Kazunobu Kojima

Electrical resistivity and magnetoresistance of CeB6

Abstract Electrical resistivity and magnetoresistance of CeB6 single crystal have been measured in the temperature range from 1.3 to 300 K under the magnetic field up to 85 kOe. Three characteristic phases are distinguished consistently with other measurements. The Kondo like behaviour in the resistivity observed in the high temperature phase is fitted by the conventional form for the dilute Kondo state with the Kondo temperature TK = 5 ∼ 10K and the unitarity limit resistivity ϱ u ≅ 110 μΩ cm/Ce-atom. The negative magnetoresistance in the middle phase is stronger with increasing magnetic field and with decreasing temperature suggesting rapid destruction of the Kondo state. The magnetoresistance in the low temperature phase exhibits some anomalies suggesting sub-phases corresponding to several kinds of spin ordering.

Quantum electrodynamics of a nanocavity coupled with exciton complexes in a quantum dot

Here, a comprehensive theory of the couplings between a nanocavity and exciton complexes in a quantum dot is developed, which successfully predicts the spectral triplet in the strong coupling regime that has been observed in several experiments but is unexpected according to conventional cavity quantum electrodynamics. The quantum anti-Zeno effect is found to play an essential role in the appearance of the central peak in the triplet under a low-excitation regime. The effect of hyperfine interactions is also discussed, which results in the cavity-mediated mixing of bright and dark exciton states. These results provide significant insights into solid-state cavity quantum electrodynamics.

Investigation and comparison of optical gain spectra of (Al,In)GaN laser diodes emitting in the 375 nm to 470 nm spectral range

We measure gain spectra for commercial (Al,In)GaN laser diodes with peak gain wavelengths of 470 nm, 440 nm, 405 nm, and 375 nm, covering the spectral range accessible with electrical pumping. For this systematic study we employ the Hakki-Paoli method, i.e. the laser diodes are electrically driven and gain is measured below threshold current densities. The measured gain spectra are reasonable for a 2D carrier system and understandable when we take into account homogeneous and inhomogeneous broadening. While inhomogeneous broadening is almost negligible for the near UV laser diode, it becomes the dominant broadening mechanism for the longer wavelength laser diodes. We compare the gain spectra with two models describing the inhomogeneous broadening. The first model assumes a constant carrier density, while the second model assumes a constant quasi Fermi level. Both are in agreement with the experimental gain spectra, but predict very different carrier densities. We see our measurements as providing a set of standard gain spectra for similar laser diodes covering a wide spectral range which can be used to develop and calibrate theoretical manybody gain simulations.

Gain Anisotropy Analysis in Green Semipolar InGaN Quantum Wells with Inhomogeneous Broadening

Polarization switching phenomena in semipolar (1122)-oriented InGaN quantum wells (QWs) were theoretically investigated by a newly formulated model considering the effect of In compositional fluctuation. The theoretical model successfully reproduced the reported polarization switching phenomena for both the emission wavelength and the excitation density in blue-green (1122) QWs, and this showed the importance of inhomogeneous broadening effects to understand polarization properties of semipolar quantum wells. Then, the model was applied for pure-green (1122) QWs, and we predicted that optical polarization was kept in the [1123] direction up to the carrier density high enough to create population inversion in such long-wavelength QWs. These results support the possibility for semipolar-oriented pure green InGaN laser diodes with cleaved facet cavity mirrors.

Defect-Resistant Radiative Performance of m-Plane Immiscible Al1−xInxN Epitaxial Nanostructures for Deep-Ultraviolet and Visible Polarized Light Emitters

Planar vacuum-fluorescent-display devices emitting polarized UV-C, blue, and green light are demonstrated using immiscible Al1-x Inx N nanostructures grown in nonpolar m-directions. Despite the presence of high concentration of nonradiative recombination centers, the Al1-x Inx N nanostructures emit polarized light with the luminescence lifetimes of 22-32 ps at 300 K. This defect-resistant radiative performance suggests supernormal localized characteristics of electron-hole pairs.

A simple theoretical approach to analyze polarization properties in semipolar and nonpolar InGaN quantum wells

By using a simple theoretical approach, the previously reported experimental results of the polarization properties in semipolar and nonpolar InGaN quantum wells (QWs) were analyzed. On the basis of the k⋅p-perturbation theory, we derived a useful analytical expression for describing the polarization properties of these QWs, and used this expression to analyze experimental data reported from various research groups. Based on these analyses, we predicted that the negative polarization degree, which is favorable for laser diodes with cleaved-facet cavity mirrors, would appear in the blue- or green-InGaN QWs on the lower-angle semipolar planes (30°–40° inclined from the c-plane).

Low-resistivity m-plane freestanding GaN substrate with very low point-defect concentrations grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method

An m-plane freestanding GaN substrate satisfying both low resistivity (ρ = 8.5 × 10−3 Ωcm) and a low point-defect concentration, being applicable to vertically conducting power-switching devices, was grown by hydride vapor phase epitaxy on a nearly bowing-free bulk GaN seed wafer synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer. Its threading dislocation and basal-plane staking-fault densities were approximately 104 cm−2 and lower than 100 cm−1, respectively. A record-long fast-component photoluminescence lifetime of 2.07 ns at room temperature was obtained for the near-band-edge emission, reflecting a significantly low concentration of nonradiative recombination centers composed of Ga vacancies.

Optical Anisotropy Control of Non-

A method to control the optical polarization of non-c-plane InGaN quantum wells (QWs) is proposed. Firstly, it is described the numerical and analytical solutions of the valence band eigenenergies and eigenfunctions for non-c-plane nitride semiconductors. It is found that anisotropic strain dominates optical anisotropy due to the small spin–orbit interaction and the large lattice mismatch. Subsequently, it is introduced the InGaN/AlGaN QW structure instead of the InGaN/GaN QW in order to manipulate the eigenenergies of the two topmost valence bands. This resulted in optical polarization switching. We conclude with a discussion of relevant computational results.

c

The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared. The intensities and lifetimes of the near-band-edge and ultraviolet luminescences associated with a MgGa acceptor of I/I GaN:Mg were significantly lower and shorter than those of the epilayers, respectively. Simultaneously, the green luminescence (GL) became dominant. These emissions were quenched far below room temperature. The results indicate the generation of point defects common to GL and nonradiative recombination centers (NRCs) by I/I. Taking the results of positron annihilation measurement into account, N vacancies are the prime candidate to emit GL and create NRCs with Ga vacancies, (VGa) m (VN) n , as well as to inhibit p-type conductivity.

-plane InGaN Quantum Wells

Omnidirectional photoluminescence (ODPL) measurement using an integrating sphere was carried out to absolutely quantify the quantum efficiency of radiation ( η) in high quality GaN single crystals. The total numbers of photons belonging to photoluminescence (PL photons) and photons belonging to an excitation source (excitation photons) were simultaneously counted in the measurement, and η was defined as a ratio of the number of PL photons to the number of absorbed excitation photons. The ODPL spectra near the band edge commonly showed a two-peak structure, which originates from the sharp absorption edge of GaN. A methodology for quantifying internal quantum efficiency ( ηint) from such experimentally obtained η is derived. A record high ηint of typically 15% is obtained for a freestanding GaN crystal grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method using an acidic mineralizer, when the excitation photon energy and power density were 3.81 eV and 60 W/cm2, r...