The best-known bottom-up method resulting in uniaxial growth of nanowires utilizes the vapor-liquid-solid mechanism in which the formation of a solid nanowire from the vapor phase is mediated by a tiny liquid metal droplet. We employ this approach (using Ga as the metal) for the synthesis of group-III-arsenide nanowires on Si(111) substrates. Selective area vapor-liquid-solid growth is readily obtained by patterning these substrates by electron beam lithography and reactive ion etching, leading to ordered nanowire arrays.

An alternative bottom-up approach exists for materials that exhibit the tendency to form nanowires under suitable growth conditions spontaneously, i.e., without any external guidance. GaN is one of these materials, and is prone to the spontaneous formation of nanowires on a wide variety of substrates, including amorphous and crystalline insulators, semiconductors, and metals. We are currently focusing on the growth of group-III nitride nanowires on metallic substrates, and in particular on TiN – both as sputtered film and flexible foil – and graphene. In either case, ensembles of vertical GaN nanowires with excellent structural and optical properties can be obtained.

Spontaneously formed GaN nanowires invariably grow along the [000-1] direction, i.e., they are N polar. In order to synthesize Ga-polar group-III-nitride nanowires, we utilize top-down processes. In particular, we fabricate ordered arrays of nanowires from high-quality group-III-nitride layers and heterostructures either by selective area sublimation or by a combination of dry and wet chemical etching. This complementary approach allows us to study phenomena depending on, for example, nanowire diameter in a systematic fashion.

These three different synthesis methods represent our platform for both fundamental investigations aimed at elucidating formation mechanisms and the fabrication of nanowire samples for dedicated studies of material properties and applications.


Selected publications

Author: M. Azadmand , T. Auzelle , J. Lähnemann , G. Gao , L. Nicolai , M. Ramsteiner , A. Trampert , S. Sanguinetti , O. Brandt , L. Geelhaar
Title: Self-assembly of well-separated AlN nanowires directly on sputtered metallic TiN films
Source: Phys. Status Solidi-Rapid Res. Lett. , 14 , 1900615 ( 2020 )
DOI: 10.1002/pssr.201900615

Author: S. Fernández-Garrido , T. Auzelle , J. Lähnemann , K. Wimmer , A. Tahraoui , O. Brandt
Title: Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation
Source: Nanoscale Adv. , 1 , 1893 ( 2019 )
DOI: 10.1039/c8na00369f

Author: H. Küpers , R. B. Lewis , A. Tahraoui , M. Matalla , O. Krüger , F. Bastiman , H. Riechert , L. Geelhaar
Title: The diameter evolution of selective area grown Ga-assisted GaAs nanowires in molecular beam epitaxy
Source: Nano Res. , 11 , 2885 ( 2018 )
DOI: 10.1007/s12274-018-1984-1 

Author: R. B. Lewis , P. Corfdir , J. Herranz , H. Küpers , U. Jahn , O. Brandt , L. Geelhaar
Title: Self-assembly of InAs nanostructures on the sidewalls of GaAs nanowires directed by a Bi surfactant
Source: Nano Lett. , 17 , 4255 ( 2017 )
DOI: 10.1021/acs.nanolett.7b01185

Author: S. Fernández-Garrido , M. Ramsteiner , G. Gao , L. A. Galves , Bharat Sharma , P. Corfdir , G. Calabrese , Z. de Souza Schiaber , C. Pfüller , A. Trampert , J. M. J. Lopes , O. Brandt , L. Geelhaar 
Title: Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene
Source: Nano Lett. , 17 , 5213 ( 2017 )
DOI: 10.1021/acs.nanolett.7b01196