Semimagnetic semiconductor oxides as materials for transparent electronics and spintronics

Article abstract of DOI:10.1002/pssa.200881789

Zinc oxide-based semimagnetic semiconductor (SMS) ZnMnO, ZnMnFeO and ZnMnSnO thin films were deposited on sapphire and glass substrate by pulsed laser deposition (PLD) and RF sputtering techniques. Further, ZnMnO nanocrystals embedded in polyvinyl alcohol

Full text of DOI:10.1002/pssa.200881789

Phys. Status Solidi A 206, No. 9, 2177–2181 (2009) / DOI 10.1002/pssa.200881789
a
p s s
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applications and materials science
Semimagnetic semiconductor oxides
as materials for transparent electronics
and spintronics
A. I. Savchuk*, V. P. Makhniy, V. I. Fediv, G. I. Kleto, and S. A. Savchuk
Chernivtsi National University, Kotsyubynsky Street 2, 58012 Chernivtsi, Ukraine
Received 19 September 2008, revised 2 January 2009, accepted 16 March 2009
Published online 28 July 2009
PACS 68.35.Ct, 68.37.Ps, 68.55.ag, 78.20.Ls, 78.66.Hf, 85.75.–d
∗ Corresponding author: e-mail a.savchuk@chnu.edu.ua, Phone: +38 0372 584 755, Fax: +38 0372 552914
Zincoxide-basedsemimagneticsemiconductor(SMS)ZnMnO, depending on the film composition and growth conditions.
ZnMnFeO and ZnMnSnO thin films were deposited on sapphire The most interesting feature was observed for the ZnMnSnO
and glass substrate by pulsed laser deposition (PLD) and RF films, inwhichtheformationofclustersincludingnanowire-like
sputtering techniques. Further, ZnMnO nanocrystals embedded structures was shown. Optical transmission spectra suggest that
in polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) metals Mn, Fe and Sn substitute for Zn2+ ions in the ZnO lattice,
matrices were prepared by chemical method employing zinc resultingintheincreaseinthebandgapenergy. Magneto-optical
acetate and manganese acetate as precursors. The morphology Faraday rotation measurements confirm ferromagnetic ordering
of the thin films was studied using atomic force microscopy in ZnMnFeO thin films and paramagnetic behaviour for the
(AFM). Large variety of the morphology images was obtained other films.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Doping of transition metals (TM) in wasexperimentallyconfirmed[24]. Inthiswork, wereporton
ZnO results in the formation of oxide-based semimagnetic complex study of electrical, structural, optical and magneto-
semiconductors (SMSs) or diluted magnetic semiconductors optical properties of ternary and quaternary SMS oxides in
(DMSs). These materials have became especially attractive form of thin films and nanocrystals.
after theoretical prediction of room temperature ferro-
magnetism in ZnO doped with Mn [1]. Namely for this
reason new functionality can be added to conventional
oxides and application in spintronic devices can be realized
[2–6]. It is very important for spintronic application that
magnetism has intrinsic nature and not associate with
secondary phase structures. Numerous research groups
have reported on experimental evidence of ferromagnetic
ordering in oxide SMS [7–14], but its exact origin
and mechanisms are still under discussion. Mechanisms
of superexchange, double exchange, free-carrier-mediated
exchange and exchange through bound magnetic polaron
have been involved to explain the ferromagnetic ordering
above room temperature [15–21]. It was shown in our
previous studies that Zn1 − xMnxO single crystals and
thin films demonstrate paramagnetic behaviour [22, 23],
whereas for Zn1 − x − yMnxFeyO thin films ferromagnetism
2 Experimental Thin films of SMS oxides under
investigations were deposited by using two different
techniques. For the deposition of ternary Zn1 − xMnxO
(0 ≤ x ≤ 0.1) thin films pulsed laser deposition (PLD)
technique was applied. An ultrahigh vacuum chamber with a
base pressure of 10−7 Pa was a key element of the used PLD
set-up. The XeCl excimer laser (λ = 308 nm, τ = 30 ns) was
operated at a repetition rate of 10 Hz. Optimal laser fluence
was measured to be about 5.5 J/cm2. The film thickness was
controlled by the number of laser pulses.
