An Inside Look at the Ti-MoS2 Contact in Ultra-thin Field Effect Transistor with Atomic Resolution

TL;DR

This study uses atomic-resolution microscopy and first-principles calculations to reveal fundamental bonding limitations at the Ti-MoS2 interface in ultra-thin FETs, impacting contact resistance and device performance.

Contribution

It provides atomic-level insights into the metal-MoS2 interface, showing that bonding interactions fundamentally limit contact quality, which cannot be improved by deposition techniques alone.

Findings

Strong Ti-S bonds cause sulfur release from MoS2

Formation of Ti/TixSy clusters damages MoS2 layers

Optimal metal selection based on bonding interactions is necessary

Abstract

Two-dimensional molybdenum disulfide (MoS2) is an excellent channel material for ultra-thin field effect transistors. However, high contact resistance across the metal-MoS2 interface continues to limit its widespread realization. Here, using atomic-resolution analytical scanning transmission electron microscopy (STEM) together with first principle calculations, we show that this contact problem is a fundamental limitation from the bonding and interactions at the metal-MoS2 interface that cannot be solved by improved deposition engineering. STEM analysis in conjunction with theory shows that when MoS2 is in contact with Ti, a metal with a high affinity to form strong bonds with sulfur, there is a release of S from Mo along with the formation of small Ti/TixSy clusters. A destruction of the MoS2 layers and penetration of metal can also be expected. The design of true high-mobility metal-MoS2 contacts will require the optimal selection of the metal or alloy based on their bonding interactions with the MoS2 surface. This can be advanced by evaluation of binding energies with increasing the number of atoms within metal clusters.