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  • 1.
    Alexander-Webber, J. A.
    et al.
    University of Oxford, England.
    Baker, A. M. R.
    University of Oxford, England.
    Janssen, T. J. B. M.
    National Phys Lab, England.
    Tzalenchuk, A
    National Phys Lab, England.
    Lara-Avila, S
    Chalmers, Sweden.
    Kubatkin, S
    Chalmers, Sweden.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Piot, B A.
    LNCMI CNRS UJF INSA UPS, France.
    Maude, D K.
    LNCMI CNRS UJF INSA UPS, France.
    Nicholas, R J.
    University of Oxford, England.
    Phase Space for the Breakdown of the Quantum Hall Effect in Epitaxial Graphene2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 9, p. e096601-Article in journal (Refereed)
    Abstract [en]

    We report the phase space defined by the quantum Hall effect breakdown in polymer gated epitaxial graphene on SiC (SiC/G) as a function of temperature, current, carrier density, and magnetic fields up to 30 T. At 2 K, breakdown currents (Ic) almost 2 orders of magnitude greater than in GaAs devices are observed. The phase boundary of the dissipationless state (ρxx=0) shows a [1-(T/Tc)2] dependence and persists up to Tc>45  K at 29 T. With magnetic field Ic was found to increase ∝B3/2 and TcB2. As the Fermi energy approaches the Dirac point, the ν=2 quantized Hall plateau appears continuously from fields as low as 1 T up to at least 19 T due to a strong magnetic field dependence of the carrier density.

  • 2.
    Alexander-Webber, J. A.
    et al.
    University of Oxford, England; University of Cambridge, England.
    Huang, J.
    University of Oxford, England.
    Maude, D. K.
    CNRS UGA UPS INSA, France.
    Janssen, T. J. B. M.
    National Phys Lab, England.
    Tzalenchuk, A.
    National Phys Lab, England; Royal Holloway University of London, England.
    Antonov, V.
    Royal Holloway University of London, England.
    Yager, T.
    Chalmers, Sweden.
    Lara-Avila, S.
    Chalmers, Sweden.
    Kubatkin, S.
    Chalmers, Sweden.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Nicholas, R. J.
    University of Oxford, England.
    Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, no 30296Article in journal (Refereed)
    Abstract [en]

    Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.

  • 3.
    Baker, A M R
    et al.
    University of Oxford, England .
    Alexander-Webber, J A
    University of Oxford, England .
    Altebaeumer, T
    University of Oxford, England .
    Janssen, T J B M
    National Phys Lab, England .
    Tzalenchuk, A
    National Phys Lab, England .
    Lara-Avila, S
    Chalmers, Sweden .
    Kubatkin, S
    Chalmers, Sweden .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lin, C-T
    Academic Sinica, Taiwan .
    Li, L-J
    Academic Sinica, Taiwan .
    Nicholas, R J
    University of Oxford, England .
    Weak localization scattering lengths in epitaxial, and CVD graphene2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 23, p. 235441-Article in journal (Refereed)
    Abstract [en]

    Weak localization in graphene is studied as a function of carrier density in the range from 1 x 10(11) cm(-2) to 1.43 x 10(13) cm(-2) using devices produced by epitaxial growth onto SiC and CVD growth on thin metal film. The magnetic field dependent weak localization is found to be well fitted by theory, which is then used to analyze the dependence of the scattering lengths L-phi, L-i, and L-* on carrier density. We find no significant carrier dependence for L-phi, a weak decrease for L-i with increasing carrier density just beyond a large standard error, and a n(-1/4) dependence for L-*. We demonstrate that currents as low as 0.01 nA are required in smaller devices to avoid hot-electron artifacts in measurements of the quantum corrections to conductivity. DOI: 10.1103/PhysRevB.86.235441

  • 4.
    Baker, A M R
    et al.
    University of Oxford, England .
    Alexander-Webber, J A
    University of Oxford, England .
    Altebaeumer, T
    University of Oxford, England .
    McMullan, S D.
    University of Oxford, England .
    Janssen, T J B M
    National Phys Lab, England .
    Tzalenchuk, A
    National Phys Lab, England .
    Lara-Avila, S
    Chalmers, Sweden .
    Kubatkin, S
    Chalmers, Sweden .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lin, C-T
    Academic Sinica, Taiwan .
    Li, L-J
    Academic Sinica, Taiwan .
    Nicholas, R J.
    University of Oxford, England .
    Energy loss rates of hot Dirac fermions in epitaxial, exfoliated, and CVD graphene2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 4, p. 045414-Article in journal (Refereed)
    Abstract [en]

    Energy loss rates for hot carriers in graphene have been measured using graphene produced by epitaxial growth on SiC, exfoliation, and chemical vapor deposition (CVD). It is shown that the temperature dependence of the energy loss rates measured with high-field damped Shubnikov-de Haas oscillations and the temperature dependence of the weak localization peak close to zero field correlate well, with the high-field measurements understating the energy loss rates by similar to 40% compared to the low-field results. The energy loss rates for all graphene samples follow a universal scaling of T-e(4) at low temperatures and depend weakly on carrier density proportional to n(-1/2), evidence for enhancement of the energy loss rate due to disorder in CVD samples.

  • 5.
    Beshkova, M.
    et al.
    Bulgarian Academic Science, Bulgaria.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Device applications of epitaxial graphene on silicon carbide2016In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 128, p. 186-197Article, review/survey (Refereed)
    Abstract [en]

    Graphene has become an extremely hot topic due to its intriguing material properties allowing for ground-breaking fundamental research and applications. It is one of the fastest developing materials during the last several years. This progress is also driven by the diversity of fabrication methods for graphene of different specific properties, size, quantity and cost. Graphene grown on SiC is of particular interest due to the possibility to avoid transferring of free standing graphene to a desired substrate while having a large area SiC (semi-insulating or conducting) substrate ready for device processing. Here, we present a review of the major current explorations of graphene on SiC in electronic devices, such as field effect transistors (FET), radio frequency (RF) transistors, integrated circuits (IC), and sensors. The successful role of graphene in the metrology sector is also addressed. Typical examples of graphene on SiC implementations are illustrated and the drawbacks and promises are critically analyzed. (C) 2016 Elsevier Ltd. All rights reserved.

