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  • 51.
    Darakchieva, Vanya
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Paskova, T.
    Institute of Solid State Physics, University of Bremen, 28359 Bremen, Germany.
    Schubert, M.
    Department of Electrical Engineering, University of Nebraska, Lincoln, NE 68588, United States.
    Paskov, Plamen
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Arwin, Hans
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Applied Optics .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Hommel, D.
    Institute of Solid State Physics, University of Bremen, 28359 Bremen, Germany.
    Heuken, M.
    Aixtron AG, D-52072 Aachen, Germany.
    Off, J.
    Institute of Physics 4, University of Stuttgart, 70569 Stuttgart, Germany.
    Haskell, B.A.
    Materials Department, University of California, Santa Barbara, CA 93106, United States.
    Fini, P.T.
    Materials Department, University of California, Santa Barbara, CA 93106, United States.
    Speck, J.S.
    Materials Department, University of California, Santa Barbara, CA 93106, United States.
    Nakamura, S.
    Materials Department, University of California, Santa Barbara, CA 93106, United States.
    Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers2007In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 300, no 1, p. 233-238Article in journal (Refereed)
    Abstract [en]

    We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials aA1 (TO), bA1 (TO), cE1 (TO) and cE1 (LO) are determined. © 2006 Elsevier B.V. All rights reserved.

  • 52.
    Darakchieva, Vanya
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Paskov, Plamen
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Arwin, Hans
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Applied Optics .
    Schubert, M
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Figge, S
    Hommel, D
    Haskell, BA
    Fini, PT
    Nakamura, S
    Assessment of phonon mode characteristics via infrared spectroscopic ellipsometry on a-plane GaN2006In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 243, no 7, p. 1594-1598Article in journal (Refereed)
    Abstract [en]

    Generalized infrared spectroscopic ellipsometry was applied to study the vibrational properties of anisotropically strained a-plane GaN films with different thicknesses. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and strain-free frequency of the GaN A,(TO) mode. These results are compared with previous theoretical and experimental findings and discussed.

  • 53.
    Darakchieva, Vanya
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Paskov, Plamen
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Ashkenov, N.
    Fak. fur Phy. and Geowissenschaften, Universität Leipzig, 04103 Leipzig, Germany.
    Schubert, M.
    Fak. fur Phy. and Geowissenschaften, Universität Leipzig, 04103 Leipzig, Germany.
    Residual strain in HVPE GaN free-standing and re-grown homoepitaxial layers2003In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 195, no 3, p. 516-522Article in journal (Refereed)
    Abstract [en]

    The lattice parameters of as-grown hydride vapor phase epitaxy GaN layers on sapphire, free-standing layers after the substrate lift-off, and homoepitaxial layers grown on the free-standing layers are measured. The in-plane and out-of-plane strains are calculated. It is found that the substrate removal leads to strain relaxation in the crack-free GaN free-standing layers to a certain extent. A small increase of the strain in the GaN homoepitaxial layers compared to the free-standing layers is observed. Cathodoluminescence (CL) spectroscopy and imaging, photoluminescence (PL) and Raman measurements are used as complementary tools in the residual strain analysis.

  • 54.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Paskova, Tanja
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Ashkenov, N.
    Schubert, M.
    Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates2005In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 97, no 1, p. 013517-Article in journal (Refereed)
    Abstract [en]

    We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase epitaxy (MOVPE) templates and on epitaxial lateral overgrown MOVPE templates are characterized by Raman scattering, high-resolution x-ray diffraction, and reciprocal space mapping. The strain relaxation in the films versus their thickness was found to proceed similarly in the GaN samples grown using the two types of templates but the strain saturates at different nonzero levels. The lattice parameters of relatively thin HVPE-GaN free-standing quasisubstrates indicate that no total strain relaxation is achieved after the sapphire removal. The lattice parameters of the thick quasisubstrates grown on different templates are not affected by the separation process and are found to have values very close to the reference strain-free lattice parameters of GaN powder. © 2005 American Institute of Physics.

  • 55.
    Darakchieva, Vanya
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Schubert, M.
    Optical phonons in a-plane GaN under anisotropic strain2008In: Group-III nitrides with nonpolar surfaces: growth, properties and devices / [ed] Tanya Paskova, Wiley , 2008, 1, p. 219-253Chapter in book (Other academic)
    Abstract [en]

    This is the first monograph to discuss in detail the current stage of development of nonpolar nitrides, with specific emphasis on the three main topics of crystal growth, properties and device studies. World–class researchers summarize their own recent achievements in their respective fields of expertise, covering both nonpolar and semipolar nitride materials. The bulk of the discussion in each chapter is related to the physical properties of the material obtained by the respective technique, in particular, defect density and properties of the defects in nonpolar nitrides. In addiiton, the optical and vibrational properties are also addressed in several chapters, as is progress in heterostructures, quantum wells and dots based on the AlGaN/GaN and the InGaN/GaN systems. Finally, an outlook of the application areas of the differently grown materials is presented in most chapters, together with the capabilities and limitations of the respective growth approaches used.

  • 56.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Schubert, M.
    Arwin, Hans
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Applied Optics .
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Hommel, D.
    Off, J.
    Scholz, F.
    Heuken, M.
    Haskell, B.A.
    Fini, P.T.
    Speck, S.J.
    Nakamura, S.
    Anisotropic strain and phonon deformation potentials in GaN2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 19, p. 195217-Article in journal (Refereed)
    Abstract [en]

    We report optical phonon frequency studies in anisotropically strained c -plane- and a -plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1 (TO), E1 (LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c E1 (TO), c E1 (LO), and c E2 are determined. A distinct correlation between anisotropic strain and the A1 (TO) and E1 (LO) frequencies of a -plane GaN films reveals the a A1 (TO), b A1 (TO), a E1 (LO), and b E1 (LO) phonon deformation potentials. The a A1 (TO) and b A1 (TO) are found to be in very good agreement with previous results from Raman experiments. Our a A1 (TO) and a E1 (LO) phonon deformation potentials agree well with recently reported theoretical estimations, while b A1 (TO) and b E1 (LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented. © 2007 The American Physical Society.

  • 57.
    Darakchieva, Vanya
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Schubert, M.
    Fak. für Phys./Geowiss., Iniversität Leipzig, 04103 Leipzig, Germany.
    Birch, Jens
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics.
    Kasic, A.
    Tungasmita, Sukkaneste
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Paskova, Tanja
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Generalized infrared ellipsometry study of thin epitaxial AlN layers with complex strain behavior2003Conference paper (Refereed)
    Abstract [en]

    The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, and a sub-layer model of the degree of strain and related defects for all films is suggested. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sub-layer model has been successfully applied to the analysis of the ellipsometry data trough model calculations of the infrared dielectric function. © 2003 Elsevier B.V. All rights reserved.

