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Influence of the contact resistance effect on the output characteristics of pentacene-based organic thin film transistors
Microelectronic Engineering 103 (2013) 76–78
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Influence of the contact resistance effect on the output characteristics of pentacene-based organic thin film transistors Yow-Jon Lin ⇑, Bo-Chieh Huang Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
a r t i c l e
i n f o
Article history: Received 28 March 2012 Received in revised form 8 August 2012 Accepted 8 September 2012 Available online 18 September 2012 Keywords: Contact resistance Organic semiconductor Thin film transistors
a b s t r a c t We study the contact resistance effect in the temperature range of 300–350 K for pentacene-based organic thin film transistors using a simple resistance model. It is shown that the ratio of contact to channel resistance decreases with increasing temperature, implying that charge transport at 300 K may be limited by contact resistance. However, the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. The linear gatevoltage-dependent output correlations may be used as a predictive tool to identify the source/drain ohmic quality. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic thin film transistors (OTFTs) have become of great interest for several electronic applications, such as activematrix flat panel displays, electronic paper, and chemical sensors, replacing the traditional inorganic-based thin film transistors. Because of its several advantages, pentacene has become one of the most widely studied molecules in both academia and industry. Recently, there has been much interest in pentacene for various applications (thin film transistors and solar cells) [1–6]. Au has been widely used as source/drain (S/D) electrodes due to its high work function [1–6]. Ohmic contacts to pentacene are a crucial part of many types of electronic and optoelectronic devices in the pentacene system. With decreasing device dimensions, the contact resistance (Rc) as part of the total device resistance will dominate compared with channel resistance (Rch) and therefore will play an important part in device operations [7]. The impact of Rc on the extracted mobility (l) has been extensively analyzed [8,9]. Klauk et al. showed that for pentacene-based OTFTs with typical dimensions, Rc can be many times greater than Rch [10]. To improve an OTFT performance it is important to understand the cause of Rc as well as the carrier injection mechanism from S/D electrodes to the channel [11]. Understanding the contact effect is of great technological importance since it limits the OTFT performance. In this study, we used a simple resistance model for considering the contact resistance effect. A link between a simple resistance model and the linear-regime output characteristics ⇑ Corresponding author. Tel.: +886 4 7232105x3379; fax: +886 4 7211153. E-mail address: [email protected] (Y.-J. Lin). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.09.001
of OTFTs was established. Drain–source voltage (VDS)-dependent drain current (ID) measurements at a constant gate–source voltage (VGS) provide a method to examine the VGS dependence of the transistor total resistance [Rt, Rt = (dID/dVDS)1] in the linear regime of OTFTs. The Rt–VGS characteristics in the linear regime were employed to demonstrate contact effects based on a simple resistance model. On the other hand, the low-temperature performances of OTFTs have been reported by Yoneya et al. [11] and Nelson et al. [12]. Yoneya et al. found that Rc(l) decreases with decreasing temperature [11]. Nelson et al. predicted contact effects to make a nearly temperature independent transport look temperature dependent [12]. Chou et al. found that the carrier mobility increases with increasing annealing temperature reaching a maximum at annealing temperature of 120 °C, and then decreases with further increasing of annealing temperature beyond that point [13]. The higher temperature than 120 °C may lead to the performance degradation of OTFTs, owing to the poor crystalline quality of the pentacene layer [13]. At thermal annealing temperature of 70 °C and above, Ahn et al. [14] found that the pentacene films lose their crystallinity and the OTFT performance is decreased. Guo et al. [15] suggested that increasing the annealing temperature to 70 °C caused obvious desorption because of the low van der Waals intermolecular forces in the pentacene film. In this study, we focused on the linear-regime output characteristics of OTFTs in the temperature range of 300–350 K for avoiding weakened crystallinity. It is found that the ratio of contact to channel resistance decreases with increasing temperature (T), implying that the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics and the device performance at 300 K can be limited more
Y.-J. Lin, B.-C. Huang / Microelectronic Engineering 103 (2013) 76–78
77
by contact physics than by the intrinsic transport physics in the pentacene itself. 2. Experimental procedure A 270 nm thick SiO2 layer was grown on the heavily n-type Si (n+-Si) wafer using a dry oxidation process as a gate oxide layer. A 70 nm thick pentacene (Luminescence Technology Corp., HsinChu, Taiwan) layer was deposited on the SiO2/n+-Si substrates by vacuum thermal evaporation and the evaporation rate was 4.2 nm/min (as monitored by a quartz crystal microbalance after thickness calibration with atomic force microscopy measurements). The substrate temperature was fixed at 30 °C. Then, the S/ D electrodes were fabricated by depositing Au metal on the pentacene layer through a shadow mask. The devices have the channel width (W) of 700 lm and the channel length (L) of 100 lm. The current–voltage characteristics of OTFTs were measured in the temperature of 300–350 K by steps of 12.5 K using a hot chuck with a temperature controller. The current–voltage measurements were performed by using a Keithley model-4200-SCS semiconductor characterization system. The VDS-dependent ID measurements at a constant VGS provide a method to examine the relationship between the threshold voltage (VTH) and l in the linear region [3]. The ID versus VDS curve was scanned from 2 to 40 V with a fixed VGS (that is, VGS = 10, 13, 16, 20, 30 and 40 V, respectively).
