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  • 1.
    Chen, Shangzhi
    et al.
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Kang, Evan S. H.
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Shiran Chaharsoughi, Mina
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Sun, Hengda
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Wang, Chuanfei
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Fahlman, Mats
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Fabiano, Simone
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Jonsson, Magnus
    Linköpings universitet, Tekniska fakulteten. Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik.
    Conductive polymer nanoantennas for dynamicorganic plasmonics2020Ingår i: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 15, artikel-id s41565-019-0583-yArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    Being able to dynamically shape light at the nanoscale is oneof the ultimate goals in nano-optics1. Resonant light–matterinteraction can be achieved using conventional plasmonicsbased on metal nanostructures, but their tunability is highlylimited due to a fixed permittivity2. Materials with switchablestates and methods for dynamic control of light–matterinteraction at the nanoscale are therefore desired. Here weshow that nanodisks of a conductive polymer can supportlocalized surface plasmon resonances in the near-infraredand function as dynamic nano-optical antennas, with their resonancebehaviour tunable by chemical redox reactions. Theseplasmons originate from the mobile polaronic charge carriersof a poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf)polymer network. We demonstrate complete and reversibleswitching of the optical response of the nanoantennasby chemical tuning of their redox state, which modulatesthe material permittivity between plasmonic and dielectricregimes via non-volatile changes in the mobile chargecarrier density. Further research may study different conductivepolymers and nanostructures and explore their usein various applications, such as dynamic meta-optics andreflective displays.

  • 2.
    Zhang, Hengfang
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Paskov, Plamen
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kordina, Olle
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. SweGaN AB, Tekn Ringen 8D, S-58330 Linkoping, Sweden.
    Chen, Jr-Tai
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. SweGaN AB, Tekn Ringen 8D, S-58330 Linkoping, Sweden.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality2020Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 580, artikel-id 411819Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (000 (1) over bar), C-face SiC (000 (1) over bar) off-cut towards the [11 (2) over bar0] by 4 degrees, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

  • 3.
    Delgado Carrascon, Rosalia
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Tran, Dat Quoc
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Sukkaew, Pitsiri
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Mock, Alyssa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Naval Res Lab, DC 20375 USA.
    Ciechonski, Rafal
    Hexagem AB, Sweden.
    Ohlsson, Jonas
    Hexagem AB, Sweden; Lund Univ, Sweden.
    Zhu, Yadan
    Lund Univ, Sweden.
    Hultin, Olof
    Lund Univ, Sweden.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Paskov, Plamen
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Samuelson, Lars
    Lund Univ, Sweden.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Optimization of GaN Nanowires Reformation Process by Metalorganic Chemical Vapor Deposition for Device-Quality GaN Templates2020Ingår i: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, artikel-id 1900581Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Herein, the potential of reformed GaN nanowires (NWs) fabricated by metalorganic chemical vapor deposition (MOCVD) for device-quality low-defect density templates and low-cost alternative to bulk GaN substrates is demonstrated. The effects of epilayer thickness and NW reformation conditions on the crystalline quality and thermal conductivity of the subsequent GaN epilayers are investigated. Smooth surfaces with atomically step-like morphologies with no spirals are achieved for GaN epilayers on the reformed NW templates, indicating step-flow growth mode. It is further found that annealing of the NWs at a temperature of 1030 degrees C in the presence of NH3 and H-2, followed by a coalescence done at the same temperature under planar growth conditions, leads to the most efficient screw dislocation density reduction by nearly an order of magnitude. At these optimized conditions, the growth takes place in a layer-by-layer fashion, producing a smooth surface with a root mean square (RMS) roughness of 0.12 nm. The highest thermal conductivity of k = 206 W m(-1) K-1, approaching the respective value of bulk GaN, is obtained for the optimized 2 mu m-thick GaN layer. The thermal conductivity results are further discussed in terms of the phonon-dislocation and the phonon-boundary scattering.

  • 4.
    Tran, Dat
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Blumenschein, Nicholas
    North Carolina State Univ, NC 27695 USA.
    Mock, Alyssa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Naval Res Lab, DC 20375 USA.
    Sukkaew, Pitsiri
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Zhang, Hengfang
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Muth, John F.
    North Carolina State Univ, NC 27695 USA.
    Paskova, Tania
    North Carolina State Univ, NC 27695 USA.
    Paskov, Plamen
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. North Carolina State Univ, NC 27695 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Thermal conductivity of ultra-wide bandgap thin layers - High Al-content AlGaN and beta-Ga2O32020Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 579, artikel-id 411810Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Transient thermoreflectance (TTR) technique is employed to study the thermal conductivity of beta-Ga2O3 and high Al-content AlxGa1-xN semiconductors, which are very promising materials for high-power device applications. The experimental data are analyzed with the Callaways model taking into account all relevant phonon scattering processes. Our results show that out-of-plane thermal conductivity of high Al-content AlxGa1-xN and (-201) beta-Ga2O3 is of the same order of magnitude and approximately one order lower than that of GaN or AlN. The low thermal conductivity is attributed to the dominant phonon-alloy scattering in AlxGa1-xN and to the strong Umklapp phonon-phonon scattering in beta-Ga2O3. It is also found that the phonon-boundary scattering is essential in thin beta-Ga2O3 and AlxGa1-xN layers even at high temperatures and the thermal conductivity strongly deviates from the common 1/T temperature dependence.

  • 5.
    Persson, Ingemar
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Halim, Joseph
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Lind, Hans
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Hansen, Thomas W.
    Tech Univ Denmark DTU, Denmark.
    Wagner, Jakob B.
    Tech Univ Denmark DTU, Denmark.
    Näslund, Lars-Åke
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Palisaitis, Justinas
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Rosén, Johanna
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    2D Transition Metal Carbides (MXenes) for Carbon Capture2019Ingår i: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 31, nr 2, artikel-id 1805472Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Global warming caused by burning of fossil fuels is indisputably one of mankinds greatest challenges in the 21st century. To reduce the ever-increasing CO2 emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO2. However, challenges remain because of limited CO2/N-2 selectivity and long-term stability. The effective adsorption of CO2 gas (approximate to 12 mol kg(-1)) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N-2 gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO2/N-2 selectivity, together with a chemical and thermal stability, identifies the archetype Ti3C2 MXene as a new material for carbon capture (CC) applications.

  • 6.
    Hilfiker, Matthew
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Mock, Alyssa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Knight, Sean
    Univ Nebraska Lincoln, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska Lincoln, NE 68588 USA.
    Mauze, Akhil
    Univ Calif Santa Barbara, CA 93106 USA.
    Zhang, Yuewei
    Univ Calif Santa Barbara, CA 93106 USA.
    Speck, James
    Univ Calif Santa Barbara, CA 93106 USA.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska Lincoln, NE 68588 USA; Leibniz Inst Polymerforsch eV, Germany.
    Dielectric function tensor (1.5 eV to 9.0 eV), anisotropy, and band to band transitions of monoclinic -(AlxGa1-x)(2)O-3 (x 0.21) films2019Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, nr 23, artikel-id 231901Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A set of monoclinic -(AlxGa1-x)(2)O-3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented -Ga2O3 substrates for compositions x0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5eV-9.0eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for -Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions.

