  Due to the chemical inertness of 2D hexagonal-Boron Nitride (h-BN), few
atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such
as Black Phosphorus (BP). However, the effects of h-BN on Schottky barrier
height, doping and contact resistance are not well known. Here, we investigate
these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co)
contacts. In sharp contrast to directly Co contacted p-type BP devices, we
observe strong n-type conduction upon insertion of the h-BN at the Co/BP
interface. First principles calculations show that this difference arises from
the much larger interface dipole at the Co/h-BN interface compared to the Co/BP
interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN
contacts exhibit low contact resistances (~ 4.5 k-ohm), and are Schottky
barrier free. This allows us to probe high electron mobilities (4,200 cm2/Vs)
and observe insulator-metal transitions even under two-terminal measurement
geometry.

Mesoscale and Nanoscale Physics