Controlling Vertical Diffusion with an Al₂O₃ Back Interface Layer for Stable High-Performance InZnO TFTs

To improve electrical performance and bias stability for low-power applications, indium-zinc oxide (IZO) thin-film transistors (TFTs) were fabricated with an Al₂O₃ back interface layer that enables vertical diffusion control, together with HfO₂/Al₂O₃ gate insulators. In our previous work, we achieved high switching performance in TFTs by employing high-k gate insulators via atomic layer deposition (ALD), but significant gate bias instability remained. To further enhance electrical performance and bias stability, vertical diffusion was precisely controlled through modulation of deposition time and oxygen partial pressure (OPP) during Al₂O₃ back interface layer formation using RF magnetron sputtering. As a result of controlled vertical diffusion, Al cations diffused from the Al₂O₃ back interface layer, significantly suppressing oxygen vacancy formation. Devices with the Al₂O₃ back interface layer deposited under optimized conditions exhibited enhanced electrical properties, including a saturation carrier mobility of 14.4 cm²/V·s and a subthreshold swing of 0.23 V/dec. Notably, under room-temperature negative bias stress, the threshold voltage shift was reduced from -1.75 to - 0.55 V, demonstrating a significant improvement over conventional IZO TFTs. We employed X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive spectroscopy (EDS) to investigate the mechanisms underlying these improvements. By engineering the Al₂O₃ back interface layer to control vertical diffusion, this work provides a viable pathway for realizing low-power, high-performance oxide TFTs for next-generation display backplanes.