  • 7.
    Halim, Joseph
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Persson, Ingemar
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Moon, Eun Ju
    SUNY Buffalo, NY 14260 USA.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Eklund, Per
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Rosén, Johanna
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Barsoum, Michel W.
    Drexel Univ, PA 19104 USA.
    Electronic and optical characterization of 2D Ti2C and Nb2C (MXene) thin films2019Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, nr 16, artikel-id 165301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) are a new class of 2D materials, with extensive opportunities for property tailoring due to the numerous possibilities for varying chemistries and surface terminations. Here, Ti2AlC and Nb2AlC MAX phase epitaxial thin films were deposited on sapphire substrates by physical vapor deposition. The films were then etched in LiF/HCl solutions, yielding Li-intercalated, 2D Ti2CTz and Nb2CTz films, whose terminations, transport and optical properties were characterized. The former exhibits metallic conductivity, with weak localization below 50 K. In contrast, the Nb-based film exhibits an increase in resistivity with decreasing temperature from RT down to 40K consistent with variable range hopping transport. The optical properties of both films were determined from spectroscopic ellipsometry in the 0.75 to 3.50 eV range. The results for Ti2Clz films confirm the metallic behavior. In contrast, no evidence of metallic behavior is observed for the Nb2CT(z) film. The present work therefore demonstrates that one fruitful approach to alter the electronic and optical properties of MXenes is to change the nature of the transition metal.

  • 8.
    Mock, Alyssa
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska, NE 68588 USA.
    Korlacki, R.
    Univ Nebraska, NE 68588 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Stokey, M.
    Univ Nebraska, NE 68588 USA.
    Fritz, A.
    Univ Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Lattice dynamics of orthorhombic NdGaO32019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, nr 18, artikel-id 184302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A complete set of infrared-active and Raman-active lattice modes is obtained from density functional theory calculations for single-crystalline centrosymmetric orthorhombic neodymium gallate. The results for infrared-active modes are compared with an analysis of the anisotropic long-wavelength properties using generalized spectroscopic ellipsometry. The frequency-dependent dielectric function tensor and dielectric loss function tensor of orthorhombic neodymium gallium oxide are reported in the spectral range of 80-1200 cm(-1). A combined eigendielectric displacement vector summation and dielectric displacement loss vector summation approach augmented by considerations of lattice anharmonicity is utilized to describe the experimentally determined tensor elements. All infrared-active transverse and longitudinal optical mode pairs obtained from density functional theory calculations are identified by our generalized spectroscopic ellipsometry investigation. The results for Raman-active modes are compared to previously published experimental observations. Static and high-frequency dielectric constants from theory as well as experiment are presented and discussed in comparison with values reported previously in the literature.

  • 9.
    Schubert, Mathias
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA; Leibniz Institute for Polymer Research, Dresden 01069, Germany; .
    Mock, Alyssa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Galazka, Zbigniew
    Leibniz Inst Kristallzuchtung, Germany.
    Wagner, Guenther
    Leibniz Inst Kristallzuchtung, Germany.
    Wheeler, Virginia
    US Naval Res Lab, DC 20375 USA.
    Tadjer, Marko
    US Naval Res Lab, DC 20375 USA.
    Goto, Ken
    Novel Crystal Technol Inc, Japan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Longitudinal phonon plasmon mode coupling in β-Ga2O32019Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, nr 10, artikel-id 102102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this letter, we investigate a set of n-type single crystals of monoclinic symmetry beta-Ga2O3 with different free electron concentration values by generalized far infrared and infrared spectroscopic ellipsometry. In excellent agreement with our previous model prediction, we find here by experiment that longitudinal-phonon-plasmon coupled modes are polarized either within the monoclinic plane or perpendicular to the monoclinic plane. As predicted, all modes change the amplitude and frequency with the free electron concentration. The most important observation is that all longitudinal-phonon-plasmon coupled modes polarized within the monoclinic plane continuously change their direction as a function of free electron concentration.

  • 10.
    Chen, Shangzhi
    et al.
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Brooke, Robert
    RISE Acreo, Sweden.
    Petsagkourakis, Ioannis
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Crispin, Xavier
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch Dresden eV, Germany.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Jonsson, Magnus
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Laboratoriet för organisk elektronik. Linköpings universitet, Tekniska fakulteten.
    On the anomalous optical conductivity dispersion of electrically conducting polymers: ultra-wide spectral range ellipsometry combined with a Drude-Lorentz model2019Ingår i: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 7, nr 15, s. 4350-4362Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electrically conducting polymers (ECPs) are becoming increasingly important in areas such as optoelectronics, biomedical devices, and energy systems. Still, their detailed charge transport properties produce an anomalous optical conductivity dispersion that is not yet fully understood in terms of physical model equations for the broad range optical response. Several modifications to the classical Drude model have been proposed to account for a strong non-Drude behavior from terahertz (THz) to infrared (IR) ranges, typically by implementing negative amplitude oscillator functions to the model dielectric function that effectively reduce the conductivity in those ranges. Here we present an alternative description that modifies the Drude model via addition of positive-amplitude Lorentz oscillator functions. We evaluate this so-called Drude-Lorentz (DL) model based on the first ultra-wide spectral range ellipsometry study of ECPs, spanning over four orders of magnitude: from 0.41 meV in the THz range to 5.90 eV in the ultraviolet range, using thin films of poly(3,4-ethylenedioxythiophene): tosylate (PEDOT: Tos) as a model system. The model could accurately fit the experimental data in the whole ultrawide spectral range and provide the complex anisotropic optical conductivity of the material. Examining the resonance frequencies and widths of the Lorentz oscillators reveals that both spectrally narrow vibrational resonances and broader resonances due to localization processes contribute significantly to the deviation from the Drude optical conductivity dispersion. As verified by independent electrical measurements, the DL model accurately determines the electrical properties of the thin film, including DC conductivity, charge density, and (anisotropic) mobility. The ellipsometric method combined with the DL model may thereby become an effective and reliable tool in determining both optical and electrical properties of ECPs, indicating its future potential as a contact-free alternative to traditional electrical characterization.

  • 11.
    Schubert, Mathias
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Mock, Alyssa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Phonon order and reststrahlen bands of polar vibrations in crystals with monoclinic symmetry2019Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, nr 4, artikel-id 041201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this Rapid Communication, we present the order of the phonon modes and the appearance of the reststrahlen bands for monoclinic symmetry materials with polar lattice vibrations. Phonon modes occur in associated pairs of transverse and longitudinal optical modes, and pairs either belong to inner or outer phonon modes. Inner modes are nested within outer modes. Outer modes cause polarization-dependent reststrahlen bands. Inner modes cause polarization-independent reststrahlen bands. The directional limiting frequencies within the Born-Huang approach are bound to within outer mode frequency regions not occupied by inner mode pairs. Hence, an unusual phonon mode order can occur where both lower-frequency as well as upper-frequency limits for the directional modes can be both transverse and/or longitudinal modes. We exemplify our findings using experimental data for the recently unraveled case of monoclinic symmetry beta-Ga2O3 [Phys. Rev. B 93, 125209 (2016)] and demonstrate excellent agreement with results from density functional theory calculations.

