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1.
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2.
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3.
  • Goss, J.P., et al. (author)
  • First principles studies of H in diamond
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 186:2, s. 263-8
  • Journal article (peer-reviewed)abstract
    • Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated impurity, the bond-centered site is found lowest in energy and posses both donor and acceptor levels. The neutral defect possesses a single local mode with very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H*2 defects which are also found to be almost IR-inactive. The complex between B and H is investigated and the activation energy for the reaction B-H → B - + H+ found to be in rasonable agreement with experiment. Hydrogen is strongly bound to dislocations which, together with H*2, may form part of the hydrogen accumulation layer detected in some plasma studies.
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4.
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5.
  • Kemerink, Martijn, et al. (author)
  • Exciton bleaching in p-type single and double quantum wells: The effects of subband occupation and wave function overlap
  • 1997
  • In: Physica status solidi. A, Applied research. - : Academic Verlag GMBH. - 0031-8965 .- 1521-396X. ; 164:1, s. 73-76
  • Journal article (peer-reviewed)abstract
    • We have studied exciton unbinding in empty and p-doped single and double quantum wells, using magneto-photoluminescence excitation measurements. The use of p-type heterostructures allows to discriminate unambiguously between exciton bleaching by Coulomb screening and by the Pauli exclusion principle. The groundstate heavy-hole exciton, which experiences both effects, is unbound at hole densities in the range (6 to 11) x 10(15) m(-2). In contrast, the groundstate light-hole and first excited heavy-hole excitons: which only experience Coulomb screening, still have a finite binding energy at these densities. We found that the excitons of excited subbands are far less efficiently screened than those of the light-hole groundstate, due to the lesser overlap of the screening groundstate heavy holes with the excited subbands than with the groundstate light holes.
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6.
  • Lloyd Spetz, Anita, et al. (author)
  • High temperature sensors based on metal-insulator-silicon carbide devices
  • 1997
  • In: Physica status solidi. A, Applied research. - : John Wiley & Sons. - 0031-8965 .- 1521-396X. ; 162:1, s. 493-511
  • Journal article (peer-reviewed)abstract
    • High temperature gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are developed both as capacitors and Schottky diodes. A maximum operation temperature of 1000 degrees C is obtained for capacitors based on 4H-SiC, and all sensors work routinely for several weeks at 600 degrees C. Reducing gases like hydrocarbons and hydrogen lower the flat band voltage of the capacitor and the barrier height of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 degrees C total combustion occurs on the sensor surface and the signal is high for fuel in excess and low for air in excess. At temperatures around 400 degrees C the response is more linear. The high temperature operation causes interdiffusion of the metal and insulator layers in these devices; and this interdiffusion has been studied. At sufficiently high temperatures the inversion capacitance shows different levels for hydrogen free and hydrogen containing ambients, which is suggested to be due to a reversible hydrogen annealing effect at the insulator-silicon carbide interface.
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7.
  • Paskov, Plamen, 1959-, et al. (author)
  • Anisotropy of the free exciton emission in GaN grown on a-plane sapphire
  • 2002
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 75-79
  • Journal article (peer-reviewed)abstract
    • The influence of the anisotropic in-plane strain on the optical response of GaN layers grown on a-plane sapphire is investigated. A splitting of the G5 states of the A and B free excitons into two components, polarized parallel and perpendicular to the strain direction, is observed. The experimental results are discussed in the framework of the effective exciton Hamiltonian formalism.
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8.
  • Jones, R, et al. (author)
  • Interaction of oxygen with threading dislocations in GaN
  • 1999
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 171:1, s. 167-173
  • Journal article (peer-reviewed)abstract
    • A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {101-0} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.
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9.
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10.
  • Arwin, Hans, et al. (author)
  • Gas sensing based on ellipsometric measurement on porous silicon
  • 2003
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 197:2, s. 518-522
  • Conference paper (other academic/artistic)abstract
    • Ellipsometry has sufficient sensitivity for sensor applications and is here used as an optical readout method in a gas sensing system. Porous silicon is used as sensing layers in which vapors of solvents can adsorb and condensate due to capillary effects. A miniaturized multi-beam ellipsometer system is proposed and the concept is demonstrated by measurements on alcohol vapors. Optimization of the sensor system is discussed and improvement of sensitivity and alteration of selectivity by metal deposition in porous silicon layers are presented.
