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Device characteristics of NiPc static induction transistor
November 2001
Materials Letters 51 Ž2001. 200–202 www.elsevier.comrlocatermatlet
Device characteristics of NiPc static induction transistor C.M. Joseph, C.S. Menon ) School of Pure and Applied Physics, Mahatma Gandhi UniÕersity, Priyadarshini Hills PO, Kottayam, Kerala 686 560, India Received 2 January 2001; accepted 4 January 2001
Abstract Static and dynamic characteristics of an organic static induction transistor ŽSIT. fabricated using nickel phthalocyanine ŽNiPc. thin films are reported. Layered structure using NiPc as active layer consists of AlŽdrain.rNiPcrAlŽgate.rAlŽoxidic.r NiPcrFTOŽsource.rglass. Excellent electrical characteristics are obtained at low negative gate voltages. q 2001 Elsevier Science B.V. All rights reserved. PACS: 72.10; 73.60.F; 73.40.S Keywords: Nickel phthalocyanine; Thin films; Static induction transistor; Device characteristics; Bandwidth
1. Introduction Phthalocyanines are organic semiconductors receiving considerable attention because of their suitability as an active layer for organic electronic devices w1–3x. These compounds have potential for various electronic components such as thin film transistors, LED and LCD. Low cost, low toxicity and high chemical stability of these compounds make them suitable for various devices w4x. The basic characteristics of static induction transistor fabricated using phthalocyanines have been reported w5–7x. To improve the device characteristics, different techniques such as optimizing the thickness of the layers in the heterostructure, doping and varying preparation methods Žgrid electrode, buried electrode. are used. In this paper, we report the static and dynamic characteristics of static induction transistor ŽSIT. fabricated using NiPc layer and electrode layer ) Corresponding author. q91-481-597-923; fax: q91-481-597494. E-mail address: [email protected] ŽC.S. Menon..
ŽAl, FTO. and an additional nonconducting oxidic Al layer. It is found that this organic NiPc transistor is having current saturation of static characteristics for higher and positive drain voltage V D , low voltage operation and high source to drain current value.
2. Experimental The organic transistor is having AlŽdrain.r NiPcrAl Ž gate . rAl Ž oxidic layer . rNiPcrFTOŽsource.rglass as layered structure. Fig. 1 shows the schematic of the sample cell structure of organic NiPc SIT. The device has been fabricated using Hind Hivac Žmodel 12 A4. coating unit. First, the NiPc film of thickness 300 nm was deposited using a molybdenum boat onto clean FTO coated Žconducting and transparent. glass substrates. Next, a very thin film of Al Ž50 nm. was deposited using a tungsten basket. This film was exposed to air and the oxide surface layer was found to be nonconducting. Next, a connection wire was fixed above the de-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 8 9 - 0
C.M. Joseph, C.S. Menonr Materials Letters 51 (2001) 200–202
201
Fig. 1. Schematic of the sample cell structure for organic NiPc transistor.
posited oxide layer for the gate electrode. Then a layer of Al Žgate. electrode of thickness 100 nm was deposited. Next, the second NiPc film Ž50 nm thickness. was deposited above the gate electrode. Next, a connection wire was fixed above the deposited NiPc layer for the drain electrode. Finally, the top Al drain electrode Ž200 nm thickness. was deposited. Suitable masks were used for each deposition. The base pressure was around 10y6 Torr for all the depositions. NiPc films were deposited at a rate of 2 nmrs with substrate kept at room temperature. The thermal probe technique yielded conductivity for NiPc films as p-type. For the NiPc thin films, activation energy Ž Ea . s 0.77 eV, band gap Ž Eg . s 3.2 eV, lattice parameters, a s b s 1.73 nm and c s 1.25 nm. For static and dynamic characteristic measurements, constant dc voltage sources, a Keithley electrometer Žmodel 617., oscilloscope and function generator were used. All the electrical measurements were done in dark at room temperature by loading the sample in a conductivity cell. The circuit diagrams for static and dynamic measurements of SIT are given elsewhere w8x.
Fig. 2. Static characteristics of organic NiPc transistor.
increase in the negative gate voltage. The current density values J are less compared to that for a previously reported SIT using CuPc w5x. Current
3. Results and discussion It is found that the geometry and thickness of Al gate electrode strongly affects the SIT performance. A very thin film of Al with continuous or discontinuous parts or grid was used as gate electrodes in the early reports for improved performance w7,8x. Here we deposited a thin nonconducting film of Al prior to the Al gate electrode. This film is expected to induce more charge carriers into the NiPc active layers and this may be the reason for a relatively higher current density J compared to that for the hetrostructure deposited without this oxidic layer. Fig. 2 shows the static characteristics of the organic NiPc SIT. It is seen that Is increases steadily with
Fig. 3. Gain bandwidth plot of organic NiPc transistor.
