High-Performance Indium Gallium Tin Oxide Transistors with an Al₂O₃ Gate Insulator Deposited by Atomic Layer Deposition at a Low Temperature of 150 °C: Roles of Hydrogen and Excess Oxygen in the Al₂O₃ Dielectric Film

In this work, high-performance amorphous In0.75Ga0.23Sn0.02O (a-IGTO) transistors with an atomic layer-deposited Al2O3 dielectric layer were fabricated at a maximum processing temperature of 150 °C. Hydrogen (H) and excess oxygen (Oi) in the Al2O3 film, which was controlled by adjusting the oxygen radical density (PO2: flow rate of O2/[Ar+O2]) in the radio-frequency (rf) plasma during ALD growth of Al2O3, significantly affected the performance and stability of the resulting IGTO transistors. The concentrations of H and Oi in Al2O3/IGTO stacks according to PO2 were characterized by secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, hard X-ray photoemission spectroscopy, and thermal desorption spectroscopy. The high concentration of H at a low PO2 of 2.5% caused heavy electron doping in the underlying IGTO during thermal annealing at 150 °C, leading to a conductive behavior in the resulting transistor without modulation capability. In contrast, a high PO2 condition of 20% introduced O2 molecules (or Oi) into the Al2O3 film, which negatively impacted the carrier mobility and caused anomalous photo-bias instability in the IGTO transistor. Through in-depth understanding of how to manipulate H and Oi in Al2O3 by controlling the PO2, we fabricated high-performance IGTO transistors with a high field-effect mobility (μFE) of 58.8 cm2/Vs, subthreshold gate swing (SS) of 0.12 V/decade, threshold voltage (VTH) of 0.5 V, and ION/OFF ratio of ∼109 even at the maximum processing temperature of 150 °C. Simultaneously, the optimized devices were resistant to exposure to external positive gate bias stress (PBS) and negative bias stress (NBS) for 3600 s, where the VTH shifts for exposure to PBS and NBS for this duration were 0.1 V and −0.15 V, respectively.