  • 6.
    Beshkova, M.
    et al.
    Bulgarian Academy of Science, Sofia, Bulgaria.
    Zakhariev, Z.
    Bulgarian Academy of Science, Sofia, Bulgaria.
    Abrashev, M. V.
    University of Sofia, Bulgaria.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Postovit, A.
    Institute of Problem Microelectronics Technology and High Purity Materials, Moskow, Russia.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Properties of AlN epitaxial layers on 6H-SiC substrate grown by sublimation in argon, nitrogen, and their mixtures2006In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 129, no 1-3, p. 228-231Article in journal (Refereed)
    Abstract [en]

    Epitaxial layers of aluminum nitride (AlN) have been grown at temperature 1900 °C on 10 mm × 10 mm 6H-SiC substrate via sublimation-recondensation in RF heated graphite furnace. The source material was polycrystalline sintered AlN. Growth of AlN layers in pure nitrogen, mixed nitrogen/argon and pure argon atmosphere of 50 mbar were compared. A maximum growth rate of about 30 µm/h was achieved in pure nitrogen atmosphere. The surface morphology reflects the hexagonal symmetry of the seed, which is characteristic of an epitaxial growth for samples grown in a pure nitrogen and mixed nitrogen/argon atmosphere. X-ray diffraction (XRD) measurements show very strong and well defined (0 0 0 2) reflection positioned at around 36° in symmetric ?-2? scans. Micro-Raman spectroscopy reveals that the films have a wurtzite structure. Secondary-ion mass spectroscopy (SIMS) results showed a low concentration of carbon incorporation in the AlN layers. This study demonstrates that nitrogen is necessary for the successful epitaxial growth of AlN on 6H-SiC by sublimation. © 2006 Elsevier B.V. All rights reserved.

  • 7.
    Beshkova, Milena
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Sublimation epitaxy of 3C-SiC grown at Si- and C-rich conditions2012In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 86, no 10, p. 1595-1599Article in journal (Refereed)
    Abstract [en]

    3C-SiC layers have been grown by using sublimation epitaxy at a source temperature of 2000 degrees C, under vacuum conditions (andlt;10(-5) mbar) on well oriented (on-axis) 6H-SiC (0001) substrates. Close space sublimation growth geometry has been used in a RF-heated furnace employing high-purity graphite crucible with a possibility to change the growth environment from Si vapor-rich to C vapor-rich. The optical microscopy in transmission mode reveals continuous 3C-domains for 3C-SiC with less than 0.4% 6H-inclusions for the layer grown at Si-rich conditions, and separate 3C-SiC domains for the layer grown at C-rich conditions. The type of 6H-inclusions for layers with continuous domain structure investigated by Atomic Force Microscopy (AFM) is discussed. 2Theta-omega scan shows 0006 and 111 peaks coming from the substrate and the layer, respectively with a higher intensity of the 111 peak for 3C-SiC grown at Si-rich conditions which is related with the continuous character of the 3C-SiC domains.

  • 8.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Grigorov, K. G.
    Zakhariev, Z.
    Abrashev, M.
    Massi, M.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Sublimation epitaxy of AlN layers grown by different conditions on 4H-SiC substrates2007In: Journal of Optoelectronics and Advanced Materials, ISSN 1454-4164, E-ISSN 1841-7132, Vol. 9, no 1, p. 213-216Article in journal (Refereed)
    Abstract [en]

    Epitaxial layers of aluminium nitride were grown at temperature 2100 degrees C on 10X10 mm(2) 4H-SiC substrates via a sublimation-recondensation method in an RF heated graphite furnace. The source material was polycrystalline sintered AIN. Growths of AIN layers in vacuum and pure nitrogen at 20 mbar were compared. MA maximum growth rate of 70 mu m/h was achieved in a pure N-2 atmosphere. The surface morphology reveals the hexagonal symmetry of the seeds, suggesting an epitaxial growth. This was confirmed by High-Resolution X-Ray Diffraction. The spectra showed a strong and well defined (0002) reflection positioned at 36.04 degrees in a symmetric theta-2 theta scan for both samples. Micro-Raman spectroscopy revealed that the films had a wurtzite structure. Rutherford Backscattering Spectrometry indicated the quality with a relative chi(min) parameter 0.68.

  • 9.
    Beshkova, Milena
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lorenzzi, J.
    UMR-CNRS.
    Jegenyes, N.
    UMR-CNRS.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Ferro, G.
    UMR-CNRS.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Properties of 3C-SiC Grown by Sublimation Epitaxy on Different Type of Substrates2010In: Materials Science Forum, Vols. 645-648, Transtec Publications; 1999 , 2010, Vol. 645-648, p. 183-186Conference paper (Refereed)
    Abstract [en]

    3C-SiC layers have been grown by using sublimation epitaxy at a temperature of 2000 degrees C, on different types of on-axis 6H-SiC(0001) substrates. The influence of the type of substrate on the morphology of the layers investigated by Atomic Force Microscopy (AFM) is discussed. Stacking faults are studied by reciprocal space map (RSM) which shows that double positions domains exists.

  • 10.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Vasiliauskas, Remigijus
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Properties of 3C-SiC Grown by Sublimation Epitaxy2009In: ECSCRM2008,2008, 2009Conference paper (Refereed)
    Abstract [en]

      

  • 11.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Vasiliauskas, Remigijus
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Structural Properties of 3C-SiC Grown by Sublimation Epitaxy2009In: ECSCRM2009,2009, Materials Science Forum Vols. 615-617: Trans Tech Publications , 2009, p. 181-184Conference paper (Refereed)
    Abstract [en]

    The present paper deals with morphological and structural investigation of 3C-SiC layers grown by sublimation epitaxy on on axis 6H-SiC(0001) at source temperature 2000 °C, under vacuum conditions (<10-5 mbar) and different temperature gradients in the range of 5-8 °C/mm. The layer grown at a temperature gradient 6 °C/mm has the largest average domain size of 0.4 mm2 assessed by optical microscope in transmission mode. The rocking curve full width at half maximum (FWHM) of (111) reflection is 43 arcsec which suggests good crystalline quality. The AFM image of the same layer shows steps with height 0.25 nm and 0.75 nm which are characteristic of a stacking fault free 3C-SiC surface and c-axis repeat height, respectively.

