Compositional Changes in the Infrared Optical Properties of Cr Doped CdZnTe Crystals
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E4.19.1
Compositional changes in the infrared optical properties of Cr doped CdZnTe crystals U. Hömmerich*,+, A.G. Bluiett*, EiEi Nyein*, S.B. Trivedi**, and R.T. Shah*** *Hampton University, Department of Physics, Hampton, VA 23668 **Brimrose Corporation of America, 19 Loveton Circle, Baltimore, MD 21152 *** M.S. University of Baroda, Applied Physics Department, Faculty of Technology and Engineering, Vadodara-390 001, INDIA +
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ABSTRACT We are currently investigating the infrared (IR) optical properties of Cr doped ternary cadmium chalcogenides for potential applications in solid-state lasers and passive optical Qswitches. In this paper, we present compositional changes in the IR optical properties of Cr doped Cd1-xZnxTe single crystals with x=0.05, 0.1, and 0.2. Undoped CdZnTe crystals were grown by vertical Bridgman technique. Cr doping of CdZnTe was achieved through either in-situ doping or through a thermal diffusion process. For comparison, Cr: CdTe and Cr: ZnTe crystals were also prepared. The optical properties of Cr2+ ions were strongly dependent on the host composition and spectral blue shifts were observed with increasing Zn content in Cr: CdZnTe. The IR absorption peak of Cr2+ ions shifted from ~1910 nm for Cr: CdTe to ~1815 nm for Cr: Cd0.8Zn0.2Te. Less pronounced blue shifts were observed for the IR emission from Cr: CdZnTe crystals. The spectral shifts can be explained by the decrease in bond-length when going from CdTe to CdZnTe leading to an increased crystal-field effect experienced by Cr2+ ions. A slight broadening of the absorption and emission was also observed in ternary Cr: CdZnTe compounds compared to Cr: CdTe, which suggests that Cr2+ ions were incorporated in multiple lattice sites in CdZnTe. Moreover, the Cr2+ emission dynamics revealed a slightly non-exponential decay behavior for Cr: CdZnTe crystals, whereas the decay time of Cr: CdTe was single-exponential. INTRODUCTION
Cr doped II-VI semiconductors continue to be of current interest as gain media for midinfrared solid-state lasers and as saturable absorbers for passive Q-switching [1-5]. The optical center in these materials was assigned to Cr2+ ions occupying tetrahedral lattice sites [1-5]. Tetrahedral sites lack inversion symmetry, which provides the odd-parity field necessary to relax the Laporte selection rule for intra-3d transitions. The increased radiative emission rate in conjunction with the small phonon energies of II-VI hosts (~250 cm-1) reduces the detrimental effect of non-radiative decay through multi-phonon relaxations. In general, Cr2+ laser crystals are characterized by a broad absorption centered around ~1700-1900 nm and emission bands extending from ~2000-3500 nm (5T2 5E transition). The reported absorption and emission cross-sections for Cr2+ doped II-VI materials are on the order of ~10-18-10-19 cm2 [1-5]. The room-temperature emission efficiencies vary between ~40-100% [1-5]. Also, excited state absorption is minimal in Cr2+ doped II-VI materials [1-5]. In an effort to extend
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