  Van der Waals materials exhibit naturally passivated surfaces and can form
versatile heterostructures, enabling observation of carrier transport
mechanisms not seen in three-dimensional materials. Here we report observation
of a "band bending junction", a new type of semiconductor homojunction whose
surface potential landscape depends solely on a difference in thickness between
the two semiconductor regions atop a buried heterojunction interface. Using
MoS2 on Au to form a buried heterojunction interface, we find that lateral
surface potential differences can arise in MoS2 from the local extent of
vertical band bending in thin and thick MoS2 regions. Using scanning ultrafast
electron microscopy, we examine the spatiotemporal dynamics of photogenerated
charge carriers and find that lateral carrier separation is enabled by a band
bending junction, which is confirmed with semiconductor transport simulations.
Band bending junctions may therefore enable new electronic and optoelectronic
devices in Van der Waals materials that rely on thickness variations rather
than doping to separate charge carriers.
