On the Contact Optimization of ALD-Based MoS₂ FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance

Abstract

Despite the extensive ongoing research on MoS 2 field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS 2 interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS 2 contacts are based on exfoliated MoS 2 , whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS 2 films and Ti/Au contacts are demonstrated, using current–voltage ( I – V ) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O 2 /Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS 2 surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O 2 /Ar plasma changes the MoS 2 chemical state and enhances the MoS 2 electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiS x layer at the Ti-to-MoS 2 interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS 2 films.

ALD-grown two-dimensional TiSx metal contacts for MoS2 field-effect transistors

“…The electrical performance of the fabricated devices were assessed and compared to a reference device, for which 5/95 nm of Ti/Au was employed as the contacts. As per a previous report, 67 this thickness combination is found to be the most optimal for the Ti/Au stacks contacted to the ALD-based MoS 2 lms. Fig.…”

“…Details of the device fabrication are described in ref. 67 . During the first EBL step, contact regions were defined on the PMMA coated MoS 2 .…”

Tapping the Performance of a Tungsten Disulfide Field-Effect Transistor with Deep Structure Optimization and Theoretical Simulation

“…The electrical properties of the WS 2 -FETs were tested using a probe station. The obtained output and transfer characteristic curves, I d – V d and I d – V g , were nonlinear, indicating that a Schottky contact had been formed by a mismatch in the work function between the WS 2 nanosheet and the metal electrode. − The carrier mobility of the WS 2 -FETs is calculated using the carrier mobility equation, as follows: μ FE = L C W V normald ∂ I ∂ V normalg where L is the device channel length, W is the channel width, C is the capacitance per unit area of the SiO 2 dielectric layer (the capacitance C per unit area of 300 nm SiO 2 is 11.5 nF·cm –2 , and the value of capacitance per unit area is inversely proportional to thickness). V d is the drain voltage, and ∂ I /(∂ V g ) is the value of the slope of the tangent line on the transfer curve.…”

Low-Energy He+ Ions Induced Functionalization of the MoS2 Surface for ALD HfO2 Growth Enhancement

“…As a result, the possibility of the ultrathin (<3 nm) and continuous film growth on the irradiated bulk MoS 2 surface was demonstrated, and for the monolayer MoS 2 film, an increase in the surface density of ALD active sites by a factor of ≈2 was observed. In addition to the HfO 2 ALD growth enhancement, this study on MoS 2 functionalization might be useful for other applications that exploit chalcogen deficiency in TMDCs such as a p-doping or lowering contact resistance in transistor applications …”

“…bulk MoS 2 surface was demonstrated, and for the monolayer MoS 2 film, an increase in the surface density of ALD active sites by a factor of ≈2 was observed. In addition to the HfO 2 ALD growth enhancement, this study on MoS 2 functionalization might be useful for other applications that exploit chalcogen deficiency in TMDCs such as a p-doping or lowering contact resistance in transistor applications 44. Center of Shared Facilities in Nanotechnology, Moscow Institute of Physics and Technology (National Research University), Dolgoprudny, Moscow Region 141701, Russian Federation; orcid.org/0000-0001-6777-5706; Email: markeev.am@mipt.ru…”