Materials Transactions, Vol. 55, No. 10 (2014) pp. 1606 to 1610
© 2014 The Japan Institute of Metals and Materials
EXPRESS REGULAR ARTICLE
Effects of O2 and N2 Flow Rate on the Electrical Properties of Fe-O-N Thin Films
Yukiko Ogawa+, Daisuke Ando, Yuji Sutou and Junichi Koike
Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
We report the dependence of electrical properties of Fe-O-N thin films on the deposition condition as well as on O2 and N2 gas flow rate.
Fe-O-N films were deposited by reactive sputtering using O2 and N2 as reactive gas. The electrical resistivity of Fe-O-N films increased with
increasing O2 and N2 gas flow rate. The resistivity increase with the O2 flow rate was due to structure change from a mixed phase of metallic
Fe+Fe3O4, to a mixed phase of FeO+¡-Fe2O3, and to a single phase of ¡-Fe2O3, as evidenced by XPS analysis of Fe 2p core excitation peaks.
Meanwhile, the resistivity increase with the N2 flow rate was due to structure change from a metallic Fe, to a mixed phase of metallic Fe+Fe3O4,
and to a single phase of Fe3O4. [doi:10.2320/matertrans.M2014089]
(Received March 7, 2014; Accepted July 15, 2014; Published August 29, 2014)
Keywords: iron oxide, nitrogen-doped, electrical resistivity, effects of oxygen and nitrogen flow rate
1.
Introduction
Transition metal oxides have been investigated for many
years because of their characteristic properties for wide range
of applications, such as transparent conductive films, resistive
random access memories, and photovoltaic cells. Among the
various oxide materials, iron oxide is an abundant and
inexpensive material. Three different structures with different
ratios of oxygen have been recognized; FeO (wüstite), Fe3O4
(magnetite) and Fe2O3, where Fe2O3 has 4 phases; ¡, ¢, £,
and ¾. It is noteworthy that ¡-Fe2O3 (hematite) and magnetite
have significantly different characteristics despite the small
difference (only 3 at%) in oxygen concentration between
these two oxides. Hematite has a corundum structure,1) while
magnetite has an inverse spinel structure.2) Both iron oxides
are n-type semiconductors.3­6) Hematite has a resistivity of
over 105 ³ cm,7) while magnetite has a lower resistivity of
approximately 10¹3 ³ cm.8,9) The bandgaps of these oxides
also differ significantly.10,11) Therefore, the electrical properties of Fe-O films can be significantly changed with the
oxygen concentration.
Electrical properties of oxides can also be changed by
doping with metals or with nitrogen. In the case of metal
doping, hematite with n-type conductivity becomes a p-type
semiconductor by doping with magnesium, copper, and
zinc.12­14) Pure hematite also has poor conductivity, which
has been improved by doping with metals such as silicon and
titanium.4,11,15,16) In the case of nitrogen doping, tin and zinc
oxides can be changed from n-type to p-type semiconductors.17,18) The conductivity of hematite has also been reported
to change from n-type to p-type with the introduction of
nitrogen.19) Nitrogen doping is also effective to control the
bandgaps of oxide materials, such as copper oxide, titanium
oxide, and indium tin oxide (ITO).20­22) Therefore, nitrogen
doping is effective for control of the electrical properties of
iron oxide materials.
Nitrogen can be easily doped into iron oxide by reactive
sputtering with N2 gas. Some researchers have reported the
properties of iron oxide films that were deposited by reactive
sputtering of an iron target in an Ar-N2-O2 gas mixture;
+
Graduate Student, Tohoku University. Corresponding author, E-mail:
[email protected]
however, the focus of their work was on the formation
and structure analysis of iron oxynitride.23­27) Recently, the
present authors have reported on the electrical and optical
properties of Fe-O-N films deposited by reactive RF magnetron sputtering under O2 gas flow rate of 0.1­0.25 sccm and
N2 gas flow rate of 1­5 sccm.28) It was found that the
resistivity of the as-deposited films increased from 102 to
over 106 ³ cm with increasing the oxygen gas flow rate.
