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@ARTICLE{Talut:88666,
author = {Talut, G. and Grenzer, J. and Reuther, H. and Shalimov, A.
and Baehtz, C. and Novikov, D. and Walz, B. and DESY},
title = {{I}n situ observation of secondary phase formation in {F}e
implanted {G}a{N} annealed in low pressure ${N}_2$
atmosphere},
journal = {Applied physics letters},
volume = {95},
issn = {0003-6951},
address = {Melville, NY},
publisher = {American Institute of Physics},
reportid = {PHPPUBDB-12678},
pages = {232506},
year = {2009},
note = {© American Institute of Physics},
abstract = {The formation of secondary phases in Fe implanted GaN upon
annealing in low pressure N$_2$-atmosphere was detected by
means of in situ x-ray diffraction and confirmed by
magnetization measurements. A repeatable phase change from
Fe$_3$N at room temperature and Fe$_3$−$_x$N at 1023 K was
observed in situ. The phase transformation is explained by
the change of lattice site and concentration of nitrogen
within nitrides. The diffusion of Fe toward sample surface
and oxidation with increasing annealing cycles limits the
availability of secondary phase and hence the repeatability.
At high temperature GaN dissolves and Ga as well as Fe
oxidize due to presence of residual oxygen in the process
gas. The ferromagnetism in the samples is related to
nanometer sized interacting Fe$_3$−$_x$Ncrystallites.GaN
is a wide band gap semiconductor that has been studied
intensively in the last years because of its potential field
of applications like in optoelectronics, plasmonics, as well
as for high power electronics. By doping with transitional
metals like Fe it might also be a diluted magnetic
semiconductor (DMS) with a Curie temperature above room
temperature (RT) and could then be used for spintronics.1
There are many experimental studies reporting ferromagnetism
at RT in Fe doped GaN. In a real DMS, with magnetic atoms
randomly substituting cation sites, ferromagnetic coupling
is supposed to be due to the indirect exchange coupling
between magnetic impurities mediated by holes.1–4
Experimental observation of strong-coupling effects in a DMS
Ga$_1$−$_x$Fe$_x$Nwas reported by Pacuski et al.5 Robust
ferromagnetism in the region of insulator-to-metal
transition was predicted for high hole densities. However,
there are also other possible sources of ferromagnetism like
spinodal decomposition of Fe or ferromagnetic secondary
phases. The detection of those is rather difficult. Bonanni
et al.6 prepared GaN:Fe layers by metalorganic chemical
vapor deposition (MOCVD) and observed ferromagnetism that
was partially accounted to the spinodal decomposition and
non-uniform distribution of Fe-rich magnetic nanocrystals.
Kuwabara et al.7 reported the formation of nanoclusters and
superparamagnetic behavior in GaN:Fe epilayers prepared by
rf-plasma-assisted molecular beam epitaxy. In case of ion
implantation the reports from different groups are quite
controversial. Theodoropoulou et al.8 and Shon et al.9,10
did not relate ferromagnetic response to secondary phases
after transition ion implantation into semiconductors. In
our experiments, however, the formation of α-Fe
nanoclusters, that were responsible for ferromagnetic
response, was observed.11 Though the appearance of such
precipitates is not desired in a DMS they might be useful
for certain applications.12 Li et al.13 detected
co-occurrence of α-Fe and ε-Fe$_3$Nin MOCVD prepared
GaN:Fe films and pointed out the role of nitrogen pressure
and structural disorder in the formation of Fe-rich phases.
