Novel Substrate Boosts AlGaN UV LED Efficiency
Patterned sapphire with silica array improves crystal quality and light extraction efficiency
The efficiency of InGaN/AlGaN UV LEDs can be significantly boosted by using a novel substrate, patterned sapphire with silica array (PSSA), according to an international team of researchers.
“The conventional patterned sapphire substrate (PSS) is facing bottlenecks toward achieving higher efficiency AlGaN-based UV LEDs,” said Shengjun Zhou, a professor at Wuhan University, who directed the research. “Our novel substrate provides an unprecedented opportunity in achieving higher electroluminescence performance in future solid-state UV light sources.”
For AlGaN-based UV LEDs grown on conventional PSS, experimental results suggest that the larger sticking coefficient of Al adatoms (than Ga adatoms with sapphire) induces misoriented growth of AlGaN on the sidewall of sapphire patterns and that the limited refractive index contrast between AlGaN and sapphire impedes the outcoupling ability of sapphire patterns. For these reasons, the state-of-the-art UV LEDs still suffer from poor crystal quality and low light extraction efficiency.
The researchers have achieved improved crystal quality and light extraction efficiency by using PSSA, which replaces the sapphire patterns with silica arrays. In comparison to PSS, PSSA reduced the threading dislocation density in AlGaN epilayer, attributing to the preferable vertical growth mode and reduced misfit at the coalescence boundary. Furthermore, PSSA serves as better reflector and refractor than PSS in redirecting light into the escape cone at top and bottom facets owing to the higher-index-contrast given by patterned silica array.
Benefiting from the reduced threading dislocation density and enhanced light extraction efficiency by using PSSA, the efficiency of InGaN/AlGaN UV LEDs has been significantly boosted.
'Boosted ultraviolet electroluminescence of InGaN/AlGaN quantum structures grown on high-index contrast patterned sapphire with silica array'; Nano Energy 69 (2020) 104427