Axis-dependent conduction polarity in crystals of air-stable semiconductor, PdSe2

Axis-dependent conduction polarity, or goniopolarity, is an electronic property of a small, but growing, group of materials, in which the charge polarity of conduction is different (p-type or n-type) depending on the direction of travel through the crystal. Most materials that exhibit goniopolarity are metals, and very few semiconducting materials with this effect exist. Here, we establish that PdSe₂, a 0.5 eV band gap semiconductor that is air- and moisture-stable exhibits goniopolarity. We show that single crystals with >5 mm scale dimensions are readily formed in Se flux. Lightly electron doped PdSe₂ exhibits p-type conduction in the cross-plane direction and n-type conduction in the in-plane directions, in agreement with DFT calculations. The cross-plane thermopower peaks at around 600 μV K^-1 near room temperature while the in-plane thermopower is -60 μV K^-1. Lightly p-doped samples do not show room-temperature goniopolarity. Instead, they show p-type thermopower in both the in-plane and cross-plane directions, where the thermopowers sit at 600 - 800 μV K^-1. However, at temperatures greater than about 400 K the goniopolarity again dominates the transport in these p-doped samples. Finally, we show that goniopolarity is sensitive to the ratio of carrier concentrations between holes and electrons and the ratios where goniopolarity can exist depend on the degree of effective mass anisotropy.