Moreover, the resistivity was found to decrease by more than
3 orders after annealing at 673 K. Such changes in the
resistivity of the films may be due to the change of crystal
structure of the films, but was not clear. Because the XRD
patterns of the as-deposited films showed only one vague
peak, it would not be appropriate to identify the crystal
structure of the as-deposited films from the X-ray diffraction
(XRD) analysis. Therefore, in this study, we systematically
investigated the electrical properties of Fe-O-N films
deposited by reactive sputtering by widely changing O2
(0.05­0.25 sccm) and N2 (0­5 sccm) gas flow rate, and the
relationships between electrical properties and crystal
structure of the Fe-O-N films were discussed based on not
only XRD measurements but also transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS)
analysis and temperature dependence of the resistivity of the
films.
2.
Experimental
Fe-O-N films were deposited by RF magnetron sputtering
of an iron (4N) target in an Ar-N2-O2 gas mixture. Oxygen
flow rate (fO2 ) was changed from 0.05 to 0.25 sccm and
nitrogen flow rate (fN2 ) was changed from 0 to 5 sccm. A
glass plate or a 100 nm thick tetraethylorthosilicate (TEOS)SiO2/Si wafer was used as a substrate. Base pressure was
approximately 3.0 © 10¹5 Pa and working pressure was kept
at 2.0 Pa by adjusting Ar flow rate at 25 sccm. Before the
deposition of the films, pre-sputtering with Ar gas was
conducted for 10 min to clean the target surface. RF power
for sputtering was maintained at 100 W.
Film thickness was fixed at approximately 100 nm by
adjusting sputtering time. Electrical resistivity was obtained
from Hall measurements using the van der Pauw method. Xray diffraction (XRD) analysis was carried out to observe the
Effects of O2 and N2 Flow Rate on the Electrical Properties of Fe-O-N Thin Films
Table 1
fO2 =sccm
Measure value of the electrical resistivity (³ cm) of as-deposited samples.
0.05
fN2 =sccm
1607
0.06
0.075
0.1
0.15
0
4.22 © 10¹5
4.39 © 10¹5
8.57 © 10¹5
8.39 © 10¹4
2.42 © 10¹1
>106
1
¹4
¹4
¹4
1.07 © 10
2.04 © 10
>106
¹2
1.36 © 10
>10
>106
¹2
1.32 © 10
>10
>106
3
5
2.11 © 10
¹3
9.93 © 10
¹3
5.61 © 10
2.40 © 10
¹2
3.03 © 10
¹2
4.44 © 10
4.54 © 10
3
4.95 © 10
4
2.27 © 10
3
5
6
6
10 3
10 1
10-1
f N2=5 sccm
f N2=3 sccm
f N2=1 sccm
f N2=0 sccm
10-3
10-5
0
0.05
0.1
0.15
0.2
0.25
Oxygen flow rate, fO2 / sccm
006
α-Fe2O3
fO2 /sccm
0.15
110
Intensity (arb. unit)
Resistivity, R / Ωcm
>10 6
10 5
104
α-Fe
0.10
0.075
0.05
30
0.3
Fig. 1 The dependence of the electrical resistivity (³ cm) of as-deposited
films on O2 and N2 gas flow rate.
crystal structure of the Fe-O-N films. Chemical bonding state
of the Fe-O-N films was determined by X-ray photoelectron
spectroscopy (XPS) using an Al radiation source and
operated at 12.5 kV and 250 W. Microstructure was observed
using transmission electron microscopy (TEM). The dependence of the electrical resistivity on temperature was
determined from electrical resistance measurements in an
Ar atmosphere. Electrical measurements were performed at
various temperatures with a heating rate of 10°C/min using
a two-point probe method. Nitrogen concentration in the FeO-N film was determined using Auger electron spectroscopy
(AES).
3.
