Tailoring Subthreshold Swing in A-IGZO Thin-Film Transistors for Amoled Displays: Impact of Conversion Mechanism on Peald Deposition Sequences

Abstract

Amorphous IGZO (a-IGZO) thin-film transistors (TFTs) are standard backplane electronics to power active-matrix organic light-emitting diode (AMOLED) televisions due to their high carrier mobility and negligible low leakage characteristics. Despite their advantages, limitations in color depth arise from a steep subthreshold swing (SS) (≤ 0.1 V/decade), necessitating costly external compensation for IGZO transistors. For mid-size mobile applications such as OLED tablets and notebooks, it is important to ensure controllable SS value (≥ 0.3 V/decade). In this study, a conversion mechanism during plasma-enhanced atomic layer deposition (PEALD) is proposed as a feasible route to control the SS. When a pulse of a diethylzinc (DEZn) precursor is exposed to the M₂ O₃ (M = In or Ga) surface layer, partial conversion of the underlying M₂ O₃ to ZnO is predicted on the basis of density function theory calculations. Notably, significant distinctions between In-Ga-Zn (Case I) and In-Zn-Ga (Case II) films are observed: Case II exhibits a lower growth rate and larger Ga/In ratio. Case II TFTs with a-IGZO (subcycle ratio of In:Ga:Zn = 3:1:1) show reasonable SS values (313 mV decade^-1 ) and high mobility (µ_FE ) of 29.3 cm2 Vs^-1 (Case I: 84 mV decade^-1 and 33.4 cm² Vs^-1 ). The rationale for Case II's reasonable SS values is discussed, attributing it to the plausible formation of In-Zn defects, supported by technology computer-aided design (TCAD) simulations.

Keywords: TCAD simulation; atomic layer deposition; conversion mechanism; defect mechanism; density functional theory; field-effect transistor; indium gallium zinc oxide; precursor.