Magnetotransport Properties of Microstructured ZnO Thin Films Grown on a- and r-Plane Sapphire Substrates

Herein, magnetotransport properties of microstructured c- and a-plane ZnO thin films grown on a- and r-plane sapphire substrates are investigated. The grain and grain boundary contributions to the electrical transport are verified using impedance spectroscopy. Photoluminescence measurements show maxima related to oxygen and zinc vacancies (V-Zn) present in both kinds of samples, which, especially V-Zn, can contribute to the ferromagnetic behavior. The temperature dependence of the resistance indicates the existence of two different regimes, a variable-range hopping mechanism at temperatures T <= 30 K, whereas thermally activated transport dominates at higher temperatures. The magnetoresistance between 2 and 250 K is negative for all samples, indicating the existence of spin-scattering processes. Hall-effect measurements reveal that the samples are n-type but have a small anomalous-like contribution related to different types of charge carriers.