Mitigating electrical degradation in ultra-thin IGZO TFTs through contact engineering with Al2O3 interlayer

• Identified contact resistance as key degradation factor in <5 nm IGZO TFTs. • Introduced ALD Al₂O₃ IL to block Ti-induced redox, the main cause of degradation. • Achieved R c reduction from 385 to 38 Ω·cm using 3 nm Al₂O₃ interlayer. • Al₂O₃ IL did not induce DIBL or thermal instability in ultra-thin IGZO TFTs. • 3 nm Al₂O₃ IL effectively blocks Ti diffusion and interfacial degradation. This study proposes an effective solution to mitigate performance degradation in ultra-thin devices after systematically investigating how the channel thickness affects the electrical characteristics of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). When the channel thickness is reduced below 5 nm, a significant decrease in mobility and current crowding are observed. This degradation primarily stems from Ti-induced oxidation and trap formation, which are not confined to the interface but extend into the IGZO channel bulk, physically damaging the electron conduction path and ultimately reducing the field-effect mobility to approximately 0.2 cm²/V·s. To address this issue, an atomic-layer-deposited (ALD) Al₂O₃ interlayer (IL) was introduced at the Ti/IGZO interface. Although the TMA precursor used in ALD has strong reactivity with IGZO components, the self-limiting surface reaction characteristic of ALD confines chemical interactions to the IGZO surface, thereby forming a uniform and dense dielectric film without damaging the underlying channel. Additionally, the resulting Al₂O₃ layer acts as a thermodynamically stable diffusion barrier that prevents spontaneous redox reactions with Ti, effectively suppressing the formation of interfacial oxides. As a result, the Al₂O₃ IL preserves the chemical and structural integrity of the IGZO channel and enables robust electron injection at the contact interface. Notably, with a 3 nm-thick IL, the field-effect mobility of ultra-thin 3 nm IGZO TFTs was significantly enhanced from ∼0.2 to ∼2.4 cm²/V·s. This study highlights the importance of interfacial engineering in addressing contact resistance issues in ultra-thin oxide semiconductors and provides a scalable and effective strategy for developing high-performance IGZO-based TFTs for next-generation electronic applications.