202
C.M. Joseph, C.S. Menonr Materials Letters 51 (2001) 200–202
saturation and low voltage operation for static characteristics are observed in this NiPc SIT as shown in Fig. 2. For dynamic characteristics measurements, a sine wave with amplitude 3.2 V ŽVi. and offset voltage y0.1 V was applied to the Al gate electrode. The drain voltage was kept at 5 V. The output voltage Vo was measured using a CRO for different input frequencies with constant Vi . The gain bandwidth plot is shown in the Fig. 3. It is a plot between 20log Ž VorVi . and applied sine wave frequency. It can be seen that gain decreases at frequencies higher than 2 kHz and cut-off frequency f c is 2.2 kHz which agrees well with the reported value of 2.3 kHz w8x. Higher cut-off frequencies and current densities may be expected by optimizing the different growth parameters for organic SITs.
4. Conclusion In the present study, organic SITs using NiPc were fabricated and its static and dynamic character-
istics were studied. We have obtained an excellent static characteristics at low gate voltages. In conclusion, optimization of the thickness of the active layers and growth parameters of the Al gate electrode can yield a higher bandwidth and current density at low gate voltages.
References w1x K. Kudo, D.X. Wang, M. Lizuka, S. Kuriyoshi, K. Tanaka, Synth. Met. 111 Ž2000. 11. w2x F. Schauer, I. Zhivkov, S. Nespurek, J. Non-Cryst. Solids 266 Ž2000. 999. w3x N.R. Armstrong, J. Porphyrins Phthalocyanines 4 Ž2000. 417. w4x W. Waclawek, M.Z. Waclawek, Thin Solid Films 146 Ž1987. 1. w5x C.M. Joseph, C.S. Menon, Mater. Lett. 50 Ž2001. 18. w6x Y. Mochida, J. Nishizawa, T. Ohmi, R.K. Gupta, IEEE Trans. Electron Devices 25 Ž1978. 761. w7x K. Kudo, T. Sumimoto, K. Hiraga, S. Kuniyoshi, K. Kudo, K. Tanaka, Jpn. J. Appl. Phys. 36 Ž1997. 6994. w8x D.X. Wang, Y. Tanaka, M. Lizuka, S. Kuniyoshi, K. Kudo, K. Tanaka, Jpn. J. Appl. Phys. 38 Ž1999. 256.
Materials Letters 51 Ž2001. 200–202 www.elsevier.comrlocatermatlet
Device characteristics of NiPc static induction transistor C.M. Joseph, C.S. Menon ) School of Pure and Applied Physics, Mahatma Gandhi UniÕersity, Priyadarshini Hills PO, Kottayam, Kerala 686 560, India Received 2 January 2001; accepted 4 January 2001
Abstract Static and dynamic characteristics of an organic static induction transistor ŽSIT. fabricated using nickel phthalocyanine ŽNiPc. thin films are reported. Layered structure using NiPc as active layer consists of AlŽdrain.rNiPcrAlŽgate.rAlŽoxidic.r NiPcrFTOŽsource.rglass. Excellent electrical characteristics are obtained at low negative gate voltages. q 2001 Elsevier Science B.V. All rights reserved. PACS: 72.10; 73.60.F; 73.40.S Keywords: Nickel phthalocyanine; Thin films; Static induction transistor; Device characteristics; Bandwidth
1. Introduction Phthalocyanines are organic semiconductors receiving considerable attention because of their suitability as an active layer for organic electronic devices w1–3x. These compounds have potential for various electronic components such as thin film transistors, LED and LCD. Low cost, low toxicity and high chemical stability of these compounds make them suitable for various devices w4x. The basic characteristics of static induction transistor fabricated using phthalocyanines have been reported w5–7x. To improve the device characteristics, different techniques such as optimizing the thickness of the layers in the heterostructure, doping and varying preparation methods Žgrid electrode, buried electrode. are used. In this paper, we report the static and dynamic characteristics of static induction transistor ŽSIT. fabricated using NiPc layer and electrode layer ) Corresponding author. q91-481-597-923; fax: q91-481-597494. E-mail address: [email protected] ŽC.S. Menon..
ŽAl, FTO. and an additional nonconducting oxidic Al layer. It is found that this organic NiPc transistor is having current saturation of static characteristics for higher and positive drain voltage V D , low voltage operation and high source to drain current value.
