Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies
We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)- sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with a SQUID magnetometer show clear ferromagnetic behav...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies / V.V. Strelchuk, V.P. Bryksa, K.A. Avramenko, P.M. Lytvyn, M.Ya. Valakh, V.O. Pashchenko, O.M. Bludov, C. Deparis, C. Morhain, P. Tronc // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 1. — С. 31-40. — Бібліогр.: 41 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-1176062025-06-03T16:28:47Z Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies Strelchuk, V.V. Bryksa, V.P. Avramenko, K.A. Lytvyn, P.M. Valakh, M.Ya. Pashchenko, V.O. Bludov, O.M. Deparis, C. Morhain, C. Tronc, P. We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)- sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with a SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature, while they are antiferromagnetic below approximately 200 K. Temperature dependences of the carrier mobility were determined using Raman line shape analysis of the longitudinal optical phonon-plasmon coupled modes. It has been show that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electron spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn₁₋xCoxO films. This work was supported by the Ukrainian-French scientific project of PHC DNIPRO 2011 N 24661 and program “Nanotechnology and Nanomaterials” (project M90/2010) by Ministry of Education and Science of Ukraine. 2011 Article Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies / V.V. Strelchuk, V.P. Bryksa, K.A. Avramenko, P.M. Lytvyn, M.Ya. Valakh, V.O. Pashchenko, O.M. Bludov, C. Deparis, C. Morhain, P. Tronc // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 1. — С. 31-40. — Бібліогр.: 41 назв. — англ. 1560-8034 PACS 63.20.kk, 75.30.Et, 75.70.-i https://nasplib.isofts.kiev.ua/handle/123456789/117606 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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We studied structural, optical and magnetic properties of high-quality 5 and 15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)- sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with a SQUID magnetometer show clear ferromagnetic behavior of the films up to room temperature, while they are antiferromagnetic below approximately 200 K. Temperature dependences of the carrier mobility were determined using Raman line shape analysis of the longitudinal optical phonon-plasmon coupled modes. It has been show that the microscopic mechanism for ferromagnetic ordering is coupling mediated by free electron spins of Co atoms. These results bring insight into a subtle interplay between charge carriers and magnetism in MBE-grown Zn₁₋xCoxO films. |
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| author |
Strelchuk, V.V. Bryksa, V.P. Avramenko, K.A. Lytvyn, P.M. Valakh, M.Ya. Pashchenko, V.O. Bludov, O.M. Deparis, C. Morhain, C. Tronc, P. |
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Strelchuk, V.V. Bryksa, V.P. Avramenko, K.A. Lytvyn, P.M. Valakh, M.Ya. Pashchenko, V.O. Bludov, O.M. Deparis, C. Morhain, C. Tronc, P. Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Strelchuk, V.V. Bryksa, V.P. Avramenko, K.A. Lytvyn, P.M. Valakh, M.Ya. Pashchenko, V.O. Bludov, O.M. Deparis, C. Morhain, C. Tronc, P. |
| author_sort |
Strelchuk, V.V. |
| title |
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| title_short |
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| title_full |
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| title_fullStr |
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| title_full_unstemmed |
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| title_sort |
ferromagnetism in co-doped zno films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2011 |
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https://nasplib.isofts.kiev.ua/handle/123456789/117606 |
| citation_txt |
Ferromagnetism in Co-doped ZnO films grown by molecular
beam epitaxy: magnetic, electrical and microstructural studies / V.V. Strelchuk, V.P. Bryksa, K.A. Avramenko, P.M. Lytvyn, M.Ya. Valakh, V.O. Pashchenko, O.M. Bludov, C. Deparis, C. Morhain, P. Tronc // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 1. — С. 31-40. — Бібліогр.: 41 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
PACS 63.20.kk, 75.30.Et, 75.70.-i
Ferromagnetism in Co-doped ZnO films grown by molecular
beam epitaxy: magnetic, electrical and microstructural studies
V.V. Strelchuk1, V.P. Bryksa1, K.A. Avramenko1, P.M. Lytvyn1, M.Ya. Valakh1, V.O. Pashchenko2,
O.M. Bludov2, C. Deparis3, C. Morhain3, P. Tronc4
1 V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
2B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine,
47, Lenin Ave., 61103 Kharkiv, Ukraine
3Centre de Recherches sur l'Heteroepitaxie et ses Applications, CNRS, F-06560 Valbonne, France
4Centre National de la Recherche Scientifique, Ecole Superieure de Physique et de Chimie Industrielles de la Ville de
Paris, 10 rue Vauquelin, 75005 Paris, France
Abstract. We studied structural, optical and magnetic properties of high-quality 5 and 15%
Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on (0001)-
sapphire substrates. Magnetic force microscopy (MFM) and magnetic measurements with a
SQUID magnetometer show clear ferromagnetic behavior of the films up to room
temperature, while they are antiferromagnetic below approximately 200 K. Temperature
dependences of the carrier mobility were determined using Raman line shape analysis of the
longitudinal optical phonon-plasmon coupled modes. It has been show that the microscopic
mechanism for ferromagnetic ordering is coupling mediated by free electron spins of Co
atoms. These results bring insight into a subtle interplay between charge carriers and
magnetism in MBE-grown Znl–xCo Ox films.
