S. Daniš

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Nanocrystalline titanium dioxide films: Influence of ambient conditions on surface- and volume-related photoluminescence

We report on systematic study of photoluminescence properties of nanocrystalline titanium dioxide films consisting of predominantly anatase nanoparticles with diameters larger than 13 nm. We measured photoluminescence under two selected excitation wavelengths (325 nm/442 nm, i.e., above/below band gap excitation), under different values of ambient air pressure (5–105 Pa), and in the temperature interval 10–300 K. On basis of our results, we are able to distinguish two different processes leading to photoluminescence: the exciton recombination and the recombination of carriers in the energy states related to the surface of nanocrystals. We propose microscopic models describing well the effects of temperature and ambient conditions on photoluminescence of nanocrystalline titanium dioxide films.

The effect of nitrogen ion implantation on the surface properties of Ti6Al4V alloy coated by a carbon nanolayer

The ion beam assisted deposition (IBAD) method was chosen for preparing a carbon thin film with a mixing area on a substrate of Ti6Al4V titanium alloy. Nitrogen ions with energy 90 keV were used. These form a broad ion beam mixing area at the interface between the carbon film and the substrate. We investigated the chemical composition by the glow discharge optical emission spectroscopy (GD-OES) method and the phases by the X-ray diffraction (XRD) method. The measured concentration profiles indicate the mixing of the carbon film into the substrate, which may have an effect on increasing the adhesion of the deposited film. The nanohardness and the coefficient of friction were measured. We found that the modified samples had a markedly lower coefficient of friction even after damage to the carbon film, and they also had higher nanohardness than the unmodified samples. The increased nanohardness is attributed to the newly created phases that arose with ion implantation of nitrogen ions.

High-resolution diffuse x-ray scattering from threading dislocations in heteroepitaxial layers

Diffuse x-ray scattering from epitaxial layers with screw dislocations perpendicular to the surface is calculated assuming correlated dislocation positions. The resulting intensity distribution was compared with a reciprocal-space map measured in a symmetric diffraction from a hexagonal GaN(0001) layer, and a good correspondence was achieved. From the fit, both the dislocation density and their correlation length were determined.

Exchange bias in UO2/Fe3O4 thin films above the Néel temperature of UO2

Magnetic exchange bias (EB) has been investigated on antiferromagnetic/ferrimagnetic heterostructures consisting of a stoichiometric UO2 film grown epitaxially on a LaAlO3 substrate and covered with highly textured Fe3O4 layers of variable thickness (tFe3O4 = 90–700 A). EB values reaching ∼2.6 kOe were observed at lowest temperatures in UO2/Fe3O4 with the thinnest Fe3O4. Magnetization measurements demonstrate that a remarkably large exchange bias in UO2/Fe3O4 is present at temperatures higher than the bulk Neel temperature of UO2 (TN = 30.8 K). Our study reveals a complex nature of the effect: we suggest that it is provided both by strong exchange coupling between UO2 and Fe3O4 and by exchange biasing within magnetite (up to 25% of the total effect at 5 K in the UO2/Fe3O4 sample with the largest EB). The former can be caused by proximity effects of Fe3O4 on TN combined with a magnetic anisotropy of UO2, persisting above a continuous magnetic-ordering transformation. For magnetite, the results indicate the...

The misfit dislocation density profile in graded SiGe/Si(001) layers prepared at different temperatures

We present a generalization of the Dodson-Tsao kinematic model of misfit dislocation for graded SixGe1-x/Si(001) layers. The layers were prepared under different growing conditions (temperature). The misfit dislocation distribution has been determined by means of high-resolution x-ray scattering. Analysis of the reciprocal space maps was compared with the kinematic Dodson-Tsao model and the equilibrium Tersoff model.

Physics of polymorphic transitions in CeRuSn

We report a detailed study of the polymorphic transitions in ternary stannide CeRuSn on high quality single crystals through a combination of X-ray diffraction experiments conducted at 300, 275 and 120 K, and measurements of the thermal expansion, magnetization, and resistivity, along main crystallographic axes. In addition, the transition was followed as a function of pressure up to 0.8 GPa. The present X-ray diffraction data show that the room temperature polymorph consists of the lattice doubled along the c axis with respect to the CeCoAl-type structure consistent with previous reports. Upon cooling, the compound undergoes two successive transitions, first to a quintuple (290 K) and than to a triple CeCoAl superstructure at 225 K. The transitions are accompanied by a tremendous volume change due to a strong shrinking of the lattice along the c axis, which is clearly observed in thermal expansion. We advance arguments that the volume collapse originates from an increasing number of crystallographically inequivalent Ce sites and the change of ratio between the short and long Ce-Ru bonds. The observed properties of the polymorphic transition in CeRuSn are reminiscent of the transition in elementary Cerium, suggesting that similar physics, i.e., a Kondo influenced transition and strong lattice vibrations might be the driving forces.

Are RENiAl hydrides metallic

Abstract Due to hydrogenation, the crystal structure of GdNiAl distorts from hexagonal to orthorhombic. The magnetic ordering temperature decreases dramatically and the magnetic entropy of this phase transition is in good agreement with theory. LuNiAl does not change its structure type, but it was found that the electronic contribution to specific heat decreases dramatically. None of the two phases loses its metallic character.

Diffuse X-ray scattering from graded SiGe/Si layers

We present a method for the determination of the density profile of misfit dislocations in graded SiGe layers based on high-resolution X-ray diffraction and simulation of diffuse X-ray scattering from misfit dislocations. From the density profile of misfit dislocations both the profile of local plastic relaxation and the profile of the Ge content were determined; the latter compares very well with the results of depth-resolved secondary-ion mass analysis.

Low-temperature transport and crystallographic studies of Er(Co1−xSix)2 and Er(Co1−xGex)2

Abstract The evolution of the magneto-transport and magneto-elastic anomalies in Si and Ge doped ErCo 2 compounds was studied by means of electrical resistivity and X-ray powder diffraction measurements. The substitution of Ge for Co causes a substantial lattice expansion, which may be one of the reasons for the observed dramatic increase of the Curie temperature observed with Ge doping. A less pronounced enhancement of T C was found with Si doping, which leaves the lattice volume nearly intact. The drop of resistivity at T C observed in ErCo 2 becomes gradually reduced with increasing Si or Ge concentration. Also, both the spontaneous magneto-volume effect and the size of rhombohedral distortion below T C decrease with increasing Si and Ge content, which probably reflects a decreasing magnetic moment of Co.