  Growth of catalyst-free AlxGa1-xN (0.15<x<0.50)/GaN nanowires by plasma
assisted molecular beam epitaxy is thoroughly structural and chemical analyzed
by using transmission electron microscopy related techniques. We found that
well-defined and defect-free core-shell structures are spontaneously formed
during the wire growth. An Al-rich shell with significantly higher Al
composition pseudomorphically encapsulates a Ga-rich AlxGa1-xN core with an
atomically-abrupt hetero-interface. Nevertheless, the energy dispersive X-Ray
spectroscopy reveals a complex chemical composition gradient along the wire
axis for both core and shell blocks which is ascribed to the adatom surface
kinetic differences and the shadow effect during the growth.

Materials Science