  Group-IV monochalcogenides have recently shown great potential for their
thermoelectric, ferroelectric, and other intriguing properties. The electrical
properties of group-IV monochalcogenides exhibit a strong dependence on the
chalcogen type. For example, GeTe exhibits high doping concentration, whereas
S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we
investigate the electrical and thermoelectric properties of gamma-GeSe, a
recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical
conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at
room temperature) owing to its high p-doping level (5x1021 cm-3), which is in
stark contrast to other known GeSe polymorphs. Elemental analysis and
first-principles calculations confirm that the abundant formation of Ge
vacancies leads to the high p-doping concentration. The magnetoresistance
measurements also reveal weak-antilocalization because of spin-orbit coupling
in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph
in which the modified local bonding configuration leads to substantially
different physical properties.