Fig. 2. Rt as a function of |(VGS–VTH)1| in the linear region for OTFTs at 300, 325 and 350 K. Note: areas of Fig. 1a–c with rectangular shapes are used for construction of this figure.
jID j ¼ ðlWC i =2LÞðV GS V TH Þ2
ð1Þ
3. Experimental results and discussion Fig. 1 shows the ID–VDS curves (VGS = 10, 13, 16, 20, 30 and 40 V) of OTFTs at 300, 325 and 350 K. The areas of Fig. 1a–c are noted with elliptic shapes because they are used for construction of Fig. 1d. Fig. 1d shows ID at VDS = 40 V as the function of VGS. The ID–VGS characteristic in the saturation region can be described by the relation
where Ci is the insulator capacitance per unit area (12.3 nF/cm2 obtained from capacitance–voltage measurement). By fitting a curve to the ID–VGS characteristic in Fig. 1d based on Eq. (1), VTH can be calculated. VTH at 300 (325 or 350) K was calculated to be 7.5 (5.4 or 0.6) V. On the other hand, by fitting the ID–VDS curve in the linear region (that is, areas of Fig. 1a–c with rectangular shapes), Rt can be calculated (that is, Rt = (dID/dVDS)1). Fig. 2 shows the
Fig. 1. ID–VDS curves of OTFTs at (a) 300, (b) 325 and (c) 350 K. (d) ID at VDS = 40 V as the function of VGS. Note: areas of (a–c) with elliptic shapes are used for construction of (d).
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Y.-J. Lin, B.-C. Huang / Microelectronic Engineering 103 (2013) 76–78
On the other hand, the ID–VDS characteristics in the nearly linear region (that is, areas of Fig. 1a–c with rectangular shapes) can be described by the relation
h i jID j ¼ jðlWC i =2LÞ 2V DS ðV GS V TH Þ V 2DS j
Fig. 3. Rc–temperature curves of OTFTs and the ratio of Rc/Rch at VGS = 30 V as a function of temperature.
ð3Þ
Fig. 4b shows the extracted l from Eq. (3) with VDS = 1.5 V at 300 (Fig. 1a), 312.5, 325 (Fig. 1b), 337.5 and 350 (Fig. 1c) K, respectively. In Fig. 4, we present the comparison between the extracted l from Eq. (2) and the extracted l from Eq. (3). We find that the extracted l from Eq. (3) is lower than the extracted l from Eq. (2), owing to the Rc effect of Au electrodes. Chou et al. have shown that the influence of the Rc effect on the extractive l at room temperature can be very large [9]. However, the difference in l extracted from Eqs. (2) and (3) decreases with increasing T and l extracted from Eq. (3) at 350 K is similar to l extracted from Eq. (2) at 350 K. This is because of a negligible ratio Rc/Rch at 350 K (Fig. 3). It is found that the contribution of Rc may lead to the underestimation of l at 300 K. However, Rc decreases with increasing T, leading to the presence of a negligible ratio Rc/Rch at 350 K. We suggest that the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. 4. Conclusions The influence of the Rc effect on the linear-regime output characteristics has been investigated in this study. It is found that the influence of the contact effect on the extractive l can be very large at 300 K. T-dependent measurements show that the ratio Rc/Rch or the difference in l extracted from Eqs. (2) and (3) decreases as T is increased, implying that the OTFT performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. It is important to identify the contact effect for understanding the actual device operation mechanism and enhancing the device performance. Preliminary measurements gave values, which seems to confirm the validity of our method. The VGS-dependent linear-regime output correlations may be used as a predictive tool to identify the S/D ohmic quality of OTFTs without the structure of a transfer length model. Acknowledgment
Fig. 4. The comparison between (a) the extracted l from Eq. (2) and (b) the extracted l from Eq. (3) at 300, 312.5, 325, 337.5 and 350 K.
