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In this study we employ Density Functional Theory (DFT) methods toinvestigate the surface energy barrier for electron emission (surface barrier)and thermodynamic stability of Ba and Ba-O species adsorption under conditionsof high temperature (approximately 1200 K) and low pressure (approximately$10^{-10}$ Torr) on the low index surfaces of bixbyite $Sc_2O_3$. The role ofBa in lowering the cathode surface barrier is investigated via adsorption ofatomic Ba and Ba-O dimers, where the highest simulated dimer coveragecorresponds to a single monolayer film of rocksalt BaO. The change of thesurface barrier of a semiconductor due to adsorption of surface species isdecomposed into two parts: a surface dipole component and doping component. Thelowest surface barrier with atomic Ba on $Sc_2O_3$ was found to be 2.12 eV and2.04 eV for the (011) and (111) surfaces at 3 and 1 Ba atoms per surface unitcell (0.250 and 0.083 Ba per surface O), respectively. The lowest surfacebarrier for Ba-O on $Sc_2O_3$ was found to be 1.21 eV on (011) for a 7 Ba-Odimer-per-unit-cell coverage (0.583 dimers per surface O). Generally, we foundthat Ba in its atomic form on $Sc_2O_3$ surfaces is not stable relative to bulkBaO, while Ba-O dimer coverages between 3 to 7 Ba-O dimers per (011) surfaceunit cell (0.250 to 0.583 dimers per surface O) produce stable structuresrelative to bulk BaO. Ba-O dimer adsorption on $Sc_2O_3$ (111) surfaces wasfound to be unstable versus BaO over the full range of coverages studied.Investigation of combined n-type doping and surface dipole modification showedthat their effects interact to yield a reduction less than the twocontributions would yield separately.

Electron Emission Energy Barriers and Stability of $Sc_2O_3$ with Adsorbed Ba and Ba-O