Light emitting diodes (LEDs) to continue the transformation of technology, whether it be through high-resolution flat-screen glow or light bulbs, which for many years. Versatility and high performance LEDs makes them more popular, but their full potential remains limited, in part due to the remaining mysteries about the exact mechanism of light emission in semiconductor materials.
One of the notable controversy surrounds the cause of the high intensity of the light from the LED semiconductor material, Indium Gallium Nitride (Gaas): researchers were split on whether or not the Indium-rich clusters form within the material and provide excellent efficiency of LEDs. Now researchers from MIT and Brookhaven National Laboratory have shown conclusively that clustering is not the cause. The results, published online in Applied Physics Letters, advance fundamental understanding of led technology and could open new avenues of research.
"This discovery helps solve a mystery in research LED and demonstrates the breakthrough experimental methods that can advance other sensitive and advanced electronics," said Silvia Gradecak, Thomas Lord Professor of materials science and engineering at MIT and co-author of the study. "Work brings us closer to this mastering of solid-state technology that can deliver light and energy with unprecedented efficiency."
The development of these technologies can have a significant impact: Gradecak indicates that about 14 percent of the electricity in the United States is used for lighting, so the sharp increase in the efficiency of light can help to achieve a corresponding reduction in electricity consumption.
Building a better bulb
Regular incandescent bulbs only convert approximately 5 per cent of their energy in visible light, the rest is lost as heat. Fluorescent push that up to about 20 per cent, still spend about 80 per cent of electricity used. In both cases, the light is a byproduct of heat-generating reaction, rather than the substantive effect, making them inherently inefficient.
The Solid-state lights convert electricity directly into photons, "said Eric stach, leader of electron microscopy at Brookhaven in the Centre of functional nanomaterials (CFN) and co-author of the study. "The effectiveness of this process, in theory, you could almost perfectly, but the pilot implementation have not reached those levels. This disconnect, helped motivate the study. "
GaAs practice especially promising led applications, but it "has long been a mystery why this stuff was so vivid, despite the fact he has a very high density of structural defects," said Gradecak. Some researchers analyzed material with electron microscopes that use high-power electron beams and found the Indies rich clusters in the material. Although some thought that those were the reason bright emissions, others thought they were artifacts caused by electron beam and usually is not present in the layers of Gaas.
To solve the mystery that was needed a way to monitor material which does not use such high-energy radiation could not cause material and heaps of these clusters. The tools available in Brookhaven "changed the way we could test these promising materials," says Gradecak. The CFN aberration corrected scanning transmission electron microscope reveals new and non-destructive testing in LED designs. For the first time, we could get details of the Angstrom level is the 10th one nanometer — without the risk of rendering process affecting the sample. "
No clusters found
Baluchi Kamal MIT postdocs, lead author of the study and Aaron Johnston-Peck CFN actually applied these methods to the same image samples, which for the first time the controversy over clustering to help resolve the issue.
"We found that the Indies there is no rich clusters actually these samples, even if they remain efficient light emitters," Baloch said. That settled the question of whether they cause bright emissions.
"The important thing is that we created a foolproof method for investigation of Gaas materials," said Baloch. "We can use these non-destructive image editing techniques to study the fundamental relationship between cluster formation and emission of light to help unlock the secrets of this amazing fusion."
Using this technique, we showed the image that this process does not produce artifacts, "said Gradecak. This means that the real reason for the material in the bright light "is to be understood, but now one dominant theory has been ruled out.
Sir Colin Humphreys, a professor and Research Director in the Department of materials science and metallurgy at Cambridge University, who was not involved in the work, said: "this document finally solves a long-standing dispute about why this type of LEDs, so bright. "This paper shows conclusively that the explanation is" based on clusters of the Indies was wrong, he said.
"This is an important piece of work that was performed carefully and meticulously," he said. "It finally puts an end to this debate which raged for the last 10 years."
GaAs is likely to remain the leading material for light-emitting diodes, but "Although commercial LEDs are very bright, their effectiveness is still below what is theoretically predicted," Baloch said. "That's why there is so much interest in finding out" precisely what accounts for their superior brightness. "If we pin down mechanism, we will be more efficient," adds the Baluchis.