  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.
