Journal of Nuclear and Radiochemical Sciences, Vol. 11, No.1, pp. 1-5, 2010

ArticlesCharacterization of 57Fe Implanted and Annealed SnO2 (3 % Sb) Films by Depth Selective
Conversion Electron Mössbauer Spectroscopy (DCEMS)
K. Nomura,*, a S. Iio,a Y. Hirose,b Z. Németh,a, c K. Yamamoto,d and H. ReuthereaSchool of Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
School of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japanc
Institute of Chemistry, Eötvös Loránd University, Pázmány P. s. 1/A, 1117 Budapest, HungarydResearch Center, Asahi Glass Co.Ltd, 1150 Hazawa-cho, Kanagawa-ku, Yokohama, 221-8755 Japane
Forschungszentrum Dresden-Rossendorf e.V., Institut für Ionenstrahlphysik und Materialforschung, Postfach
510119, D-01324 Dresden, GermanybReceived: December 1, 2009; In Final Form: February 9, 2010
SnO2 (3 % Sb) films were implanted with 5×1016 57Fe ions/cm2 at the substrate temperature of 500 °C, and annealed
at temperatures between 400 °C and 800 °C. These films were characterized by depth selective conversion
electron Mössbauer spectroscopy (DCEMS) using a back scattered type of a gas proportional counter, andmeasured by a Kerr effect magnetometer. Kerr effect measurements of the SnO2 films showed ferromagnetism atroom temperature. The Mössbauer spectra of the as-implanted films consisted of paramagnetic doublets of Fe3+
and Fe2+ species and two broad sextets, which showed site A and site B of fine grain magnetite. The Kerr rotation
angles increased step by step with post-annealing up to 700 °C. This phenomenon was attributed mainly toferromagnetic maghemite produced by post-annealing. It was found from DCEMS analysis that the maghemitewith relatively large grains exists more in top layer than deep layer.

1. Introduction
The origin of magnetic interactions in diluted magnetic
semiconductors (DMS) is an interesting issue as a basic problem in magnetism and its possible application in spintronics.1-2
We have reported different types of magnetic source in case of
Fe doped SnO2 powders, prepared by sol-gel method and post
annealing3, and also the phonon density of states (DOS) of
rutile type structures of SnO 2 and TiO 2 .4 The dilution andclustering of doping Fe species can be estimated by Mössbauer
spectra and phonon DOS. We have clarified that the diluted Fe
species probe the phonon DOS in SnO2 more faithfully than in
TiO2. When a large magnetization was obtained for diluted Fe
doped SnO 2 , a magnetic relaxation with broad lines wasobserved in room temperature Mössbauer spectra. However,when the small magnetization was observed, no magnetic
component was found in Mössbauer spectrum. In the latter
case, the magnetization disappeared by annealing for longtime.3 It is clear that defects in DMS can also contribute toenhance the saturation magnetization.
On the other hand, thin films of Sn1-x 57 FexO2-δ have been
implanted at room temperature with 1×1017 Fe ions/cm2 and at
300 °C with 5×1016 and 1×1017 Fe ions/cm2, with an implantation energy of 100 keV in each case.5 The as-implanted samples at room temperature and post-annealed samples did notshow any Kerr effect, but the sample implanted with 1×1017 Fe
ions/cm2 at substrate temperature of 300 °C showed Kerr effectalthough the magnetic sextets were not so clearly observed in
the 57Fe conversion electron Mössbauer (CEM) spectra. Kerr
effect disappeared after annealing. This suggests that the number of magnetic defects decreases by absorption of oxygen due
to annealing in air atmosphere.5 We have also shown that thebulk magnetization is enhanced by introducing Sb5+ in the Fe
doped SnO2 powder.6 We have tried to prepare 57Fe implanted
SnO2 films (containing 0.1 %Sb and 3 % Sb) at the substrate*Corresponding author. E-mail: k-nomura@t-adm.t.u-tokyo.ac.jp

