posted on 2021-01-15, 20:13authored byAndrew
M. Ochs, Prashun Gorai, Yaxian Wang, Michael R. Scudder, Karl Koster, Curtis E. Moore, Vladan Stevanovic, Joseph P. Heremans, Wolfgang Windl, Eric S. Toberer, Joshua E. Goldberger
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.