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11.
  • Arwin, Hans, et al. (author)
  • Protein adsorption in thin porous silicon layers
  • 2000
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 182:1, s. 515-520
  • Journal article (peer-reviewed)abstract
    • Porous silicon layers with thicknesses in the range 100-400 nm and average porosities in the range 38-71% were prepared by electrochemical anodization. Variable angle spectroscopic ellipsometry was used to characterize the microstructure of the layers before protein adsorption. In-situ ellipsometry was then employed tr, monitor the kinetics of fibrinogen and human serum albumin adsorption. At steady state new ellipsometric spectra were recorded to determine the total amount of adsorbed protein. Under the experimental conditions used here, the protein molecules were found to adsorb in the outermost part of the porous layer. However, human serum albumin penetrated into the porous silicon matrix at low pH and high porosity. From a methodological point of view it was found that spectroscopic ellipsometry is an appropriate tool for characterization of the microstructure of porous silicon layers and for in-situ monitoring of protein adsorption in such layers including depth profiling.
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12.
  • Arwin, Hans (author)
  • Spectroscopic ellipsometry for characterization and monitoring of organic layers
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 188:4, s. 1331-1338
  • Journal article (peer-reviewed)abstract
    • The thickness resolution and the in situ advantage of ellipsometry make this optical technique suitable for studies of thin organic layers, especially those of biological interest. With state of the art methodology it is possible to perform monolayer spectroscopy, e.g. of a protein layer at a solid/ liquid interface and also to resolve details in the kinetics of layer formation. Furthermore, complicated microstructures, like porous silicon layers, can be characterized and protein adsorption can be monitored in porous layers providing information about pore filling and penetration depths of protein molecules of different size and type. Quantification of adsorption and determination of microstructural parameters of thin organic layers on planar surfaces and in porous layers are of large interest in areas like biomaterials, basic studies of surface-based biointeraction and for biosensor development. In this report we review the use of spectroscopic ellipsometry for studies of organic layers including protein adsorption and protein monolayer spectroscopy. Included is also a discussion about possibilities for in situ spectroscopic monitoring of engineering of multilayer systems based on macromolecules.
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13.
  • Bakker, Jimmy W. P., et al. (author)
  • Improvement of porous silicon based gas sensors by polymer modification
  • 2003
  • In: Physica Status Solidi (A). - : Wiley. - 0031-8965 .- 1521-396X. ; 197:2, s. 378-381
  • Journal article (peer-reviewed)abstract
    • Gas sensing was performed using spectroscopic ellipsometry and porous silicon films. Modification of the porous layer by polymer deposition showed an increase in sensitivity to organic solvent vapor of up to 135%. The increase in sensitivity is strongly dependent on polymer concentration. At high concentrations, too much polymer is deposited, presumably blocking the pores, causing a decrease in sensitivity. At sufficiently low concentrations, the polymer causes a strong increase in sensitivity. This is assumed to be caused by the polymer being deposited inside the pores, where its interaction with the vapor influences the sensitivity. At very low concentration, the sensitivity approaches values obtained without polymer modification. The sensitivity increase is different for different vapors, pointing to possible selectivity enhancement.
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14.
  • Darakchieva, Vanya, et al. (author)
  • Residual strain in HVPE GaN free-standing and re-grown homoepitaxial layers
  • 2003
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 195:3, s. 516-522
  • Journal article (peer-reviewed)abstract
    • The lattice parameters of as-grown hydride vapor phase epitaxy GaN layers on sapphire, free-standing layers after the substrate lift-off, and homoepitaxial layers grown on the free-standing layers are measured. The in-plane and out-of-plane strains are calculated. It is found that the substrate removal leads to strain relaxation in the crack-free GaN free-standing layers to a certain extent. A small increase of the strain in the GaN homoepitaxial layers compared to the free-standing layers is observed. Cathodoluminescence (CL) spectroscopy and imaging, photoluminescence (PL) and Raman measurements are used as complementary tools in the residual strain analysis.
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15.
  • Darakchieva, Vanya, et al. (author)
  • Strain evolution in high temperature AlN buffer layers for HVPE-GaN growth
  • 2002
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 59-64
  • Journal article (peer-reviewed)abstract
    • High temperature AlN buffer layers are deposited on a-plane sapphire by reactive magnetron sputtering. The effect of the buffer thickness on the AlN structural properties and surface morphology are studied in correlation with the subsequent hydride vapour phase epitaxy of GaN. A minimum degree of mosaicity and screw dislocation density is determined for a 50 nm thick AlN buffer. With increasing the AlN thickness, a strain relaxation occurs as a result of misfit dislocation generation and higher degree of mosaicity. A blue shift of the E-1(TO) frequency evaluated by means of infrared reflection spectroscopy is linearly correlated with an increase in biaxial compressive stress in the films through the IR stress factor k(E1)(b) = 2.57 +/- 0.26 cm(-1) GPa(-1).
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16.
  • Figielski, T, et al. (author)
  • Microcircuit tailoring in ferromagnetic semiconductor (Ga,Mn)As
  • 2003
  • In: Physica Status Solidi. A, Applied Research. - : Wiley. - 0031-8965. ; 195:1, s. 228-231
  • Journal article (peer-reviewed)abstract
    • In order to search for novel giant-magnetoresistance systems, we fabricated and investigated narrow constrictions in the layers of the ferromagnetic semiconductor (Ga,Mn)As. We found that constrictions a few hundred nanometers wide, tailored by means of the electron-beam lithography and wet etching, were not conducting at liquid helium temperatures unless illuminated, probably due to the trapping action of surface states appearing on an extra surface area denuded by the etching. To avoid this, we used selective implantation of oxygen ions into the ferromagnetic layer to tailor the constrictions. We have shown that such an implantation inactivates Mn acceptors in the layer and destroys ferromagnetism. We propose an application of oxygen ion implantation as a method of fabricating microcircuits in future spin electronics based on Mn-containing III-V semiconductor compounds.
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19.
  • Georsson, K, et al. (author)
  • Transmission electron microscopy of InP Stranski‐Krastanow islands buried in GaInP
  • 1995
  • In: Physica Status Solidi (A) Applied Research. - : Wiley. - 0031-8965. ; 150:1, s. 479-487
  • Journal article (other academic/artistic)abstract
    • InP islands in a matrix of GaInP are investigated by transmission electron microscopy (TEM). The islands are uniform in size and shape, and are formed though self‐organized Stranski‐Krastanow growth. The introduction of a 4 ML intermediate GaP layer eliminates the bimodal distribution found in films where the InP was grown directly on the GaInP layer. The achieved island density is around 2 × 109 cm−2, and the basal plane of the InP islands is around 40 × 50 nm2. A model is suggested for the shape where the islands are in a truncated pyramidal form, showing {111}, {110}, and (001) facets, with an island height of 10 to 15 nm. A fast and simple specimen preparation method is suggested, based on laser assisted chemical etching.
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20.
  • Godlewski, M, et al. (author)
  • Compensation mechanisms in magnesium doped GaN
  • 2004
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:2, s. 216-220
  • Journal article (peer-reviewed)abstract
    • Compensation processes in magnesium doped GaN epilayers and bulk samples are studied. We demonstrate enhancement of potential fluctuations in Mg doped samples, from Kelvin probe atomic force microscopy measurements. Large- and small-scale light emission fluctuations are also demonstrated. Micro-photoluminescence (PL) study indicates an unusual anti-correlation between the intensities of excitonic and defect-related emission processes in p-type doped structures and also the presence of the so-called hot-PL. Hot-PL observed in compensated p-type samples, we relate to the presence of strong potential fluctuations. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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21.
  • Gogova, Daniela, et al. (author)
  • Fast growth of high quality GaN
  • 2003
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 200:1, s. 13-17
  • Journal article (peer-reviewed)abstract
    • We have grown bulk-like GaN with a thickness up to 335 μm on 2″ sapphire substrates in a vertical HVPE reactor with a bottom-fed design. A very high growth rate of 250 μm/h is reached with high crystalline quality of the grown material. The low temperature PL spectra show the free A-exciton line at 3.483 eV and rather narrow I2 lines with FWHM of 1–2 meV indicating high crystalline quality and low doping concentration. This HVPE-GaN has the potential to provide lattice-matched and thermally-matched substrates for further epitaxial growth of high quality GaN with a low dislocation density for advanced heterostructure devices.
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22.
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23.
