Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: MOCVD growth of N-polar GaN on on-axis sapphire substrate: impact of AlN
  nucleation layer on GaN surface hillock density   We report on the impact of growth conditions on surface hillock density of
N-polar GaN grown on nominally on-axis (0001) sapphire substrate by metal
organic chemical vapor deposition (MOCVD). Large reduction in hillock density
was achieved by implementation of an optimized high temperature AlN nucleation
layer and use of indium surfactant in GaN overgrowth. A reduction by more than
a factor of five in hillock density from 1000 to 170 hillocks/cm$^{-2}$ was
achieved as a result. Crystal quality and surface morphology of the resultant
GaN films were characterized by high resolution x-ray diffraction and atomic
force microscopy and found to be relatively unaffected by the buffer
conditions. It is also shown that the density of smaller surface features is
unaffected by AlN buffer conditions.

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The categories of this paper are: adap-org
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The categories of this paper are: gr-qc
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The categories of this paper are: acc-phys
29
The categories of this paper are: funct-an
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The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
14
The categories of this paper are: math-ph
19
The categories of this paper are: eess
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The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
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The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
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The categories of this paper are: stat
36
The categories of this paper are: cmp-lg