Influence of Native\nDefects on the Optical Properties\nof SnX<sub>2</sub> (X = S and Se) van der Waals Crystals

SnX<sub>2</sub> crystals (X = S and Se) are van der Waals\nsemiconductors\nwith great potential for applications as photocatalysts and thermoelectric\nmaterials as well as in two-dimensional optoelectronics. According\nto density functional theory calculations, they exhibit indirect band\ngaps in the near-infrared (SnSe<sub>2</sub>) or visible (SnS<sub>2</sub>) spectral range. The materials characterize with the Fermi level\npositioned close to the conduction band (CB), as a consequence of\nlow formation energy of native donor defects. The population of the\nbottom CB states by electrons leads to the Burstein–Moss (B–M)\neffectan effective blue shift of the optical absorption edge.\nIn our work, we exploit modulated reflectance (insensitive to the\nB–M shift) to determine the energy of the lowest direct band\ngap and verify the character of the measured absorption edge. Additionally,\nfor SnS<sub>2</sub>, a strong defect-related photoluminescence emission\nwas detected, with an unusual line shape, quenched at certain energy.\nWe postulate that the reason for such behavior is the proximity of\nthe CB valley to the defect level, acting as a trap for electrons\nin the reciprocal space. A model allowing one to reproduce the PL\nintensity spectral dependence is proposed, providing information about\nthe fundamental indirect band gap energy. In general, the obtained\nresults reveal strong influence of the structural defects on the optical\nproperties of SnX<sub>2</sub> crystals, which can be exploited to\nimprove their performance in various applications.