Obtaining strong ferromagnetism in diluted Gd-doped ZnO thin films through controlled Gd-defect complexes

We demonstrate the fabrication of reproducible long-range ferromagnetism (FM) in highly crystalline Gdx Zn 1−xO thin films by controlling the defects. Films are grown on lattice-matched substrates by pulsed laser deposition at low oxygen pressures (≤25 mTorr) and low Gd concentrations (x ≤ 0.009). These films feature strong FM (10 μB per Gd atom) at room temperature. While films deposited at higher oxygen pressure do not exhibit FM, FM is recovered by post-annealing these films under vacuum. These findings reveal the contribution of oxygen deficiency defects to the long-range FM. We demonstrate the possible FM mechanisms, which are confirmed by density functional theory study, and show that Gd dopants are essential for establishing FM that is induced by intrinsic defects in these films

Magnetic properties of HO2 thin films

We report on the magnetic and transport studies of hafnium oxide thin films grown by pulsed-laser deposition on sapphire substrates under different oxygen pressures, ranging from 10-7 to 10-1 mbar. Some physical properties of these thin films appear to depend on the oxygen pressure during growth: the film grown at low oxygen pressure (P ~= 10-7 mbar) has a metallic aspect and is conducting, with a positive Hall signal, while those grown under higher oxygen pressures (7 x 10-5 <= P <= 0.4 mbar) are insulating. However, no intrinsic ferromagnetic signal could be attributed to the HfO2 films, irrespective of the oxygen pressure during the deposition.Comment: 1

Defect-induced room temperature ferromagnetism in B-doped ZnO

ZnO microrods were grown on glass substrates by the spray pyrolysis method and boron was doped into the ZnO microrods by diffusion. X-ray diffraction results confirmed that the incorporation of B leads to a slight reduction in the deposit texture. Scanning electron microscopy measurements showed that the morphology of the ZnO samples changed from a microrod to nanocrystalline structure with B-doping. Photoluminescence data indicate that B-doping leads to a relative increase of the unstructured green band intensity. Magnetic measurements revealed that B-doped ZnO samples exhibited room temperature ferromagnetism related to defects, in agreement with first principles theoretical calculations

PLD GROWTH OF HIGH QUALITY ZINC OXIDE THIN FILMS ON SI SUBSTRATES AND DEVICE DEVELOPMENT

Growth of high quality zinc oxide thin films on silicon substrates is particularly important because it combines the unique features of zinc oxide (ZnO) with mature CMOS technology and paves the way for device developments. However, this is a challenging task due to several technical and material-related fundamental issues which exist with the growth of this highly sought after compound semiconductor. In general, metal-oxide semiconductors suffer from non-stoichiometric growth which leads to unipolar doping properties, such as ZnO grows nominally n-type while NiO grows p-type. Thus, these materials can be doped easily either n or p-type while the other polarity is hard to achieve, if not impossible. Although methods for the growth of p-type films, using extrinsic doping or exotic precursors with post growth treatments have been reported on different substrates, the problem of controlled and stable extrinsic p-type doping of ZnO films remains an open subject for research. In the present work, we have achieved the growth of undoped p-type ZnO films on Si (100) substrates by pulsed laser deposition through the optimization of growth conditions and adjustments of growth dynamics. Currently no other reports of undoped p-type ZnO on Si substrates by PLD growth exist, showing stable p-type conductivity in a repeatable process. The structural, optical, and electrical properties of the grown films were examined using techniques such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Photoluminescence spectroscopy (PL), Hall Effect four-point probe Van Der Pauw measurements, and Current-Voltage (I-V) measurements for the p-n heterojunctions. Electrical behavior of ZnO is generally attributed to intrinsic defects which include vacancies, interstitials, and anti-sites of Zn and O in addition to external contamination related defects such as hydrogen complexes. The effects of growth conditions and intrinsic defects including hydrogen contamination on the properties of the grown layers are studied. Moreover, the growth dynamics of ZnO polar planes, i.e. the stacking of O-2 and Zn+2 planes, on n and p-type Si substrates are discussed. Once material studies and growth optimizations are completed, high quality ZnO films are used in device fabrication. Two types of optoelectronic devices, a photoresistor and a Schottky diode are fabricated on Si substrates, and the electrical behavior of the devices are investigated. The high quality ZnO films also contributed to the development of a surface acoustic wave (SAW) biosensor

ZnO:Co Diluted Magnetic Semiconductor or Hybrid Nanostructure for Spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie-Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.Comment: To be published in Journal of Materials Scienc

Origin of Ferromagnetism in nitrogen embedded ZnO:N thin films

Nitrogen embedded ZnO:N films prepared by pulsed laser deposition exhibit significant ferromagnetism. The nitrogen ions contained in ZnO confirmed by Secondary Ion Microscopic Spectrum and Raman experiments and the embedded nitrogen ions can be regarded as defects. According to the experiment results, a mechanism is proposed based on one of the electrons in the completely filled d-orbits of Zn that compensates the dangling bonds of nitrogen ions and leads to a net spin of one half in the Zn orbits. These one half spins strongly correlate with localized electrons that are captured by defects to form ferromagnetism. Eventually, the magnetism of nitrogen embedded ZnO:N films could be described by a bound magnetic polaron model.Comment: 7 pages, 6 figure

Functional Oxide Thin Films and Nanostructures: Growth, Properties, and Applications

This Special Issue of Coatings is entitled “Functional Oxide Thin Films and Nanostructures: Growth, Properties, and Applications”. Recent materials nanotechnologies have created possibilities regarding the fabrication of oxide thin films at the nanometric level and other nanocomposites’ fabrication. In parallel, recent measurement technologies can characterize their unique properties arising from the limited regions of surfaces and interfaces. This Special Issue provides an opportunity to share surface-related science and engineering topics on oxide thin films and nanocomposites in an interactive and interdisciplinary manner. The ultimate goal is to elucidate the commonalities and differences between multilayer interfaces and nanocomposite grain boundaries. This Special Issue is as an effort to bridge the gap between materials science and the applications of oxide thin films and nanostructures. The topics covered in this Special Issue range from nanoparticles to thin films, heterostructures, and homojunctions and are related to various aspects of oxide materials’ preparation, characterization, and applications