Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs