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Calibration of an illuminance meter is indispensable for accurate measurement of the illuminance of indoor lighting and daylight. In recent years, because of the phasing-out of incandescent lamps and their replacement with LED lamps, it has become difficult to obtain an incandescent type standard lamp to calibrate an illuminance meter. To replace the standard lamp method, we constructed an illuminance meter calibration system based on an LED-based spectrally tunable light source. The approximate CIE Illuminant A spectrum realized by the LED-based spectrally tunable light source was controlled at various illuminance values (800–10,000 lx). A test illuminance meter was calibrated by comparison against a reference photometer with the realized approximate Illuminant A spectrum. The illuminance values measured using the reference photometer and using the test illuminance meter in the calibration system agreed within 2.5% without reference plane correction of the test illuminance meter, and within 1% with reference plane correction. Reference plane correction depends strongly on the measurement distance and the illuminance meter structure. This study demonstrated that it can be improved. Therefore, we infer that an illuminance meter calibration method using an LED-based spectrally tunable light source is a promising means of overcoming difficulties posed by the phasing-out of incandescent standard lamps.

Onsite reference infrared radiators are used to correct thermal imagers for noncontact fever screening in real time. We have developed a flat-plate reference radiator of the highest accuracy to enhance reliability of the fever screening. Our contact-durable blackbody composite with good heat transfer, which has a microcavity structured surface offering high emissivity ( > 0.999 ), was employed for the emissive surface of the reference radiator. Evaluation of the developed reference radiator regarding the spectral emissivity, in-plane uniformity, and temperature stability demonstrated an unprecedentedly small uncertainty of 0.10°C for the radiance temperature (95% level of confidence).

For total luminous flux calibration by a sphere-spectroradiometer system in 2π geometry, a new LED-based standard light source (standard LED) covering the full visible wavelength range has been developed. The developed standard LED has sufficient spectral power over the full visible wavelength range using UV-LED dies of different peak wavelengths in combination with red, green and blue phosphors. By evaluating spectrum flatness based on the magnitude of the second derivative, the spectrum of the standard LED was customised to minimise the measurement uncertainty. Properties of the standard LED such as the luminous intensity distribution, stability and reproducibility were also evaluated. The evaluation results indicate that the standard LED has good properties as a standard light source for the 2π total spectral radiant flux scale. Furthermore, the result of an operating current dependence suggests that the standard LED has benefits as a standard source that conventional standard lamps do not have.

One of the key measures to secure reliable fever screening is to calibrate a thermal imager with an accurate flat-plate blackbody device in real time. We provide durable perfect blackbody plates with both high emissivity of > 0.998 and good heat transfer, ideal for a high-precision reference radiation source. Reflectance measurements and heat transfer simulation demonstrate that a micro-cavity composite of a thin resin double layer or resin mixture with thermally conductive filler is an important solution for improving the emissivity and thermal performance of blackbody plates.

In response to the strong demand for a total spectral radiant flux (TSRF) standard from domestic lighting manufacturers, such a scale has been realized in the visible range by means of a relative gonio-spectroradiometric method at the National Metrology Institute of Japan (NMIJ). Our gonio-spectroradiometric method employs spectral irradiance as well as a luminous intensity standard as reference standards. We have investigated several models of quartz-halogen lamps from domestic manufacturers with respect to their stability and decided a set of reference standard lamps for TSRF. Our carefully selected quartz-halogen lamps have sufficient stability as the standard lamp for TSRF after a 100 h seasoning process. The relative expanded uncertainty (k = 2) for realization of the TSRF scale is between 3.1% (visible regions) and 4.1% (near ultraviolet region). We evaluated uncertainties related to the characteristics of the array spectroradiometer using experimental results and found some of those, such as effect of bandpass function, noticeably contributed to the total uncertainty.

A new traceability chain for luminance scale is proposed for light-emitting diode (LED) luminance measurement, which utilizes a newly developed LED-based luminance standard source (luminance standard LED) as a transfer standard. Compared with conventional traceability chains for LED luminance measurement that rely on either a luminance meter to transfer the scale from the primary to the user level, the proposed scheme has a two-fold advantage. Firstly, color correction is made at the traceability source and the user is freed from performing this complicated task. Secondly, the luminance standard LED acts as the reference source for comparison measurement with a luminance meter and no other extended source facilities such as integrating spheres are necessary at the user site. Performance evaluation of the luminance standard LEDs showed spatial uniformity better than ±2.0% within a 7 mm diameter area and temporal stability of 0.01% during an 8 h operation, which verified the LEDs’ capability to serve as a reference standard source. Calibration of the luminance standard LED was performed on the luminance scale realized by illuminance to luminance conversion, and the calibration uncertainty was estimated to be U = 1.4% to 1.7% (k = 2) at the traceability source. It is expected that this will enable users to obtain luminance measurement uncertainties comparable to the conventional traceability schemes while profiting from the high practicability that the proposed scheme provides. The luminous standard LEDs are now used in a new traceability chain in Japan, contributing to improved reliability of the luminance measurement of a wide range of LED-based products such as displays.

