Abstract
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Light emitting diodes (LEDs) have received great attention because LEDs are regarded as next generation solid state lighting source. LEDs exhibit low energy consumption, long life-time, and eco-friendly characteristics. Enhancement of light extraction efficiency plays a key role in achieving high-pe...
Light emitting diodes (LEDs) have received great attention because LEDs are regarded as next generation solid state lighting source. LEDs exhibit low energy consumption, long life-time, and eco-friendly characteristics. Enhancement of light extraction efficiency plays a key role in achieving high-performance LEDs. To improve the light extraction efficiency, it is one of the most important issues to develop highly reliable and thermally stable ohmic contacts for vertical-geometry GaN-based LEDs. In the first part, we investigate Pd/Zn/Ag ohmic contacts to Ga-polar p-type GaN as a function of annealing temperature. The Pd/Zn/Ag ohmic contacts exhibited a reflectance of 85% at 450 nm, which is higher than that of Ag-only contacts (57%) after annealing at 500 °C. The alleviated agglomeration of the Pd/Zn/Ag ohmic contact due to the formation of ZnO at the surface seems to be responsible for the higher reflectance. The specific contact resistances of Ag-only and Pd/Zn/Ag contacts annealed at 500 °C were 2.4 × 10−4 and 6.1 × 10−5 cm2, respectively. GaN-based light-emitting diodes (LEDs) fabricated with the Pd/Zn/Ag contacts exhibited ~20% higher output power at 20 mA than the LEDs fabricated with the Ag-only contacts annealed at 500 °C. On the basis of X-ray photoemission spectroscopy and SEM results, the improved electrical and thermal properties are described. In the second part, we explore ohmic contacts to N-polar n-GaN for GaN-based vertical light-emitting diodes (VLEDs). It is difficult to form ohmic contacts to N-polar n-GaN at temperatures below ~300 °C, due to the Ga out-diffusion. In this work, we employed graphene sheets as a diffusion barrier to form highly reliable ohmic contacts to N-polar n-GaN for high-power VLEDs. It is shown that before annealing, both the Ti/Al and Ti/Graphene/Al contacts exhibit ohmic behavior with contact resistivities of 5.2×10–4 and 6.3×10–4 Ωcm2, respectively. Unlike the Ti/Al contact, however, the Ti/Graphene/Al contacts are ohmic with a contact resistivity of 7.5×10–4 Ωcm2 after annealing at 250 °C. Secondary ion mass spectroscopy (SIMS) depth profile results show that the graphene sheets serve as a barrier to the outdiffusion of Ga atoms into the metal electrode. Based on the, SIMS, X-ray photoemission spectroscopy (XPS) and electrical results, the ohmic and degradation behavior of the Ti-based contacts are described and discussed.
주제어
#Light emitting diode;
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