Temperature dependence of the spin relaxation in highly degenerate ZnO thin films

Zinc oxide is a wide-bandgap semiconductor which is considered a potential candidate for fabricating next-generation transparent spintronic devices. However, before this can be practically achieved, a thorough, scientific understanding of the various spin transport and relaxation processes undergone in this material is essential. In the present paper we report our investigations into these processes via temperature dependent, non-local Hanle experiments. Epitaxial ZnO thin films were deposited on c-axis sapphire substrates using a pulsed laser deposition technique. Careful structural, optical, and electrical characterizations of the films were performed. Temperature dependent Hanle measurements were carried out, using an all-electrical scheme for spin injection and detection, in a non-local geometry over the temperature range of 20 - 300 K. Carrier concentration in these films, as determined by Hall effect measurements, was found to be of the order of 10^19 cm^-3. It was determined that in such a degenerately doped system it is essential to use Fermi-Dirac statistics to explain the transport of carriers in the system. From the Hanle data, spin relaxation time in the ZnO films was determined at different temperatures. Our analysis of the temperature-dependent spin relaxation time data suggests that the dominant mechanism of spin relaxation in ZnO films is the Dyakonov-Perel (DP) mechanism modified for the wurtzite crystalline structure in which a hexagonal c-axis reflection asymmetry is present. As a result of this modification the spin-relaxation rate is linear-in-momentum.