Quantum Confinement Effect in Amorphous In-Ga-Zn-O Heterojunction Channels for Thin-Film Transistors.

Daichi Koretomo,Shuhei Hamada,Yusaku Magari,Mamoru Furuta

doi:10.3390/ma13081935

Abstract

Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (ΔEc) of ~0.4 eV. A depth profile of background charge density indicated that a steep ΔEc is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (μFE) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm2 V−1 s−1, although a channel/gate insulator interface was formed by an IGZO−111 (μFE = ~12 cm2 V−1 s−1). Device simulation analysis revealed that the improvement of μFE in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ΔEc. Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.

Highlights

Thin-film transistors (TFTs) based on oxide semiconductors (OSs) have attracted considerable attention for generation flat-panel displays (FPDs) due to their advantages such as high field effect mobility, steep subthreshold swing, and low leakage current [1,2,3,4,5,6,7,8,9]
The drain current densities at the IGZO-111/GI interface reduced and that at the heterojunction interface increased with increasing the ∆Ec. These results indicate that the carrier transport in the hetero-IGZO-TFTs changed from the IGZO-111/GI interface to the heterojunction interface owing to a quantum confinement effect for electrons when the ∆Ec was formed at the heterojunction interface
The experimental and device simulation results clarified that the improvement of μFE in the hetero-IGZO TFTs was mainly caused by a quantum confinement effect for electrons, which was induced by ∆Ec at the heterojunction interface

Summary

Introduction

Thin-film transistors (TFTs) based on oxide semiconductors (OSs) have attracted considerable attention for generation flat-panel displays (FPDs) due to their advantages such as high field effect mobility (μFE ), steep subthreshold swing, and low leakage current [1,2,3,4,5,6,7,8,9]. Various types of heterojunction channels, using amorphous OSs such as IGZO/In–Zn–O (IZO), Hf–In–Zn–O/IZO, Zn–Sn–O/IZO and Al–In–Zn–Sn–O/IZO, have been proposed for TFTs [20,21,22,23,24] These heterojunction TFTs exhibited excellent μFE of over 30 cm V−1 s−1. The heterojunction channels are considered to be an appropriate approach to boost TFT performances even for amorphous OSs. the carrier transport properties in the heterojunction channel have not been discussed in detail, despite a key for the improvement of the μFE on the amorphous OS TFTs. We previously reported about the IGZO TFT with a heterojunction channel consisting of different compositions of the amorphous IGZO films. Device simulation analysis revealed that the ∆Ec , which acts as a potential barrier for electrons, strongly affects carrier transport paths in the heterojunction channel, which leads to an improvement in the μFE of the TFTs

Experimental Methods

Crystallinity of IGZO films

Band Alignment and Steepness at Heterojunction Interface

GS required to

Device Simulation

Conclusion