Synthesis of Transition Metal Dichalcogenide WSe2thin Films By Atomic Layer Deposition
Many techniques have been developed for synthesis of WSe2 films on different substrates in recent years, such as sold state reaction, elctrodepositon, RF-magnetron sputtering, electrodepostion, pulsed laser depostion, chemical vapor depositon. However, there are very few references that report atomic layer deposition (ALD) results of WSe2 films. This technique exhibits self-limiting surface reactions, which means accidental overdosing of precursors does not result in increased film deposition on the substrate surface. This self-terminating character is able to accurately control thickness and composition of ultra thin films and achieves uniformity, conformality and even monolayers of transition metal dichalcogenides.
Here we report on a large-area synthesis of WSe2 films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Tungsten hexacarbonyl (W(CO)6) and (trimethylsilyl) selenide ((Me3Si)2Se) are employed as the chemical ALD precursors for Tungsten and selenium, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 275, 300 and 325oC. 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 property was analyzed by Raman spectroscopy and photoluminescence (PL) spectroscopy. The strong Raman scattering of in-plane E12g and out-of-plane A1g vibration modes, and the dominant PL of direct transition from Kc to Kv and indirect transition from Λc to Γv with absence of Kc to Γv are typical optical spectra of layered WSe2. The reduction of out-of-plane vibration mode and the absence of PL from indirect transition provide the evidence of fewer layer WSe2.
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