Keywords: DMS, ferromagnetism, RKKY, plasmon damping.
Manuscript received 22.10.10; accepted for publication 02.12.10; published online 28.02.11.
1. Introduction
Currently one can observe a great interest in
understanding and designing the physical properties of
diluted-magnetic-semiconductor (DMS) structures.
Indeed, they have potential applications in spintronics,
where controlling the electron spin can give contribution
to new devices. Since theoretical calculations predicted
possible room-temperature ferromagnetism (FM) [1] in
transition-metal-doped Zn1–xTxO films (T = Cr2+, Mn2+,
Fe2+, Co2+ and Ni2+), they attracted a great interest.
Experimental observations of room-temperature FM in
V2+:ZnO [2], Fe2+:ZnO [3], and Co2+:ZnO [4] appeared in
literature. It seems reasonable to assume that FM is
merely due to magnetic impurities, even if some
experimental results appeared to rule this out [5]. Up to
date, the microscopic mechanism responsible for high-Tc
FM is still quite controversial for the II-VI compounds,
especially for ZnO based DMSs [6]. Various
mechanisms have been proposed for bulk materials, for
example, carrier-induced ferromagnetism [1] and
percolation of bound magnetic polarons [7]. In addition,
structural defects probably play a significant role in
controlling the ferromagnetic properties of ZnO. The
main reasons referred to in literature for appearance of a
ferromagnetic phase are substitution of Zn atoms by the
Co ones and existence of magnetic clusters of metallic
Co and/or Co oxides in ZnO host. Furthermore, the
magnetic properties of Co2+:ZnO films have a strong
dependence on synthesis and processing conditions [8].
In some cases, even the conclusion of intrinsic
ferromagnetism remains controversial [8].
The ferromagnetic properties of 3d-metal-doped
ZnO nanoparticles were explained using the core-shell
model [9]. High stability of the ferromagnetic phase in
Ni2+:ZnO nanocrystals was related to a high surface
defect concentration [10]. High-Tc ferromagnetism in
Mn2+:ZnO and Co2+:ZnO nanocrystals was interpreted
as a result of long-range exchange interaction of Mn2+
and Co2+ ions mediated by charge carriers [11]. The
important role of magnetic anisotropy of Co2+ ions in
ZnO lattice [12] has been discussed together with clearly
observed correlation between magnetism and carrier
concentration in Zn1–xCoxO films [13]. Nano-scale non-
uniform distribution of magnetic ions in the host lattice
and spinodal decomposition have been recently observed
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
31
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
in Cr-doped ZnSe films [14]. The films show
ferromagnetic ordering with high values of the Curie
temperature.
Three models have been proposed to explain room
temperature ferromagnetism in Zn1–xCoxO alloys. In the
first one, ferromagnetism is mediated indirectly via free
carriers (Ruderman-Kittel-Kasuya-Yoshida (RKKY) or
double exchange mechanism model). In the second one,
ferromagnetism originates from a secondary phase such
as metallic Co or Co-oxides. And the latter seems to be
related with bound magnetic polarons. In order to clarify
this ambiguous situation, we studied the Zn1-xCoxO thin
films by using magnetic force microscopy (MFM),
confocal micro-Raman, photoluminescence (PL) and
SQUID techniques.
Raman scattering became a very useful and
informative technique for studying different phonon
excitations in undoped and doped by Li, N, Fe, Sb, Ga,
Al ZnO films. It allows studying the influence of
structural disorder in ZnO lattice on vibrational
properties [15]. The study of Co – O – Zn local vibration
modes versus concentration of oxygen vacancies [16]
allows correlating carrier concentration and magnetic
properties. Appearance of phonon bands at 186, 491,
526, 628, and 718 cm–1 was interpreted as an indication
of ZnyCo3–yO4 [17] spinel phase. Note that nanometer-
size ZnyCo3–yO4 clusters can be very easily detected in
micro-Raman measurements whereas X-ray diffraction
method is not well suited for studying these small
clusters.
In the absence of magnetic secondary phases, the
distribution of Co2+ ions over cation sites of ZnO lattice
should play an important role for ferromagnetism. A
substituting Co2+ ion in the Zn site can have no Co
second first neighbor, i.e., it is not involved to one
(several) Co – O – Co sequence(s) or has at least one Co
second neighbor. The magnetic properties have a strong
dependence on the number of Co atoms of the first type
(isolated atoms). Assuming that Co atoms are randomly
distributed over cation sites and neglecting antisite and
interstitial-site occupation, it was shown for the 5 and
15%Co-doped ZnO films that 94 and 14% of Co atoms,
respectively, belong to the first type. Note that the
assumption rules out metallic Co clusters [18-21]. It is
expected that isolated Co atoms have ferromagnetic
interaction mediated by free carriers. In the Co – O –
Co bonding configurations, the two neighboring Co
localized spins are assumed to be coupled antiparallel
providing antiferromagnetic properties, especially at
low temperature [8, 18-21]. As a result, there is a
competition between a ferromagnetic and anti-
ferromagnetic interactions in the Zn1–xCoxO films. It is
expected that ferromagnetic interactions between Co2+
ions should take place in high quality Zn1–xCoxO films
with high electron concentrations (n > 1019 cm–3) [21].
