ELECTRICAL AND THERMOELECTRIC PROPERTIES OF VAN DER WAALS HETEROSTRUCTURES

Liangliang Yang

Two-dimensional (2D) layered semiconductors, particularly the transition metal dichalcogenides (TMDs), have been extensively explored as the material of choice for applications in electronics and optics. However, the Metal-semiconductor junctions (MSJs) based on conventional metals and TMD layers suffer from the formation of gap states, as a result of strong interactions between metal and TMD surface atoms. This has motivated efforts in using 2D metals as the contact material that bond to 2D semiconductors via the van der Waals interaction, since this weak bonding is expected to minimize the gap states formation. Particularly, in vertically integrated 2D heterostructures, the interlayer van der Waals bonding removes the constraint of lattice matching, which is a necessary condition in conventional bulk junctions to minimize interface defects. In this work, we first demonstrate a van der Waals Schottky diode defined by the phase junction of multilayer In2Se3. Particularly, with the layered α (β) crystalline phase of In2Se3 showing semiconducting (metallic) characteristics, the vertical phase hetetrostucture (α – β) exhibits ideal diode behaviors with tunable current rectification. Most interestingly, an enhanced thermoelectric power (Seebeck coefficient) was observed in this phase junction, which is more than 3 orders of magnitude higher than that in single-phase multilayer and bulk In2Se3, with the thermoelectric figure-of-merit (ZT) at room temperature approaches ~ 1. Our findings suggest that the Schottky energy barrier, formed between van der Waals materials with the same composition and similar bonding nature, plays an important role in enhancing the thermoelectric performance. In the second work, we demonstrate and use an all-van der Waals vertical metal-semiconductor Schottky junction, consisting of semiconducting MoS2(top) and metallic -In2Se3 multilayers (bottom), to elucidate the role of lateral homojunctions. Our experiments directly visualize the presence of this lateral junction and show its significant impact on the electrical transport properties of the vertical structure. Besides, the impact of the lateral junction can be tuned by the backgate. These findings shed a new light on the electron transport process in vertical van der Waals junctions.

ELECTRICAL AND THERMOELECTRIC PROPERTIES OF VAN DER WAALS HETEROSTRUCTURES

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