Tae Dong Kang - Academia.edu (original) (raw)

Papers by Tae Dong Kang

ABSTRACT We have measured the visible-ultraviolet polarized reflectivity of anisotropically align... more ABSTRACT We have measured the visible-ultraviolet polarized reflectivity of anisotropically aligned single-wall carbon nanotubes between 1.5 and 6 eV. We observed distinctively distinguished reflectivities from two type of single-wall carbon nanotubes, which were synthesized with or without sulfur. The reflectivity of single walled carbon nanotube films synthesized with sulfur was fitted assuming the lineshape of Drude conductivity, whereas that of single carbon nanotubes synthesized without sulfur was fit using the lineshape formula of simple harmonic oscillators for dielectric function. We observed several peaks in the harmonic-oscillator-type reflectivity spectra, which we assigned to pi band electronic transitions. Difference spectra of parallel and perpendicular polarization components of the reflectivities with respect to the nanotube axis ascertained anisotropic features associated with the pi band electronic transitions in the the carbon nanotubes of both types. We identified the harmonic-oscillator-type and the Drude-type single-wall carbon nanotubes as semiconductor-like and metal-like ones, respectively.

Applied Physics Letters, 2007

Single crystalline PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 thin films on SrTiO3 (001) substrates wer... more Single crystalline PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 thin films on SrTiO3 (001) substrates were grown by a combination of molecular beam epitaxy and rf sputtering methods. The authors measured the dielectric functions of the thin films using spectroscopic ellipsometry and determined the interband critical point energies using standard critical point model. They compared the critical point energies to the band structure calculations in the literature. The data suggest that anticrossing behavior occurs between Ea and Eb near Zr=0.17. This phenomenon is attributed to a coupling between X1c and X3c bands caused by intrinsic alloy disorder.

Thin Solid Films, 2004

We grew ZnO and Zn1−xMgxO thin films on (0001) sapphire substrates by using metal-organic vapor p... more We grew ZnO and Zn1−xMgxO thin films on (0001) sapphire substrates by using metal-organic vapor phase epitaxy and measured the pseudo-dielectric functions using variable-angle spectroscopic ellipsometry. We analyzed the pseudo-dielectric functions by using the multi-layer model. The dielectric functions were fitted by using a Holden model dielectric function. We used anisotropic layer modeling for the ZnO thin film, whereas we adopted the approximation of isotropic layer modeling for the Zn1−xMgxO alloys. We also discuss the Mg composition dependence of the bandgap and the binding energy in Zn1−xMgxO alloys, and consider the valence-band ordering in ZnO thin films.

Metrology, Inspection, and Process Control XXXVII

Journal of Applied Physics, 2017

The surface plasmon resonance (SPR) of Pt nanoparticles (NPs) in TiO 2-SiO 2 nanocomposite films,... more The surface plasmon resonance (SPR) of Pt nanoparticles (NPs) in TiO 2-SiO 2 nanocomposite films, in which Pt NPs of about 5 nm are incorporated, is investigated by using spectroscopic ellipsometry. After obtaining the dielectric functions of Pt NPs from the Pt-SiO 2 nanocomposite film, Pt-TiO 2-SiO 2 nanocomposite films are analyzed by applying a homogenous single layer model with an effective medium approximation. The effects of Pt NPs on the optical properties of the nanocomposite films are clearly revealed in the imaginary part of the dielectric functions, showing an increase in broadband absorption near the band gap of the films with the increasing volume fraction of Pt NPs in the films. Particularly, the maximum of extinction cross-section of Pt NPs in the films coincides with the broadband absorption, indicating that the localized SPR of Pt NPs is responsible for the enhanced light absorption at the visible-light wavelengths. This work emphasizes that, although SPR absorption of Pt NPs is not so distinctive in the visible-light wavelengths, proper tuning of the dielectric environment as well as the volume fraction of Pt NPs can enhance the photoactivity of the nanocomposite films.

We developed far-IR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron L... more We developed far-IR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and full-Mueller matrix spectra using rotating retarders, and wire-grid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm −1. Fourier-transform infrared (FT-IR) spectrometer is used for multi-wavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of θ-2θ angular rotation, χ tilt angle adjustment, and X-Y-Z translation. A LabVIEW-based software controls the motors, sample temperature, and FT-IR spectrometer and also allows to run fully automated experiments with pre-programmed measurement schedules. Data analysis is based on Berreman's 4 × 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability μ = 1. A nonlinear regression of the rotating analyzer ellipsometry and/or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thin-film samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with μ = 1 are illustrated with experimental results and simulations for TbMnO 3 and Dy 3 Fe 5 O 12 single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO 3 .

