- Email: [email protected]
The effects of microfibers on electrical characteristics of zinc oxide thin film transistor
Microelectronic Engineering 110 (2013) 25–28
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
The effects of microfibers on electrical characteristics of zinc oxide thin film transistor Zeyad A. Alahmed a, Fahrettin Yakuphanoglu b,⇑ a b
Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Saudi Arabia Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey
a r t i c l e
i n f o
Article history: Received 4 March 2013 Received in revised form 15 April 2013 Accepted 18 April 2013 Available online 28 April 2013 Keywords: Thin film transistor Solution process Field effect mobility
a b s t r a c t The high mobility n-type thin film transistor based on sol–gel processed zinc oxide (ZnO) was fabricated. The ZnO thin film was prepared by spin coating the precursor solution on a SiO2 dielectric layer. AFM results indicate that the ZnO film is formed from the microfibers. The solution-processed ZnO TFT was found to exhibit a high mobility of 0.47 cm2/V.s. This indicates that the microfiber ZnO film has a important effect to fabricate a high mobility ZnO thin film transistor. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The metal oxide semiconductors have attracted a great deal of attention for thin film transistor applications (TFTs) [1–3]. One of them is zinc oxide (ZnO) with a hexagonal wurzite structure, a wide band gap, and optical properties [4–11]. Zinc oxide based thin film transistors (ZnO TFTs) have been extensively investigated due to their superior properties that include wide band gap, transparency, and high field effect mobility compared to that of the conventional a-Si TFTs [12–16,3]. The solution process is an alternative thin film deposition method due to its simple and low-cost pathway. One of the solution processes is a sol–gel method because of its simplicity, high throughput, and low-cost and this method offers many benefits over vacuum-based processes [17,18]. In recent years, there is a growing interest on solution-processable materials for applications in which low-cost, low-temperature manufacturing is required [19–21]. Solution-processed zinc oxide (ZnO) offers high electron mobility, environmental stability, and optical transparency. Much effort has been applied toward the development of solution-based fabrication processes for ZnO thin films [22,23]. Thin-film transistors based on solution-processable semiconducting materials for the applications in which low-cost, low temperature manufacturing have been demanded [24,19,25]. The solution-processable semiconducting materials have some advantages such as low-cost, and simplicity fabrication for lowcost electronics. It is evaluated that the electrical performance of solution process based thin film transistors can be improved by ⇑ Corresponding author. Tel.: +90 424 2370000x3792; fax: +90 424 2330062. E-mail address: fyhanoglu@firat.edu.tr (F. Yakuphanoglu). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.04.023
changing preparation conditions of the active layer and this allows the development of inexpensive transparent and high mobility thin film transistors [26]. With this aim, we have synthesized microfiber ZnO film to fabricate a ZnO thin film transistor and analyzed electrical performance with output and transfer characteristics. 2. Experimental details The zinc oxide film was synthesized by sol gel method. The zinc acetate dehydrate (Zn(Ac)2), isopropanol (IPRO) and ethanolamine (EA) were used to prepare ZnO film. Firstly, (Zn(Ac)2) was dissolved in IPRO and then, EA was added to solution and the solution was stirred with a magnetic stirrer for 2 h in 60 °C and then placed in air for 24 h resulting in a clear and homogeneous sol. The film of ZnO was prepared on thermally prepared SiO2 layer of 200 nm by spin coating. Then, the ZnO film was annealed at 500 °C for 2 h. After preparation of ZnO film, the source and drain electrodes were prepared by evaporating a 100 nm thick Al layer through a shadow mask in the form of top contact geometry. The schematic diagram of ZnO TFT was shown in Fig. 1. The morphology properties of the ZnO film were investigated by a Park System XE-100E atomic force microscopy (AFM). Electrical characteristics are measured by semiconductor parameter analyzer (Keithley 4200). 3. Results and discussion The two (2D) and three dimensional (3D) AFM images (40 lm 40 lm) of ZnO film deposited on SiO2 layer are shown in Fig. 2a. As seen in Fig. 2a, the ZnO film exhibits a fiber structure
26
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
Source
150nm 100nm 200nm
Al
Drain
Al Zinc oxide SiO2 Silicon 1. Ag Gate electrode
Fig. 1. Schematic structure of the ZnO thin film transistor.
