I. Vávra - Academia.edu (original) (raw)
Papers by I. Vávra
Modern Physics Letters B, 2007
ABSTRACT We present the study on the interlayer and intergranular exchange and dipolar coupling i... more ABSTRACT We present the study on the interlayer and intergranular exchange and dipolar coupling in [Fe97Si3/SiO2]5 discontinuous multilayers by means of ferromagnetic resonance. Due to strong ferromagnetic exchange coupling (J~-3 erg/cm2) the precessional motions of magnetic moments of granules are coupled and results in an acoustic and optical mode. Moreover there is notable line splitting in optical mode under external field normal to the layer, which is explained by an interlayer dipolar coupling, only possible for discontinuous layer. Some aspects of the damping processes in discontinuous multilayers are discussed as well.
Thermal stability of WSit _x /Si multilayers (MLs) with x varying from 1 to 0.11 is studied by TE... more Thermal stability of WSit _x /Si multilayers (MLs) with x varying from 1 to 0.11 is studied by TEM and LÀXS . The structure of the prepared samples is amorphous. Doping of W sublayers with silicon affects the interdiffusion process as well as the crystallization in these sublayers. We investigated these two processes (which have a detrimental influence on e.g. X-ray mirror stability) by annealing our samples at 400°C for 40 min. The structural changes were monitored by resistance measurement in the temperature range of 1.5-300 K. A correlation between structural and electrical characteristics was found, which is based on the comparison between three different R(T) curves . It is shown in our paper that the R(T) curve of a ML lies between the R(T) curves of two extreme types of single layers . The first single layer is the analogue of a fully intermixed ML and the second one represents a parallel connection of all conductive sublayers. Thus, a simple resistance measurement can give additional information about the quality of interfaces . We claim that in MLs with ultrathin sublayers the reported highest thermal stability of the amorphous mixture WO 7 2Si o,ZS cannot be utilized because interdiffusion dominates over crystallization so that the superlattice structure is not retained. * Corresponding author . Tel. +46 46 108233, Fax +46 46 104012 . 0168-9002/94/$07 .00 © 1994 -Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00684-Y
Vacuum, 1998
We have found the deposition technology at which the interface roughness of Nb/Si multilayer is s... more We have found the deposition technology at which the interface roughness of Nb/Si multilayer is strongly correlated and decreases with deposited layer number. On the basis of our Nb/Si multilayer we succeeded to prepare ten-fold stacked Josephson junction (JJ). Basic electric properties of stacked JJ are presented.
Journal of Low Temperature Physics, 1997
A Josephson junction Nb/Si/Nb with a 10 nm thick amorphous silicon barrier is studied. The upper ... more A Josephson junction Nb/Si/Nb with a 10 nm thick amorphous silicon barrier is studied. The upper electrode contains a 2 nm thick sublayer of amorphous phase adjacent to the barrier, as revealed by cross-sectional TEM. Thus, the junction can be considered as a S-I-N-S system with the N layer represented by amorphous niobium. Peculiarities in the I–V and Δ(T) dependencies
Carbon, 2005
Carbon–nickel composite thin films (600nm thick) were prepared by dc magnetron sputtering of Ni a... more Carbon–nickel composite thin films (600nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25–800°C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni3C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the
Journal of Materials Science: Materials in Electronics, 2008
ABSTRACT In this study we investigated the effect of a NiO x layer on the electrical properties o... more ABSTRACT In this study we investigated the effect of a NiO x layer on the electrical properties of oxidized Au/NiO x /p-GaN ohmic contacts. Au/NiO x layers with a small concentration of oxygen in NiO x were deposited on p-GaN by reactive DC magnetron sputtering and annealed in a mixture of O2+N2, and in N2. Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling in combination with transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE SEM) and the circular transmission line method (CTLM) of contact resistance measurements of the contact structure with low content of oxygen in the NiO x layer have been used to explain the reduction of the contact resistance as a result of its anneal treatment. It has been found that creation of a metal/p-NiO/p-GaN contact structure due to annealing of the Au/NiO x /p-GaN structure in either N2 or O2+N2 is the main mechanism that is responsible for the ohmic nature of the system. However, lowering of the contact resistance is similarly affected also by Ga atoms leaving the vacancies at the metal/p-GaN interface after diffusion of Ga into the metallic layer. The effect of various ways of cleaning the p-GaN surface prior to metallization on the contact resistance has also been investigated.
