Dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ alloy thin films

TL;DR

This study combines spectroscopic ellipsometry and first-principles calculations to analyze the dielectric properties and electronic structure of CuBr$_x$I$_{1-x}$ alloy thin films, revealing how composition affects bandgap and spin-orbit splitting.

Contribution

It provides a comprehensive analysis of the dielectric function and electronic transitions in CuBr$_x$I$_{1-x}$ alloys, integrating experimental and theoretical approaches for the first time.

Findings

Bandgap bowing varies with alloy composition.

Spin-orbit splitting decreases with increasing Br concentration.

Electronic transitions are linked to specific k-points in the Brillouin zone.

Abstract

We study the dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ thin film alloys using spectroscopic ellipsometry in the spectral range between 0.7 eV to 6.4 eV, in combination with first-principles calculations based on density functional theory. Through the comparison of theory and experiment, we attribute features in the dielectric function to electronic transitions at specific k-points in the Brillouin zone. The observed bandgap bowing as a function of alloy composition is discussed in terms of different physical and chemical contributions. The band splitting at the top of the valence band due to spin-orbit coupling is found to decrease with increasing Br-concentration, from a value of 660 meV for CuI to 150 meV for CuBr. This result can be understood considering the contribution of copper d-orbitals to the valence band maximum as a function of the alloy composition.