Optical properties of Ho³⁺-, Er³⁺-, and Tm³⁺-doped BaIn₂S₄ and BaIn₂Se₄ single crystals

BaIn₂S₄, BaIn₂S₄:Ho³⁺, BaIn₂S₄:Er³⁺, BaIn₂S₄:Tm³⁺, BaIn₂Se₄, BaIn₂Se₄:Ho³⁺, BaIn₂Se₄:Er³⁺, and BaIn₂Se₄:Tm³⁺ single crystals were grown by the chemical transport reaction method. The optical energy gap of the single crystals was found to be 3.057, 2.987, 2.967, 2.907, 2.625, 2.545, 2.515, and 2.415 eV, respectively, at 11 K. The temperature dependence of the optical energy gap was well fitted by the Varshni equation. Broad emission peaks were observed in the photoluminescence spectra of the single crystals. They were assigned to donor–acceptor pair recombination. Sharp emission peaks were observed in the doped single crystals. They were attributed to be due to radiation recombination between the Stark levels of the Ho³⁺, Er³⁺, and Tm³⁺ ions sited in C₁ symmetry.