  • Hounsome, L.S., et al. (author)
  • Optical properties of vacancy related defects in diamond
  • 2005
  • In: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:11, s. 2182-2187
  • Journal article (peer-reviewed)abstract
    • ‹110› vacancy chains, multi-vacancy clusters and vacancy discs have been modeled using density functional theory within the AIMPRO and DFTB codes. While a connection can be established between the results on vacancy chains and previous EPR experiments, no connection can be made between the point defects and the optical properties of natural type IIa brown diamonds. However, a vacancy disc consisting of a {111} double plane of vacancies is stable and possesses an absorption spectrum similar to that found in brown diamonds.
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24.
  • Isberg, Jan, et al. (author)
  • Temperature dependence of hole drift mobility in high-purity single-crystal CVD diamond
  • 2005
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 202:11, s. 2194-2198
  • Journal article (peer-reviewed)abstract
    • Hole transport properties in high-purity single crystal CVD diamond samples were studied using the time of flight technique with optical excitation of the carriers. The measurements were taken at different temperatures in the interval 80-470 K. By varying the intensity of the optical excitation over several orders of magnitude, measurements at different carrier concentrations have been performed. In this way, measurements have been made both in the space charge limited and non space charge limited regimes, with consistent results. The temperature dependence of the low-field hole drift mobility shows a 7(alpha) dependence with a approximate to -1.5, below 350 K. This indicates that acoustic phonon scattering is the dominant scattering mechanism and a very low concentration of ionized impurities in this material.
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25.
  • Iusan, Diana, et al. (author)
  • Role of defects on the magnetic interactions in Mn-doped ZnO
  • 2007
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 204:1, s. 53-60
  • Journal article (peer-reviewed)abstract
    • Oxide based diluted magnetic semiconductors are highly controversial from the point of view of intrinsic ferromagnetism brought in by doping with transition metal ions. By ab-initio Korringa-Kohn-Rostoker-Coherent-Potential-Approximation (KKR-CPA) calculations in the framework of density functional theory, we have obtained the electronic structure and magnetic exchange interaction parameters of Mn-doped ZnO in the presence and absence of several types of defects. A weak antiferromagnetic exchange interaction is observed in pure Mn-doped ZnO in the dilute limit with an increase in the strength of interactions with increasing concentration of Mn. In the presence of donor defects, such as oxygen vacancies and interstitial Zn, the interactions remain antiferromagnetic whereas in case of acceptor defects like Zn vacancies and N substitution of O, ferromagnetic interactions are observed. We have used the ab-initio exchange parameters in Monte-Carlo simulations for calculating finite temperature properties. Due to the short ranged behavior of interactions and disorder effects, the Curie temperatures calculated from Monte Carlo simulations show moderate values, not exceeding 50 K when the Mn content is 5%.
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26.
  • Jaafar, M., et al. (author)
  • FePt thin film irradiated with high energy ions
  • 2007
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 204:6, s. 1724-1730
  • Journal article (peer-reviewed)abstract
    • The changes in structural and magnetic properties of FePt thin films due to the irradiation with high energy ions (Br7+ and Cl2+) were studied. From the hysteresis loops dominating in-plane anisotropy is derived, however, the samples present a minor out-of-plane component. The structure and the magnetic properties of the films can be tuned by selecting the appropriate irradiation parameters (different ions, energies and fluencies). For the irradiation parameters used in this study an in-plane anisotropy is favoured. Irradiation with Br7+ seems to induce minor changes in the structural ordering of the thin films, whereas the Cl2+ ions promote the amorphization of the surface of the films. In addition, a magnetic thin film pattemed at the micrometer scale was obtained after irradiation through a micrometric mask.
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27.
  • Karlsson, L M, et al. (author)
  • Adsorption of human serum albumin in porous silicon gradients
  • 2003
  • In: physica status solidi (a). - : Wiley. - 0031-8965 .- 1521-396X.
  • Conference paper (other academic/artistic)abstract
    • Backside etching has been utilized to produce gradients of pore size and layer thickness in porous silicon. Human serum albumin (HSA) was adsorbed on such gradients at two different pH values: 4.9, the pI of HSA, and 7.4, the physiological pH. The samples were investigated by scanning electron microscopy, spectroscopic ellipsometry, and autoradiography. The results show that the protein adsorbed displays a gradient along with the pore size and the thickness gradient. The higher than current density used during etching, the more sway-back shaped curves were seen for the protein adsorption pattern, independent of pH. When 50 mA/cm2 current density was used during etching, the quota between the maximal intensity value and the plateau value seen after adsorption of the HSA increased for pH 7.4.