  • 12.
    Eriksson, Martin O.
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Khromov, Sergey
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Paskov, Plamen
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Wang, X.
    Peking Univ, Peoples R China.
    Yoshikawa, A.
    Chiba Univ, Japan.
    Holtz, Per-Olof
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Recombination processes in Mg doped wurtzite InN films with p- and n-type conductivity2019Ingår i: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 9, nr 1, artikel-id 015114Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Obtaining high quality, wurtzite InN films with p-type conductivity is a challenge, and there is limited information about the photoluminescence (PL) characteristics of such films. In this study, we present a comprehensive PL study and discuss in detail the recombination processes in Mg-doped InN films with varying Mg concentrations. We find that at low Mg-doping of 1x10(18) cm(-3), which yields p-type conductivity, the PL in InN is spatially inhomogeneous. The latter is suggested to be associated with the presence of n-type pockets, displaying photoluminescence at 0.73 eV involving electrons at the Fermi edge above the conduction band edge. Increasing the Mg concentration to 2.9x10(19) cm(-3) in p-type InN yields strong and spatially uniform photoluminescence at 0.62 eV and 0.68 eV visible all the way to room temperature, indicating homogeneous p-type conductivity. An acceptor binding energy of 64 meV is determined for the Mg acceptor. Further increase of the Mg concentration to 1.8x10(20) cm(-3) leads to switching conductivity back to n-type. The PL spectra in this highly doped sample reveal only the emission related to the Mg acceptor (at 0.61 eV). In the low-energy tail of the emission, the multiple peaks observed at 0.54 - 0.58 eV are suggested to originate from recombination of carriers localized at stacking faults. (C) 2019 Author(s).

  • 13.
    Kuhne, Philipp
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Armakavicius, Nerijus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Herzinger, Craig M,
    J. A. Woollam Company, Inc., Lincoln, NE, USA.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications2018Ingår i: IEEE Transactions on Terahertz Science and Technology, ISSN 2156-342X, Vol. 8, nr 3, s. 257-270Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry, coating, and modulation. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm−1 or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

  • 14.
    Armakavicius, Nerijus
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect2018Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, nr 8, artikel-id 082103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m(perpendicular to)* = (0.205 +/- 0.013) m(0) and m(parallel to)* = (0.204 +/- 0.016) m(0) for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 +/- 0.2) x 10(19) cm(-3). Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1-xN epitaxial layers with c-plane growth orientation. Published by AIP Publishing.

  • 15.
    Knight, Sean
    et al.
    University of Nebraska, NE 68588 USA.
    Mock, Alyssa
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Yoshinao
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Goto, Ken
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    National Institute Informat and Commun Technology, Japan.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Electron effective mass in Sn-doped monoclinic single crystal beta-gallium oxide determined by mid-infrared optical Hall effect2018Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, nr 1, artikel-id 012103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The isotropic average conduction band minimum electron effective mass in Sn-doped monoclinic single crystal beta-Ga2O3 is experimentally determined by the mid-infrared optical Hall effect to be (0.2846 +/- 0.013)m(0) combining investigations on (010) and ((2) over bar 01) surface cuts. This result falls within the broad range of values predicted by theoretical calculations for undoped beta-Ga2O3. The result is also comparable to recent density functional calculations using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional, which predict an average effective mass of 0.267m(0). Within our uncertainty limits, we detect no anisotropy for the electron effective mass, which is consistent with most previous theoretical calculations. We discuss upper limits for possible anisotropy of the electron effective mass parameter from our experimental uncertainty limits, and we compare our findings with recent theoretical results. Published by AIP Publishing.

  • 16.
    Persson, Ingemar
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Näslund, Lars-Åke
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Halim, Joseph
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Drexel University, PA 19104 USA.
    Barsoum, Michel
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten. Drexel University, PA 19104 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Palisaitis, Justinas
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Rosén, Johanna
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum2018Ingår i: 2D MATERIALS, ISSN 2053-1583, Vol. 5, nr 1, artikel-id 015002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The two-dimensional (2D) MXene Ti(3)C(2)Tx is functionalized by surface groups (T-x) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 degrees C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

  • 17.
    Kang, Evan S. H.
    et al.
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Fysik och elektroteknik. Linköpings universitet, Tekniska fakulteten.
    Chen, Shangzhi
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Fysik och elektroteknik. Linköpings universitet, Tekniska fakulteten.
    Sardar, Samim
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Fysik och elektroteknik. Linköpings universitet, Tekniska fakulteten.
    Tordera, Daniel
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Fysik och elektroteknik. Linköpings universitet, Tekniska fakulteten.
    Armakavicius, Nerijus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Shegai, Timur
    Department of Physics, Chalmers University of Technology, Sweden.
    Jonsson, Magnus
    Linköpings universitet, Institutionen för teknik och naturvetenskap, Fysik och elektroteknik. Linköpings universitet, Tekniska fakulteten.
    Strong Plasmon–Exciton Coupling with Directional Absorption Features in Optically Thin Hybrid Nanohole Metasurfaces2018Ingår i: ACS Photonics, E-ISSN 2330-4022, s. 4046-4055Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Plasmons and excitons can interact to form new hybridized light–matter states, with a multitude of potential applications including optical logic circuits and single-photon switches. Here, we report the first observation of strong coupling based on optically thin plasmonic nanohole films. The absorptive plasmon resonances of these nanohole films lead to suppressed transmission and Fano-shaped extinction peaks. We prepared silver nanohole films by colloidal lithography, which enables large-scale fabrication of nanoholes distributed in a short-range order. When coated with J-aggregate molecules, both extinction and absorption spectra show clear formation of two separated polariton resonances, with vacuum Rabi splitting on the order of 300 meV determined from anticrossing experiments. In accordance with strong coupling theory, the splitting magnitude increases linearly with the square root of molecular concentration. The extinction peak positions are blue-shifted from the absorption polariton positions, as explained by additional Fano interference between the hybridized states and the metal film. This highlights that absorption measurements are important not only to prove strong coupling but also to correctly determine hybridized polariton positions and splitting magnitudes in hybrid plasmonic nanohole systems. The polariton absorption peaks also show strong dependence on illumination direction, as found related to inherent directionality of the plasmonic nanohole metasurface and differences in light interaction with nonhybridized molecules. Importantly, optical simulations could successfully reproduce the experimental results and all coupling features. Furthermore, simulated spatial distribution of the absorption provides additional evidence of strong coupling in the hybrid nanohole system. The work paves the way toward strong coupling applications based on optically thin nanohole systems, as further promoted by the scalable fabrication.

