Epitaxial growth and characterization of cobalt-doped zinc oxide and cobalt-doped titanium dioxide for spintronic applications

While conventional electronic devices utilize the charge of an electron to process data, those that are based on spin electronics, commonly referred to as spintronics, would rely on quantum mechanical spin. Many such devices would require electrical injection of a spin-polarized current into semiconductor heterostructures in order to function. Semiconducting materials that are ferromagnetic represent an ideal means of meeting that requirement, provided the material remains ferromagnetic above room temperature and exhibits a high degree of spin polarization. Thus far, only a handful of these materials have been discovered. The main focus of this research is on understanding the epitaxial growth and properties of two such materials—Co_xZn_1−xO films grown on Al₂O₃(01·2) substrates and Co _xTi_1−xO₂ thin films grown on Si(001).; The epitaxial Co_xZn_1−xO films were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. Large amounts of Co (x ≤ 0.35) can be uniformly incorporated into the film without phase segregation. Furthermore, the Co is found to be in the +2 oxidation state. Experiments show that low-resistivity, n-type films remain ferromagnetic up to 350 K, the highest temperature measured to date, and that oxygen vacancies contribute the majority of those n-type carriers. Finally, absorption and magnetic circular dichroism (MCD) spectra suggest that the film is in fact a diluted magnetic semiconductor.; The Co_xTi_1−xO₂ anatase films were deposited on Si(001) substrates through the use of a SrTiO₃ buffer layer. All of the films were synthesized by molecular beam epitaxy (MBE) and represent the first instance of epitaxially-grown Co_xTi_1−x O₂ anatase films on Si substrates. The results show that the highest quality films are grown at low Ti evaporation rates (0.027 Å of Ti metal/second) and high temperatures (948 K). However, at these conditions, the presence of a SrTiO₃ layer does not prevent oxidation of the underlying Si. All of the cobalt-doped films show the Co segregating to particles that form on the film surface during growth. Finally, band offset measurements at the TiO₂/SrTiO₃ and SrTiO₃/Si heterojunctions indicate that the overall band structure will be conducive to spin-polarized charge transport from the ferromagnetic Co_xTi_1−xO ₂ film into Si.