German team grows GaN nanowires on flexible titanium foil
Figure 1. (a) Foil after nanowire growth demonstrating a high degree of flexibility. [(b) and (c)] Scanning electron micrographs of the GaN nanowire ensemble taken in bird's eye view with (b) low and (c) high magnification. The red square in (a) is not to scale.
Researchers at the Paul-Drude Institut fur Festkorperelektronik, in Germany, have demonstrated the self-assembled growth of an ensemble of single crystalline, uncoalesced, and vertically aligned GaN nanowires on a flexible titanium foil using plasma-assisted MBE.
(In,Ga)N/GaN nanowire ensembles with a tunable bandgap from UV to infrared are very promising building blocks for solid-state lighting, photovoltaics and solar to fuel energy converters. The direct integration of such nanostructures on bendable metal foils would open the way to making devices such as bendable displays, solar cells, and flexible piezogenerators, according to the researchers.
However, due to the polycrystalline nature of metal foils, their typically rough surface, and the common occurrence of surface reactions when semiconductors are directly grown on metallic substrates, it has been unclear whether ensembles of single-crystalline and well-oriented GaN nanowires could be synthesised on metal foils.
In this work, the researchers have proved that this is possible, showing a path for the realisation of bendable GaN nanowire-based electronic and optoelectronic devices using a transfer-free approach.
Having looked at the structural and optical properties of the sample (using transmission electron microscopy and photoluminescence spectroscopy), with those of standard GaN nanowire ensembles prepared on silicon (111) substrates, the team found both the structural perfection and the low temperature photoluminescence spectra of the ensembles comparable.
Furthermore, the researchers say they did not observe any degradation of the luminescence upon bending the nanowire ensemble prepared on the foil down to a small curvature radius of 4mm.
Their conclusion is that a GaN nanowire ensemble on a titanium foil is indeed a highly flexible system suitable for the realisation of bendable GaN-based devices, and the results could pave the way for making electronic and optoelectronic devices on metal foils.