  Beta-BaZn2As2 is known to be a p-type semiconductor with the layered crystal
structure similar to that of LaZnAsO, leading to the expectation that
beta-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of
beta-BaZn2As2 (previously-reported value ~0.2 eV) is one order of magnitude
smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap
value of beta-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of
the tem-perature dependence of electrical conductivity. The origins of this
narrow bandgap are discussed based on the chemi-cal bonding nature probed by 6
keV hard X-ray photoemission spectroscopy, hybrid density functional
calculations, and the ligand theory. One origin is the direct As-As
hybridization between adjacent [ZnAs] layers, which leads to a secondary
splitting of As 4p levels and raises the valence band maximum. The other is
that the non-bonding Ba 5dx2-y2 orbitals form unexpectedly deep conduction band
minimum (CBM) in beta-BaZn2As2 although the CBM of LaZnAsO is formed mainly of
Zn 4s. These two origins provide a quantitative explanation for the bandgap
difference between beta-BaZn2As2 and LaZnAsO.
