Traditional LEDs are flat (planar) structures that are manufactured in much the same way as transistors. But if progress has been made in the production of transistors in the form of vertical channels (FinFETs), which allows high currents to pass through them, then no one has yet thought of “putting LEDs on edge” so that they shine brighter and work longer. However, the experiment has already been performed, and its results are encouraging.
The US National Institute of Standards and Technology (NIST) reported that under the leadership of its scientists, a team of researchers from the University of Maryland, Rensselaer Polytechnic Institute and IBM Research Center. Thomas J. Watson designed and tested the unusual structure of the smallest LEDs. The team’s proposed LED structure, tested in the laboratory, provided “Sharp” an increase in the brightness of the glow and even made it possible to turn the LED into a semiconductor laser without any additional tweaks (resonators, etc.).
Compared to ordinary tiny LEDs of submicron sizes, the solution proposed by the scientists showed an increase in brightness from 100 to 1000 times. This is incredible, although the scientists used the same materials as in the production of mass super-bright LEDs (gallium nitride, zinc oxide, titanium, gold). “We use the same materials as in conventional LEDs. Our difference [заключается] in their form “– said the project leader, NIST scientist Babak Nikoobakht.
Up to a certain point, applying more electricity to the LED makes it glow brighter, but soon the brightness drops, making the LED very inefficient. This problem, dubbed “efficiency degradation” by the industry, is hindering the use of LEDs in a number of promising applications, from communications technology to virus killing.
Interestingly, initially, the NIST scientists did not set themselves the task of breaking through the low efficiency barrier of LEDs. Their main goal was to create a microscopic LED for use in very small applications such as lab-on-a-chip technology, which is being developed by scientists from NIST and other organizations. But it turned out better and not at all there (see the article in Science Advances for more details).
While experimenting with a completely new design of the LED’s glowing part, the scientists constructed a light source from long thin filaments of zinc oxide, which they call fins. Each fin is only about 5 micrometers long, which is about a tenth the width of an average human hair. The length of the fins or ribs can reach one centimeter or more, which will set the characteristics of these devices.
As in the case of FinFET transistors, the channels of which are surrounded by a gate on three sides, the vertical luminous element of the LED made it possible to supply current to it through the side faces as well. The experiment literally shook the scientists. The vertical structure emitted light with a power of up to 20 µW, while a planar LED of a similar area could emit no more than 22 nW or 100–1000 times weaker.
The second surprise was that when the current increased, the luminescence of a “vertical” LED in the range of a wide spectrum of waves at the border of violet and ultraviolet turned into radiation of only two waves in the range of intense ultraviolet light. Thus, the LED actually turned into a semiconductor laser with a simple movement of the hand.
The tiny laser will be critical for chip-scale applications, not only for chemical sampling (analysis), but also for next-generation handheld communications devices, high-resolution displays and surface decontamination.
“It has great potential to become an important building block“– said Nikobakht. “Although not the smallest human-made laser, it is very bright. The lack of efficiency drop can make it useful. “
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