acta physica slovaca

Abstract: Correlation between thermal expansion and magnetic anisotropy in FeNbCuBSi alloys with 3 and 4.5 at. % Nb with various Si contents has been investigated. Besides expected influence of Nb as a crystal-growth-hampering component, specific effect of Nb in macroscopically heterogeneous ribbons is indicated. The heterogeneity is due to chemically active surfaces of the ribbons. It emerges after air annealing and varies markedly with the contents of Si. Despite promoting thermal expansion prior to crystallization, the complex chemical reactions on air exposed surfaces result in more crystallized surfaces that squeeze the underlying volume. The evidence of squeezing comes from characteristic magnetic anisotropy.

Abstract: X-ray topography as a completely nondestructive technique is frequently used to study crystal defects, deformation fields and compositional changes in semiconductors because of its high sensitivity to strains induced by defects in crystals. Principles of surface sensitive reflection Lang topography and reflection double crystal topography and their contrast mechanisms are shortly presented and three examples of their application given. The techniques have been used to characterize crystal defects such as threading dislocations arranged into cellular structure and slip bands at surfaces of semi-insulating GaAs substrate wafers, to characterize transformation of threading dislocations into misfit dislocations in a InGaAs/GaAs strained quantum well and to characterize misorientation domains in highly mismatched InGaAs/GaAs heterostructures. In the last case tilt regions have been evidenced by synchrotron double crystal topography.

Abstract: The measurement of the optical parameters of thin films deposited on absorbing substrate with dispersion is presented. Reflectance was measured for normally incident light using a commercial fiber-optics spectrometer. Suggested experimental configuration is sufficiently flexible and enables the investigation of planar samples with specular surface. The same set-up can be utilized for in-situ measurements. The obtained parameters of samples were statistically tested using resampling of the measured data set. The bootstrap method, which enables to estimate the confidence intervals for calculated parameters of layers, was used. The analysis of TiO2 films deposited on c-Si substrate is presented as an example.

Abstract: The nature of the nucleation centers formed during the so-called bias enhancement nucleation (BEN) of chemical vapor deposited diamond is still an open question. We address this problem by studying a long time nucleation process and its impact upon the nucleation density and quality of grown films deposited on Si substrates by hot filament chemical vapor deposition (HF CVD) method. It was found that the nucleation density of the growth centers on silicon wafers increased drastically with the increasing process time up to a few hours.

Abstract: Copper nitride films were deposited by reactive DC magnetron sputtering onto glass and other substrates in nitrogen atmosphere. Thickness, deposition rate, refractive index and optical band gap energy were determined from reflection and transmission spectroscopy depending on the deposition pressure and power. The crystallographic nature of films was investigated by x-ray diffraction. The films were further characterized by photoelectron spectroscopy (XPS) and multiple internal reflection (MIR) infrared spectroscopy, where the presence of Cu$_3$N nanocrystalline structure was confirmed. Considerable changes of DC conductivity and activation energy of the films before and after annealing at 500~$^\circ$C were found.

Abstract: The molecular beam epitaxy is considered to be suitable method for preparation of efficient GaAs solar cells. This paper deals with GaAs p/i/n solar cell structures prepared with different thickness of intrinsic i-region. The current-voltage characteristics in the dark and the performance under standard simulated light spectra and concentrated light were investigated. The dielectric properties at the frequency range from 50~Hz to 1~MHz and at different temperatures were investigated, as well. Preliminary results on fully processed 1cm x 1cm p/i/n GaAs solar cells have shown an increase in maximum power output by a factor of around 2.7 to 3.0 under concentrated illumination of 5~Suns. This work is being carried out as a part of an effort to fabricate efficient multiple quantum well GaAs cells based on thin film structures.

Abstract: Computer simulated small-angle neutron scattering (SANS) curves of spherical polydisperse extruded unilamellar liposomes from saturated 1,2-diacylphosphatidylcholines in the aqueous phase are evaluated by using a multishell model, which divides the lipid bilayer of liposomes into the polar head group region, and the nonpolar hydrocarbon region consisting of the chains of methylene groups and of the region of methyl groups. In the each of these regions, the coherent neutron scattering length density is supposed to be homogeneous. The evaluation is based on obtaining of radius of gyration from the Kratky-Porod plot of SANS data in the Guinier region of small scattering vector values. From radii of gyration obtained at several different molar fractions N_D2O/(N_D20 + N_H2O) in the aqueous phase (contrasts) and independent volumetric data, the lipid surface area AL (or the bilayer thickness dL and the number of water molecules NL penetrated into the bilayer polar region can be evaluated. Using this method and the SANS curves of unilamellar 1,2-dimyristoylphosphatidylcholine (DMPC) liposomes measured at 30°C and contrasts N_D2O/(N_D20 + N_H2O)=1.0, 0.8, 0.6 and 0.4 the values dL=42.6+-0.5 A^3 and AL=62.0+-0.7 A^2 at NL=7.3 were obtained.