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Excitons, which represent a type of quasi-particles consisting ofelectron-hole pairs bound by the mutual Coulomb interaction, were oftenobserved in lowly-doped semiconductors or insulators. However, realizingexcitons in the semiconductors or insulators with high charge carrier densitiesis a challenging task. Here, we perform infrared spectroscopy, electricaltransport, ab initio calculation, and angle-resolved-photoemission spectroscopystudies of a van der Waals degenerate-semiconductor Bi4O4SeCl2. A peak-likefeature (i.e., alpha peak) is present around ~ 125 meV in the opticalconductivity spectra at low temperature T = 8 K and room temperature. Afterbeing excluded from the optical excitations of free carriers, interbandtransitions, localized states and polarons, the alpha peak is assigned as theexciton absorption. Moreover, assuming the existence of weakly-boundexcitons--Wannier-type excitons in this material violates theLyddane-Sachs-Teller relation. Besides, the exciton binding energy of ~ 375meV, which is about an order of magnitude larger than those of conventionalsemiconductors, and the charge-carrier concentration of ~ 1.25 * 10^19 cm^-3,which is higher than the Mott density, further indicate that the excitons inthis highly-doped system should be tightly bound. Our results pave the way fordeveloping the optoelectronic devices based on the tightly-bound androom-temperature-stable excitons in highly-doped van der Waals degeneratesemiconductors.

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