Ryota Ishii 교수

Reflectance spectroscopy was performed for nonpolar and semipolar bulk GaN substrates under uniaxial stress. The exciton-transition energies and oscillator strengths clearly depended on uniaxial stress. The results were interpreted by group theory on excitons and analyzed in terms of the effective Hamiltonian proposed by Bir and Pikus. In the Hamiltonian, the short-range Coulomb interaction was taken into consideration. This approach allows all exciton deformation potentials to be determined wit

Reflectance spectroscopy was performed for nonpolar and semipolar bulk aluminum nitride (AlN) substrates under uniaxial stress. The exciton-polariton theory was applied to interpret the reflectivity spectra, while the Bir-Pikus deformation potential theory was used to analyze the stress dependence of exciton transition energies. This approach allowed all the deformation potentials in AlN to be determined without the quasicubic approximation.

Optical spectroscopy is performed for $c$-plane homoepitaxial aluminum nitride (AlN) films. The temperature dependence of the polarization-resolved photoluminescence spectra reveals the exciton fine structure. The experimental results demonstrate that the electron-hole exchange interaction energy ($j$) in AlN is $j=6.8\phantom{\rule{4pt}{0ex}}\text{meV}$, which is the largest value for typical III-V and II-VI compound semiconductors. We propose the effective interatomic distance as the criterion

Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and tempera

In this study, the exciton binding energies of AlN are calculated by the anisotropic effective mass theory, which considers the strong electron–hole exchange and Fröhlich-type exciton–phonon interactions. Our calculated results are in good agreement with recent experimental results, indicating that the electron–hole exchange and exciton–phonon interactions play essential roles in describing the excitonic structure of AlN. We estimate that the exciton binding energies of AlN are 53.7 and 67.3 meV

Photoluminescence (PL) spectroscopy under ideal pulsed selective and non-selective excitation conditions is used to study 265 nm AlGaN-based light-emitting diodes grown on AlN substrates. Excitation-power-density-dependent PL measurements under selective excitation conditions show that the internal quantum efficiency of the quantum-well layers is unity at cryogenic temperatures under weak excitation regime. Temperature-dependent and time-resolved PL measurements demonstrate the high internal qua

Deep-ultraviolet (DUV) microscopy and microspectroscopy have received much attention in label-free live-cell imaging, selective molecular analysis, and optical characterizations of ultrawide bandgap materials. Far-field optics approaches usually suffer from the diffraction limit of light. Meanwhile, near-field optics technology is immature in the DUV spectral region. Herein, we develop a DUV scanning near-field optical microscope (SNOM) with an excitation wavelength of 210 nm. The fourth harmoni

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large

Photoluminescence and stimulated emission spectroscopies were performed on transparent aluminum nitride (AlN) substrates grown by hydride vapor-phase epitaxy. The stimulated emission was observed from cryogenic to room temperatures and its origin was assigned on the basis of the spontaneous emission spectra and existing theories. Two stimulated emission mechanism crossovers were also found. One was a temperature-induced crossover from a purely excitonic mechanism to a carrier-involved mechanism,

Bipolar ($n$- and $p$-type) electric conductivity control is at the heart of semiconductor technologies. However, achieving such control in ultrawide-band-gap semiconductors has been a major challenge because of the very high donor and/or acceptor binding energies of these materials. In the case of aluminum nitride (AlN), which is an ultrawide-band-gap semiconductor and one of the first candidate materials for solid-state deep-ultraviolet emitters, the substitutional magnesium (Mg) acceptor bind

Abstract Low-temperature photoluminescence spectroscopy is performed for unintentionally doped and silicon-doped aluminum nitride (AlN) films grown on AlN substrates. Considering the positive electron–hole exchange interaction constant substantially changes the neutral silicon-donor bound exciton binding energy from 28.5 to 15.3 meV. The silicon-donor binding energy is also experimentally deduced as 64.8 meV from a two-electron transition, which is justified by a theoretical calculation consider

Radiative and nonradiative recombination processes are investigated in the temperature range from 10 to 500 K for AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates. Time‐integrated photoluminescence (PL) spectroscopy under selective excitation conditions demonstrates that the decrease in the radiative recombination efficiency with increasing temperature is one of the causes of the thermal droop in AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes. Time‐resolved

To elucidate the microscopic origin of the thermal droop, a blue-emitting indium gallium nitride (InGaN) quantum well grown on epitaxially laterally overgrown gallium nitride was investigated using temperature-dependent microphotoluminescence spectroscopy. Below 300 K, the sample exhibited a well-known dislocation-tolerant luminescence behavior. However, as temperature increases from 300 K to 500 K, the near band-edge emission at the wing region (with lower threading dislocation densities) was s

Abstract Photoluminescence (PL) spectroscopy is performed for natural-abundance ( N.A. C) and isotopically purified ( 13 C) synthetic diamond using a deep-ultraviolet continuous-wave laser. Because the excitation source creates low-density low-temperature excitons even under a moderate excitation power, the PL spectra show suppressed collisional and thermal broadening with a high signal-to-noise ratio. Our approach can accurately resolve the isotopic effects. The experimentally determined effect

Abstract Time-resolved photoluminescence spectroscopy under an external bias is performed on 265 nm AlGaN-based LEDs on AlN substrates. The bias dependences of the photoluminescence wavelength, intensity, and decay time are observed. Our experimental results indicate that the built-in electric field has the opposite sign as the polarization-induced electric field in the quantum-well layers. These results agree with the first-principles calculations but are contrary to a previous experimental stu