Dielectric function of CuBrxI1−x alloy thin films

We study the dielectric function of ${\mathrm{CuBr}}_{x}{\mathrm{I}}_{1\ensuremath{-}x}$ thin film alloys using spectroscopic ellipsometry in the spectral range between 0.7 eV and 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 $\mathbf{k}$-points in the Brillouin zone. The observed band gap 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.