Searching for Materials with High Refractive Index and Wide Band Gap: A First-Principles High-Throughput Study

TL;DR

This study uses first-principles high-throughput calculations on over 4000 semiconductors to explore the inverse relationship between refractive index and band gap, identifying outliers and potential ways to optimize both properties for optical applications.

Contribution

It provides a comprehensive analysis of the inverse correlation between refractive index and band gap, introduces a simple model with key descriptors, and investigates chemical influences on this relationship.

Findings

Inverse trend between refractive index and band gap confirmed

Outliers with high refractive index and wide band gap identified

Strategies to counterbalance the inverse relationship suggested

Abstract

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse correlation with high refractive index appearing in small gap materials and vice-versa. Here, we conduct a first-principles high-throughput study on more than 4000 semiconductors (with a special focus on oxides). Our data confirm the general inverse trend between refractive index and band gap but interesting outliers are also identified. The data are then analyzed through a simple model involving two main descriptors: the average optical gap and the effective frequency. The former can be determined directly from the electronic structure of the compounds, but the latter cannot. This calls for further analysis in order to obtain a predictive model. Nonetheless, it turns out that the negative effect of a large band gap on the refractive index can counterbalanced in two ways: (i) by limiting the difference between the direct band gap and the average optical gap which can be realized by a narrow distribution in energy of the optical transitions and (ii) by increasing the effective frequency which can be achieved through either a high number of transitions from the top of the valence band to the bottom of the conduction or a high average probability for these transitions. Focusing on oxides, we use our data to investigate how the chemistry influences this inverse relationship and rationalize why certain classes of materials would perform better. Our findings can be used to search for new compounds in many optical applications both in the linear and non-linear regime (waveguides, optical modulators, laser, frequency converter, etc.).