Spontaneous formation of GaN/AlN core–shell nanowires on sapphire by hydride vapor phase epitaxy

•Spontaneous GaN/AlN core–shell nanowires were synthesized on sapphire substrates by HVPE.•Aluminum is obtained by etching out the sapphire substrate by H2 or HCl.•The GaN/AlN structures are free of defects and stacking faults.•A kinetic model leads to the separation of the VLS growth of the GaN core and the VS growth of the AlN shell.

Spontaneous GaN/AlN core–shell nanowires with high crystal quality were synthesized on sapphire substrates by vapor–liquid–solid hydride vapor phase epitaxy (VLS-HVPE) without any voluntary aluminum source. Deposition of aluminum is difficult to achieve in this growth technique which uses metal-chloride gaseous precursors: the strong interaction between the AlCl gaseous molecules and the quartz reactor yields a huge parasitic nucleation on the walls of the reactor upstream the substrate. We open up an innovative method to produce GaN/AlN structures by HVPE, thanks to aluminum etching from the sapphire substrate followed by redeposition onto the sidewalls of the GaN core. The paper presents the structural characterization of GaN/AlN core–shell nanowires, speculates on the growth mechanism and discusses a model which describes this unexpected behavior.