Optical Properties of Transition Metal Dichalcogenide MoS2thin Films Synthesized By Atomic Layer Deposition

Abstract

Transition metal dichalcogenides (TMDCs), such as MoS2, WS2, WSe2 and MoSe2, exhibit a special structure in which a transition metal layer sandwiched between two chalcogen atomic layers. The in-plane covalent bonds between neighboring layer are strong and interplane van der Waals forces which keep the layer together are relatively weak. Especially, the monolayers commonly called as two-dimensional (2D) materials, are highly attractive due to their unique and tunable material properties. Therefore, the 2D TMDCs has promising potentials for the application of sensors, filed effect transistors, opto-electronics, piezoelectric devices, energy storage and hydrogen generation. Recently, monolayer or few-layer TMDC films have been fabricated by many techniques, such as exfoliation, pulsed laser deposition (PLD), chemical vapor deposition (CVD). However, very little is known about atomic layer deposition (ALD) synthesis of 2D films and there are very only few references. ALD technique exhibits self-limiting atomic layer reactions in each introduced ALD cycle. ALD is the ability to control film thickness at angstrom level. Therefore, it offers better control of parameters like the composition, stoichiometry, uniformity and sharp interface. Furthermore, ALD also can be used to deposit conformal film onto very complex structures. For this study MoS2 thin films have been prepared on silicon with native oxide and quartz substrates by Atomic Layer Deposition using Molybdenum hexacarbonyl (Mo(CO)6) and dimethyldisulfide (CH3SSCH3, DMDS) as the chemical ALD precursors for Molybdenum and Sulfur, respectively. For our experiments, 20 sccm N2 was used as a carrier gas flow for the precursors. This carrier gas flow enabled proper transport of precursors from the precursor cylinder to the reaction chamber. The growth temperature was set at 200, 230 and 250 oC. The chamber base pressure was kept at 40 mTorr. The crystallinity of as-deposited MoS2 thin film could be improved by post-annealing under saturated sulfur vapor. The Raman spectroscopy analysis show the MoS2 thin films the characteristic of E12g and A1g Raman modes. Multilayered MoS2 films were investigated with femtosecond laser pump-probe spectroscopy for the effects of increasing layer thickness on the electronic relaxation dynamics. It was found that the decay processes and valence band splitting are in accord with the known energy scheme for MoS2 thin films. A band gap of about 1.8 eV of MoS2 thin film is in the range of 1.1 eV (bulk MoS2) to 1.9 eV (monolayer MoS2). The band gap of ALD MoS2 thin films is much closer to the monolayer limit than the bulk limit.