  • 12.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Zakhariev, Z.
    Abrashev, M.V.
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Low-pressure sublimation epitaxy of AlN films - growth and characterization2004In: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 76, p. 143-146Article in journal (Refereed)
    Abstract [en]

    Epitaxial layers of aluminum nitride have been grown at temperatures 1900-2400degreesC on 10 x 10 mm(2) 4H-SiC substrate via sublimation recondensation in an RF heated graphite furnace. The source material was polycrystalline sintered AlN. A maximum growth rate of about 100 mum/h was achieved at 2400degreesC and seed to source distance of 1 mm. The surface morphology reflects the hexagonal symmetry of the seed suggesting an epitaxial growth. This was confirmed by X-ray diffraction (XRD). The spectra showed very strong and well-defined (0002) reflection position at around 36.04degrees in symmetric Theta-2Thetascans for all samples. Micro-Raman spectroscopy reveals that the films have a wurtzite structure. It is evidenced by the appearance of the A(1) (TO) (at 601 cm(-1)) and E-2((2)) (at 651 cm(-1)) lines in the spectra. Secondary-ion mass spectroscopy (SIMS) results showed a low concentration of carbon incorporation in the AlN films. A correlation between the growth conditions and properties of the AlN layers was established.

  • 13.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Zakhariev, Z
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Properties of AlN layers grown by sublimation epitaxy2003In: Materials Science Forum, Vols. 433-436, 2003, Vol. 433-4, p. 995-998Conference paper (Refereed)
    Abstract [en]

    Epitaxial layers of aluminum nitride (AlN)less than or equal to 80 mum thick have been grown at the temperatures 1900 and 2100 degreesC on 10x10mm(2) 4H-SiC substrates via sublimation recondensation in a RF heated graphite furnace. The source material was polyerystalline sintered AlN. A maximum growth rate of 80 mum/h was achieved at 2100degreesC and seed to source separation of I mm. The surface morphology reflects the hexagonal symmetry of the seed that suggesting an epitaxial growth. All crystals show strong and well defined single crystalline XRD patterns. Only the (002) reflection positioned at around 36.04 was observed in symmetric Theta-2Theta scan. The rocking curves FWHM (full width half maximum) and peak positions arc reported.

  • 14.
    Beshkova, Milena
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Zakhariev, Z
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Sublimation epitaxy of AIN layers on 4H-SiC depending on the type of crucible2003In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 14, no 10-12, p. 767-768Article in journal (Refereed)
    Abstract [en]

    Epitaxial layers of aluminum nitride less than or equal to335 mum thick have been grown attemperatures of 1900 and 2100degreesC on 10 x 10 mm(2) (0001)-oriented alpha(4H) silicon carbide (SiC), with growth times of 1 and 4h, via sublimation-recondensation in a RF-heated graphite furnace. The source material was polycrystalline AIN. The sublimation process was performed in three types of graphite (C) crucible: C-1, C-2 with inner diameters of 35 and 51 mm, respectively, and C-3 with the same inner diameter as C-1, but coated with a layer of TaC. The surface morphology reflects the hexagonal symmetry of the substrate, suggesting an epitaxial growth for samples grown in C-1 and C-3 crucibles for all growth conditions. The same symmetry is observed for AIN layers grown in the C-2 crucible, but only at 2100degreesC. X-ray diffraction analyses confirm the epitaxial growth of AIN samples with the expected hexagonal symmetry. A high-resolution X-ray diffractometer was used to assess the quality of the single crystals. A full width at half maximum of 242 arcsec was achieved for an AIN layer grown in the crucible coated with TaC. (C) 2003 Kluwer Academic Publishers.

  • 15. Bikbajevas, V
    et al.
    Grivickas, V
    Stolzer, M
    Velmre, E
    Udal, A
    Grivickas, P
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Impact of phonon drag effect on Seebeck coefficient in p-6H-SiC: Experiment and simulation2003In: Materials Science Forum, Vols. 433-436, 2003, Vol. 433-4, p. 407-410Conference paper (Refereed)
    Abstract [en]

    The temperature dependence of Seebeck coefficient (S) for p-6H-SiC has been obtained. It increases from 2 up to 5.2 mV/K when temperature decreases from 400 down to 240 K. It is shown that phonon drag effect makes critical contribution to the S value. Improved theoretical model involving 4-phonon scattering process has been proposed for the simulation of Seebeck coefficient phonon pail.

  • 16.
    Bohnen, T.
    et al.
    Radboud University Nijmegen.
    Yazdi, Gholamreza
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    van Dreumel, G W G
    Radboud University Nijmegen.
    Hageman, P R
    Radboud University Nijmegen.
    Vlieg, E.
    Radboud University Nijmegen.
    Algra, R E
    Radboud University Nijmegen.
    Verheijen, M A
    Philips Res Labs.
    Edgar, J H
    Kansas State University.
    ScAlN nanowires: A cathodoluminescence study2009In: JOURNAL OF CRYSTAL GROWTH, ISSN 0022-0248, Vol. 311, no 11, p. 3147-3151Article in journal (Refereed)
    Abstract [en]

    Wurtzite ScAlN nanowires, grown on a scandium nitride (ScN) thin film by hydride vapor phase epitaxy (HVPE), were analyzed by energy dispersive analysis of X-rays (EDX), CL, high resolution transmission electron spectroscopy (HRTEM), and scanning electron microscopy (SEM). The wires were grown along the [0 0 0 1] axis, had an average length of 1 mu m, a diameter between 50 and 150 run, and a ScAlN composition with a 95:5 Al:Sc ratio. Cathodoluminescence studies on the individual wires showed a sharp emission near 2.4 eV, originating from the Sc atoms in the aluminum nitride (AlN) matrix. The formation of such a semiconducting ScAlN alloy could present a new alternative to InAlN for optoelectronic applications operating in the 200-550 nm range.

