Exciton and Trion Emission in Wafer-Scale MoS₂ Layers – The Role of Bilayer Islands

2D materials, especially the group of semiconducting transition metal dichalcogenides (TMDCs), have emerged as highly promising for electronic and optoelectronic devices over the past decades [1][2]. For industrially relevant applications, wafer-scale growth of high quality TMDCs is essential. Until today, the control of defects, crystallinity and grain size in wafer-scale synthesis via metal organic chemical vapor deposition (MOCVD) is still challenging. E.g., in molybdenum disulfide (MoS₂) or related sulfur based TMDCs, sulfur vacancies are known to lead to a background n-doping, thus affecting device performance significantly. While there are various approaches for post-growth treatments of defective TMDCs (e.g. super-acid treatment or partial oxidization), little is known about controlling sulfur vacancies and thus background doping in the growth process.<br/>In this contribution we use confocal photoluminescence (PL) measurements with &lt; 500 nm spatial resolution to monitor the local charge carrier distribution in MOCVD grown MoS₂ via the ratio of neutral exciton and trion emission. A 2” c-plane sapphire substrate with 1° off-axis cut is used as the growth substrate in MOCVD. The wafer-scale growth results in MoS₂ monolayers with crystal grain sizes above 50 µm as extracted from grain boundary visualization in transmission electron microscopy (TEM), and a ~15 % coverage with bilayer domains of ~500 nm in size. Despite the large grain sizes, the average PL quantum yield of the MoS₂ monolayer sample is only about 10^-3 %. This is attributed to the high point defect density of (4.1±1.6)*10¹³ cm^-2 as determined by atomically resolved scanning tunneling microscopy (STM) [3] and the impact of interfacial defects and substrate-induced doping, respectively [4]. Surprisingly, our spatially resolved PL mappings indicate an unusual behavior for monolayer and bilayer regions on the sample. The PL obtained from monolayers is dominated by trion emission, indicating a strong influence of n-type background doping. In contrast, the PL on bilayer domains mainly shows neutral exciton emission, combined with reduced line width and a significantly enhanced intensity. This on one hand confirms literature reports on preferred nonradiative recombination via trions [5], and on the other hand hints to a suppression of background doping and thus defect formation in bilayer areas. The latter might be highly relevant for achieving less defective TMDCs via MOCVD growth. Low temperature micro-PL mapping is used to investigate the defect structure of the samples in more detail and to verify this interpretation.<br/><br/>Acknowledgement:<br/>The authors greatly acknowledge the MoS₂ growth and sample supply by imec Leuven, Belgium.<br/><br/>References:<br/><br/>[1] Lemme, M.C., Akinwande, D., Huyghebaert, C. et al. 2D materials for future heterogeneous electronics. Nature Communications 13, 1392 (2022)<br/><br/>[2] Andrzejewski, D., Myja, H., Heuken, M., et al. Scalable Large-Area p–i–n Light-Emitting Diodes Based on WS₂ Monolayers Grown via MOCVD. ACS Photonics 6, 1832 (2019)<br/><br/>[3] Rybalchenko, Y., Minj, A., Medina, H., et al. Scanning tunneling microscopy for imaging and quantification of defects in as-deposited MoS2 monolayers on sapphire substrates. Solid-State Electronics 209, 108781 (2023)<br/><br/>[4] Yu, Y., Yu, Y., Xu, C. et al. Engineering Substrate Interactions for High Luminescence Efficiency of Transition-Metal Dichalcogenide Monolayers. Advanced Functional Materials 26, 4733 (2016)<br/><br/>[5] Lien, D.-H., Uddin, S.Z., Yeh, M., et al. Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors. Science 364, 468 (2019)