Rt |(VGS VTH)1| curves of OTFTs at 300, 325 and 350 K. Rt in the linear regime can be expressed as [6,11]
L Rt ¼ Rch þ Rc ¼ þ Rc WC i jðV GS V TH Þjl
ð2Þ
By fitting a curve to the Rt |(VGS VTH)1| characteristic in Fig. 2 based on Eq. (2), Rc and can be calculated Based on the experimental data, the device parameters (Rc and l) have been extracted from Eq. (2) and compared, as demonstrated in Figs. 3 and 4a, respectively. Fig 3 also shows the ratio Rc/Rch at VGS = 30 V as a function of T. In agreement with the previously reported results [6,9,16], we found that the extracted Rc values were varied in the range of 1–100 MX (Fig. 3). As can be seen in Fig. 3, Rc (the ratio Rc/Rch) decreases with increasing T, implying that charge transport at 300 K may be limited by Rc and the influence of the contact effect on the device performance at 300 K can be very large. However, the OTFT performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. In addition, relative to the channel, the contacts actually improve at high temperatures, so that a device is bulk (contact) limited at 350 (300) K.
The authors acknowledge the support of the National Science Council of Taiwan (Contract No. 100-2112-M-018-003-MY3) in the form of grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
H. Klauk, Organic Electronics, Wiley-VCH, Weinheim, 2006. M.Y. Tsai, Y.J. Lin, Microelectron. Eng. 96 (2012) 24. Y.J. Lin, C.L. Tsai, B.C. Huang, Appl. Phys. Lett. 97 (2010) 203509. Y.J. Lin, Synth. Met. 160 (2010) 2628. B.C. Huang, Y.J. Lin, Appl. Phys. Lett. 99 (2011) 113301. S.D. Wang, T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, Appl. Phys. Lett. 91 (2007) 203508. D.J. Yun, D.K. Lee, H.K. Jeon, S.W. Rhee, Org. Electron. 8 (2007) 690. L. Li, P. Heremans, J. Jang, Jpn. J. Appl. Phys. 50 (2011) 021602. W.Y. Chou, C.W. Kuo, H.L. Cheng, Y.R. Chen, F.Y. Yang, D.Y. Shu, C.C. Liao, F.C. Tang, Jpn. Appl. Phys. 45 (2006) 7922. H. Klauk, G. Schmid, W. Radlik, W. Weber, L. Zhou, C.D. Sheraw, J.A. Nichols, T.N. Jackson, Solid-State Electron. 47 (2003) 297. N. Yoneya, M. Noda, N. Hirai, K. Nomoto, M. Wada, J. Kasahara, Appl. Phys. Lett. 85 (2004) 4663. S.F. Nelson, Y.Y. Lin, D.J. Gundlach, T.N. Jackson, Appl. Phys. Lett. 72 (1998) 1854. D.W. Chou, C.J. Huang, C.M. Su, C.F. Yang, W.R. Chen, T.H. Meen, Solid-State Electron. 61 (2011) 76. T. Ahn, H. Jung, H.J. Suk, M.H. Yi, Synth. Met. 159 (2009) 1277. D. Guo, S. Ikeda, K. Saiki, H. Miyazoe, K. Terashima, J. Appl. Phys. 99 (2006) 094502. J. Park, J.M. Kang, D.W. Kim, J.S. Choi, Thin Solid Films 518 (2010) 6232.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Influence of the contact resistance effect on the output characteristics of pentacene-based organic thin film transistors Yow-Jon Lin ⇑, Bo-Chieh Huang Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
a r t i c l e
i n f o
Article history: Received 28 March 2012 Received in revised form 8 August 2012 Accepted 8 September 2012 Available online 18 September 2012 Keywords: Contact resistance Organic semiconductor Thin film transistors
a b s t r a c t We study the contact resistance effect in the temperature range of 300–350 K for pentacene-based organic thin film transistors using a simple resistance model. It is shown that the ratio of contact to channel resistance decreases with increasing temperature, implying that charge transport at 300 K may be limited by contact resistance. However, the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. The linear gatevoltage-dependent output correlations may be used as a predictive tool to identify the source/drain ohmic quality. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, organic thin film transistors (OTFTs) have become of great interest for several electronic applications, such as activematrix flat panel displays, electronic paper, and chemical sensors, replacing the traditional inorganic-based thin film transistors. Because of its several advantages, pentacene has become one of the most widely studied molecules in both academia and industry. Recently, there has been much interest in pentacene for various applications (thin film transistors and solar cells) [1–6]. Au has been widely used as source/drain (S/D) electrodes due to its high work function [1–6]. Ohmic contacts to pentacene are a crucial part of many types of electronic and optoelectronic devices in