According to reports, the research team of Wuhan University recently announced the use of PSSA (patterned sapphire with silica array) substrate to reduce the problem of gallium nitride junction boundary mismatch. It is proposed that PSSA substrate can improve indium gallium nitride and gallium nitride ( InGaN/GaN) flip-chip visible light LED efficiency.
PSSA is a patterned sapphire substrate with a silicon dioxide array. Earlier this year, Professor Zhou Shengjun, the head of the research team, proposed that the PSSA substrate can greatly improve the efficiency of indium gallium nitride and aluminum gallium nitride (InGaN/AlGaN) UV LEDs.
This study describes the great potential of PSSA substrates in improving the efficiency of Group III gallium nitride LEDs. Compared with the formal chip, flip chip LED can solve the problems of heat dissipation and uneven current distribution. In the flip-chip structure, light is mainly emitted through a transparent substrate.
Professor Zhou pointed out that using traditional patterned sapphire substrate (PSS) to grow gallium nitride, because gallium nitride is grown on the patterned sidewalls, there will be positioning errors, and reducing the threading dislocation density of gallium nitride films has always been A big challenge, which largely limits the further improvement of internal quantum efficiency.
At the same time, it is difficult to achieve breakthroughs in light extraction efficiency for flip-chip LEDs on PSS substrates, because the large refractive index contrast at the interface between sapphire and air is predetermined.
In contrast to the PSSA substrate, since the sidewalls of the silicon dioxide array cones do not form gallium nitride islands, the LED grown on the PSSA substrate can effectively reduce the contact between the sidewall area and the gallium nitride on the C-plane area of the substrate. The mismatch problem that appears on the joint boundary.
Professor Zhou said that compared to the traditional PSS substrate solution, the silicon array in the flip-chip LED on the PSSA substrate has a smaller refractive index contrast between the silicon array and the air, so more light is refracted from the silicon array to the air. In order to improve the light extraction efficiency.
In addition, based on the improvement of crystal quality and light extraction efficiency, the external quantum efficiency of the flip-chip LED on the PSSA substrate is also higher than that of the flip-chip LED on the PSS substrate.
Two studies from Wuhan University have shown that PSSA substrates can perform better reflection and refraction functions than PSS substrates. The use of PSSA substrate reduces threading dislocation density and improves light extraction efficiency, thereby improving the efficiency of InGaN/AlGaN UV LEDs and InGaN/GaN flip chip LEDs.
