Structural and Electrical Investigation of MoS2 Thin Films Formed By Thermal Assisted Conversion of Mo Metal
Authors: Duffy Ray , Foley Patrick , Filippone Bruno , Mirabelli Gioele , O'Connell Dan , Sheehan Brendan , Carolan Patrick , Schmidt Michael , Cherkaoui Karim , Gatensby Riley , Hallam Toby , Duesberg Georg S. , Nagle Roger , Hurley P. K. 
Published: February 2020

Due to the performance and economic benefit obtained by scaling, future semiconductor electron devices for logic functions will progress toward ultra-thin-body channels and 2-dimensional (2D) high carrier mobility materials. The significance is that small devices can be made to yield higher performance and greater energy-efficiency. To put this trend in perspective, fin-field-effect-transistor[i],[ii] (FinFET) technology was a research topic 7-8 years ago, but is now at the heart of the microprocessor in high-end smartphones.[iii] While the positioning of TMDs in electronic products of the future is still unsure, perhaps they will be more suited to low power, it is still worthwhile to understand how a relatively undeveloped system can reach maturity in less than a decade.

Graphene is semi-metallic, which makes it difficult to switch off electron devices such as FETs. Bandgap engineering is required to open a bandgap of graphene, which is not an easy undertaking. This has motivated the scientific community to search for alternate 2D layer materials with semiconducting properties and better tuneability. Many TMDs are natural semiconductors with thicknesses on the nanometre scale. TMD semiconductors are now emerging as potentially useful materials, where more research is needed, in order to explore their properties and potential applications.

Large-area synthesis is of great demand for the preparation of high-performance transition-metal-dichalcogenides (TMD) devices, however reports of device operation on large-area TMDs are sparse. In this work we fabricate MoS2 devices based on Thermal Assisted Conversion (TAC) of metal layers, and characterise the thin-films with material analysis combined with electrical device parameter extraction. Specifically we perform parameter extraction for MoS2 thin-films to determine sheet resistance (Rsh), resistivity (ρ), and activation energy (EA) of on-state current flow. For undoped MoS2, ρ was determined to be 191 Ω.cm at 25 °C. EA of the on-state current found to be 0.18 eV, points to a current generation mechanism possibly linked to a deep level in MoS2.

In Fig. 1 are representative images of the 10 nm sulphurised Mo, forming approximately 20 nm of MoS2. In (a) a wide view of MoS2 on a SiO2/Si substrate. On the right-hand side the Ti/Au metal contact is visible. The MoS2 layer is continuous across the surface. In (b) the layered structure of MoS2 is visible in the higher resolution image. In Fig. 2 are representative measured current versus voltage characteristic in the MoS2 circular Transfer Length Method structure as a function of contact spacing. The current is linear with respect to voltage and passes through the origin. The current increases with decreased contact spacing as expected. The inset shows an optical images of the circular TLM structure.

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[i]  J. –P. Colinge (Ed.), “FinFETs and other Multi-Gate Transistors” ISBN 978-0-387-71752-4, Springer (2008).

[ii]  N. Collaert (Ed.), “CMOS Nanoelectronics: Innovative Devices, Architectures, and Applications” ISBN 9789814364027,  Pan Stanford (2012).

[iii]  J. Borland, ECS Transactions, 69 (10) 11-20 (2015).

Figure 1