This paper is organized as follows. Section II
describes the growth procedure and setup for micro-
Raman, MFM and magnetic measurements. In Section
III, we focus on the magnetic, structural, optical and
electronic properties of the MBE-grown 5% and 15%
Co-doped Zn1–xCoxO thin films. Using the MFM and
SQUID technique, magnetic interactions in the films are
studied. In the films, a broad emission peak at 1.816 eV
(683 nm) is ascribed to electron transitions within
substitional Co2+ ions. These results confirm that the
Co2+ ions are located at the Zn sites in the wurtzite ZnO
structure. The micro-Raman measurements confirm the
crystalline wurtzite structure in Co-doped ZnO films.
Also, the temperature-dependent Raman measurements
of longitudinal optical phonon-plasmon coupled modes
(LOPCMs) were perfomed. Modeling the Raman spectra
for LOPCMs allows determining the temperature
dependence of the carrier mobility. The results show that
ferromagnetism in Zn1–xCoxO films is due to free carriers
with high mobility and supports an indirect interaction of
localized magnetic moments of isolated Co2+ ions in the
ZnO lattice. Section IV is devoted to summary and
outlook.
2. Experimental details
The Zn1–xCoxO films were grown on c-sapphire
substrates in a Riber Epineat MBE system equipment
with conventional effusion cells for elemental Zn and
Co. Atomic oxygen was supplied via an Addon radio-
frequency plasma cell equipped with a high-purity
quartz cavity [19]. The film thickness was about 1.7 μm.
The epilayer crystalline quality was attested by low full-
width-at-half-maximum values in high-resolution X-ray
diffraction scans for high-symmetry as well as oblique
directions (see Table I). Lattice parameters of the pure
ZnO sample match well with the values of ZnO single
crystal (a = 3.2495 Å, c = 5.2069 Å). After Co
substitution with Zn atom, both a- and c-axis lattice
constants are changed (a = 3.266 (3.259) Å and c =
5.197 (5.195) Å for 5 (15) at.% Co). Some discrepancy
between the concentrations determined using contactless
submicrometer Raman and macro-Hall measurements
(Table 1) can be caused by differences in local regions
of measurements and possible changes of electric
parameters due to heating under contact formation for
Co-doped ZnO films.
Confocal micro-Raman and PL measurements were
performed using the 488.0 nm line of the Ar+ /Kr+ laser
and recorded with a Jobin-Yvon T64000 spectrometer
equipped with a CCD detector. Spatial resolution (lateral
and axial) was about 1 μm. The temperature-dependent
micro-Raman spectra (80-500 K) were obtained using a
Linkam THM600 temperature stage.
The MFM measurements were performed by
Dimension 3000 Nano-Scope IIIa scanning probe
microscope for spatial mapping of the magnetization
structure of the out-of-plane component of the magnetic
stray field of the Zn1–xCoxO sample surface at room
temperature. Before measurements, the probe was
magnetized using a strong permanent magnet with the
field aligned along the tip axial axis. Then the MFM
images of the sapphire substrate surface were taken, and
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
32
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
no magnetic signal was registered. The magnetic force
gradients were measured using a two-pass technique (Lift
Mode), where the topography was scanned at the first pass
in the tapping mode and then magnetic field gradients
were obtained using the oscillation frequency shift of the
probe moving over surface. The cobalt coated Veeco
magnetic tips with a coercivity of ≈400 Oe, magnetic
moment of ~10–13 emu and 25 nm nominal tip apex radius
were used. The two opposite orientations of probe
magnetization were used (i.e. North or South pole on the
tip apex). This allows distinguishing the signal of the
gradient magnetic fields from other artefacts of long-range
electrostatic fields detected by the magnetic tip apex. The
value of lift scan height was optimized for maximal
sensitivity and minimal topography effects and was about
100 nm. The chip structure of Zn1–xCoxO samples were
also studied using a ZEISS EVO-50 scanning electron
microscope (SEM). The magnetic measurements were
carried out using a Quantum Design SQUID
magnetometer MPMS-XL5.
Table 1. Parameters of MBE growth for undoped and Co-
doped ZnO films and determined values of the carrier
mobility and concentrations obtained by Hall and Raman
measurements at room temperature.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Sample number 226 283 288
Co concentration (%) – 5 15
Growth rate (μm/hour) 0.66 0.43 0.43
Growth temperature (°C) 510 560 560
X-ray line
FWHM (degree)
– [002]: 0.29
[–105]: 0.28
[102]: 0.78
Twist: ±0.54
[002]: 0.32
[–105]: 0.21
[102]: 0.57
Twist: ±0.35
Mobility (cm2/Vs)
measured by
Hall
Raman*
32
–
47
98
29
130
Electron density (cm-3)
measured by
Hall
Raman*
1×1018
–
0.1×1020
1.2×1020
0.7×1020
1.3×1020
* The accuracy of determining the carrier concentration from
analysis of modelled ω+ LOPCM was about ±20%.
