Optical and electrical characterization of cadmium selenide doped with cobalt

The optical and electrical properties of Co2+ ions in CdSe have been investigated. Absorption, photoluminescence, electron paramagnetic resonance, and Hall measurements were used to characterize a cobalt-doped (1 x 10(19) cm(-3)) single crystal. Infrared absorption and emission spectra associated with transitions between the (4)A(2)(F) ground state and the T-4(1)(F) and T-4(2)(F) excited states are described. At 10 K, spin-orbit splittings result in three structured absorption bands associated with the (4)A(2)(F) to T-4(1)(F) transition having zero-phonon lines at 4926, 5101, and 5724 cm(-1). The (4)A(2)(F) to T-4(2)(F) transition shows two zero-phonon lines at 2874 and 3286 cm(-1), also accompanied by vibronic structure. Intrinsic lattice modes explain most of the sharp-line structure observed at low temperature, except for a subset of peaks where local modes (25-30 cm(-1)) are invoked. Using below-band-gap light, selective excitation allows detection of the T-4(1)(F) to (4)A(2)(F) recombination at liquid-helium temperatures. The activation of free carriers in our n-type material containing shallow donors is affected by the presence of cobalt, and we find the Co+/++ level to be about 34 meV below the conduction band of CdSe. (c) 2006 Elsevier Ltd. All rights reserved.