Luminescence properties of CdTe/ZnSe fractional monolayer grown by atomic layer epitaxy have been investigated. To investigate the origin of the highly efficient luminescence, various optical spectroscopic methods such as, photoluminescence (PL), temporal/spatial resolved PL, temperature dependence PL, and excitation power dependence PL have been used. It is found that structural inhomogeneities affect dominant influence on the line width and line shape of luminescence. The luminescence intensity greatly enhanced by the localization of exciton at the disorder induced localized states.

We propose ZnSe/BeTe:N superlattice (SL) as a new p-type cladding layer with improved p-type dopability, in which nitrogen-doped BeTe (N-doped) layers are inserted into undoped ZnSe layers. We have optimized the growth temperature for ZnSe/BeTe:N SLs. Both equivalent beam pressure (BEP) ratio and growth rate are further optimized in low-temperature growth of BeTe:N layers. The structural properties of BeTe:N layers were analyzed by reflection high-energy electron diffraction and high-resolution X-ray diffraction. Electrical properties were evaluated by the van der Pauw method and capacitance–voltage ( C – V ) measurements. We achieve the net acceptor concentrations of 1.2×10 19 cm −3 at the growth temperature of 350°C, BEP ratio of around 6, and growth rate of 35 nm/h.

We study the growth and optical properties of Cr-doped CdTe/ZnTe nanostructures grown on ZnTe (001) substrates by molecular beam epitaxy. In-situ reflection high-energy electron diffraction is used to study the growth processes and strain relaxation behaviors of Cr-doped CdTe quantum dots (QDs). After 4.5�ML deposition, the surface lattice parameter begins to increase remarkably, which indicates that the two-dimensional growth mode is terminated and the CdTe layer grows in a three-dimensional mode. Low temperature photoluminescence spectra of Cr-doped CdTe QDs (Tcr = 900 °C) show a broad emision. With increasing the Cr cell temperature above 1000 °C, the luminescence from CdTe QDs disappears and the broad luminescence at around 1.6 eV becomes dominant.

We study the electrical properties of heavily Al-doped ZnSe grown by molecular beam epitaxy, whose electron concentration is saturated. Hall measurements show that electron mobility decreases with the increase of Al cell temperature, while it is almost independent of measurement temperature. These phenomena can be explained in terms of scattering mechanisms by ionized impurities and point defects. The point defects, which are induced by the incorporation of excess Al atoms, result in carrier compensation in heavily Al-doped ZnSe and decrease the electron mobility. As Al concentration increases, deep level emission around 2.25 eV, associated with a point defect such as a self-activated center [Al Zn –V Zn ], develops in photoluminescence spectrum. The line width of (0 0 4) X-ray rocking curve is broadened due to degradation of crystal quality, which is induced by the increase of point defects with an increase of Al concentration. As a result, it is suggested that the carrier compensation in highly Al-doped ZnSe is dominated by generation of complex point defects such as self-activated centers [Al Zn –V Zn ].

Highly efficient luminescence at room temperature is achieved from a novel CdTe(thickness ≤2 ML)/ ZnSe fractional monolayer heterostructures (FMS) grown by atomic layer epitaxy. Various optical spectroscopic methods such as, photoluminescence (PL), PL-excitation, spatially-resolved PL and temperature-dependent PL are used to investigate the origin of the luminescence. Structural inhomogeneities such as alloying and thickness fluctuation affect the line width and line shape of luminescence, also the luminescence intensity is greatly enhanced by the localization of excitons at disorder induced localized states. The radiative recombination mechanism is explained in terms of a simple model which includes localized states originating from the inhomogeneity of CdTe FMS.

