P. Vasek

Institute of Chemical Technology Prague, Praha, Praha, Czech Republic

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Publications (44)55.48 Total impact

  • J. Nahlik · Z. Soban · J. Voves · V. Jurka · P. Vasek
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    ABSTRACT: Graphene is perspective material for future carbon based electronics, flexible electronics and other applications. The necessary condition for the commercial use is the high quality graphene growth and semiconductor technology compatible process of whole field effect transistor (FET). One of suitable method for large scale graphene monolayer preparation is the thermal annealing of semi-insulating SiC substrate. One important task of graphene FET process is reliable, cheap and simple gate structure preparation. In this work we present our results of using MicroChem Lift-Off Resist (LOR) layer as a dielectric layer for SiC graphene FETs. LOR resist is based on polydimethylglutharimide. Its unique properties enable to perform exceptionally well resolution imaging, easy process tuning, high yields and superior deposition line width control. In the case of polymer based dielectric layers the breakdown voltage is important parameter. We prepared two sets of different capacitor structures with LOR dielectric layer and Au/Cr electrodes. The first set exhibits very low breakdown voltages (about 3 V). The optimisation of the LOR layer deposition process in the second set increased the breakdown voltage over 40 V keeping the leakage current lower than 2 nA. The second process with LOR layer was used for the preparation of graphene FETs on SiC substrates. The first measurements show resistivity dependence on gate voltage.
    No preview · Conference Paper · Oct 2014
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    ABSTRACT: We have studied the transport properties of graphene grown on silicon side of SiC. Samples under study have been prepared by two different growth methods in two different laboratories. Magnetoresistance and Hall resistance have been measured at temperatures between 4 and 100 K in resistive magnet in magnetic fields up to 22 T. In spite of differences in sample preparation, the field dependence of resistances measured on both sets of samples exhibits two periods of magneto-oscillations indicating two different parallel conducting channels with different concentrations of carriers. The semi-quantitative agreement with the model calculation allows for conclusion that channels are formed by high-density and low-density Dirac carriers. The coexistence of two different groups of carriers on the silicon side of SiC was not reported before.
    Full-text · Article · Oct 2012 · Journal of Physics Conference Series
  • O. Smrckova · D. Sykorova · P. Svoboda · P. Vasek
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Oct 2010 · ChemInform
  • D. SYKOROVA · O. SMRCKOVA · P. SVOBODA · P. VASEK
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Sep 2010 · ChemInform
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    ABSTRACT: We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band. Comment: 22 pages, 14 figures
    Full-text · Article · Jul 2010
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    ABSTRACT: We explore the basic physical origins of the noncrystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. The sign of the noncrystalline AMR is found to be determined by the form of spin-orbit coupling in the host band and by the relative strengths of the nonmagnetic and magnetic contributions to the Mn impurity potential. We develop experimental methods yielding directly the noncrystalline and crystalline AMR components which are then analyzed independently. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. Generic implications of our findings for other dilute moment systems are discussed.
    Full-text · Article · Nov 2007 · Physical Review Letters
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    ABSTRACT: Series of the samples with nominal compositions Bi3.2Pb0.8Sr4Ca5Cu7Ox and Bi3.2Pb0.8Sr4Ca5Cu7Ox/Ag (10 wt%) were prepared by the standard solid state reaction at the same calcination and sintering temperature and time. Three starting mixtures were used, carbonates, oxides and silver i) with nanoparticles, ii) with micrometer size of particles and iii) powder obtained by modified sol-gel process. The main phase in all samples was Bi-2223 and silver formed the separate phase. The sol-gel process was found to be effective way in term of porosity, bulk density and connectivity of the grains. The effect of the microstructure on the transport properties were described by the temperature dependence of resistivity and critical current densities.
    No preview · Article · Jul 2007 · International Journal of Modern Physics B