with diameter of ZnO microfiber. The diameter of the fiber is changed from 0.883 lm to 2.1 lm from one fiber to another. The grain size distribution and histograms for the ZnO film were analyzed using XEI software program of AFM microscopy and are given in Fig. 2b. It is seen in Fig. 2a that the ZnO film has a microstructure. The surface roughness of the film was determined to be 117.23 nm using a PARK system XEI software program. This is rather high due to the distribution of microfibers on the substrate surface. These parameters affect the performance of the transistor and it was discussed in detail later. Fig. 3 shows the output characteristics of the as-deposited ZnO TFT. The drain current of the transistor increases with drain voltage and then, reached to a saturation. The saturation of the drain
Fig. 2. (a) AFM images of the ZnO film deposited on SiO2 layer (40 lm 40 lm, left in 2D image and right in 3D image) and (b) Grain size distribution and histograms of ZnO film.
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
current is taken place when the drain voltage reaches a voltage where the channel is pinched off. The drain current increases with positive gate voltages. This indicates that the transistor works in an n-channel operational mode. The output characteristics confirm that ZnO active layer used in TFT is an n-type semiconductor material. At lower voltages, the drain current–voltage curves exhibit a good linearity between the drain current and drain voltage. This confirms that a good ohmic contact is formed between ZnO film and Al contacts. The electrical parameters of ZnO TFT for saturation region can be obtained by the following relation [27–29],
Ids ¼
W lC i ðV g V th Þ2 2L
ð1Þ
where l is the saturation mobility, Ci (17.25 nF/cm2) is the capacitance per unit area of the 200 nm SiO2 insulator layer, Ids is the drain-source current, W is the width of channel, L is the channel length, Vg is the gate voltage, and Vth is the threshold voltage. The mobility value of the transistor was determined from the plot of Ids1/2 vs Vg under Vds = 40 V, as shown in Fig. 4 and was found to be 0.47 cm2/V.s. The obtained mobility value of the ZnO transistor is higher than that of a solution processed ZnO thin film transistor (2 105 cm2/V.s) [30] using zinc acetate and other ZnO thin film transistors with the mobility (2 103 cm2/V.s) [31] and (2 105 cm2/V.s) [32] and sol–gel processed zinc oxide TFT (4.2 102 annealed 500 °C, 3.6 103 at 400 °C and
27
3.6 102 cm2/V.s at 300 °C) [33] and ZnO TFT with ZnO nanoparticles [34]. This indicates that the studied ZnO transistor has the higher mobility. The improved performance is attributed the microfibers of the ZnO film, because the fiber diameter of the ZnO film (0.8–2.1 lm) used for studied transistor is higher than that of a solution processed ZnO thin film transistor (33 nm) [30] and another ZnO thin film transistor (50 nm) [31]. This indicates that the mobility is highly sensitive to the microstructure with the microfibers. It is evaluated that low temperature solution-processed zinc oxide thin film transistor using SiO2 gate dielectric produced a high mobility due to a high carrier density. The high carrier density is sufficient to fill the disorder-induced carrier traps produced by grain boundaries or oxygen vacancies and therefore, the ZnO TFT exhibits a high performance regardless of the annealed temperature. Ion/Ioff ratio of the ZnO TFT was determined from Fig. 4 and was found to be 3.99 102. This value is higher than that of the ZnO-TFT containing poly-4-vinylphenol (PVP) [35], whereas it is lower than that of the TFT containing PVP/Al2O3 (7.9 104) [36]. We believe that the low Ion/off ratio of the transistor is due to the leakage current. From the transfer characteristics, the sub threshold gate swing which is an important device performance parameter for highspeed and low-power operation is determined using the following relation [27],
S¼
C dep þ C it kT ln 10ð1 þ Þ q C ox
ð2Þ
where S is the subthreshold gate swing defined as the voltage required to increase the drain current by a factor of 10. Cdep is the depletion capacitance, Cit is the interface capacitance and Cox is the capacitance of oxide layer. The S value for the transistor was determined by means of Fig. 4 and was found to be 5.46 V/decade. This is rather high and it is related to the interface trap states at the ZnO/SiO2 interface. The interface trap density for the transistor can be calculated by the following relation [37,38],
Dit ¼
Fig. 3. Output characteristics of the ZnO thin film transistor.
