Ambient effects on electrical characteristics of CVD-grown monolayer MoS2 field-effect transistors

Jae-Hyuk Ahn,William M Parkin,A T Charlie Johnson,Marija Drndić,Carl H Naylor

doi:10.1038/s41598-017-04350-z

Jae-Hyuk Ahn, William M Parkin + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-04350-z

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Abstract

Monolayer materials are sensitive to their environment because all of the atoms are at their surface. We investigate how exposure to the environment affects the electrical properties of CVD-grown monolayer MoS2 by monitoring electrical parameters of MoS2 field-effect transistors as their environment is changed from atmosphere to high vacuum. The mobility increases and contact resistance decreases simultaneously as either the pressure is reduced or the sample is annealed in vacuum. We see a previously unobserved, non-monotonic change in threshold voltage with decreasing pressure. This result could be explained by charge transfer on the MoS2 channel and Schottky contact formation due to adsorbates at the interface between the gold contacts and MoS2. Additionally, from our electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 1012 cm−2 eV−1, degrading the electrical properties of monolayer MoS2.

Highlights

Two-dimensional transition metal dichalcogenides (TMDCs) have received significant attention due to a wide range of unique physical and electronic properties[1,2,3]
As the measurement environment of the monolayer MoS2 field-effect transistors (FETs) was changed from normal pressure to high vacuum (~10−7 Torr), we observed the gradual improvement of the electrical properties including increased field-effect mobility, reduced hysteresis, and decreased subthreshold slope
We infer from our measurements that it is likely that water and oxygen molecules serve as acceptor-type, shallow traps with a level of several 1012 cm−2 eV−1 based on extraction of the interface trap density using a subthreshold slope method

Summary

Introduction

Two-dimensional transition metal dichalcogenides (TMDCs) have received significant attention due to a wide range of unique physical and electronic properties[1,2,3]. Monolayer MoS2 is especially attractive for applications in electronics and optoelectronics[1,2,3,4,5,6] It is a promising channel material for field-effect transistors (FETs) because of a large bandgap for effective switching with a high on/off ratio and low power operation, a strong electrostatic control of the channel potential due to atomic thickness, and high mobility potentially allowing fast operation[4, 5]. We monitor changes in the electrical parameters of the MoS2 FETs under different vacuum pressures Based on these measurements, we discuss the likely underlying charge transfer mechanism governing the MoS2 channel properties and the modification of MoS2-metal contact nature from Schottky to ohmic contacts in the absence of adsorbed gas molecules. This study provides a more comprehensive analysis of the interaction between ambient gases and MoS2 FETs than previously considered and helps guide the future design of high-performance MoS2 FETs and for highly sensitive MoS2-based chemical and gas sensors

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