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Annealing Induced Dissociation of N 2 Formed in Indium Implanted GaN
M. Katsikini 1* , E. C. Paloura 1 , F. Boscherini 2 , E. Wendler 3 , W. Wesch 3 .
1 Aristotle University of Thessaloniki, School of Physics, Section of Solid State Physics, 54124 Thessaloniki, Greece
2
Department of Physics and CNISM, University of Bologna, viale C. Berti Pichat 6/2, 40127 Bologna, Italy
3
Institut fur Festkörperphysik, Friedrich-Schiller Universität Jena, Max-Wien-Platz 1, 07743, Germany
*katsiki@auth.gr
InGaN/GaN heterostructures find applications in the fabrication of high efficiency light emitting diodes as well as in high –
frequency, high – power microelectronic devices [1]. Ion implantation followed by annealing is a promising method for the
growth of nanomaterials [2,3], like InGaN quantum dots, which enhance the emission characteristics of InGaN – based laser
diodes [4], and InGaN/GaN heterostructures. For example, high dose Ge implantation in Si has been proposed as a method
for the fabrication of Si x Ge y /Si heterostructures [5]. Although ion implantation has the advantage of precise dose and profile
control, it causes lattice damage which can be recovered via annealing. Here, we apply near edge x-ray absorption fine
structure (NEXAFS) spectroscopy at the N K edge, in order to determine the implantation induced changes in the electronic
structure of GaN samples. XAFS spectroscopies are ideally suited for the study of highly defective or amorphous materials,
such as implanted samples, since the main mechanism involved is the backscattering of the outgoing photoelectron wave
from the neighboring atoms which does not depend on the periodicity of the lattice. More specifically, we focus on the
identification and thermal stability of N 2 gaseous bubbles that have been proposed [6] as an implantation induced defect in
ion implanted GaN.
The 450nm – thick – GaN sample was grown on Al 2 O 3 by Plasma Enhanced Molecular Beam Epitaxy using an AlN
buffer layer [7]. The sample was diced and the
individual pieces were implanted at 77 K with
700 keV In ions with a fluence of 5 × 10 15 cm -2 .
Rutherford backscattering characterization has
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shown that a 200 µm thick amorphous layer is
formed on the surface of the sample, while the
remaining GaN sample is severely defected.
as grown
The implanted samples were subjected to rapid
thermal annealing (RTA) at 800 o and 900 o C
for 15 sec. The N K edge NEXAFS
3
ann. (900 o C)
measurements were conducted at the ALOISA
undulator beamline of the Synchrotron
Radiation facility ELETTRA in Trieste. The
spectra were recorded in the fluorescence yield
ann. (800 o C)
mode using a high purity Ge detector cooled at
RL
77K. In order to avoid polarisation effects, the
2
angle of incidence was equal to the magic
as implanted
angle (55 o to the sample surface). The
information depth of the fluorescence photons
is approximately equal to 40nm.
The N K edge NEXAFS spectra of the
as – implanted and the annealed samples are
shown in Fig. 1. In the same Figure the
spectrum of N 2 , which fills an ionization
chamber (2×10 -8 mbar) positioned in front of
the sample, is also included. As shown in the
Figure, implantation smears out the NEXAFS
peaks that characterize the spectrum of the asgrown
sample while at the same time causes
the evolution of a sharp resonance line (RL),
which appears above the absorption edge. The
energy center of the RL coincides with the one
which corresponds to 1s π * transitions of
N 2 , as it is evident in Fig. 1. Thus, the
implantation induced RL can be attributed to
the implantation induced formation of N 2 .
Similar RLs have been reported in the
Intensity (arb. units)
1
0
390 400 410 420 430 440
Energy (eV)
literature in a number of occasions. Katsikini et al reported the evolution of a sharp RL above the absorption edge in the
fluorescence spectra of GaN implanted with O, N, Mg and Si [8]. The RL was attributed to transitions to final states related
to N dangling bonds due to the independency of its energy position and full width at half maximum from the nature of the
projectile and its similarity with the RL observed in the electron energy loss spectra recorded from the core of threading
dislocations [9]. Furthermore, a similar RL, attributed to 1s π* transitions of N 2 trapped in the semiconductor, has been
detected in the surface sensitive electron yield spectra of bulk semiconductors bombarded with low energy N 2 + [10] and in N
N 2
Figure 1: N K edge NEXAFS spectra of the as-grown, as-implanted
and annealed GaN:In samples. The spectrum of molecular nitrogen is
also included in the figure.
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