S logðeÞ Ci 1 kT=q q
ð3Þ
where k is the Boltzmann constant, q is the electronic charge and T is the temperature. The Dit value for the transistor was found to be 9.81 1012 cm2 eV1. The obtained Dit value of the studied transistor is lower than that of ZnO TFT containing PVP (1.0 1013 cm2), whereas it is higher than that of ZnO TFT device containing PVP/Al2O3 (8.1 1011 cm2). The mobility (0.47 cm2/ V.s) of the studied transistor is lower than that of the mobility (0.8 cm2/V.s) of the ZnO containing PVP/Al2O3 having low Dit value (8.1 1011 cm2) [36]. This indicates that the performance of the ZnO TFT depends on the interface trap states at the ZnO/SiO2 interface. The low Dit of the ZnO TFT gives a high mobility. 4. Conclusions A ZnO thin film transistor based on sol–gel processed zinc oxide was fabricated. The structural properties of the ZnO film were investigated by AFM. AFM results indicate that the ZnO film is formed from the microfibers and their diameter is changed from one to another. The ZnO transistor exhibits a high mobility regardless of the thermal annealing of the transistor after its fabrication. It is evaluated that the high mobility ZnO transistor can be prepared using microfiber ZnO film. References
Fig. 4. Plot of (Ids)1/2 vs Vg of the ZnO thin film transistor.
[1] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Goncalves, A.J. S. Marques, L.M.N. Pereira, R.F.P. Martins, Adv. Mater. 17 (2005) 590.
28
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
[2] R.E. Presley, C.L. Munsee, C.-H. Park, D. Hong, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 37 (2004) 2810. [3] P.F. Carcia, R.S. McLean, M.H. Reilly, G. Nunes, Appl. Phys. Lett. 82 (2003) 1117. [4] S.J. Lim, S. Kwon, H. Kim, Thin Solid Films 516 (2008) 1523. [5] J.H. Chung, J.Y. Lee, H.S. Kim, N.W. Jang, J.H. Kim, Thin Solid Films 516 (2008) 5597. [6] V. Tvarozek, I. Novotny, P. Sutta, S. Flickyngerova, K. Schtereva, E. Vavrinsky, Thin Solid Films 515 (2007) 8756. [7] E.Q.B. Macabebe, C.J. Sheppard, E.E. van Dyk, Phys. B 404 (2009) 4466. [8] H.W. Kim, N.H. Kim, Mater. Sci. Eng. B103 (2003) 297. [9] E.S. Shim, H.S. Kang, J.S. Kang, J.H. Kim, S.Y. Lee, Appl. Surf. Sci. 186 (2002) 474. [10] S. Flickyngerova, K. Shtereva, V. Stenova, D. Hasko, I. Novotny, V. Tvarozek, P. Sutta, E. Varinsky, Appl. Surf. Sci. 254 (2008) 3643. [11] Y.M. Lu, W.S. Hwang, W.Y. Lin, J.S. Yang, Mater. Chem. Phys. 72 (2001) 269. [12] J. Nishi, F.M. Hossain, S. Takagi, T. Aita, K. Saikusa, Y. Ohmaki, I. Ohkubo, S. Kishimoto, A. Ohtomo, T. Fukumura, F. Matsukura, Y. Ohno, H. Koinuma, H. Ohno, M. Kawasaki, Jpn. J. Appl. Phys. 42 (2003) L347. [13] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [14] M.G. McDowell, R.J. Sanderson, I.G. Hill, Appl. Phys. Lett. 92 (2008) 013502. [15] R.L. Hoffman, B.J. Norris, J.F. Wager, Appl. Phys. Lett. 82 (2003) 733. [16] P.F. Carcia, R.S. McLean, M.H. Reilly, M.K. Crawford, E.N. Blanchard, A.Z. Kattamis, S. Wagner, J. Appl. Phys. 102 (2007) 074512. [17] Y.H. Hwang, J.H. Jeon, S. Seo, B. Bae, Electrochem. Solid-State Lett. 12 (2009) H336. [18] B.J. Morris, J. Anderson, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 36 (2003) L105. [19] A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Am. Chem. Soc. 124 (2002) 8812. [20] M. Mushrush, A. Facchetti, M. Lefenfeld, H.E. Katz, T.J. Marks, J. Am. Chem. Soc. 125 (2003) 9414.
