Microscopic Quantum Transport Processes of Out-of-Plane Charge Flow in 2D Semiconductors Analyzed by a Fowler–Nordheim Tunneling Probe

Weak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out-of-plane charge flow, the information carrier in vdW-assembled vertical electronic and optical devices, from the in-plane band transport processes. Here, the out-of-plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few-layer SCTMDs, sequential layer-by-layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub-nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.