Journal of Low Temperature Physics, 1997
We report on the technology and basic electrical properties of ten-fold stacked Nb/Si/Nb Josephso... more We report on the technology and basic electrical properties of ten-fold stacked Nb/Si/Nb Josephson junction (J J). The problem of making a large number of stacked ,Lls with identical properties was solved by metallic superlattice preparation technology. The uniformity of Si barriers thickness was examined by low angle X-ray diffraction and cross-sectional transmission electron microscopy. To prevent pinholes in the Si barriers thc Nb/Si superlattice was sputtered at the regime at which smoothing of inteTfacial roughness occurs. The stacked junction exhibit both ac and dc Josephson effects. In the Ie( B) diffraction pattern there is an extra periodicity of about 2 -3 G in addition to a larger period of about 21 G.
Journal of Crystal Growth, 2011
The magnetic and structural properties of InMnAs self-assembled dots and epitaxial layers prepare... more The magnetic and structural properties of InMnAs self-assembled dots and epitaxial layers prepared from the identical composition of the gas phase were investigated. All structures under study were grown by low pressure metal organic vapor phase epitaxy at the growth temperature of 500 1C. Magnetization measurements indicated that the Curie temperature of InMnAs dots prepared under the Stranski-Krastanov growth mode varied from 7 to 235 K in relation to the material composition. The Curie temperature of epitaxial layers prepared under layer by layer growth mode was found to be independent from the material composition. Transmission electron microscopy and microstructure analysis revealed that the dots have a zinc blende structure and they are plastically relaxed.
Journal of Crystal Growth, 2004
Graded InxGa1−xP buffer layers were grown on GaP substrate in order to prepare optically transpar... more Graded InxGa1−xP buffer layers were grown on GaP substrate in order to prepare optically transparent substrate for optoelectronic structures. In our contribution we present results from the optimization of the growth process. We found different optimal growth conditions for GaP buffer layer growth and subsequent deposition of InxGa1−xP layers with graded value of xIn. Structures with final composition xIn=0.33 and
We considered simple 2D model of the magnetic dot array with uniaxial anisotropic dots. For this ... more We considered simple 2D model of the magnetic dot array with uniaxial anisotropic dots. For this model we formulated the inverse magnetic problem, where desired static ground-state properties are attained by varying directions of the local anisotropy vectors. The problem is investigated numerically using a method based on evolutionary optimization. 85.42.+m, 75.10., 41.20.G
Modern Physics Letters B, 2007
ABSTRACT We present the study on the interlayer and intergranular exchange and dipolar coupling i... more ABSTRACT We present the study on the interlayer and intergranular exchange and dipolar coupling in [Fe97Si3/SiO2]5 discontinuous multilayers by means of ferromagnetic resonance. Due to strong ferromagnetic exchange coupling (J~-3 erg/cm2) the precessional motions of magnetic moments of granules are coupled and results in an acoustic and optical mode. Moreover there is notable line splitting in optical mode under external field normal to the layer, which is explained by an interlayer dipolar coupling, only possible for discontinuous layer. Some aspects of the damping processes in discontinuous multilayers are discussed as well.
Thermal stability of WSit _x /Si multilayers (MLs) with x varying from 1 to 0.11 is studied by TE... more Thermal stability of WSit _x /Si multilayers (MLs) with x varying from 1 to 0.11 is studied by TEM and LÀXS . The structure of the prepared samples is amorphous. Doping of W sublayers with silicon affects the interdiffusion process as well as the crystallization in these sublayers. We investigated these two processes (which have a detrimental influence on e.g. X-ray mirror stability) by annealing our samples at 400°C for 40 min. The structural changes were monitored by resistance measurement in the temperature range of 1.5-300 K. A correlation between structural and electrical characteristics was found, which is based on the comparison between three different R(T) curves . It is shown in our paper that the R(T) curve of a ML lies between the R(T) curves of two extreme types of single layers . The first single layer is the analogue of a fully intermixed ML and the second one represents a parallel connection of all conductive sublayers. Thus, a simple resistance measurement can give additional information about the quality of interfaces . We claim that in MLs with ultrathin sublayers the reported highest thermal stability of the amorphous mixture WO 7 2Si o,ZS cannot be utilized because interdiffusion dominates over crystallization so that the superlattice structure is not retained. * Corresponding author . Tel. +46 46 108233, Fax +46 46 104012 . 0168-9002/94/$07 .00 © 1994 -Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00684-Y
Vacuum, 1998
We have found the deposition technology at which the interface roughness of Nb/Si multilayer is s... more We have found the deposition technology at which the interface roughness of Nb/Si multilayer is strongly correlated and decreases with deposited layer number. On the basis of our Nb/Si multilayer we succeeded to prepare ten-fold stacked Josephson junction (JJ). Basic electric properties of stacked JJ are presented.
