Effect of Different Doping Concentrations on the Properties\nof Fe<sup>2+</sup>:ZnSe and Fe<sup>2+</sup>, Cr<sup>2+</sup>:ZnSe\nCrystals Grown by the Chemical Vapor Transport Method

The\nFe<sup>2+</sup>:ZnSe crystal is an important material for 3–5\nμm mid-infrared lasers. In this work, we have grown different\ndoping concentrations of Fe<sup>2+</sup>:ZnSe and Fe<sup>2+</sup>,\nCr<sup>2+</sup>:ZnSe by the chemical vapor transport method. The doped\ncrystals have the same structure. The lattice constant increases slightly\nwith a higher Fe<sup>2+</sup> concentration, while the doped Cr<sup>2+</sup> has a great influence on the lattice constant. Besides,\nwith a higher Fe<sup>2+</sup> concentration, the TO mode has no change\nand the LO mode presents a small blue shift. The doped ZnSe crystals\nall have a wide absorption peak in the range of 2.3–4.5 μm,\nand these absorption peaks widen with the increase of the Fe<sup>2+</sup> doping concentration. Besides, in the range of 5–20 μm,\nthe crystals have good transparency. The concentrations of 3, 2, and\n1% Fe<sup>2+</sup> samples are calculated to be 4.15 × 10<sup>19</sup>, 3.27 × 10<sup>19</sup>, and 3.02 × 10<sup>19</sup> cm<sup>–3</sup>, respectively, and the Fe<sup>2+</sup> concentration\nof the Fe<sup>2+</sup>, Cr<sup>2+</sup>:ZnSe sample is 3.71 ×\n10<sup>19</sup> cm<sup>–3</sup>. The band gap decreases from\n2.52 to 2.27 eV with a higher Fe<sup>2+</sup> doping concentration.\nTherefore, we can modulate the absorption bandwidth and band gap by\ndoping the ion concentration, which is more suitable for developing\nmid-infrared gain medium applications.