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28.
  • Kasic, A., et al. (author)
  • Micro-Raman scattering profiling studies on HVPE-grown free-standing GaN
  • 2004
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:12, s. 2773-2776
  • Journal article (peer-reviewed)abstract
    • Free-standing GaN of ∼330 μm thickness with low defect density was prepared by hydride vapor-phase epitaxy (HVPE) on sapphire in a vertical atmospheric-pressure reactor and a subsequent laser-induced lift-off process. The structural and optical properties of the material were assessed by various characterization techniques, like X-ray diffraction, photo- and cathodoluminescence, spectroscopic ellipsometry, positron annihilation spectroscopy, and transmission electron microscopy. Here, we focus on μ-Raman scattering profiling studies providing the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness. Profiles of the free-carrier concentration are obtained from monitoring the LO-phonon plasmon coupled mode. Comparative investigations are performed on the material before and after separation of the sapphire substrate. The GaN material presented here is well capable of serving as a substrate for further homoepitaxial strain-relaxed and crack-free growth needed for fabrication of high-quality III-nitride device heterostructures.
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29.
  • Lastras-Martí­nez, L. F., et al. (author)
  • Reflectance difference spectroscopy of GaAs asymmetric surface quantum wells above the fundamental gap
  • 1998
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 170:2, s. 317-321
  • Journal article (peer-reviewed)abstract
    • We report Reflectance Difference (RD) measurement on (001) GaAs surface quantum wells (QW) under Ultra High Vacuum (UHV) conditions from 1.7 to 5.0eV. The QW is embedded between an arsenic-rich reconstructed GaAs surface and an AlAs barrier. The samples, grown by MBE with a protective arsenic cap layer, were heated to 320 and 430°C to desorb the As layer and form c(4 x 4) and (2 x 4) surface reconstructions, respectively. By modifying the surface reconstructure, we are able to separate the contributions to the optical anisotropy from the surface region (mainly associated with the As dimers) from those originating below the surface.
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30.
  • Lloyd-Spets, Anita, et al. (author)
  • SiC based field effect gas sensors for industrial applications
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 185:1, s. 15-25
  • Journal article (peer-reviewed)abstract
    • The development and field-testing of high-temperature sensors based on silicon carbide devices have shown promising results in several application areas. Silicon carbide based field-effect sensors can be operated over a large temperature range, 100-600 degreesC, and since silicon carbide is a chemically very inert material these sensors can be used in environments like exhaust gases and flue gases from boilers. The sensors respond to reducing gases like hydrogen, hydrocarbons and carbon monoxide. The use of different temperatures, different catalytic metals and different structures of the gate metal gives selectivity to different gases and arrays of sensors can be used to identify and monitor several components in gas mixtures. MOSFET sensors based on SIC combine the advantage of simple circuitry with a thicker insulator, which increases the long term stability of the devices. In this paper we describe silicon carbide MOSFET sensors and their performance and give: examples of industrial applications such as monitoring of car exhausts and flue gases. Chemometric methods have been used for the evaluation of the data.
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31.
  • Malmhäll, Roger, et al. (author)
  • Transport properties of amorphous ferromagnets : Hall and ohmic resistivities
  • 1979
  • In: Physica status solidi. A, Applied research. - : John Wiley & Sons. - 0031-8965 .- 1521-396X. ; 53:2, s. 641-651
  • Journal article (peer-reviewed)abstract
    • Simultaneous measurements are made of the ohmic resistivity (ϱ) and the Hall resistivity (ϱH) of a number of amorphous ferromagnetic Fe–Ni based alloys prepared by the rapid quenching technique. The temperature range of the experiments is 100 through 700 K and the applied magnetic fields are varied up to 2.5 T. It turns out that for all the amorphous alloys: (i) R1 ≈ 5 × 10−8 m3/As, (ii) ϱ ≈ 1.5 μΩm, (iii) ϱH has a sharp drop from which the ferromagnetic transition temperature (TC) can be determined to ≈ 5 K, (iv) the critical exponent γ ≈ 1.6 for (TC + 1) < < T < (TC + 50) K, and (v) the extraordinary Hall conductivity γHs is of the same order (≈ 5 × 104 Ω−1 m−1) as in crystalline Fe alloys. Further, in some cases (dϱ/dT) exhibits a well-defined anomaly at TC.
