| dc.description.abstract | GaN-based materials have leaped to a brand new stage in the past two decades. The single crystalline and direct band-gap GaN film can be grown on the sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The wavelength of the emitting light from GaN ranges from ultra-violate (UV) to blue light region by doping various indium content. Nowadays, the material of the blue light emitting diodes is based on the GaN material. Pumping phosphors or other wavelength converter by blue light, the white light can be generated. Hence, the GaN material is the key material for white solid-state lighting.
For the solid-state lighting applications, the GaN-based LED operates under a high electric power. Under such a high operation power, the heat dissipation is a critical issue. The sapphire substrate of the conventional LED has a poor thermal conduction. Also, the degradation of the electric and optical property would be very serious due to the high operation temperature. Therefore, in this study, the thin-GaN LED device is produced by the wafer bonding process and the laser lift-off process, which are used to transfer GaN thin film from the sapphire substrate to a better thermal conductive Si substrate. Owing to the better thermal dissipation, the thin-GaN LED structure is a very promising candidate for developing high-power GaN LED. Two main topics of the studied thin-GaN LED structure in this work: (1) Design a suitable p-GaN contacts and reflector for thin-GaN LED structure. (2) Increase the light extraction efficiency of thin-GaN LED.
In thin-GaN LED process, the wafer bonding process is necessary and it is a high temperature and high pressure process. Furthermore, the Si substrate is a non-transparent material for the blue light region. So, the p-type GaN contact should consist of an ohmic contact layer and a reflector as well. It is very important to develop a high thermally stable p-GaN contact. In this study, the Ni/Au/Ni/Al p-GaN contacts and Ni/Ag(Al) p-GaN contacts are investigated. These two thermally stable p-GaN contacts can reduce the degradation of the specific contact resistance and the reflection upon the thermal process. The specific contact resistance of the Ni/Au/Ni/Al p-GaN contact keeps on the order of 10-2 Ω-cm2 after 500 ℃ annealing. The reflectance of the Ni/Au/Ni/Al metal scheme is 60 % after 500 ℃ annealing. This high thermally stable Ni/Au/Ni/Al p-GaN contact is very suitable for the thin-GaN LED structure.
Another critical issue is the low light extraction efficiency due to large refraction index difference between GaN and air. The light emitted from the active layer in GaN is significantly trapped in the GaN epi-layer, and a serious total internal reflection occurs. In this study, the aluminum oxide and silicon oxide honeycomb structure are produced on the n-GaN emitting surface by poly-styrene spheres template and sol-gel method. The aluminum oxide and silicon oxide honeycomb structures capping on the n-GaN surface can increase the external quantum efficiency by 35 % and 19 %, respectively. The mechanism of increasing light out-put by the oxide honeycomb structure would be discussed. | en_US |