Fabrication, Photoluminescence, and Electrical Measurements of Molybdenum DiSulphide Films and Heterostructures

Abstract

Transition-metal dichalcogenides (TMDs) exhibit unique optical and electronic properties that make them highly appealing to the scientific community. Like graphene, they have strong in-plane bonds allowing for easy fabrication of complex single layer structures or molecular heterostructures. In particular, MoS2 is a TMD semiconductor that displays emerging photoluminescence (PL) as it transitions from an indirect band gap with a bulk thickness to a direct band gap at few- to mono-layer thickness. Its potential to be used in novel nanoelectronics and optical devices necessitate a better understanding of its excitonic properties. One issue is the reported variability of PL and electron mobility in MoS2; we aimed to investigate influential factors affecting our 2D material and to optimize our devices. We also study the effects of hexagonal boron nitride (h-BN) encapsulation on MoS2 membranes as they are reported to mitigate surface roughness, increase PL, and protect against degradation. In the current work, we report our exfoliation and membrane transfer technique: a modified mask aligner and dry PDMS transfer for assembly of heterostructures. We developed simple optical methods to quantify layer number, which we correlated with Raman spectroscopy, Atomic force microscopy, and PL spectrum analysis. We then compared layer number, h-BN encapsulated samples, and exfoliated vs. grown MoS2 via chemical vapour deposition (CVD). We found red shifted PL peaks with increasing layer number and in CVD-grown MoS2, indicative of fewer defects in exfoliated devices. Also, PL intensity increased with decreasing layer number. We further discuss the lithographic process used to integrate monolayer samples into a bottom-gated field effect transistor, followed by their field effect mobility and resistance trends.