Evolution of Metastable Defects and Its Effect on the Electronic Properties of MoS2 Films

M Precner,C Di Giorgio,I Cone,G Karapetrov,Y Zhu,T Polaković,X X Xi,D J Trainer,Qiao Qiao,A V Putilov,M Iavarone

doi:10.1038/s41598-018-24913-y

Abstract

We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS2 films. Scanning tunneling microscopy, Kelvin probe force microscopy, and transmission electron microscopy were used to obtain high resolution images and quantitative measurements of the local density of states, work function and nature of defects in MoS2 films. We track the evolution of defects that are formed under heating and electron beam irradiation. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The experiments show that sulfur vacancies formed after exposure to elevated temperatures diffuse, coalesce, and migrate bringing the system from a metastable to equilibrium ground state. The process could be thermally or e-beam activated with estimated energy barrier for sulfur vacancy migration of 0.6 eV in single unit cell MoS2. Even at equilibrium conditions, the work function and local density of states values are strongly affected near grain boundaries and edges. The results provide initial estimates of the thermal budgets available for reliable fabrication of MoS2-based integrated electronics and indicate the importance of defect control and layer passivation.

Highlights

Transition metal dichalcogenides (TMDs) and other 2D materials are currently receiving wide attention from the scientific community due to their unique physical[1] and chemical[2] properties
As the metastable surface potentials are generated in conditions that are common in device fabrication, their presence might be considered detrimental to the stable performance of MoS2-based electronic devices
In our case we investigate mono- and few-layer single crystalline MoS2 films grown on highly ordered pyrolytic graphite (HOPG) using chemical vapor deposited (CVD) method

Summary

Introduction

Transition metal dichalcogenides (TMDs) and other 2D materials are currently receiving wide attention from the scientific community due to their unique physical[1] and chemical[2] properties. Changes of surface structure and tunneling spectra was observed at edges and grain boundaries with presence of mid-gap electronic states[23,24] This led to observation of both p-type and n-type doping with varying contact resistance in molybdenite and synthetic MoS212,15,16,19,22. Prolonged exposure to electron beam irradiation in ultra-high vacuum conditions has been shown to cause complete dissociation of MoS2, with sulfur atoms evaporating from the material, decoupling the molybdenum sheet and allowing it to recrystallize into its native bcc crystal structure[28] Understanding of these processes in synthetically grown MoS2 is important for design of thin film MoS2 electronic devices, for device stability and the choice of material for contacts and gates[16], where homogeneity and predictable behavior of the work function and doping is crucial. The importance of passivation of MoS2 surface by different methods like chemical stabilization with thiol molecules[29] might be considered

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Results

Conclusion