Oriented growth and oxygen incorporations in CVD-grown WS₂

Ping Lien Lee^1,2*, Chao Yang Chu^1,2, He Shin Pan^1,2, Wei Chen Tseng^1,2, Wen Hao Chang^1,2

¹Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan

* Presenter:Ping Lien Lee, email:archielee.sc10@nycu.edu.tw

We demonstrate here an in-situ surface step engineering for epitaxial growth of unidirectional two-dimensional (2D) transition metal dichalcogenides (TMDs). Previous studies have proposed that oriented growth of aligned TMD domains can be guided by surface step edges or surface atomic-plane symmetry, but the dominant mechanism of the oriented growth remains unclear. On the other hand, incorporating oxygen during chemical vapor deposition (CVD) can enable the growth of TMDs with larger domain size, yet the impact of oxygen on film quality remains unclear. Our investigation focuses on CVD-grown WS₂, revealing unidirectionally aligned growth of triangular WS₂ domains through sapphire surface step engineering at the early stage of the growth. Atomic force microscopy (AFM) shows that aligned domains resulted from the guiding by the sapphire atomic steps formed at early growth stages in the H₂S environment. Introducing oxygen not only increases domain sizes but also induces isoelectronic doping in WS₂. Low-temperature photoluminescence measurements on oxygen-incorporated WS₂ show suppressed trion and defect emissions, indicating the passivation of n-type defects by the incorporated oxygens. X-ray photoelectron spectroscopy (XPS) also shows W-O bonds substituting W-S bonds. Field-effect transistors (FETs) based on the CVD-grown WS₂ with oxygen incorporation exhibit a notable positive shift in the threshold voltage, which is consistent with the doping effects observed in photoluminescence and XPS analyses.

Keywords: Incorporation, Chemical Vapor Deposition, Unidirectional growth, Doping, WS2