Electron Emission Energy Barriers and Stability of Sc2O3 with Adsorbed Ba and Ba-O

In this study we employ density functional theory (DFT) methods to investigate the surface energy barrier for electron emission (surface barrier) and thermodynamic stability of Ba and Ba-O species adsorption (relative to formation of bulk BaO) under conditions of high temperature (approximately 1200 K) and low pressure (approximately 10(-10) Ton) on the low-index surfaces of bixbyite Sc2O3. We employ both the standard generalized gradient approximation (GGA) and the hybrid HSE functional to calculate accurate surface barriers from relaxed GGA structures. The role of Ba in lowering the cathode surface barrier is investigated via adsorption of atomic Ba and Ba-O dimers, where the highest simulated dimer coverage corresponds to a single-monolayer film of rocksalt BaO. The change of the surface barrier of a semiconductor due to adsorption of surface species is decomposed into two parts: a surface dipole component and a doping component. The dipole component is the result of charge rearrangement at the surface and described by the electrostatic Helmholtz equation. The doping component is due to charge transfer from the surface species, which changes the Fermi level and thereby changes the surface barrier. Different initial geometries, adsorption sites, and coverages were tested for the most stable low-index Sc2O3 surfaces ((011) and (111)) for both atomic Ba and Ba-O dimers. The lowest surface barrier with atomic Ba on Sc2O3 was found to be 2.12 and 2.04 eV for the (011) and (111) surfaces at 3 and 1 Ba atoms per surface unit cell (0.250 and 0.083 Ba per surface O), respectively. The lowest surface barrier for Ba-O on Sc2O3 was found to be 1.21 eV on (011) for a 7 Ba-O dimer-per-unit-cell coverage (0.583 dimers per surface O). Generally, we found that Ba in its atomic form on Sc2O3 surfaces is not stable relative to bulk BaO, while Ba-O dimer coverages between 3 and 7 Ba-O dimers per (011) surface unit cell (0.250 and 0.583 dimers per surface O) produce stable structures relative to bulk BaO. Ba-O dimer adsorption on Sc2O3 (111) surfaces was found to be unstable versus BaO over the full range of coverages studied. Investigation of combined n-type doping and surface dipole modification showed that their effects interact to yield a reduction less than the two contributions would yield separately.