Hole-induced electronic and optical transitions in L a1-x S rxFe O3 epitaxial thin films

We have investigated the electronic and optical properties of epitaxial La1-xSrxFeO3 for 0≤x≤1 prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O K-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a p-type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at x=1. Low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.