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32.
  • Mlyuka, Nuru, et al. (author)
  • Thermochromic VO2-Based Multilayer Films with Enhanced Transmittance and Solar Modulation
  • 2009
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 206:9, s. 2155-2160
  • Journal article (peer-reviewed)abstract
    • Vanadium dioxide (VO2) shows an abrupt and reversible change in optical   and electrical properties when the temperature is raised beyond a   critical point of similar to 68 degrees C. Films made from this   material have a potential to be used in energy efficient "smart"   windows with temperature-dependent throughput of solar radiation. Two   of the drawbacks of this material have been its low luminous   transmittance and limited solar modulation of transmittance during   switching. In this work we report calculations and experiments on   multilayers Of VO2 and TiO2, produced by reactive DC magnetron   sputtering, that significantly improve the luminous transmittance and   solar modulation of the films during switching. We also explore the   angular-dependent transmittance of five-layer TiO2/VO2/TiO2/VO2/TiO2   films and demonstrate that the modulation of luminous and solar   transmittance can be enhanced at non-normal angles of incidence.
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33.
  • Monemar, Bo, et al. (author)
  • Influence of depletion fields on photoluminescence of n-doped InGaN/GaN multiple quantum well structures
  • 2002
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 192:1, s. 21-26
  • Journal article (peer-reviewed)abstract
    • We report on a detailed study of low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si-doped barriers of In0.01Ga0.99N. The entire MQW structure was grown at 800 degreesC. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the OW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample, 5 QWs of width 3 nm and with 6 nm highly Si-doped In0.01Ga0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction, i.e. a complete LED structure with semitransparent top metallisation. Two PL peaks are observed also in this case, of similar origin as described above. With forward bias, this structure shows lower-energy PL emission, indicating the gradual activation of the other QWs closer to the pn-junction. At high forward bias the low-energy part of the PL spectrum becomes similar to the electroluminescence (EL) spectrum.
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34.
  • Monemar, Bo, et al. (author)
  • Optical investigation of AlGaN/GaN quantum wells and superlattices
  • 2004
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2251-2258
  • Journal article (peer-reviewed)abstract
    • We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 x 10(18) cm(-3) free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.
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35.
  • Monemar, Bo, et al. (author)
  • Photoluminescence of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells
  • 2002
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 161-166
  • Journal article (peer-reviewed)abstract
    • We report on a detailed study of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01-0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1-yN barriers had a Si doping of about 5 x 10(18) cm(-3) . The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p-n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can e interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.
  •  
36.
  • Monemar, Bo, et al. (author)
  • Photoluminescence of InGaN/GaN and AlGaN/GaN multiple quantum well structures : Role of depletion fields and polarization fields
  • 2003
  • In: Physica status solidi. A, Applied research. - Weinheim, Germany : Wiley-VCH Verlagsgesellschaft. - 0031-8965 .- 1521-396X. ; 195:3, s. 523-527
  • Journal article (peer-reviewed)abstract
    • We report on low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. The MQW was placed either in the near surface depletion field or at the n-side depletion field of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears in PL. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for Al0.07Ga0.93N/GaN structures, with near surface MQWs with 5 QWs, both Si-doped and nominally undoped. These structures show clear spectral features related to nonequivalent QWs in a graded depletion field.
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37.
  • Paskov, Plamen, et al. (author)
  • Polarized photoluminescence of exciton-polaritons in free-standing GaN
  • 2004
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:4, s. 678-685
  • Journal article (peer-reviewed)abstract
    • We report on the polarization properties of the exciton-polariton modes in GaN. The dispersion curves and the expected emission lineshape of polaritons for all polarization configurations are calculated taking into account the spatial dispersion and the simultaneous exciton-photon coupling of all optically active states. An experimental study of the exciton-polariton luminescence in a free-standing GaN layer is also performed. The spectra reveal a clear difference between the emissions polarized perpendicular and parallel to the c-axis of the crystal. The experimental results are discussed in terms of optical selection rules and population of the polariton states. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  •  
38.
