Tightly bound and room-temperature-stable excitons in van der Waals degenerate-semiconductor Bi₄O₄SeCl₂ with high charge-carrier density

Excitons, which represent a type of quasi-particles consisting of electron-hole pairs bound by the mutual Coulomb interaction, are often observed in lowly-doped semiconductors or insulators. However, realizing excitons in semiconductors or insulators with high charge-carrier densities is challenging. Here, we perform infrared spectroscopy, electrical transport, ab initio calculations, and angle-resolved-photoemission spectroscopy study of the van der Waals degenerate-semiconductor Bi₄O₄SeCl₂. A peak-like feature (α peak) is present around ~125 meV in the optical conductivity spectra at low temperature T = 8 K and room temperature. After being excluded from the optical excitations of free carriers, interband transitions, localized states and polarons, the α peak is assigned as an exciton absorption. Assuming the existence of weakly-bound Wannier-type excitons in this material violates the Lyddane-Sachs-Teller relation. Moreover, the exciton binding energy of ~375 meV, which is about an order of magnitude larger than those of conventional semiconductors, and the charge-carrier concentration of ~1.25 × 10¹⁹ cm⁻³, which is higher than the Mott density, further indicate that the excitons in this highly-doped system should be tightly bound. Our results pave the way for developing optoelectronic devices based on tightly bound and room-temperature-stable excitons in highly-doped van der Waals degenerate semiconductors.