Deposition of quaternary co-doped Zn1 − x − yMnxFeyO
andZn1 − x − yMnxSnyOthinfilmshavebeenperformedbyRF-
diodesputteringtechniquewithplanarcathodeand1.76 MHz
power source in argon–oxygen gas mixture. The ratio of
sputtering power, substrate temperature and oxygen content
in gas mixture were those critical parameters, which mainly
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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A. I. Savchuk et al.: Semimagnetic semiconductor oxides for transparent electronics and spintronics
affected structural and electrical properties of the films
deposited by this technique.
Composite targets for both technological methods were
prepared by pressing technique with using of ZnO, Mn3O4,
Fe3O4 and SnO2 pure powders as initial components. Two
different kinds of substrates (glass and sapphire) have been
used in both the methods. The substrate temperature could
be varied from 20 to 300 ◦C.
Zn1 − xMnxO nanocrystals embedded in polymer matrix
were prepared by chemical method. As a source of
Zn2+ and Mn2+ ions served zinc acetate (ZnAc2 · 4H2O)
and manganese acetate (MnAc2 · 4H2O), respectively. Then
15 mL of H2O2 solution (30%) was added. After reaction
for more than 10 h with continuous strong stirring at the
boiling point of water, the solution was filtered and dried
at 90 ◦C. As a polymer matrix for oxide nanoparticles was
used the mixture of polyvinyl alcohol (PVA) and polyvinyl
pyrrolidone (PVP).
The surface morphology of oxide layers was investigated
by atomic force microscopy (AFM) system, which can
operate in semicontact mode, error mode and phase contrast.
Optical transmission and absorption spectra were
measured in range of 0.2–25 ␮m at temperatures of
Figure 1 (online color at: www.pss-a.com) AFM micrograph
(phase contrast) of Zn0.9Mn0.1O thin film deposited by PLD on
sapphire substrate.
Electrical resistivity was also found to decrease in the
4.2–300 K. Magneto-optical Faraday rotation was measured case of Mn, Sn co-doping. In addition, it was found for
in range of 0.4–0.7 ␮m in magnetic field up to 50 kOe.
the Zn0.9Mn0.1O film that the presence of pure oxygen
atmosphere during PLD deposition leads to increase in
electrical resistivity, i.e. explained by decrease in the oxygen
vacancy concentration.
3 Results and discussion
3.1 Electrical and magnetic characteristics In the
Table 1, we have summarized main electrical and magnetic
characteristics of the grown thin films.
3.2 AFM characterization The surface morphology
of the layers studied by AFM shows strong dependence on
deposition technique and conditions. The most homogenous
surface was found for Zn0.9Mn0.1O thin films grown by PLD
on sapphire substrate at temperature of 300 ◦C (Fig. 1).
It should be noticed that for this kind of samples surface
roughness is increased, when pure oxygen atmosphere
during deposition is created into chamber. The surface of
Zn0.9Mn0.1O film sample grown at oxygen ambient has a
maximum height of 30.62 nm and a root mean square (Rms)
roughness of 4.31 nm.
The electrical properties were probed by four-point
probe resistance and Hall measurements. Undoped ZnO as
a rule exhibits n-type conductivity that can be explained
by the inner defects. It is well known, that the oxygen
vacancy and the zinc interstitial act as donors, while the
zinc vacancy and the oxygen antisite act as acceptors.
Therefore, electron type of conduction in undoped ZnO is
determined the relation between several kinds of defects. In
particular, Zn excess at interstitial sites can provide donor
states.
Surface morphology of quaternary thin films is
characterized more complicated AFM images. The most
interesting feature was observed for Zn0.93Mn0.03Sn0.04O thin
films deposited by RF sputtering on glass substrate at room
temperature. From Figs. 2 and 3, we can see that this kind of
deposition gives rise to the formation of elongated clusters,
which in turn consist of nanowires with average length of
a few micrometers and average diameter of a few tenth of
nanometers.
For the quaternary oxide with composition Zn0.88Mn0.05
Fe0.07O we have observed p-type of conductivity and room
temperature ferromagnetic behaviour.
We suggest, that in this case the introducing of Mn and Fe
in ZnO lattice leads to the change of relation between native
defects to side of those defects which give acceptor states.
Table 1 Main electrical and magnetic characteristics.
The layer height for the Zn0.93Mn0.03Sn0.04O film shown
in Fig. 3 is about 3 nm and the Rms roughness is 1.5 nm.
