(Invited) Overview of ALD Synthesized MoS2 Thin Films

Abstract

Two-dimensional (2-D) layered transition metal dichalcogenides, such as MoS2, WSe2, WS2 and MoSe2 have received a lot of attention recently due to their significant thermal, mechanical, electrical and optical properties. Molybdenum Disulfide (MoS2), one of the transition metal dichalcogenides of interest for 2-D electronic systems, exhibits not only good thermal stability and high melting point, but also excellent light absorption, fast photo-response and sizeable band gap properties. Therefore, these unique properties make MoS2 a promising material for various electronic devices, photodetectors, sensors and catalysis applications. Many techniques have been developed for the synthesis of MoS2 films on different substrates in recent years, such as pulsed laser deposition (PLD), chemical vapor deposition (CVD). However, very little is known about atomic layer deposition (ALD) synthesis of MoS2 films there are very only few references. ALD technique exhibits self-limiting atomic layer reactions in each introduced ALD cycle. It can accurately control film layer thickness stoichiometry, composition, uniformity, and sharp interface. Furthermore, ALD also can be used to deposit conformal film onto very complex structures. Here we report on a large-area synthesis of MoS2 films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Molybdenum hexacarbonyl (Mo(CO)6) and dimethyldisulfide (CH3SSCH3, DMDS) are employed as the chemical ALD precursors for Molybdenum and Sulfur, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 200, 230 and 250 oC. The crystal structure and chemical composition of WSe2 films were identified by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The film morphology was characterized by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The optical properties of ALD MoS2 films were analyzed by Raman spectroscopy and femtosecond transient absorption spectroscopy.