  • 58.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Schubert, M
    University of Nebraska.
    Hofmann, T
    University of Nebraska.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Hsiao, Ching-Lien
    National Taiwan University.
    Liu, Ting-Wei
    National Taiwan University.
    Chen, Li-Chyong
    National Taiwan University.
    Schaff, W J
    Cornell University.
    Takagi, Y
    Ritsumeikan University.
    Nanishi, Y
    Ritsumeikan University.
    Electron accumulation at nonpolar and semipolar surfaces of wurtzite InN from generalized infrared ellipsometry2009In: APPLIED PHYSICS LETTERS, ISSN 0003-6951, Vol. 95, no 20, p. 202103-Article in journal (Refereed)
    Abstract [en]

    The free electron properties of nonpolar (1120)-oriented and semipolar (1011)-oriented wurtzite InN films are studied by generalized infrared ellipsometry (GIRSE). We demonstrate the sensitivity of GIRSE to the surface charge accumulation layer and find a distinct surface electron accumulation to occur at all surfaces. The obtained surface electron sheet densities are found to vary from 0.9x10(13) to 2.3x10(14) cm(-2) depending on the surface orientation and bulk electron concentration. The upper limits of the surface electron mobility parameters of 417-644 cm(2)/V s are determined and discussed in the light of electron confinement at the surface.

  • 59.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Valcheva, E.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Schubert, M.
    Paskova, Tanja
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Amano, H.
    Akasaki, I.
    Phonon mode behavior in strained wurtzite AlN/GaN superlattices2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 11, p. 115329-Article in journal (Refereed)
    Abstract [en]

    We have studied phonons in AlN/GaN superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified E1(TO), A 1(LO) and E2 localized, and E1(LO) and A 1(TO) delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the A1(TO) and E1(LO) phonons, as well as the free-carrier effect on the E1(LO) phonon are also discussed. ©2005 The American Physical Society.

  • 60.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Xie, Mengyao
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Franco, N
    Institute Tecnol and Nucl, Sacavem, Portugal .
    Giuliani, F
    University London Imperial College of Science Technology and Medicine.
    Nunes, B
    Institute Super Tecn, Lisbon, Portugal .
    Alves, E
    Institute Tecnol and Nucl, Sacavem, Portugal .
    Hsiao, C L
    National Taiwan University.
    Chen, L C
    National Taiwan University.
    Yamaguchi, T
    Ritsumeikan University.
    Takagi, Y
    Ritsumeikan University.
    Kawashima, K
    Ritsumeikan University.
    Nanishi, Y
    Ritsumeikan University.
    Structural anisotropy of nonpolar and semipolar InN epitaxial layers2010In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979, Vol. 108, no 7, p. 073529-Article in journal (Refereed)
    Abstract [en]

    We present a detailed study of the structural characteristics of molecular beam epitaxy grown nonpolar InN films with a- and m-plane surface orientations on r-plane sapphire and (100) gamma-LiAlO2, respectively, and semipolar (10 (1) over bar1) InN grown on r-plane sapphire. The on-axis rocking curve (RC) widths were found to exhibit anisotropic dependence on the azimuth angle with minima at InN [0001] for the a-plane films, and maxima at InN [0001] for the m-plane and semipolar films. The different contributions to the RC broadening are analyzed and discussed. The finite size of the crystallites and extended defects are suggested to be the dominant factors determining the RC anisotropy in a-plane InN, while surface roughness and curvature could not play a major role. Furthermore, strategy to reduce the anisotropy and magnitude of the tilt and minimize defect densities in a-plane InN films is suggested. In contrast to the nonpolar films, the semipolar InN was found to contain two domains nucleating on zinc-blende InN(111) A and InN(111) B faces. These two wurtzite domains develop with different growth rates, which was suggested to be a consequence of their different polarity. Both, a- and m-plane InN films have basal stacking fault densities similar or even lower compared to nonpolar InN grown on free-standing GaN substrates, indicating good prospects of heteroepitaxy on foreign substrates for the growth of InN-based devices.

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  • 61.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Xie, Mengyao
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Rogalla, D
    Ruhr-Universität Bochum, Bochum, Germany.
    Becker, H-W
    Ruhr-Universität Bochum, Bochum, Germany.
    Lorenz, K
    Instituto Tecnológico e Nuclear, Sacavém, Portugal.
    Alves, E
    Instituto Tecnológico e Nuclear, Sacavém, Portugal.
    Ruffenach, S
    Groupe d'Etude des Semiconducteurs, Université Montpellier II, Montpellier, France.
    Moret, M
    Groupe d'Etude des Semiconducteurs, Université Montpellier II, Montpellier, France.
    Briot, O
    Groupe d'Etude des Semiconducteurs, Université Montpellier II, Montpellier, France.
    Free electron properties and hydrogen in InN grown by MOVPE2011In: PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, ISSN 1862-6300, Vol. 208, no 5, p. 1179-1182Article in journal (Refereed)
    Abstract [en]

    In this work we present a comprehensive study on the hydrogen impurities, free electron, and structural properties of MOVPE InN films with state-of-the-art quality. We find a correlation between the decrease of free electron concentration and the reduction of bulk hydrogen in the films upon thermal annealing, while no changes in the dislocation densities and strain are observed. Our results suggest that hydrogen is a major source for the unintentional n-type doping in MOVPE InN.

  • 62.
    Darakchieva, Vanya
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Xie, Mengyao
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kamimura, J
    Sophia University.
    Kishino, K
    Japan Science & Technology Agency.
    Lattice parameters, deviations from Vegards rule, and E-2 phonons in InAlN2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, no 26, p. 261908-Article in journal (Refereed)
    Abstract [en]

    The lattice parameters of InxAl1-xN in the whole compositional range are studied using first-principle calculations. Deviations from Vegards rule are obtained via the bowing parameters, delta(a)=0.0412 +/- 0.0039 A and delta(c)=-0.060 +/- 0.010 A, which largely differ from previously reported values. Implications of the observed deviations from Vegards rule on the In content extracted from x-ray diffraction are discussed. We also combine these results with x-ray diffraction and Raman scattering studies on InxAl1-xN nanocolumns with 0.627 <= x <= 1 and determine the E-2 phonon frequencies versus In composition in the scarcely studied In-rich compositional range.