  • Paskova, Tanja, et al. (author)
  • Defect reduction in HVPE growth of GaN and related optical spectra
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 183:1, s. 197-203
  • Journal article (peer-reviewed)abstract
    • GaN technology is still based on highly mismatched heteroepitaxial growth on foreign substrates, and therefore needs to overcome a high defect density and a high level of stress in the epitaxial layers. Various attempts have been made to reduce the defects and stress in thick GaN layers. We here report a reduction of the defect density in thick GaN layers grown by hydride vapour phase epitaxy, using regrowth on free-standing GaN films, as well as introducing an AlN buffer and AlN interlayer in the growth sequence. Special focus is put on the optical properties of the material.
  •  
39.
  • Paskova, Tanja, et al. (author)
  • Mass transport growth and properties of hydride vapour phase epitaxy GaN
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 188:1, s. 447-451
  • Journal article (peer-reviewed)abstract
    • We report a comparative study of the optical and structural properties of mass-transport and conventionally grown GaN by hydride vapor phase epitaxy. A strong donor-acceptor pair emission is observed from the mass-transport regions with a distinctive intensity contrast between the exciton and donor-acceptor bands from mass-transport and nontransport regions. Secondary ion mass spectroscopy was employed to investigate the impurity incorporation into different regions. A moderate increase of residual impurity incorporation or redistribution was found in mass-transport regions related to different growth modes.
  •  
40.
  • Paskova, Tanja, et al. (author)
  • Polar and nonpolar GaN grown by HVPE : Preferable substrates for nitride-based emitting devices
  • 2004
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2265-2270
  • Journal article (peer-reviewed)abstract
    • We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.
  •  
41.
  • Pehlivan, Esat, et al. (author)
  • Ageing of electrochromic WO3 coatings characterized by electrochemical impedance spectroscopy
  • 2010
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 207:7, s. 1772-1776
  • Journal article (peer-reviewed)abstract
    • We have developed a method for characterization of ageing of electrochromic materials by electrochemical impedance spectroscopy (EIS). Electrochromic WO3 thin films have been electrochemically cycled in propionic acid electrolyte and probed by EIS and optical measurements. A very small amount of optical degradation was observed in both the bleached and coloured states. The samples exhibited a few hundred times higher impedance in the bleached state than in the coloured state. It was observed that, in the bleached state, impedance values at low frequencies increased significantly with increasing number of cycles.
  •  
42.
  • Perraud, Simon, et al. (author)
  • Silicon nanocrystals : Novel synthesis routes for photovoltaic applications
  • 2013
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 210:4, s. 649-657
  • Journal article (peer-reviewed)abstract
    • Novel processes were developed for fabricating silicon nanocrystals and nanocomposite materials which could be used as absorbers in third generation photovoltaic devices. A conventional high-temperature annealing technique was studied as a reference process, with some new insights in crystallisation mechanisms. Innovative methods for silicon nanocrystal synthesis at much lower temperature were demonstrated, namely chemical vapour deposition (CVD), physical vapour deposition (PVD) and aerosol-assisted CVD. Besides the advantage of low substrate temperature, these new techniques allow to fabricate silicon nanocrystals embedded in wide bandgap semiconductor host matrices, with a high density and a narrow size dispersion.
  •  
43.
  •  
44.
  • Porro, Samuele, et al. (author)
  • Electrical Analysis and Interface States Evaluation of of Ni Schottky diodes on 4H-SiC thick epilayers
  • 2005
  • In: Physica Status Solidi (a) applications and materials science. - : John Wiley & Sons. - 1862-6300 .- 1862-6319. ; 202:13, s. 2508-2514
  • Journal article (peer-reviewed)abstract
    • This work has been focused on characterization of thick 4H-SiC layers produced by sublimation epitaxy. Nickel Schottky contacts have been fabricated in order to characterize the grown material and evaluate the interfacial layer between metal and semiconductor. The characterization study includes current-voltage and capacitance-voltage high temperature measurements, from which Schottky barrier, net donor concentration and on-state resistance values have been extracted. The diodes show a typical behavior of J-V and C-V curves with temperature, with Schottky barrier heights of 1.3 eV ÷ 1.4 eV and net donor concentration of 4 × 1015 cm-3 ÷ 1 × 1016 cm-3. From the Bardeen's model on reverse J-V, the density of states of the interfacial layer has been estimated to 7 × 1011 eV-1 cm-2 ÷ 8 × 1011 eV-1 cm-2, a value that is similar to the density of states of oxide layers in deliberated MOS structures realized on the same epilayers.
  •  
45.
