the device performance, we have carried out I–V measurements with and without a-Si. It can be seen from Fig. 8. that the device having a-Si as a contact shows rectifying behaviour whereas without a-Si, it shows ohmic or linear behaviour. The asymmetric and non-linear I–V curve shows that the device exhibits rectification behavior. The rectification ratio (RR) is a ratio of forward and reverse bias current at the same voltage. A very high rectification ratio of 1.78 105 is obtained at 0.9 V. Fig. 9 shows the reverse bias I–V characteristic of a-Si/PCz/ITO device. The ideality factor of the device was extracted from the slope of ln (I) versus V plot as shown in Fig. 10. The ideality factor was found to be 1.3. The value of ideality factor deviates from unity due to the inhomogeneous surface or
irregularities in the roughness profile of polycarbazole film which can be seen from the SEM and AFM images. As discussed in [8], the ideality factor h of organic Schottky diodes was reported to vary from 2 to 11 [16–21]. However, this ideality factor can be improved largely by improving the synthesis of the polymer film and quality of the interface. Electrochemical method, Langmuir–Blodgett technique and vacuum deposition have been widely used to prepare thin, smooth and homogeneous polymer films. The value of reverse saturation current is obtained by intercept of the ln (I) versus V plot on ln (I) axis and the value is found to be 6.9 1014 A (reverse saturation current density J0 is equal to 8.78 1012 A/ cm2). The barrier height fB evaluated from Eq. (2) at room temperature is 0.96 eV. A comparison is made among the proposed a-Si/PCz/ITO and reported Al/PCz/ITO [7], Al/PCz-MWNT/ITO [11] and Al/PCznanoclay/ITO [22] devices in respect of ideality factor, barrier height and reverse saturation current density. The values for the three devices are listed Table 1. The study reveals that the proposed device can outperform the existing similar devices based on PCz.
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Fig. 9. Reverse bias I–V characteristic of a-Si/PCz/ITO device.
Fig. 8. I–V characteristics of the device (a) with a-Si as a contact, (b) without a-Si.
Fig. 10. ln (I) versus V plot of a-Si/PCz/ITO device.
Table 1 Comparison of electrical parameters for different devices. Parameters
Al/PCz/ITO [7]
Al/PCz-MWNT/ ITO [11]
Al/PCz-nanoclay/ITO [22]
a-Si/PCz/ITO (proposed)
Ideality factor Barrier height (eV) Reverse saturation current density (A/cm2)
1.9 0.85 5.23 1010
1.71 0.77 1.13 108
1.83 0.85 5.34 1010
1.3 0.96 8.78 1012
4. Conclusions It is confirmed from the I–V characteristic that the configuration a-Si/PCz/ITO shows an excellent rectifying behavior. The configuration a-Si/PCz/ITO shows better results in terms of ideality factor, barrier height and reverse saturation current density as compared to contemporary Schottky diodes based on
PCz. The use of a-Si contact in place of conventional metal contact is expected make the device especially attractive for photodetection as well as photovoltaic applications. The simple structure outperforms existing Schottky diodes based on PCz. The near ideal characteristic of the proposed device implies that the quality of the PCz-a-Si interface is much better than other PCzmetal contacts.
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Acknowledgements The authors gratefully acknowledge the financial support provided by the Defence Research and Development Organization (DRDO) (Grant No. ERIP/ER/0803699/M/01/1423) of the Government of India in the form a Research Project. The authors are thankful to the Centre for Interdisciplinary Research (CIR), Motilal Nehru National Institute of Technology Allahabad for extending the experimental facilities. References [1] [2] [3] [4] [5] [6]
J.C. Bose, U.S. Patent 775840, 1904. W. Schottky, Naturwissenschaften 26 (1938) 843. N.F. Mott, Proc. Camb. Philos. Soc. 34 (1938) 568–572. V.L. Rideout, Thin Solid Films 48 (1978) 261–399. S.M. Sze, Physics of Semiconductor Devices, John Wiley, 1981. A.K. Singh, R. Prakash, A.D.D. Dwivedi, P. Chakrabarti, IEEE Electron Device Lett. 29 (2008) 571–574. [7] A.K. Singh, A.D.D. Dwivedi, P. Chakrabarti, R. Prakash, J. Appl. Phys. 105 (2009) 114506-1–1145065-5.