  • 63.
    Del Castillo, Ragnar Ferrand-Drake
    et al.
    Chalmers Univ Technol, Sweden.
    Chen, Ding-Yuan
    SweGaN AB, S-58278 Linkoping, Sweden.
    Chen, Jr-Tai
    SweGaN AB, S-58278 Linkoping, Sweden.
    Thorsell, Mattias
    Chalmers Univ Technol, Sweden; SAAB AB, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Rorsman, Niklas
    Chalmers Univ Technol, Sweden.
    Characterization of Trapping Effects Related to Carbon Doping Level in AlGaN Back-Barriers for AlGaN/GaN HEMTs2024In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646Article in journal (Refereed)
    Abstract [en]

    The impact of different carbon concentrations in the Al 0.06 Ga 0.94 N graded back-barrier and GaN buffer of high electron mobility transistors (HEMTs) is investigated. Four epi-wafers with different carbon concentrations, ranging from 1 x 10(17) to 5 x 10(17) cm( -3) , were grown by metal organic chemical vapor deposition (MOCVD). HEMTs with 100 and 200 nm gate lengths were fabricated and characterized with dc, Pulsed-IV, drain current transient spectroscopy (DCTS), and large-signal measurements at 30 GHz. It is shown that the back-barrier effectively prevents buffer-related electron trapping. The highest C-doping provides the best 2DEG confinement, while lower carbon doping levels are beneficial for a high output power and efficiency. A C-doping of 1 x 10(17)cm( -3) offers the highest output power at maximum power added efficiency (PAE) (1.8 W/mm), whereas 3 x 10(17) cm( -3) doping provides the highest PAE ( &gt; 40%). The C-profiles acquired by using secondary ion mass spectroscopy (SIMS), in combination with DCTS, is used to explain the electron trapping effects. Traps associated with the C-doping in the back-barrier are identified and the bias ranges for the trap activation are discussed. The study shows the importance of considering the C-doping level in the back-barrier of microwave GaN HEMTs for power amplification and generation.

  • 64.
    Delgado Carrascon, Rosalia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Nawaz, Muhammad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Hitachi Energy, Sweden.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies2022In: Crystal Growth & Design, ISSN 1528-7483, E-ISSN 1528-7505, Vol. 22, no 12, p. 7021-7030Article in journal (Refereed)
    Abstract [en]

    Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 x 1015 cm-3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

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  • 65.
    Delgado Carrascon, Rosalia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat Quoc
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Sukkaew, Pitsiri
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, Faculty of Science & Engineering.
    Mock, Alyssa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. 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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Samuelson, Lars
    Lund Univ, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Optimization of GaN Nanowires Reformation Process by Metalorganic Chemical Vapor Deposition for Device-Quality GaN Templates2020In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 257, no 4, article id 1900581Article in journal (Refereed)
    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.

  • 66.
    Eriksson, Martin O.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Khromov, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Wang, X.
    Peking Univ, Peoples R China.
    Yoshikawa, A.
    Chiba Univ, Japan.
    Holtz, Per-Olof
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Recombination processes in Mg doped wurtzite InN films with p- and n-type conductivity2019In: AIP Advances, E-ISSN 2158-3226, Vol. 9, no 1, article id 015114Article in journal (Refereed)
    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).

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  • 67.
    Furlan, Andrej
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Gueorguiev, Gueorgui Kostov
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Czigány, Zsolt
    Research Institute for Technical Physics and Materials Science, P.O. Box 49, Budapest, H-1525, Hungary.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Braun, Slawomir
    Linköping University, Department of Physics, Chemistry and Biology, Surface Physics and Chemistry. Linköping University, The Institute of Technology.
    Correia, Rosario
    I3N and Physics Department, University of Aveiro, 3810-193 Aveiro, Portugal.
    Högberg, Hans
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Structure and properties of phosphorus-carbide thin solid films2013In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 548, no 2, p. 247-254Article in journal (Refereed)
    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.

  • 68.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ghezellou, Misagh
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat Q.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    ul-Hassan, Jawad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Kordina, Olof
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hilfiker, Matthew
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Epitaxial growth of β-Ga2O3 by hot-wall MOCVD2022In: AIP Advances, E-ISSN 2158-3226, Vol. 12, no 5, article id 055022Article in journal (Refereed)
    Abstract [en]

    The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.

  • 69.
    Gogova-Petrova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Jokubavicius, Valdas
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Vines, L.
    Univ Oslo, Norway.
    Schubert, M.
    Lund Univ, Sweden; Univ Nebraska, NE 68588 USA.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    High crystalline quality homoepitaxial Si-doped β-Ga2O3(010) layers with reduced structural anisotropy grown by hot-wall MOCVD2024In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 42, no 2, article id 022708Article in journal (Refereed)
    Abstract [en]

    A new growth approach, based on the hot-wall metalorganic chemical vapor deposition concept, is developed for high-quality homoepitaxial growth of Si-doped single-crystalline beta-Ga2O3 layers on (010)-oriented native substrates. Substrate annealing in argon atmosphere for 1 min at temperatures below 600 degrees C is proposed for the formation of epi-ready surfaces as a cost-effective alternative to the traditionally employed annealing process in oxygen-containing atmosphere with a time duration of 1 h at about 1000 degrees C. It is shown that the on-axis rocking curve widths exhibit anisotropic dependence on the azimuth angle with minima for in-plane direction parallel to the [001] and maximum for the [100] for both substrate and layer. The homoepitaxial layers are demonstrated to have excellent structural properties with a beta-Ga2O3(020) rocking curve full-widths at half-maximum as low as 11 arc sec, which is lower than the corresponding one for the substrates (19 arc sec), even for highly Si-doped (low 1019 cm -3 range) layers. Furthermore, the structural anisotropy in the layer is substantially reduced with respect to the substrate. Very smooth surface morphology of the epilayers with a root mean square roughness value of 0.6 nm over a 5 x 5 mu m(2) area is achieved along with a high electron mobility of 69 cm 2 V -1 s -1 at a free carrier concentration n = 1.9 x 10(19) cm -3. These values compare well with state-of-the-art parameters reported in the literature for beta-Ga2O3(010) homoepitaxial layers with respective Si doping levels. Thermal conductivity of 17.4 Wm(-1)K(-1) is determined along the [010] direction for the homoepitaxial layers at 300 K, which approaches the respective value of bulk crystal (20.6 Wm(-1)K(-1)). This result is explained by a weak boundary effect and a low dislocation density in the homoepitaxial layers.

