Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.Comment: 13 pages, 14 figure

Fe-implanted ZnO: Magnetic precipitates versus dilution

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present paper we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences and temperatures and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to the Fe concentration and the process temperature are found: 1) Disperse Fe$^{2+}$ and Fe$^{3+}$ at low Fe concentrations and low processing temperatures, 2) FeZn$_2$O$_4$ at very high processing temperatures and 3) an intermediate regime with a co-existence of metallic Fe (Fe$^0$) and ionic Fe (Fe$^{2+}$ and Fe$^{3+}$). Ferromagnetism is only observed in the latter two cases, where inverted ZnFe$_2$O$_4$ and $\alpha$-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier mediated coupling. However, their separation is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison investigations of Fe-implanted epitaxial ZnO thin films are presented.Comment: 14 pages, 17 figure

Compositional, structural and morphological modifications of N-rich Cu3N films induced by irradiation with Cu at 42 MeV

N-rich Cu3N films were irradiated with Cu at 42 MeV in the fluences range from 4 × 1011 to 1 × 1014 cm−2. The radiation-induced changes in the chemical composition, structural phases, surface morphology and optical properties have been characterized as a function of ion fluence, substrate temperature and angle of incidence of the incoming ion by means of ion-beam analysis (IBA), x-ray diffraction, atomic force microscopy, profilometry and Fourier transform infrared spectroscopy techniques. IBA methods reveal a very efficient sputtering of N whose yield (5 × 103 atom/ion) is almost independent of substrate temperature (RT-300 °C) but slightly depends on the incidence angle of the incoming ion. The Cu content remains essentially constant within the investigated fluence range. All data suggest an electronic mechanism to be responsible for the N depletion. The release of nitrogen and the formation of Cu2O and metallic Cu are discussed on the basis of existing models

Structural and magnetic properties of Mn-implanted Si

Structural and ferromagnetic properties in Mn implanted, p-type Si were investigated. High resolution structural analysis techniques like synchrotron X-ray diffraction revealed the formation of MnSi1.7 nanoparticles already in the as implanted samples. Depending on the Mn-fluence, the size increases from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. The observed ferromagnetism yields a saturation moment of 0.21 mu_B per implanted Mn at 10 K, which could be assigned to MnSi1.7 nanoparticles as revealed by a temperature dependent magnetization measurement.Comment: 21 pages, 6 figures, accepted for publicaiton at Phys. Rev.

Reactive Magnetron Sputtering of CuInS2 Absorbers for Thin Film Photovoltaic Relation between Deposition Parameters and Film Morphology

For the preparation of high quality CuInS2 absorber films, two film properties are decisive large crystallites and compact film morphologies. In this work we investigate the influence of the deposition parameters discharge power and substrate bias, on the morphology and chemical composition of CuInS2 films, which were prepared by reactive magnetron sputtering radio frequency in H2S Ar atmosphere. A lateral concentration distribution along the substrate axis, which is caused by the arrangement the copper and indium cathodes in our sputtering equipment, was used to study the influence of the Cu to In ratio on the film growth. At a Cu to In ratio of one, which can be adjusted by the discharge powers at the copper and indium target to be positioned at the middle of the substrate, the crystallites in the film are small in size and separated by crevices from each other. In order to achieve compact CuInS2 films, the negative substrate bias was varied systematically 15V to 150 V . The influence of the ion assistance, i.e. the impact of positive argon ions onto the growing film, was investigated with respect to the chemical composition and the morphology of the film

Post-implantation defects instability under 1 MeV electron irradiation in GaAs

The influence of 1 MeV electron irradiation on the stability of post-implantation defects in GaAs has been investigated. The n-type GaAs wafers of orientation were implanted with 150 keV As+ ions below the amorphization threshold at RT using the implantation dose of 2×1013 ions cm–2 at a constant flux of 0.1 žA cm–2. Then the implanted samples were irradiated with a scanned beam of 1 MeV electrons from a Van de Graaff accelerator in a dose range (0.5–5.0)×1017 cm–2 at 320 K. RBS and channeling spectroscopy of 1.7 MeV 4He+ ions were used to determine the depth distribution of defect concentration before and after 1 MeV irradiations. New results of an "oscillatory" behaviour of the damage level as a function of 1 MeV electron fluence are presented

Metabolic shifts in hypersaline microbial mats upon addition of organic substrates

The responses of hypersaline microbial mats to the addition of acetate, glycolate or glucose were investigated using oxygen, pH and sulphide microsensors. Changes in community structure were investigated with molecular techniques. Acetate addition inhibited respiration in the photic zone, stimulated respiration in the aphotic zone and had no effect on gross photosynthesis. Glycolate addition strongly increased both respiration and gross photosynthesis in the photic zone. Thus, glycolate and acetate were probably consumed in those regions of the mat where these substrates are usually formed. Moreover, photosynthesis was only stimulated by increased respiration and concomitant CO2 production in the photic zone which indicates that the photosynthetic and respiratory populations must be present in close proximity to each other. Glucose addition had an unexpected negative effect on the microbial population, strongly inhibiting both respiration and gross photosynthesis within hours. After four days, oxygen profiles in the light were equal to those measured in the dark. After replacing the water phase with unamended water, photosynthesis and respiration recovered within a week. None of the physiological changes were accompanied by detectable shifts in the cyanobacterial or the overall microbial community. The mechanism of inhibition of photosynthesis by glucose requires further investigation