Koh Era

Ultraviolet light‐emitting diode of a cubic boron nitride pn junction made at high pressure

Injection luminescence in the ultraviolet is observed from a cubic boron nitride pn junction diode made at high pressure. Microscopic observation and spectroscopic studies show that the light emission occurs near the junction region only in the forward‐bias condition. The spectra extend from ∼215 nm to the red, having a few peaks mainly in the ultraviolet.

Nature of Luminescence Transitions in ZnS Crvstals

With the purpose to elucidate the nature of luminescence transitions in ZnS crystals, emission spectra from typical luminescence centers, “Green-Cu”, “Blue-Cu”, Self-Activated, “Red-Cu”, and “Red-Cu, In” centers, were measured at various temperatures. The results show that these luminescences can be divided into two growps, A and B, the former including “G-Cu” and “B-Cu”, while the latter SA, “R-Cu” and “R-Cu, In”. In B group, emission peaks shift towards higher energies with increase in temperature from 4°K accompanied by the remarkable increase of half-widths, while in A group emission peaks shift towards lower energies, half-widths increasing only slightly. These characteristics in B group were accounted for from the theoretical viewpoint based on the configurational coordinate model, indicating that the transition responsible for the luminescences takes place between two localized levels. As for A group it may be concluded that the transition is that of free electron or hole to localized level. Taking...

Biexciton lasing in CuCl quantum dots

Lasing of CuCl microcrystals embedded in a NaCl single crystal was observed for the first time. The lasing takes place at 77 K in a sample sandwiched between dielectric mirrors under pulsed ultraviolet laser excitation. The lasing transition is that from biexciton to exciton. The lasing is observed up to 108 K. The optical gain of the sample is almost the same as that of a CuCl bulk crystal in spite of the low concentration of CuCl in the NaCl matrix.

Residual strain in epitaxially grown ZnSe/GaAs studied by two‐photon absorption spectroscopy

We apply two‐photon absorption spectroscopy as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in‐plane tensile strain of the order of 10−4. Deformation potential for the ZnSe layer is determined directly from two‐photon data to high accuracy.

Fast luminescence from carbon-related defects of hexagonal boron nitride

Abstract Luminescence from carbon-related defects of hexagonal boron nitride powders is studied as to transient characteristics. Lifetimes of four luminescence groups classified previously range from 1.15 ns to 17 ns. The transitions determining the lifetimes are assumed to be totally or almost radiative. The transient characteristics provide some new aspects of the luminescence and support for the previous classification.

Estimation of misfit strain in epitaxially grown ZnSe/GaAs with two-photon absorption spectroscopy

Abstract Two-photon absorption spectroscopy is used as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in-plane tensile strain of the order of 10 -4 , while the epilayer thinner than 1 μm undergoes biaxial compression. The measured thickness dependence of misfit strains is compared to that obtained from X-ray diffraction.

Dynamics of excitons localized by disorder in semiconductors

Abstract The dynamics of excitons at temperatures down to 1.6 K in three different (stacking, interface and composition) disordered systems are investigated in the pico- and nanosecond time domains by measuring time- and energy-resolved luminescence. With the aid of theoretical calculations based on the rate equation, the observed spectral diffusion within an inhomogeneously broadened exciton band is interpreted as phonon-assisted transfer of excitons between states localized by random potential fluctuations.

Exciton resonance of hyper-Raman and hyper-Rayleigh scatterings in II–VI compounds

Strong signals of both hyper-Raman scattering by optic phonons and hyper-Rayleigh scattering (SHG), the latter of which is forbidden by the usual selection rules, were found to emerge in two-photon resonance with excitation energy of the 2P excitons in a thin film sample of ZnSe on GaAs substrate. Both excitation spectra, which are found to be basically similar to each other, indicate an energy-splitting of the 2P excitons. As a result, it is found that the sample of 5 μm thickness suffer in-plane strains due to lattice misfit between ZnSe and GaAs, an order of which has been estimated thereby as e ≈ 5 × 10-4. A similar resonant hyper-Raman scattering was also found in a bulk sample of CdS, and is utilized for gaining into insight the microscopic origin of the phenomenon. On the basis of the results, it is discussed that the present phenomena, found for the first time in solids, would serve as novel spectroscopic methods for characterizing sample quality of II–VI compounds recently developed by state-of-the-art methods, through the strain-induced structure of the 2P excitons free from polariton effects.

Existence of Mobility Edge in Amalgamation Bands of Mixed Crystals

It is reported that the temperature dependence of luminescence intensity in AI x Ga 1- x As and Cd 1- x Mn x Te obeys the same relation 1/(1+ A exp T / T 0 )) as is observed in amorphous Si:H and chalcogenide glasses. This temperature dependence is well understood by the assumption that a mobility edge exists near the edge of amalgamation-type bands in mixed crystals, and that localized states exist continuously below the mobility edge. The same dependence is observed for the luminescence intensity from bound magnetic polarons in Cd .95 Mn .05 Te.

Electron-phonon interactions in layered hexagonal boron nitride

Abstract The characteristics of electron-phonon coupling in layered hexagonal boron nitride are studied for vibronic transitions in typical defects, in view of the strong anisotropy inherent in its crystal structure. Participation of low-frequency vibrational modes is explained in connection with an approximation in which the motions of atoms in a layer containing a defect can be separated from low-frequency vibrations of atoms in surrounding layers. Two kinds of vibronic transitions, formerly ascribed to carbon acceptors and nitrogen-site vacancies, are examined.