  • 70.
    Gonschorek, M.
    et al.
    Institute of Quantum Electronics and Photonics, Ecole Polytechnique F´d´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    Carlin, J.-F.
    Institute of Quantum Electronics and Photonics, Ecole Polytechnique F´d´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    Feltin, E.
    Institute of Quantum Electronics and Photonics, Ecole Polytechnique F´d´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    Py, M.A.
    Institute of Quantum Electronics and Photonics, Ecole Polytechnique F´d´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    Grandjean, N.
    Institute of Quantum Electronics and Photonics, Ecole Polytechnique F´d´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
    Darakchieva, Vanya
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Lorenz, M.
    Institut für Experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnstraße 3-5, 04103 Leipzig, Germany.
    Ramm, G.
    Institut für Experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnstraße 3-5, 04103 Leipzig, Germany.
    Two-dimensional electron gas density in Al1-x InMx N/AlN/GaN heterostructures (0.03=x=0.23)2008In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 9, p. 093714-Article in journal (Refereed)
    Abstract [en]

    Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegard's law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ~17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1-x Inx N/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03=x=0.23 for 6 nm thick barriers and 0.07=x=0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1) × 1013 cm-2 and (2.2±0.1) × 1013 cm-2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1) × 1013 cm-2 is obtained for a nearly LM AlInN barrier with ~14.5% indium on GaN as thin as 6 nm. © 2008 American Institute of Physics.

  • 71.
    Gopalan, Prashanth
    et al.
    Univ Utah, UT 84112 USA.
    Knight, Sean Robert
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Chanana, Ashish
    Univ Utah, UT 84112 USA.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Ranga, Praneeth
    Univ Utah, UT 84112 USA.
    Scarpulla, Michael A.
    Univ Utah, UT 84112 USA.
    Krishnamoorthy, Sriram
    Univ Utah, UT 84112 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Galazka, Zbigniew
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Irmscher, Klaus
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Fiedler, Andreas
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Blair, Steve
    Univ Utah, UT 84112 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Sensale-Rodriguez, Berardi
    Univ Utah, UT 84112 USA.
    The anisotropic quasi-static permittivity of single-crystal <bold>beta</bold>-Ga2O3 measured by terahertz spectroscopy2020In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 117, no 25, article id 252103Article in journal (Refereed)
    Abstract [en]

    The quasi-static anisotropic permittivity parameters of electrically insulating beta gallium oxide (beta -Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200GHz to 1THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c and b and reciprocal lattice direction a *. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high-frequency contributions and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta -Ga2O3 in high-frequency electronic devices.

  • 72.
    Gribisch, Philipp
    et al.
    Lund Univ, Sweden.
    Delgado Carrascon, Rosalia
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Lind, Erik
    Lund Univ, Sweden.
    Capacitance and Mobility Evaluation for Normally-Off Fully-Vertical GaN FinFETs2023In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 70, no 8, p. 4101-4107Article in journal (Refereed)
    Abstract [en]

    In this work, we present the fabrication and analysis of fully-vertical GaN FinFETs with a gate length of 550 nm. The devices with fin widths of around 100 nm reveal normally-OFF behavior and subthreshold swings (SSs) very close to the 60-mV/dec limit. Low hysteresis values indicate low defect densities at the oxide/GaN interface. The devices exhibit low specific ON-resistances at a maximum of around 90 V breakdown voltage, which is reasonable for the drift layer thickness of 1 mu m. The capacitances in the devices were modeled and identified with capacitance voltage measurements, which could also be used to approximate the effective and field effect mobility in the channel and reveal to around 164 and 54 cm(2)/(Vs) at higher gate voltages, which is a slight improvement to reported values for similar devices.

  • 73.
    Gribisch, Philipp
    et al.
    Department of Electrical and Information Technology and NanoLund, Lund University, Lund, Sweden.
    Delgado Carrascon, Rosalia
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. NanoLund and the Physics Department, Lund University, Lund, Sweden.
    Lind, Erik
    Department of Electrical and Information Technology and NanoLund, Lund University, Lund, Sweden.
    Tuning of Quasi-Vertical GaN FinFETs Fabricated on SiC Substrates2023In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 70, no 5, p. 2408-2414Article in journal (Refereed)
    Abstract [en]

    In this work, we present the fabrication and investigation of the properties of quasi-vertical gallium nitride (GaN) fin field effect transistors (FinFETs) on silicon carbide (SiC) substrates and the influence of a postgate metallization annealing (PMA). The devices reveal low subthreshold swings (SSs) down to around 70 mV/dec. For a 1- μm -thick drift layer, a low ON-resistance below 0.05 mΩ⋅ cm2 (normalized on the fin area) and a breakdown voltage of 60 V were obtained. Devices with included PMA show a decreased threshold voltage and ON-resistance and by several orders of magnitude reduced gate leakage current compared to non-annealed devices. The devices show ohmic contact behavior and slightly negative threshold voltages, which indicates normally- ON behavior. The effective and field-effect mobility of the fin channel was obtained with a modeled carrier concentration and reveal to around 70 and 13 cm2/(Vs) at high gate voltages, which is in a good comparison to so far reported similar devices.

  • 74.
    Gustafsson, Anders
    et al.
    Lund Univ, Sweden.
    Persson, Axel
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Bi, Zhaoxia
    Lund Univ, Sweden; Future Display Inst Xiamen, Peoples R China.
    Samuelson, Lars
    Lund Univ, Sweden; Southern Univ Sci & Technol, Peoples R China.
    Cathodoluminescence investigations of dark-line defects in platelet-based InGaN nano-LED structures2024In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 35, no 25, article id 255703Article in journal (Refereed)
    Abstract [en]

    We have investigated the optical properties of heterostructured InGaN platelets aiming at red emission, intended for use as nano-scaled light-emitting diodes. The focus is on the presence of non-radiative emission in the form of dark line defects. We have performed the study using hyperspectral cathodoluminescence imaging. The platelets were grown on a template consisting of InGaN pyramids, flattened by chemical mechanical polishing. These templates are defect free, whereas the dark line defects are introduced in the lower barrier and tend to propagate through all the subsequent layers, as revealed by the imaging of different layers in the structure. We conclude that the dark line defects are caused by stacking mismatch boundaries introduced by multiple seeding and step bunching at the edges of the as-polished, dome shaped templates. To avoid these defects, we suggest that the starting material must be flat rather than dome shaped.

