Optical, electrical, and EPR studies of Polycrystalline Al:Cr:ZnSe Gain Elements

Middle infrared (Mid-IR) lasers are at the forefront of new technological developments in optics. Transition metal-doped chalcogenides (TM:II-VI) are promising gain media for mid-IR lasers. Cr:ZnSe, a key representative of (TM:II-VI) materials, is considered a material of choice for optically pumped mid-IR lasers. In addition to effective mid-IR lasing under optical excitation, Cr:ZnSe, being a wide band semiconductor, holds potential for direct electrical excitation. We report on optimizing the doping technique for providing n-type conductivity to polycrystalline ZnSe via Al diffusion. These crystals were doped with an alloy of 0.5% Al in Zn at 1050°C for 1-5 days. These parameters yielded samples with resistivities ranging from 3.5 to 1.5x10³ W*cm. Conductive Al:ZnSe crystals were co-doped through diffusion from Cr thin film deposited on crystal facets. Optical characterization of these samples demonstrated that the level of Cr²⁺ was much smaller than the expected value of 1.3x10¹⁹ cm^-3 measured in non-conductive ZnSe samples. It was suggested that a decrease of the Cr²⁺ ions could result from the formation of Cr⁺ ions by capturing an electron from the conductive band. EPR measurements supported this assumption. The maximum Cr⁺ concentration was measured to be 10¹⁸ cm^-3.