[21] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2002) 2123. [22] B.S. Ong, C.S. Li, Y.N. Li, Y.L. Wu, R. Loutfy, J. Am. Chem. Soc. 129 (2007) 2750. [23] B. Sun, H. Sirringhaus, Nano Lett. 5 (2005) 2408. [24] M. Mushrush, A. Facchetti, M. Lefenfeld, H.E. Katz, T.J. Marks, J. Am. Chem. Soc. 125 (2003) 9414. [25] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123. [26] E. Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, A. Marques, L. Pereira, R. Martins, Adv. Mater. 17 (2005) 590. [27] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley-Interscience, New York, 1981. [28] G. Horowitz, P. Delannoy, J. Appl. Phys. 70 (1991) 469. [29] M. Shur, Physics of Semiconductor Devices _Prentice-Hall, Englewood Cliffs, NJ, 1990. [30] S. Lee, S. Jeong, D. Kim, B.K. Park, J. Moon, Superlattices Microstruct. 42 (2007) 361. [31] S. Jeong, Y. Jeong, J. Moon, J. Phys. Chem. C 112 (2008) 11082. [32] S. Lee, Y. Jeong, S. Jeong, J. Lee, M. Jeon, J. Moon, Superlattices Microstruct. 44 (2008) 761. [33] H. Bong, W.H. Lee, D.Y. Lee, B.J. Kim, J.H. Cho, K. Cho, Appl. Phys. Lett. 96 (2010) 192115. [34] H.-C. Huang, Nanotechnology 21 (2010) 295707. [35] K. Wolff, U. Hilleringmann, Solid-State Electron. 62 (2011) 110–114. [36] Seokhwan Bang, Seungjun Lee, Sunyeol Jeon, Semyung Kwon, Wooho Jeong, Honggyu Kim, Iksup Shin, Ho Jung Chang, Hyung-ho Park, Hyeongtag Jeon, Semicond. Sci. Technol. 24 (2009) 025008. [37] G. Horowitz, Adv. Funct. Mater. 13 (2003) 53. [38] R.N. Christopher, C.D. Frisbie, D.A. da Silva Filho, J.L. Bredas, C.E. Paul, R.M. Kent, Chem. Mater. 16 (2004) 4436.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
The effects of microfibers on electrical characteristics of zinc oxide thin film transistor Zeyad A. Alahmed a, Fahrettin Yakuphanoglu b,⇑ a b
Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Saudi Arabia Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey
a r t i c l e
i n f o
Article history: Received 4 March 2013 Received in revised form 15 April 2013 Accepted 18 April 2013 Available online 28 April 2013 Keywords: Thin film transistor Solution process Field effect mobility
a b s t r a c t The high mobility n-type thin film transistor based on sol–gel processed zinc oxide (ZnO) was fabricated. The ZnO thin film was prepared by spin coating the precursor solution on a SiO2 dielectric layer. AFM results indicate that the ZnO film is formed from the microfibers. The solution-processed ZnO TFT was found to exhibit a high mobility of 0.47 cm2/V.s. This indicates that the microfiber ZnO film has a important effect to fabricate a high mobility ZnO thin film transistor. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The metal oxide semiconductors have attracted a great deal of attention for thin film transistor applications (TFTs) [1–3]. One of them is zinc oxide (ZnO) with a hexagonal wurzite structure, a wide band gap, and optical properties [4–11]. Zinc oxide based thin film transistors (ZnO TFTs) have been extensively investigated due to their superior properties that include wide band gap, transparency, and high field effect mobility compared to that of the conventional a-Si TFTs [12–16,3]. The solution process is an alternative thin film deposition method due to its simple and low-cost pathway. One of the solution processes is a sol–gel method because of its simplicity, high throughput, and low-cost and this method offers many benefits over vacuum-based processes [17,18]. In recent years, there is a growing interest on solution-processable materials for applications in which low-cost, low-temperature manufacturing is required [19–21]. Solution-processed zinc oxide (ZnO) offers high electron mobility, environmental stability, and optical transparency. Much effort has been applied toward the development of solution-based fabrication processes for ZnO thin films [22,23]. Thin-film transistors based on solution-processable semiconducting materials for the applications in which low-cost, low temperature manufacturing have been demanded [24,19,25]. The solution-processable semiconducting materials have some advantages such as low-cost, and simplicity fabrication for lowcost electronics. It is evaluated that the electrical performance of solution process based thin film transistors can be improved by ⇑ Corresponding author. Tel.: +90 424 2370000x3792; fax: +90 424 2330062. E-mail address: fyhanoglu@firat.edu.tr (F. Yakuphanoglu). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.04.023