  • 75.
    Halim, Joseph
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Ingemar
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Moon, Eun Ju
    SUNY Buffalo, NY 14260 USA.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Rosén, Johanna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Barsoum, Michel W.
    Drexel Univ, PA 19104 USA.
    Electronic and optical characterization of 2D Ti2C and Nb2C (MXene) thin films2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 16, article id 165301Article in journal (Refereed)
    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.

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  • 76.
    Hilfiker, Matthew
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Mock, Alyssa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. 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) films2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, no 23, article id 231901Article in journal (Refereed)
    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.

  • 77.
    Hofmann, T.
    et al.
    Institut fur Experimentelle Physik II, Universität Leipzig, Leipzig, Germany.
    Chavdarov, T.
    Institut fur Experimentelle Physik II, Universität Leipzig, Leipzig, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Lu, H.
    Department of Electrical and Computer Engineering, Cornell University, USA.
    Schaff, W.J.
    Department of Electrical and Computer Engineering, Cornell University, USA.
    Schubert, M.
    Department of Electrical Engineering and Center for Materials Research and Analysis (CMRA), University of Nebraska-Lincoln, Lincoln, USA.
    Anisotropy of the Γ-point effective mass and mobility in hexagonal InN2006In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 3, no 6, p. 1854-1857Article in journal (Refereed)
    Abstract [en]

    We determine the anisotropic electron effective mass and mobility parameters in wurtzite InN thin films with free electron concentration Nfrom 1.8 × 1017 cm–3 to 9.5 × 1018 cm–3 using Infrared Magneto-optic Generalized Ellipsometry. The room-temperature measurements were carried out with magnetic fields up to 4.5 T. For the Γ-point we estimate m* = 0.047m0 and m* = 0.039m0 for polarization perpendicular and parallel to the c -axis, respectively. Scattering by impurities or ionized donors may explain the decrease of mobility for polarization parallel to the c -axis from 1600 cm2/(Vs) to 800 cm2/(Vs) with increase in N , where the perpendicular mobility is further decreased, likely caused by additional grain boundary scattering.

  • 78.
    Hofmann, T.
    et al.
    Univ Nebraska, Dept Elect Engn, Lincoln, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Lu, H.
    Cornell Univ, Dept Elect & Comp Engn, Ithaca, NY USA.
    Schaff, W. J.
    Cornell Univ, Dept Elect & Comp Engn, Ithaca, NY USA.
    Schubert, M.
    Univ Nebraska, Dept Elect Engn, Lincoln, NE 68588 USA.
    Optical hall effect in hexagonal InN2008In: Journal of Electronic Materials, ISSN 0361-5235, E-ISSN 1543-186X, Vol. 37, no 5, p. 611-615Article in journal (Refereed)
    Abstract [en]

    Measurements of the optical Hall effect in naturally doped high-quality wurtzite-structure InN thin films by generalized ellipsometry reveal that both the surface and the interior (bulk) free electron densities decrease with power-law dependencies on the film thickness. We discover a significant deviation between the bulk electron and dislocation densities. This difference is attributed here to the existence of surface defects with activation mechanism different from bulk dislocations and identifies the possible origin of the so far persistent natural n-type conductivity in InN. We further quantify the anisotropy of the Gamma-point effective mass.

  • 79. Hofmann, T.
    et al.
    Darakchieva, Vanya
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Lu, H.
    Schaff, W.J.
    Chen, L.-C.
    Nanishi, Y.
    Schubert, M.
    Assessment of the surface electron properties of polar and non-polar InN surfaces2008In: MRS Fall Meeting,2008, 2008, p. 124-Conference paper (Other academic)
  • 80.
    Hofmann, T.
    et al.
    University of Nebraska, USA .
    Kuehne, P.
    University of Nebraska, USA .
    Schöche, S.
    University of Nebraska, USA .
    Chen, Jr-Tai
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Forsberg, Urban
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Ben Sedrine, N.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Health Sciences.
    Herzinger, C. M
    JA Woollam Co Inc, USA .
    Woollam, J. A
    JA Woollam Co Inc, USA .
    Schubert, M.
    University of Nebraska, USA .
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Temperature dependent effective mass in AlGaN/GaN high electron mobility transistor structures2012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 19Article in journal (Refereed)
    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.

  • 81. Honda, Y.
    et al.
    Hikosaka, T.
    Yamaguchi, M.
    Sawaki, N.
    Pozina, Galia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Karlsson, Fredrik
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Darakchieva, Vanya
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Paskov, Plamen
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    DAP emission band in a carbon doped (1-101)GaN grown ob (001) Si substrate2009In: Phys. Stat. Sol. (c) Vol. 6, 2009, Vol. 6, p. S772-S775Conference paper (Refereed)
    Abstract [en]

    Optical spectra of a C-doped (1-101) GaN are investigated via time resolved photoluminescence spectroscopy. Samples with different C-doping levels were prepared by metalorganic vapour phase epitaxy using C2H2 as the doping precursor. A carbon related emission peak is identified at 375 nm which shows typical behaviours for a donor-acceptor-pair emission band. The acceptor level is estimated to be 190 meV which is at 43 meV shallower than that in an Mg doped GaN. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

  • 82.
    Junige, Marcel
    et al.
    Technical University of Dresden, Germany.
    Oddoy, Tim
    Technical University of Dresden, Germany.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    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 graphene2015In: NANOTECHNOLOGY VII, Society of Photo-optical Instrumentation Engineers (SPIE) , 2015, Vol. 9519, no 951915Conference paper (Refereed)
    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.

  • 83.
    Kakanakova-Georgieva, Anelia
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD2022In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 259, no 10, article id 2200137Article in journal (Refereed)
    Abstract [en]

    Herein, metal-organic chemical vapor deposition (MOCVD) of GaN layers doped with Mg atoms to the recognized optimum level of [Mg] approximate to 2 x 10(19) cm(-3) is performed. In a sequence of MOCVD runs, operational conditions, including temperature and flow rate of precursors, are maintained except for intentionally larger flows of hydrogen carrier gas fed into the reactor. By employing the largest hydrogen flow of 25 slm in this study, the performance of the as-grown Mg-doped GaN layers is certified by a room-temperature hole concentration of p approximate to 2 x 10(17) cm(-3) in the absence of any thermal activation treatment. Experimental evidence is delivered that the large amounts of hydrogen during the MOCVD growth can regulate the incorporation of the Mg atoms into GaN in a significant way so that MgH complex can coexist with a dominant and evidently electrically active isolated Mg-Ga acceptor.

