Fundamental absorption edges in heteroepitaxial YBiO$_3$ thin films

The dielectric function of heteroepitaxial YBiO$_3$ grown on $a$-Al$_2$O$_3$ single crystals via pulsed laser deposition is determined in the spectral range from 0.03 eV to 4.5 eV by simultaneous modeling of spectroscopic ellipsometry and optical transmission data of YBiO$_3$ films of different thickness. The (111)-oriented YBiO$_3$ films are nominally unstrained and crystallize in a defective fluorite-type structure with $Fm\bar{3}m$ space group. From the calculated absorption spectrum, a direct electronic bandgap energy of 3.6(1) eV and the signature of an indirect electronic transition around 0.5 eV are obtained. These values provide necessary experimental feedback to previous conflicting electronic band structure calculations predicting either a topologically trivial or non-trivial insulating ground state in YBiO$_3$.