  • 17.
    Boosalis, A.
    et al.
    University of Nebraska, USA .
    Hofmann, T.
    University of Nebraska, USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Schubert, M.
    University of Nebraska, USA .
    Visible to vacuum ultraviolet dielectric functions of epitaxial graphene on 3C and 4H SiC polytypes determined by spectroscopic ellipsometry2012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 1Article in journal (Refereed)
    Abstract [en]

    Spectroscopic ellipsometry measurements in the visible to vacuum-ultraviolet spectra (3.5-9.5 eV) are performed to determine the dielectric function of epitaxial graphene on SiC polytypes, including 4H (C-face and Si-face) and 3C SiC (Si-face). The model dielectric function of graphene is composed of two harmonic oscillators and allows the determination of graphene quality, morphology, and strain. A characteristic van Hove singularity at 4.5 eV is present in the dielectric function of all samples, in agreement with observations on exfoliated as well as chemical vapor deposited graphene in the visible range. Model dielectric function analysis suggests that none of our graphene layers experience a significant degree of strain. Graphene grown on the Si-face of 4H SiC exhibits a dielectric function most similar to theoretical predictions for graphene. The carbon buffer layer common for graphene on Si-faces is found to increase polarizability of graphene in the investigated spectrum.

  • 18.
    Boosalis, Alexander
    et al.
    Department of Electrical Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska, U.S.A..
    Hofmann, Tino
    Department of Electrical Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska, U.S.A..
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositza
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Tiwald, Tom
    J.A. Woollam Co., Lincoln, Nebraska, U.S.A..
    Schubert, Mathias
    Department of Electrical Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska, U.S.A..
    Spectroscopic Mapping Ellipsometry of Graphene Grown on 3C SiC2012In: MRS Proceedings Volume 1407, 2012, p. aa20-43Conference paper (Refereed)
    Abstract [en]

    Spectroscopic mapping ellipsometry measurements in the visible spectrum (1.25 to 5.35 eV) are performed to determine the lateral variations of epitaxial graphene properties as grown on 3C SiC. Data taken in the visible spectrum is sensitive to both the Drude absorption of free charge carriers and the characteristic exciton enhanced van Hove singularity at 5 eV. Subsequent analysis with simple oscillator models allows the determination of physical parameters such as free charge carrier scattering time and local graphene thickness with a lateral resolution of 50 microns.

  • 19.
    Bouhafs, Chamseddine
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Persson, Ingemar
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Tiberj, A.
    University of Montpellier 2, France.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Paillet, M.
    University of Montpellier 2, France.
    Zahab, A. -A.
    University of Montpellier 2, France.
    Landois, P.
    University of Montpellier 2, France.
    Juillaguet, S.
    University of Montpellier 2, France.
    Schoeche, S.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Schubert, M.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 8, p. 085701-Article in journal (Refereed)
    Abstract [en]

    Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn. (C) 2015 AIP Publishing LLC.

  • 20.
    Burnett, Tim L
    et al.
    National Phys Lab, England .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kazakova, Olga
    National Phys Lab, England .
    Identification of epitaxial graphene domains and adsorbed species in ambient conditions using quantified topography measurements2012In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 112, no 5, p. 054308-Article in journal (Refereed)
    Abstract [en]

    We discuss general limitations of topographical studies of epitaxial graphene in ambient conditions, in particular, when an accurate determination of the layers thickness is required. We demonstrate that the histogram method is the most accurate for measurements of small vertical distances (andlt;0.5 nm) and generally should be applied to epitaxial graphene and similar types of samples in order to get the correct and reproducible values. Experimental determination of the step height between different domains of epitaxial graphene shows excellent agreement with the predicted values once the adsorption of a 2D monolayer is taken into account on top of the one layer graphene. In contrast to general limitations of AFM topography, electrostatic force microscopy imaging allows a straightforward identification of domains of epitaxial graphene of different thickness.

  • 21.
    Burnett, Tim
    et al.
    National Physics Lab, Teddington, England .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kazakova, Olga
    National Physics Lab, Teddington, England .
    Mapping of Local Electrical Properties in Epitaxial Graphene Using Electrostatic Force Microscopy2011In: NANO LETTERS, ISSN 1530-6984, Vol. 11, no 6, p. 2324-2328Article in journal (Refereed)
    Abstract [en]

    Local electrical characterization of epitaxial graphene grown on 4H-SiC (0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.

  • 22.
    Chua, Cassandra
    et al.
    University of Cambridge, England .
    Connolly, Malcolm
    University of Cambridge, England National Phys Lab, England .
    Lartsev, Arseniy
    Chalmers, Sweden .
    Yager, Tom
    Chalmers, Sweden .
    Lara-Avila, Samuel
    Chalmers, Sweden .
    Kubatkin, Sergey
    Chalmers, Sweden .
    Kopylov, Sergey
    University of Lancaster, England .
    Falko, Vladimir
    University of Lancaster, England .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pearce, Ruth
    National Phys Lab, England .
    Janssen, T.J. B. M.
    National Phys Lab, England .
    Tzaenchuk, Alexander
    National Phys Lab, England University of London, England .
    Smith, Charles G.
    University of Cambridge, England .
    Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene2014In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 14, no 6, p. 3369-3373Article in journal (Refereed)
    Abstract [en]

    We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.

  • 23.
    Ciechonski, Rafal
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Effect of boron on the resistivity of compensated 4H-SiC2003In: Journal of electronic materials, ISSN 0361-5235, Vol. 32, no 5, p. 452-457Article in journal (Refereed)
    Abstract [en]

    High-resistivity 4H-SiC samples grown by sublimation with a high growth rate are studied. The measurements show resistivity values up to a high of 104 Ωcm. The secondary ion mass spectroscopy (SIMS) results revealed a presence of only common trace impurities such as nitrogen, aluminum, and boron. To understand the compensation mechanism in these samples, capacitance deep-level transient spectroscopy (DLTS) on the p-type epilayers has been performed. By correlation between the growth conditions and SIMS results, we apply a model in which it is proposed that an isolated carbon vacancy donorlike level is a possible candidate responsible for compensation of the shallow acceptors in p-type 4H-SiC. A relation between cathodoluminescence (CL) and DLTS data is taken into account to support the model.

  • 24.
    Ciechonski, Rafal
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Porro, Samuele
    Polytechnic of Turin, Physics Department, Torino, Italy.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Evaluation of On-state Resistance and Boron-related Levels in n-type 4H-SiC2005In: Materials Science Forum, Vols. 483-485, 2005, Vol. 483-485, p. 425-428Conference paper (Refereed)
    Abstract [en]

    Specific on-resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick layers exhibits variations from tens of mΩ.cm2 to tens of Ω.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with a literature reported values. Decrease in mobility with the temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples respectively were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy, an influence of shallow boron related levels and D-center on dependence of on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism thus resulting in high Ron.