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[8] P. Syed Abthagir, R. Saraswathi, J. Appl. Polym. Sci. 81 (2001) 2127–2135. [9] A.D.D. Dwivedi, A.K. Singh, R. Prakash, P. Chakrabarti, Curr. Appl. Phys. 10 (2010) 900–903. [10] K. Gurunathan, A. Vadivel Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar, Mater. Chem. Phys. 61 (1999) 173–191. [11] A.K. Singh, P. Chakrabarti, R. Prakash, IEEE Electron Device Lett. 32 (2011) 593– 595. [12] P. Chakrabarti, A. Srivastava, A novel a-silicon/polycarbazole organic Schottky diode (OSD) and the method of fabrication thereof, Patent filed (F.N. 2332/DEL/ 2014). [13] M.O. Thompson, G.J. Galvin, J.W. Mayer, P.S. Peercy, J.M. Poate, D.C. Jacobson, A. G. Cullis, N.G. Chew, Phys. Rev. Lett. 52 (1984) 2360–2363. [14] R. Kumar, H. Yan, R.L. McCreery, A.J. Bergren, Phys. Chem. Chem. Phys. 13 (2011) 14318–14324. [15] R. Clergereaux, I. Seguy, P. Jolinat, J. Farenc, P. Destruel, J. Phys. D 33 (2000) 1947–1952. [16] J. Kanicki, Mol. Cryst. Liq. Cryst. 105 (1984) 203. [17] P.M. Grant, T. Tani, W.D. Gill, M. Krounbi, T.C. Clarke, J. Appl. Physiol. 52 (1981) 869. [18] A. Turut, F. Koleli, J. Appl. Phys. 72 (1992) 818. [19] I.B. Nazarova, V.I. Krinichnyi, L.M. Goldenberg, Synth. Met. 53 (1993) 399. [20] S.C.K. Misra, S. Chandra, Indian J. Chem. 33A (1994) 583. [21] S.A. Chen, Y. Fang, H.T. Lee, Synth. Met. 55–57 (1993) 4082. [22] A.K. Singh, R. Prakash, RSC Adv. 2 (2012) 5277–5283.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
An organic Schottky diode (OSD) based on a-silicon/polycarbazole contact Aditi Srivastavaa , P. Chakrabartib,* ,1 a b
Department of Electronics and Communication Engineering, National Institute of Technology Allahabad, Allahabad 211004, India Department of Electronics Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2015 Received in revised form 15 May 2015 Accepted 23 May 2015 Available online xxx
In this paper a novel organic Schottky diode (OSD) based on polycarbazole is proposed. The article presents fabrication and experimental characterization of an amorphous silicon/polycarbazole organic Schottky diode, particularly with configuration a-Si/p-PCz/ITO, wherein in the said novel Schottky diode amorphous silicon (a-Si) contact has been used in place of conventional metal contact on the semiconducting polymer to form a rectifying contact. The characterization of polymer film was done in terms of morphology, absorbance versus wavelength, bandgap and thickness. I–V measurement was performed by Semiconductor Device Analyzer. The electrical parameters such as barrier height, ideality factor, rectification ratio and reverse saturation current as extracted from I–V characteristics of the device were found to be much better than those reported earlier for Schottky diodes based on polycarbazole. The proposed organic Schottky diode (OSD) exhibits a very low dark current (1014 A) and is therefore expected to have great commercial applications. ã 2015 Published by Elsevier B.V.
