Exciton spin dynamics in ZnO epilayers

Abstract

We used time-resolved optical orientation experiments to study the low temperature spin dynamics of a ZnO epilayer. The sample shows a circular polarisation of the donor-bound exciton of 11% with a decay time of 275 ps. A very narrow spectral dependence of the initial polarisation and a rapid decrease of the polarisation decay time with temperature are also observed.

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

“…Recently, Chen et al [58] pointed out that two dominating factors may limit the efficiency of optical spin detection in ZnO-based materials, which are the weak spin-orbit interactions and the fast carrier/exciton spin relaxations in ZnO. 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].…”