The potential of indium gallium nitride/gallium nitride (InGaN/GaN) blue light-emitting diodes to have higher energy conversion efficiency has over the years been identified. Indium gallium nitride/gallium nitride light-emitting diodes grown on a c-plane sapphire substrate are known to possess a piezo-electric polarization field induced by the lattice mismatch between active indium gallium nitride and underlying gallium nitride layers. Moreover, the Wurtzite crystal structure of gallium nitride generates a spontaneous polarization field in the light-emitting diodes, which consequently forms tiled energy bands within the InGaN/GaN multi-quantum wells, leading to reduced spatial distribution of electron and hole wave functions and thus reduced radiative recombination rates. The techniques used to compensate such polarization fields require complicated device design, expensive substrates and skillful epitaxy techniques. Recently, stacked aluminum nitride/silica (AlN/Al2O3) showed potential to significantly improve the passivation quality. Unfortunately, no published reports on the influences of AlN/Al2O3 polarization layers on the performance of InGaN/GaN light-emitting diodes can be found in the existing plethora of literature.
Researchers led by Professor Zhenqiang Ma from University of Wisconsin-Madison in collaboration with Hong Kong University of Science and Technology scientists led by Professor Kevin Chen investigated the impact of plasma-enhanced atomic-layer deposition AlN/Al2O3 on the energy conversion efficiency of InGaN/GaN blue light-emitting diodes. They hoped that the AlN/Al2O3 stacked layers would significantly improve the light emitting diode-energy efficiency. Their work is currently published in the research journal, Nano Energy.
The research method employed entailed the use of X-ray photoelectron spectroscopy to characterize the surface potential changes inside the p-Gallium Nitride layer of InGaN/GaN light emitting diodes coated with AlN/Al2O3 stacked layers. The researchers then proceeded to fabricate the three types of gallium nitride light emitting diodes. Eventually, the energy efficiencies of the three types of fabricated LEDs wafers were characterized.
The authors made significant findings whereby the peak efficiency values of wall-plug efficiency, external quantum efficiency, and efficacy of the gallium nitride light emitting diodes were improved by 29%, 29% and 30%, respectively, and their corresponding efficiency droop rates were decreased by 13%, 6% and 3%, separately. In addition, the AlN/Al2O3 layers were noted to reduce the surface defect states of p-Gallium Nitride layer and enhance hole injection rate into the multi-quantum wells by increasing the surface potential of gallium nitride.
The study successfully reported the development of InGaN/GaN light emitting diodes with improved energy efficiency. Such significant energy improvements can be credited to the simple deposition of multifunctional ultrathin AlN/Al2O3 layers on top of p-type gallium nitride using remote plasma pretreatment and plasma-enhanced atomic-layer deposition. To be more specific, the AlN/Al2O3 stacked layers deposited on the gallium nitride: magnesium have overall improved the radiative recombination rate of the InGaN/GaN light emitting diodes and thus the improved light-emission efficiency. Therefore, the reported technique yields excellent results and should be adopted for further improvements.
Kwangeun Kim, Mengyuan Hua, Dong Liu, Jisoo Kim, Kevin J. Chen, Zhenqiang Ma. Efficiency enhancement of InGaN/GaN blue light-emitting diodes with top surface deposition of AlN/Al2O3. Nano Energy volume 43 (2018) pages 259–269.
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