21. – 23. 9. 2011, Brno, Czech Republic, EU
CHARACTERIZATION OF THE NANOSTRUCTURED NICKEL OXIDE LAYERS PREPARED BY
ION BEAM SPUTTERING
Pavel HORÁKa,b, Václav BEJŠOVECa, Vasyl LAVRENTIEVa, Josef KHUNb, Martin VRŇATAb
a
Nuclear Physics Institute, Academy of Sciences of the Czech Republic, 250 68 Řež, Czech Republic
b
ICT Prague, Technická 5, 166 28 Prague 6, Czech Republic
Abstract
Nanostructured nickel oxide layers (thickness cca 100 nm) were prepared by bombardment of nickel foil with
ion beam created from a mixture of argon and oxygen. Different volume ratios of argon:oxygen mixture were
used, ranging from 4:1 to 1:3. Composition of the resulting layers was analyzed by RBS, morphology
by AFM and main crystal orientation of the sample by XRD. The electrophysical properties (resistivity,
concentration of charge carriers) were measured by four point Van der Pauw technique and Hall
measurement respectively. Prepared samples were characterized in as-deposited state and after annealing
with varying temperature of treatment.
Chemical composition (i.e. stoichiometry) of the as-deposited samples with different argon:oxygen ratio was
related to their electrophysical parameters. Hall measurements are showing majority charge carriers to be
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electrons - surface concentration (0.5 - 2.3) x 10 m - suggesting prevailing metallic conductivity. Resistivity
of the sample is increasing with higher amount of oxygen in gas mixture. The as-deposited layer is almost
amorphous with no visible grains on AFM.
Comparison of the as-deposited and annealed sample is presented. Annealing of the sample causes
reorganization accentuating several crystalline orientation planes hence suggesting polycrystalline structure
is formed. Annealing process also leads to significant increase of the layer transmittance.
Keywords: Nickel oxide, Ion Beam Sputtering, Van der Pauw
1.
INTRODUCTION
Nickel oxide due to its interesting electric and magnetic properties is a extensively studied material. Catalytic
activity of Nickel oxide makes it promising material as a functional sensor layer for gas sensors [1]. Various
preparation techniques are used for fabrication, such as pulsed laser deposition [2,3], atomic layer epitaxy
[4] or magnetron sputtering [5]. Properties of prepared structures were studied with conclusion that that the
preparation method and the deposition parameters play an important role in the physical properties of metaloxide based gas sensors. Our research is aimed at preparation and characterization of the NiO layers
prepared by ion beam sputtering.
2.
PREPARATION
Samples were prepared by Ion Beam Sputtering (IBS). 1 mm thick nickel foil (99,99%) was bombarded by
ion beam composed of a mixture of argon and oxygen.
Different ratios of Ar : O2 were used - 4:1, 3:1, 2:1, 1:1, 1:2, 1:4, and the test sample was prepared using
pure oxygen. However, the utilization of oxygen as an only gas significantly reduces the lifetime of the
cathode wire (Tungsten), from tens of hours to tens of minutes. The substrates were Si (1 0 0) and glass
plates.Samples were annealed at 400°C and 600°C in air for 5 hours.
Extraction voltage:
25 kV
Ion beam current:
400 mA (without secondary emission)
The layer thickness: ~ 100 nm.
21. – 23. 9. 2011, Brno, Czech Republic, EU
3.
RESULTS AND DISCUSSION
3.1
RBS characterization
1 Ar : 2 O2
Nickel oxide layer deposited on PE foil
0,020
1200
Nickel oxide deposited on Si
0,018
1000
0,016
Normalized yield
O
Counts
800
Si edge
Ni
600
400
0,014
Ni
0,012
0,010
0,008
0,006
C edge O
0,004
200
0,002
0,000
0
0
100
200
300
400
500
600
700
800
900
1000
0
100
200
300
400
500
600
700
800
900
1000
Channel
Channel
Composition of the prepared nanolayer was measured by the RBS (Rutherford Back-Scattering) technique.
Measurement was performed using an incident beam of the 2 MeV alpha particles with a nominal current
10 nA and a fluence 5 mC.
