III-V semiconductor materials have the best external quantum efficiency (EQE) in the blue range of the visible spectrum and thus show promise for use in solid-state lighting. Indium gallium nitride/gallium nitride (InGaN/GaN)-based light-emitting diodes, for example, have a high luminance of 106 cd/m2 and an EQE of around 80%. They also last for more than 105 hours, which is much longer than other common light sources.

The performance of LEDs across the whole visible range could be improved by employing III-nitride-based nanowires (NWs) because they have outstanding optoelectronic properties thanks to their high surface-to-volume aspect ratio. Another of their good points is that they can withstand mechanical strain without deforming. Since these mechanical properties are closely connected with optoelectronic characteristics, researchers would like to find out more about the inner structure of NWs.

Bragg X-ray ptychography images lattice distortions in thin films

One way to do this is using electron diffraction, which is a powerful tool to characterize nanostructures on the atomic scale. Scanning electron microscopy can provide detailed information of the surface of an NW, while transmission electron microscopy resolves the atomic structure of a crystal. However, the problem is that the low penetration depth of electrons means that NWs must be sliced and prepared outside the environment in which they are grown to be able to study the entire length of the samples.

X-ray-diffraction-based methods overcome the limitations of electron microscopy as X-rays penetrate deeper into a material than electrons, so samples do not need to be sectioned and destroyed. Researchers, led by Ivan Vartanyants of the Coherent X-Ray Scattering and Imaging Group at DESY, have now used one such technique, Bragg X-ray ptychography, to study how strain is distributed within single InGaN/GaN NWs. The technique can image lattice distortions in thin films non-destructively at spatial resolutions of less than 20 nm using coherent nanofocused hard X-rays.

Seeing how strain is distributed in a nanowire

“We performed the experiment at the PETRA III synchrotron radiation source at DESY,” explains Vartanyants. “We scanned the nanofocused X-ray beam of the instrument over a single free-standing core-shell nanowire and reconstructed the diffraction patterns obtained during the measurement to obtain essential information about the internal structure of the sample. This information was enough to build a 3D model of the nanowire and learn how strain was distributed in the wire.”

The technique allowed the researchers to determine the shape of the nanowire with the strain inside it with high spatial resolution in 3D. “Alternative techniques, such as scanning X-ray diffraction microscopy, can provide similar information but with much lower resolution, defined by the size of the X-ray beam,” says Vartanyants. “And as mentioned, although electron microscopy provides high resolution, for 400 nm-sized samples it requires destructive sample preparation – like sectioning.”

Towards more efficient LEDs

The new work could help in the development of more efficient LED in the future, he tells nanotechweb.org. “We now know that the efficiency of LEDs drastically drops at the wavelength from green to yellow (this is called the ‘green gap’ problem) and our research allows us to better understand the core-shell nanowire structure and how it affects the performance of nanowire-based LEDs at these wavelengths. Our results provide essential structural information that could be used to tune growth parameters when manufacturing nanowires and could allow the efficiency of nanowire-based LEDs to be tuned. The energy they consume and their cost could drop a lot as a result.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.6b08122, says that it will now be looking at how functional devices based on NWs operate in real-world conditions. “We also plan on in situ and in operando studies of single NWs under an applied voltage to follow how strain evolves inside a sample in this context,” reveals Vartanyants.