Amorphous oxide channel TFTs Hideya Kumomi a, ⁎ , Kenji Nomura b , Toshio Kamiya b , Hideo Hosono b,c a Canon Research Center, Canon Inc., 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan c ERATO-SORST, Japan Science and Technology Agency, Japan Available online 5 April 2007 Abstract Thin film transistors (TFTs) using amorphous oxides of post-transition metals: indium, gallium, and zinc for the channel materials are fabricated with radio-frequency magnetron sputtering methods for the deposition of the channel and the gate insulator layers, at room temperature with no high-temperature post-deposition annealing process. The TFTs operate as n-channel field-effect transistors with various structures of top/ bottom gate and top/bottom source-and-drain contact including the inverse-stagger types, and with various materials for the gate insulators, the electrodes, and the substrates. The TFTs having smoother channel interfaces show the better performance at the saturation mobility beyond 10 cm 2 V − 1 s − 1 and the on-to-off current ratio over 10 8 than the rough channel interfaces. The ring oscillator circuits operate with five-stage inverters of the top-gate TFTs or the inverse-stagger TFTs. Organic light-emission diode cells are driven by a simple circuit of the TFTs. It isalso found by a combinatorial approach to the material exploration that the TFT characteristics can be controlled by the composition ratio of the metals in the channel layers. The amorphous oxide channel TFTs fabricated with sputtering deposition at low temperature could be a candidate for key devices of large-area flexible electronics. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous; Oxides; Semiconductors; Transparent; TFT 1. Introduction Large-area electronics composed of thin-film transistors (TFTs) have been fabricated mainly on rigid glass substrates. The substrates and the housing bodies to protect the substrates from the damage occupy the greater part of the electronics in volume and determine their total weight. As the dimension of the electronic devices gets large, typically as has been developed in flat-panel displays, the devices become too heavy to hang on walls or to easily move even by a short distance. Portable devices essentially require the reduction of the weight and volume of their electronic components. There is, however, a finite limit of thickness reduction of the substrates, because the glass is fragile. One of the approaches to these problems is the electronics on non-fragile and soft substrates made from materials which are of lower mass density than the glass, and of flexible properties, such as plastics (synthetic resins) or metal foils. Generally, such materials possesses the large coefficients of thermal expansion and cannot endure high-temperature processes for conventional silicon TFTs. Even below the maximum endurable temperature, the large thermal expansion makes the alignment of the device patterns difficult in the fabrication processes. We have to fabricate the TFTs on the glass substrate at high temperature, and then to transfer only the thin device layer onto the soft substrate [1]. Otherwise, it is necessary to lower the process temperature of the TFTs, if fabricated directly on the substrates. There have been many attempts to the low-temperature TFTs, by lowering the deposition temperature of hydrogenated amorphous silicon (a-Si:H) [2–4] for a-Si:H TFTs, or for precursors of laser-crystallized polycrystalline Si TFTs [5]. The lowest process temperature for a-Si:H TFTs exceeds 100 °C or more, while the saturation mobility, μ sat , does not exceed 1 cm 2 V − 1 s − 1 . The device fabrication processes of polycrystalline Si TFTs generally require more thermal budgets than the a-Si:H TFTs. Organic semiconductor TFTs [6] have been widely investigated for years due to their materials variation and printable nature, but are still faced with the stability problems to atmosphere and the difficulty in raising μ sat beyond that of the a-Si:H TFTs. Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 1516 – 1522 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: kumomi.hideya@canon.co.jp (H. Kumomi). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.161