temperature of 500 °C, which showed Kerr effect.7 The Kerr
effect did not disappear after annealing.
In the present work, the SnO2 (3 % Sb) films annealed atvarious temperatures are characterized by depth selective
conversion electron Mössbauer spectroscopy (DCEMS) using a
back-scattered type of gas flow counter in order to study
especially the effect of post annealing.
2. Experimental
SnO2 (3 % Sb) films with thickness of 200 nm were preparedon quartz glass by DC sputtering, and implanted with 5×1016
57
Fe ions/cm2 at the substrate temperature of 500 °C in vacuum,using an energy of 100 keV. 8 From TRIM calculations ofimplantation conditions of 5×1016 Fe ions/cm2, the iron profile
peak is expected to be located at about 40 nm depth with amaximum Fe concentration of 5 at. %. Some samples were
step by step post-annealed at 400 °C, 500 °C, 600 °C, 700 °C,
and 800 °C. Polar Kerr effect of these samples was measured
with magnetic circular dichroism (MCD) mode. Three CEM
spectra were simultaneously observed on each sample fromdifferent depths by discriminating the resonance electrons withthree energy regions (2–6.5 keV, 6.5–11 keV, and 11–20 keV)using homemade Mössbauer system and He + 5 % CH4 gas
flow counter.9,10 This method provides the rough depth profile
of layers of interest, which is named as DCEMS. In contrast to
DCEMS, conventional CEMS, which detects all electrons, iscalled in this article Integrated CEMS (ICEMS). Doppler
velocity was calibrated with standard α-Fe foil at room temperature and a γ source of 57Co/Cr matrix was used.
3. Results and Discussion
In order to confirm whether the bulk ferromagnetism
appeared or not, polar Kerr rotation curves were measured by
MCD mode for SnO2 (3 % Sb) film, implanted with 57 Fe at
substrate temperature of 500 °C, and annealed at 400 °C for 6hours and further at 500 °C for 6 hours. As shown in Figure 1,© 2010 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/18/2010

NomuraJ. Nucl. Radiochem. Sci., Vol. 11, No. 1, 2010

it is clear that the Kerr rotation angles at light wave length of λ= 300 nm for SnO2 (3 % Sb) films increased with the increaseof annealing temperatures. In order to know the origin offerromagnetism at room temperature and the effect of
annealing on the ferromagnetism, three CEM spectra withdifferent energies were measured step by step on the asimplanted sample after annealing at 400 °C for 2 hours and for
4 hours, and further at 500 °C for 2 hours and 4 hours. The
Mössbauer parameters of the as-implanted SnO2 (3 % Sb) film
and the post-annealed one at 400 °C for 2 hours are listed inTables 1 and 2, respectively.
DCEM spectra were obtained by detecting emitted electrons
with the high, middle, and low energy regions, which reflect
the top, middle, and deep layers, respectively. These spectra
were decomposed to four subspectra of two doublets and two
sextets. The doublets with small isomer shift (δ = 0.39 mm/s)
and large isomer shift (δ = 1.00 mm/s) are assigned to paramagnetic Fe3+ and Fe2+ species, respectively. Two broad sextets
with δ = 0.33 mm/s and δ = 0.64 mm/s can be assigned to Fe3+
at tetrahedral site A and Fe2.5+ at octahedral site B of magnetite(Fe3O4) because the area ratio of the two sextets is almost 1:2
in the middle energy CEMS. The broadening of these peaks is
considered to be due to the fine grain size of the iron compounds.
As shown in Table 1, the subspectra areas of magnetite were
34 %, 30 %, and 21 % in high energy, middle energy, and low
energy CEM spectra of 3 % Sb doped SnO2 film, respectively,
whereas the subspectra areas of paramagnetic Fe2+ were 33 %,48 % and 54 %, respectively. These suggest that magnetite
layers are produced rather in upper layers of implanted regions,and that paramagnetic Fe2+ species prefer the deep layers of
implanted regions of SnO2 films.
Kerr effect of the films after annealed at 400 °C for 6 hours
showed the larger rotation angles compared to that before theannealing treatment. In the case of implantation at substratetemperature of 300 °C, only a small Kerr effect was observed
for the as-implanted film, and it disappeared by post
annealing. 5 However, in the present case Kerr effect wasobserved even after annealing. The CEM spectra were measured after the heat treatment at 400 °C for 2 hours. The magnetite peaks (two sextets with different isomer shifts), found in
the case of the as-prepared samples, changed into two sextets
with similar isomer shifts, quadrupole splitting values of nearzero quadrupole shift values, and different ratios as shown in
Table 2. Those sextets can be considered as a superposition ofseveral sextets with small tailing towards lower velocities. The
magnetic components of samples annealed up to 500 °C wereanalyzed by using hyperfine field distributions. The magneticod ) 700 C, 2h