  • 18.
    Persson, Ingemar
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    El Ghazaly, Ahmed
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Tao, Quanzheng
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Halim, Joseph
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Kota, Sankalp
    Drexel Univ, PA 19104 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Palisaitis, Justinas
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Barsoum, Michel W.
    Drexel Univ, PA 19104 USA.
    Rosén, Johanna
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases2018Ingår i: Small, ISSN 1613-6810, E-ISSN 1613-6829, Vol. 14, nr 17, artikel-id 1703676Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The exploration of 2D solids is one of our times generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo2/3Y1/3)(2)AlC, the resulting MXene can be either: i) (Mo2/3Y1/3)(2)C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3Y1/3)(2)C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolytea volumetric capacitance exceeding 1500 F cm(-3) is obtained, which is 40% higher than that of its Mo1.33C counterpart. With H2SO4, the trend is reversed, with the latter exhibiting the higher capacitance (approximate to 1200 F cm(-3)). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

  • 19.
    Mock, Alyssa
    et al.
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Briley, Chad
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Yoshinao
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Goto, Ken
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    National Institute Informat and Commun Technology, Japan.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic beta-Ga2O32017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 24, artikel-id 245205Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic beta-Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75-9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

  • 20.
    Armakavicius, Nerijus
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Bouhafs, Chamseddine
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kühne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Knight, Sean
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA / Department of Physics and Optical Science, University of North Carolina at Charlotte, USA.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies2017Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, s. 357-360Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmᅵᅵᅵ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmᅵᅵᅵ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.

  • 21.
    Schoche, S.
    et al.
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA.
    Nilsson, Daniel
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kakanakova-Gueorguie, Anelia
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Lorenz, K.
    University of Lisbon, Portugal.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Infrared dielectric functions, phonon modes, and free-charge carrier properties of high-Al-content AlxGa1-xN alloys determined by mid infrared spectroscopic ellipsometry and optical Hall effect2017Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, nr 20, artikel-id 205701Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content AlxGa1-xN layers on 4H-SiC. For high-Al-content AlxGa1-xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E-1 symmetry, while a single mode behavior is found for the longitudinal modes with A1(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content AlxGa1-xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters mu. Published by AIP Publishing.

  • 22.
    Knight, Sean
    et al.
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of N Carolina, NC 28223 USA.
    Bouhafs, Chamseddine
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Armakavicius, Nerijus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Ivanov, Ivan Gueorguiev
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Wimer, Shawn
    University of Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; Leibniz Institute Polymerforsch Dresden eV, Germany.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene2017Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, artikel-id 5151Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

  • 23.
    Bouhafs, Chamseddine
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Zakharov, A. A.
    Lund University, Sweden.
    Ivanov, Ivan Gueorguiev
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Giannazzo, F.
    CNR IMM, Italy.
    Eriksson, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Iakimov, Tihomir
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska Lincoln, NE 68588 USA.
    Roccaforte, F.
    CNR IMM, Italy.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC2017Ingår i: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 116, s. 722-732Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, we report a multi-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 x 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiOx layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation. (C) 2017 Elsevier Ltd. All rights reserved.

  • 24.
    Schubert, M.
    et al.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Korlacki, R.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Knight, S.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Schoeche, S.
    JA Woollam Corp Inc, Japan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Tokyo University of Agriculture and Technology, Japan.
    Gogova, D.
    Bulgarian Academic Science, Bulgaria; Leibniz Institute Crystal Growth, Germany.
    Thieu, Q. -T.
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Togashi, R.
    Tokyo University of Agriculture and Technology, Japan.
    Murakami, H.
    Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Y.
    Tokyo University of Agriculture and Technology, Japan.
    Goto, K.
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Kuramata, A.
    Tamura Corp, Japan.
    Yamakoshi, S.
    Tamura Corp, Japan.
    Higashiwaki, M.
    National Institute Informat and Communicat Technology, Japan.
    Anisotropy, phonon modes, and free charge carrier parameters in monoclinic beta-gallium oxide single crystals2016Ingår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 12, s. 125209-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic beta-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ((2) over bar 01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent beta-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with A(u) and B-u symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in beta-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in beta-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for beta-Ga2O3.

  • 25.
    Bouhafs, Chamseddine
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Zakharov, A. A.
    Lund University, Sweden.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, USA.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Iakimov, Tihomir
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Schubert, Mathias
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. University of Nebraska, USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Decoupling and ordering of multilayer graphene on C-face 3C-SiC(111)2016Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, nr 20, artikel-id 203102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 x 10(11) cm(-2). Low-energy electron diffraction mu-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of amp;lt;= 200 nm.mu-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack. Published by AIP Publishing.

  • 26.
    Schubert, Mathias
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research IPF Dresden, Germany.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Optical Hall effect-model description: tutorial2016Ingår i: Optical Society of America. Journal A: Optics, Image Science, and Vision, ISSN 1084-7529, E-ISSN 1520-8532, Vol. 33, nr 8, s. 1553-1568Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The optical Hall effect is a physical phenomenon that describes the occurrence of magnetic-field-induced dielectric displacement at optical wavelengths, transverse and longitudinal to the incident electric field, and analogous to the static electrical Hall effect. The electrical Hall effect and certain cases of the optical Hall effect observations can be explained by extensions of the classic Drude model for the transport of electrons in metals. The optical Hall effect is most useful for characterization of electrical properties in semiconductors. Among many advantages, while the optical Hall effect dispenses with the need of electrical contacts, electrical material properties such as effective mass and mobility parameters, including their anisotropy as well as carrier type and density, can be determined from the optical Hall effect. Measurement of the optical Hall effect can be performed within the concept of generalized ellipsometry at an oblique angle of incidence. In this paper, we review and discuss physical model equations, which can be used to calculate the optical Hall effect in single- and multiple-layered structures of semiconductor materials. We define the optical Hall effect dielectric function tensor, demonstrate diagonalization approaches, and show requirements for the optical Hall effect tensor from energy conservation. We discuss both continuum and quantum approaches, and we provide a brief description of the generalized ellipsometry concept, the Mueller matrix calculus, and a 4 x 4 matrix algebra to calculate data accessible by experiment. In a follow-up paper, we will discuss strategies and approaches for experimental data acquisition and analysis. (C) 2016 Optical Society of America

  • 27.
    Armakavicius, Nerijus
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Chen, Jr-Tai
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Knight, Sean
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Nilsson, Daniel
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Forsberg, Urban
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect2016Ingår i: Physica Status Solidi C-Current Topics in Solid State Physics, Vol 13 No 5-6, Wiley-VCH Verlagsgesellschaft, 2016, Vol. 13, nr 5-6, s. 369-373Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined tobe in the range of 0.30-0.32m0 and 4.3-5.5×1012 cm–2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm2/Vs to values above 2200 cm2/Vs. A record mobility of 2332±61 cm2/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG.

