One of the biggest challenges in creating a better light-emitting diode (LED) is the search for a way to efficiently extract the light generated in the semiconductor device into the surrounding air, while avoiding the internal light reflection that is cause for a considerable waste of energy. A team of Japanese researchers have recently managed to achieve just that, in what is believed to be a huge step toward significantly more energy-efficient LEDs.
All of the materials currently used for the production of LEDs are characterized by a high refractive index. Air, by contrast, has a very low refractive index. According to the laws of optics, this means that when light is extracted from the one to the other, a vast portion of the light emitted by the semiconductor will be inevitably reflected back into the semiconductor, where it usually degenerates into heat.
The portion of light that is wasted is indeed very substantial. When a light-emitting semiconductor is deposited on a flat surface, it is only possible to extract a small percentage (~2 percent with gallium arsenide, ~4 percent with gallium nitride) of the total light generated. Researchers have therefore gone out of their way to avoid this waste of energy by trying to maximize the portion of the light that is released outward, leading to higher-efficiency devices.
Some of the strategies used so far to counter the unwanted reflection include embedding the device in a hemispheric package, so that the light rays strike its surface perpendicularly; the addition of an anti-reflective coating; designing the LED so that it will reabsorb and re-emit the reflected light (a process known as photon recycling); creating random roughness in the reflective surface with so-called moth-eye patterns; and even using nano-imprint lithography to create billions of tiny holes in the device to allow more photons out.
While all these techniques have led to some improvement, the one developed by the Japanese research team is by far the most promising yet, as it allows for an astounding 50 percent of the light to be extracted. They managed to do so by fabricating narrow ridges on the semiconductor surface, and then coating them with a layer of silicon dioxide (SiO2), whose refractive index is lower than that of the semiconductor.
The improved efficiency is due to the double coupling of so-called "evanescent waves" – a special kind of light existing only in the vicinity of the reflection interface – generated at two interfaces. Two evanescent waves are generated symmetrically on the two sidewalls of a ridge upon the total reflection of light. Coupling of these two evanescent waves occurs when they meet at the flat plane at the top of the ridge, and allows the evanescent waves to be efficiently transformed into light that propagate into the air.
The results were obtained on GaAs/AlGaAs-based materials, which emit light outside the visible spectrum. The team is now working on enhancing light-extraction efficiency in materials used in visible LEDs, such as AlGaInP-based and GaN-based materials, and to develop visible LEDs capable of high-efficiency light extraction.
The research was carried out by the National Institute of Advanced Industrial Science and Technology (AIST) and its Nanosystem Research Institute. The study was in part financed by the Japan Society for the Promotion of Science.
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