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    ABSTRACT: We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.
    Full-text · Article · Jul 2007 · Microelectronics Journal
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    ABSTRACT: The magnetoresistance of the MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state, and after brief illumination by a red-light diode at low temperature, T is approximately 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also the coupling among 2D electron layers in neighboring quantum wells has been reduced.
    Full-text · Article · Jul 2007 · Physical review. B, Condensed matter
  • V. Jakes · D. Sykorova · O. Smrckova · P. Vasek
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    ABSTRACT: In the first part of the work two routes of preparation of undoped superconductor Bi-2223 are compared. The influence of addition of extra PbO is studied as well. The route that leads to higher content of the phase 2223 is then chosen as the appropriate way for preparation of fluorine doped samples of general stoichiometry Bi1.6Pb0.4Sr2Ca2.5Cu3.5Ox-z Fz (z = 0, 0.3, 0.6 and 0.9). These samples are characterized by measuring of their temperature dependence of electrical resistance and critical current density. Their phase composition is determined by XRD. While greater amount of fluorine widenes the superconducting transition and worsenes transport properties, the less doped sample shows increasement of critical current density.
    No preview · Article · Sep 2006 · physica status solidi (c)
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    ABSTRACT: We present an experimental study of electron transport in inversion layers of high-mobility Si(001) samples with occupied excited subbands. The second series of oscillations, observed in addition to the main series of Shubnikov-de Hass oscillations, is tentatively attributed to the occupation of a subband associated with the $E_{0'}$ level. Besides, a strong negative magnetoresistance and nonlinear field dependence of the Hall resistance accompany the novel oscillations at high carrier concentrations. The heating of the 2D electron layers leads to suppression of the observed anomalies. Comment: 4 pages, 6 figures, elsart/PHYEAUTH macros; to be presented on the EP2DS-15 Conference in Nara, Japan
    Full-text · Article · Jun 2003 · Physica E Low-dimensional Systems and Nanostructures
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    ABSTRACT: We have investigated experimentally the magnetoresistance of strongly asymmetric double-wells. The structures were prepared by inserting a thin Al$_{0.3}$Ga$_{0.7}$As barrier into the GaAs buffer layer of a standard modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. The resulting double-well system consists of a nearly rectangular well and of a triangular well coupled by tunneling through the thin barrier. With a proper choice of the barrier parameters one can control the occupancy of the two wells and of the two lowest (bonding and antibonding) subbands. The electron properties can be further influenced by applying front- or back-gate voltage. Comment: 4 pages, 5 figures, elsart/PHYEAUTH macros; to be presented on the EP2DS-15 Conference in Nara, Japan. Revised version. To appear in Physica E
    Full-text · Article · Jun 2003 · Physica E Low-dimensional Systems and Nanostructures
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    ABSTRACT: Samples with the nominal composition Bi1.8−xAxPb0.26Sr2Ca2Cu3Oy (x=0–0.5; A=B, Al, Ga, In) were prepared. Measurements of resistivity, AC susceptibility, critical current density and magnetization have been used for characterization of the samples. All boron-substituted samples had a 2223 phase and narrow transition with the critical temperature Tc≈106–108K and the critical current density Jc (77K)≈105A/m2. The presence of aluminium caused decrease of Tc. The 2212 phase became evident and Jc (77K) sharply decreased at x⩾0.3. The resistivity curves of all the samples with Ga were steeper. The Tc values ranging from 103 to 60K were considerably influenced by the amount of dopant. The volume of 2212 phase increased rapidly at x=0.2 and Jc decreased to zero below 77K. Introducing indium into the samples had a rather positive effect on Tc since the resistivity–temperature curves were always shifted towards higher temperatures with the narrow transition at ∼107K although the XRD patterns showed that the samples also contained the 2212 phase. The samples with x=0.05–0.4 had Jc (77K)≈105A/m2. The highest content of In worsened the quality of the samples and decreased the Jc (77K) to zero.
    Full-text · Article · Aug 2002 · Physica B Condensed Matter
  • P Svoboda · P Vasek
