Papers by Author: A. Csik

Abstract: Results of the study of the structural and magnetic properties of periodic Fe/V heterostructures with periods D = 9.4nm and D = 6.3nm are present. The study has shown that ferromagnetic islands are formed on the interfaces of Fe and V. These islands have different magnetic properties as sub-layers of pure iron. Islands in the sample with the period D = 9.4 nm have anisotropic shape, which leads to the anisotropy of magnetic properties.

Abstract: It was shown more recently in our Laboratory [1,2,3] that having a substrate/diffusant/thin-film/cap-layer structure (the thin film was typically several 10 nm thick, with the same order of magnitude of grain size; the refractory metal cap layer was used just to avoid the oxidation), first the diffusant atoms migrated very fast across the thin film and segregated at the film/cap-layer interface. The accumulated atoms at the film/cap layer interface form a secondary diffusion reservoir and atoms diffuse back to the layer. Later on, the thin film was gradually filled up with the diffusing atoms and composition depth profiles, determined by Secondary Neutral Mass Spectroscopy (SNMS), showed a maximum at the cap layer-thin film interface. The accumulated atoms at this interface formed a secondary diffusion reservoir and atoms diffused back to the layer. These observations can be interpreted supposing a bimodal grain boundary structure with different (fast and low) diffusivities. The observed grain boundary diffusion phenomena can be classified as C-type diffusion. The appearance of the peak observed at the cap layer interface can be used as a tool to determine the grain boundary diffusivity along the fast boundaries. Because the fast boundaries were saturated in the first stage of the process, this back-diffusion took place along the low-diffusivity boundaries only. Thus the SNMS depth-profiling is a good method to determine grain boundary diffusivities in a bimodal structure. In addition, from the overall impurity content inside the film the segregation can also be estimated, if the bulk solubility is low and the GB density is known. Numerical simulations of C-type GB diffusion in thin films with a bimodal structure confirmed that the interpretation of the result depicted above is reasonable [4]. In order to estimate roughly the GB diffusion data we determined the fast diffusivity using the first appearance method. The lower diffusivity was determined from the time evolution of the broadening of the diffusant/thin film interface. In addition both (slow and fast) diffusivities were also estimated from fitting numerical solutions obtained in [4] too.

Abstract: The influence of hydrogen on the structural stability of multilayers made of ultrathin (3 nm) Si and Ge amorphous layers submitted to annealing to activate Si and Ge intermixing has been studied by TEM and AFM. By energy dispersive microanalysis the interdiffusion of Si and Ge has been observed. The Si/Ge multilayers, however, underwent remarkable structural degradation because of the formation of hydrogen bubbles which give rise to surface bumps and eventually craters when the bubbles blow up because of too high internal pressure in samples with high H content and annealed at high temperatures. The hydrogen forming the bubbles comes from the rupture of the Si-H and Ge-H bonds activated by the thermal energy of the annealing and by the energy released by the recombination of thermally generated electron hole pairs.

Abstract: X-ray standing wave technique has been used to measure the kinetics of CoSi intermetallic phase growth in a-Si/Co/a-Si sandwich structure. The a-Si/Co/a-Si arrangement were placed into a waveguide structure formed by two Ta films. X-ray fluorescence and extended X-ray absorption fine structure analysis has been used in a combination with X-ray standing wave technique for depth profiling with sub-nanometer resolution of specimens annealed at 493K for different annealing time. The position and the thickness of the growing CoSi intermetallic phase have been monitored.

Abstract: Epitaxial, coherent Mo/V multilayers were deposited by magnetron sputtering on (001) oriented MgO substrates at 873K (sample MoV-T), 923K (sample MoV-U) and 973K (sample MoV-V), respectively. In order to estimate the concentration profiles in our multilayers, a superlattice refinement modelling procedure has been used on high-angle XRD symmetric scans. The Mo/V interfaces were always sharper than V/Mo ones (in this notation the order of element reflects the sequence of deposition: e.g. the Mo/V interface was formed by the deposition of the V on the Mo surface). Furthermore the interface diffuseness was only slightly different at the lowest substrate temperature, but the difference increased with increasing temperature and an abrupt concentration jump could be observed at the Mo/V interface in the sample, sputtered at the 973 K. This indicates that during deposition the interfacial mixing by impact exchange events is important and thermally activated processes (surface diffusion and/or jumps driven by segregation) are less effective. With increasing substrate temperature the thickness of the V/Mo interfaces were unchanged while the Mo/V interface became shaper and sharper i.e. thermally activated jumps were more active during deposition of V atoms. Thus in forming Mo/V interfaces the segregation tendency of V to the Mo surface results in enhanced exchanges between V atoms (buried in the near surface layers of the Mo substrate) and surface Mo atoms, leading to more sharper interface with increasing temperature. On the other hand during the formation of the V/Mo interfaces the chemical thickness of the interface, provided again by impact exchanges, was practically unchanged.

