TECHNOLOGY UV LEDs
Figure 4. To enable efficient light extraction, deep-UV LEDs need to have a markedly different architecture from their blue cousins. The conventional p-type contact, GaN, absorbs UV light, which is partially reflected back into the device at AlN/sapphire and sapphire/air interfaces
Despite these significant improvements, devices still produced a low output power due to poor light extraction efficiency. There are two reasons for this: All the light that exits the quantum well towards the top surface of the chip is absorbed by the p-GaN contact layer; and much of the light that exits towards the substrate is reflected back into the device, due to significant refractive index differences at both the sapphire/air and AlN/ sapphire interfaces (see Figure 4). A result of all of this is that light extraction efficiency is restricted to just 8 percent.
To address the absorption of light by the p-GaN contact layer, we switched to a transparent p-AlGaN contact (see Figure 5). Making this adjustment in isolation can increase light extraction efficiency to more than 40 percent, but at the expense of hole injection efficiency. When high-aluminium-content p-AlGaN is used, the deep acceptor level of the magnesium dopant can limit the hole density to less than 1015
cm-3 . This meant
that when we fabricated 265 nm DUV LEDs with a high- aluminium-content p-AlGaN contact layer, the EQE of the device was no better than its predecessors. However, the EQE
All the light that exits the quantum well towards the top surface of the chip is absorbed by the p-GaN contact layer; and much of the light that exits towards the substrate is reflected back into the device, due to significant refractive index differences at both the sapphire/air and AlN/sapphire interfaces. A result of all of this is, light extraction efficiency is restricted to just 8 percent
“ October 2013
www.compoundsemiconductor.net 45 ”
Figure 5. Light extraction in deep UV LEDs can be improved by: switching from p-GaN to p-AlGaN, a transparent contact layer; using a highly reflective mirror for this wavelength range; and forming the device on connected AlN pillars that channel light out of the chip
was not significantly worse either, and as we shall see in the next paragraph, that held the key to a generation of brighter devices. To first determine what the appropriate compositional wavelength of p-AlGaN is, we varied this between 290 nm and 270 nm, which corresponds to aluminium compositions of approximately 48-60 percent. This short study revealed that p-AlGaN with aluminium composition as high as 60 percent is useful for the contact layer of a DUV LED (see Figure 6).
Validation of this choice of aluminium composition came from another set of experiments, where we fixed the compositional wavelength of p-type AlGaN at 270 nm, corresponding to an aluminium composition of 60 percent, and changed the emission of the quantum well from 265 nm to 282 nm. This revealed that the 270 nm p-AlGaN contact layer is transparent for emission at wavelengths longer than 280 nm. Our final step in this particular effort was to address the aforementioned reduction in the hole density that resulted from the switch from p-GaN by p-AlGaN. We compensated for this by adding a higher electron-blocking structure: A higher-aluminium-
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