Achieving a Low-Voltage, High-Mobility IGZO Transistor through an ALD-Derived Bilayer Channel and a Hafnia-Based Gate Dielectric Stack

Abstract

Ultrahigh-resolution displays for augmented reality (AR) and virtual reality (VR) applications require a novel architecture and process. Atomic-layer deposition (ALD) enables the facile fabrication of indium–gallium zinc oxide (IGZO) thin-film transistors (TFTs) on a substrate with a nonplanar surface due to its excellent step coverage and accurate thickness control. Here, we report all-ALD-derived TFTs using IGZO and HfO2 as the channel layer and gate insulator, respectively. A bilayer IGZO channel structure consisting of a 10 nm base layer (In0.52Ga0.29Zn0.19O) with good stability and a 3 nm boost layer (In0.82Ga0.08Zn0.10O) with extremely high mobility was designed based on a cation combinatorial study of the ALD-derived IGZO system. Reducing the thickness of the HfO2 dielectric film by the ALD process offers high areal capacitance in field-effect transistors, which allows low-voltage drivability and enhanced carrier transport. The intrinsic inferior stability of the HfO2 gate insulator was effectively mitigated by the insertion of an ALD-derived 4 nm Al2O3 interfacial layer between HfO2 and the IGZO film. The optimized bilayer IGZO TFTs with HfO2-based gate insulators exhibited excellent performances with a high field-effect mobility of 74.0 ± 0.91 cm2/(V s), a low subthreshold swing of 0.13 ± 0.01 V/dec, a threshold voltage of 0.20 ± 0.24 V, and an I ON/OFF of ∼3.2 × 108 in a low-operation-voltage (≤2 V) range. This promising result was due to the synergic effects of a bilayer IGZO channel and HfO2-based gate insulator with a high permittivity, which were mainly attributed to the effective carrier confinement in the boost layer with high mobility, low free carrier density of the base layer with a low V O concentration, and HfO2-induced high effective capacitance.

Effect of HfO2-Based Multi-Dielectrics on Electrical Properties of Amorphous In-Ga-Zn-O Thin Film Transistors

“…The µ sat of the device is larger (33.76 cm 2 V −1 s −1 ), but the I on /I off (1.96 × 10 7 ) and ∆V th (0.3 V) are not as good as our results [24]. Min Hoe Cho et al reported a bilayer IGZO TFT, consisting of a 10 nm base layer (In 0.52 Ga 0.29 Zn 0.19 O) with good stability and a 3 nm boost layer (In 0.82 Ga 0.08 Zn 0.10 O) with extremely highest mobility (74.0 ± 0.91 cm 2 V −1 s −1 ) [13]. As mentioned above, rich a-IGZO films will produce uncontrollable conductivity in the channel layer so that the film mobility is greatly improved.…”

“…The ∆V th and ∆SS are 0.09 V and 0.057 V/dec after 10 4 s PBST, respectively, illustrating the long-term stability, excellent electrical characteristics device for a wide perspective application. [13] 0.2 74.0 3 × 10 8 0.17 1.9 × 10 11 0.66…”

“…The reduction of TFT driving voltage, that is to say, using high dielectric constant (high k) materials, is the most effective way to realize lower power consumption of OLED. High k materials such as HfO 2 [13], Al 2 O 3 [14], Y 2 O 3 [15] can reduce the equivalent oxide layer thickness (EOT). However, high-k dielectrics have some inherent defects, such as high leakage current, poor surface morphology, high trap charge density, and these defects will deteriorate the device characteristics and degrade the long-term stability.…”

15.2: Invited Paper: Atomic Layer Deposited Oxide Semiconductor Enabling High Mobility and Low Driving Voltage in TFTs

“…[9] Finally, the high-mobility, low-driving-voltage IGZO TFTs by adopting bilayer channel and hafnia-based gate dielectric stack were investigated. [10] Figure 1. Required field-effect mobility to satisfy the trend towards ultra-high-definition and high frame rate driving in the next generation flat-panel displays.…”

“…Summary of the electrical parameters of the base-layer IGZO TFTs with different gate insulator. Data adapted from reference[10].…”