Defect Mediated Ferromagnetism in Zinc Oxide Thin Film Heterostructures

Recent developments in the field of spintronics (spin based electronics) have led to an extensive search for materials in which semiconducting properties can be integrated with magnetic properties to realize the objective of successful fabrication of spin-based devices. Since zinc oxide (ZnO) posits a promising player, it is important to elucidate the critical issues regarding the origin and nature of magnetism in ZnO thin film heterostructures. Another critical issue in the development of practical devices based on metal oxides is the integration of high quality epitaxial thin films on the existing technology based on Si (100) substrates, which requires appropriate substrate templates.;The present research work is focused on the study of room temperature ferromagnetism (RTFM) caused by intrinsic defects and precise control of RTFM using thermal treatments and laser and ion irradiation. We performed a systematic study of the structural, chemical, electrical, optical and magnetic properties of undoped ZnO films grown under different conditions as well as the films that were annealed in various environments. Oxygen annealed films displayed a sequential transition from ferromagnetism to diamagnetism as a function of the annealing temperature. An increase in the green band intensity has been observed in oxygen annealed ZnO films. Reversible switching of room-temperature ferromagnetism and n-type conductivity have been demonstrated by oxygen and vacuum annealing. Detailed electron energy loss spectroscopy and secondary ion mass spectroscopy studies have been presented to rule out the possibility of external source of magnetism. Electron-Paramagnetic Resonance (EPR) measurements indicate the presence of a broad peak at g=2.01. This would be most consistent with the magnetic moment arising from the oxygen vacancies (g=1.996), although the possible contribution from Zn vacancies (g=2.013) cannot be entirely ruled out. The magnetic moment in these films may arise from the unpaired 2p electrons at the O sites surrounding the zinc vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.49 to 0.74 muB and the oxygen vacancies may provide the coupling mechanism. Results of EPR study are found to be in agreement with the results of magnetization and conductivity measurements. The effect of UV Excimer laser irradiation on electrical, magnetic and optical properties of ZnO thin films has been studied. Increases in the electrical conductivity and magnetic moment have been controlled precisely with the number of laser pulses, without altering the Wurtzite crystal structure and n-type semiconducting characteristics of the films. The laser-induced ferromagnetism and concomitant conductivity enhancement can be reversed through subsequent thermal annealing. It has also been shown that heavy swift ion irradiation can also create room temperature ferromagnetism in oxygen annealed insulating ZnO films. Saturation magnetic moments increase with increasing ion dose. A systematic study of the thickness dependency of the structural, electrical and magnetic properties of undoped ZnO thin films has been presented. The role of film/substrate interface in magnetism has been discussed. It has been shown by EPR study and oxidative quenching of ferromagnetism that oxygen vacancies are the key mediating defect in ferromagnetic ZnO thin films.;Finally growth of epitaxial ZnO on Si (100) substrates has been achieved using a titanium nitride (TiN)/strontium titanate (STO) template layer. It has been shown that TiN can be grown epitaxially on silicon substrates. It was observed that, crystallographic orientations of ZnO on STO can be controlled by the oxygen pressure and substrate temperature during the deposition. The detailed x-ray diffraction, transmission electron microscopy (TEM), electrical and magnetic characterization results for the deposited films have been carried out.;The above mentioned methods provide a controlled way to study changes in magnetic, electrical and optical properties of ZnO films and determine the mechanisms associated with RTFM in ZnO based materials and visualize exciting new areas ranging from spintronics to biomedical applications.