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. 
D. Gonzalez, R. Fath, T. Ben, and R. Songmuang
cond-mat.mtrl-sci