Japanese Journal of Applied Physics, 2003

We have measured the visible-ultraviolet polarized reflectivity of anisotropically aligned single... more We have measured the visible-ultraviolet polarized reflectivity of anisotropically aligned single-walled carbon nanotubes between 1.5 and 6 eV. We observed distinctively distinguished reflectivities from two types of single-walled carbon nanotubes, synthesized with and without sulfur. The reflectivity of single-walled carbon nanotube films synthesized with sulfur was fitted under the assumption of the lineshape of Drude conductivity, whereas that of single-walled carbon nanotubes synthesized without sulfur was fit using the lineshape formula of simple harmonic oscillators for dielectric function. We observed several peaks in the harmonic-oscillator-type reflectivity spectra, which we assigned to band electronic transitions. Difference spectra of the parallel and perpendicular polarization components of the reflectivities with respect to the nanotube axis revealed anisotropic features associated with the band electronic transitions in the carbon nanotubes of both types. We identified the harmonic-oscillator-type and the Drude-type single-wall carbon nanotubes as semiconductor-like and metal-like ones, respectively.

Applied Physics Letters, 2010

Journal of the Korean Physical Society, 2002

ABSTRACT Using spectroscopic ellipsometry, we measured the pseudo-dielectric function of self-ass... more ABSTRACT Using spectroscopic ellipsometry, we measured the pseudo-dielectric function of self-assembled InAs/GaAs quantum dot systems at the onset of quantum-dot nucleation at room temperature from 0.8 to 6 eV. The nominal In coverage varied from 1 to 2.6 monolayers. In addition to quantum-dot-related feature at 1 eV, we observd two transitions at 1.34 eV and 1.38 eV, respectively, which arose from InAs wetting layer. We also found GaAs-related structure near 1.43 eV which might arise from resonance states in GaAs cap layer. We determined the transition energies accurately by performing curve-fitting to the second-order derivative spectra of pseudo-dielectric function. Furthermore we fitted the dielectric function of InAs wetting layer in the visible range performing multi-layer analysis and found strong excitonic enhancement of E1 peak and damping of E2 peak .

Advanced Materials, 2017

The real-time evolution of the electronic phase during the topotactic phase transformation in a S... more The real-time evolution of the electronic phase during the topotactic phase transformation in a SrFeO x thin film is shown. In article number 1606566, Woo Seok Choi and co-workers report the metal-insulator transition in epitaxial SrFeO x thin films. A pressure-temperature phase diagram of the topotactic transformation is constructed based on real-time optical spectroscopy and thermodynamic calculation.

Journal of the Korean Physical Society, 2008

Advanced materials (Deerfield Beach, Fla.), Jan 31, 2017

Topotactic phase transformation enables structural transition without losing the crystalline symm... more Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeOx epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy co...

Physical Review Letters, 2015

Barium bismuth oxide (BaBiO 3) is the end member of two families of high-T c superconductors, i.e... more Barium bismuth oxide (BaBiO 3) is the end member of two families of high-T c superconductors, i.e., BaPb 1−x Bi x O 3 and Ba 1−x K x BiO 3. The undoped parent compound is an insulator, exhibiting a charge density wave that is strongly linked to a static breathing distortion in the oxygen sublattice of the perovskite structure. We report a comprehensive spectroscopic and x-ray diffraction study of BaBiO 3 thin films, showing that the minimum film thickness required to stabilize the breathing distortion and charge density wave is ≈11 unit cells, and that both phenomena are suppressed in thinner films. Our results constitute the first experimental observation of charge density wave suppression in bismuthate compounds without intentionally introducing dopants.

Physics and High Technology, 2012

MRS Proceedings, 2004

Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as... more Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO2/SiOx multilayers (MLs). The SiO2(2nm)/SiOx(2nm) MLs have been prepared under various deposition temperature by ion beam sputtering. The annealing at temperatures ≥ 1100°C leads to the formation of Si nanocrystals (nc-Si) in the SiOx layer of MLs. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentzian lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO2/nc-Si MLs as a mixture of nc-Si and SiO2. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiOx layer. This result is compared to expected nc-Si volume fraction, which was estimated from stoichiometry of SiOx.

Micro-crystalline silicon thin films are currently investigated world-wide to realize integrated ... more Micro-crystalline silicon thin films are currently investigated world-wide to realize integrated circuits in TFT-LCD. Various crystallization methods are being developed using amorphous silicon thin films. We measured the pseudo-dielectric function of micro-crystalline silicon film formed on glass which were crystallized using various methods. Performing effective medium approximation, we determined the volume fraction of crystalline phase and amorphous phase of the

Thin Solid Films, 2011

Forward models for the Mueller Matrix (MM) components of materials with relative magnetic permeab... more Forward models for the Mueller Matrix (MM) components of materials with relative magnetic permeability tensor 1 µ ≠ are studied. 4 4 × matrix formalism is employed to produce general solutions for the complex reflection coefficients and MMs of dielectric-magnetic materials having arbitrary crystal symmetry and arbitrary laboratory orientation. For certain orientations of materials with simultaneously diagonalizable ε and µ tensors (with coincident principal axes), analytic solutions to the Berreman equation are available. For the single layer thin film configuration of these materials, analytic formulas for the complex reflection and transmission coefficients are derived for orthorhombic or higher crystal symmetry. The separation of the magnetic and dielectric contributions to the optical properties of a material are demonstrated using measurements of the MM at varying angles of incidence.