  Van der Waals (vdW) heterojunctions composed of 2-dimensional (2D) layered
materials are emerging as a solid-state materials family that exhibit novel
physics phenomena that can power high performance electronic and photonic
applications. Here, we present the first demonstration of an important building
block in vdW solids: room temperature (RT) Esaki tunnel diodes. The Esaki
diodes were realized in vdW heterostructures made of black phosphorus (BP) and
tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap
energy band offset. The presence of a thin insulating barrier between BP and
SnSe2 enabled the observation of a prominent negative differential resistance
(NDR) region in the forward-bias current-voltage characteristics, with a peak
to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence
of the NDR indicates electron tunneling being the dominant transport mechanism,
and a theoretical model shows excellent agreement with the experimental
results. Furthermore, the broken-gap band alignment is confirmed by the
junction photoresponse and the phosphorus double planes in a single layer of BP
are resolved in transmission electron microscopy (TEM) for the first time. Our
results represent a significant advance in the fundamental understanding of vdW
heterojunctions, and widen the potential applications base of 2D layered
materials.
