<em>(Invited) </em>Approach to Oxide Tfts with High Mobility & Stability

Hosono, Hideo

Thin film transistors (TFTs) with the channel of amorphous oxide semiconductors (AOSs) represented by amorphous Indium-Gallium-Zinc Oxide (a-IGZO) are now applied to drive pixels of the state-of the art display based on high mobility, good homogeneity, easy fabrication and optical transparency [1]. Recently, larger mobility TFTs are highly demanded for emerging applications such as ultra-high vision and memories. However, there is a critical obstacle to be overcome, i.e., an empirical trade-off rule in oxide TFTs between mobility and stability. Higher mobility TFTs possess less stability to various types of stress (negative bias-thermal stress instability, NBTS, positive bias-thermal stress instability, PBTS, and negative bias-illumination stress instability, NBIS). The Elucidation of origin for this trade-off rule and new approach are required to realize high mobility and high stability TFTs.

We clarify that the origin of this trade-off through comparison in TFTs between IGZO with low mobility and high stability and ITZO with high mobility and low stability. A critical phenomenon was noted during the conventional TFT fabrication; CO-related impurity donates electrons to Indium-Tin-Zinc Oxide (ITZO) with high mobility under the negative bias stress condition but such CO-related impurity effect is negligible in IGZO with lower mobility than ITZO [2]. This finding may be understood by considering that large difference in E_CBM determines the transfer of electron generated by adsorbed CO-related species on the back surface of ITZO [3]. Based on this model and this finding, we successfully fabricated the most stable ITZO TFT with a high mobility of 70 cm²/Vs by eliminating CO-related impurity and further passivation; NBTS (-20V, 60 ^oC, 3600 s, ΔV_th: -0.02 V), PBTS (+20 V, 60 ^oC, 3600 s, ΔVth: 0.12 V) and NBIS (-20V, 15000 lx, 3600s, ΔVth: -1.33 V).

[1] Hosono, H., & Kumomi,K., (ed), Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory (Wiley, 2022); Hosono, H. (2018). How we made the IGZO transistor. Nature Electronics, 1(7), 428-428.

[2]Shiah, Y. S., Sim, K., Ueda, S., Kim, J., & Hosono, H. (2021). Unintended Carbon-Related Impurity and Negative Bias Instability in High-Mobility Oxide TFTs. IEEE Electron Device Letters, 42(9), 1319-1322.

[3] Shiah, Y. S., Sim, K., Shi, Y., Abe, K., Ueda, S., Sasase, M., Kim,J., & Hosono, H. (2021). Mobility–stability trade-off in oxide thin-film transistors. Nature Electronics, 4(11), 800-807.

(Invited) Approach to Oxide Tfts with High Mobility & Stability