Advanced Optoelectronic Device Processing
Over the last 20 years, great advancements have been made in nitride-based devices. The achievement can be credited to the development of material growth, as well as the innovation of device fabrication technologies. Sapphire is the most widely used substrate for GaN-based LEDs with excellent mechanical property and chemical stability. Consequently, the original substrate for GaN is hard to be removed during device fabrication process. Compared with other conventional III-V semiconductors, III-nitrides have much higher bond energies, resulting in excellent chemical stability and good mechanical properties, which is advantageous for application of GaN-based devices in harsh environments and under extreme operating conditions. However, it also causes great impediments to device processing. The sapphire substrate of a conventional lateral LED functions as the carrier of the chip, where the backside is mounted on the heat sink. Therefore, the backside of sapphire should be covered with reflective mirror to enhance light extraction efficiency.