Keywords: Electrochemical deposition Organic semiconductors Schottky diode Vacuum evaporation coating
1. Introduction Schottky diode is a metal-semiconductor contact that exhibits rectification property like p–n junction diode. A Schottky diode is a majority carrier device and, therefore, has a fast switching speed. A primitive form of this device in the form of cat’s-whisker was reported to be used in early days of radio detection [1,2]. In principle, both p-type and n-type semiconductors can be used for making Schottky diodes using a variety of semiconducting materials including organic semiconductors. Metals like Pd, Pt, Au, Ti, Al, W, Cr, molybdenum and a few metal silicides are also used for making Schottky contact with semiconductors. A variety of Schottky diode configurations have been studied, fabricated and tested in the past [3–5]. A conducting polymer, polyanthranilic acid (PANA) was synthesized and used for fabrication of contacts with configurations of (Al, Ti, Sn metal)/PANA/indium tin oxide-coated glass [6]. The I–V characteristics of the proposed configuration exhibited a rectifying contact for the case of Al and Ti metal contacts but an ohmic contact for the case of Sn metal contact. The bandgap of PANA was estimated to be 3.8 eV. The fabrication and characterization of an organic Schottky diode with configuration
* Corresponding author. E-mail address: [email protected] (P. Chakrabarti). 1 The author is currently with the Motilal Nehru National Institute of Technology Allahabad, Allahabad 211004, India. http://dx.doi.org/10.1016/j.synthmet.2015.05.024 0379-6779/ ã 2015 Published by Elsevier B.V.
ITO/PCz/Al has also been reported [7]. The value of ideality factor, reverse saturation current density and barrier height were found to be 1.9, 5.32 1010 A/cm2 and 0.85 eV respectively. An Au/ polypyrrole/Al (or In) Schottky barrier diode in the form of sandwich configuration was fabricated and characterized [8]. The values of ideality factor for these configurations are 1.2 and 2.1 and the reverse saturation current densities are 3.2 1010 and 8.7 109 A/cm2 for the junctions with Al and In, respectively. The respective barrier potentials are reported to be 0.97 and 0.89 eV. Organic semiconducting polymers like polyaniline, polythiophene, polyacetylene, polypyrrole, polyindole, polycarbazole etc. have been gaining increasing interest in the fabrication of junction devices, optoelectronics and energy storage devices [9–11]. Low-cost manufacturing methods and compatibility with flexible substrates are two of the exciting features of organic electronics. In the present work an organic Schottky diode (OSD) based on electrochemically deposited p-type polycarbazole semiconductor and e-beam deposited amorphous Si contact is proposed, fabricated and characterized [12]. 2. Experimental procedure The two processes i.e., polymerization of carbazole monomer and electrochemical deposition of polymer on the working electrode, i.e., ITO coated glass were performed simultaneously by using electrochemical workstation (Metrohm Autolab B.V.). The
A. Srivastava, P. Chakrabarti / Synthetic Metals 207 (2015) 96–101
Fig. 1. Schematic diagram of a-Si/PCz/ITO device.
reaction solution was prepared by dissolving 60 mM carbazole monomer, 0.1 M tetrabutylammonium perchlorate (TBAP) in dichloromethane. Experiment was performed potentiostatically at 1.3 V (employing chronoamperometry technique) in an electrochemical cell having three electrodes. The electrode, where the electrochemical reaction should take place (in our case ITO coated glass), is called the working electrode (WE). Pt electrode, which is closing the circuit, is called the counter electrode (CE). Ag/AgCl used to measure the voltage between the electrolyte and WE, is called the reference electrode (RE). Amorphous-silicon dots were deposited on PCz/ITO sample using Vacuum Coating unit (model 12A4-D HINDVAC, India) by electron beam vacuum evaporation method. These a-Si dots have been deposited with the help of a mask having 1 mm diameter (area 0.785 mm2) circular pattern in it. The thickness of a-Si was kept in the range of 150–200 nm with
97
the help of the built-in thickness monitor. In this device polycarbazole film acts as an anode and a-Si dots acts as a cathode. The schematic diagram of the device has been shown in Fig. 1. The morphological studies of polycarbazole film were carried out by SEM (Zeiss, Model EVO MA 15) and atomic force microscopy (Agilent Technologies N9445A). Ellipsometer (J.A. Wollam, VB-400) has been used to perform transmittance and thickness measurement. With the measured value of transmittance we have calculated the bandgap of the semiconducting polymer. The amorphous nature of Si was confirmed by XRD (Rigaku Smart Lab 3KW) studies at ambient conditions. The XRD image does not show the existence of any peak in the entire range of measurement. The I–V characteristics have been obtained by using Semiconductor Device Analyzer (Agilent Technologies B 1500A). The other electronic parameters such as barrier height, ideality factor, reverse saturation current and rectification ratio were extracted from the measured I–V characteristics. 3. Results and discussion SEM and AFM analyses have been performed to know the surface morphology and roughness profile of the electrochemically polymerized polycarbazole film on ITO coated glass substrate. The SEM analysis has been performed at 11,000 magnification. Fig. 2(a) shows surface morphology of a typical bare ITO coated glass plate. The image shows a smooth surface of indium tin oxide. Fig. 2(b) shows the surface morphology of electrochemically
Fig. 2. (a) Surface morphology of bare ITO, (b) surface morphology of electrochemically deposited polycarbazole film on ITO.