The observed nanowire-like structures have not certain
orientation in plane of film surface, but they are oriented only
along the surfaces.
film composition
conductivity resistivity
magnetism
type
(ꢀ cm)
ZnO
n
n
n
p
n
107
106
107
104
104
dia
Zn1 − xMnxO (vacuum)
Zn1 − xMnxO (oxygen)
Zn1 − x − yMnxFeyO
Zn1 − x − yMnxSnyO
para
para
ferro
para
3.3 Optical and magneto-optical spectra The
observed optical transmission spectra of oxide SMS thin
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Phys. Status Solidi A 206, No. 9 (2009)
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Figure 2 (online color at: www.pss-a.com) AFM micrographs
(phase contrast and topography) of Zn0.93Mn0.03Sn0.04O thin film
deposited by RF sputtering on glass substrate at 300 ◦C.
Figure 4 Transmission spectra of the oxide SMS films with
different compositions.
Figure 3 (online color at: www.pss-a.com) AFM image
(topography) of Zn0.93Mn0.03Sn0.04O thin films showing presence
of nanowire-like nanostructures.
Figure 5 Transmission spectrum of Zn0.9Mn0.1O thin film
deposited by PLD on sapphire substrate in infrared region.
filmswithdifferentcompositionshavetypicalformnearband
gap absorption edge. The absorption edge is shifted towards
the lower wavelength side with the increase in Mn, Fe and
Sn content. Obviously, this is indication that the band gap Eg spectrum for Zn0.9Mn0.1O thin film is shown in Fig. 5.
of the materials increases with the doping concentration of It was found that the value of transmission coefficient is
the used impurity ions.
As an example, Fig. 4 shows optical transmission spectra was observed in infrared region the absorption band with
of Zn0.9Mn0.1O, Zn0.93Mn0.03Sn0.04O and Zn0.88Mn0.05Fe0.07 maximum at 7.32 ␮m (0.169 eV). Presently origin of this
thin films at liquid helium temperature. From this figure, we absorption band is unclear.
slowly decreased with increase in wavelength. In addition, it
O
can see that the absorption edge is more abrupt and shifted
Previous studies of magneto-optical Faraday effect in
towards the lower wavelength side for Zn0.9Mn0.1O sample. oxide SMS bulk crystals and thin films [22–24] allowed
The estimated values of band gap Eg for Zn0.93Mn0.03Sn0.04O, to find paramagnetic behaviour in ternary Zn1 − xMnxO and
Zn0.88Mn0.05Fe0.07O and Zn0.9Mn0.1O films are 3.54, 3.52 ferromagnetic ordering in Zn0.88Mn0.05Fe0.07O thin films. The
and 3.48 eV, respectively. From temperature studies of the present results confirm room temperature ferromagnetism
Zn0.9Mn0.1O sample, we have estimated the temperature in co-doped Zn0.88Mn0.05Fe0.07O thin films, but suggest of
coefficient of band gap dEg/dT = 0.4 meV/K.
absence of similar magnetic behaviour for new quaternary
In order to evaluate transparency of the oxide SMS thin Zn0.93Mn0.03Sn0.04O oxides. The latter is characterized the
films, we have performed optical measurements in infrared other conductivity type (n-type) and it is possible reason for
region up to wavelength 25 ␮m. Appropriate transmission the exhibition of paramagnetic state.
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images demonstrate different surface morphology including
exhibition of nanowire-like structures. Optical transmission
spectra suggest of blue shift of the absorption edge and
increase in band gap Eg with increase in content of
impurity ions. The estimated values of Eg for Zn0.9Mn0.1O,
Zn0.88Mn0.05Fe0.07O and Zn0.93Mn0.03Sn0.04O films are 3.48,
3.52 and 3.54 eV, respectively. High transparency is shown
for SMS oxides in infrared region. Faraday rotation
spectra have typical for SMS materials character and
indicate different magnetic behaviour depending on film
composition. Further experimental studies are in progress
in order to elucidate the connection between the observed
nanostructures in AFM images and magnetism of SMS
oxides.
Acknowledgements The work has been supported in part
by the Ministry of Education and Science of Ukraine.
Figure 6 Spectral dependence of the Faraday rotation for
Zn0.88Mn0.05Fe0.07O thin film on glass substrate.
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simple method of colloidal chemistry. The observed AFM
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