Since the world's first GaN-based blue LED was introduced in 1993, the development of LED manufacturing technology has been remarkable. Currently, commercially available GaN-based LEDs are all fabricated on sapphire substrates or SiC substrates. However, due to high hardness, poor conductivity and thermal conductivity, sapphire brings a lot of inconvenience to the processing and application of later devices. SiC also has the disadvantages of high hardness and high cost, and the Si substrate with relatively cheap price has excellent The advantages of thermal conductivity and mature device processing technology, so Si substrate GaN-based LED manufacturing technology has received widespread attention in the industry.
At present, Japan Nichia Corporation monopolizes the patented technology of GaN-based LEDs on sapphire substrates. CREE Corporation of the United States monopolizes the patented technology of GaN-based LEDs on SiC substrates. Therefore, the development of GaN-based LED production technology on other substrates has become a hot spot in the world. Nanchang University cooperated with Xiamen Hualian Electronics Co., Ltd. to undertake the National 863 Program “Power-based GaN-based LED manufacturing technology based on Si substrateâ€. After nearly three years of research and development, it has passed the acceptance of the Ministry of Science and Technology.
1. Si substrate LED chip manufacturing
1.1 Technical route
GaN is grown on a Si substrate to produce an LED blue chip.
Process flow: growing AlN buffer layer on Si substrate→ growing n-type GaN→ growing InGaN/GaN multi-quantum well light-emitting layer→ growing p-type AIGaN layer→ growing p-type GaN layer→ bonding with Ag reflective layer and forming p-type Ohmic contact electrode → stripping the substrate and removing the buffer layer → making an ohmic contact electrode of an n-type doped Si layer → alloy → passivation → dicing → testing → packaging.
1.2 Main manufacturing processes
A GaN-based MQW structure was grown on a 50 mm si (111) substrate using a Thomas Swan CCS low voltage MOCVD system. Use trimethylgallium (TMGa) as Ga source, trimethylaluminum (TMAI) as Al source, trimethylindium (TMIn) as In source, ammonia (NH3) as N source, silane (SiH4) and ferrocene Magnesium (CP2Mg) is used as an n-type and p-type dopant, respectively. First, an AlN buffer layer is epitaxially grown on a Si (111) substrate, and then an n-type GaN layer, an InGaN/GaN multiple quantum well light-emitting layer, a p-type AlGaN layer, and a p-type GaN layer are sequentially grown, and then an Ag mirror is formed on the p-plane. And forming a p-type ohmic contact, and then transferring the epitaxial layer to the conductive substrate by a thermocompression bonding method, then etching the Si substrate with a Si etching solution to remove the n-type GaN layer, and roughening the n-type surface with an alkali etching solution After that, an n-type ohmic contact is formed, thus completing the fabrication of the vertical structure LED chip. The structure is shown in Figure 1.
Figure 1 Schematic diagram of GaN-based LED chip on Si substrate
As seen from the structure diagram, the Si substrate chip is a flip-chip structure, and from the bottom to the top are a back Au electrode, a Si substrate, a bonding metal, a metal mirror (p ohmic electrode), a GaN epitaxial layer, a roughened surface, and Au electrode. The current distribution of the structure chip is vertical, the substrate has high thermal conductivity and high reliability; the back surface of the luminescent layer is a metal mirror, the surface has a roughened structure, and the light extraction efficiency is high.
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