the pentacene system. With decreasing device dimensions, the contact resistance (Rc) as part of the total device resistance will dominate compared with channel resistance (Rch) and therefore will play an important part in device operations [7]. The impact of Rc on the extracted mobility (l) has been extensively analyzed [8,9]. Klauk et al. showed that for pentacene-based OTFTs with typical dimensions, Rc can be many times greater than Rch [10]. To improve an OTFT performance it is important to understand the cause of Rc as well as the carrier injection mechanism from S/D electrodes to the channel [11]. Understanding the contact effect is of great technological importance since it limits the OTFT performance. In this study, we used a simple resistance model for considering the contact resistance effect. A link between a simple resistance model and the linear-regime output characteristics ⇑ Corresponding author. Tel.: +886 4 7232105x3379; fax: +886 4 7211153. E-mail address: [email protected] (Y.-J. Lin). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.09.001
of OTFTs was established. Drain–source voltage (VDS)-dependent drain current (ID) measurements at a constant gate–source voltage (VGS) provide a method to examine the VGS dependence of the transistor total resistance [Rt, Rt = (dID/dVDS)1] in the linear regime of OTFTs. The Rt–VGS characteristics in the linear regime were employed to demonstrate contact effects based on a simple resistance model. On the other hand, the low-temperature performances of OTFTs have been reported by Yoneya et al. [11] and Nelson et al. [12]. Yoneya et al. found that Rc(l) decreases with decreasing temperature [11]. Nelson et al. predicted contact effects to make a nearly temperature independent transport look temperature dependent [12]. Chou et al. found that the carrier mobility increases with increasing annealing temperature reaching a maximum at annealing temperature of 120 °C, and then decreases with further increasing of annealing temperature beyond that point [13]. The higher temperature than 120 °C may lead to the performance degradation of OTFTs, owing to the poor crystalline quality of the pentacene layer [13]. At thermal annealing temperature of 70 °C and above, Ahn et al. [14] found that the pentacene films lose their crystallinity and the OTFT performance is decreased. Guo et al. [15] suggested that increasing the annealing temperature to 70 °C caused obvious desorption because of the low van der Waals intermolecular forces in the pentacene film. In this study, we focused on the linear-regime output characteristics of OTFTs in the temperature range of 300–350 K for avoiding weakened crystallinity. It is found that the ratio of contact to channel resistance decreases with increasing temperature (T), implying that the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics and the device performance at 300 K can be limited more
Y.-J. Lin, B.-C. Huang / Microelectronic Engineering 103 (2013) 76–78
77
by contact physics than by the intrinsic transport physics in the pentacene itself. 2. Experimental procedure A 270 nm thick SiO2 layer was grown on the heavily n-type Si (n+-Si) wafer using a dry oxidation process as a gate oxide layer. A 70 nm thick pentacene (Luminescence Technology Corp., HsinChu, Taiwan) layer was deposited on the SiO2/n+-Si substrates by vacuum thermal evaporation and the evaporation rate was 4.2 nm/min (as monitored by a quartz crystal microbalance after thickness calibration with atomic force microscopy measurements). The substrate temperature was fixed at 30 °C. Then, the S/ D electrodes were fabricated by depositing Au metal on the pentacene layer through a shadow mask. The devices have the channel width (W) of 700 lm and the channel length (L) of 100 lm. The current–voltage characteristics of OTFTs were measured in the temperature of 300–350 K by steps of 12.5 K using a hot chuck with a temperature controller. The current–voltage measurements were performed by using a Keithley model-4200-SCS semiconductor characterization system. The VDS-dependent ID measurements at a constant VGS provide a method to examine the relationship between the threshold voltage (VTH) and l in the linear region [3]. The ID versus VDS curve was scanned from 2 to 40 V with a fixed VGS (that is, VGS = 10, 13, 16, 20, 30 and 40 V, respectively).