0.001
0.000
-0.001o0.001c ) 500 C, 6h

0.000-0.001

[rad]

2

o

0.001

b ) 400 C, 6h

0.000-0.0010.001a ) as implanted

0.000-0.001-10

0
H / kOe10

Figure 1. Polar Kerr rotation curves of SnO 2 (3 % Sb) film,
implanted with 57 Fe at substrate temperature of 500 °C (a), postannealed at 400 °C for 6 hours (b), at 500 °C for 6 hours (c) , and at
700 °C for 2 hours (d) measured by MCD mode at λ=300 nm.

subspectra of the heated samples can be fitted with a small distribution of magnetic hyperfine fields as shown in Figure 2.
Therefore, the magnetic subspectra of the post-annealed samples up to 500 °C are considered to be due to one component offine maghemite (γ-Fe2O3). It is understood that the maghemite
is easily produced from magnetite by heating at 400 °C becauseof the same spinel structure of magnetite and maghemite. The
magnetic hyperfine field distributions are shown in Figure 2.
With increasing the annealing temperature and time, the maximum probability of hyperfine fields increased a little and thehyperfine field increased by 1 T. This suggests that the grain
size of fine maghemite increased a little with increase of

TABLE 1: Mössbauer parameters of SnO2 (3 % Sb) film, implanted with 5×1016 Fe ions/cm2
Doublets
Area ratio.
(%)
Top layer
22.8%
32.9%
Middle layer
22.9%
48.1%
Deep layer
25.4%
53.8%
ICEMS
23.2%
44.5%

δ
(mm/s)∆(mm/s)

Γ(mm/s)

Sextets
Area ratio(%)δ(mm/s)

Bhf(T)

2ε(mm/s)

Γ(mm/s)

0.38 (1)0.97 (1)0.74 (2)1.97 (2)0.59 (2)0.76 (2)

18.6%
25.6%

0.33 (1)0.64 (1)47.1 (1)42.7 (2)-0.03 (2)-0.05 (3)0.60 (3)1.00 (5)0.39 (1)1.00 (1)0.74 (2)1.98 (1)0.63 (2)0.72 (2)

9.6%
19.5%

0.32 (1)0.64 (1)47.4 (1)43.3 (1)-0.02 (2)-0.14 (3)0.49 (4)1.06 (5)0.38 (3)1.03 (1)0.73 (5)2.03 (2)0.70 (6)0.68 (3)

11.0%
9.8%

0.34 (6)0.6 (1)46.4 (3)43.0 (9)0.1 (2)-0.0 (3)0.5 (3)0.9 (4)0.39 (1)1.00 (1)0.73 (1)1.99 (1)0.62 (1)0.72 (1)

12.4%
19.8%

0.33 (1)0.64 (2)47.2 (1)43.1 (2)-0.02 (2)-0.08 (3)0.55 (4)1.00 (5)CEMS spectra were fitted assuming two doublets and two sextets. (δ : isomer shift, ∆ : quadrupole splitting, 2ε : quadrupole shift,
Bhf : hyperfine field, Γ : line width at half maximum peak)Characterization of 57Fe Implanted and Annealed SnO2 (3 % Sb) Films

J. Nucl. Radiochem. Sci., Vol. 11, No. 1, 2010

3

TABLE 2: Mössbauer parameters of 57Fe implanted SnO2 (3 % Sb) film, annealed at 400 °C for 2 hours
Doublets
Area ratio.
(%)
Top layer
28.0%
30.7%
Middle layer
28.5%
45.7%
Deep layer
25.7%
55.4%
ICEMS
28.2%
44.0%

Sextets
Area ratio(%)δ(mm/s)

Bhf(T)

2ε(mm/s)

Γ(mm/s)

0.59 (1)0.96 (2)

28.9%
12.5%

0.32 (3)0.31 (1)47.4 (1)42.1 (2)-0.00 (2)-0.09 (3)0.82 (3)1.0 (1)0.80 (2)2.02 (1)0.64 (1)0.77 (2)