0.25
Results and Discussion
Table 1 lists the electrical resistivity dependence of the asdeposited Fe-O-N films on fO2 and fN2 . The films deposited
at fO2 ¼ 0:25 sccm have very high resistivity of over 106
³ cm, which is beyond the measurable range of the Hall
measurement equipment employed. For fO2 ¼ 0:15 sccm,
and fN2 ¼ 3 and 5 sccm, the resistivities were also too high to
be measured. The resistivity of conventional hematite is
typically higher than 105 ³ cm, which suggests that these
films have a hematite structure. Figure 1 shows the plots of
the film resistivity vs. oxygen gas flow rate, fO2 , depending
on the nitrogen gas flow rate, fN2 . It is seen that the resistivity
of the films at fN2 ¼ 0 sccm monotonically increase with
increasing fO2 . Meanwhile, the films at fixed fN2 ¼ 1, 3 and
35
40
45
50
55
Diffraction angle, 2 θ / degree
60
Fig. 2 XRD spectra of the films deposited at a fixed value of fN2 ¼ 1 sccm
and at the indicated values of fO2 .
5 sccm show drastic increase in the resistivity at over fO2 ¼
0:1 sccm. It is noteworthy that the resistivity of the films at a
fixed value of fO2 becomes higher with increasing fN2 in the
range of below fN2 ¼ 3 sccm and is almost saturated at fN2
of over 3 sccm.
XRD measurements were performed to observe the crystal
structure of the Fe-O-N films. Figure 2 shows the XRD
patterns of the films deposited at fN2 ¼ 1 sccm and fO2 ¼
0:05, 0.075, 0.10 and 0.15 sccm. In the films at fO2 ¼ 0:05
and 0.075 sccm, a very weak peak was observed at around 44
degree, indicating 110 peak of ¡-Fe. Meanwhile, in the films
at fO2 ¼ 0:10 and 0.15 sccm, very weak 104 and 006 peaks
of hematite are observed, respectively. However, it would not
be appropriate to identify the crystal structure from only one
vague peak. Therefore, XPS measurements were performed
to clarify the crystal structure of the obtained films in terms of
chemical bonding state.
Figure 3(a) shows the XPS spectra for the Fe 2p energy
range at the fixed value of fN2 ¼ 1 sccm and varied fO2 . The
spectra were calibrated with the peak position of C 1s at
285 eV. For fO2 ¼ 0:05 and 0.075 sccm, the observed peak
positions are 707.2, 711 and 724.4 eV. The 707.2 eV peak is
nearly identical to the reference peak position of 707.1 eV for
metallic Fe 2p3/2.29) This result corresponds to the result of
XRD which shows the presence of ¡-Fe. The other two peaks
can be attributed to either Fe3O4 or Fe2O3. However, the
binding energies of Fe 2p3/2 and Fe 2p1/2 are around 710.6
and 724.4 eV for both phases.30) It has been reported that a
satellite peak located at approximately 8 eV higher than the
Fe 2p3/2 peak was observed in Fe2O3, while such a satellite
1608
Y. Ogawa, D. Ando, Y. Sutou and J. Koike
× 103
30 (a)
25 fO2 /sccm
2p1/2
2p3/2
Intensity (counts)
0.15
20
0.10
15
0.075
10
0.05
5
Fe2O3
Fe3O4
FeO
Fe 2p3/2
0
735
725
715
Binding energy, Eb / eV
4 0.10
3.5
Fe-N
Adsorbed N
Intensity (counts)
× 102
5 (b)
fO2 /sccm
4.5 0.15
705
3 0.075
2.5 0.05
2
410
405
400
395
Binding energy, Eb / eV
390
Fig. 3 XPS spectra of (a) Fe 2p, (b) N 1s of the films formed at a fixed
value of fN2 ¼ 1 sccm and at the indicated values of fO2 .
peak was absent in Fe3O4.31,32) It is seen that the satellite peak
is not observed for fO2 ¼ 0:05 and 0.075 sccm. From the
XPS analysis, these films are determined to be composed of a
mixed phase of metallic Fe and Fe3O4. For fO2 ¼ 0:10 sccm,
the observed peak positions are 709.8, 710.9, and 724.2 eV.
The 708.8 eV peak is nearly identical to the peak position of
709.7 eV for Fe 2p3/2 of FeO.31) In addition, this film has a
satellite peak at 718.9 eV, which shows the feature of Fe2O3.
From the results combined with XRD measurement, the film
at fO2 ¼ 0:10 sccm is determined to have a mixed phase
of FeO and ¡-Fe2O3 (hematite). For fO2 ¼ 0:15 sccm, the
observed peak positions are 711.1 and 724.6 eV, and a
satellite peak is observed at 719.2 eV. Therefore, from the
results, together with the XRD pattern, it is determined that
this film is a single phase of hematite.
