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.