Fig. 2. Rt as a function of |(VGS–VTH)1| in the linear region for OTFTs at 300, 325 and 350 K. Note: areas of Fig. 1a–c with rectangular shapes are used for construction of this figure.
jID j ¼ ðlWC i =2LÞðV GS V TH Þ2
ð1Þ
3. Experimental results and discussion Fig. 1 shows the ID–VDS curves (VGS = 10, 13, 16, 20, 30 and 40 V) of OTFTs at 300, 325 and 350 K. The areas of Fig. 1a–c are noted with elliptic shapes because they are used for construction of Fig. 1d. Fig. 1d shows ID at VDS = 40 V as the function of VGS. The ID–VGS characteristic in the saturation region can be described by the relation
where Ci is the insulator capacitance per unit area (12.3 nF/cm2 obtained from capacitance–voltage measurement). By fitting a curve to the ID–VGS characteristic in Fig. 1d based on Eq. (1), VTH can be calculated. VTH at 300 (325 or 350) K was calculated to be 7.5 (5.4 or 0.6) V. On the other hand, by fitting the ID–VDS curve in the linear region (that is, areas of Fig. 1a–c with rectangular shapes), Rt can be calculated (that is, Rt = (dID/dVDS)1). Fig. 2 shows the
Fig. 1. ID–VDS curves of OTFTs at (a) 300, (b) 325 and (c) 350 K. (d) ID at VDS = 40 V as the function of VGS. Note: areas of (a–c) with elliptic shapes are used for construction of (d).
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Y.-J. Lin, B.-C. Huang / Microelectronic Engineering 103 (2013) 76–78
On the other hand, the ID–VDS characteristics in the nearly linear region (that is, areas of Fig. 1a–c with rectangular shapes) can be described by the relation
h i jID j ¼ jðlWC i =2LÞ 2V DS ðV GS V TH Þ V 2DS j
Fig. 3. Rc–temperature curves of OTFTs and the ratio of Rc/Rch at VGS = 30 V as a function of temperature.
ð3Þ
Fig. 4b shows the extracted l from Eq. (3) with VDS = 1.5 V at 300 (Fig. 1a), 312.5, 325 (Fig. 1b), 337.5 and 350 (Fig. 1c) K, respectively. In Fig. 4, we present the comparison between the extracted l from Eq. (2) and the extracted l from Eq. (3). We find that the extracted l from Eq. (3) is lower than the extracted l from Eq. (2), owing to the Rc effect of Au electrodes. Chou et al. have shown that the influence of the Rc effect on the extractive l at room temperature can be very large [9]. However, the difference in l extracted from Eqs. (2) and (3) decreases with increasing T and l extracted from Eq. (3) at 350 K is similar to l extracted from Eq. (2) at 350 K. This is because of a negligible ratio Rc/Rch at 350 K (Fig. 3). It is found that the contribution of Rc may lead to the underestimation of l at 300 K. However, Rc decreases with increasing T, leading to the presence of a negligible ratio Rc/Rch at 350 K. We suggest that the device performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. 4. Conclusions The influence of the Rc effect on the linear-regime output characteristics has been investigated in this study. It is found that the influence of the contact effect on the extractive l can be very large at 300 K. T-dependent measurements show that the ratio Rc/Rch or the difference in l extracted from Eqs. (2) and (3) decreases as T is increased, implying that the OTFT performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. It is important to identify the contact effect for understanding the actual device operation mechanism and enhancing the device performance. Preliminary measurements gave values, which seems to confirm the validity of our method. The VGS-dependent linear-regime output correlations may be used as a predictive tool to identify the S/D ohmic quality of OTFTs without the structure of a transfer length model. Acknowledgment
Fig. 4. The comparison between (a) the extracted l from Eq. (2) and (b) the extracted l from Eq. (3) at 300, 312.5, 325, 337.5 and 350 K.
