Robustness of Passivated ALD Zinc Tin Oxide TFTs to Aging and Bias Stress

Thin-film electronics fabricated with amorphous oxide semiconductors (AOS) are being studied for beyond-display technologies because of their superior electron transport. To widen the commercial applications, robust devices fabricated using scalable deposition techniques are required. Here, we fabricate bottom-gate, top contact thin-film transistors (TFTs) using zinc tin oxide, an AOS deposited by atomic layer deposition (ALD), and study robustness to aging and bias stress for devices stored in a dark air ambient with and without Al2O3 passivation. All samples exhibit excellent mobility (<inline-formula> <tex-math notation="LaTeX">$\mu _{\text {FE}}$ </tex-math></inline-formula>) and subthreshold swing (SS) robustness to aging. Most of the device aging occurs within the first 25 days, after which the performance stabilizes. Changes due to positive bias stress, which are dominated by interactions with ambient molecules, are greatly reduced by passivation. Passivation, however, results in an increase in contact resistance (<inline-formula> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula>), SS, and negative bias illumination stress (NBIS) instability. The increase in <inline-formula> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula> is attributed to the diffusion of oxygen vacancies that occurs during the passivation process, while the increase in SS and NBIS instability is attributed to subgap interface defects. The experimental results are compared to Silvaco ATLAS simulations to confirm the proposed physical mechanisms and allow quantification of the associated defects and non-idealities.