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 approximate to 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.