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Optical processes in α-ZnAl2S4 spinel-type single crystals doped by transition metals ions : ti, Co et V

Abstract : Spectroscopic properties of α-ZnAl2S4 spinel-type single crystals of the wide band gap semiconductor doped by the transition metals Ti, Co and V are investigated and their interpretation is presented. The crystals, grown by the chemical vapour transport method, with activator impurities concentrations 0.01 – 0.1% at., correspond to optically homogeneous octahedrons with (111) - oriented mirror-like faces. The x-ray analyses confirm that all samples crystallised into the normal spinel type structure with Fd3m (Oh7) cubic symmetry. It is found out that α-ZnAl2S4:Ti single crystals exhibit luminescence in the IR spectral range 0.8–1.4μm. The spectroscopic results obtained in the temperature range 10-300K (steady-state and time resolved photoluminescence, optical absorption and excitation luminescence spectra) are interpreted in terms of a cluster composed of the central Ti4+ ion in an octahedral coordination of six sulphur ions. The observed spectral bands are assigned to the electronic transitions arising from the ligand – Ti4+ charge transfer for octahedral sites of titanium that is in agreement with the experimental evidence for the absence of the EPR signal from Ti ions. The vibronic coupling constant for the excited levels and the barrier height between them are calculated. The structure of the absorption and luminescence spectra of α-ZnAl2S4:Co crystals is determined by the electronic transitions of Co2+ ions located in tetrahedral sites. Four radiative spectral components are revealed using the time-resolved spectroscopy at different excitation wavelengths and it is shown that the luminescence of α-ZnAl2S4:Co crystals is due to the electronic transitions between the excited levels of Co2+ ions. The calculated values of the spin-orbit coupling constants of the excited levels indicate a weak crystal field influence and a strong spin-orbit coupling. It is determined that the absorption and luminescent properties of α-ZnAl2S4:V spinel type crystals are the result of electronic transitions of trivalent vanadium ions located in octahedral sites. The rise of temperature leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centered at λ =1.4μm. Three main spectral components of the α-ZnAI2S4:V IR spectra revealed at low temperatures are caused by electronic transitions of V3+ ions. The coexistence of a broad band with the narrow lines at low temperatures, when the thermal energy kBT is much less than the height of the potential barrier between the excited states, is explained assuming that there is a phonon assisted tunnelling between these states. On the base of the comparative analysis of spectroscopic properties of α-ZnAl2S4 spinel type crystals doped with transition metals Ti, Co, and V it is established that α-ZnAl2S4:V3+ compounds are the most appropriate for applications as active media for solid state IR-lasers tunable in the 1.2-1.6μm wavelength range, which corresponds to the spectral region used in the fibre-optics communication systems.
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Submitted on : Wednesday, October 10, 2012 - 2:52:11 PM
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  • HAL Id : tel-00740578, version 1



Sergiu Anghel. Optical processes in α-ZnAl2S4 spinel-type single crystals doped by transition metals ions : ti, Co et V. Other [cond-mat.other]. Université Claude Bernard - Lyon I, 2011. English. ⟨NNT : 2011LYO10224⟩. ⟨tel-00740578⟩



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