  • 25.
    Ciechonski, Rafal
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    ul-Hassan, Jawad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Structural instabilities in growth of SiC crystals2005In: Journal of Crystal Growth, ISSN 0022-0248, Vol. 275, no 1-2, p. e461-e466Article in journal (Refereed)
    Abstract [en]

    Misoriented grains, which may occur on the growth front of 6H–SiC boules have been studied in relation to their appearance during sublimation growth. The effect was obtained by applying growth conditions at which the source powder was gradually approaching graphitisation and the vapour becoming C-rich. The high off-orientation of the grains is demonstrated through etching in molten KOH and transmission light optical microscopy. Micropipes propagating in the single crystal area and facing the misoriented grain have been studied, and it is shown that they may either be terminated at the grain or their propagation is altered to be parallel with the grain boundary. It has been found that the polytype of the grains may switch from 6H to 4H, which is explained by the change of the Si/C ratio in the vapour.

  • 26.
    Ciechonski, Rafal
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Wahab, Qamar Ul
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Evaluation of MOS structures processed on 4H–SiC layers grown by PVT epitaxy2005In: Solid-State Electronics, ISSN 0038-1101, E-ISSN 1879-2405, Vol. 49, no 12, p. 1917-1920Article in journal (Refereed)
    Abstract [en]

    MOS capacitors have been fabricated on 4H–SiC epilayers grown by physical vapor transport (PVT) epitaxy. The properties were compared with those on similar structures based on chemical vapor deposition (CVD) layers. Capacitance–voltage (CV) and conductance measurements (GV) were performed in the frequency range of 1 kHz to 1 MHz and also at temperatures up to 475 K. Detailed investigations of the PVT structures indicate a stable behaviour of the interface traps from room temperature up to 475 K. The amount of positive oxide charge QO is 6.83 × 109 cm−2 at room temperature and decreases with temperature increase. This suggests that the processed devices are temperature stable. The density of interface states Dit obtained by Nicollian–Brews conductance method is lower in the structure based on the PVT grown sample.

  • 27.
    Ciechonski, Rafal
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Effect of Ambient on 4H-SiC Bulk Crystals grown by Sublimation2003In: Materials Science Forum, Vols. 433-436 / [ed] Peder Bergman and Erik Janzén, 2003, Vol. 433-436, p. 75-78Conference paper (Refereed)
    Abstract [en]

    Sublimation bulk growth in vacuum using graphite crucibles and such with tantalum shielding of the crucible walls has been studied. Residual nitrogen, aluminum and boron doping in the material grown in vacuum is presented. Activation energies of growth rate in respect to growth temperature in vacuum are deduced. The estimated values are 21 kcallmole for growth temperatures below 2075°C and 128 kcal/mole in the range of growth temperatures between 2075°C and 2275°C. Cathodoluminescence spectra taken from samples grown in the graphite crucible in absence of tantalum under different pressures show nitrogen-alurninum DAP transition and strong luminescence from deep boron. This is not the case for samples grown in the tantalum environment.

  • 28. Dannefaer, S.
    et al.
    Avalos, V.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    The role of nitrogen in the annealing of vacancies in 4H-SiC2005In: Materials Science Forum, Vols. 483-485, 2005, Vol. 483-485, p. 481-484Conference paper (Refereed)
  • 29. Dannefaer, S
    et al.
    Avalos, V
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Vacancies in As-grown and electron-irradiated 4H-SiC epilayers investigated by positron annihilation2003In: Materials Science Forum, Vols. 433-436, 2003, Vol. 433-4, p. 173-176Conference paper (Refereed)
    Abstract [en]

    Epilayers of 4H-SiC were investigated by positron annihilation spectroscopies: four epilayers and their substrates were investigated. The epilayers (47 to 220 mum thick) contained significantly lower grown-in vacancy concentration than did their substrates, and there was no dependency on layer thickness. Upon electron irradiation silicon vacancies were introduced at the same rate in epilayer and in substrate.

  • 30.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Boosalis, A.
    Department of Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
    Zakharov, A. A.
    Lund University, Maxlab, Lund, Sweden.
    Hofmann, T.
    Department of Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
    Schubert, M.
    Department of Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, USA.
    Tiwald, T. E.
    J. A. Woollam Co., Lincoln, Nebraska, USA.
    Iakimov, Tihomir
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Vasiliauskas, Remigijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, no 21, p. 213116-Article in journal (Refereed)
    Abstract [en]

    Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temperature sublimation on 3C-SiC (111) substrates is reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at ∼4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.

  • 31. Davydov, SY
    et al.
    Savkina, NS
    Lebedev, Alexander
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Simple model for calculation of SiC epitaxial layers growth rate in vacuum.2004In: Materials Science Forum, Vols. 457-460, 2004, Vol. 457-460, p. 249-252Conference paper (Refereed)
    Abstract [en]

    Within the frame of a simple model, based on Hertz-Knudsen equation with account of temperature dependant sticking coefficient, temperature dependence of silicon carbide epitaxial layers growth rate in vacuum has been calculated. Calculation results are in a good agreement with the experimental data.

  • 32.
    Davydov, S.Yu.
    et al.
    Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg 194021, Russian Federation.
    Lebedev, A.A.
    Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg 194021, Russian Federation.
    Savkina, N.S.
    Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg 194021, Russian Federation.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    A Simple Model for Calculating the Growth Rate of Epitaxial Layers of Silicon Carbide in Vacuum2004In: Semiconductors (Woodbury, N.Y.), ISSN 1063-7826, E-ISSN 1090-6479, Vol. 38, no 2, p. 150-152Article in journal (Refereed)
    Abstract [en]

    The temperature dependence of the growth rate of epitaxial layers of silicon carbide in vacuum was calculated within the simple model based on the Hertz-Knudsen equation, taking into account the temperature-dependent sticking coefficient. The calculation results fit the experimental data well. © 2004 MAIK "Nauka/Interperiodica".