Low dislocation density ZnTe heteroepitaxial layers are achieved by inserting a thin ZnTe buffer layer at low growth temperature (LT-buffer) followed by annealing. The ZnTe heteroepitaxy layer with a LT-buffer shows a narrow line width of X-ray rocking curve as narrow as 42 arcsec in (0 0 4) reflection and the dislocation density is estimated as 1.3×10 6 cm −2 . A cross-sectional view of transmission electron microscopy shows high structural quality of ZnTe layer with LT-buffers.

Formation processes of self-organized CdTe quantum dots (QDs) grown on ZnTe (001) substrates by molecular beam epitaxy were studied. In situ reflection high-energy electron diffraction (RHEED) was used to study the initial growth processes and strain relaxation behaviors of CdTe QDs. The growth process of CdTe layer at the initial stage can be divided into three stages: (1) stable two-dimensional (2D) growth, (2) metastable 2D growth just before the QD formation, and (3) QD formation. It was found that the critical thickness for the 2D–3D transition is about 5.5 monolayers (ML) at the growth rate of 0.12 ML/s. The results of photoluminescence (PL) and μ-PL agree with the RHEED observations. In addition, by investigating the dependence of PL peak energy on the growth temperature, we found that interdiffusion between the cations (Cd and Zn) is activated at a higher growth temperature.

The optical properties and band alignment of ZnSe/ZnMgBeSe heterostructures are investigated. Two sets of ZnSe/ZnMgBeSe multi-quantum-well structures that consist of five or three wells with different well thicknesses and 100-nm-thick ZnMgBeSe barrier layers are grown on GaAs (100) substrates by molecular beam epitaxy. Low-temperature photoluminescence spectra show dominant sharp excitonic emission whose peak position systematically shifts to the higher energy side with decreasing the well thickness. Photoluminescence excitation spectra show optical transitions between excited quantum levels in addition to the ground levels. The standard analysis based on the effective-mass approximation gives the valence band offset of ΔEυ=(0.4±0.1)ΔEg. This is consistent with calculated values based on Harrison’s linear combination of atomic orbitals theory.

We have investigated the effects of band filling and thermal escape on the temperature dependence and excitation power dependence of photoluminescence of CdTe QDs grown by molecular beam epitaxy on ZnTe(100) substrate. The absorption spectrum indicates that the PL spectrum consists of two peaks. The experimental results of excitation power dependence were well simulated by the rate equation model taking into account the effect of band filling. The temperature dependence of PL showed significant difference for different excitation powers. It is suggested that either the wetting layer or excited states play an important role in band filling and thermal escape.

We have investigated the ZnTe-based material system for the application to light-emitting devices. To this end, ZnTe homoepitaxy techniques have been developed to grow high-quality epitaxial layers. The conductivity control of ZnTe and ZnMgSeTe layers have been investigated. High structural quality n-type ZnTe layers with high carrier concentration are achieved by aluminum doping. Ambipolar conductivity control of quaternary layers is achieved. Aluminum doped ZnMgSeTe layers show a net carrier concentration of 5 × 10 16 cm -3 , while a high hole concentration of 2.5 x 10 19 cm -3 is achieved by p-type doping using a nitrogen plasma source. Based on those results, Zn 1-x Cd x Te/ZnMgSeTe triple-quantum-well(TQW) LED structures were fabricated. Bright electroluminescence was obtained at room temperature at the wavelength of 604 nm from Zn 0.7 Cd 0.3 Te and at 566 nm from Zn 0.85 Cd 0.15 Te TQW-LED.

Molecular beam epitaxy of Al doped n-type ZnSe is studied. Growth conditions such as Al cell temperature and substrate temperature are systematically examined. The electrical, optical and structural properties of ZnSe : Al layers are investigated. ZnSe : Al layers show high electron concentration of 8.37 × 10 18 cm -3 with low electrical conductivity of 6.92 x 10 -3 Ω cm along with high crystallinity.

We have investigated the optical and structural properties of ZnMgBeSe/ZnSe/ZnMgBeSe quantum-well (QW) structures grown on GaAs (001) substrates by molecular beam epitaxy. The structural property of the QWs is confirmed by a sharp (line width is 21 arcsec) X-ray diffraction peak from ZnMgBeSe cadding layer. Photoluminescence (PL) and photoluminescence-excitation (PLE) spectra of QWs suggest the effective collection of excited carriers due to Type-I band alignment and high quality of ZnMgBeSe cladding layers. The bandoffset in these samples are evaluated as ΔEc=0.6ΔEg(ΔEv=0.4ΔEg) from the optical investigation of multi-quantum-well (MQW) structures and numerical calculation of the optical transition energies. From the optical pumping experiment, room temperature stimulated emission is observed at short wavelength (444 nm) with low threshold (16kWcm−2). Laser action is observed up to 473 K and the characteristic temperature (T0) is determined to be as high as 166 K. We believe that the low threshold and high characteristic temperature are due to the efficient electron and hole confinement in the quantum-well and high crystalline quality of the cladding layer as revealed from high excitation pumping experiments at low temperature.

