Strain Measurement Optimizes Nanowire LEDs

Tiny defects can affect the performance of nanowire LEDs. Now researchers have a way to detect these defects and potentially correct them.

A team from the Niels Bohr Institute at the University of Copenhagen used x-ray microscopy to pinpoint exactly how the nanowires should be designed for optimal light output.

Nanowire LEDs produce more natural light and use much less power than other light sources and could see widespread use in as few as five years, the researchers said.

X-ray images of each nanowire show the distribution of the scattering intensity and the mechanical strain in the GaN core and InGaN shell. The strain shows that the shell fits perfectly with the core. Courtesy of Tomas Stankevic/Niels Bohr Institute.

The nanowires studied were about 2 μm high and 10 to 500 nm in diameter. Each featured a GaN inner core and InGaN outer layer.

"The light in such a diode is dependent on the mechanical strain that exists between the two materials, and the strain is very dependent on how the two layers are in contact with each other," said professor Robert Feidenhans'l. "We have examined a number of nanowires using x-ray microscopy, and even though the nanowires should in principle be identical, we can see that they are different and have very different structure."

The researchers studied nanowires at DESY, the electron synchrotron research center in Hamburg, Germany.

X-ray microscopy is usually very time consuming, and the results are often limited to very few study subjects. Here, the researchers managed to measure a series of upright nanowires simultaneously using nondestructive, nanofocused x-rays.

"We measured 20 nanowires, and when we saw the images, we were very surprised because you could clearly see the details of each nanowire," said doctoral student Tomas Stankevic. "You can see the structure of both the inner core and the outer layer. If there are defects in the structure, or if they are slightly bent, they do not function as well. So we can identify exactly which nanowires are the best and have the most efficient core-shell structure."

The research was published in ACS Nano (doi: 10.1021/acsnano.5b01291).

For more information, visit www.nbi.ku.dk.