3. Results and discussion
AFM is used to characterize surface morphology, root-
mean-square (rms) roughness, and to verify the micro-
structures of Zn1–xCoxO films. AFM morphology image
of Zn1–xCoxO films in the regions of chipped film edge
changes with increase of the Co concentration (Fig. 1).
As seen from Fig. 1a, morphology of 5%-doped ZnO
films shows very tiny crystal grains (20–30 nm) due to
the vertical columnar growth mode and the rms
roughness of about 1.8 nm. For 15% Co-doped ZnO
films (Fig. 1b) the rms roughness is close to 2 nm, the
larger domain structures with sizes from 100 to 400 nm
are formed by binding the smaller crystal grains. Let us
note that in this case the smaller crystal grain size is
practically unchanged with increasing the Co
concentration. Similar morphology was reported for Co-
and Al-doped ZnO films [22, 23].
The surface sensitive MFM method was used to
study magnetization in the vicinity of the chipped
sample edge of the Co-doped ZnO films. We deal with
the area of pure substrate and sharp film edge (Fig. 2).
As seen from the profiles (Fig. 2c), magnetization of the
15%Co-doped ZnO film exhibits a sharp jump in
magnetic signal at the chipped edge for the South and
North pole of the probe (Fig. 2a,b). The MFM
magnetization map is independent of the AFM
topography image of the Zn1–xCoxO surface films. For
the 5%Co-doped ZnO film, similar changes in the MFM
image take place, but changes are not so sharp, and their
value is ~10 times lower than for the 15%Co-doped ZnO
film (Fig. 2c).
Fig. 1. 3 × 3 μm2 AFM images for ZnO films doped with 5% (a)
and 15% (b) on the sapphire substrate. The inserts show SEM
(left) and 3D AFM (right) images of the chipped edge.
33
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
Fig. 2. MFM images at room temperature of the chipped edge
of 15%Co-doped ZnO films scanned under North (a) and
South (b) tip apex magnetization (deep blue color corresponds
to higher MFM signal). Profiles of surface magnetic field
gradients (c) along horizontal dashed lines on the (a) and (b)
magnetic maps recorded for the North(N) and South(S) tip
apex magnetization of the 15% (1a(N) and 1b(S) curves) and
5% (2a(N) and 2b(S) curves) Co-doped ZnO films.
In the area of film sharp edge, the MFM probe
interacts with the studied surface not only by the tip apex
but by some area of side surface too, which causes local
increase of the MFM signal amplitude. As shown in
Fig._2c, magnetization of the Zn1–xCoxO films for the
South pole has a lower value when compared with that
of the North pole. This fact can be explained by the
hysteresis of the magnetic film (curves 1b(S) and 1a(N)
in Fig. 2c) and agrees well with the SQUID data at T =
300 K and H || c geometry in magnetic field with HMFM ~
15 Oe (insert in Fig. 4). It does indicate ferromagnetic
behavior at room temperature in the films.
The MFM investigations did not reveal any fine
magnetic surface structure of the films even in the high
resolution mode with the lift height close to 10 nm. The
observed uniform contrast of the MFM picture can
testify for homogeneous distribution of the doping Co
impurity over the surface of the Zn1–xCoxO alloys at least
with the precision of our MFM experiments (20 nm). In
the opposite case, the magnetic contrast and surface
topography image would be correlated in some manner.
It was observed, for instance, for V2+:ZnO nano-rods,
when the pattern of separate vertically oriented magnetic
dipoles were correlated with topographical AFM images
of the nano-rods [24].
In order to study the magnetic properties of the
films, we performed SQUID measurements. It is seen
from Fig. 3 that the temperature dependences for
undoped and 15%Co-doped ZnO films drastically differ.
The observed higher value for magnetization of 15%Co-
doped ZnO film is caused by strong interaction between
Co2+ ions with the magnetic moment close to 3μB per
Co2+ ion in Zn1–xCoxO film [12, 19]. The magnetic
susceptibility in the undoped ZnO film is negative
(Fig._3) and practically does not depend on temperature.
For the 15%Co-doped sample one can observe a strong
dependence of the magnetic susceptibility on
temperature that is described with the Curie-Weiss law.
The diamagnetic contribution χ = C/(T – Θ) + χsub, where
C is the Curie constant, Θ – Curie-Weiss temperature,
and χsub – diamagnetic susceptibility of substrate, were
taken into account when analyzing SQUID data for the
Co-doped samples. As a result, for the undoped ZnO
film we obtained low magnetization values < 0.1μB
magnetic moment per defect, which is often related with
oxygen vacancies [25]. Weak ferromagnetism for
undoped ZnO film is clearly demonstrated by
magnetization reversal loops in the insert of Fig. 3. For
the case of Co-doped ZnO films, the obtained results for
magnetization are shown in Figs 4 and 5 with account of
the substrate diamagnetic contribution.
