Electrical and structural characterisation of electrodeposited 2D transition metal dichalcogenide

Abstract

Semiconductors are integral to modern technology and the chips that provide the processing power that drive devices that modern society relies on such as smartphones, tablets, and PCs. The demand for smaller, faster and lower power devices places an ever-increasing demand on the development of nanoscale semiconductors. The 2D-layered transition metal dichalcogenides (TMDCs) represent an exciting class of highly tunable semiconductors for implementation into next generation electronic devices. TMDCs adopt layered structures, with strong covalent bonding within layers and weak van der Waals interactions between the layers. The layered structure allows their exceptional semiconducting “bulk” properties to remain when scaled down to single mono-layer.
TMDC layer-by-layer growth approaches include techniques such as chemical vapour deposition (CVD) and atomic layer deposition (ALD). These are highly versatile and scalable approaches that enable production of ultrathin TMDCs with high crystal quality, scalable size, controllable thickness, and excellent electronic properties. However, grain boundaries occurred during CVD process weakens the electrical performance of TMDCs, and ALD requires further annealing for crystallinity improvement. These processes also offer very little scope for substrate-selective growth and have been motivation to explore alternative growth methods in the form of electrodeposition.
As a well-established industrial manufacturing process, electrodeposition offers the capability of producing wafer-scale continuous films of mono-/few-layer TMDCs. Its substrate-selective deposition nature enables precise and controlled growth over any electrode surface, making it particularly well-suited for depositing materials inside complex structures and for creating micro-and nano-scale features in advanced devices. Moreover, electrodeposition provides exceptional control over the material, with the flexibility to fine-tune properties by selecting the appropriate precursors in the electrolyte, adjusting the deposition potential and time, and the capability to produce heterostructures. Therefore, it is an extremely promising technique to produce 2D-layered TMDCs with qualities that are suitable for different applications.
In this thesis, I report MoS2, WSe2 and WS2 can be successfully electrodeposited on TiN using [NnBu4]2[MoS4], WSeCl4 and [NEt4]2[WS2Cl4] as a single-source precursor, respectively. The as electrodeposited TMDCs requires further high temperature annealing at 500 ℃ to 900 ℃ to transfer the amorphous state to the crystal state. Then the MoS2 shows its two characteristic Raman peaks, E_2g^1 (383 cm-1) and A1g (406 cm-1). For the WSe2, the in-plane and out-of-plane vibrations give one major peak located around 250 cm-1 and three minor second order peaks between 350 cm-1 to 400 cm-1. Higher annealing temperature leads to the higher crystallinity based on FWHM analysis performed by Lorentz fitting. AFM image analysis shows correlation between thickness and deposition time.
I demonstrate the electrodeposition of atomic layers of WS2 and WSe2 over graphene electrodes using a single source precursor. Using conventional microfabrication techniques, graphene was patterned to create micro-electrodes where WS2 and WSe2 was site-selectively deposited to form 2D heterostructures. We used various characterization techniques, including atomic force microscopy, transmission electron microscopy, Raman spectroscopy and x-ray photoelectron spectroscopy to show that our electrodeposited WS2 and WSe2 layers are highly uniform and can be grown over graphene at a controllable deposition rate. This technique to selectively deposit TMDCs over microfabricated graphene electrodes paves the way towards wafer-scale production of 2D material heterostructures for nanodevice applications.
Both MoS2 and WSe2 can be successfully laterally electrodeposited on the TiN patterned over the SiO2 layer. Both MoS2 and WSe2 exhibit dominant vertical growth near the electrode edge during the initial electrodeposition process. However, the lateral growth gradually surpasses the vertical growth during the process. I developed a dual-electrode patterned substrates to allow the growth of MoS2 and WSe2 heterostructures where each TMDC is grown subsequently from its own electrode. The heterostructures are electrically characterised and show some asymmetry due to Schottky barriers or p/n junction effects.
Transistor structures were designed and fabricated. The patterned dimension of fabricated substrate is close to the designed specification. Capacitance-Voltage measurement, breakdown voltage measurement and resistivity measurements confirm the good quality of SiO2 gate dielectric, sputtered TiN electrodes and e-beam evaporated Al gate. Exfoliated MoS2 draped over the fabricated structure shows significant transistor behaviour with more than two orders swing between on and off current, which confirms the fabricated device are of good standard. Electrodeposited MoS2 with ionic liquid gate shows some signs of gating effect, but the result has not been convincing enough.

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Identifiers

ORCID for Kees De Groot: orcid.org/0000-0002-3850-7101

ORCID for Yasir Noori: orcid.org/0000-0001-5285-8779

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