  • 33.
    Drexler, C
    et al.
    University of Regensburg, Germany .
    Tarasenko, S A.
    Russian Academic Science, Russia .
    Olbrich, P
    University of Regensburg, Germany .
    Karch, J
    University of Regensburg, Germany .
    Hirmer, M
    University of Regensburg, Germany .
    Mueller, F
    University of Regensburg, Germany .
    Gmitra, M
    University of Regensburg, Germany .
    Fabian, J
    University of Regensburg, Germany .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lara-Avila, S
    Chalmers, Sweden .
    Kubatkin, S
    Chalmers, Sweden .
    Wang, M
    Rice University, TX USA .
    Vajtai, R
    Rice University, TX USA .
    Ajayan, P M
    Rice University, TX USA .
    Kono, J
    Rice University, TX USA .
    Ganichev, S D.
    University of Regensburg, Germany .
    Magnetic quantum ratchet effect in graphene2013In: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 8, no 2, p. 104-107Article in journal (Refereed)
    Abstract [en]

    A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations(1). This so-called ratchet effect(2) has fascinating ramifications in engineering and natural sciences(3-18). Graphene(19) is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.

  • 34.
    Eless, V
    et al.
    National Phys Lab, England.
    Yager, T
    Chalmers, Sweden.
    Spasov, S
    University of London, England.
    Lara-Avila, S
    Chalmers, Sweden.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kubatkin, S
    Chalmers, Sweden.
    Janssen, T J B M.
    National Phys Lab, England.
    Tzalenchuk, A
    National Phys Lab, England.
    Antonov, V
    University of London, England.
    Phase coherence and energy relaxation in epitaxial graphene under microwave radiation2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 9Article in journal (Refereed)
    Abstract [en]

    We have performed low-temperature magnetotransport measurements on monolayer epitaxial graphene under microwave radiation and extracted the radiation-induced effective temperatures, energy relaxation, and the dephasing times. We established that the response of the graphene sample is entirely bolometric at least up to 170 GHz. Dynamic dephasing, i.e., the time-reversal symmetry breaking effect of the ac electromagnetic field rather than mediated by heating, may become significant in the terahertz frequency range and in samples with longer phase coherence time.

  • 35.
    Eless, V.
    et al.
    National Physical Laboratory, Teddington, United Kingdom, Royal Holloway, University of London, United Kingdom.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pearce, R.
    National Physical Laboratory, Teddington, United Kingdom.
    Controlling the carrier concentration of epitaxial graphene by ultraviolet illumination2014In: SILICON CARBIDE AND RELATED MATERIALS 2013, PTS 1 AND 2 / [ed] Okumura, H; Harima, H; Kimoto, T; Yoshimoto, M; Watanabe, H; Hatayama, T; Matsuura, H; Funaki, T; Sano, Y, Switzerland: Trans Tech Publications , 2014, Vol. 778-780, p. 1137-1141Conference paper (Refereed)
    Abstract [en]

    Silicon carbide (SiC) is a well-known material for UV detection however the effect of UV illumination on the electron donation between the substrate, interfacial (or buffer layer) and graphene is not well understood. The effect of ultraviolet (UV) illumination on the carrier concentration of an epitaxial graphene hall bar device is investigated by scanning Kelvin probe microscopy (SKPM) and transport measurements in ambient and vacuum conditions. Modulation of the carrier concentration is demonstrated and shown to be due to both substrate and environmental effects.

  • 36.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    Khranovskyy, Volodymyr
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Söderlind, Fredrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, Faculty of Science & Engineering.
    Käll, Per-Olov
    Linköping University, Department of Physics, Chemistry and Biology, Physical Chemistry. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lloyd-Spets, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    ZnO nanoparticles or ZnO films: A comparison of the gas sensing capabilities2009In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 137, no 1, p. 94-102Article in journal (Refereed)
    Abstract [en]

    Zinc oxide is an interesting material for bio and chemical sensors. it is a semiconducting metal oxide with potential as an integrated multisensing sensor platform, which simultaneously detects Parameters like change in field effect, mass and Surface resistivity. in this investigation we have used resistive sensor measurements regarding the oxygen gas sensitivity in order to characterize sensing layers based on electrochemically produced ZnO nanoparticles and PE-MOCVD grown ZnO films. Proper annealing procedures were developed in order to get stable sensing properties and the oxygen sensitivity towards operation temperature was investigated. The ZnO nanoparticles showed a considerably increased response to oxygen as compared to the films. Preliminary investigations were also performed regarding the selectivity to other gases present in car exhausts or flue gases.

  • 37.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    Pearce, Ruth
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    Iakimov, Tihomir
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Virojanadara, Chariya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Leibniz Institute of Crystal Growth, Berlin, Germany .
    Andersson, Mike
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
    Yakimova, Rositza
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC2012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 24, p. 241607-Article in journal (Refereed)
    Abstract [en]

    A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.

  • 38.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Puglisi, Donatella
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Fashandi, Hossein
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Gas sensing with epitaxial graphene on silicon carbide: performance tuning for air quality control2014In: Proc. E-MRS 2014, Lille, France, May 26-30, 2014Conference paper (Refereed)
  • 39.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Puglisi, Donatella
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Hsuan Kang, Yu
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Adjusting the electronic properties and gas reactivity of epitaxial graphene by thin surface metallization2014In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 439, p. 105-108Article in journal (Refereed)
    Abstract [en]

    Graphene-based chemical gas sensors normally show ultra-high sensitivity to certain gas molecules but at the same time suffer from poor selectivity and slow response and recovery Limes. Several approaches based on functionalization or modification of the graphene surface have been demonstrated as means to improve these issues, but most such measures result in poor reproducibility. In this study we investigate reproducible graphene surface modifications by sputter deposition of thin nanostructured Au or Pt layers. It is demonstrated that under the right metallization conditions the electronic properties of the surface remain those of graphene, while the surface chemistry is modified to improve sensitivity, selectivity and speed of response to nitrogen dioxide.

