Nanoscale Origin of Strong Charge Carrier Scattering at Grain Boundaries in Orthorhombic SnSe Semiconductor Thin Films

Tin monoselenide (SnSe) is one of the high-performance thermoelectric materials, and recent progress has been made in its application to thin-film thermoelectric devices. However, the performance of SnSe polycrystalline and epitaxial films has been limited by their electronic conductivity being lower than the single crystal due to strong carrier scattering. In this paper, we investigated the origin of carrier scattering through nanoscale characterization of a -axis oriented SnSe films with different grain boundary (GB) structures (i.e., a -axis misorientations and in-plane rotational GBs). The a -axis oriented SnSe polycrystalline films grown on glass substrates and epitaxial films on MgO (001) substrates show p-type conduction. In-grain carrier mobility increases with the increased grain size and improved out-of-plane a -axis orientation at higher growth temperatures, but the hole transport in the films is dominated by GB scattering. For SnSe films with out-of-plane misorientation angles larger than 5°, GBs form potential barriers with a height ( E b ) of up to 62 meV and significantly reduce carrier mobility (μ). Meanwhile, when the out-of-plane misorientation angles are less than 5°, the effect of a -axis orientation fluctuations on μ becomes small, but the room-temperature μ is limited to ∼50 cm 2 /(Vs), regardless of the different in-plane misorientations. It is found that (100) SnSe epitaxial films on a MgO (001) substrate form 90° rotational domains composed of in-plane [011] and [01-1] oriented grains and have GB potential barriers with E b = 30 meV, which is lower than those in the a -axis misoriented SnSe polycrystalline films but still deteriorates μ. Density functional theory calculations show that the 90° rotational domain boundary causes a shift of SnSe molecular layers between the [011] and [01-1] oriented grains, and the distorted Sn–Se 3 polyhedra form hole trap states above the fundamental valence band maximum, which can explain the formation of the GB potential barriers. Controlling the crystallite orientation and size is a critical factor to realize high μ SnSe films for thermoelectric device applications.