Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil

(In,Ga)N/GaN nanowire ensembles with a tunable bandgap from the UV to the 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 path to the realization of devices with improved performances and functionalities such as bendable displays, solar cells, and flexible piezogenerators. 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 was unclear whether ensembles of single–crystalline and well–oriented GaN nanowires could be synthesized on metal foils. In this work we prove this is actually possible, showing a path for the realization of bendable GaN nanowire–based electronic and optoelectronic devices using a transfer–free approach.

Figure 1. (a) Photograph of the Ti foil after nanowire growth demonstrating a high degree of flexibility. [(b) and (c)] Scanning electron micrographs of the GaN nanowire ensemble grown on the Ti foil taken in bird’s eye view with (b) low and (c) high magnification. The red square in (a) is not to scale.

Figure 2. (a) Low temperature (9 K) photoluminescence spectrum of the GaN nanowire ensemble grown on the Ti foil. The corresponding spectrum of a GaN nanowire ensemble prepared on Si(111) is included for comparison. The collected spectra, normalized and shifted vertically for clarity, indicate that nanowires grown on Ti foils exhibit a similar crystalline perfection, a lower degree of coalescence, a higher concentration of stacking faults, and a reduced density of inversion domain boundaries than those prepared on Si substrates. (b) Room-temperature photoluminescence spectra of the GaN nanowire ensemble grown on the Ti foil for an infinite and a convex curvature of the substrate with a 4mm radius. The collected spectra indicate that as–grown nanowires are well anchored to the Ti foil and do not degrade upon substrate bending. The inset shows a photograph of the bent Ti foil taken during the acquisition of the experimental data.

In this work, we demonstrate the selfassembled growth of an ensemble of single crystalline, uncoalesced, and vertically aligned GaN nanowires on a flexible Ti foil using plasmaassisted molecular beam epitaxy. The structural and optical properties of the sample, investigated by transmission electron microscopy and photoluminescence spectroscopy, are compared with those of standard GaN nanowire ensembles prepared on Si(111) substrates. We find that both the structural perfection and the low temperature photoluminescence spectra of the nanowire ensembles prepared on Ti foils and Si substrates are fairly comparable. Furthermore, we do not observe any degradation of the luminescence upon bending the nanowire ensemble prepared on the foil down to a small curvature radius of 4mm. Therefore, a GaN nanowire ensemble on a Ti foil is indeed a highly flexible system suitable for the realization of bendable GaN-based devices. The present results pave the way for the realization of flexible GaN nanowire-based electronic and optoelectronic devices on metal foils.

1 Author G. Calabrese , P. Corfdir , G. Gao , C. Pfüller , A. Trampert , O. Brandt , L. Geelhaar , S. Fernández-Garrido

Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil

Source Appl. Phys. Lett. , 108 , 202101 ( 2016 )
DOI : 10.1063/1.4950707 | Download: PDF | 2794 Cite : Bibtex RIS
G. Calabrese, P. Corfdir, G. Gao, C. Pfüller, A. Trampert, O. Brandt, L. Geelhaar, and S. Fernández-Garrido