Monolayer molybdenum diselenide with ultralow defect density grown on Au(111) using oxygen-free precursors

Guan-Hao Chen^1,2*, Li-Syuan Lu², Po-Sen Mao^1,2, Pei-Yu Chuang³, Cheng-Rong Hsing⁴, Yu Shiuan Su^1,5, Juhn-Jong Lin¹, Cheng Maw Cheng³, Ching-Ming Wei⁴, Chun-Liang Lin¹, Wen-Hao Chang^1,2,6

¹Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
³National synchrotron radiation research center, Hsinchu, Taiwan
⁴Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
⁵Taiwan Instrument Research Institute, 20, R&D Rd. VI, Hsinchu Science Park, Hsinchu, Taiwan
⁶Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

* Presenter:Guan-Hao Chen, email:a0913635603@gmail.com

Large-scale synthesized transition-metal dichalcogenides (TMDs) hold great promise for advancing transistor technology and enabling further device scaling. However, synthetic TMDs are often plagued by abundant point defects, aside from grain boundaries. Recent studies using scanning tunneling microscopy/spectroscopy (STM/STS) challenge the prevailing notion that chalcogen vacancies are the predominant point defects in CVD-grown TMDs. Instead, these studies identify oxygen-substituted chalcogen vacancies as the dominant point defects in freshly grown samples. This prompts the essential question of when and how oxygen is introduced during TMD growth. To reduce the density of point defects in TMDs, it is crucial to clarify the stage at which oxygen substitutions occur. We present here a comparative study aimed at unraveling the primary pathway for the formation of oxygen-substituted selenium vacancies (O_Se) in CVD-grown MoSe₂ on Au(111) substrates. Our investigation reveals that using oxygen-free Mo precursors and implementing a Se-rich cooling process result in an impressively low point defect density down to 2 x 10¹⁰ cm^-². This defect density is about three orders of magnitude lower than that observed in CVD-grown MoSe₂ utilizing MoO₃ precursors. Furthermore, cooling samples from the high growth temperature under Se-rich conditions can suppress the formation of selenium vacancies (V_Se), which could evolve into O_Se after exposure to ambient conditions. The utilization of oxygen-free precursors in CVD growth thus offers a viable pathway to achieving exceptionally low defect densities in synthetic 2D TMDs.

Keywords: Oxygen-free precursor, Ultralow defect density, Scanning tunneling Microscope