Substrate boosts efficiency of flip-chip LEDs
Wuhan team shows how a patterned sapphire with silica array (PSSA) substrate decreased the misfit in the coalescence boundary of GaN grown on the sidewall regions and c-plane region of the substrate
Researchers from Wuhan University in China have reported a strategically constructed substrate, patterned sapphire with silica array (PSSA), to boost the efficiency of InGaN/GaN flip-chip visible LEDs.
“Our PSSA is a promising substrate that has the potential to be exploited in boosting efficiency of the III-nitride LEDs,” said Shengjun Zhou, a professor at Wuhan University who directed the research.
The flip-chip LEDs, which are inverted compared to top-emitting LEDs, can overcome the thermal issues and non-uniform current spreading. In this configuration, rays emit mainly from the transparent substrate. Decrease in threading dislocation density of GaN film grown on conventional patterned sapphire substrate (PSS) remains a challenge due to the misoriented growth of GaN on the patterned sidewall, which is a major limitation for further improvement in internal quantum efficiency. Moreover, breakthroughs in light extraction are limited in flip-chip LEDs on PSS because the large refractive index contrast at sapphire-air interface is predetermined.
Since no GaN islands formed on the silica array cone sidewall regions, the LED grown on PSSA effectively decreased the misfit existing in the coalescence boundary of GaN grown on the sidewall regions and c-plane region of the substrate.
In the flip-chip LED on PSSA, the smaller refractive index contrast between silica array and air led to that more rays refract from silica array to air, resulting in a higher light extraction efficiency, as compared to that between sapphire array and air. Owing to the improvement of crystal quality and light extraction efficiency, the external quantum efficiency of the flip-chip LED on PSSA was higher than that of the flip-chip LED on PSS.
'Strategically constructed patterned sapphire with silica array to boost substrate performance in GaN-based flip-chip visible light-emitting diodes' by Shuyu Lan et al; Optics Express, Vol. 28, Issue 25, 38444-38455 (2020).