Peculiarities of hysteresis curves of the Zn1–xCoxO
samples are observed in magnetization measurements up
to 300 K (Fig. 4). The temperature dependence of
inverse magnetic susceptibility in magnetic field of
1000 Oe is shown in Fig. 5. The magnetic susceptibility
has two well distinguished temperature regimes (at low
(LT) and high (HT) temperatures, respectively) with a
typical Curie-Weiss behavior for both H ⊥ c and H || c
geometry of magnetic field. The Curie temperatures Θ
obtained from extrapolation to the temperature axis
show clearly the HT and LT regimes of effective
magnetic interactions (Fig. 5 and Table II). As a result of
the dominant ferromagnetic interactions between the
Co2+ ions, one can observe hysteresis loops with
coercivity values of ≈ 20 Oe and ≈ 10 Oe at
300 K (inserts in Fig. 4). On the other hand, at low
temperature (T < 200 K), one obtains a negative Curie-
Weiss temperatures (Fig. 5 and Table 2), which can be
considered as a result of antiferromagnetic behavior of
Co – O – Co sequences and has been previously
observed for the Zn
⊥
cH ||
cH
1–xCoxO films [19, 20] and powders
[26]. So, when analyzing the magnetic properties of Co-
doped ZnO films, the change in magnetic behavior of the
Zn1–xCoxO films can be understood as a result of
competition between the ferro- and antiferromagnetic Co
interactions that are caused with to isolated Co ions and
Co – O – Co sequences [20, 21], respectively. Effective
indirect exchange interaction between isolated Co2+ ions
decreases with decreasing the carrier mobility, and at
low temperatures the dominant magnetic interaction is
antiferromagnetic due to Co – O – Co sequences. The
carrier concentration in our films is ~1020 cm-3 at any
temperature. This value was estimated from the ω+
LOPCM in the Raman spectra. Therefore, for these high
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
34
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
quality MBE-grown Zn1–xCoxO films, the high-
temperature ferromagnetism is expected to be related
with to the high electron concentration in the conduction
band.
Table 2. Values of the Curie-Weiss temperatures for the
Zn Col–x xO film with x = 15 % at magnetic field geometries
of H ⊥ c and H || c.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Θ||
LT (K) Θ⊥
LT (K) Θ||
HT (K) Θ⊥
HT (K)
–200 –40 150 270
Fig. 3. Magnetic susceptibility of the 15%Co-doped ZnO film
(filled square) and undoped ZnO film (open circle) versus
temperature in magnetic field of H = 1000 Oe with H ⊥ c
geometry. The insert shows a dependence of the magnetization
for the undoped ZnO film versus magnetic field at 5 (filled
circles) and 300 K (open circles) for H ⊥ c geometry.
Fig. 4. Magnetization of the Zn1–xCoxO film with x = 15%
versus magnetic field. M(H) were taken at 5 (squares) and
300 K (circles) with H ⊥ c (filled symbols) and H || c (open
symbols) geometry of a magnetic field, respectively. The
inserts show the region near the zero field for both geometries
in more details.
It is well known that for Zn1–xCoxO films the
magnetization curves at low magnetic field, which is a
more favorable for ferromagnetism observation, can be
different from the prediction of the effective spin model
[19] used for interpretation of the para- and
antiferromagnetic Co behaviors. For the studied films,
the magnetization (Fig. 4) and magnetic susceptibility
(Fig. 5) curves reveal the presence of significant
magnetic anisotropy with the magnetic moment M ⊥ c as
observed in Refs [19, 20]. The fact that M ⊥ c is higher
than M || c at low temperatures is in good agreement
with theoretical predictions that antiferromagnetic
interactions of Co–O–Co sequences are less stable than
ferromagnetic interactions along the c direction [10, 27].
The ferromagnetic interaction in this direction is more
favorable for ferromagnetism, at least for temperatures
above 200 K, as observed in our SQUID measurements
(Fig. 5).
Fig. 5. Inverse magnetic susceptibility of the Znl–xCo O x film
with x = 15% versus temperature at the magnetic field H =
1000 Oe with H ⊥ c (filled square) and H || c (open triangle)
geometry.
Fig. 6 shows PL spectra of undoped (curve 1) and
Co-doped (5 and 15%Co, curve 2 and 3, respectively)
ZnO films at T = 300K (in insert at T = 6 K). In the PL
spectra of undoped ZnO films, a broad green emission
band is observed at ~ 2.18 eV, which is associated with
intrinsic deep-level defects in ZnO, namely: oxygen
vacancies, interstitial zinc atoms, and antisite oxygen
atoms [10, 28]. For 5%Co-doped ZnO, the red emission
peak at ~ 1.816 eV (683 nm) (Fig. 6, curve 2)
corresponds to electron transitions between d-levels [29]
of isolated Co2+ ions tetrahedrally coordinated to oxygen
atoms. Increasing the Co concentration to 15% induces a
red-emission weakening due to decrease in the amount
of isolated Co2+ ions and increase of Co – O – Co
sequences [8, 18] that don’t contribute to the red band
emission. In addition, the effect of decreasing the
intensity of red emission cannot be related with
formation of secondary phases such as octahedral Co
oxides, since no indication of additional structure phases
35
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
were observed in Raman and X-ray diffraction
measurements within the detection limit.
