Fermi-level depinning in metal-2D multilayered semiconductor junctions

TL;DR

This paper investigates how the Fermi-level pinning in metal-2D semiconductor junctions depends on the number of layers, revealing a depinning mechanism that influences contact properties and band alignment, with implications for electronics and optoelectronics.

Contribution

The authors develop an extended Fermi-level pinning theory for multilayered 2D semiconductor junctions, explaining layer-dependent depinning and its effects on device characteristics.

Findings

Pinning strength varies with layer number in 2D semiconductor junctions.

Depinning occurs between layers, weakening overall pinning.

Layer-dependent depinning enables p-type contacts and type II band alignment.

Abstract

Thicknesses-dependent performance of metal-two-dimensional (2D) semiconductor junctions (MSJ) in electronics/optoelectronics have attracted increasing attention, but till present, people have little knowledge about the micro-mechanism of the thicknesses (or layer-number) dependence. Here, by first-principles calculations based on density functional theory, we show that the Fermi-level pinning (FLP) factor of MSJ depends sensitively on the layer-number of few-layer 2D semiconductors, and, an extended FLP theory is proposed for metal-2D multilayered semiconductor junctions (MmSJ). Taking multilayered MoS2 as a typical example for van der Waals (vdW) semiconductor in MmSJ, the extended FLP theory has the following character: strong pinning right at the metal-1st-layer semiconductor interface while depinning occurs between MoS2 layers. This depinning effect between vdW layers has several important consequences: 1) the overall pinning in MmSJ is greatly weakened; 2) p-type contact, rarely obtained in metal-monolayer MoS2 junctions, becomes favored in MmSJ, which is important for CMOS logical devices; 3) depinning between MoS2 layers result in type II band alignment in MoS2 'homojunction' supported on metals, which is useful for optoelectronics. Moreover, our extended FLP theory sheds light on the recent controversial experimental observation and paves a new and universal route to type II band alignment in vdW 'homojunctions'.