Hisashi Yoshikawa

Bio: Hisashi Yoshikawa is an academic researcher from Gunma University. The author has contributed to research in topics: Plasmon & Physics. The author has an hindex of 3, co-authored 3 publications receiving 709 citations.

Optical constants of ZnO

Abstract: The complex dielectric functions, e(E)=e1(E)+ ie2(E), of ZnO have been measured by spectroscopic ellipsometry (SE) in the photon-energy range between 1.5 and 5.0 eV at room temperature. The SE measurements are carried out on the surface parallel to the optic axis c, which allows the determination of the optical constants for light polarized perpendicular (E⊥c) and parallel to the c -axis (E//c). The measured SE spectra show the exciton peaks at ∼3.4 eV (E0 edge). These e(E) spectra are analyzed on the basis of a simplified model of the interband transitions. Excellent agreement is achieved between the calculated and experimental results over the entire range of photon energies. Dielectric-function-related optical constants, such as the complex refractive index n*(E)=n(E)+ ik(E), absorption coefficient α(E) and normal-incidence reflectivity R(E), of ZnO have also been reported.

Optical properties of hexagonal GaN

Abstract: Single-crystalline hexagonal GaN (α-GaN) films have been grown on (0001) sapphire substrates by metalorganic chemical vapor deposition at 1040 °C. The complex dielectric functions, e(E)=e1(E)+ie2(E), of the epitaxial films have been measured by spectroscopic ellipsometry (SE) for E⊥c in the region between 1.5 and 5.0 eV at room temperature. Previously published ultraviolet SE spectra of α-GaN are examined by considering the effects of surface roughness using an analysis based on an effective medium model. Ex situ atomic force microscopy is used to assess independently surface flatness. By mathematically removing the effects of surface roughness, the most reliable e(E) values for α-GaN are presented in the 1.25–10 eV photon–energy range. Theoretical dispersion analysis suggests that the E0 structure could be characterized by a three-dimensional M0 critical point and the E1α (α=A,B,C) structures by two-dimensional M1 critical points. To facilitate design of various optoelectronic devices, dielectric-functio...

Evaluation of dielectric function models for calculation of electron inelastic mean free path

Abstract: This work investigates the detailed difference between dielectric function models, the Mermin model and the full Penn algorithm (FPA) model, for the determination of an electron inelastic mean free path (IMFP) with optical energy loss function (ELF), as an extension of our previous study [Da et al., Surf. Interface Anal. 51, 627 (2019)] by using the simple Drude-type ELF. In the conventional normal Mermin (NM) model, the approximations of ELF by the Drude equation will introduce inevitable fitting error. In order to enhance the accuracy of the NM model, our previous proposed extended Mermin model [Da et al., Phys. Rev. Lett. 113, 063201 (2014)], which is renamed as a super-extended Mermin algorithm (SE-MA) now, is employed to eliminate the error by expanding the definition of Drude oscillators used in the NM. In the SE-MA, the Drude-like oscillators allow the existence of negative strengths to express the fine structures of phonon–electron scattering and the plasmon lifetime broadening effect. Because in our previous study, the simple Drude-type ELF cannot include these complex structures, in this work, the electron IMFPs are calculated for five realistic materials, Al, Si, Cu, Au, and MgO. The difference between IMFPs calculated by the SE-MA model and the FPA model is material dependent and is significant in the low energy region, which is analyzed by using the Fano plot. This is due to the more important role played by the plasmon lifetime broadening effect.

Analysis of ellipsometric and thermoreflectance spectra for P-based III-V compounds GaP and InP

Abstract: Spectroscopic-ellipsometry (SE) and thermoreflectance (TR) spectra of GaP and InP are presented. Both measurements are carried out on the same samples in the 1.4–5.6-eV photon-energy range at room temperature. The measured SE and TR spectra reveal distinct structures at energies of the E0, E0+Δ0, E1, E0', E2 and E2+δ critical points. The Γ8v→X6c indirect-gap peak at 2.25 eV can also be clearly seen in the TR spectrum of GaP. These spectra are analyzed based on a simplified model of the interband transitions. The results agree well with the experimental SE and TR data over the entire range of photon energies.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Surface-Plasmon-Enhanced Light Emitters Based on InGaN Quantum Wells

Abstract: Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP–QW coupling. Large enhancements of the internal quantum efficiencies (etaint) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.

ZnO nanostructures as efficient antireflection layers in solar cells.

Abstract: An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.

A nanocomposite ultraviolet photodetector based on interfacial trap-controlled charge injection.

Abstract: A solution-processed ultraviolet photodetector with a nanocomposite active layer composed of ZnO nanoparticles blended with semiconducting polymers can significantly outperform inorganic photodetectors

Infrared dielectric functions and phonon modes of high-quality ZnO films

Abstract: Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...