Fig. 6. PL spectra of the undoped (1), 5 and 15%Co-doped
(2 and 3, respectively) ZnO films excited by Eexc = 2.54 eV at
T = 300 K and
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
T T = 6 K (insert).
Raman measurements were performed to analyze
the vibrational modes and lattice structure of the Co-
doped ZnO films. They confirm that the films do have
the wurtzite structure. Indeed, Fig. 7 exhibits micro-
Raman spectra taken from an undoped (curve 1) and Co-
doped (5 and 15%Co, curves 2 and 3, respectively) ZnO
films. According to group theory, four Raman-active
modes A1, E1 and 2E2 (E2
low and E2
high) are expected for
the wurtzite-type ZnO structure, which belongs to the
space group P63mc. The polar nature of A1 and E1 modes
leads to a splitting into TO and LO components. The
E2
low and E2
high modes are non-polar. In the
backscattering geometry for (0001) ZnO, both E2 and
A1(LO) modes can be detected. The A1(LO) mode at
~ 574 cm-1 shows a very low intensity for high-quality
ZnO films. The most pronounced peaks in ZnO originate
from E2
low and E2
high phonon modes at ~100 and
~437 cm-1, respectively. The E2
high - E2
low modes are
observed at ~ 333 cm-1. Fig. 7 illustrates also the
presence of three phonon modes of the sapphire
substrate (denoted asterisks) with the A1g (418 cm-1) and
Eg (379 and 578 cm-1) symmetries, respectively.
The Raman non-polar E2
low and E2
high modes in
undoped ZnO films are very sensitive to disorder in zinc
and oxygen sublattices, respectively. According to Fig. 7
(curve 1), for undoped ZnO films the E2
low mode at
~ 100.5 cm-1, involving mainly Zn motion, displays a
very narrow linewidth (~1.6 cm-1). After Co doping, the
E2
low mode intensity strongly decreases. The mode is
broadened up to ~ 2.4 cm-1 and undergoes a red shift (up
to ~ 0.7 cm-1) with respect to undoped ZnO. It is the
effect of compositional fluctuations induced by random
substitution of Co ions into Zn sites in host lattice. Such
an alloying effect does not usually involve any
precipitation of other crystalline phases and occurs, for
example, in (Ga, In)N, where In-rich quantum-dot-like
regions arise [30]. The spectra also exhibit an intense
E2
high mode associated with oxygen-atom vibrations
which appear at ~ 439.5 cm-1 with a full-width-at-half-
maximum Γ ~ 5 cm-1 for undoped ZnO film. With the
increase of Co amount, the E2
high mode shows a red shift
up to ~ 1.2 cm-1 and broadens up to ~ 13 cm-1 due to
disorder effects in the oxygen sublattice (vacancies,
interstitials) inducing a change in coordination numbers
of some Co atoms due to oxygen vacancies.
In the Raman spectra of most heavily doped ZnO,
there is often observed the intense signal in the region
between TO and LO modes, the interpretation of this
band is ambiguous. For Co-doped samples, additional
overlapping the broad and intense bands in the region of
450-580 cm-1 is detected (Fig. 7). These bands were
observed in different scattering geometries and therefore
could be attributed to the ZnO phonon states due to
disorder-activated Raman scattering [31]. It is also
assumed [32, 33] that for Co-doped ZnO nanostructures
a broad feature at 470-500 cm-1 may be assigned to the
surface optical phonon mode (SOP) (Fig. 7). When the
crystallite size of ZnO lies within the range 10 nm < L <
100 nm, the SOP can appear, and its intensity increases
with reducing the nano-column diameter [34]. It is
noteworthy that the SOP peak is reliably detected in
resonant multi-phonon Raman spectra (Eexc = 3.81) of
undoped and Co-doped ZnO films, the frequency of this
mode being independent of the Co concentration (is not
shown). Difficulties in SOP detection in non-resonant
Raman spectra of undoped ZnO films can be also caused
both by large crystallite sizes (150-200 nm) and
considerable disordering at the boundaries of the internal
grain structure, as compared with Co-doped films.
Additional bands at ~ 488, 550 and ~ 708 cm-1 in
Co-doped bulk samples and thin ZnO films grown by
various methods have been reported in the literature [17,
35]. These bands arise from secondary structural phases.
They can be clusters of Co3O4 or isometric compounds
ZnxCo3–xO4. However, in our samples, additional Raman
bands of these secondary phases are not present.
36
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
Fig. 7. Room temperature Raman spectra of undoped (1) and
doped with 5 and 15%Co (2 and 3, respectively) ZnO films.
Eexc = 2.54 eV. T = 300 K.
