Bias Stability of zinc-tin-oxide thin film transistors with Al2O3 gate dielectrics

Zinc-tin-oxide (ZTO) thin film transistors (TFTs) with atomic layer deposition (ALD) Al₂O₃ gate dielectrics were fabricated and compared to devices with PECVD SiO₂ gate dielectrics. The Al₂O₃ devices showed acceptable mobility (~14 cm²/V·s), subthreshold slope (~0.4 V/dec), and I_ON/I_OFF (~10⁷). However, a pronounced positive V_T shift was observed during initial gate voltage sweeps, consistent with electron trapping. In an effort to stabilize operation, devices were made with varying Al₂O₃ deposition temperature, O₂ plasma exposure, and a plasma-enhanced chemical vapor deposited (PECVD) SiO₂ capping layer. It was found that capping of the Al₂O₃ with a thin (2-3 nm) layer of PECVD SiO₂ altered the trapping behavior and reduced the bias instability significantly, suggesting that the Al₂O₃/ZTO interface is the source of the trapping.