Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

12C/2003G273

Structural and magnetic properties of Co doped GaN
Junji Sawahata*, Katsuhiro Akimoto
Univ.of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Introduction
Diluted magnetic semiconductors (DMSs) haveattracted considerable interest as materials for spintronicdevices. Among the various DMSs, GaN-based DMSs
have received great attention since the Curie temperature(Tc) higher than room temperature was predicted for ptype GaN doped with Mn[1]. Since then several
experimental studies were reported regarding themagnetic properties of GaN:Mn. Theoretical prediction
for magnetic properties of GaN incorporating various
concentrations of V, Cr, Mn, Fe, Co, and Ni assumingthat magnetic atoms were randomly substituted with Ga
atoms has also been reported, and the systematic variationof the magnetic properties were suggested. Recently, theTc higher than room temperature obtained experimentallyin Cr doped GaN has been reported. Co may beinteresting since the Tc of Co doped TiO2 and ZnO havebeen reported to be above room temperature, however,experimental reports of Co doped GaN are very few. In
this study, we report the structural and magnetic
properties of Co doped GaN.
Experimental and Result
Co doped GaN were grown on sapphire(0001)
substrates by gas source molecular beam epitaxy(GSMBE) using uncracked ammonia as a nitrogen source.
Metallic Ga of 6N-purity, and Co of 4N-purity wereevaporated from conventional Knudsen cells, andammonia gas of 6N-purity was introduced to the growthsurface. Prior to the growth of the Co doped GaN,
sapphire substrate was thermally cleaned at 950°C for 5min and then at the same temperature substrate was-4nitrided under 4×10 Torr of NH3 for 20 min. The Co
doped GaN films were grown at 700°C.
The concentration of Co were varied between 0.3at%
and 4.7at%. The thickness of the epitaxial layer was about
0.5µm. Co-K edge extended x-ray absorption finestructure (EXAFS) measurements were carried out influorescence mode using beam line 12C at the HighEnergy Accelerator Research Organization (KEK) tostudy the local structures around Co atoms. The analysisof the EXAFS data were performed Fourier filtering andcurve fitting techniques using FEFF8 program. The bondlength, coordination number, and Debye-Waller factor
were used as fitting parameters to yield optimum values.
We obtained the EXAFS function with k weighting
value of three of Co doped GaN with Co concentration of
1at%. Fourier transform was applied to the region from
3.0 to 12.6 in k-space to get radial distribution in distancespace. A large peak at about 2.3Å which is attributed to
the first-nearest-neighbor atoms was observed. If the Coatom was incorporated into a substitutional Ga lattice site,peaks corresponding to the first- and second-nearestneighbour atoms should be observed at about 1.9Å and3.2Å, respectively, since the atomic radius of Co isalmost the same as that of Ga. Therefore, it seems that Co
atoms are incorporated into other site than thesubstitutional Ga lattice site in GaN. We carried out backfourier transform to the range between 1.8Å and 2.5Å inthe radial distribution function, and curve fitting to yield
the optimum value using several fitting models such asCoGa, hcp- and fcc-Co. A good curve fitting wasobtained only for the model of CoGa, and it was notpossible to yield a good fitting in case of the model ofhcp-, fcc-Co. The analytical results and theoretical valueof CoGa were listed in table I.
Table I Analytical results of Co-K edge measurements.
1.0%
1.8% 4.7%
CoGa
NearestGa
Neighboratom
Bond length
2.5
2.5
2.5
2.5
Coordination
8.4
9.3
9.7
8
number
Debye0.077
0.1
0.086
Waller factor
The coordination numbers for Co in CoGa and hcp-Co
are 8 and 12, respectively. Therefore, the increase in thecoordination number with increasing Co concentration
may suggest the formation of hcp-Co. From these results,
it can be concluded that the secondary phase mainly
formed is CoGa
The magnetic properties of these Co doped GaN
showed ferromagnetic behavior. The origin of thismagnetic property is unclear at present. It may bepossible to consider the contribution from CoGa since theexistence of secondary phase of CoGa was confirmed byEXAFS analysis. However, the magnetic property ofCoGa is reported to be paramagnetic [2]. Another originof the ferromagnetism could be caused by Co clusters
those are too small to be detected by EXAFS analysis.
Therefore, we considered that the most plausible reason
for the magnetic property is Co clusters.
References[1] T. Dietl et al, Science 287, 1019 (2000).
[2] J. G. Booth et al, Phys. Lett. A32, 149, (1970).

* bk002038@s.bk.tsukuba.ac.jp

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