Unveiling Defect-Mediated Carrier Dynamics in Monolayer Semiconductors by Spatiotemporal Microwave Imaging

Zhaodong Chu, Chun-Yuan Wang, Jiamin Quan, Chenhui Zhang, Chao Lei, Ali Han,Xuejian Ma, Hao-Ling Tang, Dishan Abeysinghe, Matthew Staab, Xixiang Zhang, Allan H. MacDonald, Vincent Tung, Xiaoqin Li, Chih-Kang Shih, Keji Lai
Proceedings of the National Academy of Sciences, (2020)

Keywords

Carrier Dynamics, Spatiotemporal Microwave Imaging

Abstract

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 WS2
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 photo-excited 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 photoconductivity is stronger in CVD samples than exfoliated monolayers with a lower
defect density. Our work reveals the intrinsic time and length scales of electrical response to
photo-excitation in van der Waals materials, which is essential for their applications in novel
optoelectronic devices


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