Stoichiometry, band alignment and electronic structure of Eu_2O_3 thin films studied by direct and inverse photoemission: A reevaluation of the electronic band structure

摘要

The electronic structure of Eu sesquioxide (Eu_2O_3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu_2O_3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu~(2+) are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O_2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu_2O_3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu_2O_3/GaN interface is discussed.