changing preparation conditions of the active layer and this allows the development of inexpensive transparent and high mobility thin film transistors [26]. With this aim, we have synthesized microfiber ZnO film to fabricate a ZnO thin film transistor and analyzed electrical performance with output and transfer characteristics. 2. Experimental details The zinc oxide film was synthesized by sol gel method. The zinc acetate dehydrate (Zn(Ac)2), isopropanol (IPRO) and ethanolamine (EA) were used to prepare ZnO film. Firstly, (Zn(Ac)2) was dissolved in IPRO and then, EA was added to solution and the solution was stirred with a magnetic stirrer for 2 h in 60 °C and then placed in air for 24 h resulting in a clear and homogeneous sol. The film of ZnO was prepared on thermally prepared SiO2 layer of 200 nm by spin coating. Then, the ZnO film was annealed at 500 °C for 2 h. After preparation of ZnO film, the source and drain electrodes were prepared by evaporating a 100 nm thick Al layer through a shadow mask in the form of top contact geometry. The schematic diagram of ZnO TFT was shown in Fig. 1. The morphology properties of the ZnO film were investigated by a Park System XE-100E atomic force microscopy (AFM). Electrical characteristics are measured by semiconductor parameter analyzer (Keithley 4200). 3. Results and discussion The two (2D) and three dimensional (3D) AFM images (40 lm 40 lm) of ZnO film deposited on SiO2 layer are shown in Fig. 2a. As seen in Fig. 2a, the ZnO film exhibits a fiber structure
26
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
Source
150nm 100nm 200nm
Al
Drain
Al Zinc oxide SiO2 Silicon 1. Ag Gate electrode
Fig. 1. Schematic structure of the ZnO thin film transistor.
with diameter of ZnO microfiber. The diameter of the fiber is changed from 0.883 lm to 2.1 lm from one fiber to another. The grain size distribution and histograms for the ZnO film were analyzed using XEI software program of AFM microscopy and are given in Fig. 2b. It is seen in Fig. 2a that the ZnO film has a microstructure. The surface roughness of the film was determined to be 117.23 nm using a PARK system XEI software program. This is rather high due to the distribution of microfibers on the substrate surface. These parameters affect the performance of the transistor and it was discussed in detail later. Fig. 3 shows the output characteristics of the as-deposited ZnO TFT. The drain current of the transistor increases with drain voltage and then, reached to a saturation. The saturation of the drain
Fig. 2. (a) AFM images of the ZnO film deposited on SiO2 layer (40 lm 40 lm, left in 2D image and right in 3D image) and (b) Grain size distribution and histograms of ZnO film.
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
current is taken place when the drain voltage reaches a voltage where the channel is pinched off. The drain current increases with positive gate voltages. This indicates that the transistor works in an n-channel operational mode. The output characteristics confirm that ZnO active layer used in TFT is an n-type semiconductor material. At lower voltages, the drain current–voltage curves exhibit a good linearity between the drain current and drain voltage. This confirms that a good ohmic contact is formed between ZnO film and Al contacts. The electrical parameters of ZnO TFT for saturation region can be obtained by the following relation [27–29],
Ids ¼
W lC i ðV g V th Þ2 2L
ð1Þ
