Dimensionality-Reduced Fermi Level Pinning in Coplanar 2D Heterojunctions

Electronic transport through a metal|semiconductor (M vertical bar S) heterojunction is largely determined by its Schottky barrier. In 3D M vertical bar S junctions, the barrier height determines the turn-on voltage and is often pinned by the interface states, causing Fermi level pinning (FLP). The pinning strength in 3D depends on the ratio C-i/C-M between the interface quantum capacitance C-i and the metal surface capacitance C-M. In 2D, the interface dipole does not influence the band alignment, but still affects the Schottky barrier and transport. In light of the general interest in building 2D electronics, in this work we discover the relevant material parameters which dictate the behavior and strength of FLP in 2D M vertical bar S contacts. Using a multiscale model combining first-principles, continuum electrostatics, and transport calculations, we study a realistic Gr vertical bar MoS2 interface as an example with high interface state density (C-i/C-M >> 1). Transport calculations show partial pinning with a strength P similar to 0.6, while a 3D junction with similar heterogeneity gives full pinning with P = 1 as expected. We further show that in 2D M vertical bar S contacts the turn-on voltage and pinning strength are affected by a physical parameter l/lambda(D), the ratio between the interface width l, and the thermal de Broglie wavelength lambda(D). Pinning is absent for ideal line-contacts (l/lambda(D) = 0), but increases for realistic l/lambda(D) values.

Automated summary

The electronic transport through a metal|semiconductor heterojunction is mainly determined by the Schottky barrier, with the pinning strength in 3D structures depending on the ratio between interface quantum capacitance and metal surface capacitance. In 2D structures, the interface dipole does not affect band alignment but influences the Schottky barrier and transport. The turn-on voltage and pinning strength in 2D contacts are affected by the physical parameter l/lambda(D), the ratio between interface width and thermal de Broglie wavelength.