Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated proces...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2020-06, Vol.117 (25), p.13908-13913
Main Authors: Chu, Zhaodong, Wang, Chun-Yuan, Quan, Jiamin, Zhang, Chenhui, Lei, Chao, Han, Ali, Ma, Xuejian, Tang, Hao-Ling, Abeysinghe, Dishan, Staab, Matthew, Zhang, Xixiang, MacDonald, Allan H., Tung, Vincent, Li, Xiaoqin, Shih, Chih-Kang, Lai, Keji
Format: Article
Language:English
Subjects:
Carrier lifetime
Carrier transport
charge carriers
Chemical vapor deposition
Conduction
Conduction bands
Defects
Diffusion length
Electronics industry
ENGINEERING
laser-illuminated microwave impedance microscopy
Microwave imaging
Monolayers
Optoelectronic devices
Photoconductivity
Photoexcitation
Physical Sciences
Recombination
Signal processing
spatiotemporal dynamics
Temporal relaxation
Transition metal compounds
transition-metal dichalcogenides
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Summary:The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WSâ‚‚ monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.2004106117