where l is the saturation mobility, Ci (17.25 nF/cm2) is the capacitance per unit area of the 200 nm SiO2 insulator layer, Ids is the drain-source current, W is the width of channel, L is the channel length, Vg is the gate voltage, and Vth is the threshold voltage. The mobility value of the transistor was determined from the plot of Ids1/2 vs Vg under Vds = 40 V, as shown in Fig. 4 and was found to be 0.47 cm2/V.s. The obtained mobility value of the ZnO transistor is higher than that of a solution processed ZnO thin film transistor (2 105 cm2/V.s) [30] using zinc acetate and other ZnO thin film transistors with the mobility (2 103 cm2/V.s) [31] and (2 105 cm2/V.s) [32] and sol–gel processed zinc oxide TFT (4.2 102 annealed 500 °C, 3.6 103 at 400 °C and
27
3.6 102 cm2/V.s at 300 °C) [33] and ZnO TFT with ZnO nanoparticles [34]. This indicates that the studied ZnO transistor has the higher mobility. The improved performance is attributed the microfibers of the ZnO film, because the fiber diameter of the ZnO film (0.8–2.1 lm) used for studied transistor is higher than that of a solution processed ZnO thin film transistor (33 nm) [30] and another ZnO thin film transistor (50 nm) [31]. This indicates that the mobility is highly sensitive to the microstructure with the microfibers. It is evaluated that low temperature solution-processed zinc oxide thin film transistor using SiO2 gate dielectric produced a high mobility due to a high carrier density. The high carrier density is sufficient to fill the disorder-induced carrier traps produced by grain boundaries or oxygen vacancies and therefore, the ZnO TFT exhibits a high performance regardless of the annealed temperature. Ion/Ioff ratio of the ZnO TFT was determined from Fig. 4 and was found to be 3.99 102. This value is higher than that of the ZnO-TFT containing poly-4-vinylphenol (PVP) [35], whereas it is lower than that of the TFT containing PVP/Al2O3 (7.9 104) [36]. We believe that the low Ion/off ratio of the transistor is due to the leakage current. From the transfer characteristics, the sub threshold gate swing which is an important device performance parameter for highspeed and low-power operation is determined using the following relation [27],
S¼
C dep þ C it kT ln 10ð1 þ Þ q C ox
ð2Þ
where S is the subthreshold gate swing defined as the voltage required to increase the drain current by a factor of 10. Cdep is the depletion capacitance, Cit is the interface capacitance and Cox is the capacitance of oxide layer. The S value for the transistor was determined by means of Fig. 4 and was found to be 5.46 V/decade. This is rather high and it is related to the interface trap states at the ZnO/SiO2 interface. The interface trap density for the transistor can be calculated by the following relation [37,38],
Dit ¼
Fig. 3. Output characteristics of the ZnO thin film transistor.
S logðeÞ Ci 1 kT=q q
ð3Þ
where k is the Boltzmann constant, q is the electronic charge and T is the temperature. The Dit value for the transistor was found to be 9.81 1012 cm2 eV1. The obtained Dit value of the studied transistor is lower than that of ZnO TFT containing PVP (1.0 1013 cm2), whereas it is higher than that of ZnO TFT device containing PVP/Al2O3 (8.1 1011 cm2). The mobility (0.47 cm2/ V.s) of the studied transistor is lower than that of the mobility (0.8 cm2/V.s) of the ZnO containing PVP/Al2O3 having low Dit value (8.1 1011 cm2) [36]. This indicates that the performance of the ZnO TFT depends on the interface trap states at the ZnO/SiO2 interface. The low Dit of the ZnO TFT gives a high mobility. 4. Conclusions A ZnO thin film transistor based on sol–gel processed zinc oxide was fabricated. The structural properties of the ZnO film were investigated by AFM. AFM results indicate that the ZnO film is formed from the microfibers and their diameter is changed from one to another. The ZnO transistor exhibits a high mobility regardless of the thermal annealing of the transistor after its fabrication. It is evaluated that the high mobility ZnO transistor can be prepared using microfiber ZnO film. References
Fig. 4. Plot of (Ids)1/2 vs Vg of the ZnO thin film transistor.
[1] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Goncalves, A.J. S. Marques, L.M.N. Pereira, R.F.P. Martins, Adv. Mater. 17 (2005) 590.