  • 40.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Puglisi, Donatella
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Strandqvist, Carl
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering. Graphensic AB Linköping, Sweden.
    Gunnarsson, Rickard
    Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, Faculty of Science & Engineering.
    Ekeroth, Sebastian
    Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, Faculty of Science & Engineering.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Helmersson, Ulf
    Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, Faculty of Science & Engineering.
    Uvdal, Kajsa
    Linköping University, Department of Physics, Chemistry and Biology, Molecular Surface Physics and Nano Science. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Graphensic AB Linköping, Sweden.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Modified Epitaxial Graphene on SiC for Extremely Sensitive andSelective Gas Sensors2016In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 858, p. 1145-1148Article in journal (Refereed)
    Abstract [en]

    Two-dimensional materials offer a unique platform for sensing where extremely high sensitivity is a priority, since even minimal chemical interaction causes noticeable changes inelectrical conductivity, which can be used for the sensor readout. However, the sensitivity has to becomplemented with selectivity, and, for many applications, improved response- and recovery times are needed. This has been addressed, for example, by combining graphene (for sensitivity) with metal/oxides (for selectivity) nanoparticles (NP). On the other hand, functionalization or modification of the graphene often results in poor reproducibility. In this study, we investigate thegas sensing performance of epitaxial graphene on SiC (EG/SiC) decorated with nanostructured metallic layers as well as metal-oxide nanoparticles deposited using scalable thin-film depositiontechniques, like hollow-cathode pulsed plasma sputtering. Under the right modification conditions the electronic properties of the surface remain those of graphene, while the surface chemistry can betuned to improve sensitivity, selectivity and speed of response to several gases relevant for airquality monitoring and control, such as nitrogen dioxide, benzene, and formaldehyde.

  • 41.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Puglisi, Donatella
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Vasiliauskas, Remigijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Thickness uniformity and electron doping in epitaxial graphene on SiC2013In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 740-742, p. 153-156Article in journal (Refereed)
    Abstract [en]

    Large variations have been observed in the thickness uniformity and carrier concentration of epitaxial graphene grown on SiC by sublimation for samples grown under identical conditions and on nominally on-axis hexagonal SiC (0001) substrates. We have previously shown that these issues are both related to the morphology of the graphene-SiC surface after sublimation growth. Here we present a study on how the substrate polytype, substrate surface morphology and surface restructuring during sublimation growth affect the uniformity and carrier concentration in epitaxial graphene on SiC. These issues were investigated employing surface morphology mapping by atomic force microscopy coupled with local surface potential mapping using scanning Kelvin probe microscopy.

  • 42.
    Eriksson, Jens
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Puglisi, Donatella
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Lloyd Spetz, Anita
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    SiC-2D-material-hybrids as a Platform for Extremely Sensitive and Selective Gas Sensors2016In: Proceedings EMRS 2016, 2016Conference paper (Refereed)
  • 43.
    Eriksson, Susanna K.
    et al.
    Uppsala University, Sweden .
    Hahlin, Maria
    Uppsala University, Sweden .
    Kahk, Juhan Matthias
    University of London Imperial Coll Science Technology and Med, England .
    Villar-Garcia, Ignacio J.
    University of London Imperial Coll Science Technology and Med, England .
    Webb, Matthew J.
    Uppsala University, Sweden .
    Grennberg, Helena
    Uppsala University, Sweden .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Rensmo, Hakan
    Uppsala University, Sweden .
    Edstrom, Kristina
    Uppsala University, Sweden .
    Hagfeldt, Anders
    Uppsala University, Sweden .
    Siegbahn, Hans
    Uppsala University, Sweden .
    Edwards, Marten O. M.
    VG Scienta AB, Sweden .
    Karlsson, Patrik G.
    VG Scienta AB, Sweden .
    Backlund, Klas
    VG Scienta AB, Sweden .
    Ahlund, John
    VG Scienta AB, Sweden .
    Payne, David J.
    University of London Imperial Coll Science Technology and Med, England .
    A versatile photoelectron spectrometer for pressures up to 30 mbar2014In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 85, no 7, p. 075119-Article in journal (Refereed)
    Abstract [en]

    High-pressure photoelectron spectroscopy is a rapidly developing technique with applications in a wide range of fields ranging from fundamental surface science and catalysis to energy materials, environmental science, and biology. At present the majority of the high-pressure photoelectron spectrometers are situated at synchrotron end stations, but recently a small number of laboratory-based setups have also emerged. In this paper we discuss the design and performance of a new laboratory based high pressure photoelectron spectrometer equipped with an Al Ka X-ray anode and a hemispherical electron energy analyzer combined with a differentially pumped electrostatic lens. The instrument is demonstrated to be capable of measuring core level spectra at pressures up to 30 mbar. Moreover, valence band spectra of a silver sample as well as a carbon-coated surface (graphene) recorded under a 2 mbar nitrogen atmosphere are presented, demonstrating the versatility of this laboratory-based spectrometer.

  • 44. Ewing, D.J.
    et al.
    Porter, L.M.
    Wahab, Qamar Ul
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Ciechonski, Rafal
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Inhomogeneous electrical characteristics in 4H-SiC Schottky diodes2007In: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 22, no 12, p. 1287-1291Article in journal (Refereed)
    Abstract [en]

    Hundreds of current-voltage (I-V) measurements of Ni, Pt and Ti Schottky diodes on 4H-SiC were conducted at low applied voltages. The SiC substrates contained homoepitaxial layers grown by either chemical vapor deposition or sublimation. While near-ideal contacts were fabricated on all samples, a significant percentage of diodes (∼7%-50% depending on the epitaxial growth method and the diode size) displayed a non-ideal, or inhomogeneous, barrier height. These 'non-ideal' diodes occurred regardless of growth technique, pre-deposition cleaning method, or contact metal. In concurrence with our earlier reports in which the non-ideal diodes were modeled as two Schottky barriers in parallel, the lower of the two Schottky barriers, when present, was predominantly centered at one of the three values: ∼0.60, 0.85 or 1.05 eV. The sources of these non-idealities were investigated using electron-beam- induced current (EBIC) and deep-level transient spectroscopy (DLTS) to determine the nature and energy levels of the defects. DLTS revealed a defect level that corresponds with the low- (non-ideal) barrier height, at ∼0.60 eV. It was also observed that the I-V characteristics tended to degrade with increasing deep-level concentration and that inhomogeneous diodes tended to contain defect clusters. Based on the results, it is proposed that inhomogeneities, in the form of one or more low-barrier height regions within a high-barrier height diode, are caused by defect clusters that locally pin the Fermi level. © 2007 IOP Publishing Ltd.

