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Group III-nitride semiconductors have been recognized as very important materials for opto-electronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) for applications in the visible and ultra-violet (UV) regions. To produce highly efficient light emission in LEDs, it is important to realize pure and almost perfect semiconductor materials in epitaxial growth. In most LED materials, it is desirable to have dislocation densities of 10 4 cm -2 or less for highly efficient light emission.In practice, GaN layers are grown on sapphire substrates (Al 2 O 3 ) by metalorganic chemical vapor deposition (MOCVD) at high temperature. Because of the difference in crystal structure and atomic spacing of sapphire substrate and GaN layer, so-called lattice mismatch, the GaN layers are formed and grown with a high density of threading dislocations with a typical density of 10 8 -10 9 cm -2 . This high density of threading dislocations associated with these GaN devices was a major obstacle in commercializing blue and green LEDs. Extended defects such as threading dislocations decrease the efficiency of LEDs because threading dislocations are associated with non-radiative recombination centers. Therefore, the efficiency of LEDs made in hetero-epitaxial growth decreases dramatically as the dislocation density increases.But it was discovered that adding a certain amount of In (indium) to the GaN layers increases the LED efficiency dramatically despite very high dislocation densities. This discovery has been the most important advance in allowing high efficiency blue and green LEDs. To explain the unexpected highly efficient light emission in InGaN based LEDs despite the existence of high dislocation density above 10 8 -10 9 cm -2 , a high density of In clustering or In rich regions was proposed to be essential in InGaN based active layers. Because the bandgap of the In rich regions is smaller than that of the surrounding InGaN materials, radiative carrier recombination is favored in these regions. The existence of In clustering has been often observed by photo-luminescence, cathodoluminescence, and high resolution transmission electron microscopy (HR-TEM) measurements. Especially, cross-sectional TEM and X-ray fluorescence measurements have been believed to be direct tools for detecting In clustering and used to optimize the density of In clusters or In rich quantum dots in the InGaN active layers to increase the internal quantum efficiency of InGaN based LEDs.The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). The average In mole fraction by three-dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis.The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be nonuniform in the InGaN active layer. In clustering or In ...

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Passive daytime radiative cooling (PDRC) devices have enabled subambient cooling of terrestrial objects without any energy input, offering great potential to future clean energy technology. Among various PDRC structures, random dielectric particles in a polymer matrix or paint-like coatings have displayed powerful radiative cooling performances with excellent scalability and easy fabrication. While modeling and analyzing such a system is nontrivial to enhance the cooling effect and engineer the structures to be utilized in various applications, it is essential to understand its complex physical relations and determine the optimal design conditions. In this work, we have thoroughly analyzed the optical properties and radiative cooling performances of PDRC paints composed of two-material particles (SiO2 and Al2O3) using 2D FDTD simulation and investigated the optimal design conditions. Specifically, we have studied the effects of design parameters, such as particle size, size distribution, binder volume ratio, and coating thickness. Subsequently, we have conducted an outdoor cooling measurement of the fabricated PDRC paints to demonstrate their radiative cooling potential and to analyze and understand their performance based on our numerical investigations. The fabricated PDRC paints exhibited high solar reflectance (0.958) and strong long-wave infrared emission (0.937) in the atmospheric transparency window, achieving a maximum temperature drop of 9.1 °C. This comprehensive study provides a detailed characterization of the structure and material parameters of the multimaterial PDRC paint system.

In order to achieve good conductivity and high transmittance of transparent conductive oxide (TCO), we attempted to fabricate Mg-doped ZnO:Al (AZO) films and analyzed their structural, electrical, and optical properties. The Mg-doped AZO films were successfully deposited on top of the glass substrates by using an ion beam sputter system. The Mg concentration was controlled by varying the number of MgO chips attached on the AZO target. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of Mg incorporated into the AZO films. The structural, electrical, and optical properties of the Mg-doped AZO films were found to depend on the Mg concentration. As the Mg concentration increased, the optical band gap also increased because of the replacement of Mg with Zn in the TCO films.