Transparent amorphous oxide semiconductors (TAOSs) such as amorphous In-Ga-Zn-O (a-IGZO) are superior to the conventional semiconducting materials used in flat-panel displays (FPDs) because they have the advantages of high electron mobility, good uniformity, and a simple manufacturing process based on matured sputtering techniques [1]. To apply TAOS thin-film transistors (TFTs) to the mass production of next-generation displays, the stress stability during the a-IGZO TFT operation is a crucial issue that must be resolved.

It has been reported that the concentration of the hydrogen atoms in the a-IGZO is huge and the diffusion coefficient of hydrogen is two orders of magnitude larger than that of oxygen [2]. It means that hydrogen atoms easily diffuse into the channel region of the a-IGZO TFTs from hydrogen-containing layers such as a passivation layer (PV) during the fabrication. Hydrogen atoms also easily absorb to the channel region during the fabrication. Therefore, clarification of the effect of hydrogen on the electronic properties of the a-IGZO is particularly important to improve the TFT performance such as various stress stabilities against negative bias thermal illumination stress (NBTIS), negative bias thermal stress (NBTS) and positive bias thermal stress (PBTS).

The hump-shaped variation of the transfer curves under the NBTIS test was observed in both a-IGZO etch-stop layer (ESL)-type and back channel etch (BCE)-type TFTs. According to the results of photo-induced transient spectroscopy (PITS) analysis, it was observed that the density of hydrogen-related traps in the a-IGZO thin film was changed with the increase and/or decrease of hydrogen incorporated by the TFT manufacturing process [3, 4]. In this presentation, the impact of hydrogen-related trap levels on stress reliability with humps will be reviewed. Furthermore, the parallel-shaped variation of the transfer curves under the NBTS and PBTS test changed depending on the temperature of post-process annealing. It is believed that these behaviors are caused by interface defects due to dissociation and diffusion of hydrogen atoms at the interface [5, 6].

[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature Vol. 432, pp. 488-492 (2004).
[2] K. Nomura, T. Kamiya, and H. Hosono, ECS J. Solid State Sci. Technol. Vol. 2, No. 1, pp. 5-8 (2013).
[3] K. Hayashi, M. Ochi, A. Hino, H. Tao, H. Goto, and T. Kugimiya, Jpn. J. Appl. Phys. 56, 03BB02 (2017).
[4] M. Ochi, A. Hino, H. Goto, K. Hayashi, and T. Kugimiya, ECS J. Solid State Sci. Technol. 6, P247 (2017).
[5] K. Hayashi, M. Ochi, A. Hino, H. Tao, H. Goto, and T. Kugimiya, Proc. IDW ’17, pp. 320-323 (2017).
[6] M. Ochi, A. Hino, H. Goto, K. Hayashi, and T. Kugimiya, Jpn. J. Appl. Phys. 57, 02CB06 (2018).