Rt |(VGS VTH)1| curves of OTFTs at 300, 325 and 350 K. Rt in the linear regime can be expressed as [6,11]
L Rt ¼ Rch þ Rc ¼ þ Rc WC i jðV GS V TH Þjl
ð2Þ
By fitting a curve to the Rt |(VGS VTH)1| characteristic in Fig. 2 based on Eq. (2), Rc and can be calculated Based on the experimental data, the device parameters (Rc and l) have been extracted from Eq. (2) and compared, as demonstrated in Figs. 3 and 4a, respectively. Fig 3 also shows the ratio Rc/Rch at VGS = 30 V as a function of T. In agreement with the previously reported results [6,9,16], we found that the extracted Rc values were varied in the range of 1–100 MX (Fig. 3). As can be seen in Fig. 3, Rc (the ratio Rc/Rch) decreases with increasing T, implying that charge transport at 300 K may be limited by Rc and the influence of the contact effect on the device performance at 300 K can be very large. However, the OTFT performance at 350 K can be limited more by the intrinsic transport physics in the pentacene itself than by contact physics. In addition, relative to the channel, the contacts actually improve at high temperatures, so that a device is bulk (contact) limited at 350 (300) K.
The authors acknowledge the support of the National Science Council of Taiwan (Contract No. 100-2112-M-018-003-MY3) in the form of grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
H. Klauk, Organic Electronics, Wiley-VCH, Weinheim, 2006. M.Y. Tsai, Y.J. Lin, Microelectron. Eng. 96 (2012) 24. Y.J. Lin, C.L. Tsai, B.C. Huang, Appl. Phys. Lett. 97 (2010) 203509. Y.J. Lin, Synth. Met. 160 (2010) 2628. B.C. Huang, Y.J. Lin, Appl. Phys. Lett. 99 (2011) 113301. S.D. Wang, T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, Appl. Phys. Lett. 91 (2007) 203508. D.J. Yun, D.K. Lee, H.K. Jeon, S.W. Rhee, Org. Electron. 8 (2007) 690. L. Li, P. Heremans, J. Jang, Jpn. J. Appl. Phys. 50 (2011) 021602. W.Y. Chou, C.W. Kuo, H.L. Cheng, Y.R. Chen, F.Y. Yang, D.Y. Shu, C.C. Liao, F.C. Tang, Jpn. Appl. Phys. 45 (2006) 7922. H. Klauk, G. Schmid, W. Radlik, W. Weber, L. Zhou, C.D. Sheraw, J.A. Nichols, T.N. Jackson, Solid-State Electron. 47 (2003) 297. N. Yoneya, M. Noda, N. Hirai, K. Nomoto, M. Wada, J. Kasahara, Appl. Phys. Lett. 85 (2004) 4663. S.F. Nelson, Y.Y. Lin, D.J. Gundlach, T.N. Jackson, Appl. Phys. Lett. 72 (1998) 1854. D.W. Chou, C.J. Huang, C.M. Su, C.F. Yang, W.R. Chen, T.H. Meen, Solid-State Electron. 61 (2011) 76. T. Ahn, H. Jung, H.J. Suk, M.H. Yi, Synth. Met. 159 (2009) 1277. D. Guo, S. Ikeda, K. Saiki, H. Miyazoe, K. Terashima, J. Appl. Phys. 99 (2006) 094502. J. Park, J.M. Kang, D.W. Kim, J.S. Choi, Thin Solid Films 518 (2010) 6232.