Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Photoluminescence from surface GaN/AlGaN quantum wells: Effect of the
  surface states   We report on photoluminescence (PL) measurements at 85 K for GaN/AlGaN
surface quantum wells (SQW's) with a width in the range of 1.51-2.9 nm. The PL
spectra show a redshift with decreasing SQW width, in contrast to the blueshift
normally observed for conventional GaN QW's of the same width. The effect is
attributed to a strong coupling of SQW confined exciton states with surface
acceptors. The PL hence originates from the recombination of
surface-acceptor-bound excitons. Two types of acceptors were identified.

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The categories of this paper are: q-bio
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The categories of this paper are: stat
29
The categories of this paper are: funct-an
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
26
The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
19
The categories of this paper are: eess
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
7
The categories of this paper are: hep-th