Synthesis of Transition Metal Dichalcogenide WSe2thin Films By Atomic Layer Deposition

Two-dimensional layered transition metal dichalcogenides, such as MoS₂, WSe₂ and MoSe₂  have received a lot of attention recently due to their promising unique thermal, mechanical, electrical, optical properties. Tungsten Diselenide (WSe₂), 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 optical and electrical properties. WSe₂  is a   semiconductor with a bandgap  which could be used for the fabrication of solar cells and  LEDs of various colors [1, 2].

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 325^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 property was analyzed by Raman spectroscopy and photoluminescence (PL) spectroscopy. The strong Raman scattering of in-plane E¹_2g and out-of-plane A_1g vibration modes, and the dominant PL of direct transition from K_c to K_v and indirect transition from Λ_c to Γ_v with absence of K_c to Γ_v 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.

References:

1. H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, and Y. Cui, Nano Lett. 13, 3426 (2013).
2. C. Huang, S. Wu, A. M. Sanchez, J. J. P. Peters, R. Beanland, J. S. Ross, P. Rivera, W. Yao, D. H. Cobden and X. Xu, Nature materials 13, 1096 (2014).