Computationally Guided Discovery of Axis-Dependent Conduction Polarity in NaSnAs Crystals

Most electronic materials exhibit
a single dominant charge carrier
type, either holes or electrons, along all crystallographic directions.
However, there are a small number of compounds, mostly metals, that
exhibit simultaneous p-type and n-type conduction behavior along different
crystallographic directions. We demonstrate that the experimental
discovery of semiconductors with this axis-dependent conduction polarity
can be facilitated by identifying a large anisotropy of either the
electron or hole effective masses (m*) or both, providing
the electron and hole masses dominate along different crystallographic
directions. We calculated the layered semiconductor NaSnAs to have
a lower electron m* in-plane than the cross-plane
and a very large hole m* in-plane and small hole m* cross-plane. We established the growth of >3 mm-sized
NaSnAs crystals via Sn flux and confirmed the band gap to be 0.65
eV, in agreement with theory. NaSnAs exhibits p-type thermopowers
cross-plane and n-type thermopowers in-plane, confirming that the
large anisotropy in the effective mass at the band edges is an excellent
indicator for axis-dependent conduction polarity. Overall, this work
shows that the discovery of semiconductors with such a phenomenon
can be accelerated by computationally evaluating the anisotropic curvatures
of the band edges, paving the way for their future discovery and application.