• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.157-157
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• 2010

Vertical-structure light-emitting diodes (V-LEDs) by laser lift-off (LLO) have been exploited for high-efficiency GaN-based LEDs of solid-state lightings. In V-LEDs, emitted light from active regions is reflected-up from reflective ohmic contacts on p-GaN. Therefore, silver (Ag) is very suitable for reflective contacts due to its high reflectance (>95%) and surface plasmon coupling to visible light emissions. In addition, low contact resistivity has been obtained from Ag-based ohmic contacts annealed in oxygen ambient. However, annealing in oxygen ambient causes Ag to be oxidized and/or agglomerated, leading to degradation in both electrical and optical properties. Therefore, preventing Ag from oxidation and/or agglomeration is a key aspect for high-performance V-LEDs. In this work, we demonstrate the enhanced thermal stability of Ag-based Ohmic contact to p-GaN by reducing the thermal compressive stress. The thermal compressive stress due to the large difference in CTE between GaN ($5.6{\times}10^{-6}/^{\circ}C$) and Ag ($18.9{\times}10^{-6}/^{\circ}C$) accelerate the diffusion of Ag atoms, leading to Ag agglomeration. Therefore, by increasing the additional residual tensile stress in Ag film, the thermal compressive stress could be reduced, resulting in the enhancement of Ag agglomeration resistance. We employ the thin Ni layer in Ag film to form Ni/Ag mutli-layer structure, because the lattice constant of NiO ($4.176\;{\AA}$ is larger than that of Ag ($4.086\;{\AA}$). High-resolution symmetric and asymmetric X-ray diffraction was used to measure the in-plane strain of Ag films. Due to the expansion of lattice constant by oxidation of Ni into NiO layer, Ag layer in Ni/Ag multi-layer structure was tensilely strained after annealing. Based on experimental results, it could be concluded that the reduction of thermal compressive stress by additional tensile stress in Ag film plays a critical role to enhance the thermal stability of Ag-based Ohmic contact to p-GaN.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.23-23
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• 2003

GaN-related semiconductors are of great technological importance for the fabrication of optoelectronic devices, such as blue and ultra violet light emitting diodes (LEDs), laser diodes, and photo-detectors. One of the most important applications of GaN-based LEDs is solid-state lighting, which could replace incandescent bulbs and ultimately fluorescent lamps. For solid-state lighting applications, the achievement of high extraction efficiency in LED structures is essential. For flip-chip LEDs (FCLEDS), the formation of low resistance and high reflective p-GaN contact is crucial. So far, a wide variety of different methods have been employed to improve the ohmic properties of p-type contacts to GaN. For example, surface treatments using different chemical solutions have been successfully used to produce high-quality ohmic contacts, Metallization schemes, such as Ta/Ti contacts to p-GaN, were also investigated. For these contacts, the removal of hydrogen atoms from the Mg atoms doped n the GaN was argued to be responsible for low contact resistances.

• Journal of the Korean Vacuum Society
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• v.20 no.2
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• pp.127-134
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• 2011

We investigate the crystallographic orientation and strain states of the Ni/Ag ohmic contacts on p-type GaN. The Ag film in the Ni/Ag contact was severely agglomerated during high temperature annealing in air ambient. As a results, after annealing for 24 h, the Ni/Ag contact shows non-linear I-V curve and low light reflectance of ~21% at 460 nm wavelength. High-resolution X-ray diffraction results show that the interplanar spacing of Ag (111) planes is almost same to that of bilk Ag after annealing for 24 hrs, indicating that the in-plane tensile strain in the Ag film was fully relaxed due to the Ag agglomeration.