These iron oxide structures determined with XRD and
XPS can be confirmed by electrical resistivity values in
Table 1. In literatures, the electrical resistivity of FeO and
¡-Fe2O3 (hematite) was reported to be about 4 © 10¹2 ³ cm
and >105 ³ cm, respectively.33,34) The films deposited at
fO2 ¼ 0:10 and 0.15 sccm had relatively high resistivities of
1.07 © 103 ³ cm and >105 ³ cm, which suggest a mixed
phase of FeO and hematite, and a single phase of hematite,
respectively. Here, £-Fe2O3 (maghemite) has been reported to
show a high resistivity of over 104 ³ cm,35) but the formation
of maghemite phase was not observed by XRD measurements in the obtained films.
The role of N2 gas flow on the film structure was also
investigated. Figure 3(b) shows XPS spectra for the N 1s
energy range. The films deposited at fO2 ¼ 0:05 and
0.075 sccm have relatively strong peaks at around 397,
397.9 and 400 eV. For fO2 ¼ 0:10 sccm, the observed peak
positions are 395.6 and 400.3 eV, while for fO2 ¼ 0:15 sccm,
only a small peak is observed at 400 eV. According to a
previous study, the peak position for Fe-N bond is 395­
398.6 eV.36,37) Therefore, the peak at 400 eV is due to
molecular N adsorbed on the surface.36,37) These results
suggest that N is incorporated into the Fe-O film and forms
Fe-N bonds. The amount of the incorporated N decreases
with increasing fO2 at the same fN2 . It is noted that the
intensity of the N 1s peak is much weaker than that of Fe 2p.
Using AES, the N/Fe concentration ratio of the film
deposited at fO2 ¼ 0:15 sccm and fN2 ¼ 3 sccm was determined to be 0.0123. Since this film is considered to be
hematite having the Fe concentration of 40 at%, the N
concentration is estimated to be only 0.5 at%. The fO2 =fN2
ratio for this film is the same as that for the film deposited at
fO2 ¼ 0:05 sccm and fN2 ¼ 1 sccm, which has a Fe-N peak
in the N 1s spectra. In addition, oxygen is much more
reactive than nitrogen, and nitride is difficult to form.
Therefore, despite the clear indication of the Fe-N bonds, it is
likely that a nitride phase is not formed in the film.
To understand the cause of the resistivity increase with
increasing fN2 , the crystal structure of the film at fO2 ¼
0:075 sccm and fN2 ¼ 5 sccm was investigated by TEM
observation. Figure 4(a) shows a cross-sectional TEM image
of the Fe-O-N film, which indicates a columnar crystalline
structure. Figure 4(b) shows a diffraction pattern taken from
the square area in Fig. 4(a), which indicates a spinel type
structure. It is known that Fe3O4 (magnetite) and £-Fe2O3
(maghemite) have an inverse spinel structure. Moreover,
neither metallic Fe nor nitride is evident; therefore, the film
deposited at fO2 ¼ 0:075 sccm and fN2 ¼ 5 sccm is concluded to have a magnetite or maghemite structure. The
resistivity of magnetite was reported to be 10¹2 ³ cm,34)
while that of maghemite was reported to be >104 ³ cm.35)
The resistivity of the film at fO2 ¼ 0:075 and 5 sccm is
2.27 © 10¹2 ³ cm. Therefore, the film at fO2 ¼ 0:075 and
5 sccm is supposed to be a magnetite. In contrast, as shown
earlier with XPS, the film deposited at the same fO2 ¼
0:075 sccm and the lower fN2 ¼ 1 sccm is determined to have
a mixed phase of metallic Fe and magnetite. These results
indicate that the iron oxide structure is more easily formed at
higher fN2 . The increase in resistivity with fN2 in Fig. 1 is
due to this change in crystal structure.
