Nanowires Can Be Tuned to Range of Wavelengths for Optoelectronics

A research team at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in collaboration with researchers at Technische Universität (TU) Dresden and Deutsches Elektronen-Synchrotron (DESY) in Hamburg, has produced nanowires with operating wavelengths that can be tuned over a wide range by altering the structure of the nanowire’s shell. Fine-tuned nanowires could take on several roles in an optoelectronic component, making the component more powerful, cost-effective, and easier to integrate.

The researchers grew gallium arsenide nanowires epitaxially on silicon substrates. Then they enclosed the wafer-thin wires in another layer of material to which they added indium. The mismatched crystal structure of the materials was intended to induce a mechanical strain in the wire core that would change the electronic properties of the gallium arsenide — for example, cause the bandgap to become smaller or the electrons to become more mobile. To magnify this effect, the scientists kept adding indium to the shell, increasing the shell’s thickness.

Cross-section of a nanowire featuring a gallium arsenide core, an indium aluminum arsenide shell, and an indium gallium arsenide capping layer (gallium is shaded blue, indium red, and aluminum cyan). The image was produced by energy-dispersive x-ray spectroscopy. Courtesy of HZDR/R. Huebner.

The gallium arsenide core sustained unusually large tensile strain, and the magnitude of the strain could be engineered by varying the composition and thickness of the shell. At this level of strain, the researchers expected to see disorders occurring in the semiconductors — defects or bends in the wire core, for example. They believe that the experimental conditions were the reason for the absence of such disorders: First, they grew extremely thin wires (around 5000 times finer than a human hair). Second, they produced the wire shell at very low temperatures. The surface diffusion of atoms was more or less frozen, forcing the shell to grow evenly around the core.

“What we did was take a known effect to extremes,” researcher Emmanouil Dimakis said. “The 7% of strain achieved was tremendous.” The team confirmed its discovery by conducting several independent series of measurements at facilities in Dresden and at the high-brilliance x-ray light sources PETRA III in Germany and Diamond in England.

The researchers next examined what triggered the extremely high strain in the nanowire core, and how this could be applied. They found that the high strain let them shift the bandgap of the gallium arsenide semiconductor to very low energies, making it compatible even for wavelengths of fiber optic networks — a spectral range that could previously only be achieved using alloys containing indium. The nanowires exhibited a reduction of their bandgap by up to 40% when overgrown with lattice-mismatched indium gallium arsenide or indium aluminium arsenide shells.

The researchers believe that the resulting bandgap reduction could make gallium arsenide nanowires suitable for photonic devices across the near-infrared (NIR) range, including telecom photonics at 1.3 μm and potentially 1.55 μm, with the additional possibility of monolithic integration in silicon-CMOS chips. “Scientists have been aware of gallium arsenide as a material for years, but nanowires are special. A material may exhibit completely new properties at the nanoscale,” Dimakis said.

The research was published in Nature Communications (http://dx.doi.org/10.1038/s41467-019-10654-7).