The Significance on Structural Modulation of Buffer and Gate Insulator for ALD Based InGaZnO TFT Applications

Atomic layer deposition (ALD) has been studied extensively to employ oxide semiconductor thin film transistor (TFT) including both active layer and gate insulator (GI). Herein, we developed an ALD sandwich structure, which deposits both semiconductor and GI by ALD. In contrast to the previous results using sputter In-Ga-Zn-O (IGZO), ALD sandwich structure IGZO TFT exhibited severe deterioration in its electrical performance when the Al ₂ O ₃ was adopted for both buffer layer and GI application. Through measurement of hydrogen permeability of ALD insulators and secondary ion mass spectroscopy of each sandwich structure after annealing, we found a hydrogen accumulation effect between Al ₂ O ₃ and ALD IGZO interface layer, which caused deterioration of electrical performance. In contrast, TFTs with ALD SiO ₂ , which has proper hydrogen diffusivity, chosen as the buffer and GI had favorable electric properties of 28.17 cm ² / $\text{V}\cdot \text{s}$ , 0.20 V/dec, 0.96, and 0.12 V for the mobility, ${V}_{\text {th}}$ , subthreshold swing (SS), and hysteresis. In this regard, an optimized GI structure via the ALD SiO ₂ and Al ₂ O ₃ in situ process based on excellent interface formation with the semiconductor and hydrogen barrier performance, respectively, was developed. This functional GI structure consisting of SiO ₂ and Al ₂ O ₃ exhibited excellent TFT characteristics (27.52 cm ² / $\text{V}\cdot \text{s}$ , 0.24 V/dec, and 1.07 V for the mobility, SS, and ${V}_{\text {th}}$ , respectively) with improved stability even after hydrogen annealing, which was used to examine the resistance to external hydrogen, showing a threshold voltage shift of −0.15 V and a SS shift of 0.01 V/dec.