Charged carrier spin dynamics in ZnO quantum wells and epilayers

Longitudinal charged carrier spin dynamics is studied for ZnO quantum wells and epilayers using the optical transition of the negatively charged exciton X^- and the neutral donor bound exciton D⁰X , respectively. The hole spin relaxation is derived from the optical orientation of X^- and D⁰X photoluminescence, whereas the spin relaxation of the resident electrons and donor electrons is accessed via the bleaching of the spin selective excitation process. Hole spin relaxation times of τ₁^{s ,h} of 80 and 140 ps are found for D⁰X and X^-, respectively, which are practically independent of a magnetic field B_∥ applied along the ZnO c ⃗ axis. Much longer longitudinal electron spin relaxation times in the 1 μ s range are uncovered if the hyperfine interaction is suppressed by a proper B_∥. A field strength of ≈2 mT is large enough proving the extremely small value of the Overhauser field in ZnO. This is related to the very restricted number of magnetic nuclei interacting with the electron inside the volume of the exciton complex.