# Limitations of In$_2$O$_3$ as a transparent conducting oxide

## TL;DR

This study uses first-principles calculations to analyze the fundamental limitations of In$_2$O$_3$ as a transparent conducting oxide, focusing on free-carrier absorption mechanisms and their dependence on phonons and charged defects.

## Contribution

It provides a detailed theoretical investigation of the absorption processes limiting In$_2$O$_3$ transparency, highlighting the roles of phonons, charged defects, and screening effects at high doping levels.

## Key findings

- Phonon-assisted absorption mainly involves phonon emission, making it temperature-insensitive.
- Charged defect-assisted absorption dominates at impurity concentrations above 10$^{20}$ cm$^{-3}.
- The wavelength dependence follows a power law, influenced by band-structure effects.

## Abstract

Sn-doped In$_2$O$_3$ or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 10$^{20}$ cm$^{-3}$. The differences in the photon-energy dependence of the two processes can be explained by band-structure effects.