# Observation of Excitonic Doublet Structure, Biexcitons and Their Temperature Dependence in High-Quality β-InSe Single Crystals

## TL;DR

This study explores the optical properties of high-quality InSe crystals, revealing excitonic structures and their behavior at different temperatures.

## Contribution

The paper reports the observation of excitonic doublet structure and biexcitons in β-InSe single crystals with detailed temperature dependence.

## Key findings

- A free exciton resonance is prominent in the temperature-dependent absorption and PL spectra of InSe.
- At 17 K, the PL spectrum shows a fine-structure splitting of the Wannier–Mott exciton into triplet and singlet states.
- A biexciton is identified at 1.322 eV based on nonlinear intensity dependence on excitation power density.

## Abstract

We present a systematic study of the fundamental optical properties of indium selenide (InSe) single crystals over a temperature range of 17 K to 300 K. The high structural quality of the β-polytype crystals was confirmed through X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy, demonstrating excellent crystallinity and a nearly stoichiometric In:Se ratio. The temperature-dependent absorption and photoluminescence (PL) spectra are characterized by a prominent free exciton (FX) resonance. At 17 K, the photoluminescence spectrum exhibits a distinct fine-structure splitting of the Wannier–Mott exciton, yielding a triplet state at 1.333 eV and a singlet state at 1.336 eV. Additionally, a biexciton (XX) is localized at an energy of 1.322 eV as confirmed by the nonlinear dependence of intensity on excitation power density. At low temperatures, the absorption spectrum exhibits the free exciton ground state (n = 1) at 1.338 eV together with the first excited state (n = 2) at 1.350 eV. We systematically tracked and analyzed the temperature evolution of these quasiparticle energies. These findings enhance our understanding of the intrinsic many-body interactions in high-quality InSe, providing essential parameters for advancing its applications in innovative optoelectronic and quantum light-emitting devices.