Transition Metal Doped MOCVD-Grown ZnO Epitaxial Films and Nanostructures for Spintronic Applications
Professor Robert A. Bartynski
Department of Physics and Astronomy
Laboratory for Surface Modification, NanoPhysics Laboratory
ZnO is a wide bandgap (~3.3 eV) semiconductor which, when doped with transition metals (TM), becomes a promising candidate diluted magnetic semiconductor for room temperature spintronics applications. We have characterized the chemical, compositional, and magnetic properties of TM-doped ZnO epitaxial thin films and nanostructures grown by MOCVD. The films and nanopillars were doped with Mn or Fe either by ion implantation or in-situ during MOCVD growth. RBS ion channeling shows a minimum yield < 2% for the ZnO epi films indicating excellent crystallinity. The minimum yield is much higher for the ion implanted samples, but improves dramatically upon annealing. Soft x-ray absorption spectroscopy (SXAS) indicates that the TM dopant is primarily in the 2+ oxidation state when implanted, but becomes dominate by higher oxidation states after annealing. In-situ doped films exhibit oxidation states similar to ion implanted films that have been annealed. SQUID magnetometry measurements show that both the implanted and annealed films and nanostructures exhibit hysteretic M vs. H curves at temperatures as high as liquid nitrogen temperature. M(T) curves show a small paramagnetic component at 5 K, but the majority of the magnetization remains up to room temperature. TM-ion implanted MOCVD-grown ZnO nanopillars show relatively uniform TM concentration (<~ 5%) throughout the pillar. X-ray diffraction and TEM images show no indication of secondary phase formation or metal clustering upon annealing to temperatures as high as 700C.