28
Z.A. Alahmed, F. Yakuphanoglu / Microelectronic Engineering 110 (2013) 25–28
[2] R.E. Presley, C.L. Munsee, C.-H. Park, D. Hong, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 37 (2004) 2810. [3] P.F. Carcia, R.S. McLean, M.H. Reilly, G. Nunes, Appl. Phys. Lett. 82 (2003) 1117. [4] S.J. Lim, S. Kwon, H. Kim, Thin Solid Films 516 (2008) 1523. [5] J.H. Chung, J.Y. Lee, H.S. Kim, N.W. Jang, J.H. Kim, Thin Solid Films 516 (2008) 5597. [6] V. Tvarozek, I. Novotny, P. Sutta, S. Flickyngerova, K. Schtereva, E. Vavrinsky, Thin Solid Films 515 (2007) 8756. [7] E.Q.B. Macabebe, C.J. Sheppard, E.E. van Dyk, Phys. B 404 (2009) 4466. [8] H.W. Kim, N.H. Kim, Mater. Sci. Eng. B103 (2003) 297. [9] E.S. Shim, H.S. Kang, J.S. Kang, J.H. Kim, S.Y. Lee, Appl. Surf. Sci. 186 (2002) 474. [10] S. Flickyngerova, K. Shtereva, V. Stenova, D. Hasko, I. Novotny, V. Tvarozek, P. Sutta, E. Varinsky, Appl. Surf. Sci. 254 (2008) 3643. [11] Y.M. Lu, W.S. Hwang, W.Y. Lin, J.S. Yang, Mater. Chem. Phys. 72 (2001) 269. [12] J. Nishi, F.M. Hossain, S. Takagi, T. Aita, K. Saikusa, Y. Ohmaki, I. Ohkubo, S. Kishimoto, A. Ohtomo, T. Fukumura, F. Matsukura, Y. Ohno, H. Koinuma, H. Ohno, M. Kawasaki, Jpn. J. Appl. Phys. 42 (2003) L347. [13] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, T. Kawai, J. Appl. Phys. 93 (2003) 1624. [14] M.G. McDowell, R.J. Sanderson, I.G. Hill, Appl. Phys. Lett. 92 (2008) 013502. [15] R.L. Hoffman, B.J. Norris, J.F. Wager, Appl. Phys. Lett. 82 (2003) 733. [16] P.F. Carcia, R.S. McLean, M.H. Reilly, M.K. Crawford, E.N. Blanchard, A.Z. Kattamis, S. Wagner, J. Appl. Phys. 102 (2007) 074512. [17] Y.H. Hwang, J.H. Jeon, S. Seo, B. Bae, Electrochem. Solid-State Lett. 12 (2009) H336. [18] B.J. Morris, J. Anderson, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 36 (2003) L105. [19] A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Am. Chem. Soc. 124 (2002) 8812. [20] M. Mushrush, A. Facchetti, M. Lefenfeld, H.E. Katz, T.J. Marks, J. Am. Chem. Soc. 125 (2003) 9414.
[21] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2002) 2123. [22] B.S. Ong, C.S. Li, Y.N. Li, Y.L. Wu, R. Loutfy, J. Am. Chem. Soc. 129 (2007) 2750. [23] B. Sun, H. Sirringhaus, Nano Lett. 5 (2005) 2408. [24] M. Mushrush, A. Facchetti, M. Lefenfeld, H.E. Katz, T.J. Marks, J. Am. Chem. Soc. 125 (2003) 9414. [25] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123. [26] E. Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, A. Marques, L. Pereira, R. Martins, Adv. Mater. 17 (2005) 590. [27] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley-Interscience, New York, 1981. [28] G. Horowitz, P. Delannoy, J. Appl. Phys. 70 (1991) 469. [29] M. Shur, Physics of Semiconductor Devices _Prentice-Hall, Englewood Cliffs, NJ, 1990. [30] S. Lee, S. Jeong, D. Kim, B.K. Park, J. Moon, Superlattices Microstruct. 42 (2007) 361. [31] S. Jeong, Y. Jeong, J. Moon, J. Phys. Chem. C 112 (2008) 11082. [32] S. Lee, Y. Jeong, S. Jeong, J. Lee, M. Jeon, J. Moon, Superlattices Microstruct. 44 (2008) 761. [33] H. Bong, W.H. Lee, D.Y. Lee, B.J. Kim, J.H. Cho, K. Cho, Appl. Phys. Lett. 96 (2010) 192115. [34] H.-C. Huang, Nanotechnology 21 (2010) 295707. [35] K. Wolff, U. Hilleringmann, Solid-State Electron. 62 (2011) 110–114. [36] Seokhwan Bang, Seungjun Lee, Sunyeol Jeon, Semyung Kwon, Wooho Jeong, Honggyu Kim, Iksup Shin, Ho Jung Chang, Hyung-ho Park, Hyeongtag Jeon, Semicond. Sci. Technol. 24 (2009) 025008. [37] G. Horowitz, Adv. Funct. Mater. 13 (2003) 53. [38] R.N. Christopher, C.D. Frisbie, D.A. da Silva Filho, J.L. Bredas, C.E. Paul, R.M. Kent, Chem. Mater. 16 (2004) 4436.