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  • 84.
    Kang, Evan S. H.
    et al.
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Chen, Shangzhi
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Sardar, Samim
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Tordera, Daniel
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Shegai, Timur
    Department of Physics, Chalmers University of Technology, Sweden.
    Jonsson, Magnus
    Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
    Strong Plasmon–Exciton Coupling with Directional Absorption Features in Optically Thin Hybrid Nanohole Metasurfaces2018In: ACS Photonics, E-ISSN 2330-4022, p. 4046-4055Article in journal (Refereed)
    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.

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  • 85.
    Karki, Akchheta
    et al.
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Cincotti, Giancarlo
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Chen, Shangzhi
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Wang, Chuanfei
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Fahlman, Mats
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Jonsson, Magnus
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas2022In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 34, no 13, article id 2107172Article in journal (Refereed)
    Abstract [en]

    Nanostructures of conventional metals offer manipulation of light at the nanoscale but are largely limited to static behavior due to fixed material properties. To develop the next frontier of dynamic nano-optics and metasurfaces, this study utilizes the redox-tunable optical properties of conducting polymers, as recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Electrically tunable conducting polymer nano-optical antennas are presented, using nanodisks of poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system. In addition to repeated on/off switching of the polymeric nanoantennas, the concept enables gradual electrical tuning of the nano-optical response, which was found to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The resonance position of the PEDOT:Sulf nanoantennas can be conveniently controlled by disk size, here reported down to a wavelength of around 1270 nm. The presented concept may be used for electrically tunable metasurfaces, with tunable farfield as well as nearfield. The work thereby opens for applications ranging from tunable flat meta-optics to adaptable smart windows.

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  • 86.
    Karki, Akchheta
    et al.
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Yamashita, Yu
    Univ Tokyo, Japan; Natl Inst Mat Sci NIMS, Japan.
    Chen, Shangzhi
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kurosawa, Tadanori
    Univ Tokyo, Japan.
    Takeya, Jun
    Univ Tokyo, Japan; Natl Inst Mat Sci NIMS, Japan.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Watanabe, Shun
    Univ Tokyo, Japan.
    Jonsson, Magnus
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Doped semiconducting polymer nanoantennas for tunable organic plasmonics2022In: Communications Materials, ISSN 2662-4443, Vol. 3, no 1, article id 48Article in journal (Refereed)
    Abstract [en]

    Optical nanoantennas based on organic plasmonics are promising for their higher degree of tunability over metallic nanostructures. Here, nanodisks of polythiophene-based semiconducting polymers provide nanooptical antennas with resonances that are tunable over a 1000 nm wavelength range and can be switched off or on by doping modulation. Optical nanoantennas are often based on plasmonic resonances in metal nanostructures, but their dynamic tunability is limited due to the fixed permittivity of conventional metals. Recently, we introduced PEDOT-based conducting polymers as an alternative materials platform for dynamic plasmonics and metasurfaces. Here, we expand dynamic organic plasmonic systems to a wider class of doped polythiophene-based semiconducting polymers. We present nanodisks of PBTTT semiconducting polymer doped with a dicationic salt, enabling a high doping level of around 0.8 charges per monomer, and demonstrate that they can be used as nanooptical antennas via redox-tunable plasmonic resonances. The resonances arise from the polymer being optically metallic in its doped state and dielectric in its non-conducting undoped state. The plasmonic resonances are controllable over a 1000 nm wavelength range by changing the dimensions of the nanodisks. Furthermore, the optical response of the nanoantennas can be reversibly tuned by modulating the doping level of the polymer. Simulations corroborate the experimental results and reveal the possibility to also modulate the optical nearfield response of the nanoantennas.

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  • 87. Kasic, A.
    et al.
    Schubert, M.
    Inst. f. Experimentelle Physik II, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany.
    Off, J.
    4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany, OSRAM Opto Semiconductors, Wernerwerkstr. 2, 93049 Regensburg, Germany.
    Kuhn, B.
    4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany, Robert Bosch GmbH, Tübinger Str. 123, 72762 Reutlingen, Germany.
    Scholz, F.
    4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany, Abteilung Optoelektronik, Universität Ulm, 89069 Ulm, Germany.
    Einfeldt, S.
    Inst. für Festkörperphysik, Universität Bremen, Kufsteiner Straße NW 1, 28359 Bremen, Germany.
    Bottcher, T.
    Böttcher, T., Inst. für Festkörperphysik, Universität Bremen, Kufsteiner Straße NW 1, 28359 Bremen, Germany.
    Hommel, D.
    Inst. für Festkörperphysik, Universität Bremen, Kufsteiner Straße NW 1, 28359 Bremen, Germany.
    As, D.J.
    Fachbereich Physik, Universität Paderborn, Warburger Straße 100, 33095 Paderborn, Germany.
    Kohler, U.
    Köhler, U., Fachbereich Physik, Universität Paderborn, Warburger Straße 100, 33095 Paderborn, Germany.
    Dadgar, A.
    Inst. für Experimentelle Physik, Otto-von Guericke Univ. Magdeburg, Universitätsplatz 2, 39016 Magdeburg, Germany.
    Krost, A.
    Inst. für Experimentelle Physik, Otto-von Guericke Univ. Magdeburg, Universitätsplatz 2, 39016 Magdeburg, Germany.
    Saito, Y.
    Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan.
    Nanishi, Y.
    Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan.
    Correia, M.R.
    Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
    Pereira, S.
    Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
    Darakchieva, Vanya
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Amano, H.
    High-Tech Research Center, Dept. of Mat. Sci. and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan.
    Akasaki, I.
    High-Tech Research Center, Dept. of Mat. Sci. and Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan.
    Wagner, G.
    Inst. f. Nichtklassische Chemie, Universität Leipzig, Permoserstraße 15, 04318 Leipzig, Germany.
    Phonons and free-carrier properties of binary, ternary, and quaternary group-III nitride layers measured by infrared Spectroscopic Ellipsometry2003In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 0, no 6 SPEC. ISS., p. 1750-1769Conference paper (Other academic)
    Abstract [en]

    This work reviews recent ellipsometric investigations of the infrared dielectric functions of binary, ternary, and quaternary group-III nitride films. Spectroscopic Ellipsometry in the mid-infrared range is employed for the first time to determine phonon and free-carrier properties of individual group-III nitride heterostructure components, including layers of some ten nanometer thickness. Assuming the effective carrier mass, the free-carrier concentration and mobility parameters can be quantified upon model analysis of the infrared dielectric function. In combination with Hall-effect measurements, the effective carrier masses for wurtzite n- and p-type GaN and n-type InN are obtained. The mode behavior of both the E1(TO) and A1(LO) phonons are determined for ternary compounds. For strain-sensitive phonon modes, the composition and strain dependences of the phonon frequencies are differentiated and quantified. Information on the crystal quality and compositional homogeneity of the films can be extracted from the phonon mode broadening parameters. A comprehensive IR dielectric function database of group-III nitride materials has been established and can be used for the analysis of complex thin-film heterostructures designed for optoelectronic device applications. Information on concentration and mobility of free carriers, thickness, alloy composition, average strain state, and crystal quality of individual sample constituents can be derived. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  • 88.
    Khromov, Sergey
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Wang, X.
    Peking University, Peoples R China.
    Yoshikawa, A.
    Chiba University, Japan.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Rosén, Johanna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Correlation between switching to n-type conductivity and structural defects in highly Mg-doped InN2015In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 23, article id 232102Article in journal (Refereed)
    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.

