Spin-dependent recombination of defects in bulk ZnO crystals and ZnO nanocrystals as studied by optically detected magnetic resonance

Long-lasting (several hours) afterglow at helium temperatures was observed in ZnO nanocrystals, 36 nm in size, after switching off the UV excitation. A quenching of the afterglow intensity in the magnetic field was found, which appears because of the preferential orientation of the recombining spins along the field and the spin-dependence of the recombination. ODMR was detected as resonance increase in the afterglow intensity at saturation of the EPR transitions of defects, which take part in the recombination. Shallow donors and different types of acceptors responsible for the afterglow in Al doped ZnO nanocrystals were identified. ODMR of donors and Li acceptors was recorded via the afterglow of bulk ZnO:Li, which could last for about an hour. In the photoluminescence-detected ODMR of bulk ZnO:Li crystals exchange-coupled donor-acceptor pairs were observed in addition to the Li acceptor and donor signals. Exchange-coupled pairs dominated the ODMR spectra recorded on the photoluminescence of ZnO nanocrystals.