  • 28.
    Junige, Marcel
    et al.
    Technical University of Dresden, Germany.
    Oddoy, Tim
    Technical University of Dresden, Germany.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Wenger, Christian
    IHP GmbH, Germany.
    Lupina, Grzegorz
    IHP GmbH, Germany.
    Kitzmann, Julia
    IHP GmbH, Germany.
    Albert, Matthias
    Technical University of Dresden, Germany.
    Bartha, Johann W.
    Technical University of Dresden, Germany.
    Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene2015Ingår i: NANOTECHNOLOGY VII, Society of Photo-optical Instrumentation Engineers (SPIE) , 2015, Vol. 9519, nr 951915Konferensbidrag (Refereegranskat)
    Abstract [en]

    Graphene has been considered for a variety of applications including novel nanoelectronic device concepts. However, the deposition of ultra-thin high-k dielectrics on top of graphene has still been challenging due to graphenes lack of dangling bonds. The formation of large islands and leaky films has been observed resulting from a much delayed growth initiation. In order to address this issue, we tested a pre-treatment with NF3 instead of XeF2 on CVD graphene as well as epitaxial graphene monolayers prior to the Atomic Layer Deposition (ALD) of Al2O3. All experiments were conducted in vacuo; i. e. the pristine graphene samples were exposed to NF3 in the same reactor immediately before applying 30 (TMA-H2O) ALD cycles and the samples were transferred between the ALD reactor and a surface analysis unit under high vacuum conditions. The ALD growth initiation was observed by in-situ real-time Spectroscopic Ellipsometry (irtSE) with a sampling rate above 1Hz. The total amount of Al2O3 material deposited by the applied 30 ALD cycles was cross-checked by in-vacuo X-ray Photoelectron Spectroscopy (XPS). The Al2O3 morphology was determined by Atomic Force Microscopy (AFM). The presence of graphene and its defect status was examined by in-vacuo XPS and RAMAN Spectroscopy before and after the coating procedure, respectively.

  • 29.
    Knight, S.
    et al.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Schoeche, S.
    JA Woollam Co Inc, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Kuhne, Philipp
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Carlin, J. -F.
    Ecole Polytech Federal Lausanne, Switzerland.
    Grandjean, N.
    Ecole Polytech Federal Lausanne, Switzerland.
    Herzinger, C. M.
    JA Woollam Co Inc, NE 68588 USA.
    Schubert, M.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies2015Ingår i: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, nr 12, s. 2688-2691Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of a tunable, externally coupled Fabry-Perot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this Letter. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example, by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high-electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry-Perot cavity, are presented and discussed. Tuning the externally coupled Fabry-Perot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence, and magnetic field direction and strength. (C) 2015 Optical Society of America

  • 30.
    Khromov, Sergey
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Wang, X.
    Peking University, Peoples R China.
    Yoshikawa, A.
    Chiba University, Japan.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Rosén, Johanna
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Correlation between switching to n-type conductivity and structural defects in highly Mg-doped InN2015Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, nr 23, artikel-id 232102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of Mg doping on the microstructure of InN epitaxial films in relation to their free-charge carrier properties has been investigated by transmission electron microscopy (TEM) and aberration corrected scanning TEM. We observe a direct correlation between Mg concentration and the formation of stacking faults. The threading dislocation density is found to be independent of Mg concentration. The critical Mg concentration for the on-set of stacking faults formation is determined and found to correlate with the switch from p- to n-type conductivity in InN. Potential mechanisms involving stacking faults and point defect complexes are invoked in order to explain the observed conductivity reversal. Finally, the stacking faults are structurally determined and their role in the reduction of the free electron mobility in highly doped InN: Mg is discussed. (C) 2015 AIP Publishing LLC.

  • 31.
    Ben Sedrine, Nabiha
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan. University of Aveiro, Portugal; University of Aveiro, Portugal.
    Zukauskaite, Agne
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten. Fraunhofer Institute Appl Solid State Phys, Germany.
    Birch, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Jensen, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Schoeche, S.
    University of Nebraska, NE 68588 USA.
    Schubert, M.
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Infrared dielectric functions and optical phonons of wurtzite YxAl1-xN (0 less than= x less than= 0.22)2015Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 48, nr 41, s. 415102-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    YAlN is a new member of the group-III nitride family with potential for applications in next generation piezoelectric and light emitting devices. We report the infrared dielectric functions and optical phonons of wurtzite (0001) YxAl1-xN epitaxial films with 0 less than= x less than= 0.22. The films are grown by magnetron sputtering epitaxy on c-plane Al2O3 and their phonon properties are investigated using infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The infrared-active E-1(TO) and LO, and the Raman active E-2 phonons are found to exhibit one-mode behavior, which is discussed in the framework of the MREI model. The compositional dependencies of the E-1(TO), E-2 and LO phonon frequencies, the high-frequency limit of the dielectric constant, epsilon(infinity), the static dielectric constant, epsilon(0), and the Born effective charge Z(B) are established and discussed.

  • 32.
    Bouhafs, Chamseddine
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Persson, Ingemar
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Tiberj, A.
    University of Montpellier 2, France.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    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
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)2015Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, nr 8, s. 085701-Artikel i tidskrift (Refereegranskat)
    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.

  • 33.
    Xie, Mengyao
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Schubert, M.
    University of Nebraska, NE 68588 USA.
    Lu, Jun
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Persson, Per O A
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Stanishev, Vallery
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Hsiao, Ching-Lien
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Chen, L. C.
    National Taiwan University, Taiwan.
    Schaff, W. J.
    Cornell University, NY 14853 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Assessing structural, free-charge carrier, and phonon properties of mixed-phase epitaxial films: The case of InN2014Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 19, s. 195306-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We develop and discuss appropriate methods based on x-ray diffraction and generalized infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende polymorphs, and quantify their volume fractions in mixed-phase epitaxial films taking InN as an example. The spectral signatures occurring in the azimuth polarization (Muller matrix) maps of mixed-phase epitaxial InN films are discussed and explained in view of polymorphism (zinc-blende versus wurtzite), volume fraction of different polymorphs and their crystallographic orientation, and azimuth angle. A comprehensive study of the structural, phonon and free electron properties of zinc-blende InN films containing inclusions of wurtzite InN is also presented. Thorough analysis on the formation of the zinc-blende and wurtzite phases is given and the structural evolution with film thickness is discussed in detail. The phonon properties of the two phases are determined and discussed together with the determination of the bulk free-charge carrier concentration, and electron accumulation at the mixed-phase InN film surfaces.

  • 34.
    Xie, Mengyao
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Sedrine, Ben
    IST/ITN Instituto Superior Técnico, Universidade Técnica de Lisboa, Portugal.
    Hong, L.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Monemar, Bo
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Schöche, S.
    Department of Electrical Engineering, Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, U.S.A.
    Hofmann, T.
    Department of Electrical Engeneering, University of Nebraska, Lincoln, Nebraska 68588.
    Schubert, M.
    Department of Electrical Engeneering, University of Nebraska, Lincoln, Nebraska 68588.
    Wang, X
    Graduate School of electrical and Electronics Engineering and InN-Project as a CREST program of JST, Chiba University, Japan.
    Yoshikawa, A.
    Graduate School of electrical and Electronics Engineering and InN-Project as a CREST program of JST, Chiba University, Japan.
    Wang, K.
    Research Organization of Science and Engineering, Ritsumeikan University, Japan.
    Araki, T.
    Department of Photonics, Ritsumeikan University, Japan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Nanishi, Y.
    Department of Photonics, Ritsumeikan University, Japan/WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Republic of Korea.
    Effect of Mg doping on the structural and free-charge carrier properties of InN2014Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, nr 16, s. 163504-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the structural and free-charge carrier properties of two sets of InN films grown by molecular beam epitaxy doped with different Mg concentrations from 1x1018 cm-3 to 3.9x1021 cm-3. We determine the effect of Mg doping on surface morphology, lattice parameters, structural characteristics and carrier properties. We show that infrared spectroscopic ellipsometry can be used to evidence successful p-type doping in InN, which is an important issue in InN. High resolution X-ray diffraction, combined with atomic force microscopy measurements reveals a drastic decrease in structural quality of the film for Mg concentrations above 1020 cm-3, accompanied with a significant increase in surface roughness. In addition, a decrease of the c-lattice parameter and an increase of the a-lattice parameter are found with increasing Mg concentration. Different contributions to the strain are discussed and it is suggested that the incorporation of Mg leads to a change of growth mode and generation of tensile growth strain. At high Mg concentrations zinc-blende InN inclusions appear which are suggested to originate from higher densities of stacking faults. Infrared spectroscopic ellipsometry analysis shows a reduced LPP-coupling, manifested as a characteristic dip in the IRSE data, and qualitatively different broadening behavior for Mg concentrations between 1.1x1018 cm−3 and 2.9x1019 cm−3 indicate the existence of a p-type conducting bulk InN layer for these Mg concentrations.

