Optical Signatures of Many-body Interactions in Few-layer Semiconductor

The family of III-VI metal monochalcogenides, for example, Indium selenide (InSe), emerge as new two-dimensional materials with a layer dependent band structure, high electron mobility and strong light matter interaction. Monolayer InSe is predicted to be an indirect band gap semiconductor. However, few-layer InSe maintains direct band gap optical properties and hosts a flat valence band dispersion. Here we report the observation of the electrical field controlled neutral and charged excitons in high-quality hBN encapsulated few-layer InSe devices. Near the charge neutrality point, the photoluminescence (PL) spectra display a strong and narrow peak with 4meV linewidth (at 20K) associated with the recombination of neutral excitons. By increasing the carrier density, positive (negative) charged trions are observed and manifest a red (blue) energy shift. The observation is a direct signature of the many-body interaction induced by the flat valence band.