Quantum oscillation in carrier transport in two-dimensional junctions

Two-dimensional (2D) junction devices have recently attracted considerable attention. Here, we show that most 2D junction structures, whether vertical or lateral, act as a lateral monolayer-bilayer-monolayer junction in their operation. In particular, a vertical structure cannot function as a vertical junction as having been widely believed in the literature. Due to a larger electrostatic screening, the bilayer region in the junction always has a smaller bandgap than its monolayer counterpart. As a result, a potential well, aside from the usual potential barrier, will form universally in the bilayer region to affect the hole or electron quantum transport in the form of transmission or reflection. Taking black phosphorus as an example, our calculations using a non-equilibrium Green function combined with density functional theory show a distinct oscillation in the transmission coefficient in a two-electrode prototypical device, and the results can be qualitatively understood using a simple quantum well model.