Copper phthalocyanine-based organic thin-film transistors (OTFTs) with zirconium oxide (ZrO2) as gate dielectric have been fabricated on glass substrates. The gate dielectric is annealed in N2 at different durations (5, 15, 40, and 60？min) to investigate the effects of annealing time on the electrical properties of the OTFTs. Experimental results show that the longer the annealing time for the OTFT, the better the performance. Among the devices studied, OTFTs with gate dielectric annealed at 350°C in N2 for 60？min exhibit the best device performance. They have a small threshold voltage of ？0.58？V, a low subthreshold slope of 0.8？V/decade, and a low off-state current of 0.73？nA. These characteristics demonstrate that the fabricated device is suitable for low-voltage and low-power operations. When compared with the TFT samples annealed for 5？min, the ones annealed for 60？min have 20% higher mobility and nearly two times smaller the subthreshold slope and off-state current. The extended annealing can effectively reduce the defects in the high-k film and produces a better insulator/organic interface. This results in lower amount of carrier scattering and larger CuPc grains for carrier transport. 1. Introduction Owing to light weight, mechanical flexibility, and low-cost fabrication, organic thin-film transistors (OTFTs) have a wide range of applications such as sensors, flat-panel displays, and RFID tags [1–3]. The first OTFT based on organic semiconductor polythiophene was reported in 1986 [4]. Heavily doped silicon substrates are generally used for the fabrication of OTFT as they are highly conductive and can act as the gate of the devices. In addition, high-quality gate dielectric SiO2 can be thermally grown directly on the Si substrate. However, for the next generation of OTFTs and oxide TFTs fabricated on glass or plastic substrates, a metal gate electrode is necessary. By fabricating OTFTs on flexible plastic substrates, there is a potential in the future for producing roll-up displays that can be integrated into a small device such as a pen. There are three major types of dielectric used in OTFTs: inorganic dielectric, polymeric dielectric, and self-assemble layer. For inorganic dielectric materials, silicon dioxide is commonly used as the gate insulator in OTFT. However, this kind of OTFT requires a relatively high voltage (about 100 V) for operation. In order to reduce the operating voltage and hence the power consumption, high-k material is often used as a gate dielectric in OTFTs. Several high-k dielectrics have been employed to fabricate OTFT,
References
[1]
P. Mach, S. J. Rodriguez, R. Nortrup, P. Wiltzius, and J. A. Rogers, “Monolithically integrated, flexible display of polymer-dispersed liquid crystal driven by rubber-stamped organic thin-film transistors,” Applied Physics Letters, vol. 78, no. 23, pp. 3592–3594, 2001.
[2]
M. Halik, H. Klauk, U. Zschieschang et al., “Low-voltage organic transistors with an amorphous molecular gate dielectric,” Nature, vol. 431, no. 7011, pp. 963–966, 2004.
[3]
T. Someya and T. Sakurai, “Integration of organic field-effect transistors and rubbery pressure sensors for artificial skin applications,” in Proceedings of the IEEE International Electron Devices Meeting, pp. 203–206, December 2003.
[4]
A. Tsumura, H. Koezuka, and T. Ando, “Macromolecular electronic device: field-effect transistor with a polythiophene thin film,” Applied Physics Letters, vol. 49, no. 18, pp. 1210–1212, 1986.
[5]
J. Tardy, M. Erouel, A. L. Deman et al., “Organic thin film transistors with HfO2 high-k gate dielectric grown by anodic oxidation or deposited by sol-gel,” Microelectronics Reliability, vol. 47, no. 2-3, pp. 372–377, 2007.
[6]
J. B. Koo, J. W. Lim, S. H. Kim et al., “Pentacene thin-film transistors and inverters with plasma-enhanced atomic-layer-deposited Al2O3 gate dielectric,” Thin Solid Films, vol. 515, no. 5, pp. 3132–3137, 2007.
[7]
G. Wang, D. Moses, A. J. Heeger, H. M. Zhang, M. Narasimhan, and R. E. Demaray, “Poly(3-hexylthiophene) field-effect transistors with high dielectric constant gate insulator,” Journal of Applied Physics, vol. 95, no. 1, pp. 316–322, 2004.
