Hyeon Seob So

Optical properties of amorphous and crystalline Sb-doped SnO2 thin films studied with spectroscopic ellipsometry: Optical gap energy and effective mass

We investigated the optical properties of amorphous and crystalline antimony (Sb)-doped tin dioxide (SnO2) thin films grown using the co-sputtering deposition method at room temperature. We used undoped and Sb-doped (8 wt. %) SnO2 targets. Varying the relative power ratio of the two targets, we controlled the Sb-composition of the SnO2:Sb thin films up to 2.3 at. % of Sb contents. Through annealing, the as-grown amorphous SnO2:Sb thin films were transformed to crystalline thin films. Dielectric functions were obtained from the measured ellipsometry angles, Ψ and Δ, using the Drude and parametric optical constant models. We determined the absorption coefficients and optical gap energies of the SnO2:Sb thin films from the dielectric functions. We found increasing optical gap energy with increasing Sb composition. Increases in the Drude tail amplitudes, a signature of free carrier concentrations, were found in annealed, crystalline thin films with increasing Sb composition. The increase in the optical gap en...

Transition from a nanocrystalline phase to an amorphous phase in In-Si-O thin films: The correlation between the microstructure and the optical properties

We investigated the structural and optical properties of In-Si-O thin films as the phase abruptly changes from nanocrystalline (nc) to amorphous (a) with increasing Si content. In-Si-O thin films were deposited on Si substrate using a co-sputtering deposition method. The RF power of the In2O3 target was fixed at 100 W, while the power applied to the SiO2 target was varied between 0 W and 60 W. At the Si = 2.8 at. %, i.e., at the onset of amorphous phase, the optical properties, including the dielectric functions, optical gap energies, and phonon modes, changed abruptly which were triggered by changes in the crystallinity and surface morphology. X-ray diffraction (XRD) spectra showed crystalline (c-) In2O3-like peaks below Si = 2.2%. Additionally, a broad peak associated with an amorphous (a-) In2O3 phase appeared above 2.8%. However, the Raman spectra of In-Si-O showed very weak peaks associated with c-In2O3 below 2.2%, and then showed a strong Raman peak associated with a-In-Si-O above 2.8%. X-ray photoe...

Spectroscopic ellipsometry-based study of optical properties of amorphous and crystalline ZnSnO alloys and Zn2SnO4 thin films grown using sputtering deposition: Dielectric function and subgap states

We investigated the optical properties of amorphous and crystalline zinc tin oxide (ZTO) thin films grown on SiO2/Si substrates with varying compositions via a co-sputtering deposition method at room temperature. The co-sputtering targets consist of SnO2 and ZnO. By varying the relative power ratio of the two targets, we demonstrate the ability to control the Sn and Zn composition of the resulting ZTO thin films. The ratio of [Sn]/([Sn] + [Zn]) atomic compositions was estimated at 11%, 29%, 42%, 54%, and 60%. Using a 600 °C annealing process, the as-grown amorphous ZTO films were transformed into crystalline ZTO films. The dielectric functions were obtained based on the measured ellipsometric angles, ψ and Δ. We determined the dielectric functions, absorption coefficients, and optical gap energies of ZTO thin films with varying compositions. The dielectric functions, absorption coefficients, and optical gap energies of amorphous and crystalline Zn2SnO4 thin films were obtained at 29 at. % of Sn. Subgap st...

Enhancement of Thermoelectric Performance in Na-Doped Pb0.6Sn0.4Te0.95–xSexS0.05 via Breaking the Inversion Symmetry, Band Convergence, and Nanostructuring by Multiple Elements Doping

Topological insulators have attracted much interest in topological states of matter featuring unusual electrical conduction behaviors. It has been recently reported that a topological crystalline insulator could exhibit a high thermoelectric performance by breaking its crystal symmetry via chemical doping. Here, we investigate the multiple effects of Na, Se, and S alloying on thermoelectric properties of a topological crystalline insulator Pb0.6Sn0.4Te. The Na doping is known to be effective for breaking the crystalline mirror symmetry of Pb0.6Sn0.4Te. We demonstrate that simultaneous emergence of band convergence by Se alloying and nanostructuring by S doping enhance the power factor and decrease lattice thermal conductivity, respectively. Remarkably, the high power factor of 22.3 μW cm-1 K-2 at 800 K is achieved for Na 1%-doped Pb0.6Sn0.4Te0.90Se0.05S0.05 mainly due to a relatively high Seebeck coefficient via band convergence by Se alloying as well as the suppression of bipolar conduction at high temperatures by the increase of energy band gap. Furthermore, the lattice thermal conductivity is significantly suppressed by PbS nanoprecipitates without deteriorating the hole carrier mobility, ranging from 0.80 W m-1 K-1 for Pb0.6Sn0.4Te to 0.17 W m-1 K-1 at 300 K for Pb0.6Sn0.4Te0.85Se0.10S0.05. As a result, the synergistically combined effects of breaking the crystalline mirror symmetry of topological crystalline insulator, band convergence, and nanostructuring for Pb0.6Sn0.4Te0.95- xSe xS0.05 ( x = 0, 0.05, 0.1, 0.2, and 0.95) give rise to an impressively high ZT of 1.59 at 800 K for x = 0.05. We suggest that the multiple doping in topological crystalline insulators is effective for improving the thermoelectric performance.

Low temperature growth of amorphous VO2 films on flexible polyimide substrates with a TiO2 buffer layer

Amorphous VO2 thin films were grown on anatase TiO2-buffered polyimide (PI) films using radio-frequency magnetron sputtering deposition with a VO2 target as low as at 175 °C. For comparison, the authors grew VO2 films on TiO2-buffered SiO2/Si substrates. The structural and morphological properties of the VO2 films were evaluated by x-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. VO2 films grown on TiO2/SiO2/Si were crystalline at 200 and 250 °C and were amorphous at 175 °C. VO2 films grown on TiO2/PI were amorphous. No peak corresponding to the monoclinic phase of VO2 appeared in the Raman spectra of VO2/TiO2/PI films grown at 175 or 200 °C. The chemical compositions of VO2 and the binding energy spectra of V and O atoms were probed by x-ray photoelectron spectroscopy. The authors discussed the multivalence states of V atoms and oxygen vacancies based on the x-ray photoemission spectroscopy of crystalline and amorphous VO2 films. Th...