  • 35.
    Schoehe, S.
    et al.
    University of Nebraska, NE 68588 USA.
    Hofmann, T.
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Wang, X.
    Chiba University, Japan.
    Yoshikawa, A.
    Chiba University, Japan.
    Wang, K.
    Ritsumeikan University, Japan.
    Araki, T.
    Ritsumeikan University, Japan.
    Nanishi, Y.
    Ritsumeikan University, Japan.
    Schubert, M.
    University of Nebraska, NE 68588 USA.
    Free-charge carrier parameters of n-type, p-type and compensated InN:Mg determined by infrared spectroscopic ellipsometry2014Ingår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 571, s. 384-388Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Infrared spectroscopic ellipsometry is applied to investigate the free-charge carrier properties of Mg-doped InN films. Two representative sets of In-polar InN grown by molecular beam epitaxy with Mg concentrations ranging from 1.2 x 10(17) cm(-3) to 8 x 10(20) cm(-3) are compared. P-type conductivity is indicated for the Mg concentration range of 1 x 10(18) cm(-3) to 9 x 10(19) cm(-3) from a systematic investigation of the longitudinal optical phonon plasmon broadening and the mobility parameter in dependence of the Mg concentration. A parameterized model that accounts for the phonon-plasmon coupling is applied to determine the free-charge carrier concentration and mobility parameters in the doped bulk InN layer as well as the GaN template and undoped InN buffer layer for each sample. The free-charge carrier properties in the second sample set are consistent with the results determined in a comprehensive analysis of the first sample set reported earlier [Schoche et al., J. Appl. Phys. 113, 013502 (2013)]. In the second set, two samples with Mg concentration of 2.3 x 10(20) cm(-3) are identified as compensated n-type InN with very low electron concentrations which are suitable for further investigation of intrinsic material properties that are typically governed by high electron concentrations even in undoped InN. The compensated n-type InN samples can be clearly distinguished from the p-type conductive material of similar plasma frequencies by strongly reduced phonon plasmon broadening.

  • 36.
    Yakimova, Rositsa
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Iakimov, Tihomir
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Yazdi, Gholamreza
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Bouhafs, Chamseddine
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Eriksson, J.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Zakharov, A.
    MaxLab, Sweden .
    Boosalis, A.
    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 .
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Morphological and electronic properties of epitaxial graphene on SiC2014Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 439, s. 54-59Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the structural and electronic properties of graphene grown on SiC by high-temperature sublimation. We have studied thickness uniformity of graphene grown on 4H-SiC (0 0 0 1), 6H-SiC (0 0 0 1), and 3C-SiC (1 1 1) substrates and investigated in detail graphene surface morphology and electronic properties. Differences in the thickness uniformity of the graphene layers on different SiC polytypes is related mainly to the minimization of the terrace surface energy during the step bunching process. It is also shown that a lower substrate surface roughness results in more uniform step bunching and consequently better quality of the grown graphene. We have compared the three SiC polytypes with a clear conclusion in favor of 3C-SiC. Localized lateral variations in the Fermi energy of graphene are mapped by scanning Kelvin probe microscopy It is found that the overall single-layer graphene coverage depends strongly on the surface terrace width, where a more homogeneous coverage is favored by wider terraces, It is observed that the step distance is a dominating, factor in determining the unintentional doping of graphene from the SiC substrate. Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene on 3C-SiC (1 1 1) is also 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 similar to 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 (1 1 1) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.

  • 37.
    Muhammad, Junaid
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Sandström, Per
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Palisaitis, Justinas
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Hsiao, Ching-Lien
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Persson, Per
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Birch, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Stress Evolution during Growth of GaN (0001)/Al2O3 (0001) by Reactive DC Magnetron Sputter Epitaxy2014Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 47, nr 14, s. 145301-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the real time stress evolution, by in-situ curvature measurements, during magnetron sputter epitaxy of GaN (0001) epilayers at different growth temperatures, directly on Al2O3 (0001) substrates. The epilayers are grown by sputtering from a liquid Ga target in a mixed N2/Ar discharge. For 600 °C, a tensile biaxial stress evolution is observed, while for 700 °C and 800 °C, compressive stress evolutions are observed. Structural characterization by crosssectional transmission electron microscopy, and atomic force microscopy revealed that films grew at 700 °C and 800 °C in a layer-by-layer mode while a growth temperature of 600 °C led to an island growth mode. High resolution Xray diffraction data showed that edge and screw threading dislocation densities decreased with increasing growth temperature with a total density of 5.5×1010 cm-2. The observed stress evolution and growth modes are explained by a high adatom mobility during magnetron sputter epitaxy at 700 - 800 °C. Also other possible reasons for the different stress evolutions are discussed.

  • 38.
    Ben Sedrine, Nebiha
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Zukauskaite, Agne
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Birch, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Bandgap Engineering and Optical Constants of YxAl1-xN Alloys2013Ingår i: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 52, nr 8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study wurtzite Yx Al1-xN (0 andlt;= x andlt;= 0:22) films with (0001) orientation deposited by magnetron sputtering epitaxy on Si(100) substrates and we determine the alloys band gap energies and optical constants. Room temperature spectroscopic ellipsometry (SE) is employed in the energy range from 1 to 6.3 eV, and data modeling based on the standard dielectric function model is used. As a result of the SE data analysis the Yx Al1-xN refractive index and extinction coefficient are determined. The band gap of Yx Al1-xN is found to decrease linearly from 6.2 eV (x=0) down to 4.5 eV (x=0:22). We further observe an increase of the refractive index with increasing Y content; from 1.93 to 2.20 (at 2 eV) for x=0 and 0.22, respectively, reflecting the increase in material density.

