Optical properties of undoped and chromium-doped VA-VIA-VIIA single crystals

SbSl∶Cr, SbSel∶Cr, BiSl∶Cr, and BiSel∶Cr single crystals were grown by the vertical Bridgman technique using the ingots synthesized from the high-purity (99.9999%) elements antimony, bismuth, sulphur, selenium, iodine and chromium. The grown single crystals crystallized in the orthorhombic structure and had an indirect energy-band structure. The temperature dependence of the optical energy gap was well fitted by the Varshni equation. When chromium was doped into the single crystals, impurities in the optical absorption peaks appeared, and were attributed to the electron transitions between the energy levels of Cr²⁺ ions sited at the T_d symmetry point in the host lattice.