Suitability of ZnO for future spin-based applications

Abstract

During the last decades development in semiconductor technology was based on the miniaturization of already existing devices. This approach will soon reach its limits since further miniaturization will effectively lead to the observation of quantum effects even at room temperature. Future technology demands newdevices which avail themselves of these effects. A property suitable as control mechanism is the spin of charge carriers rather than their charge itself since devices based on spin, e.g., consume less energy. The field of research which investigates the controlled manipulation of spins is called spintronics and has gained much attention during the last years. Spintronics does not only demand new devices, but also new classes of materials.This thesis is devoted to the investigation of (Zn,Mn)O, (Zn,Co)O and ZnO quantum dots as new materials for future spintronic applications.Since a few years researchers have tried to establish diluted magnetic semiconductors by doping ZnO with transition metal ions. First attempts by various research groups were promising. However, it soon became evident that the results were only poorly reproducible. Furthermore, the mechanism which should mediate ferromagnetic coupling in transition metal doped ZnO remains under debate up to now.In this thesis (Zn,Mn)O and (Zn,Co)O mixed oxides have been prepared by radio-frequency magnetron sputtering. Characterization of structural properties was accomplished by X-ray diffraction and various techniques based on transmission electron microscopy. The latter was carried out at the laboratory for transmission electron microscopy at the Karlsruhe Institute of Technology in the group of Prof. Dagmar Gerthsen. To some extent, both mixed oxides showed formation of secondary phases in the form of transition metal monoxides, depending on thepreparation conditions and the composition of the samples.Magnetic properties were charaterized by SQUID measurements and—in case of (Zn,Co)O—X-ray magnetic circular dichroism. Magnetic behavior of the (Zn,Co)O samples which did not show phase segregation was hardly reproducible. Both paramagnetism as well as weak ferromagnetism was observed for different samples prepared under the same conditions. The (Zn,Co)O samples containing a secondary phase of CoO were purely paramagnetic. Despite reports of ferromagnetism in CoO nanoclusters, this behavior was not observed for samples prepared in the course of this work. In contrast, strong ferromagnetic behavior was found for (Zn,Mn)O samples containing MnO which could be linked to the secondary phase.In addition to the mixed oxides, sputtered ZnO quantum dots have been fabricated and investigated. A preparation routine was established to produce ZnO quantum dots at room temperature. Preparation without additional heating is important with regard to future integration into devices, especially optical resonators which could be destroyed by the exposure to heat. In a first step, ZnO nanocrystals with a large size distribution of 1-16 nm were prepared which did not yet show quantum effects. Later a smaller size distribution of 2-5 nm and a blue-shift of the bandgap with respect to bulk ZnO was achieved. The quantum dots were embedded into optical resonators composed of two Bragg mirrors. Their optical properties were characterized by transmittance measurements.