High-Performance Solution-Processed Amorphous Zinc−Indium−Tin Oxide Thin-Film Transistors

Abstract

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>I</i>_on/<i>I</i>_off ratio ∼10⁵, a subthreshhold swing of ∼0.2 V/dec, and operating voltage <2 V for patterned devices with <i>W</i>/<i>L</i> = 50. The microstructural and electronic properties of ZITO semiconductor film compositions in the range Zn_{9−2<i>x</i>}In_<i>x</i>Sn_<i>x</i>O_{9+1.5<i>x</i>} (<i>x</i> = 1−4) and ZnIn_8−<i>x</i>Sn_<i>x</i>O_{13+0.5<i>x</i>} (<i>x</i> = 1−7) were systematically investigated to elucidate those factors which yield optimum mobility, <i>I</i>_on/<i>I</i>_off, and threshold voltage parameters. It is shown that structural relaxation and densification by In³⁺ and Sn⁴⁺ 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, ZnIn₄Sn₄O₁₅ TFTs fabricated with SiO₂ gate insulators exhibit electron mobilities of only ∼11 cm² V⁻¹ s⁻¹ with <i>I</i>_on/<i>I</i>_off ratios ∼10⁵, and a subthreshhold swing of ∼9.5 V/dec.