19.0%
6.8%

0.35 (1)0.39 (3)47.6 (1)42.3 (1)-0.00 (2)-0.13 (6)0.79 (4)0.9 (1)0.42 (3)1.01 (1)0.82 (3)2.01 (2)0.62 (4)0.75 (2)

12.4%
6.5%

0.36 (4)0.3 (1)47.8 (4)43. (1)0.09 (9)-0.1 (3)0.8 (2)0.9 (1)0.41 (1)0.99 (1)0.81 (1)2.02 (1)0.63 (1)0.79 (1)

19.5%
8.3%

0.34 (1)0.35 (2)47.6 (1)42.5 (4)0.01 (1)-0.10 (5)0.78 (4)0.9 (1)δ(mm/s)∆(mm/s)

0.40 (1)0.90 (1)0.79 (2)2.04 (2)0.41 (1)1.00 (1)Γ(mm/s)The CEM spectra were fitted by assuming two doublets and two sextets.
0.14

Probability

0.12

1.08

500 o C, 6h
500 o C, 2h
400 o C, 6h

0.10
0.08

ICEMS
SnO 2 (3%Sb) filmb) 700 °C, 2h

ICEMSa) 600 °C, 2h

ICEMS

1.04

0.06

1.00

0.04

0.00

30

1.12
1.08

40

B hf / T50d) SnO 2 (3%Sb) film
500 oC, 6h

60

ICEMS

1.00
1.08

1.08

1.04
1.00
1.12

1.04
ICEMS

c) SnO 2 (3%Sb) film
500 oC, 2h

1.00
-10

1.04
1.00
1.12

1.08

1.08

1.04
1.12

Relative intensity

0.02

Relative intensityc) 800 °C, 2h

-5

0

5

10

Velocity / mm/sb) SnO 2 (3%Sb) film
400 oC, 6h

ICEMS

Figure 3. ICEM spectra of the samples annealed at each temperatureof 600 °C (a), 700 °C (b), and 800 °C (c) for 2 hours.

1.04
1.00
1.08

ICEMS

a) As implanted
SnO 2 (3%Sb) film

1.04
1.00

-10

-5
0
Velocity / mm/s

510Figure 2. ICEM spectra of the samples: as implanted, annealed at
400 °C for 6 hours, at 500 °C for 2 hours and for 6 hours, and thehyperfine field distributions of fine maghemite produced by heating
the 57Fe implanted SnO2 (3 % Sb) films at each temperature.

annealing temperature and time.
When heating above 600 °C, the sextet due to hematite (αFe2O3, with the estimated values: δ = 0.38 mm/s, 2 ε = -0.19mm/s, Bhf = 50.5 T. Γ = 0.35 mm/s) was observed in addition
to that of the maghemite (δ = 0.33 mm/s, 2ε = -0.04 mm/s, Bhf= 47.9 T. Γ = 0.9 mm/s) as shown in Figure 3. The doublet
peaks of Fe2+ almost disappeared, although some small Fe2+
peaks remained only in the deepest layer, and two kinds ofparamagnetic Fe3+ peaks (δ = 0.34–0.37 mm/s, ∆ = 0.77–0.85mm/s, Γ = 0.44–0.49 mm/s and δ = 0.26–0.33 mm/s, ∆ =1.35–1.51 mm/s, Γ = 0.65–0.90 mm/s) were clearly observed
as shown in Figure 4. The component with large isomer shifts
may be superparamagnetic α-Fe2 O3 and γ-Fe2 O3, while theother with lower isomer shifts and large quadrupole splitting
may be dilute Fe3+ in SnO2 matrix.

