Materials Science and Engineering B82 (2001) 9 – 11
Structural quality and ordering of MBE grown Al
x
Ga
1 - x
N-layers
L. Kirste
a
, D.G. Ebling
a,
*, Ch. Haug
b
, R. Brenn
b
, K.W. Benz
a
, K. Tillmann
c
a
Freiburg Materials Research Centre, Uniersity of Freiburg, Stefan -Meier -Str. 21, D-79104 Freiburg, Germany
b
Faculty of Physics, Herman -Herder -Str., D-79104 Freiburg, Germany
c
Centre for Microanalysis, Uniersity of Kiel, Kaiserstr 2, D-24143 Kiel, Germany
Abstract
Epitaxial Al
x
Ga
1 -x
N-layers (x =1–0) with the hexagonal wurzite structure were grown on 6H-SiC- and -Al
2
O
3
-substrates for
the development of semiconductor UV detectors. The experiments were performed in a plasma enhanced molecular beam epitaxy
system (MBE) by varying substrate temperature, III/V ratio and growth rates. A detailed analysis of dislocations was performed
for pure AlN-layers by RBS-channeling and was compared to the analysis of cross sectional and plan view TEM images. The
annihilation of threading dislocations during the growth process was observed by RBS-channeling depending on the type of
substrate and the growth mechanism. Lowest dislocation densities are obtained for 2D-growth on SiC substrate in the range of
2 ×10
8
cm
-2
. The decrease of the growth temperature from 1000 to 900°C lead to an decrease of the dislocation density by about
an order of magnitude. Depending on the growth conditions chemical ordering was observed for ternary Al
x
Ga
1 -x
N-layers. The
space group symmetry P6
3
mc of the wurzite structure is reduced to P3m1 due to the ordering of the group III sublattice into
alternating aluminium and gallium layers. The phenomenon was studied by XRD-measurements of the symmetric superlattice
peaks. © 2001 Elsevier Science S.A. All rights reserved.
Keywords: AlN; RBS-channeling; Al
x
Ga
1 -x
N ordering; Dislocation density
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1. Introduction
The continuous alloy Al
x
Ga
1 -x
N offers a wide field
of applications due to a bandgap from 3.4 (GaN) to 6.1
eV (AlN) ranging from optical UV filters and insulating
layers to optoelectronic devices like LED’s, laser diodes
or detectors [1]. A basic problem for device fabrication
is the usually high dislocation density of the layers in
the range of up to several 10
10
cm
-2
. It is shown in
reference [2] that the free carrier concentration and
their mobility are affected seriously by the dislocation
density. A 2D epitaxial growth along monoatomic steps
is suitable to obtain high crystal quality. To reduce the
amount of threading dislocations (TD) homogeneously
their formation process and the corresponding growth
conditions have to be studied in detail. Some results are
reported for the growth of GaN [3] but only little has
been published for the growth of AlN. The system
Al
x
Ga
1 -x
N offers the possibility if bandgap engineer-
ing by changing the composition. It was recently shown
that depending on the growth condition superstructures
of the wurzite structure can be observed for hexagonal
Al
x
Ga
1 -x
N-layers [4].
2. Experimental
The Al
x
Ga
1 -x
N-layers (x =1–0) were grown by
molecular beam epitaxy (MBE) on (0001) 6H-SiC- and
(0001) -Al
2
O
3
-substrates in a Riber P32 machine
equipped with gallium and aluminium effusion cells.
Nitrogen radicals were produced by the OAR CARS25
RF-Plasma source with a nitrogen flow rate of 1.4 sccm
and a RF-power of 440 W. The substrate temperature
was varied between 900 and 1050°C for the growth of
AlN and 750–800°C for the growth of Al
x
Ga
1 -x
N. A
350 nm AlN buffer layer was deposited prior to growth
of the alloy films. Ordering in Al
x
Ga
1 -x
N was
analysed by measuring the intensity relation of /2
scans of the 001 superlattice peaks and the 004 sublat-
tice peaks using a four circle X-ray diffractometer with
Cu-K
radiation. The thickness and the composition of
* Corresponding author. Tel.: +49-761-2034772; fax: +49-761-
2034700.
E-mail address: ebling@fmf.uni-freiburg.de (D.G. Ebling).
0921-5107/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved.
PII:S0921-5107(00)00770-4