Effects of Annealing Time on the Performance of OTFT on Glass with ZrO2 as Gate Dielectric

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,