| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 94 === In this dissertation, the nitride based epitaxy material was grown by metalorganic chemical vapor deposition (MOCVD). Improvements of brightness, reliability and electrostatic discharge (ESD) ability on InGaN/GaN LED devices have been investigated. In order to increase LED output intensity, 300oC-RTA annealing was applied on Ni/ITO p-contact. It was found that the EL-intensity could be improved. Moreover, we could use transparent indium-tin-oxide (ITO) to replace semi-transparent Ni/Au or NiITO. However, good ohmic contact is difficult to achieve for ITO deposited on p-GaN. By growing such SPS structure on top of the p-GaN cap layer, one could achieve a good “ohmic” contact through tunneling when the n+(InGaN/GaN)-p(GaN) junction was properly reverse biased. The 20 mA output power and wall-plug efficiency (WPE) of LEDs with ITO p-contact was about 8.4mW and 13.9 %, for blue LEDs and 4.98 mW and 8.2 % for green LEDs. In the other way, light extraction efficiency is also significant to the LED output intensity. Similar concept should also be applied to chip side walls. A 10 % output power enhancement from LEDs by the introduction of the wavelike textured side walls could be achieved.
The slightly poor reliability of the LED with ITO could be attributed to the slightly larger specific contact resistance and sheet resistance of the ITO on n+-SPS. As a result, more heat would be generated at the ITO/n+-SPS interface and thus a shorter lifetime for the devices.
Therefore, the better reliability of LEDs could be achieved due to the better current spreading and thus less heating effect by the use of metal finger n-electrodes. Besides, by the deposition of a SiO2 layer on top of the ITO LEDs, a longer lifetime and a smaller leakage current (IR) could be achieved.
Large size (i.e. 1 mm x 1 mm) nitride-based power LEDs with ITO transparent p-contacts was also fabricated. The 350 mA output power was 84.8 mW (wall-plug efficiency = 7.2 %) for the ITO power chip with Al reflector. After more than 1000 hours RT-500mA lifetime testing, the luminous intensity only was decreased by 10 %. By using flip-chip technology, we should be able to achieve a larger output power since no bonding pads or wires exist on top of the devices so that photons could be emitted freely from the substrates. The flip-chip LED was also more reliable due to the shorter thermal path. It would be a challenge to take account of the ESD ability for the outdoor application of LEDs. Moreover, in my investigation, electrostatic discharge capability of the LED with ESD protection could be greatly improved from several hundred to over 1500 V in human body model (HBM).
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