TECHNOLOGY UV LEDs


Increasing ultraviolet efficiency with nano-patterned sapphire


Switching from conventional patterning of sapphire to a nano-scale variant trims epitaxial growth times and materials costs while boosting extraction efficiency.


By JIANCHANG YAN, PENG DONG, AND JUNXI WANG from the INSTITUTE OF SEMICONDUCTORS AT THE CHINESE ACADEMY OF SCIENCES.


DEEP ULTRAVIOLET (DUV) sources can serve many applications, including air and water purification, disinfection, bioagent detection (see image above), curing and non-line- of-sight communication. Traditionally, the mercury lamp has served this spectral range, but it is bulky, fragile and unsuitable for providing a modulated light source.


All of these weaknesses can be addressed with a DUV LED. Following several years of research into this class of solid-state emitter, chips now span 365 nm to 210 nm, with some devices producing milliwatt levels of light output while operating for thousands of hours. It is a level of performance that is adequate for some applications, but far higher levels of performance will drive significant market penetration for the DUV LED. Today


50 www.compoundsemiconductor.net January / February 2014


it is held back by a modest internal quantum efficiency and a low light extraction efficiency, which combine to limit this chip’s external quantum efficiency and output power.


The low internal quantum efficiency stems from the high density of crystal defects – they are typically of the order of 1010


-1011 cm-2


. DUV LEDs are riddled with these defects because there is a large lattice mismatch and a significant thermal expansion mismatch between the sapphire substrate and the AlN and AlGaN epilayers that form the LED (see Figure 1). Meanwhile, light extraction is poor, due to the combination of low internal total reflection at the epi-layers’ flat interfaces and absorption of the emission from the active region by the top p-GaN.


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