Optically detected magnetic resonance of the triplet state of copper-center—donor pairs in CdS

Eight distinct types of triplet-state optically detected magnetic resonance (ODMR) spectra have been obtained at 2 K by monitoring the intensity of a broad emission band (peak wavelength 720 nm) observed for undoped, vapor-phase CdS crystals. Several of the ODMR spectra show hyperfine structure due to copper. The signals are attributed to exchange-coupled pairs consisting of a shallowly bound electron and a hole that is localized in the $3d$ shell of a copper ion. The copper ion is part of an associate center $\mathrm{Cu}X$, which can be oriented either parallel to or at 109° to the crystal $c$ axis. The eight different spectra correspond to different geometrical arrangements of this center and of a nearby donor impurity. Recombination of the electron bound to this complex defect with the copper-center hole gives rise to the optical emission. It is shown how the hyperfine and $g$ tensors for the triplet centers can be related to the corresponding quantities for the individual $S=\frac{1}{2}$ electron and hole centers from which they are constructed.