Physico-Chemical nanomaterials science

Raman and infrared spectroscopy of ZnSe crystal doped with cobalt ions
B.I. Tsykaniuk1, O.F. Kolomys1, P.M. Lytvyn1, V.V. Strelchuk1, V.P. Makhniy2, M.M. Berezovskiy2, V.Tkach3
1 V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41 prospect Nauky, 03028 Kyiv, Ukraine.
E-mail: btsykaniuk@gmail.com
2 Yu. Fedkovych Chernivtsi National University, 2, Kotsyubinskogo str., 58012 Chernivtsi, Ukraine
3 V. Bakul Institute for Superhard Materials, National Academy of Science of Ukraine, 2 Avtozavodskaya str.,04074 Kyiv, Ukraine.

Zinc selenide (ZnSe) doped with transition elements have received considerable attention as a promising material for the lasers of mid-infrared (mid-IR) spectral region. The ZnSe:Co bulk crystals grown with the Bridgman-Stockbarger technique from the melt of stoichiometric composition were investigated using SEM and EDS, photoluminescence, Raman and FTIR spectroscopy. Doping of ZnSe substrates with Co atoms was carried out in pumped up to 10-4 Torr and soldered quartz ampoule at 1230 and 1450° C for several hours.
The EDS results confirmed the presence of Co impurities in the bulk crystals. The average Co concentration in ZnSe:Co determined by the EDS technique was found as 1.5-3 at. %. The increase of the growth temperature leads to increase of average Co concentration. The first- and second-order Raman spectra of the initial and annealed ZnSe:Co crystals were registered. In comparison with the LO phonon of bulk ZnSe at 252 cm-1 the LO mode for ZnSe:Co was found to be shifted to low frequency range and broadened due to cobalt doping and/or LO-phonon–plasmon coupling caused by free charge carriers. Raman and IR reflection spectra of ZnSe:Co annealed at 1230 and 1450°C revealed an additional low-frequency shoulder of LO(ZnSe) band at about 242 cm-1 which may be attributed to a trigonal Se phase or to surface optical (SO) phonons. Study of the infrared absorption and emission bands associated with transitions between the 4A2(F) and 4T1(F) or 4T(F) states of the Co2+ ions in ZnSe was performed and spectral features due to intrinsic lattice vibrations were identified. As the composition is increased, the triplet structure of 4T1(F) term shifts to lower energies whereas that of 4T1(P) term shifts to higher energies. The influence of doping on the optical and structural properties ZnSe:Co are discussed.