P. Ziemann

Optical investigations on the annealing behavior of gallium- and nitrogen-implanted ZnO

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.

Sequential ion-induced stress relaxation and growth: A way to prepare stress-relieved thick films of cubic boron nitride

It is shown that the bombardment of high quality cubic (c-) BN films with 300 keV Ar+ ions leads to a strong relaxation of their compressive stresses without destroying the cubic phase if the total ion fluence is kept below an upper limit. In addition, it was found that on top of such a stress-relieved film a further pure c-BN layer can be grown, but it builds up compressive stress again. Based on both results, a procedure is developed to grow thick (>1 μm) c-BN films (>80% c-BN) exhibiting low residual stress and long term stability under ambient conditions.

Phase stability and stress relaxation effects of cubic boron nitride thin films under 350 keV ion irradiation

Abstract To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) films, samples either prepared by r.f. magnetron sputtering or ion beam-assisted sputter deposition (IBAD) were irradiated at room temperature with 350 keV inert ions (Kr + ). FTIR spectra taken before and after each irradiation step clearly demonstrate that c-BN is stable under this medium energy bombardment. Furthermore, additional AES measurements show that the average film stoichiometry is not affected by the ion fluences used in the present experiments. While the observed broadening of the different lines in the IR spectra with increasing ion fluences points to a build-up of disorder and/or a decreasing average grain size due to the bombardment, the additionally found significant shift of the line related to the c-BN TO-mode towards smaller wave numbers, indicates a strong relief of the compressive stress present in the as-prepared films. This irradiation-induced stress relief could be independently confirmed by profilometer measurements yielding post-bombardment values of 5.1 GPa as compared to 20.5 GPa before irradiation.

The effect of substrate surface nanotopography on the behavior of multipotnent mesenchymal stromal cells and osteoblasts.

Hexagonally arranged Gold nanoparticles with controllable diameters and inter-particle distances were deposited on thick SiO2 layers on top of Si wafers and used as masks during subsequent reactive ion etching. In this way, arrays of nanopillars are obtained with well-defined diameters (10/30 nm), inter-pillar distances (50-120 nm) and heights (20-35 nm), all on the nanoscale. Such nanotopographies served as substrate for multipotent mesenchymal stromal cells (MSC) and human osteoblasts (OB) allowing to study cellular responses to purely topographically patterned interfaces. Focus was put on adhesion, proliferation and differentiation of the cells. It turned out experimentally that adhesion is comparable for both cell types practically independent of topographical details at the substrate surface. Topography induced proliferation enhancement, however, is again independent of geometrical details in case of MSC, but significantly sensitive to pillar height in case of OB with a clear preference towards short nanopillars (20 nm). A high sensitivity to topographic details is also observed for osteogenic differentiation of MSC, in that case with a preference towards higher nanopillars (50 nm). The present experimental data also allow the important conclusion that cell proliferation and differentiation can be optimized simultaneously by fine-tuning nanoscaled topographical parameters.

Spin wave resonance in Ga1−xMnxAs

We report ferromagnetic resonance experiments on Ga1−xMnxAs thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10−9 Oe cm2 for the spin stiffness and JMnMn≈1 meV for the exchange integral between Mn spins are obtained.

Lowering of the L10 ordering temperature of FePt nanoparticles by He+ ion irradiation

Arrays of FePt particles (diameter 7nm) with mean interparticle distances of 60nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L10 phase is tracked for 350kV He+ ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He+ ion exposure yields a significant reduction of the ordering temperature by more than 100K.

Chemically Induced Metal-to-Insulator Transition in Au55 Clusters

The cluster compound Au55(PPh3)12Cl6 has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.

