From the high-resolution glow of flat screen televisions to light bulbs that last for years, light-emitting diodes (LEDs) continue to transform technology. The celebrated efficiency and versatility of LEDs and other solid-state technologies including laser diodes and solar photovoltaics make them increasingly popular. Their full potential, however, remains untapped, in part because the semiconductor alloys that make these devices work continue to puzzle scientists.
A contentious controversy surrounds the high intensity of one leading LED semiconductor—indium gallium nitride (InGaN)— with experts split on whether or not indium-rich clusters within the material provide the LED's remarkable efficiency. Now, researchers from the Massachusetts Institute of Technology (MIT) and the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have demonstrated definitively that clustering is not the source. The results—published online May 16 in Applied Physics Letters—advance fundamental understanding of LED technology and open new research pathways.
Incandescent lights, the classic bulbs that use glowing wires of tungsten or other metals, convert only about five percent of their energy into visible light, with the rest lost as heat. Fluorescent lights push that efficiency up to about 20 percent, still wasting 80 percent of the electricity needed to keep homes and businesses bright. In both of these instances, light is only the byproduct of heat-generating reactions rather than the principal effect, making the technology inherently inefficient.
Solid-state lights, on the other hand, convert electric current directly into photons through a process called electroluminescence. The efficiency of this process could, in theory, be nearly perfect, but the experimental realization has not reached those levels. That prompted scientists to look at the LED compound InGaN (pronounced in-gan), which is particularly promising for practical applications. InGaN alloys contain dislocations, structural imperfections that could inhibit electricity flow and light production. Nevertheless, the alloy performs exceptionally well. To understand the light-emitting reactions, physicists needed to understand what was happening on the atomic scale. After researchers started to investigate, however, not everyone reached the same conclusions.
According to Eric Stach, leader of the Electron Microscopy Group at Brookhaven Lab's Center for Functional Nanomaterials (CFN), a team of researchers some years ago used electron microscopes to examine InGaN samples, and they discovered that the material appeared to be spontaneously decomposing and forming isolated indium-rich clusters. It was initially theorized that this behavior could explain the efficient light emission, as the clusters might help electrons avoid the structural problems in the InGaN. But things became really interesting when another group proposed that the electron microscope itself caused the clustering decomposition.
Rather than using light to examine materials, electron microscopes bombard samples with finely tuned beams of electrons and detect their interactions when they pass through a sample to reveal atomic structures. To achieve high enough resolution to examine the InGaN alloys, the electron microscopes used in the older experiments needed high-voltage beams. The controversy revolved around whether or not the experiment itself produced the clusters, rather than discovering the mechanism behind efficient light emission.
The researchers at Brookhaven Lab’s Center for Functional Nanomaterials used the Center’s aberration-corrected scanning transmission electron microscope (STEM), with its ability to provide Ångstrom-level details (one tenth of one nanometer), to resolve the controversy. The researchers combined the leading STEM techniques with high-resolution electron energy loss spectroscopy (EELS), which measured the energy lost by electrons as they passed through the sample. Post-doctoral researchers Kamal Baloch of MIT and Aaron Johnston-Peck of CFN actually applied these imaging techniques to the same samples that first launched the controversy over clustering, helping further settle the issue.
What they found is that the indium-rich clusters do not actually exist in these samples, even though they remain efficient light emitters. Although clustering might still occur in other samples that have been prepared in different ways, the important point is that the team established a foolproof method for investigating InGaN materials. They can now use these non-destructive imaging techniques to explore the fundamental relationship between cluster formation and light emission to help unlock the secrets of InGaN.
Beyond the advanced imaging instruments, researchers used the expertise of Brookhaven Lab physicist Kim Kisslinger, who specializes in nanoscale sample preparation. The InGaN samples were reduced to a thickness of just 20 nanometers, an essential step in priming the materials for STEM and EELS experimentation. The samples were also painstakingly cleaned and polished to eliminate artifacts that might impact image resolution.