MAGNETIC PROPERTIES OF Mn-
AND Fe-IMPLANTED p-GaN
N.
Theodoropoulou (a), M.E. Overberg (b)
S.N.G. Chu (c), A.F. Hebard (a),
C.R.
Abernathy (b), R.G. Wilson (d), J.M. Zavada (e), K.P. Lee (b) and S.J. Pearton [1]
(b)
(a) Department of Physics, University of Florida, Gainesville, FL 32611
(b) Department of Materials Science and Engineering, University of
Florida, Gainesville, FL 32611
(c) Bell Laboratories, Lucent
Technologies, Murray Hill, NJ 07974
(d) Consultant, Stevenson Ranch, CA 91381
(e) Army Research Office, Research Triangle Park, NC 27709
(Submitted
June 11, 2001)
Subject
classification: 75.30.Hx, 75.50.Pp
Abstract The structural and magnetic
properties of p-GaN implanted with high doses of Mn+ or Fe+
(0.1-5 at.%) and subsequently annealed at 700-1000°C were
examined by transmission electron microscopy, selected-area diffraction
patterns, x-ray diffraction and SQUID magnetometry. The implanted samples showed paramagnetic behavior on a large
diamagnetic background signal for implant doses below 3 at.% Mn or Fe. At higher doses the samples showed
signatures of ferromagnetism with Curie temperatures < 250K for Mn and <150 K for Fe implantation. Structural analysis of the Mn-implanted GaN
showed regions consistent with the formation of GaxMn1-xN
platelets occupying ~5% of the implanted volume. An estimate of ~5.5 ± 1.9 Bohr magnetrons per Mn was obtained, consistent with
the expected value (5.0) for a half-filled shell. The formation of the secondary phases such as MnxGay
or MnxNy was excluded by careful diffraction
analysis. The implantation process may
have application in forming selected-area contact regions for spin-polarized
carrier injection in device structures and in enabling a quick determination of
the Curie temperatures in dilute magnetic semiconductor host materials.
Introduction Recent advances in the carrier-induced
ferromagnetism in dilute magnetic compound semiconductors such as (In, Mn)As
and (Ga,Mn)As has promoted interest in their potential application to new
classes of devices based on spin-polarized transport or integration of
magnetic, optical and electronic functions on a single chip [1-5]. The ability to control the magnetic
properties through application of an electric field (i.e. field gating to
manipulate the carrier density) to the material has recently been demonstrated
by Ohno et al. [6]. The Curie
temperatures, TC, of (In,Mn)As [2] and (Ga,Mn)As [3,7] are
relatively low (~35 and 110 K, respectively) and from practical considerations
it is desirable to find materials with higher values.
Recent calculations based on a Zener model of ferromagnetism predict the
possibility that wide bandgap systems such as (Ga,Mn)N and (Zn,Mn)O might have
TC values above room temperature [7].
Currently, little is known
about the properties of GaN doped with impurities that might induce
ferromagnetic behavior. Some initial
reports have appeared on microcrystalline Ga1-xMnxN with
Mn contents up to x = 0.005 which exhibited ferromagnetic behavior [8,9]
Akinaga et al. [10] reported ferromagnetic properties at <100 K in heavily
Fe-doped GaN grown by low temperature (380°C) Molecular Beam
Epitaxy. We have found apparent
ferromagnetic behavior in Mn-implanted p-GaN at temperatures up to ~250 K for implanted Mn
concentrations of 3-5at.% [11]. The
implantation process is a simple approach for introducing magnetic ions into
different host materials and could readily be used for making selected-area
contact regions for injection of spin-polarized current into device structures.
In this paper we report on
the properties of p-GaN implanted with Fe or Mn at doses designed to produce
concentrations of 3-5at.% at the peak of the implanted profile. Under these conditions, samples annealed at
700°C do not show any evidence of secondary phase
formation (at least to the sensitivity of transmission electron microscopy and
selected area diffraction pattern analysis).