[8]
C. Bartic, H. Jansen, A. Campitelli, and S. Borghs, “Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors,” Organic Electronics, vol. 3, no. 2, pp. 65–72, 2002.
[9]
N. Hiroshiba, R. Kumashiro, K. Tanigaki et al., “Rubrene single crystal field-effect transistor with epitaxial BaTiO3 high- k gate insulator,” Applied Physics Letters, vol. 89, no. 15, Article ID 152110, 2006.
[10]
R. K. Nahar, V. Singh, and A. Sharma, “Study of electrical and microstructure properties of high dielectric hafnium oxide thin film for MOS devices,” Journal of Materials Science, vol. 18, no. 6, pp. 615–619, 2007.
[11]
B. H. Lee, S. C. Song, R. Choi, and P. Kirsch, “Metal electrode/high-κ dielectric gate-stack technology for power management,” IEEE Transactions on Electron Devices, vol. 55, no. 1, pp. 8–20, 2008.
[12]
C. C. Fulton, T. E. Cook, G. Lucovsky, and R. J. Nemanich, “Interface instabilities and electronic properties of ZrO2 on silicon (100),” Journal of Applied Physics, vol. 96, no. 5, pp. 2665–2673, 2004.
[13]
W. J. Qi, R. Nieh, B. H. Lee et al., “MOSCAP and MOSFET characteristics using ZrO2 gate dielectric deposited directly on Si,” in Proceedings of the 1999 IEEE International Devices Meeting (IEDM), pp. 145–148, December 1999.
[14]
W. J. Qi, R. Nieh, B. H. Lee et al., “Performance of MOSFETs with ultra thin ZrO2 and Zr silicate gate dielectrics,” in Proceedings of the 2000 Symposium on VLSI Technology, pp. 40–41, June 2000.
[15]
H. E. Katz, L. Torsi, and A. Dodabalapur, “Synthesis, material properties, and transistor performance of highly pure thiophene oligomers,” Chemistry of Materials, vol. 7, no. 12, pp. 2235–2237, 1995.
[16]
H. E. Katz, A. Dodabalapur, L. Torsi, and D. Elder, “Precursor synthesis, coupling, and TFT evaluation of end-substituted thiophene hexamers,” Chemistry of Materials, vol. 7, no. 12, pp. 2238–2240, 1995.
[17]
R. Jiang, E. Q. Xie, and Z. F. Wang, “Effect of inner oxygen on the interfacial layer formation for HfO2 gate dielectric,” Journal of Materials Science, vol. 42, no. 17, pp. 7343–7347, 2007.
[18]
L. Pereira, P. Barquinha, E. Fortunato, and R. Martins, “Low temperature processed hafnium oxide: structural and electrical properties,” Materials Science in Semiconductor Processing, vol. 9, no. 6, pp. 1125–1132, 2006.
[19]
D. Knipp, R. A. Street, A. V？lkel, and J. Ho, “Pentacene thin film transistors on inorganic dielectrics: morphology, structural properties, and electronic transport,” Journal of Applied Physics, vol. 93, no. 1, pp. 347–355, 2003.
[20]
S. Steudel, S. De Vusser, S. De Jonge et al., “Influence of the dielectric roughness on the performance of pentacene transistors,” Applied Physics Letters, vol. 85, no. 19, pp. 4400–4402, 2004.
[21]
M. C. Kwan, K. H. Cheng, P. T. Lai, and C. M. Che, “Improved carrier mobility for pentacene TFT by NH3 annealing of gate dielectric,” Solid-State Electronics, vol. 51, no. 1, pp. 77–80, 2007.
[22]
K. H. Cheng, W. M. Tang, L. F. Deng, C. H. Leung, P. T. Lai, and C. M. Che, “Correlation between carrier mobility of pentacene thin-film transistor and surface passivation of its gate dielectric,” Journal of Applied Physics, vol. 104, no. 11, Article ID 116107, 2008.
