Improvement of Hysteresis in Aluminum Doped Indium-Tin-Zinc-Oxide Thin Film Transistors By Modulating Intrinsic Film Stress of Mo S/D Electrodes
Authors: Bae Jaehan , Park Sang-Hee Ko 
Published: February 2020

Since first novel report from Hosono’s research group in 2004, numerous studies have been conducted on amorphous oxide TFTs [1]. Because of their various advantages such as high electron mobility, large on-off ratio, and low fabrication temperature, oxide TFTs are expected to play an important role in the next generation display applications. Among several important parameters of oxide TFTs, hysteresis in the transfer curve can give us critical information about types and densities of defects in the TFTs. In the case of clockwise hysteresis, it is generally known that the hysteresis occurs when there are electron traps in the bulk region of the active layer, or interface between the active front channel and the gate insulator. Therefore, until now, studies have mainly focused on reducing the hysteresis by optimizing several engineering conditions such as deposition method, deposition temperature, post-annealing temperature.
On the other hand, as far as we know, the intrinsic film stress, which is determined by the process conditions during the thin film deposition, has not been discussed much in terms of performance of oxide TFT. Especially in the large screen display applications, the intrinsic film stress of metal contacted with active layer, needs to be carefully handled because carrier trap sites may be generated due to high film stress , resulting in degradation of device performance. Nevertheless, there have been few studies about the change of device properties caused by the intrinsic film stress in the oxide TFT. Thus, in this study, we investigated the relationship between the intrinsic film stress of metal and the hysteresis phenomenon in the aluminum doped indium-tin-zinc-oxide (Al-ITZO) TFT.
First, we fabricated molybdenum (Mo) thin films with different intrinsic film stress by intentionally controlling the argon working pressure during the sputtering process [2]. The intrinsic film stress values of Mo thin films were calculated by Stoney equation which uses the change of substrate curvature before and after the thin film deposition. According to the calculated results, high-stress Mo and low-stress Mo showed intrinsic tensile stress values of 1920.7 MPa and 143.9 MPa, respectively. Then, we fabricated back-channel-etched structured Al-ITZO TFTs by applying those molybdenum Mo thin films as source and drain (S/D) electrodes, and compared hysteresis of each TFT. It was confirmed that the modulation of the intrinsic film stress of S/D electrodes affected the clockwise hysteresis characteristics of the TFTs. As shown in Figure 1, in the case of TFT using high-stress S/D, hysteresis was about 1.4V larger than the TFT with low-stress S/D in both of short and long channel TFT. This result is interesting because the S/D electrodes with the same material but different film stress on the back surface of the active layer induced a change of the hysteresis. Therefore, we assumed that the stronger tensile stress of high-stress S/D induced larger hysteresis in the TFT, and X-ray photoelectron spectroscopy (XPS) depth profile method was used for further analysis. By analyzing the XPS results, it was possible to compare the difference of the chemical state at the front channel after depositing S/D electrodes with different film stress values, and consequently, hysteresis difference could be explained. Through this research, we expect new engineering perspective can be provided in display industries.
[1] Nomura, Kenji, et al. "Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors." Nature 432.7016 (2004): 488.
[2] Windischmann, Henry. "Intrinsic stress in sputter-deposited thin films." Critical Reviews in Solid State and Material Sciences 17.6 (1992): 547-596.
This work has supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT)(2018R1A2A3075518).

Figure 1