Thermal annealing effect on spin coherence in ZnO single crystals

Abstract

The spin coherence time (T2*) in ZnO single crystals at 8.5 K decreases significantly from ∼11.2 ns to ∼2.3 ns after annealing at 500 °C, as indicated by time-resolved Kerr-rotation pump-probe magneto-optical spectroscopy. The annealing-induced spin coherence degradation in ZnO arises neither from crystallinity degradation during the annealing process, as confirmed by x-ray rocking curves; nor from reflection variations of the probe laser beam induced by surface roughness changes during the annealing process, as confirmed by atomic force microscopy. Temperature-dependent Hall-effect studies indicate that decreased mobility and increased shallow-donor concentration in the annealing-induced surface conducting layer on top of the bulk ZnO are most likely to be the reasons for the spin coherence degradation in ZnO during the annealing process.

Epitaxial Mn-doped ZnO diluted magnetic semiconductor thin films grown by plasma-assisted molecular-beam epitaxy

“…According to their analyses [58], the weak spinorbit coupling (À 3.5-16 meV [58][59][60][61][62][63][64]) leads to the cancellation of the circular polarization from the optical transitions between the conduction band and valence band states in ZnO and the spin relaxation is very fast (45-80 ps at 2 K [58]) especially when the ZnO is of high impurity density [58]. Our recent studies [65] based on time-resolved optical orientation measurements also show that the spin coherence time in ZnO is significantly decreased when the density of ''impurity'' states increases. These may be the possible reasons for no observation of spin polarization in our magneto-PL studies.…”

“…However, some successful magneto-optical studies [60,61,[65][66][67][68][69][70][71] have also been reported, despite of the two claimed ''dominating factors'' [58]. For example, long electron spin coherence times (up to 20 ns at 30 K [66]) were observed in ''relatively clean'' undoped ZnO samples using time-resolved Faraday/Kerr rotation spectroscopy at low-temperatures [65][66][67][68], and even at elevated temperatures (188 ps at 280 K [66]); a not short hole spin coherence time (350 ps at 1.7 K) in ZnO [61] and a not small polarization (11% at 20 K [60]) together with a not short decay time (275 ps at 20 K [60]) of donor-bound exciton were observed in ZnO using time-resolved magneto-PL studies; evident free [69] and bound [70] exciton splittings were observed in ZnO-based DMS materials using magnetic-optical techniques and well explained [71]. So these distinct results (unsuccessful vs. successful optical spin detections of ZnO) suggest that the ''intrinsic background impurity/defect states'' of ZnO, which differs significantly from samples to samples prepared with different methods, more or less determine the possibility of optical spin detections in ZnO also.…”