2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron–hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm2 range based on MoS2/WS2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5–6 orders of magnitude with respect to reference devices based on either MoS2 or WS2 monolayers only. The large gain enhancement is attributed to efficient charge carrier separation at the MoS2/WS2 heterointerface combined with hole trapping, leading to an improved electron transport in the heterostructure under illumination.
A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS2 with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal–organic chemical vapor deposition (MOCVD) to directly grow MoS2 on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene–MoS2 photodetectors. Two-color optical pump–electrical probe experiments allow for separation of light-induced carrier transfer across the graphene–MoS2 heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS2 to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.
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