4

NomuraJ. Nucl. Radiochem. Sci., Vol. 11, No. 1, 20103+

T he a rea ratios of pa ra magnetic Fe components,
paramagnetic Fe2 + component, magnetic maghemite, andhematite; produced in top surface, middle, and deep layers of
the samples heated step by step in air; are shown in Figure 5.
The area ratio of paramagnetic Fe3+ increased a little in deep
layers but decreased in top layers with heating at hightemperature ranges of 600–800 °C. The difference of area
ratios of maghemite between layers was relatively large in caseof heating at low temperatures of 400–500 °C, but decreased
with the further increase of heating temperature. This suggeststhat the fine grains of maghemite are growing by heating.
The Kerr rotation angles observed for the annealed samples
are considered to attribute mainly to the fine maghemite produced in the SnO2 films, and annealing up to 700 °C is foundto be effective as transparent and ferromagnetic film. However,heating at 800 °C was not good for room temperature ferromagnetism because weak ferromagnetic hematite was produced
from maghemite while the ratio of paramagnetic or superparamagnetic Fe 3 + species was not so much increased. The
annealed sample at 800 °C did not show any Kerr effect, andthe Mössbauer parameters of the sextet with low intensity andlow hyperfine field (Bhf = 48.4 T, and δ = 0.34 mm/s, 2ε = -0.13mm/s, Γ = 1.06 mm/s) as shown in Figures 3 and 4 were different from those for the annealed sample at 700 °C (Bhf = 47.8
T, and δ = 0.33 mm/s, 2ε = -0.03 mm/s, Γ = 0.9 mm/s), which
latter is corresponding to maghemite. The hematite peaks canbe distinguished from the maghemite peaks because the quadrupole shift’s values are different. The area ratios of the sextets
with high magnetic field and low magnetic field were almost
the same among DCEM spectra of top, middle, and deep layersof the annealed sample at 800 °C. Therefore, the sextet withlow hyperfine field for the annealed sample at 800 °C is considered to be a tailing part due to fine particle of hematite in
SnO2 film produced by annealing at the high temperatures, notmaghemite.
The magnetic peaks observed by Fe ion implantations andpost-annealing at high temperatures can be assigned to sextets
due to magnetite and maghemite, and clearly distinguishedfrom the magnetic relaxation peaks observed in dilute Fe doped
SnO2 powders.

600 °C, 2h

1.30High energy CEMS

1.20
1.10

Relative intensity

1.00
1.20
Middle energy CEMS

1.15
1.10
1.05
1.00
1.06

Low energy CEMS

1.04
1.02
1.00

-10

-5

0
5
Velocity / mm/s

10

Figure 4. DCEM spectra of 57Fe implanted SnO2 (3 % Sb) sample,
annealed at 600 °C for 2 hours. High energy CEMS: top layer,
Middle energy CEMS: middle layer, Low energy CEMS: deep layer.

60
paramag.Fe 3+ in deepparamag.Fe 3+ in middle
paramag.Fe 3+ in top
paramag.Fe 2+ in deepparamag.Fe 2+ in middle
paramag.Fe 2+ in top
maghemite in deep
maghemite in middle
maghemite in top
hematite in deep
hematite in middle
hematite in top

Ratio / %

50
40
30
20
10
0

400

500

600

700

800

Annealing temperature / °C
Figure 5. Ratios of four divided Fe species produced in top, middle, and deep layers of SnO2 film implanted with 57Fe, annealed step by step at
400 °C for 6 h, 500 °C for 6 h, 600 °C for 2 h, 700 °C for 2 h, and 800 °C for 2 h. Two paramagnetic Fe3+ species observed for the samples heatedat the higher temperatures than 600 °C are summed up. The ratio of maghemite for the heated sample at 800 °C may be a part of fine hematite.

Characterization of 57Fe Implanted and Annealed SnO2 (3 % Sb) Films

4. Conclusions
SnO 2 (3 % Sb) film doped with 5×1016 57 Fe ions/cm 2 atsubstrate temperature of 500 °C with an energy of 100 keV waspost-annealed step by step from 400 °C to 800 °C. These samples except the one heated at 800 °C showed Kerr effect, thatis, bulk ferromagnetism at room temperature. The Kerr
rotation angles increased with the increase of annealing
temperature up to 700 °C. The room temperature ferromagnetic
phenomena were attributed mainly to maghemite produced
from magnetite and mixed Fe3+ and Fe2+ oxides by post-annealing. It was found from DCEMS analysis that the maghemitewith relatively large grains exists more in top layer than in deep
layer. The transparent magnetic films at room temperature
were obtained by implantation at substrate temperature of 500°C and post-annealing up to 700 °C although these films were
not intrinsic dilute magnetic semiconductors.
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