Fe oxidation versus Pt segregation in FePt nanoparticles and thin films

Metallic nanoparticles containing 3d elements are generally susceptible to oxidation leading to a deterioration of desired properties. Here, the oxidation behavior of differently sized FePt nanoparticles is experimentally studied by x-ray photoelectron spectroscopy (XPS) and compared to a FePt reference film. For all as-prepared metallic samples the common features are the formation of Fe(3+), becoming detectable for exposures to pure oxygen above 10(6) langmuir whereas under identical conditions the Pt(0) signal is conserved. Most notably, these features are independent of particle size. Annealing at 650 degrees C, however, affects small and large FePt particles differently. While large particles as well as the reference film show a 100-1000 times enhanced resistance against oxidation, small FePt particles (diameter 5 nm) exhibit no such enhancement due to the thermal treatment. Additional XPS intensity analysis in combination with model calculations leads to an explanation of this observation in terms of Pt segregating to the surface. In large particles and films the thickness of the resulting Pt layer is sufficient to strongly impede oxidation, while in small particles this layer is incomplete and no longer provides protection against oxidation.

Monodispersed NiO nanoflowers with anomalous magnetic behavior

Nickel oxide (NiO) nanoflowers, prepared by thermal decomposition, exhibit anomalous magnetic properties far below the blocking temperature, i.e., a cusp in both the zero-field-cooled and field-cooled curves at about 21 K. Detailed characterization discloses that the individual NiO nanoflower consists of porous crystals with holes (1.0-1.5 nm in size) inside. We believe that the low temperature magnetic feature observed here could be a new kind of spin transition for the uncompensated spins around the holes and will trigger more studies in other nanostructured antiferromagnetic materials.

Growth of thin, flat, epitaxial (111) oriented gold films on c-cut sapphire

AbstractAnewgrowthprocedurehasbeendevelopedtopreparethin(<50nm),flat(roughness 0.3–1nm rms ),epitaxialAufilmsontopofaninsulatingsubstratewithoutusingamagneticseedlayer.ThiscouldbeobtainedbydepositingfirstaNbseedlayer(61nm)atroomtemperatureontopofc-cutsapphirefollowedbyevaporatingAuatarateof0.05–0.1nm/sat300 C.Thisprocedureresultedin(111)orientedflatAufilmsevenforthicknesswellbelow50nm.Theflatnessofthesefilmswasconsistentlyconfirmedbyatomicforceandscanningtunnelingmicroscopy,theirexcellentepitaxialquality(rocking width 0.1 –0.3 ) byX-raydiffractometryand reflected high energyelectron diffraction. 2001ElsevierScienceB.V.Allrightsreserved. Keywords:Gold;Niobium;Aluminumoxide;Epitaxy;Growth;Metallicfilms;X-rayscattering,diffraction,andreflection;Reflectionhigh-energyelectrondiffraction(RHEED) 1. IntroductionThere is immense interest in flat epitaxial andthin, yet electrically conducting, metallic films,which then can be used for experiments likee.g. attenuated total reflection (ATR) [1], electro-depositionofnanoclusters[2,3]orscanningtunnel-ing microscopy(STM) on organic monolayers [4].In this context, Au films are especiallyattractivedue to their chemical stability, which is essential,if, after film preparation under ultrahigh vacuum(UHV),experimentsdemandfurtherexsitusteps.In our case, aiming at studying surface sensitiveelectrical transport properties of such thin films,the application of magnetic seedlayers had to beexcluded, since otherwise additional spin-depen-dent scattering mechanisms would contributeto the resistance. Furthermore, to make surfacescatteringthedominantprocess,competingdefectscatteringprocesseshavetobeminimizedleadingtothedemandofthin,flat,highqualityepitaxialAu films. As a consequence of these combinedqualityrequirements, the well known substratesusedtogrowepitaxialAufilms,i.e.mica[5–7]andMgO [8] turn out to be unsuitable. On mica, thepercolation thickness of epitaxial Au films neces-saryto observe electrical conductance appears tobe close to 30 nm. Thus, for epitaxial Au filmsbelow30nmelectricalconductancecannolongerbeguaranteed.ForMgO,ontheotherhand,Feor