Negative Photoresponse Switching via Electron-Hole Recombination at The Type III Junction of MoTe₂ Channel/SnS₂ Top Layer

Extensive study on 2D van der Waals (vdW) heterojunctions has primarily focused on PN diodes for fast-switching photodetection, while achieving the same from 2D channel phototransistors is rare despite their other advantages. Here, a high-speed phototransistor featuring a type III junction between p-MoTe₂ channel and n-SnS₂ top layer is designed. The photodetecting device operates with a basis of negative photoresponse (NPR), which originates from the recombination of photoexcited electrons in n-SnS₂ and accumulated holes in the p-MoTe₂ channel. For the NPR to occur, high-energy photons capable of exciting SnS₂ (band gap ≈2.2 eV) are found to be effective because lower-energy photons simply penetrate the SnS₂ top layer only to excite MoTe₂ , leading to normal positive photoresponse (PPR) which is known to be slow due to the photogating effects. The NPR transistor showcases 0.5 ms fast photoresponses and a high responsivity over 5000 A W^-1 . More essentially, such carrier recombination mechanism is clarified with three experimental evidences. The phototransistor is finally modified with Au contact on n-SnS₂ , to be a more practical device displaying voltage output. Three different photo-logic states under blue, near infrared (NIR), and blue-NIR mixed photons are demonstrated using the voltage signals.