An intense wide band appears in the frequency
range of 550-600 cm-1 for the Co-doped samples
(Fig. 7). At least two Lorentzian profiles are necessary
for fitting the band which splits into two subbands at
frequencies ~ 550.8 cm-1 (Γ ~ 33 cm-1) and ~ 576.3 cm-1
(Γ ~ 22 cm-1) for the 5%Co and 546.2 cm-1 (Γ ~ 46 cm-1)
and ~ 572.9 cm-1 (Γ ~ 27 cm-1) for the 15%Co
concentrations, respectively. It is interesting to note that
with increasing the Co concentration from 5 to 15%, the
intensity of both modes is substantially increased. This
gives a clear evidence for the Co substitution in ZnO
host lattice [17]. A similar increase in the intensity of the
Raman band was reported for the multiphonon mode at
540 cm-1 and E1(LO) mode at 584 cm-1 in Co-doped [17]
ZnO. In our opinion, these bands are related with the
resonant Raman effect at subband excitation caused by
d-d transitions in Co2+ ions as well as by defect levels in
ZnO host within the energy range 2.2 – 3.0 eV [36]. The
extrinsic Fröhlich interaction mediated by the localized
electronic states bounded to defect, impurities and d-
related levels of Co2+ ions could enhance the scattering
efficiency independently on the phonon wave vector q.
The zone-center LO phonons are affected by the n-
type conductivity which is due to the oxygen vacancy
(V0) and interstitial Zn(Zni) [37] of the Co-doped ZnO
films, since we deal with electron concentrations higher
than 1019 cm-3. In polar semiconductors, when the
frequency of longitudinal plasma oscillations approaches
to the LO phonon frequencies, their macroscopic electric
fields strongly interact, which results in appearance of
the ω- and ω+ LOPCMs. However, owing to poor carrier
mobility of the doped ZnO epilayers, it is expected that
the LOPCMs are overdamped due to existence of many
structural defects.
In order to assign the bands in films with 5 and
15%Co at ~ 342 cm-1 and ~ 368 cm-1, respectively, to
plasmon modes, we performed the Raman measurements
at temperatures from 80 to 500 K (Fig. 8) analyzing the
band shapes by using the semiclassical theory of Raman
scattering [38]. Both the electro-optic and deformation
potentials (IDP-EO) as well as charge-density (ICDF)
contributions to the processes of light scattering were
taken into account. The ω- plasma-like modes are fitted
by using the following set of equations:
( ) ( ) ( )
,
)(
)(
,
)(
1Im
)1(
2
22
22
2
22
22
2
2
22
22
1
21
γ−ωω
ω
ε−
ωΓ−ω−ω
ω−ω
ε+ε=ωε
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
ωε
−×
×
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎝
⎛
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
ω−ω
ω−ω
+
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
ω−ω
ω−+ω
=ω+ω=ω
∞∞∞
−
ii
ACA
IAIAI
p
TO
TOLO
TO
LO
TO
FHTO
CDFEODP
(1)
where ε∞ is the high-frequency dielectric constant, CFH is
the Faust-Henry coefficient, ωLO and ωTO are frequencies
of LO and TO phonons, Γ(γ) is a phonon (plasmon)
damping coefficient, ωp is the plasmon frequency.
We used the prefactors of Im(–1/ε) in Eq.(1) for the
light-scattering efficiency from Ref. [38]. By using the
optimization procedure for the ω- plasma-like mode [38],
one finds the A1, A2 coefficients and the γ parameter for
which the sum of the χ2 values, Σ(Iexp(ω) – I(ω))2 is
minimal at fixed ωp and γ values. The ω- band shape
fitting analysis was made for each of the Raman spectra
at a given temperature in order to get the plasmon
damping, γ versus temperature.
Two calculated LOPCMs modes (dashed lines) are
shown in Fig. 8 and demonstrate very good agreement
with the experimental spectra. For modeling the ω-
LOPCM band, we used the following parameter values:
C = 6.4 [39], ωTO = 381.6 cm-1 and ωLO = 574.2 cm-1,
ε∞ = 3.67 and the effective mass of electron m* = 0.27m0
where m0 is the electron mass in vacuum. The plasmon
damping value provides the carrier mobility value (μ =
e/m*γ), and the plasma frequency ωp is related with the
carrier concentration n by the relation ωp = 4πe2n/ε∞m*.
Therefore, one can obtain also the carrier mobility value,
μ, versus temperature.
Fig. 9 shows the temperature dependence of the
electron mobility for the Zn1–xCoxO films with 5 and
15%Co, respectively, obtained from modeling the ω-
LOPCMs band. We found ωp and Γ values equal to
3400 cm-1 and 47 cm-1, respectively, at any temperature.
The value of ωp corresponds to the electron
concentration 1.3 ×1020 cm-3. This high value for ωp is in
good agreement with a spectral position of the ω+
LOPCMs (ωp ≈ ω+) observed in the experimental Raman
spectra for the Zn1-xCoxO films with 5 and 15%Co (see
insert in Fig. 7). The plasmon damping parameter γ has a
strong temperature dependence, which arises from the
temperature dependence of the electron mobility. In
order to determine the influence of ferromagnetic
ordering on the carrier mobility, we calculate
contributions to the mobility, which are due to the carrier
scattering process on the acoustic ~(kT)-3/2 and optic
~(exp(hωLO/kT) – 1) phonons (Fig. 9) in high quality
epitaxial undoped ZnO films [37]. Even if the mobility
in our Zn1–xCoxO films decreases with the temperature
increase up to 500 K, its value is comparable to that in
structurally perfect ZnO films at temperatures around
500 K. It is interesting to note that the electron mobility
in the films with 15%Co is higher than that with 5%Co
at any temperature (Fig. 9). Correlation between
magnetic and transport properties was published for
DMS based on A3B5 semiconductors [40]. For example,
for p-GaMnAs the maximum value of Curie temperature
(Tc = 110 K) was obtained for a metallic type
conductivity and with a higher value of the charge
carrier mobility [40]. This correlation between magnetic
properties and electron mobility takes place in the
studied Zn1–xCoxO films, too (Fig. 9).
