Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation

TL;DR

This study uses first-principles calculations to analyze how crystal symmetry, chemical composition, and doping influence the electronic and optical properties of complex transparent conducting oxides, revealing pathways to optimize their performance.

Contribution

It provides a detailed theoretical analysis of factors affecting TCO properties and demonstrates how multicomponent oxides and aliovalent doping can enhance conductivity and transparency.

Findings

Crystal symmetry impacts transport and optical properties.

Multicomponent oxides can overcome electron localization.

Doping with transition metals improves conductivity without losing transparency.

Abstract

The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In$_2$O$_3$, $\alpha$- and $\beta$-Ga$_2$O$_3$, rock salt and wurtzite ZnO, and layered InGaZnO$_4$ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s) -- $s$, $p$ or $d$ -- which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p-states of the neighboring oxygen atoms. The results not only explain the experimentally observed trends in the electrical conductivity in the single-cation TCO, but also demonstrate that multicomponent oxides may offer a way to overcome the electron localization bottleneck which limits the charge transport in wide-bandgap main-group metal oxides. Further, the advantages of aliovalent substitutional doping -- an alternative route to generate carriers in a TCO host -- are outlined based on the electronic band structure calculations of Sn, Ga, Ti and Zr-doped InGaZnO$_4$. We show that the transition metal dopants offer a possibility to improve conductivity without compromising the optical transmittance.