Electronic Structure of Epitaxial Thin Films of the Transparent Conducting Oxide La:BaSnO$_3$ Measured By In-Situ Angle-Resolved Photoemission Spectroscopy

Lanthanum-doped barium stannate (La:BaSnO$_3$) is a transparent conducting oxide where single crystals have exhibited unusually high mobility and oxygen stability. Here we present in-situ angle-resolved photoemission (ARPES) measurements of La:BaSnO$_3$ epitaxial films that were co-deposited onto lattice-matched rare-earth scandate substrates by molecular-beam epitaxy (MBE). Density functional theory (DFT) calculations agree well with the observed valence bands and predict a parabolic conduction band. However, the features observed near the Fermi energy ($E_F$) are non-dispersive yet localized in momentum space. This unusual appearance may be the result of quasi-localized charge carriers or out-of-plane momentum broadening. Over long measurement periods, we also observe changes to the valence band and near-$E_F$ feature that bear a strong resemblance to the beam-induced two-dimensional electron gases previously reported in SrTiO$_3$ and KTaO$_3$. The origin of these unexpected phenomena and their relationship to the structural and transport properties of these films will be discussed.