3018 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 9, SEPTEMBER 2011 Extraction of Trap Densities in ZnO Thin-Film Transistors and Dependence on Oxygen Partial Pressure During Sputtering of ZnO Films Mutsumi Kimura, Senior Member, IEEE, Mamoru Furuta, Member, IEEE, Yudai Kamada, Takahiro Hiramatsu, Tokiyoshi Matsuda, Hiroshi Furuta, Chaoyang Li, Shizuo Fujita, and Takashi Hirao Abstract—Trap densities in the channel layers D t of ZnO thin-film transistors have been extracted. First, the low-frequency (low-f) capacitance–voltage C –V characteristics are measured using the customized measurement system. Next, the surface po- tential is calculated from the low-f C –V characteristic, and the surface potential gradient is calculated by applying Gauss’s law. Finally, the spatial profile of the electric potential is calculated by applying Poisson equation and carrier density equations, and D t is extracted. It is found that, generally, the deep states are flatly distributed in the energy gap apart from the conduction band, and the shallow states seem to be the tail states. More- over, the dependence on the oxygen partial pressure during the sputtering of ZnO films [p(O 2 )] has been analyzed. It is found that for p(O 2 )= 0.50 ∼ 0.75 Pa, D t changes little, whereas for p(O 2 )= 0.17 ∼ 0.33 Pa, D t increases. It is clarified that the abnormal shapes of the current–voltage and low-f C –V charac- teristics originate from the increase of D t . Index Terms—Oxygen partial pressure, sputtering, thin-film transistor (TFT), trap density, ZnO. Manuscript received March 24, 2011; revised May 3, 2011 and May 21, 2011; accepted May 25, 2011. Date of publication July 18, 2011; date of current version August 24, 2011. This work was supported in part by a research project of the Joint Research Center for Science and Technology of Ryukoku University, by a grant for research facility equipment for private universities from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), by a grant for special research facilities from the Faculty of Science and Technology of Ryukoku University, and by a grant from the High-Tech Research Center Program for private universities from MEXT. A part of the ZnO TFT research was supported by the Japan Science and Technology Agency (JST). The review of this paper was arranged by Editor B. Kaczek. M. Kimura is with the Department of Electronics and Informatics, and the Joint Research Center for Science and Technology, and also with the Inno- vative Materials and Processing Research Center, High-Tech Research Cen- ter, Ryukoku University, Otsu 520-2194, Japan (e-mail: mutsu@rins.ryukoku. ac.jp). M. Furuta, H. Furuta, C. Li, and T. Hirao are with the Research Institute for Nanodevices, Kochi University of Technology, Kami 782-8502, Japan. Y. Kamada was with the Graduate School of Engineering and the Photonics and Electronics Science and Engineering Center, Kyoto University, Kyoto 615-8520, Japan. T. Hiramatsu was with the Research Institute for Nanodevices, Kochi Uni- versity of Technology, Kami 782-8502, Japan. T. Matsuda is with the Department of Electronics and Informatics, Ryukoku University, Otsu 520-2194, Japan. S. Fujita is with the Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2158546 I. I NTRODUCTION O XIDE–SEMICONDUCTOR thin-film transistors (TFTs), such as ZnO TFTs [1] and amorphous In–Ga–Zn–O (α-IGZO) TFTs [2], are promising next-generation giant mi- croelectronic elements because they are transparent devices, exhibit high performance, and can be fabricated on plastic substrates at low temperatures. Therefore, ZnO TFTs have been actively studied and extensively developed not only for flat-panel displays [3], [4] but also for image sensors [5] and transparent electronics [6], [7]. Trap densities in the channel layers D t are some of the main determinants of transistor performance. Therefore, it is impor- tant to extract D t to improve transistor performance, diagnose fabrication processes, etc. Several methods have been devel- oped to extract D t , such as electron paramagnetic resonance [8], deep-level transient spectroscopy [9], isothermal capaci- tance transient spectroscopy [10], constant photocurrent meth- ods [11], device simulation fitting [12], field-effect methods, [13]–[15], and capacitance methods [16], [17]. However, these methods require some hypotheses on the optical and electrical properties of the channel layers, which are often doubtful for complex semiconductors, such as ZnO films. Recently, we have developed an extraction technique of D t by measuring the low- frequency (low-f) capacitance–voltage C–V characteristics and using a novel extraction algorithm [18]. The unique merit is that the charge densities in ZnO TFTs are directly counted from the transistor characteristics, and no doubtful model is necessary, unlike the conventional techniques. Although the extraction technique has first been applied to α-IGZO TFTs, it can also be applied to ZnO TFTs [19], [20]. Deposition methods for ZnO films are pulsed laser deposition [21], [22], atomic layer deposition [23], [24], direct-current or radio-frequency (RF) sputtering [1], [3], [25], [26], etc. Among these deposition methods, sputtering is one of the most available methods to deposit ZnO films on large substrates at low temperature. The electrical resistivity of intrinsic ZnO films strongly depends on the oxygen partial pressure during sputtering of ZnO films [p(O 2 )] [25]. An abrupt transition from semiconductor to insulator occurs as p(O 2 ) exceeds a critical pressure. Therefore, it is necessary to choose an appropriate p(O 2 ) to obtain superior switching properties, such as low leakage current and high ON/OFF current ratio. In this paper, D t in ZnO TFTs has been extracted, and the dependence on p(O 2 ) has been analyzed. Particularly, in this 0018-9383/$26.00 © 2011 IEEE