TECHNOLOGY UV LEDs
percent for Crystal IS’ DUV LED that was fabricated on a single-crystal AlN substrate, and efficiencies of 14 percent and 11 percent from UV Craftory and Sensor Electric Technology Inc. So, at this stage, we are slightly behind the state-of-the-art values for external quantum efficiency. However, we have a program in place that could enable us to overtake the leaders.
Just reaching 7 percent EQE was not easy, and required improvements to many aspects of the device, including the internal quantum efficiency (IQE), electron injection efficiency and light-extraction efficiency – it is the product of these three that determines the EQE.
Figure 1. The ultraviolet spectrum can be sub-divided into the UVA, UVB, UVC and vacuum UV ranges. Emission at 270 nm can be used for sterilization, due to the damage that it can cause to DNA
Now we are taking radical steps in LED design, such as the formation of the device on an array of AlN hexagonal pillars, to take device EQE to double-digit efficiencies. In addition, we have started a programme to slash the cost of DUV LEDs by growing the devices on silicon substrates.
Increasing internal efficiencies
Between 2006 and 2010, we focused our efforts on increasing the internal quantum efficiency (IQE) of our devices. When we started, this was below 1 percent, and it has been increased through improvements in material quality. The key has been the development of a low threading-dislocation density AlN buffer on sapphire, which is produced with ammonia pulse-flow multi- layer growth. With this deposition technology, we have formed AlN layers with atomically flat surfaces and threading-dislocation densities of just 3×108
cm-2 .
Figure 2. The efficiency of LEDs emitting in the deep UV is significantly less than that for the blue
There are also other commercial opportunities for ultraviolet, solid-state sources. They could be used for medical treatments, such as skin care; enable high-speed dissociation of pollutant materials; play a role in high colour-rendering illuminations; and form the heart of a new generation of high-density optical storage devices.
But to make significant inroads into every one of these promising markets will require a significant improvement in the efficiency of UV LEDs operating at around 280 nm and below (see Figure 2). Back in 2010, several groups announced efficiencies for these DUV LEDs of more than 1 percent, and recently values of 5-14 percent have been reported (see Figure 3). If this level of improvement is maintained, the DUV LED market could take off in 2015.
Our team at RIKEN, a research institute just outside Tokyo, Japan, is one of the leading developers of these devices. Working in partnership with Panasonic, we are developing 270 nm LEDs that will be launched on the market that combine efficiency in excess of 2 percent with a lifetime of more than 10,000 hours.
Results in our lab for this class of LED include an external quantum efficiency (EQE) of 7 percent. This compares to reports at the International Conference on Nitride Semiconductors in 2013 (ICNS-10) of an EQE of about 7
44
www.compoundsemiconductor.net October 2013
Figure 3. Some of the leading deep UV developers, including US firms Sensors Electronic Technology and Crystal IS, plus UV Craftory and RIKEN from Japan, reported EQE results at the International Workshop on Nitride Semiconductors in 2012 and ICNS 2013
Diminishing this threading-dislocation density increased the IQE from the AlGaN quantum well to over 60 percent. Even higher values are possible with InAlGaN wells: We estimate that they can produced an IQE in excess of 80 percent, thanks to indium segregation effects in the quaternary alloy. Segregation causes carrier localisation, and this supresses non-radiative recombination. Further improvements to DUV LED performance have resulted from increases to the electron injection efficiency through the introduction of a multi-quantum barrier, electron- blocking layer. This electron-blocking structure is especially beneficial for DUV LEDs emitting below 250 nm.
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