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How LEDs Got Their Shine Back
8 May 2012 5:10 pm
Tiny and efficient, light-emitting diodes (LEDs) are supposed to be the bright future of illumination. But they perform best at only low power, enough for a flashlight or the screen of your cellphone. If you increase the current enough for them to light a room like an old-fashioned incandescent bulb, their vaunted efficiency nosedives. It's called LED droop, and it's a real drag on the industry. Now, researchers have found a way to grow more efficient LEDs that get more kick from the same amount of current—especially in the hard-to-manufacture green and blue parts of the spectrum.
LEDs look like a sandwich, with layers of semiconductors slapped between metal electrodes. When a voltage is applied between the electrodes, negatively charged electrons and positively charged "holes" are pushed into the middle of the sandwich and combine, giving off light.
It sounds straightforward. But there's often a problem. In blue and green LEDs, the semiconductor layer at the center is typically made from gallium nitride. As the current passing through the LED increases, the diode's gallium nitride layer generates its own electric field. This second field pushes apart positive and negative charges rushing in from the electrodes and prevents them from combining and giving off light. The larger the current grows, the bigger the second field gets, and the harder it is for electrons and the holes to combine. The efficiency goes down. This droop quickly becomes so bad that LEDs lose their cost advantage over the far less efficient but cheaper-to-manufacture fluorescent and regular incandescent bulbs.
Now, materials scientist Yuji Zhao and colleagues at the University of California, Santa Barbara, have figured out a way to minimize the efficiency-sucking electric field. Gallium nitride for LEDs is usually grown on sapphire or silicon carbide substrates. Unfortunately, the easiest way to grow it—and the way every commercial supplier does it—encourages the gallium nitride crystals to form along a "polar" orientation that maximizes the efficiency-sucking electric field.
Zhao's group obtained a special substrate from Mitsubishi Chemical Corporation. This substrate, itself made of gallium nitride, encouraged the gallium nitride diodes to grow in a very specific crystalline orientation, close to but different than the polar orientation usually used. It eliminated most of the unwanted electric field. Other teams have made diodes with a nonpolar and semipolar orientation, but none of them have been as successful at reducing droop. Zhao's team created a blue LED with a peak efficiency of 52% at 20 mA. Even when the current is increased by 10 times that, the efficiency drops by less than 14%. A typical LED would lose more than 60% of its efficiency with that much current pumping through it. The team will present its research on 10 May at the Conference on Lasers and Electro-Optics in San Jose.
"This is a very promising solution that could bring down the cost of LEDs," says Emmanouil Kioupakis, a computational materials scientist at the University of Michigan, Ann Arbor. He says that Zhao's results are not just a direct effect of the diminished unwanted electric field, but also because the reoriented gallium nitride reduces unwanted electron-hole interactions that produce heat instead of light.
Right now, the specially prepared substrate Zhao and colleagues used is expensive and hard to get. But the potential market is huge: Efficient LEDs capable of lighting rooms would appeal to people both in developed countries who want more efficiency, and in developing countries who want to run a light on limited power.