Analysis of Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors with Bi-Layer Gate Dielectric Stacks Using Maxwell-Wagner Instability Model

Abstract

Aluminium oxide (Al2O3) and hafnium oxide (HfO2) have been grown, using atomic layer deposition (ALD), as single and bi-layer gate dielectric films. Electrical and structural characterisation indicates that the material properties depend on layer thickness and growth order, when deposited as bi-layers. Charge trapping at the interface between the bi-layer stacks results from the Maxwell-Wagner (MW) instability, which states a difference of conductivities at the dielectric-dielectric interface with respect to the bulk dielectric films. Hence a build-up of this interface charge compensates for the condition that the current densities of individual dielectric films match that of the overall stack. This causes electrical instabilities in stack behaviour. Bottom-gate amorphous indium-gallium-zinc-oxide thin-film transistors are fabricated with these gate oxides, single and bi-layer stacks, with low VTH and gate leakage current to understand this instability. By empirically adopting ideal MOSFET equations and single gate dielectric film behaviour as a reference, this MW instability in bi-layer gate dielectric stacks has been accounted for in amorphous indium-gallium-zinc-oxide thin-film transistor transfer characteristics with a good fit in both above threshold and sub-threshold regimes.