  • 39.
    Yakimova, Rositsa
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Yazdi, Gholamreza
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Iakimov, Tihomir
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Eriksson, Jens
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska fakulteten.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    Challenges of Graphene Growth on Silicon Carbide2013Ingår i: ECS Transactions, Vol. 53, nr 1, s. 9-16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    One of the main challenges in the fabrication of device quality graphene is the achievement of large area monolayer graphene that is processing compatible. Here, the impact of the substrate properties on the thickness uniformity and electronic characteristics for epitaxial graphene on SiC produced by high temperature sublimation has been evidenced and discussed. Several powerful techniques have been used to collect data, among them large scale ellipsometry mapping has been demonstrated for the first time. The study is covering all three SiC polytype, e.g. 4H-, 6H- and 3C-SiC in order to reveal eventual peculiarities that have to be controlled during graphene growth. The advantage of the cubic polytype is unambiguously demonstrated.

  • 40.
    Schoeche, S
    et al.
    University of Nebraska, NE 68588 USA .
    Kuehne, P
    University of Nebraska, NE 68588 USA .
    Hofmann, T
    University of Nebraska, NE 68588 USA .
    Schubert, M
    University of Nebraska, NE 68588 USA .
    Nilsson, Daniel
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Kakanakova-Gueorguie, Anelia
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Electron effective mass in Al0.72Ga0.28N alloys determined by mid-infrared optical Hall effect2013Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, nr 21, s. 212107-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effective electron mass parameter in Si-doped Al0.72Ga0.28N is determined to be m* = (0.336 +/- 0.020) m(0) from mid-infrared optical Hall effect measurements. No significant anisotropy of the effective electron mass parameter is found supporting theoretical predictions. Assuming a linear change of the effective electron mass with the Al content in AlGaN alloys and m* = 0.232m(0) for GaN, an average effective electron mass of m* = 0.376m(0) can be extrapolated for AlN. The analysis of mid-infrared spectroscopic ellipsometry measurements further confirms the two phonon mode behavior of the E-1(TO) and one phonon mode behavior of the A(1)(LO) phonon mode in high-Al-content AlGaN alloys as seen in previous Raman scattering studies.

  • 41.
    Schöche, S.
    et al.
    University of Nebraska-Lincoln,USA .
    Hofmann, T.
    University of Nebraska-Lincoln, USA .
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Sedrine, N. Ben
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Wang, X.
    Peking University, Peoples R China .
    Yoshikawa, A.
    Chiba University, Japan .
    Schubert, M.
    University of Nebraska-Lincoln, USA .
    Infrared to vacuum-ultraviolet ellipsometry and optical Hall-effect study of free-charge carrier parameters in Mg-doped InN2013Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, nr 1, s. 013502-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Infrared to vacuum-ultraviolet spectroscopic ellipsometry and far-infrared optical Hall-effect measurements are applied to conclude on successful p-type doping of InN films. A representative set of In-polar Mg-doped InN films with Mg concentrations ranging from 1.2 x 10(16) cm(-3) to 3.9 x 10(21) cm(-3) is investigated. The data are compared and discussed in dependence of the Mg concentration. Differences between n-type and p-type conducting samples are identified and explained. p-type conductivity in the Mg concentration range between 1.1 x 10(18) cm(-3) and 2.9 x 10(19) cm(-3) is indicated by the appearance of a dip structure in the infrared spectral region related to a loss in reflectivity of p-polarized light as a consequence of reduced LO phonon plasmon coupling, by vanishing free-charge carrier induced birefringence in the optical Hall-effect measurements, and by a sudden change in phonon-plasmon broadening behavior despite continuous change in the Mg concentration. By modeling the near-infrared-to-vacuum-ultraviolet ellipsometry data, information about layer thickness, electronic interband transitions, as well as surface roughness is extracted in dependence of the Mg concentration. A parameterized model that accounts for the phonon-plasmon coupling is applied for the infrared spectral range to determine the free-charge carrier concentration and mobility parameters in the doped bulk InN layer as well as the GaN template and undoped InN buffer layer. The optical Hall-effect best-match model parameters are consistent with those obtained from infrared ellipsometry analysis.

  • 42.
    Darakchieva, Vanya
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    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
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Vasiliauskas, Remigijus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)2013Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 102, nr 21, s. 213116-Artikel i tidskrift (Refereegranskat)
    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.

  • 43.
    Kuehne, P
    et al.
    University of Nebraska, NE USA.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Tedesco, J D.
    ABB Inc, VA USA.
    Myers-Ward, R L.
    US Naval Research Lab, DC USA.
    Jr Eddy, C R.
    US Naval Research Lab, DC USA.
    Gaskill, D K.
    US Naval Research Lab, DC USA.
    Herzinger, C M.
    JA Woollam Co Inc, NE USA.
    Woollam, J A.
    JA Woollam Co Inc, NE USA.
    Schubert, M
    University of Nebraska, NE USA.
    Hofmann, T
    University of Nebraska, NE USA.
    Polarization Selection Rules for Inter-Landau-Level Transitions in Epitaxial Graphene Revealed by the Infrared Optical Hall Effect2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, nr 7, s. e077402-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the polarization selection rules of inter-Landau-level transitions using reflection-type optical Hall effect measurements from 600 to 4000  cm-1 on epitaxial graphene grown by thermal decomposition of silicon carbide. We observe symmetric and antisymmetric signatures in our data due to polarization preserving and polarization mixing inter-Landau-level transitions, respectively. From field-dependent measurements, we identify that transitions in coupled graphene monolayers are governed by polarization mixing selection rules, whereas transitions in decoupled graphene monolayers are governed by polarization preserving selection rules. The selection rules may find explanation by different coupling mechanisms of inter-Landau-level transitions with free charge carrier magneto-optic plasma oscillations.

  • 44.
    Furlan, Andrej
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Gueorguiev, Gueorgui Kostov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Czigány, Zsolt
    Research Institute for Technical Physics and Materials Science, P.O. Box 49, Budapest, H-1525, Hungary.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Braun, Slawomir
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Ytors Fysik och Kemi. Linköpings universitet, Tekniska högskolan.
    Correia, Rosario
    I3N and Physics Department, University of Aveiro, 3810-193 Aveiro, Portugal.
    Högberg, Hans
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Structure and properties of phosphorus-carbide thin solid films2013Ingår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 548, nr 2, s. 247-254Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Phosphorus-carbide (CPx) thin films have been deposited by unbalanced reactive magnetron sputtering and investigated by TEM, XPS, SEM, ERDA, Raman scattering spectroscopy, nanoindentation testing, and four-point electrical probe techniques. As-deposited films with x=0.1 are electron amorphous with elements of FL structure and high mechanical resiliency with hardness of 34.4 GPa and elastic recovery of 72%. The electrical resistivity of the films are in the range 0.4-1.7 Ωcm for CP0.027, 1.4-22.9 Ωcm for CP0.1, and lower than the minimal value the four-point probe is able to detect for CPx with x≥0.2.

  • 45.
    Zhou, Mi
    et al.
    University of N Texas.
    Pasquale, Frank L
    University of N Texas.
    Dowben, Peter A
    University of Nebraska Lincoln.
    Boosalis, Alex
    University of Nebraska Lincoln.
    Schubert, Mathias
    University of Nebraska Lincoln.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Yakimova, Rositsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Kong, Lingmei
    University of Nebraska Lincoln.
    Kelber, Jeffry A
    University of N Texas.
    Direct graphene growth on Co3O4(111) by molecular beam epitaxy2012Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, nr 7, s. 072201-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Direct growth of graphene on Co3O4(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp(2) carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp2 carbon film with a lattice constant of 2.5(+/-0.1) angstrom characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of similar to 1800 angstrom, and show an incommensurate interface with the Co3O4(111) substrate, where the latter exhibits a lattice constant of 2.8(+/-0.1) angstrom. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected pi to pi* satellite feature, but with a binding energy for the 3 ML film of 284.9(+/-0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO2 or grown on SiC substrates, but with the pi to pi* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.

