Optimization of Metal Capping Layer Design for Enhanced Mobility in Indium Gallium Zinc Oxide (IGZO) Thin Film Transistors
Authors: Lee JinKyu , Lee Soo-Yeon 
Published: December 2024

Abstract

Given the increasing demand for high frame rates and fast response times in high-end displays, various strategies have been explored to enhance the mobility of IGZO TFTs. In general, plasma treatment and elemental doping are widely utilized methods for this purpose [1], [2]. However, these approaches often encounter a trade-off between electrical properties and reliability, prompting ongoing research to overcome such challenges. Another approach to improve mobility involves increasing the electron concentration of the back channel by incorporating a metal capping layer (MCL) between the source and drain [3]. However, excessive electron concentration in the back channel can induce increased off-current and hump phenomenon, resulting in degraded electrical properties [4]. Most research on MLC has primarily focused on improving mobility by using different metals, while there has been little research on the trade-off between electrical properties and carrier concentration based on MCL design [5]. In this paper, we investigated the influence of MCL design on electrical properties and present the optimized metal capping layer design which does not degrade electrical properties.

Experiment

A heavily doped P++ silicon with thermally oxidized SiO2 (200 nm) was used as a substrate, onto which a 20 nm thick IGZO was deposited by RF sputtering. Subsequently, the IGZO channel was patterned using a standard photolithography process, and a Ti/Al/Ti layer was deposited via e-gun evaporation to form the source and drain (S/D) electrodes. Finally, an Al-metal capping layer was deposited in the middle of the back channel between the source and drain to form the MCL, which covers a range of 6.6% to 70.8% of the IGZO back channel.

Result and Discussion

Based on experimental results, IGZO TFTs with an MCL coverage of 70.8% exhibited a more than twofold increase in mobility compared to pristine samples. This enhancement is attributed to the increased electron concentration that results from the redox reaction between the MCL and IGZO. However, a specific threshold of MCL coverage can trigger the hump phenomenon. In addition, this threshold varied depending on the MCL design. This suggests that the formation of additional sub-channels and electron distributions contributing to the hump phenomenon differs with each design. Therefore, we explored various MCL designs to identify the optimal structure that enhances mobility without compromising electrical properties. These findings suggest a design strategy for high-mobility devices in high-end displays, potentially applicable to backplane technology.

Reference

[1] Zan, H. W., Tsai, W. W., Chen, C. H., & Tsai, C. C, Advanced Materials, 23(37), 4237-4242, 2011.
[2] Kawai, H., Kataoka, J., Saito, N., Ueda, T., Ishihara, T., & Ikeda, K, MRS Bulletin, 46(11), 1044-1052, 2021.
[3] Yoon, C. S., Kim, H. T., Kim, M. S., Yoo, H., Park, J. W., Choi, D. H., ... & Kim, H. J, ACS Applied Materials & Interfaces, 13(3), 4110-4116, 2021.
[4] Lee, B. H., Sohn, A., Kim, S., & Lee, S. Y, Scientific reports, 9(1), 886, 2019.
[5] Kim, J., Park, J. B., Zheng, D., Kim, J. S., Cheng, Y., Park, S. K., ... & Facchetti, A, Advanced Materials, 34(45), 2205871, 2022.
Fig.1 The transfer characteristics of IGZO TFTs with various MCL coverage, indicating two MCL types: (a) increasing MCL width with fixed length and (b) increasing MCL length with fixed width.

Figure 1