Fig. 3. 2-D and 3-D AFM images of electrochemically polymerized PCz film.
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Fig. 4. XRD studies of a-Si film.
deposited polycarbazole film on ITO coated glass substrate which confirms the deposition of polycarbazole film on ITO. It can be seen from the SEM image that PCz film exhibits porous and fibrous structures that help to enhance the sensing reaction because the film has a large surface area. The film can be used to develop a variety of devices such as photodetectors, gas sensors and solar cells. The polycarbazole fibers form a porous mesh that can serve as a flexible semiconducting film which can be deposited on flexible plastic substrate. It can be seen from the SEM image that electrochemically polymerized film is a little rough. So AFM analysis has been performed to measure the roughness profile of electrochemically polymerized polycarbazole film under contact mode (8 8 mm2 scan size) as shown in Fig. 3. Uniformly organized globular structure can be seen in the AFM images, which are due to polycarbazole chains bundles. From the AFM analysis, mean height of the roughness profile elements, arithmatic mean deviation of the roughness profile and root-mean square (RMS) deviation of the roughness profile are found to be 0.213 mm, 0.066 mm and 0.078 mm respectively. The XRD pattern of e-beam deposited silicon contact is shown in Fig. 4. It can be easily seen from the figure that in place of a clear peak, a broad hump is observed in the XRD pattern. This confirms the amorphous nature of the deposited silicon contact. The purpose of using amorphous silicon as a contact is that it melts at a temperature around 1487 K [13] which is very high as compared to melting points of contact metals such as Al, Ag and tin. So thermal stability of amorphous silicon is better than that of metals. Furthermore, it retains the material properties of silicon. a-Si layers can be made thinner than c-Si, which may
Fig. 6. Bandgap estimation of polymerized PCz film.
produce savings on silicon material cost. Since a-Si is full of defects naturally, any other defects such as impurities, do not affect the overall material characteristics too drastically. It has been demonstrated by others that the electron beam evaporated Si films are amorphous in nature and have suitable conductivity for molecular junction fabrication [14]. Fig. 5 shows the absorbance versus wavelength plot of polymer film. The bandgap of polycarbazole film was obtained from the (ahy)2 versus hy plot by extrapolating the linear portion of the plot to intersect the hy axis (X-axis) as shown in Fig. 6. Here hy is the energy of absorbed light, a is absorbance coefficient which is obtained from the measured value of transmittance using the ellipsometer. Estimated bandgap of the polymer was found to be 2.7 eV. The estimated thickness of the polycarbazole film was around 200 nm as obtained by ellipsometric measurement. Rectifying behavior of the Schottky diode can be described by thermionic emission-diffusion theory and/or field emission theory in the case of heavily doped semiconductor [5]. Since the organic semiconducting polymer behaves as a lightly doped p-type material [6], the electrical characteristics of ITO/PCz/a-Si junction have been analyzed by assuming the standard emission-diffusion theory. According to this theory, the I–V relationship is expressed as: qV I ¼ I0 exp 1 (1) hkT where I is the forward current, I0 is reverse saturation current, h is ideality factor, q is the electronic charge, V is the applied voltage, T is the absolute temperature and k is the Boltzmann constant. Further, Schottky barrier height related to reverse saturation current I0 as: qfB I0 ¼ AA T 2 exp (2) kT where A* is the effective Richardson constant. The value of A* for polycarbazole thin film is 1.2 A cm2 K2 [15], and A is the area of a single a-Si dot.Under the assumption (qV/hkT) 1, Eq. (1) can be approximated as lnðIÞ ¼ lnðI0 Þ þ
Fig. 5. Absorbance versus wavelength plot of PCz film.
q
hkT
V
(3)
The I–V measurement was performed by using Semiconductor Device Analyzer. The I–V characteristic of the prepared ITO/PCz/a-
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Fig. 7. Energy band-diagram of the proposed device (a) before contact of a-Si and PCz, (b) after contact of a-Si and PCz.