  • 46.
    Xie, Mengyao
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Tasnadi, Ferenc
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Abrikosov, Igor
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Hultman, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Elastic constants, composition, and piezolectric polarization in InxAl1-xN: From ab initio calculations to experimental implications for the applicability of Vegards rule2012Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, nr 15, s. 155310-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a theoretical analysis on the applicability of Vegards linear rule in InxAl1-xN alloys in relation to strain related elastic and piezoelectric properties. We derive the elastic stiffness constants and biaxial coefficients, as well as the respective deviations from linearity (Vegards rule) by using ab initio calculations. The stress-strain relationships to extract composition from the lattice parameters are derived in different coordinate systems for InxAl1-xN with an arbitrary surface orientation. The error made in the composition extracted from the lattice parameters if the deviations from linearity are not taken into account is discussed for different surface orientations, compositions and degrees of strain in the InxAl1-xN films. The strain induced piezoelectric polarization is analyzed for InxAl1-xN alloys grown pseudomorphically on GaN. The polarization values are compared with those obtained from our experimental data for the lattice parameters. We establish the importance of the deviation from linearity to correctly determine the piezoelectric polarization and also a smooth, not particular piezoelectric response at GaN lattice matched conditions.

  • 47.
    Catarino, N
    et al.
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU .
    Nogales, E
    Departamento de Física de Materiales, Universidad Complutense - 28040 Madrid, Spain, EU .
    Franco, N
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU and Centro de Física Nuclear da Universidade de Lisboa - Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, EU.
    Darakchieva, Vanya
    Instituto Tecnol´ogico e Nuclear/IST, Sacavem, Portugal, EU; Centro de F´ısica Nuclear da Universidade de Lisboa - Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, EU .
    Miranda, S. M. C.
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU .
    Mendez, B
    Departamento de Física de Materiales, Universidad Complutense - 28040 Madrid, Spain, EU .
    Alves, E
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU and Centro de Física Nuclear da Universidade de Lisboa - Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, EU.
    Marques, J. G.
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU and Centro de Física Nuclear da Universidade de Lisboa - Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, EU.
    Lorenz, K
    Instituto Tecnológico e Nuclear/IST - Estrada Nacional 10, 2686-953 Sacavém, Portugal, EU and Centro de Física Nuclear da Universidade de Lisboa - Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, EU.
    Enhanced dynamic annealing and optical activation of Eu implanted a-plane GaN2012Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, nr 6, s. 68004-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The implantation damage build-up and optical activation of a-plane and c-plane GaN epitaxial films were compared upon 300 keV Eu implantation at room temperature. The implantation defects cause an expansion of the lattice normal to the surface, i.e. along the a-direction in a-plane and along the c-direction in c-plane GaN. The defect profile is bimodal with a pronounced surface damage peak and a second damage peak deeper in the bulk of the samples in both cases. For both surface orientations, the bulk damage saturates for high fluences. Interestingly, the saturation level for a-plane GaN is nearly three times lower than that for c-plane material suggesting very efficient dynamic annealing and strong resistance to radiation. a-plane GaN also shows superior damage recovery during post-implant annealing compared to c-plane GaN. For the lowest fluence, damage in a-plane GaN was fully removed and strong Eu-related red luminescence is observed. Although some residual damage remained after annealing for higher fluences as well as in all c-plane samples, optical activation was achieved in all samples revealing the red emission lines due to the 5D0 7F2transition in the Eu3+ ion. The presented results demonstrate a great promise for the use of ion beam processing for a-plane GaN based electronic devices as well as for the development of radiation tolerant electronics.

  • 48.
    Schöche, Stefan
    et al.
    Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, U.S.A..
    Hofmann, Tino
    Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, U.S.A..
    Sedrine, Nebiha Ben
    Instituto Tecnológico e Nuclear, Sacavém, Portugal.
    Darakchieva, Vanja
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Wang, Xinqiang
    State Key Lab of Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing, China.
    Yoshikawa, Akihiko
    Graduate School of Electrical and Electronics Engineering, Venture Business Laboratory, Chiba University, Chiba, Japan.
    Schubert, Mathias
    Instituto Tecnológico e Nuclear, Sacavém, Portugal.
    Infrared ellipsometry and near-infrared-to-vacuum-ultraviolet ellipsometry study of free-charge carrier properties in In-polar p-type InN2012Ingår i: MRS Proceedings Volume 1396, 2012, s. o07-27Konferensbidrag (Refereegranskat)
    Abstract [en]

    We apply infrared spectroscopic ellipsometry (IRSE) in combination with near-infrared to vacuum-ultraviolet ellipsometry to study the concentration and mobility of holes in a set of Mg-doped In-polar InN samples of different Mg-concentrations. P-type behavior is found in the IRSE spectra for Mg-concentrations between 1x1018 cm-3 and 3x1019 cm-3. The free-charge carrier parameters are determined using a parameterized model that accounts for phonon-plasmon coupling. From the NIR-VUV data information about layer thicknesses, surface roughness, and structural InN layer properties are extracted and related to the IRSE results.

  • 49.
    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
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Yakimova, Rositza
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    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 SiC2012Ingår i: MRS Proceedings Volume 1407, 2012, s. aa20-43Konferensbidrag (Refereegranskat)
    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.

  • 50.
    Hofmann, T.
    et al.
    University of Nebraska, USA .
    Kuehne, P.
    University of Nebraska, USA .
    Schöche, S.
    University of Nebraska, USA .
    Chen, Jr-Tai
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Forsberg, Urban
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Ben Sedrine, N.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Hälsouniversitetet.
    Herzinger, C. M
    JA Woollam Co Inc, USA .
    Woollam, J. A
    JA Woollam Co Inc, USA .
    Schubert, M.
    University of Nebraska, USA .
    Darakchieva, Vanya
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Temperature dependent effective mass in AlGaN/GaN high electron mobility transistor structures2012Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, nr 19Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The temperature-dependence of free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas in a AlGaN/GaN heterostructure deposited on SiC substrate is determined using the THz optical Hall effect in the spectral range from 0.22 to 0.32 THz for temperatures from 1.5 to 300 K. The THz optical Hall-effect measurements are combined with room temperature mid-infrared spectroscopic ellipsometry measurements to determine the layer thickness, phonon mode, and free-charge carrier parameters of the heterostructure constituents. An increase of the electron effective mass from (0.22 +/- 0.01)m(0) at 1.5 K to (0.36 +/- 0.03)m(0) at 300 K is observed, which is indicative for a reduction in spatial confinement of the two-dimensional electron gas at room temperature. The temperature-dependence of the mobility and the sheet density is in good agreement with electrical measurements reported in the literature.

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