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  • 89.
    Kimel, Alexey
    et al.
    Radboud Univ Nijmegen, Netherlands.
    Zvezdin, Anatoly
    Russian Acad Sci, Russia.
    Sharma, Sangeeta
    Max Born Inst Nichtlineare Opt & Kurzzeitspektros, Germany.
    Shallcross, Samuel
    Max Born Inst Nichtlineare Opt & Kurzzeitspektros, Germany.
    de Sousa, Nuno
    Donostia Int Phys Ctr DIPC, Spain.
    Garcia-Martin, Antonio
    CEI UAM CSIC, Spain.
    Salvan, Georgeta
    Tech Univ Chemnitz, Germany.
    Hamrle, Jaroslav
    Charles Univ Prague, Czech Republic.
    Stejskal, Ondrej
    Charles Univ Prague, Czech Republic.
    McCord, Jeffrey
    Univ Kiel, Germany.
    Tacchi, Silvia
    Univ Perugia, Italy.
    Carlotti, Giovanni
    Univ Perugia, Italy.
    Gambardella, Pietro
    Swiss Fed Inst Technol, Switzerland.
    Salis, Gian
    IBM Res Zurich, Switzerland.
    Muenzenberg, Markus
    Univ Greifswald, Germany.
    Schultze, Martin
    Graz Univ Technol, Austria.
    Temnov, Vasily
    CNRS, France.
    Bychkov, Igor V
    Chelyabinsk State Univ, Russia.
    Kotov, Leonid N.
    Syktyvkar State Univ, Russia.
    Maccaferri, Nicolo
    Umea Univ, Sweden; Univ Luxembourg, Luxembourg.
    Ignatyeva, Daria
    Russian Quantum Ctr, Russia; VI Vernadsky Crimean Fed Univ, Russia; Lomonosov Moscow State Univ, Russia.
    Belotelov, Vladimir
    Russian Quantum Ctr, Russia; VI Vernadsky Crimean Fed Univ, Russia; Lomonosov Moscow State Univ, Russia.
    Donnelly, Claire
    Max Planck Inst Chem Phys Solids, Germany.
    Rodriguez, Aurelio Hierro
    Univ Oviedo, Spain; Univ Oviedo, Spain.
    Matsuda, Iwao
    Univ Tokyo, Japan.
    Ruchon, Thierry
    Univ Paris Saclay, France.
    Fanciulli, Mauro
    CY Cergy Paris Univ, France; Sorbonne Univ, France.
    Sacchi, Maurizio
    Synchrotron SOLEIL, France.
    Du, Chunhui Rita
    Univ Calif San Diego, CA 92093 USA; Univ Calif San Diego, CA 92093 USA.
    Wang, Hailong
    Univ Calif San Diego, CA 92093 USA.
    Armitage, N. Peter
    Johns Hopkins Univ, MD 21210 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Liu, Bilu
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China.
    Huang, Ziyang
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China.
    Ding, Baofu
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China; Chinese Acad Sci, Peoples R China.
    Berger, Andreas
    CIC nanoGUNE BRTA, Spain.
    Vavassori, Paolo
    CIC nanoGUNE BRTA, Spain; IKERBASQUE, Spain.
    The 2022 magneto-optics roadmap2022In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 55, no 46, article id 463003Article, review/survey (Refereed)
    Abstract [en]

    Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of todays magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton-Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light-matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future.

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  • 90.
    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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. 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 frequencies2015In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 12, p. 2688-2691Article in journal (Refereed)
    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

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  • 91.
    Knight, Sean
    et al.
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; University of N Carolina, NC 28223 USA.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Wimer, Shawn
    University of Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; Leibniz Institute Polymerforsch Dresden eV, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene2017In: Scientific Reports, E-ISSN 2045-2322, Vol. 7, article id 5151Article in journal (Refereed)
    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.

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  • 92.
    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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. 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
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. 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 effect2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 1, article id 012103Article in journal (Refereed)
    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.

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  • 93.
    Knight, Sean Robert
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Guo, Shiqi
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Rindert, Viktor
    Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Persson, Per O. Å.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden; Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, 68588, NE, United States.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in AlxGa1−xN/GaN heterostructures (0.07 ≤ x ≤ 0.42)2023In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 134, no 18, article id 185701Article in journal (Refereed)
    Abstract [en]

    Al xGa 1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the Al xGa 1−xN barrier layers between x=0.07 and x=0.42⁠. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42⁠, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3 m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer.

  • 94.
    Knight, Sean Robert
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Lund Univ, Sweden; Lund Univ, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Lund Univ, Sweden.
    Terahertz permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate2024In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 124, no 3, article id 032101Article in journal (Refereed)
    Abstract [en]

    The anisotropic permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate, Lu2SiO5 (LSO), have been determined in the terahertz spectral range. Using terahertz generalized spectroscopic ellipsometry (THz-GSE), we obtained the THz permittivities along the a, b, and c? crystal directions, which correspond to the ea; eb, and ec? on-diagonal tensor elements. The associated off diagonal tensor element eac? was also determined experimentally, which is required to describe LSO's optical response in the monoclinic a-c crystallographic plane. From the four tensor elements obtained in the model fit, we calculate the direction of the principal dielectric axes in the a-c plane. We find good agreementwhen comparing THz-GSE permittivities to the static permittivity tensors from previous infrared and density functional theory studies.

  • 95.
    Knight, Sean
    et al.
    Univ Nebraska, NE 68588 USA.
    Schoche, Stefan
    JA Woollam Co Inc, NE 68508 USA.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hofmann, Tino
    Univ N Carolina, NC 28223 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Tunable cavity-enhanced terahertz frequency-domain optical Hall effect2020In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 91, no 8, article id 083903Article in journal (Refereed)
    Abstract [en]

    Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry-Perot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.

