Trion induced negative photoconductivity in monolayer MoS2

TL;DR

This paper reports a transient negative photoconductivity in doped monolayer MoS2 caused by trion formation, where photoexcited electron-hole pairs increase effective mass and reduce conductivity.

Contribution

It reveals a novel negative photoconductivity mechanism in 2D semiconductors due to many-body interactions and trion formation under femtosecond laser excitation.

Findings

Conductivity drops to 30% of equilibrium under high fluence

Negative photoconductivity caused by increased effective mass from trions

Strong many-body interactions dominate the transient response

Abstract

Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We have, however, observed a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS2), a two-dimensional (2D) semiconductor, under femtosecond laser excitation. In particular, the conductivity is reduced dramatically down to only 30% of its equilibrium value with high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the carrier conductivity.