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
37
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
Up to date, the physical mechanism of
ferromagnetic ordering in n-type Zn1–xCoxO is not
ascertained yet. One can offer the following microscopic
mechanism of ferromagnetism related to the electron
concentration in the conduction band. The mechanism
foresees long-range interaction between two localized
magnetic moments iS
r
and jS
r
of isolated Co2+(S = 3/2)
ions at a distance of ji RR
rr
− via free electrons in the
conduction band. For these magnetic moments, an
important role is played by the parameter of exchange
interaction Jij that can oscillate in the direct space, as for
RKKY interaction mechanism [40, 41]. The exchange
interaction J( ji RR
rr
− ), which is responsible for the
Curie temperature, depends on the electronic subsystem
of Zn1–xCoxO semiconductor [41]. For the samples to be
ferromagnetic, most of the Co-atom spins should be
parallel one to another. In other words, the symmetry of
the total system including the crystalline lattice and Co-
atom spins should be higher in the ferromagnetic phase.
Since the symmetry of the total system for the
ferromagnetic phase is maximal, one should expect
larger electron mobility than for a disordered
configuration. However, the situation is more complex
for the studied anisotropic wurtzite Co-doped ZnO films.
There takes place considerable anisotropy of
magnetization with the easy-axis magnetization H ⊥ c
[12, 19] (Fig. 4).
For ferromagnetic phase of Co-doped ZnO films
(H ⊥ c, T > 40 K , Table 2), we performed the analysis
of Raman spectra of LOCPMs and found an increase of
the carrier mobility with decreasing temperature (Fig. 9).
This effect could be explained by phonon mechanism of
the carrier scattering [37]. Note that the carrier mobility
(and magnetization) is higher for 15% sample as
compared with 5% sample (Fig. 9). It remains to explain
why ferromagnetism takes place only at rather high
temperatures. One can suggest that coupling between
electron spins and Co-atom spins is favored by collisions
with phonons. Phonon population increases with
temperature which increases the collision probability.
Fig. 8. Raman spectra for the Zn1–xCo Ox films at 80 K. The
dashed lines correspond to the modelled ω- LOPCMs band at
~342 and ~368 cm-1 with 5 (2) and 15%Co (1), respectively.
Fig. 9. Temperature dependence of the plasmon damping (a)
and electron mobility(b) obtained from the analysis of Raman
spectra for Zn1–xCo Ox films with 5% (1) and 15% (2). The
solid, dashed and dot-dashed lines show the dependences for
the electron mobilities limited by processes of scattering on
acoustic and optic phonons as well as limited by their joint
contribution, respectively. ωp = 3400 cm .-1 Γ = 47 cm . -1
4. Summary and outlooks
In this work, we have studied magnetic, structural,
optical and electron properties of high quality MBE-
grown Zn1–xCoxO films with x = 0 (undoped), x = 5%
and x = 15%. We provide experimental evidence for
important role of electrons that enhance ferromagnetism
with Curie temperature up to the room one.
From MFM magnetization maps and SQUID
measurements, ferromagnetic behavior of films at room
temperature is clearly put into evidence. SQUID data
show a complicated temperature dependence of the
magnetic susceptibility, which is due to two different
kinds of coordination, at the range of second first
neighbors, for Co2+ ions within the ZnO host. Co–O–Co
sequences contribute to antiferromagnetic behavior
whereas isolated Co2+ ions contributes to ferromagnetic
properties of the films. High temperature
ferromagnetism results from interaction between isolated
ions at cation sites mediated by conduction electrons
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
38
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 1. P. 31-40.
(RKKY-like mechanism). On the contrary, at low
temperatures (temperatures below 150 K), the
antiferromagnetic effect of Co – O – Co sequences is
dominant.
The Raman measurements confirm the high
crystalline quality of both undoped and Co-doped ZnO
films as well as their wurtzite structure. Raman bands of
the antiferromagnetic Co oxygen spinel clusters have not
been observed. The red Co emission exhibits a broad
peak at 1.816 eV (683 nm), which can be ascribed to
electron transitions within isolated Co2+ ion.
Raman investigation of LOPCMs versus
temperature has been used to probe the free-carrier
properties in films. A modeling of the ω- LOPCMs band
was performed, which allows determining the
temperature dependence of the charge carrier mobility.
Curie temperature increases with the Co concentration
within the range 5% to 15%, and the magnetic films with
a higher value of magnetization have a higher electron
mobility.
5. Acknowledgments
This work was supported by the Ukrainian-French
scientific project of PHC DNIPRO 2011 N 24661 and
program “Nanotechnology and Nanomaterials” (project
M90/2010) by Ministry of Education and Science of
Ukraine.
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© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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2. Experimental details
4. Summary and outlooks
5. Acknowledgments
|