  • 45.
    Fornari, R.
    et al.
    Leibniz-Institute for Crystal Growth, Institute of Physics, Humboldt University, Berlin, Germany.
    Carlos, Rojo J.
    Carlos Rojo, J., General Electric Global Research, NY, United States.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Preface2008In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 310, no 5, p. 875p. 875-875Article in journal (Other academic)
  • 46.
    Giannazzo, F.
    et al.
    CNR-IMM, Catania, Italy.
    Deretzis, I.
    CNR-IMM, Catania, Italy.
    La Magna, A.
    CNR-IMM, Catania, Italy.
    Di Franco, S.
    CNR-IMM, Catania, Italy.
    Piluso, N.
    ETC, Catania, Italy.
    Fiorenza, P.
    CNR-IMM, Catania, Italy.
    Roccaforte, F.
    CNR-IMM, Catania, Italy.
    Schmid, P.
    Centrotherm thermal solutions GmbH + Co. KG, Blaubeuren, Germany.
    Lerch, W.
    Centrotherm thermal solutions GmbH + Co. KG, Blaubeuren, Germany.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Impact of substrate steps and of monolayer-bilayer junctions on the electronic transport in epitaxial graphene on 4H-SiC (0001)2013In: Silicon Carbide and Related Materials 2012 / [ed] Alexander A. Lebedev, Sergey Yu. Davydov, Pavel A. Ivanov and Mikhail E. Levinshtein, Trans Tech Publications Inc., 2013, Vol. 740-742, p. 113-116Conference paper (Refereed)
    Abstract [en]

    Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.

  • 47.
    Giannazzo, F
    et al.
    CNR IMM, Italy .
    Deretzis, I
    CNR IMM, Italy .
    La Magna, A
    CNR IMM, Italy .
    Roccaforte, F
    CNR IMM, Italy .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Electronic transport at monolayer-bilayer junctions in epitaxial graphene on SiC2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 23Article in journal (Refereed)
    Abstract [en]

    Two-dimensional maps of the electronic conductance in epitaxial graphene grown on SiC were obtained by calibrated conductive atomic force microscopy. The correlation between morphological and electrical maps revealed the local conductance degradation in epitaxial graphene over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at the 1L/2L junction is a purely quantum-mechanical effect independent on the interaction with the substrate. First-principles transport calculations indicate that the weak wave-function coupling between the 1L pi/pi* bands with the respective first bands of the 2L region gives rise to a strong suppression of the conductance for energies within +/- 0.48 eV from the Dirac point. Conductance degradation at 1L/2L junctions is therefore a general issue for large area graphene with a certain fraction of inhomogeneities in the layer number, including graphene grown by chemical vapor deposition on metals. DOI: 10.1103/PhysRevB.86.235422

  • 48.
    Giusca, Cristina E.
    et al.
    National Phys Lab, England .
    Spencer, Steve J.
    National Phys Lab, England .
    Shard, Alex G.
    National Phys Lab, England .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kazakova, Olga
    National Phys Lab, England .
    Exploring graphene formation on the C-terminated face of SiC by structural, chemical and electrical methods2014In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 69, p. 221-229Article in journal (Refereed)
    Abstract [en]

    The properties of epitaxial graphene on the C-face of SiC are investigated using comprehensive structural, chemical and electrical analyses. By matching similar nanoscale features on the surface potential and Raman spectroscopy maps, individual domains have been assigned to graphene patches of 1-5 monolayers thick, as well as bare SiC substrate. Furthermore, these studies revealed that the growth proceeds in an island-like fashion, consistent with the Volmer-Weber growth mode, illustrating also the presence of nucleation sites for graphene domain growth. Raman spectroscopy data shows evidence of large area crystallites (up to 620 nm) and high quality graphene on the C-face of SiC. A comprehensive chemical analysis of the sample has been provided by X-ray photoelectron spectroscopy investigations, further supporting surface potential mapping observations on the thickness of graphene layers. It is shown that for the growth conditions used in this study, 5 monolayer thick graphene does not form a continuous layer, so such thickness is not sufficient to completely cover the substrate.

  • 49. Glans, P.-A.
    et al.
    Balasubramanian, T.
    MAX-Lab, Lund University, S-221 00, Lund, Sweden.
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Johansson, Leif
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Core level and valence band photoemission study of the (1 1 1) and (1¯ 1¯ 1¯) surfaces of 3C-SiC2001In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 470, no 3, p. 284-292Article in journal (Refereed)
    Abstract [en]

    A core level and valence band photoemission study of thick 3C-SiC(1 1 1) and 3C-SiC(1¯ 1¯ 1¯) epilayers grown by sublimation epitaxy is reported. The as introduced samples show threefold 1×1 low-energy electron diffraction patterns. For the Si face v3 and 6v3 reconstructed surfaces develop after in situ heating to 1100 °C and 1300 °C, respectively. For the C face a 3×3 reconstruction form after heating to 980 °C. A semiconducting behavior is observed for the v3 and 3×3 reconstructed surfaces while the 6v3 reconstruction show a Fermi edge and thus a metallic-like behavior. The surface state on the v3 surface is investigated and found to have ?1 symmetry and a total band width of 0.10 eV within the first surface Brillouin zone. For the Si2p and C 1s core levels binding energies and surface shifted components are extracted and compared to earlier reported results for 6H- and 4H-SiC.

  • 50.
    Gogova, D
    et al.
    Leibniz Institute Kristallzuchtung.
    Albrecht, M
    Leibniz Institute Kristallzuchtung.
    Remmele, T
    Leibniz Institute Kristallzuchtung.
    Irmscher, K
    Leibniz Institute Kristallzuchtung.
    Siche, D
    Leibniz Institute Kristallzuchtung.
    Rost, H-J
    Leibniz Institute Kristallzuchtung.
    Schmidbauer, M
    Leibniz Institute Kristallzuchtung.
    Fornari, R
    Leibniz Institute Kristallzuchtung.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Microscopic lateral overgrowth by physical vapour transport of GaN on self-organized diamond-like carbon masks2009In: CRYSTAL RESEARCH AND TECHNOLOGY, ISSN 0232-1300, Vol. 44, no 10, p. 1078-1082Article in journal (Refereed)
    Abstract [en]

    Techniques to reduce dislocation densities in GaN grown on foreign substrates are an interesting alternative to bulk growth as long as efficient bulk crystals growth techniques are not available, In this paper a new approach for epitaxial lateral overgrowth (ELO) of GaN through an in-situ grown self-organized amorphous (diamond-like) carbon mask is demonstrated. The ELO was done for the first time by physical vapour transport of Ga, using NH3 as a nitrogen source. The overgrowth results in a decrease of the threading dislocation density by at least one order of magnitude compared to that of the MOCVD GaN/sapphire templates.

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