  • Pozina, Galia, et al. (author)
  • Multiple peak spectra from InGaN/GaN multiple quantum wells
  • 2000
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 180:1, s. 85-89
  • Journal article (peer-reviewed)abstract
    • Optical properties of In0.12Ga0.88N/GaN multiple quantum well (MQW) structures grown by metalorganic vapor phase epitaxy at 820 degrees C are reported. The transmission electron microscopy measurements show a roughness of the sample surfaces containing small pit like defects of a size 100 to 200 Angstrom with a density far exceeding the dislocation density. The photoluminescence (PL) spectra in this set of samples are dominated by strong multiple peak emissions associated both with the MQW exciton recombination and with strongly localized states of energies much lower than the QW bandgap. We suggest that the low energy PL peaks are due to (i) photocarriers localized in quantum islands closely related to the threading dislocations, and possibly associated with V-defect induced side-wall QWs, (ii) strongly localized excitons in OD quantum dots in the disordered surface region, formed by uncontrolled surface etching processes.
  •  
46.
  • Pozina, Galia, et al. (author)
  • Optical study of AlGaN/GaN multiple quantum well structures grown on laterally overgrown GaN templates
  • 2002
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 107-111
  • Journal article (peer-reviewed)abstract
    • We report on optical studies of Al0.1Ga0.9N/GaN structures with five 30 (A) over circle thick GaN quantum wells (QWs) grown by metal organic vapor phase epitaxy using the lateral overgrowth technique. Overgrown regions demonstrate better structural and optical properties. The low temperature photoluminescence (PL) is dominated by the multiple quantum well (MQW) emission at 3.53 eV with the linewidth of similar to50 meV. The PL decay time for this line was measured to be similar to600 ps. Comparison with an AlGaN/GaN MQW light-emitting diode (LED) structure is done. The LED structure was grown with a p-type doped AlGaN top layer and a p-GaN contact layer. The PL spectrum of the LED structure shows besides the donor-acceptor pair recombination from the top layer an additional 60 meV wide line at 3.64 eV. The presence of two MQW peaks may be related to the potential gradient present across the MOW structure.
  •  
47.
  • Pozina, Galia, et al. (author)
  • Time-resolved photoluminescence in strained GaN layers
  • 2001
  • In: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 183:1, s. 151-155
  • Journal article (peer-reviewed)abstract
    • A set of GaN epilayers grown by metalorganic chemical vapor deposition on 6H-SiC substrates was studied by time-resolved photoluminescence (PL) spectroscopy. The PL spectra are dominated by the free A exciton (FEA) and by the neutral-donor-bound exciton (D0X) transitions. The position of FEA indicates that the GaN layers are under tension. We observe that the recombination lifetime for the FEA is about 40-50 ps in all the layers, whereas the recombination time for the D0X varies for different samples. We found that the recombination lifetimes for D0X have a clear dependence on the position of FEA, i.e. the recombination lifetime increases with decreasing strain in the layers. The results can be explained in terms of the character of the hole states involved in the donor-bound exciton recombination.
  •  
48.
  • Ren, Yi, et al. (author)
  • Influence of the Cu2ZnSnS4 absorberthickness on thin film solar cells
  • 2015
  • In: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 212:12, s. 2889-2896
  • Journal article (peer-reviewed)abstract
    • In this study, we investigate the influence of absorber thickness on Cu2ZnSnS4 (CZTS) solar cells, ranging from 500 to 2000 nm, with nearly constant metallic composition. Despite the observed ZnS and SnS phases on the surface and backside of all absorber films, scanning electron microscopy, Raman scattering, and X-ray diffraction show no large variations in material quality for the different thicknesses. The open-circuit voltage (V-oc), short-circuit current and overall power conversion efficiency of the fabricated devices show an initial improvement as the absorber thickness increases but saturate when the thickness exceeds 750 nm. External quantum efficiency (EQE) measurements suggest that the current is mainly limited by collection losses. This can result from non-optimal bulk quality of the CZTS absorber (including the presence of secondary phases), which is apparently further reduced for the thinnest devices. The observed saturation of V-oc agrees with the expected influence from strong interface recombination. Finally, an effective collection depth of 750-1000 nm for the minority carriers generated in the absorber can be estimated from EQE, indicating that the proper absorber thickness for our device process is approximately 1000 nm. Performance could be improved for thicker films, if the collection depth can be increased.
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49.
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50.
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