Electronic Structure and Band Alignment of LaMnO3/SrTiO3 Polar/Nonpolar Heterojunctions

The behavior of polar LaMnO3 (LMO) thin films deposited epitaxially on nonpolar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X‐ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built‐in electric field in LMO films of any thickness is found. Measurements of the Mn valence by Mn L‐edge X‐ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanning transmission electron microscopy images reveal that Mn2+ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface. The mechanism that alleviates the polar catastrophe of LaMnO3/SrTiO3 heterojunctions is revealed using in situ and ex situ spectroscopies. The valence band offset is much larger than previously theorized, which impacts carrier confinement. Mn2+ is observed on the LaMnO3 surface, indicating that oxygen vacancies have formed. These oxygen vacancies compensate the built‐in electric field arising from the polar catastrophe.