A team of researchers from Lund University and Northwestern University in the United States have used the nano focused beam at the NanoMAX beamline to construct a 2D map of the distribution of material strain in individual InP-GaInP heterostructure nanowires. Understanding the strain that forms in heterostructure nanowires is essential for tailoring their electronic properties to applications in electronics and for energy materials.
Semiconductor materials are essential for everything from electronics such as computers and mobile phones to LED-lights and solar cells. Different types of semiconductor materials often need to be combined in a so-called heterostructure to realise the advanced functions required for these devices.
Typically the combination is done by growing layers of one semiconductor material on top of another. However, since the distances between the atoms, the lattice spacing, is different in the different materials, it often leads to mismatch and strain in the materials when they are combined in this way. The mismatch puts a limit on what materials are possible to mix and how thick the layers can be.
Nanomaterials offer advantages
Nanowires, tiny fibres with a diameter of tens of nanometers, are increasingly used as the active structures in modern electronics. They offer miniaturisation but also have an advantage when it comes to combining materials. The small diameter allows the strain to relax perpendicular to the wires axial direction. The possibility for strain relaxation means that even material combinations with a relatively large mismatch are possible without the materials getting too distorted.
When the material combination results in strain to a semiconductor material, its electronic properties change. So, to be able to combine the materials we want and get the desired properties for the device, we need to understand what type of strain will form.
“We image strain in nanowires to see structural changes in the lattice, which has an impact on the transport properties of electronic devices built from nanowires, such as transistors,” says first author Susanna Hammarberg, PhD candidate at Synchrotron Radiation Research and Nano Lund at Lund University.
X-ray diffraction is a perfect fit
X-ray diffraction is an excellent tool for measuring lattice spacings and strain. With a small, focused spot of X-ray light, it’s possible to examine the segments and interfaces within an individual nanowire. The researchers used the beamline NanoMAX for their experiments.
“We needed the high coherent flux and the focusing abilities of NanoMAX. We also required incredible precision in where the beam hits the nanowire under rotation and translation,” explains Hammarberg.
The researchers studied nanowires consisting of a combination of the semiconductor materials indium phosphide, InP, and gallium indium phosphide, GaInP. Scanning X-ray diffraction resulted in high-resolution 2D strain maps of the heterostructured nanowire.
“We’re excited that we managed to image strain and lattice tilt of an axially heterostructured nanowire with 0.01% relative strain sensitivity and 90 nm spatial resolution, says Hammarberg. “We also found nice agreement with finite element modelling simulations.”
The measurements confirm that the strain relaxes at the surface of the wire. The research team also found that the length of the material segments has a significant impact on the distribution of strain in the wire.
“In future experiments, we will go to 3D imaging, to be able to resolve strain in all dimensions. In an experiment we did, after the one presented in this paper, we performed Bragg ptychography to do just that and with a higher resolution than before,” concludes Hammarberg.
Hammarberg, S., Dagytė, V., Chayanun, L., Hill, M. O., Wyke, A., Björling, A., Johansson, U., Kalbfleisch, S., Heurlin, M., Lauhon, L. J., Borgström, M. T., and Wallentin J. (2020). High resolution strain mapping of a single axially heterostructured nanowire using scanning X-ray diffraction. Nano Res. 13, 2460–2468.