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High brightness, low voltage GaN LEDs with efficient current spreading layers

Authors Chong, Wing Cheung
Issue Date 2007
Summary In this thesis, Tin-doped Indium Oxide (ITO) current spreading layer with ultra-thin silver interlayer has been studied for high brightness and low voltage GaN light-emitting diodes. Silver layer thickness and annealing condition were optimized. Our results show that the insertion of 0.5nm silver can remarkably reduce the forward voltage of LEDs. Additionally, high transmittance of ITO can be retained. By comparing with LEDs using Ni/Au (5/5nm) p-contact, 34.6% light output improvement and 29.5% wall plug efficiency gain on average were achieved. To further improve the device performance, two-step annealing was developed to modify the reversible sheet resistance of ITO layer. LEDs treated with optimized two-step annealing show 20.5% wall plug efficiency gain at 100mA compared with LEDs treated with optimized one-step annealing. Low cost ATO-based current spreading layer has been investigated. Near-UV LEDs with Ag/ITO/ATO (0.5/5/150nm) composite demonstrated 70% light output enhancement with acceptable low forward voltage compared with traditional Ni/Au contact. It can be attributed to its high light transmittance near UV wavelength regime. Reverse leakage current is greatly suppressed by using such thin ITO interlayer and thick ATO overlayer because of lower indium in-diffusion which is confirmed by Secondary Ions Mass Spectrometry (SIMS). High brightness 1 x 1 mm2 top-emitting interdigitated light-emitting diodes with different finger designs were fabricated in this study. Optimum number of finger can compromise the tradeoffs between uniform current spreading and light blocking. Experimental results show the best interdigitated LEDs can afford high current loading up to 320mA. Furthermore, aluminium back reflector and patterned MESA structure were investigated to enhance the light output power of LEDs. Heat dissipation of high power chip can be improved by using thermal conductive silver paste and/or thinner sapphire substrate. Improved LED chips can bear even higher current injection and show higher light output power. A novel simplified LED fabrication process was proposed and presented in the latest part of this thesis. This processing can save two photolithography steps and a metal evaporation. LEDs fabricated by our simplified process show lower forward voltage and exceptionally high light output power. The forward voltage of LEDs fabricated by simplified process is 3.51V at 20mA, compared to 3.67V obtained in conventional process. 50.92% light intensity improvement at 20mA and ~72% wall plug efficiency gain at high current injection level were achieved.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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