CVT growth and optical properties of Cr,Fe:ZnSe single crystals for mid-infrared laser applications

At present, the research of transition metal ions doped ZnSe (TM:ZnSe) materials is confronted with the uneven TM-doping and the difficulty of growing high-quality single crystals. In this work, Cr,Fe:ZnSe single crystals with expected elemental doping concentrations were grown at lower temperature by the CVT method using NH₄Cl as a transport agent. The UV–VIS–NIR absorption spectrum shows that the concentrations of Cr²⁺ and Fe²⁺ ions are 9.61 × 10¹⁸ cm⁻³ and 1.45 × 10¹⁹ cm⁻³, respectively. The element doping efficiency is as high as 96–97 %. Compared with other doping methods, the CVT method can effectively control the doping concentrations of Cr and Fe elements of the as-grown Cr,Fe:ZnSe single crystals, the ratio between which is also very close to the design value. The XPS results demonstrate that the Cr and Fe elements are mainly present as Cr²⁺ and Fe²⁺ ions in the Cr,Fe:ZnSe single crystals. The fluorescence spectra under 1770 nm excitation at room temperature show a luminescence peak at about 3400–4900 nm resulting from the energy transfer between Cr²⁺ and Fe²⁺ ions. Based on the fluorescence decay lifetime, the energy transfer process can be explained by a non-radiative energy transfer mechanism. The maximum intensity ratio of the luminescence peaks of Fe²⁺ and Cr²⁺ ions is about 1:2, and the ratio of the emission peak areas of Fe²⁺ and Cr²⁺ ions is 0.852, which indicating that the energy transfer process is more effective. The fluorescence lifetimes of Cr,Fe:ZnSe single crystals at 2200 nm and 4400 nm are 6.3 μs and 6.5 μs, and the emission cross sections are calculated to be 2.02 × 10⁻¹⁸ cm² and 1.80 × 10⁻¹⁷ cm², respectively. Therefore, it is believed that the unintentional doping of Cl⁻ ions has no obvious effect on the optical and luminescent properties of the as-grown crystals.