  Group-IV monochalcogenides are emerging as a new class of layered materials
beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus
(BP). In this paper, we report experimental and theoretical investigations of
the band structure and transport properties of GeSe and its heterostructures.
We find that GeSe exhibits a markedly anisotropic electronic transport, with
maximum conductance along the armchair direction. Density functional theory
calculations reveal that the effective mass is 2.7 times larger along the
zigzag direction than the armchair direction; this mass anisotropy explains the
observed anisotropic conductance. The crystallographic orientation of GeSe is
confirmed by angleresolved polarized Raman measurements, which are further
supported by calculated Raman tensors for the orthorhombic structure. Novel
GeSe/MoS2 p-n heterojunctions are fabricated, combining the natural p-type
doping in GeSe and n-type doping in MoS2. The temperature dependence of the
measured junction current reveals that GeSe and MoS2 have a type-II band
alignment with a conduction band offset of ~0.234 eV. The anisotropic
conductance of GeSe may enable the development of new electronic and
optoelectronic devices, such as high-efficiency thermoelectric devices and
plasmonic devices with resonance frequency continuously tunable through light
polarization direction. The unique GeSe/MoS2 p-n junctions with type-II
alignment may become essential building blocks of vertical tunneling
field-effect transistors for low-power applications. The novel p-type layered
material GeSe can also be combined with n-type TMDCs to form heterogeneous
complementary metal oxide semiconductor (CMOS) circuits.
