Influence of Passivation Layers on Characteristics of High Mobility Amorphous Indium-Zinc-Tin-Oxide Thin-Film Transistors

Amorphous oxide semiconductors (AOS) have recently attracted much attention as potential channel layer in thin film transistors (TFTs) due to high carrier mobility, transparency in visible light and room-temperature deposition. Among various materials, the TFTs based on amorphous Indium Gallium Zinc Oxide (a-IGZO) are expected to be a promising technology for the next generation novel flat panel display development [1], in particular for high resolution Liquid Cristal Display (LCD) [2]. The average field-effect mobility of TFT with sputter-deposited a-IGZO thin film as a channel layer is around 10 cm2/Vs and it’s not high enough (20 cm2/Vs) for future display applications such as ultra-high definition (UHD) display, active matrix organic light-emitting diode displays (AMOLEDs) and system driving circuits integrated with the TFT array process [3, 4]. Compared with other amorphous oxide semiconductor materials, amorphous indium-zinc-tin oxide (a-IZTO) is promising candidate for achieving high mobility oxide TFTs [5, 6]. Although the properties of oxide semiconductor films can be easily controlled by radio-frequency (RF) sputtering, the subsequent processes, particularly the passivation layer growth, often cause destructive impacts on channel layer [7]. The challenge in fabricating high performance a-IZTO TFTs actually lies in the post processes rather than in the a-IZTO film deposition. Therefore, it is necessary to investigate the effect of the passivation layers on the characteristics of a-IZTO TFTs. In this study, we investigated the effect of the plasma enhanced chemical vapor deposition (PECVD) deposited passivation layer (SiO2) on the characteristics of a-IZTO TFTs. The threshold voltage (Vth ) of the TFTs was found to be highly dependent on the deposition condition of passivation layer, which is ascribed to ion bombardment induced by PECVD process. By adjusting the post annealing temperature, the Vth of the TFTs can be well controlled to the desired range. We investigated the influence of passivation-layer deposition on the characteristics of a-InZnSnO thin-film transistors (TFTs). The threshold voltage (Vth ) of the TFTs shifted markedly as a result of the ion bombardment induced by the passivation layers above. By adjusting the post annealing temperature, the performance of the TFTs can be modulated. The a-InZnSnO TFTs after passivation exhibited good performance with a field-effect mobility of 21.34 cm2 /Vs, a threshold voltage of -5.03V, and a subthreshold slope of 0.52 V/decade. Reference [1] M. A. Marrs, C. D. Moyer, E. J. Bawolek, et al., " Control of Threshold Voltage and Saturation Mobility Using Dual-Active-Layer Device Based on Amorphous Mixed Metal–Oxide–Semiconductor on Flexible Plastic Substrates," IEEE Transactions on Electron Devices, vol. 58, pp. 3428-3434, 2011. [2] T. Kamiya, K. Nomura, and H. Hosono, “Present status of amorphous In-Ga-Zn-O thin-film transistors,” Sci. Technol. Adv. Mater., vol. 11, no. 4, Aug. 2010, Art ID 044305. [3] J. U. Bae, D. H. Kim, K. Kim, K. Jung, W. Shin, I. Kang and S. Yeo, “Development of oxide TFT’s structures,” in SID Symp. Dig., Jun. 2013, vol. 44, no. 1, pp. 89–92. [4] L.F. Teng, P.T. Liu, Y.J. Lo, and Y.J. Lee, “Effects of microwave annealing on electrical enhancement of amorphous oxide semiconductor thin film transistor,” Appl. Phys. Lett., vol. 101, no. 13, p. 132901, Sep. 2012. [5] K. Jang, J. Raja, Y.J. Lee, D. Kim, and J. Yi, “Effects of carrier concentration, indium content, and crystallinity on the electrical properties of indium-tin-zinc-oxide thin-film transistors,” IEEE Electron Device Lett., vol. 34, no. 9, pp. 1151–1153, Sep. 2013. [6] N. Morosawa et al., “High-mobility self-aligned top-gate oxide TFT for high-resolution AM-OLED,” in SID Symp. Dig., Jun. 2013, vol. 44, no. 1, pp. 85–88. [7] T. T. T. Nguyen, B. Aventurier, T. Terlier, J. P. Barnes, and F. Templier, “Impact of Passivation Conditions on Characteristics of Bottom-Gate IGZO Thin-Film Transistors,” J. Display Technol., vol. 11, no. 6, pp. 554–558, June. 2015