Si device shows nonlinear, asymmetric and rectifying behavior. This is due to the difference between the work function of PCz (fos = 5.08 eV) and a-Si (fa-Si = 4.5 eV) and their interfacial properties. For the formation of a rectifying contact, the work function of a-Si must be smaller than that of a p-type semiconductor i.e., polycarbzole. Energy band-diagram of the proposed device is shown in Fig. 7. In order to explore the effect of a-Si on the device performance, we have carried out I–V measurements with and without a-Si. It can be seen from Fig. 8. that the device having a-Si as a contact shows rectifying behaviour whereas without a-Si, it shows ohmic or linear behaviour. The asymmetric and non-linear I–V curve shows that the device exhibits rectification behavior. The rectification ratio (RR) is a ratio of forward and reverse bias current at the same voltage. A very high rectification ratio of 1.78 105 is obtained at 0.9 V. Fig. 9 shows the reverse bias I–V characteristic of a-Si/PCz/ITO device. The ideality factor of the device was extracted from the slope of ln (I) versus V plot as shown in Fig. 10. The ideality factor was found to be 1.3. The value of ideality factor deviates from unity due to the inhomogeneous surface or
irregularities in the roughness profile of polycarbazole film which can be seen from the SEM and AFM images. As discussed in [8], the ideality factor h of organic Schottky diodes was reported to vary from 2 to 11 [16–21]. However, this ideality factor can be improved largely by improving the synthesis of the polymer film and quality of the interface. Electrochemical method, Langmuir–Blodgett technique and vacuum deposition have been widely used to prepare thin, smooth and homogeneous polymer films. The value of reverse saturation current is obtained by intercept of the ln (I) versus V plot on ln (I) axis and the value is found to be 6.9 1014 A (reverse saturation current density J0 is equal to 8.78 1012 A/ cm2). The barrier height fB evaluated from Eq. (2) at room temperature is 0.96 eV. A comparison is made among the proposed a-Si/PCz/ITO and reported Al/PCz/ITO [7], Al/PCz-MWNT/ITO [11] and Al/PCznanoclay/ITO [22] devices in respect of ideality factor, barrier height and reverse saturation current density. The values for the three devices are listed Table 1. The study reveals that the proposed device can outperform the existing similar devices based on PCz.
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Fig. 9. Reverse bias I–V characteristic of a-Si/PCz/ITO device.
Fig. 8. I–V characteristics of the device (a) with a-Si as a contact, (b) without a-Si.
Fig. 10. ln (I) versus V plot of a-Si/PCz/ITO device.
Table 1 Comparison of electrical parameters for different devices. Parameters
Al/PCz/ITO [7]
Al/PCz-MWNT/ ITO [11]
Al/PCz-nanoclay/ITO [22]
a-Si/PCz/ITO (proposed)
Ideality factor Barrier height (eV) Reverse saturation current density (A/cm2)
1.9 0.85 5.23 1010
1.71 0.77 1.13 108
1.83 0.85 5.34 1010
1.3 0.96 8.78 1012
4. Conclusions It is confirmed from the I–V characteristic that the configuration a-Si/PCz/ITO shows an excellent rectifying behavior. The configuration a-Si/PCz/ITO shows better results in terms of ideality factor, barrier height and reverse saturation current density as compared to contemporary Schottky diodes based on
PCz. The use of a-Si contact in place of conventional metal contact is expected make the device especially attractive for photodetection as well as photovoltaic applications. The simple structure outperforms existing Schottky diodes based on PCz. The near ideal characteristic of the proposed device implies that the quality of the PCz-a-Si interface is much better than other PCzmetal contacts.
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Acknowledgements The authors gratefully acknowledge the financial support provided by the Defence Research and Development Organization (DRDO) (Grant No. ERIP/ER/0803699/M/01/1423) of the Government of India in the form a Research Project. The authors are thankful to the Centre for Interdisciplinary Research (CIR), Motilal Nehru National Institute of Technology Allahabad for extending the experimental facilities. References [1] [2] [3] [4] [5] [6]
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