  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 under conditions
of high temperature (approximately 1200 K) and low pressure (approximately
$10^{-10}$ Torr) on the low index surfaces of bixbyite $Sc_2O_3$. 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 doping component. The
lowest surface barrier with atomic Ba on $Sc_2O_3$ was found to be 2.12 eV 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 $Sc_2O_3$ 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 $Sc_2O_3$ surfaces is not stable relative to bulk
BaO, while Ba-O dimer coverages between 3 to 7 Ba-O dimers per (011) surface
unit cell (0.250 to 0.583 dimers per surface O) produce stable structures
relative to bulk BaO. Ba-O dimer adsorption on $Sc_2O_3$ (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.