Journal of Low Temperature Physics, 1997
A Josephson junction Nb/Si/Nb with a 10 nm thick amorphous silicon barrier is studied. The upper ... more A Josephson junction Nb/Si/Nb with a 10 nm thick amorphous silicon barrier is studied. The upper electrode contains a 2 nm thick sublayer of amorphous phase adjacent to the barrier, as revealed by cross-sectional TEM. Thus, the junction can be considered as a S-I-N-S system with the N layer represented by amorphous niobium. Peculiarities in the I–V and Δ(T) dependencies
Carbon, 2005
Carbon–nickel composite thin films (600nm thick) were prepared by dc magnetron sputtering of Ni a... more Carbon–nickel composite thin films (600nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25–800°C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni3C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the
Journal of Materials Science: Materials in Electronics, 2008
ABSTRACT In this study we investigated the effect of a NiO x layer on the electrical properties o... more ABSTRACT In this study we investigated the effect of a NiO x layer on the electrical properties of oxidized Au/NiO x /p-GaN ohmic contacts. Au/NiO x layers with a small concentration of oxygen in NiO x were deposited on p-GaN by reactive DC magnetron sputtering and annealed in a mixture of O2+N2, and in N2. Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling in combination with transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE SEM) and the circular transmission line method (CTLM) of contact resistance measurements of the contact structure with low content of oxygen in the NiO x layer have been used to explain the reduction of the contact resistance as a result of its anneal treatment. It has been found that creation of a metal/p-NiO/p-GaN contact structure due to annealing of the Au/NiO x /p-GaN structure in either N2 or O2+N2 is the main mechanism that is responsible for the ohmic nature of the system. However, lowering of the contact resistance is similarly affected also by Ga atoms leaving the vacancies at the metal/p-GaN interface after diffusion of Ga into the metallic layer. The effect of various ways of cleaning the p-GaN surface prior to metallization on the contact resistance has also been investigated.
Journal of Low Temperature Physics, 1997
We report on the technology and basic electrical properties of ten-fold stacked Nb/Si/Nb Josephso... more We report on the technology and basic electrical properties of ten-fold stacked Nb/Si/Nb Josephson junction (J J). The problem of making a large number of stacked ,Lls with identical properties was solved by metallic superlattice preparation technology. The uniformity of Si barriers thickness was examined by low angle X-ray diffraction and cross-sectional transmission electron microscopy. To prevent pinholes in the Si barriers thc Nb/Si superlattice was sputtered at the regime at which smoothing of inteTfacial roughness occurs. The stacked junction exhibit both ac and dc Josephson effects. In the Ie( B) diffraction pattern there is an extra periodicity of about 2 -3 G in addition to a larger period of about 21 G.
Journal of Crystal Growth, 2011
The magnetic and structural properties of InMnAs self-assembled dots and epitaxial layers prepare... more The magnetic and structural properties of InMnAs self-assembled dots and epitaxial layers prepared from the identical composition of the gas phase were investigated. All structures under study were grown by low pressure metal organic vapor phase epitaxy at the growth temperature of 500 1C. Magnetization measurements indicated that the Curie temperature of InMnAs dots prepared under the Stranski-Krastanov growth mode varied from 7 to 235 K in relation to the material composition. The Curie temperature of epitaxial layers prepared under layer by layer growth mode was found to be independent from the material composition. Transmission electron microscopy and microstructure analysis revealed that the dots have a zinc blende structure and they are plastically relaxed.
Journal of Crystal Growth, 2004
Graded InxGa1−xP buffer layers were grown on GaP substrate in order to prepare optically transpar... more Graded InxGa1−xP buffer layers were grown on GaP substrate in order to prepare optically transparent substrate for optoelectronic structures. In our contribution we present results from the optimization of the growth process. We found different optimal growth conditions for GaP buffer layer growth and subsequent deposition of InxGa1−xP layers with graded value of xIn. Structures with final composition xIn=0.33 and
We considered simple 2D model of the magnetic dot array with uniaxial anisotropic dots. For this ... more We considered simple 2D model of the magnetic dot array with uniaxial anisotropic dots. For this model we formulated the inverse magnetic problem, where desired static ground-state properties are attained by varying directions of the local anisotropy vectors. The problem is investigated numerically using a method based on evolutionary optimization. 85.42.+m, 75.10., 41.20.G