2. Experimental The organic transistor is having AlŽdrain.r NiPcrAl Ž gate . rAl Ž oxidic layer . rNiPcrFTOŽsource.rglass as layered structure. Fig. 1 shows the schematic of the sample cell structure of organic NiPc SIT. The device has been fabricated using Hind Hivac Žmodel 12 A4. coating unit. First, the NiPc film of thickness 300 nm was deposited using a molybdenum boat onto clean FTO coated Žconducting and transparent. glass substrates. Next, a very thin film of Al Ž50 nm. was deposited using a tungsten basket. This film was exposed to air and the oxide surface layer was found to be nonconducting. Next, a connection wire was fixed above the de-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 8 9 - 0
C.M. Joseph, C.S. Menonr Materials Letters 51 (2001) 200–202
201
Fig. 1. Schematic of the sample cell structure for organic NiPc transistor.
posited oxide layer for the gate electrode. Then a layer of Al Žgate. electrode of thickness 100 nm was deposited. Next, the second NiPc film Ž50 nm thickness. was deposited above the gate electrode. Next, a connection wire was fixed above the deposited NiPc layer for the drain electrode. Finally, the top Al drain electrode Ž200 nm thickness. was deposited. Suitable masks were used for each deposition. The base pressure was around 10y6 Torr for all the depositions. NiPc films were deposited at a rate of 2 nmrs with substrate kept at room temperature. The thermal probe technique yielded conductivity for NiPc films as p-type. For the NiPc thin films, activation energy Ž Ea . s 0.77 eV, band gap Ž Eg . s 3.2 eV, lattice parameters, a s b s 1.73 nm and c s 1.25 nm. For static and dynamic characteristic measurements, constant dc voltage sources, a Keithley electrometer Žmodel 617., oscilloscope and function generator were used. All the electrical measurements were done in dark at room temperature by loading the sample in a conductivity cell. The circuit diagrams for static and dynamic measurements of SIT are given elsewhere w8x.
Fig. 2. Static characteristics of organic NiPc transistor.
increase in the negative gate voltage. The current density values J are less compared to that for a previously reported SIT using CuPc w5x. Current
3. Results and discussion It is found that the geometry and thickness of Al gate electrode strongly affects the SIT performance. A very thin film of Al with continuous or discontinuous parts or grid was used as gate electrodes in the early reports for improved performance w7,8x. Here we deposited a thin nonconducting film of Al prior to the Al gate electrode. This film is expected to induce more charge carriers into the NiPc active layers and this may be the reason for a relatively higher current density J compared to that for the hetrostructure deposited without this oxidic layer. Fig. 2 shows the static characteristics of the organic NiPc SIT. It is seen that Is increases steadily with
Fig. 3. Gain bandwidth plot of organic NiPc transistor.
202
C.M. Joseph, C.S. Menonr Materials Letters 51 (2001) 200–202
saturation and low voltage operation for static characteristics are observed in this NiPc SIT as shown in Fig. 2. For dynamic characteristics measurements, a sine wave with amplitude 3.2 V ŽVi. and offset voltage y0.1 V was applied to the Al gate electrode. The drain voltage was kept at 5 V. The output voltage Vo was measured using a CRO for different input frequencies with constant Vi . The gain bandwidth plot is shown in the Fig. 3. It is a plot between 20log Ž VorVi . and applied sine wave frequency. It can be seen that gain decreases at frequencies higher than 2 kHz and cut-off frequency f c is 2.2 kHz which agrees well with the reported value of 2.3 kHz w8x. Higher cut-off frequencies and current densities may be expected by optimizing the different growth parameters for organic SITs.
4. Conclusion In the present study, organic SITs using NiPc were fabricated and its static and dynamic character-
istics were studied. We have obtained an excellent static characteristics at low gate voltages. In conclusion, optimization of the thickness of the active layers and growth parameters of the Al gate electrode can yield a higher bandwidth and current density at low gate voltages.
References w1x K. Kudo, D.X. Wang, M. Lizuka, S. Kuriyoshi, K. Tanaka, Synth. Met. 111 Ž2000. 11. w2x F. Schauer, I. Zhivkov, S. Nespurek, J. Non-Cryst. Solids 266 Ž2000. 999. w3x N.R. Armstrong, J. Porphyrins Phthalocyanines 4 Ž2000. 417. w4x W. Waclawek, M.Z. Waclawek, Thin Solid Films 146 Ž1987. 1. w5x C.M. Joseph, C.S. Menon, Mater. Lett. 50 Ž2001. 18. w6x Y. Mochida, J. Nishizawa, T. Ohmi, R.K. Gupta, IEEE Trans. Electron Devices 25 Ž1978. 761. w7x K. Kudo, T. Sumimoto, K. Hiraga, S. Kuniyoshi, K. Kudo, K. Tanaka, Jpn. J. Appl. Phys. 36 Ž1997. 6994. w8x D.X. Wang, Y. Tanaka, M. Lizuka, S. Kuniyoshi, K. Kudo, K. Tanaka, Jpn. J. Appl. Phys. 38 Ž1999. 256.