Improvement of TFT Characteristics for Low-Temperature Solution-Processed Oxide Semiconductors with Hydrogen Injection and Oxidation Process

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Thin-film transistors (TFTs) using oxide semiconductors have attracted much attention for switching and driving devices in large-screen, high-resolution liquid crystal display (LCD) and organic light-emitting diode (OLED) displays because of their high mobility1-3. As an advanced oxide TFT fabrication method, the solution-process is widely researched due to advantages such as its low process cost, large-scale fabrication capacity, and the simplicity of the process. However, several drawbacks of the solution-process must be addressed for display applications. One of the key points is the trade-off between the processing temperature and the TFT characteristics. From previous research on solution-processed oxide TFTs, it has been reported that low-temperature-processed TFTs are generally inactive below 400 °C due to a lack of metal-oxide-metal bond formation and decomposition of the residual species4,5. However, a low-temperature process is strongly required for application to plastic substrates for flexible displays and flexible electronics devices.In addition, a low-temperature process is important to achieve low fabrication costs. Here, we demonstrate improvement of the TFT characteristics by application of a hydrogen injection and oxidation (HIO) process for low-temperature solution-processed oxide TFTs. From a technical standpoint, we consider that the important factor is how to efficiently decompose the residual species. The solution-processed oxide TFTs exhibit inferior device performance when a few residual species remain. Therefore, a reduction reaction is initiated by a hydrogen injection process to decompose the residual spices. The characteristics of a TFT based on indium gallium zinc oxide (IGZO) were evaluated, such as the field effect mobility and the threshold voltage (Vth) shift under positive and negative bias stress. The maximum temperature for the entire fabrication process was as low as 300 °C. Solution-processed IGZO thin films were fabricated by a spin-coating method onto 200 nm thick thermally oxidized SiO2/n+-Si substrates and annealing at 300 °C for 1 h. The IGZO precursor was prepared from 0.3 M solutions of indium nitrate hydrate (In(NO3)3·xH2O), gallium nitrate hydrate (Ga(NO3)3·xH2O), and zinc nitrate hydrate (Zn(NO3)2·xH2O) in pure water. The composition ratio for In:Ga:Zn was set to 4:1:1. Hydrogen plasma treatment was adopted for the hydrogen injection process with subsequent annealing at 300 °C for 1 h. After fabrication of the IGZO film, the active area of the IGZO film was defined using a conventional photolithography method. Patterned source/drain (S/D) electrodes (Mo) were formed on the IGZO thin films using a shadow mask. Figure 1 shows the transfer characteristics of the IGZO TFT with and without the HIO process. The transfer characteristics of these IGZO TFTs for gate voltages (Vg) ranging from -30 to 30 V were measured at a fixed drain voltage (Vd) of 30 V. After the HIO process for the IGZO TFTs, the field effect mobility was significantly improved from 2.1 cm2/Vs to 4.8 cm2/Vs. Figure 2 shows the results for the threshold voltage shift (ΔVth) under a positive gate bias stress (PBS) of 20 V and a negative gate bias stress (NBS) of -20 V, with and without the HIO process. The stress time was set to 1 h at the maximum. Improvement of the reliability was confirmed: the Vth shift was less than 0.25 V for PBS and -2 V for NBS. The trap sites of any residual species or defect states in active channels were thus efficiently suppressed by the hydrogen injection process. In addition, the oxidation process may enhance the metal-oxide-metal bond formation. In conclusion, the HIO process as a combination of hydrogen reduction reaction and oxidation was demonstrated as effective for the improvement of solution-processed oxide TFTs. Improvement of the TFT characteristics was confirmed in terms of the field effect mobility and the Vthshift under PBS and NBS. Therefore, we consider that this new approach will lead to the development of high-performance solution-processed oxide TFTs in future. 1K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004). 2K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, Science 300, 1269 (2003). 3L. Wang, M. Yoon, G. Lu, Y. Yang, A. Facchetti, and T.J. Marks, Nature materials 5, 893 (2006). 4 M. G. Kim, M. G. Kanatzidis, A. Facchetti, and T. J. Marks, Nature materials 5, 382 (2011) 5 S. Jeong, Y. G. Ha, J. Moon, A. Faccchetti, T. J. Marks, Advanced materials 22, 1346 (2010) Figure 1