Evidence for the Eu 4f Character of Conduction-Band Edge at the Eu2O3 Surface Studied by Scanning Tunneling Spectroscopy

•Stoichiometric Eu2O3 requires annealing at 600 °C in O2 after exposure in air.•The conduction-band edge of Eu2O3 is formed by Eu 4f rather than 6s5d states.•The bottom of unoccupied Eu 5d6s band lies 2.3-2.6 eV higher than that of Eu 4f.•Origin of states can be identified by variation of tip-sample distance in STS. Effects of localized Eu 4f levels on the band gap properties of Eu2O3 have attracted significant fundamental and technological interest, and the band structure of such thin films has been thoroughly studied by photoelectron spectroscopies (T. Hadamek et al., J. Appl. Phys. 127 (2020) 074101). Here we apply a scanning tunneling spectroscopy (STS) to clarify the character of the conduction band (CB) bottom at the surface of epitaxial Eu2O3 grown on GaN(0001)/Si(111) substrates. It is shown that the CB edge is formed solely by an unoccupied Eu 4f state 0.8 eV above the Fermi level at the Eu2O3 surface and does not overlap with unoccupied Eu 5d6s states laying more than 2 eV higher than the bottom of the 4f band. [Display omitted]