Figure 5 shows the temperature dependence of the
resistance for the Fe-O-N films deposited at (a) fO2 ¼
Effects of O2 and N2 Flow Rate on the Electrical Properties of Fe-O-N Thin Films
1609
30
(a)
(a) f N2=0 sccm
Resistance. ρ / Ω
28
Fe-O
TEOS
24
22
100 nm
Si
26
20
(b)
40
440
333
400
311
220
111
200
60
160
7100
(b)
fN2 =5 sccm
6100
Resistance. ρ / Ω
5100
180
4100
170
3100
fN2 =1 sccm
2100
160
Fig. 4 (a) A Cross sectional TEM image and (b) a diffraction pattern of the
Fe-O-N films at fO2 ¼ 0:075 sccm, fN2 ¼ 5 sccm.
1100
100
150
40
ð1Þ
where µo is a pre-exponential factor, Ea is activation energy
for electrical transport, k is the Boltzmann constant and T
is temperature. Optical bandgap Eg, is calculated from the
activation energy using:38)
ð2Þ
where ¦E is the depth of a trap state. From eq. (1), Ea for the
film deposited at fN2 ¼ 1 sccm is estimated to be 0.0126 eV.
From eq. (2), Eg is calculated to be 0.025 eV. Figure 6 shows
ln µ ¹ 1/T plot for the film deposited at fN2 ¼ 5 sccm. The
decrease in resistance for this film can be separated into two
states. It is assumed that the effect of ¦E is larger for the low
temperature state than the high temperature state. Therefore,
the high temperature state (dotted line in Fig. 6) is used to
estimate Ea and Eg for the film deposited at fN2 ¼ 5 sccm,
which are 0.043 and 0.086 eV, respectively. Ea for electron
transport in a bulk single crystal of magnetite was reported to
be approximately 0.06 eV39) and the bandgap of magnetite is
0.1 eV,10) which are close to the obtained values from Fig. 6.
60
80
100 120
Temperature, T / °C
140
160
Fig. 5 Temperature dependence of resistance of the film at fO2 ¼
0:075 sccm and at (a) fN2 ¼ 0 sccm, (b) fN2 ¼ 1; 5 sccm.
8.8
8.7
Lnρ (Ω)
0:075 sccm and fN2 ¼ 0 sccm, (b) fO2 ¼ 0:075 sccm and
fN2 ¼ 1 sccm, and (c) fO2 ¼ 0:075 sccm and fN2 ¼ 5 sccm.
For fN2 ¼ 0 sccm, the resistance increases with the temperature, which indicates metallic behavior. For fN2 ¼ 1 and
5 sccm, the resistance decreases with increasing temperature,
which indicates semiconductor-like behavior. The temperature dependence of the resistance in a semiconductor is
given by:38)
Ea ¼ Eg =2 þ E;
140
190
Fe3O4
µ ¼ µ o expðEa =kT Þ;
80
100 120
Temperature, T / °C
8.6
8.5
8.4
8.3
0.0020
Ea=0.043 eV
0.0025
0.0030
1/T (K-1)
0.0035
Fig. 6 1/T ¹ ln µ plot of the film at fO2 ¼ 0:075 sccm and fN2 ¼ 5 sccm
(Ea and Eg was calculated from the slope of dotted line.).
On the other hand, Ea and Eg for the film deposited at
fN2 ¼ 1 sccm are much smaller than those for the film
deposited at fN2 ¼ 5 sccm. This is explained by the
formation of the mixed phase of metallic Fe and magnetite
at fN2 ¼ 1 sccm, while single phase magnetite is formed at
fN2 ¼ 5 sccm.
1610
4.
Y. Ogawa, D. Ando, Y. Sutou and J. Koike
Conclusion
Fe-O-N thin films were deposited by reactive sputtering in
Ar-N2-O2 gas mixture. The electrical resistivity of the asdeposited films was investigated by changing the oxygen and
nitrogen flow rates. At the fixed fN2 ¼ 1 sccm, resistivity
increased with increasing fO2 from 0.05 to 0.15 sccm. The
increase was due to structure change from a mixed phase of
metallic Fe+Fe3O4, to a mixed phase of FeO+¡-Fe2O3, and
to a single phase of ¡-Fe2O3. Meanwhile at the fixed
fO2 ¼ 0:075 sccm, resistivity increased with increasing fN2
from 0 to 5 sccm. The increase was due to structure change
from a single phase of metallic Fe, to a mixed phase of
metallic Fe+Fe3O4, and to a single phase of Fe3O4.
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