TECHNOLOGY UV LEDs
The DUV LED’s efficiency and output power can be increased with a combination of superior heat dissipation, optimisation of the dimensions of the nano-patterned sapphire, and improvements to the growth processes used to deposit AlN onto the substrate.
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Our devices built on patterned sapphire emit a peak wavelength of 282 nm and, when operated at 20 mA, produce an output power and external quantum efficiency of 3.03 mW and 3.45 percent, respectively. Output saturates at 60 mA, hitting 6.56 mW. In comparison, the equivalent LED formed on conventional sapphire is limited to 2.53 mW at 50 mA, primarily due to inferior heat dissipation.
Driven at 20 mA, our DUV LED grown on the novel platform delivers an external quantum efficiency that is 98 percent higher than that of the control sample (see Figures 5 and 6). Since external quantum efficiency is a product of internal quantum efficiency, light extraction efficiency and carrier injection efficiency – and the measurements for ‘top-GaN-less’ multiple quantum wells show a 60 percent gain in internal quantum efficiency with the patterned sapphire – it is highly likely that the patterning has boosted extraction efficiency. This is to be expected, because light scattering at the interface between AlN and nano-patterned sapphire should decrease total internal reflection and absorption in the p-GaN layer, while increasing the photon’s opportunity for escaping from the sapphire backside.
We will now try to build even more impressive devices. The DUV LED’s efficiency and output power can be increased with a combination of superior heat dissipation, optimisation of the dimensions of the nano-patterned sapphire, and improvements
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Figure 6. Nano-patterning increases the output power and the efficiency of DUV LEDs
to the growth processes used to deposit AlN onto the substrate. In addition, we will investigate the impact of nano-patterning on the reliability of our devices.
© 2014 Angel Business Communications. Permission required.
Figure. 5 The patterning of sapphire leads to an increase in electroluminescence intensity
Measuring ultraviolet LED output January / February 2014
www.compoundsemiconductor.net 53
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