The hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (Co2+: ZnO) is mediated by charge carriers was tested by controlled introduction and removal of the shallow donor interstitial zinc. Using oriented epitaxial Co2+: ZnO films grown by chemical vapor deposition, kinetics measurements demonstrate a direct correlation between the oxidative quenching of ferromagnetism and the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of Co2+: ZnO and, in the process, unambiguously reveal this ferromagnetism to be dependent upon charge carriers. The distinction between defect-bound and free carriers in Co2+: ZnO is discussed.
Co-doped ZnO͑Co x Zn 1−x O͒ is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial Co x Zn 1−x O films on Al 2 O 3 ͑012͒ substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co͑x ഛ 0.35͒ can be uniformly incorporated into the film without phase segregation. Co is found to be in the +2 oxidation state, independent of x, by both surface-sensitive core-level x-ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown n-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ϳ1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small ͑Ͻ0.1 B /Co͒, even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co͑II͒ ions behave as magnetically isolated S =3/2 ions.
We have used x-ray photoelectron spectroscopy with high energy resolution to determine band discontinuities at the two buried interfaces of the epitaxial TiO2 (anatase)/SrTiO3/Si(001) system. The valence band offsets are −2.1±0.1 eV and +0.2±0.1 eV at the SrTiO3/Si and TiO2/SrTiO3 heterojunctions, respectively. Assuming bulk band gaps for the SrTiO3 and TiO2 epitaxial films, the associated conduction band offsets are +0.1±0.1 eV and +0.1±0.1 eV. Si at the interface is in a flatband state, indicating a very low density of electronic states. These results suggest that spin-polarized electron injection from ferromagnetic Co-doped TiO2 anatase into Si should be facile.
Oxidation of silicon with neutral atomic oxygen species generated in a rare gas plasma has recently been shown to produce high-quality thin oxides. It has been speculated that atomic oxygen in the first excited state, O(1D), is a dominant reactive species in the oxidation mechanism. In this study, we investigate the role of O(1D) in silicon oxidation in the absence of other oxidizing species. The O(1D) is generated by laser-induced photodissociation of N2O at 193 nm. We find that, at 400 °C, O(1D) is effective in the initial stages of oxidation, but the oxide growth rate decreases dramatically past 1.5 nm. Oxide films thicker than 2 nm were not achieved regardless of oxidation time or N2O partial pressure (0.5–90 mTorr), indicating O(1D) cannot be a dominant reactive species in thicker rapid oxidation mechanisms. We suggest that quenching of O(1D) to O(3P) (ground state) during diffusion through thicker oxides results in drastically slower oxidation kinetics. In contrast, oxidation with a vacuum ultraviolet excimer lamp operating at 172 nm resulted in rapid oxide growth up to 4 nm. Thus, other species produced in plasmas and excimer lamps, such as molecular and atomic ions, photons, and free and conduction band electrons, likely play a dominant role in the rapid oxidation mechanism of thicker oxides (>2 nm).
Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial Co x Zn 1Ϫx O films on Al 2 O 3 (012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with Al. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5-300 K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor ͑DMS͒ superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/ semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasithree-dimensional or quasitwo-dimensional intergranular hopping, and superparamagnetism.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.