Optical and electrical characterization of cadmium selenide doped with cobalt Original Research Article

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 (1×1019 cm−3) single crystal. Infrared absorption and emission spectra associated with transitions between the 4A2(F) ground state and the 4T1(F) and 4T2(F) excited states are described. At 10 K, spin–orbit splittings result in three structured absorption bands associated with the 4A2(F) to 4T1(F) transition having zero-phonon lines at 4926, 5101, and 5724 cm−1. The 4A2(F) to 4T2(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 4T1(F) to 4A2(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.