High-performance solution-processed amorphous zinc-indium-tin oxide thin-film transistors

Films of the high-performance solution-processed amorphous oxide semiconductor a-ZnIn₄Sn₄O₁₅, grown from 2-methoxyethanol/ethanolamine solutions, were used to fabricate thin-film transistors (TFTs) in combination with an organic self-assembled nanodielectric as the gate insulator. This structurally dense-packed semiconductor composition with minimal Zn²⁺ incorporation strongly suppresses transistor off-currents without significant mobility degradation, and affords field-effect electron mobilities of ∼90 cm² V^-1 s^-1 (104 cm² V^-1 s^-1 maximum obtained for patterned ZITO films), with I_on/I_off ratio ∼10⁵, a subthreshhold swing of ∼0.2 V/dec, and operating voltage <2 V for patterned devices with W/L = 50. The microstructural and electronic properties of ZITO semiconductor film compositions in the range Zn_9-2xIn _xSn_xO_9+1.5x (x = 1-4) and ZnIn _8-xSn_xO_13+0.5x (x = 1-7) were systematically investigated to elucidate those factors which yield optimum mobility, I _on/I_off, and threshold voltage parameters. It is shown that structural relaxation and densification by In³⁺ and Sn ⁴⁺ mixing is effective in reducing carrier trap sites and in creating carrier-generating oxygen vacancies. In contrast to the above results for TFTs fabricated with the organic self-assembled nanodielectric, ZnIn ₄Sn₄O₁₅ TFTs fabricated with SiO₂ gate insulators exhibit electron mobilities of only ∼11 cm² V^-1 s^-1 with I_on/I_off ratios ∼10⁵, and a subthreshhold swing of ∼9.5 V/dec.