Indium-Free Amorphous Ca-Al-O Thin Film as a Transparent Conducting Oxide

Transparent conducting oxide (TCO) is a promising material system for transparent electrodes, which is one of the most essential elements in current electronic and energy devices. In particular, In-based TCOs such as Sn:In₂O₃ (ITO) and In-Ga-Zn-O (IGZO) have shown large optical band gap and high electrical conductivity, sufficient for the applications. However, In is an expensive element, which hampers its facile industrial application. Moreover, In-based TCOs show an abrupt decrease in conductivity when their thickness decreases below ∼100 nm, possibly due to inhomogeneity within the thin films. Here, we introduce an amorphous Ca-Al-O (CAO) thin film as a promising candidate for the In-free TCOs. The amorphous CAO thin film has very low resistivity (∼10^-5 ω cm) at room temperature, as well as high transparency in the visible region of the light spectrum (>80%). The isotropic Ca s-orbital in the conduction band is found to be responsible for the superior performance of CAO as a TCO. Owing to the exceptional structural homogeneity of the CAO thin film, thickness-independent transport characteristics are observed, maintaining its TCO performance down to 10 nm of thickness.