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    ABSTRACT: The absolute thermoelectric power of the Pd82Si18 and Pd80Si20 metallic glasses has been measured at 1.8-8.5K using a SQUID. The thermopower of Pd82Si18 becomes negative below about 7.7K. This anomaly has been attributed to scattering at magnetic impurities present in the sample. Their negative contribution to the thermopower prevails at sufficiently low temperatures over a positive one from the PdSi matrix. The results for two glasses Pd80Si20 of different purity support this conclusion.
    No preview · Article · Nov 2000 · Journal of Physics F Metal Physics
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    ABSTRACT: Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure. Comment: 6 pages, 5 figures
    Full-text · Article · Apr 2000 · Physical review. B, Condensed matter
  • P. Svoboda · P. Chrobok · P. Vasek
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    ABSTRACT: We have developed a simple, computer controlled DC measuring system for calibration of wire-wound 10 kΩ resistors against a QHE structure. The system employs a high-accuracy DMM in both voltage and four-terminal resistance measurement modes. In both cases, resolution down to 10-8 and combined standard uncertainty at the level of 10-7 could be achieved. The system proved to be also useful for testing new QHE samples fabricated for quantum resistance metrology
    No preview · Article · Jan 2000
  • D. Sýkorová · O. Smrcková · P. Vasek
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    ABSTRACT: Formation of the high temperature Bi-2223 phase has been studied for samples with nominal composition Bi3,2-xPb0.8MxSr4Ca5Cu7Oy. M represented V, Nb, Ta, their contents was altered from 0 to 0.6 mol %. The effect of the additives on the growth of the phase 2223, lattice parameters and the superconducting properties of the samples was detected. It was found that the additives enhanced the volume of the 2223 phase, improved the intergrain connection resp. the critical current density. The additives did not affect the lattice parameters and Tc significantly.
    No preview · Article · Dec 1999 · International Journal of Modern Physics B
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    ABSTRACT: The effect of the sintering temperature, the cooling rate and the additive on the critical parameters was studied on samples with nominal composition Bi1.8(Pb0.26)Sr2Ca2Cu3O8+x both undoped and with addition of 5, 9, 12.7 mol % of boron (as B2O3), respectively. The sample with 5 mol % of boron cooled slowly had the critical current density five times higher than undoped sample. The rapid cooling of the samples caused enhancement of the volume of the 2223 phase.
    No preview · Article · Dec 1999 · International Journal of Modern Physics B
  • J. Bohacek · P. Svoboda · P. Vasek
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    ABSTRACT: Influence of temperature on frequency performance of an AlGaAs-GaAs heterostructure fabricated by Laboratoires d'Electronique Philips has been investigated. The structure was placed into a <sup>4 </sup>He bath and significant improvement of its frequency performance has been observed when the operational temperature was reduced from 4.2 K to 1.6 K. For T=1.6 K and I=30 μA, an approximately linear frequency dependence of the quantized Hall resistance R<sub>H</sub>(2) was found, its slope being 0.11 ppm/kHz
    No preview · Conference Paper · Jun 1998
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    ABSTRACT: At electron densities N S>610−2 cm−22 a second series of oscillations, which are tentatively attributed to population of the second energy subband, is observed in addition to the main series of Shubnikov-de Haas oscillations. A change in phase of the oscillations of the second series is observed at some angle of inclination αe of the field. The measured value of αe is used to calculate the ratio of the cyclotron mass to the effective g factor. The maximum possible cyclotron mass is also determined as m Hm e. On this basis it is concluded that the second series of oscillations is due to electrons which have an in-plane effective mass m*≈0.2m e and which belong to the same valleys of the Fermi surface as in the case of the main oscillations.
    Full-text · Article · Jan 1998 · JETP Letters

Publication Stats

308 Citations
55.48 Total Impact Points

Institutions

  • 1999-2010
    • Institute of Chemical Technology Prague
      • Department of Inorganic Chemistry
      Praha, Praha, Czech Republic
  • 1995-2007
    • Academy of Sciences of the Czech Republic
      • Institute of Physics
      Praha, Praha, Czech Republic
  • 1997
    • Czech Technical University in Prague
      • Chair of Electrical Engineering (FS)
      Praha, Hlavni mesto Praha, Czech Republic
  • 1992
    • National Center for Scientific Research Demokritos
      Athínai, Attica, Greece
  • 1990
    • Humboldt-Universität zu Berlin
      • Department of Physics
      Berlín, Berlin, Germany