Experimental The p-GaN
(3x1017 cm-3) samples were grown by Metal Organic
Chemical Vapor Deposition on sapphire.
The total epilayer thickness was 4mm. Fe+
or Mn+ ions were implanted at an energy of 250 keV at dose of
~3-5x1016 cm-2 to produce average volume concentrations
of ~3-5at.% in the top ~2000Å of the GaN.
Amorphization of the implanted region was avoided by holding the samples
at ~350°C during the implantation.
Annealing was performed at 700°C for 5 mins under flowing N2 gas with the
samples face-down on another GaN wafer.
The samples were examined by transmission electron microscopy (TEM) and
selected area diffraction pattern (SADP) analysis, while the magnetic
properties were measured in a Quantum Design MPMS SQUID magnetometer.
Results and
Discussion Figure 1
shows the hysteresis curve at 10 K for the 5at.% Fe-implanted GaN annealed at
700°C. From the difference in magnetization for
field-cooled versus zero field-cooled samples, we could observe a ferromagnetic
contribution present in the films below ~50 K (Figure 2). The origin of the magnetic behavior in the
Fe-implanted GaN is not yet clear, since the effective hole concentration is
certainly less than the 2x1017 cm-3 present in the
starting sample due to residual implant damage. Theory suggests much higher hole densities (³1020
cm-3) are necessary for carrier-mediated ferromagnetism [7]. In the 3at.% Fe-implanted samples, the
ferromagnetism was present at < 150 K, which might be due to lower level of
implant damage and higher high concentration.
TEM cross-sectional
view of Mn-implanted samples (3x1016 cm2) showed a buried
band of dislocation loops and platelet structures. Similar results were
obtained after 1000°C anneals, with the platelet region being, on average,
slightly larger than for the 700°C annealed samples. While the end-of-range
damage is a typically feature of implanted semiconductors, the platelet
structures appear to the related to the chemical nature of the Mn since when
impurities such as Si or Au are
implanted at similar doses platelets are not observed.
Figure 1. Hysteresis loop at 10 K of GaN
implanted with 5x1016 cm-2 Fe+ and annealed at
700°C.
Figure 2. Temperature
dependence of the difference in magnetization between field-cooled (B = 0.1T)
and zero field-cooled conditions for the sample of Figure 1. The inset shows the raw data.
Figure 3 (bottom) shows SADP results from a GaN sample
implanted with a Mn+ ion dose of 5x1016 cm-2
and annealed at 1000°C. The diffraction pattern from the GaN away from the
platelet region (center) shows clear spots in [2īī0] zone diffraction. The
same analysis on a platelet region shows streaks in [2īī0] zone diffraction
(bottom), but no extra spots due to different phases. The fact that the diffraction pattern shows only satellite spots
around those of the GaN, with hexagonal symmetry, is an indication that the
platelet structures are most likely Ga1-xMnxN with the
same lattice structure as GaN but different lattice constant. The SADP patterns combined with energy
dispersion x-ray (EDX) spectra identified these regions as GaxMn1-xN
with a different (smaller) lattice constant than the host GaN. The presence of the GaMnN corresponded to
ferromagnetic behavior of the samples below ~ 250 K. The only secondary phase with hexagonal symmetry that could form
in the Mn-implanted GaN would be Mn3Ga, but this is ruled out on the
grounds that it was not consistent with the EDX measurements or with the
magnetization data since Mn3Ga is ferromagnetic with a very high
Curie temperature (> 600K).
Figure 3. SADP’s
from unimplanted (left) and implanted regions (right) of Mn-implanted (3x1016
cm-2) GaN.
Conclusions In summary, high dose
implantation of Fe and Mn into p-GaN produced ferromagnetism. This behavior is consistent with previous
results on epitaxial GaN(Fe). Future
work should focus on the improvement in Curie temperature by increasing both Fe
an Mn solubility and the hole concentration in the GaN.
Acknowledgments The work at UF is partially supported by NSF, while that of RGW is
partially supported by ARO.
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