The composition dependence of the energy gap (Eg) of Zn1−x−yMgxBeySe quaternary alloys grown by molecular-beam epitaxy was investigated. The energy gap of Zn1−x−yMgxBeySe can be controlled in a range of 2.7 eV

A ZnCdTe/ZnTe/ZnMgSeTe quantum-well (QW) structure lattice matched to ZnTe is proposed for the light-emitting devices in the pure-green wavelength region. Thin ZnTe layers are inserted in between the ZnCdTe QW layer and ZnMgSeTe cladding layers, which improve the quality of the QW structure as demonstrated by its narrow photoluminescence line width (6.5 meV at 10 K). Optically pumped lasing at 552 nm at room temperature with a threshold optical power of 215 kW cm−2 is achieved. The present results clearly show the feasibility of ZnTe-based QW structures for the application to light-emitting devices in the pure-green wavelength region.

We have achieved low-threshold optically pumped lasing at 444 nm at room temperature with high characteristic temperature (T0) from ZnSe/ZnMgBeSe single-quantum-well structures. The threshold intensity is as low as 15 kW cm−2, while T0 value is as high as 166 K. Lasing is observed up to 473 K. Lasing wavelength of 444 nm at room temperature is the shortest wavelength ever achieved in ZnSe-based laser diode structures. The laser structure includes a single ZnMgBeSe/ZnSe/ZnMgBeSe quantum well with a ZnSe well thickness of 4 nm. The (004) x-ray diffraction rocking curve of the ZnMgBeSe quaternary cladding layers shows a sharp diffraction peak with a full width at half maximum of 21 arcsec which is in contrast to that from a ZnMgSSe cladding layer showing much broader multiple peaks. The observed lasing features are partly ascribed to high crystal quality of the ZnMgBeSe layers and type-I band alignment, as has been supported by photoluminescence in addition to x-ray diffraction measurements.

Conductivity control is one of the most important issues in wide band gap semiconductors such as ZnSe. Recently, we have reported the successful growth of high carrier concentration n-type ZnSe layers doped with Al. However, the carrier compensation mechanisms and limitation of carrier mobility has not been understood yet. In this work, a series of Al doped ZnSe samples were grown on semi-insulating [001] GaAs substrate by molecular beam epitaxy (MBE). High resolution X-ray diffraction (HRXRD) measurements were performed to investigate the doping induced structural changes. Optical properties of deep/near band emission were investigated by photoluminescence (PL). The electrical properties were investigated by Hall measurement at the elevating temperature.

Although there have been successful demonstrations of buffer layer growth for the heteroepitaxy of large mismatched systems, however, general selection rule for the buffer layer is still unknown because the role of a buffer layer is not fully understood. In this article, heteroepitaxy ZnTe layers are grown on GaAs (001) substrates by molecular beam epitaxy (MBE). Prior to the main growth, a thin (~30nm) ZnTe buffer layer is introduced at low temperature (LT-buffer) and the effect of LT-buffer to the high temperature grown ZnTe layers have been investigated.

Although the application of semiconductor quantum dots to light emitters has been expected, only a few successful achievements have been reported mainly due to the low luminescence efficiency. This paper will report on the strong luminescence of CdTe/ZnSe fractional-monolayer (FM) heterostructures. Various optical-spectroscopic methods are used to investigate the origin of the luminescence in this structure.

By strain relaxation measurements using reflection high-energy electron diffraction, it is observed that development of the self-assembled InAs∕GaAs(001) dots continues after In deposition is stopped just above the critical thickness under As4 atmosphere. Transmission electron microscope and photoluminescence measurements reveal that dot size increases considerably during this postdeposition process. On the other hand, the dot size increases only slightly during the postdeposition if the amount of In is greater than the critical thickness. Energetics within a finite surface area for a dot shows that the energy barrier for the transformation from the metastable InAs wetting layer to the optimum dot on the thinner wetting layer is high and suppresses the transformation near the critical thickness. Development of the dot density is also illustrated by calculating the nucleation rate from the thickness-dependent barrier height.