The effect of chip size on the thermal-optical properties of GaN-based light emitting diodes was investigated. An increase in chip size was associated with a decrease of total series resistance due to enhancement of current spreading area. Consequently, the junction temperature linearly increased with an increase of the driving current, and the increase rate was slower for lager chip size. Moreover, we found out that the driving current and chip size affect the dominant emission wavelength shift that was understood to be a competition between blue shift behaviors of piezoelectricity-induced quantum confined stark effect and red shift behavior of self-heating effect. Thus, the operating current for color stabilization was increased with increasing chip size such as 80mA, 140mA and 160mA for 350×350μm2, 600×600μm2 and 1000×1000μm2 chip sizes, respectively. Herein, the operating current for color stabilization was determined at the driving current, where blue shift of piezoelectricity-induced quantum confined stark effect became in balance with the temperature induced band gap shrinkage resulted from self-heating effect.
Low-defect AlGaN films were grown by metal organic chemical deposition (MOCVD) over patterned sapphire substrates (PSS) with V-grooves fabricated by wet chemical etching based on a mixed solution of H2SO4 and H3PO4. Three high-temperature (HT) growth steps were performed on the wet-etched PSS. In the 1st HT-growth step, GaN layers with triangular cross-sections were grown on sapphire mesas of the surface of the wet-etched PSS. In the 2nd HT-growth step, the GaN layers were more grown in the lateral direction and coalescent at the bottom, but corrugated at the surface. Finally, in the 3rd HT-growth step, AlGaN layer was grown on the corrugated surface of GaN and coalesced completely into AlGaN layer with flat surface. By employing the corrugated surface of GaN grown on the wet-etched PSS as the initial surface for the growth of AlGaN layers, the tensile stress of AlGaN was remarkably reduced. Additionally, in order to completely eliminate the cracks of the AlGaN layer, a low-temperature (LT) AlN layer was inserted between the AlGaN layer and the GaN layer with the corrugated surface. By inserting the LT-AlN interlayer, no crack did generate in the AlGaN film, and the density of threading dislocations in the film was remarkably decreased after the lateral epitaxy using the wet-etched PSS with V-grooves.
Fluorescent polymer-zinc oxide hybrid (PZOH) materials were successfully prepared using simple chemical bath deposition method at low temperature. The formation of ZnO nanoparticles and their deposition on the surface of fluorescent polymer was confirmed by SEM and TEM images, which were further clarified by performing selected area diffraction patterns in TEM image that confirms the incorporation of crystalline ZnO material. The optical emission efficiency showed significant enhancement by employing different ratio of ZnO composition with respect to fluorescent polymer. Finally, bright and efficient white light emitting diodes have been fabricated using these PZOH, as a luminescence converter (LUCO) with the help of commercially available Nitride based blue LED, as a primary pumping source. The output light efficiency of PZOHs fabricated White LED showed drastic improvement when compared to pure fluorescent polymer, due to higher PL quantum efficiency (86 %) of PZOH then the pure fluorescent polymer (54 %).
The homoepitaxial growth InGaN/GaN double heterostructure (DH) light emitting diode (LED) by low pressure metalorganic chemical vapor deposition is reported for the first time. Hydride Vapor Phase Epitaxy prepared 350micrometers -thick GaN single crystal was polished down to a surface roughness of 10 angstrom rms to serve as the substrate. The LED exhibited luminous intensity of 850mcd and forward voltage of 5V at a current of 20mA. The peak wavelength and full width at half maximum of electroluminescence were 490nm and 83nm, respectively.
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