Light-emitting Area Research Articles

可視シンチレーション光によるX線CTの性能評価

We proposed a new method of performance evaluation for X-ray CT using visible scintillation light and examined its usefulness in this study. When we scanned a plastic scintillator disk in a gantry opening of the X-ray CT, we could observe visible scintillation light. The rotation of the light-emitting area of the disk corresponded to that of the X-ray tube. We were able to record the scintillation light by digital video camera. By analyzing the area of visible scintillation light, the rotation speed of the X-ray tube, angular spread of the X-ray beam, uniformity of the incident X-rays, and change in X-ray energy were measured. No other method is available to obtain the above parameters of X-ray CT during a single CT scan. In the measurements of the uniformity of incident X-rays and change of X-ray energy, our method showed good accuracy in detecting the attenuation caused by the couch between the X-ray tube and the plastic scintillator disc. The proposed method is inexpensive and easy-to-use. We conclude that the method is a useful tool for performance evaluation as well as a maintenance tool for X-ray CT.

Comparison of Optical Properties of GaN/AlGaN and InGaN/AlGaN Single Quantum Wells

Static, field-modulated and time-resolved spectroscopies were carried out to compare the electronic states between GaN/AlGaN binary and InGaN/AlGaN ternary single quantum wells (SQWs). The internal field that exists across the quantum well (QW) naturally induces a quantum-confined Stark effect (QCSE), namely the redshift of the QW resonance energy and decrease of the electron-hole wavefunction overlap. The GaN/AlGaN SQW exhibited a weak emission due to QCSE. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character was maintained for thin QWs with the well width nearly the same as the bulk free exciton Bohr radius even under an electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited a bright luminescence peak in spite of an effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence (CL) mapping method. The lateral interval of each light-emitting area was estimated from the spatial resolution of the CL mapping to be smaller than 60 nm. Such local potential minima is considered to be formed due to the presence of a structure similar to quantum-disks [M. Sugawara: Phys. Rev. B 51 (1995) 10743]. Carriers generated in the InGaN QWs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.

Emission mechanisms of bulk GaN and InGaN quantum wells prepared by lateral epitaxial overgrowth

The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs.

Light emission from semiconducting polymers: Light-emitting diodes, light-emitting electrochemical cells, lasers and white light for the future

High performance photonic devices fabricated from conjugated polymers have been demonstrated, including diodes, light emitting diodes, photodiodes, field-effect transistors, polymer grid triodes, light emitting electrochemical cells, optocouplers and lasers, i.e. essentially all the categories which characterize the field of photonic devices. In many cases, semiconducting polymer-based devices have reached performance levels comparable to their inorganic counterparts. This short review focuses on recent progress in the area of light emission from semiconducting polymers.

The spatial distribution of light from silicon LEDs

Well-defined light patterns emitted from circular guard-ring avalanche silicon photodioles made on CZ substrates provide an experimental proof that the reverse current does not flow all over the junction area. This fact should be taken into account in calculations of current densities in future device design considerations. The light patterns originate from zones of crystal striations. These striations, which are usually considered to be a negative factor, are utilized here to control the overall light-emission intensity. It is shown that the light-patter dimensions and intensity are current controlled. The percentage of light area coverage, the overall emitted light intensity and the average reverse-current density are interrelated and determined as a function of the operating current. Four distinct regions of operation are identified. It is shown that the emitted light intensity is directly proportional to the area of light emission in three of the regions (low, medium and high currents) even at high junction temperatures. The results are application oriented with regard to the design of future silicon light-emitting diodes (LEDs).

High-efficiency InGaAlP/GaAs visible light-emitting diodes

High-efficiency InGaAlP surface emission light emitting diodes (LEDs) have been successfully fabricated. Newly designed double-heterostructure LEDs with a GaAlAs current spreading layer were employed to expand the light emission area, which is necessary to take out the light efficiently. The external quantum efficiency was 1.5% at 620 nm orange light for an In0.5 (Ga0.8Al0.2)0.5P active layer LED. This LED is five times more efficient at 620 nm than that of the GaAlAs and GaAsP LEDs. 563 nm green electroluminescence, which is the shortest wavelength ever reported for InGaAlP LEDs, was also achieved with an In0.5(Ga0.5Al0.5)0.5P active layer.

High-resolution reflective photointerrupter

Abstract A high-resolution reflective photointerrupter is presented. In this photointerrupter a newly developed LED chip with a small light-emitting area is used. The combination of the LED an aspherical lens gives a resolution almost one order of magnitude higher than that of conventional units.

Charge transfer reaction inverse photoemission spectroscopy (CTRIPS) Part II. Electrode specificity

A comparison of CTRIPS measurements for Au (111) and highly orientated stress-annealed pyrolytic graphite, in acetonitrile containing tetrabutylammonium tetrafluoroborate and thianthrene, is reported. The light emission spectra of the two samples are shown to be very different and characteristic of the electrode material. Special emphasis is placed upon the extension of the spectral range towards the near-ultraviolet, for high injection rates of excited holes, which is attributed to multi-particle radiative processes occurring very close to the solid surface and therefore providing enhanced surface sensitivity. In addition, the relationship between the injection current density and the time constant for double layer charging is analysed and future developments in the area of light emission from charge transfer reactions are discussed.

Small-area GaAs-GaAlAs heterostructure light-emitting diode with improved current confinement

A new type of high-radiance GaAs-GaAlAs heterostructure light-emitting diode (l.e.d.) with confinement of the light-emitting area by contact resistance is described. Single heterostructure l.e.d.s of this type with active-region doping of p=3×1018 cm?3 exhibit external quantum efficiency of 1% and a 3 dB modulation cutoff frequency of 66 MHz. Second-harmonic distortion was measured at below 40 dB at a 30 mA peak-peak modulation current and 50 mA direct current (=7.0 kA/cm2).

Hemispherical GaP:N green electroluminescent diodes

Hemispherically shaped, reduced active junction area, GaP green light-emitting diodes have been fabricated by mechanical polishing and electrochemical etching from p-n junctions grown by double liquid phase epitaxy on pulled crystal substrates. The hemispherical structure with the light-emitting area restricted to the central portion of the flat surface represents an idealized geometry in which to study luminescent generation and optical coupling phenomena in green-emitting material and devices. Experiments reported here show that the efficiency with which internally generated light is extracted from these GaP structures is at least twice as large as in encapsulated reduced junction area slurry cut cubical dice or mesa diodes. The junction reduction procedure is described and the properties of reduced junction area cubical diodes examined. A maximum external quantum efficiency of 0·41 per cent has been achieved with the hemispherical structure from a crystal which for other geometric devices yielded a maximum external efficiency of 0·17 per cent.

A Light Source for the Primary Standard of Wave-Length*

A light source for the red cadmium line, 6438.4696 IA, was developed in which the important objections against the cadmium lamp of Michelson type were eliminated. The intensity of the standard line was strong as shown by the short exposure time required to produce an interferometer spectrogram. The effective light-emitting area was 50 × 2.5 mm2 which was large enough to cover most spectrograph slits. The cadmium standard line was narrow as shown by the interferometer spectrograms. The Doppler-effect broadening was reduced by means of observing the light in a direction perpendicular to that of the discharge current.