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

## TL;DR

This paper shows how adding an Al2O3 back interface layer improves the performance and stability of InZnO transistors for low-power electronics.

## Contribution

A novel method to control vertical diffusion using an Al2O3 back interface layer, enhancing both performance and bias stability in IZO TFTs.

## Key findings

- Devices with the Al2O3 layer showed a saturation carrier mobility of 14.4 cm²/V·s and a subthreshold swing of 0.23 V/dec.
- Threshold voltage shift under stress was reduced from −1.75 to −0.55 V, showing improved bias stability.
- XPS, TEM, and EDS confirmed the suppression of oxygen vacancies due to controlled Al cation diffusion.

## Abstract

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.