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 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 Â 10 16 cm À3 , while a high hole concentration of 2.5 Â 10 19 cm À3 is achieved by p-type doping using a nitrogen plasma source. Based on those results, Zn 1Àx 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.Introduction Devices emitting light at around 550 nm are strongly needed for several important applications such as medical, plastic optical fiber communication, and full color laser display applications. Although laser diodes (LDs) for violet to blue-green and red wavelength ranges are now commercially available, no succesful report on LDs in the wavelength range from the pure-green to yellow (l = 540-580 nm) have been reported yet. Moreover, the conversion efficiency of light-emitting diodes (LEDs) in this wavelength region is extremely low in comparison with that of blue and red LEDs [1].ZnTe has a direct band gap of 2.27 eV (546 nm) at room temperature, therefore it is one of the most important materials for the pure-green wavelength (l = 550 nm) region. Because of the importance of ZnTe, there have been continuous attempts to fabricate ZnTe-based LEDs. Homo-junction LEDs were fabricated by diffusion of donor impurities [2,3]. However, ZnTe-based heterojunction LEDs have not been demonstrated yet, mainly due to the problems in growing highly doped n-type epitaxial layers with high crystal quality.In this paper, molecular beam epitaxy of ZnTe-based high-quality thin films by homoepitaxy technique is reported. The conductivity control of ZnTe-based materials is investigated using aluminum and nitrogen plasma sources as a n-and p-type dopants. Heterojunction LED strutures are fabricated and current injected operation at room temperature is demonstrated.

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.

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