  • 96.
    Korlacki, R.
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Knudtson, J.
    Univ Nebraska Lincoln, NE 68588 USA.
    Stokey, M.
    Univ Nebraska Lincoln, NE 68588 USA.
    Hilfiker, M.
    Univ Nebraska Lincoln, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Linear strain and stress potential parameters for the three fundamental band to band transitions in beta-Ga2O32022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, no 4, article id 042103Article in journal (Refereed)
    Abstract [en]

    We report the strain and stress relationships for the three lowest energy direct band to band transitions at the Brillouin zone center in monoclinic beta-Ga2O3. These relationships augment four linear perturbation parameters for situations, which maintain the monoclinic symmetry, which are reported here as numerical values obtained from density functional theory calculations. With knowledge of these perturbation parameters, the shift of each of the three lowest band to band transition energies can be predicted from the knowledge of the specific state of strain or stress, thus providing a useful tool for modeling performance of power electronic devices and rational strain engineering in heteroepitaxy.

  • 97.
    Korlacki, R.
    et al.
    Univ Nebraska, NE 68588 USA.
    Stokey, M.
    Univ Nebraska, NE 68588 USA.
    Mock, A.
    NRC Res Associateship Programs, DC 20001 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Strain and stress relationships for optical phonon modes in monoclinic crystals with beta-Ga2O3 as an example2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 18, article id 180101Article in journal (Refereed)
    Abstract [en]

    Strain-stress relationships for physical properties are of interest for heteroepitaxial material systems, where strain and stress are inherent due to thermal expansion and lattice mismatch. We report linear perturbation theory strain and stress relationships for optical phonon modes in monoclinic crystals for strain and stress situations which maintain the monoclinic symmetry of the crystal. By using symmetry group analysis and phonon frequencies obtained under various deformation scenarios from density-functional perturbation theory calculations on beta-Ga2O3, we obtain four strain and four stress potential parameters for each phonon mode. We demonstrate that these parameters are sufficient to describe the frequency shift of the modes regardless of the stress or strain pattern which maintain the monoclinic symmetry of the crystal. The deformation potentials can be used together with experimentally determined phonon frequency parameters from Raman or infrared spectroscopy to evaluate the state of strain or stress of beta-Ga2O3, for example, in epitaxial heterostructures.

  • 98.
    Korlacki, Rafal
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Hilfiker, Matthew
    Univ Nebraska Lincoln, NE 68588 USA.
    Knudtson, Jenna
    Univ Nebraska Lincoln, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    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 Nebraska Lincoln, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Strain and Composition Dependencies of the Near-Band-Gap Optical Transitions in Monoclinic (AlxGa1-x)2O3 Alloys with Coherent Biaxial In-Plane Strain on Ga2O3(010)2022In: Physical Review Applied, E-ISSN 2331-7019, Vol. 22, no 6, article id 064019Article in journal (Refereed)
    Abstract [en]

    The bowing of the energy of the three lowest band-to-band transitions in beta-(AlXGa1-X)2O3 alloys is resolved using a combined density-functional theory (DFT) and generalized spectroscopic ellipsometry approach. The DFT calculations of the electronic band structure of both beta-Ga2O3 and theta-Al2O3 allow the linear portion of the energy shift in the alloys to be extracted, and provide a method for quantifying the role of coherent strain present in the beta-(AlXGa1-X)2O3 thin films on (010) beta-Ga2O3 substrates. The energies of band-to-band transitions are obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain, which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic beta-(AlXGa1-X)2O3 are found.

  • 99.
    Korlacki, Rafal
    et al.
    Univ Nebraska, Canada.
    Mock, Alyssa
    US Navy, DC 20375 USA.
    Briley, Chad
    Univ Nebraska, Canada.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Tokyo Univ Agr and Technol, Japan.
    Kumagai, Yoshinao
    Tokyo Univ Agr and Technol, Japan.
    Goto, Ken
    Tokyo Univ Agr and Technol, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    Natl Inst Informat and Commun Technol, Japan.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, Canada; Leibniz Inst Polymer Res Dresden, Germany.
    Comment on "Characteristics of Multi-photon Absorption in a beta-Ga2O3 Single Crystal" [J. Phys. Soc. Jpn. 88, 113701 (2019)]2020In: Journal of the Physical Society of Japan, ISSN 0031-9015, E-ISSN 1347-4073, Vol. 89, no 3, article id 036001Article in journal (Other academic)
    Abstract [en]

    n/a

  • 100.
    Kuang, Chaoyang
    et al.
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Chen, Shangzhi
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Luo, Min
    Univ Elect Sci & Technol China, Peoples R China.
    Zhang, Qilun
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Sun, Xiao
    Univ Elect Sci & Technol China, Peoples R China.
    Han, Shaobo
    Wuyi Univ, Peoples R China.
    Wang, Qingqing
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Crispin, Reverant
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Fahlman, Mats
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Zhao, Dan
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Wen, Qiye
    Univ Elect Sci & Technol China, Peoples R China; Univ Elect Sci & Technol China, Peoples R China.
    Jonsson, Magnus
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering. Stellenbosch Univ, South Africa.
    Switchable Broadband Terahertz Absorbers Based on Conducting Polymer-Cellulose Aerogels2024In: Advanced Science, E-ISSN 2198-3844, Vol. 11, no 3, article id 2305898Article in journal (Refereed)
    Abstract [en]

    Terahertz (THz) technologies provide opportunities ranging from calibration targets for satellites and telescopes to communication devices and biomedical imaging systems. A main component will be broadband THz absorbers with switchability. However, optically switchable materials in THz are scarce and their modulation is mostly available at narrow bandwidths. Realizing materials with large and broadband modulation in absorption or transmission forms a critical challenge. This study demonstrates that conducting polymer-cellulose aerogels can provide modulation of broadband THz light with large modulation range from approximate to 13% to 91% absolute transmission, while maintaining specular reflection loss &lt; -30 dB. The exceptional THz modulation is associated with the anomalous optical conductivity peak of conducting polymers, which enhances the absorption in its oxidized state. The study also demonstrates the possibility to reduce the surface hydrophilicity by simple chemical modifications, and shows that broadband absorption of the aerogels at optical frequencies enables de-frosting by solar-induced heating. These low-cost, aqueous solution-processable, sustainable, and bio-friendly aerogels may find use in next-generation intelligent THz devices.

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