Abstract: Solid state reactions between amorpous Si and crystalline Co have been investigated by synchrotron radiation at Bessy (Berlin, Germany). The multilayered samples (with 10 periods of a-Si(15 nm)/Co(15 nm) layers) were produced by magnetron sputtering and isothermally heat treated at temperatures between 523 and 593 K. From the time evolution of the XRD spectra first the growth rate of the CoSi phase as well as the decay rate of the Co layer we determined (at 523 and 543 K). The kinetics were described by a power law; tk, and for the growth of CoSi k=0.65 while for the loss of the Co the k=0.77 was obtained, respectively. At higher temperatures (at 573 and 593 K) the formation and growth of the Co2Si layer, at the expense of the Co and already existing CoSi layers, was observed with exponents of about 1 for all the above kinetics. These results, together with the results of resistance kinetics measurements, in similar multilayered as well as bi-layered samples at similar temperatures, providing similar exponents will be presented. Possibility of the interface reaction control and/or the effect of the diffusion asymmetry (which was recently published for the interpretation of solid state reactions with non-parabolic kinetics on the nanoscale) will be discussed.

Abstract: Solid state reactions between amorphous Si and crystalline Co have been investigated by 4W electrical resistance and TEM. Multilayered (with 10 periods of 5nm a-Si/5nm Co and 10 nma- Si/10nm Co layers) as well as tri-layered samples (20nm a-Si/3nmCoSi/6nm Co) were produced by magnetron sputtering and isothermally heat treated at different temperatures between 473 and 523K. From the time evolution of the normalized resistance the kinetics of the process were determined by fitting a power law, tk, and k was between 0.8 and 1. Possibility of the interface reaction control and/or the effect of the diffusion asymmetry (which was recently published for the non-parabolic interface shifts on the nanoscale) will be discussed.

Abstract: In nanostructured materials, where the density of grain- and interphase-boundaries is high, the diffusion and kinetics of surface segregation, i.e. the effective material flow is always influenced by the contributions of these boundaries [1]. Diffusion on the nano/atomic scales in multilayers, thin films has many challenging features even if the role of structural defects can be neglected and ‘only’ the effects related to the nano/atomic scale arise. Different examples for diffusional nanoscale effects discovered recently by the authors will be given in this paper. We show that the continuum descriptions of diffusion cannot be applied automatically on such short distances, the classical continuum approximations (Fick's laws) cannot describe correctly the atomic movements. [2-4] They predict faster kinetics than the atomistic models and the interface shift is always proportional to the square-root of time (x ∝ t1/2 ⇒ x2 ∝ t: parabolic or Fickian kinetics). As we will show, however, the kinetics can be even linear (x ∝ t) on the nano/atomic scale. [3, 4] Furthermore, the continuum descriptions foretell infinitely fast kinetics as the time goes to zero (v=dx/dt∝1/t1/2), which is a long standing paradox of diffusion theory. We will show a possible resolution of this paradox. [5] Moreover, we will show that an initially diffused interface can sharpen even in completely miscible systems. [6, 7]

Abstract: Diffusion on the nano/atomic scales in multilayers, thin films has many challenging features even if the role of structural defects can be neglected and ‘only’ the effects related to the nano/atomic scale raise. Different examples for diffusional nanoscale effects we have discovered recently will be summarized in this paper. We illustrate that the continuum descriptions of the diffusion cannot be applied automatically on such short distances, the classical continuum approximations (Fick's laws) cannot describe correctly the atomic movements. [1-4] They predict faster kinetics than the atomistic models and the interface shift is always proportional to the squareroot of the time (x ∝ t1/2 ⇒ x2 ∝ t: parabolic or Fickian kinetics). However, the kinetics can be even linear (x ∝ t) on the nano/atomic scale. [3, 4] Furthermore, the continuum descriptions foretell infinitely fast kinetics as the time goes to zero (v=dx/dt∝1/t1/2), which is a long standing paradox of the diffusion theory. Very recently a possible resolution of this paradox has been offered [5], moreover, it was also shown that an initially diffused interface can sharpen even in completely miscible systems. [6, 7] We will also review the possible stress effects on the above phenomena.