Bonanni et al.14 have shown that the controlled aggregation
of magnetic ions in a semiconductor can be affected by the
growth rate and doping with shallow impurities.Recently we
reported predominant formation of epitaxially oriented
α-Fenanoclusters if Fe-doped samples were annealed in a
N$_2$ flow at 1.1 bar pressure.11 In this paper we report
the formation of ε-Fe$_3$−$_x$N with x<1 that builds up
during annealing at 1073 K in 0.5 bar N2 and the reversible
transformation to ε-Fe$_3$Nduring cooling down to RT.P-type
(Mg) doped (∼2×10$^{17}$cm$^{−3}$)single crystalline
wurtzite GaN(001) films of about 3μm thickness epitaxially
grown by metal organic vapor phase epitaxy on sapphire (001)
were used. Samples, 7° tilted relative to the ion beam to
avoid channeling, were implanted with 195 keV $^{57}$Fe ions
with fluence Φ=4×10$^{16}$cm$^{−2}$ (peak Fe
concentration of 4 at. \% at the projected range
R$_p$=85nm according to TRIM$^{15}$), keeping the samples at
RT. In order to reduce the implantation damage and to
investigate the formation of secondary phases the implanted
samples were annealed at 1073 K in a low pressure
N$_2$-atmosphere(0.5 bar) within several minutes. The
annealing experiments along with the in situ x-ray
diffraction characterization were performed at the
Rossendorf beamline at the ESRF in Grenoble with a x-ray
wavelength of λ=0.124nm. The annealing chamber was equipped
with a boron nitride heater, controlled by a Eurotherm
controller, gas inlet and a half sphere beryllium dome. The
temperature was measured by a PtRh/Pt thermocouple placed on
top of the heater. The gas pressure in the chamber was
limited to 0.5 bar with a flow of about 40l⋅min$^{−1}$.
The purity of the N$_2$gas (99.9999\%) was limited by the
setup with an oxygen contamination in the ppm range.A
Pilatus 100 K two-dimensional (2D) pixel detector was used
to record 2D diffraction pattern. Additionally, a
scintillation counter was used for 2θ-ω-scans. Generally,
for clusters in the range of some nm the signal to
background ratio is very low. In order to increase the
signal to background ratio to an acceptable level the
acquisition time of the 2D detector was set to 10 s and
those of the scintillation counter to 5 s per point. The
investigations of the magnetic properties were performed
with a Quantum Design MPMS superconducting quantum
interference device magnetometer. For the evaluation of the
2D detector exposures rectangular areas with dimensions 2θ
from 32.3° to 36.5° and χ=±0.2° (angle ⊥ to the
scattering plane) were integrated over χ and assumed as
line scans. Those line scans are represented over
annealing/cooling time with the color/grayscale coded
intensity in Fig. 1(a). Single examples of line scans are
given in Figs. 1(b) and 1(c) for RT and 1073 K,
respectively. Prior to the annealing procedure (time t=0 at
RT) no reflexes from secondary phases were detected [see
Fig. 1(a)]. After about 200 s annealing, at 1073 K, a broad
reflex starts to evolve in the region between 32.5° and
35° with a local maximum at about 33.3°. The peak shift
due to the lattice expansion from RT to 1073 K is in the
order of 0.1° and can be neglected. The position of the
maximum fits well to the pattern of FeN(200) (33.46° at
RT). However, because of the broadness of the reflex, other
nitrides like disordered ε-Fe$_3$−$_x$N(002) or
ζ-Fe$_2$N(102) can also be taken into account. Disordered
in this sense means the redistribution of mainly N atoms
within the structure and can be described by the transfer of
N from Fig. 2(b) and 2(c) Wyckoff site position in the
ε-Fe$_3$−$_x$N-phase.16 From symmetry reasons the
formation of ε-Fe$_3$−$_x$N is more probable since it
features the same type of structure (wurtzite) as GaN. The
strain caused by the lattice mismatch is supposed to relax
by generating misfit dislocations, as was shown in Ref. 13.},
cin = {HASYLAB(-2012)},
ddc = {530},
cid = {$I:(DE-H253)HASYLAB_-2012_-20130307$},
pnm = {DORIS Beamline BW1 (POF1-550)},
pid = {G:(DE-H253)POF1-BW1-20130405},
experiment = {EXP:(DE-H253)D-BW1-20150101},
typ = {PUB:(DE-HGF)16},
UT = {WOS:000272627700057},
doi = {10.1063/1.3271828},
url = {https://bib-pubdb1.desy.de/record/88666},
}
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