Spontaneous formation of well-defined Al rich shell structures in AlxGa1-xN/GaN nanowires

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.