Samples were deposited on the silicon substrate and thin PE foil as well. The PE foil enabled to analyze O
presence and its content; at the NiOx/Si samples the low O content was not possible analyze due to high
background from thick Si buffer.
The RBS analysis confirms that oxygen is incorporated into the Ni layer. The incorporation proceeds during
the sputtering using a mixture of argon and oxygen.
3.2
Resistivity
Resistivity of the prepared samples was measured by the 4-point Van der Pauw method. For this
measurement, samples were deposited on the glass substrate (15x15 mm).
The resistivity of the as-deposited samples (fig. 1)
shows that the oxygen concentration in the gas mixture
leads to an increase in resistivity of the NiO x layer.
Lower oxygen concentration, however, does not
significantly influence NiOx resistivity. Major charge
carriers were measured to be electrons with surface
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carrier concentration (0.5 - 2.3) x 10 m . It suggests
prevailing metallic behavior of the deposited NiO x
layers. Annealing for 5 hours at 600°C causes
a significant increase in the resistivity (see table) :
the resistivity of the annealed sample is almost 6 orders
higher than that for the as-deposited one.
Fig. 1 The resistivity of the as-deposited samples
with different mixture ratios
Table 1 Results from resistivity measurements of the annealed samples
Ratio Ar:O2
As-deposited
400°C for 5 h
600°C for 5 h
2:1
48.56 Ω
4.08 MΩ
9.17 MΩ
1:2
56.42 Ω
1.99 MΩ
2.50 MΩ
21. – 23. 9. 2011, Brno, Czech Republic, EU
3.3
Surface
Fig. 2 Comparison of the as-deposited and annealed sample (600°C) for the ratio 1 Ar : 2O2
Observed parameters were:
Table 2 AFM results - observed parameters
Ratio Ar:O2
RMS Roughness
Grain size
As-deposited
2:1
0.58 nm
25-35 nm
600°C for 5h
2:1
3.4 nm
30-60 nm
As-deposited
1:2
0.23 nm
20-25 nm
600°C for 5h
1:2
5.85 nm
cca 40 nm
AFM shows uniform grain formation after the annealing. The structure transformation correlates
with observed change of electrical properties.
3.4
Structure
Fig. 3 XRD results for the ratio Ar:O2 – 1:2
XRD measurements show quasi-amorphous structure of the as-deposited samples. After annealing the
deposited layers become polycrystalline: several strong NiO XRD peaks appear in the spectra of the
annealed samples.
4.
SUMMARY
Ion beam sputtering was exploited for fabrication of the NiO layers. Effects of the gas ratio and the postdeposition annealing on structure and electrical properties of the layers have been studied. It was found:
21. – 23. 9. 2011, Brno, Czech Republic, EU
 As-deposited layers compose mainly of amorphous nickel oxide. Higher content of oxygen results in
higher resistivity of the as-deposited layer.
 High-temperature annealing induces abundant growth of the nickel oxide grain in the layers.
The layer produced with higher content of oxygen includes uniform NiO grains with size of about
40 nm. In contrast the NiO layer produced with lower oxygen content includes various NiO grains
with size range of 30–60 nm.
 There is evident correlation between structure and electrical properties: the sample containing larger
grain (deposited with lower oxygen content) possesses higher resistivity.
This study shows the attractive opportunity of the ion beam sputtering method to produce NiO with controlled
nanostructure which is very important for sensoric application of the material.
ACKNOWLEDGEMENTS
This study received Financial support from specific university research - MSMT no.21/2011, from
Ministry of Education of the Czech Republic - project MSM 604 613 7306 and from GACR - project
GAP108/11/1298.
LITERATURE
[1.]
Hotovy I., Huran J., Siciliano P., Capone S., Spiess L., Rehacek V., Sensors and Actuators B 78 (2001) 126-132
[2.]
Gupta K. R., Ghosh K., Kahol P. K., Physica E 41 (2009) 617–620
[3.]
N. Brilis et al., Thin Solid Films 515 (2007) 8484–8489
[4.]
Utriainen M., Kröger-Laukkanen M., Niinistö L., Materials Science and Engineering B54 (1998) 98–103
[5.]
Sato H., Minami T., Takata S., Yamada T., Thin Solid Films 236 (1993) 27-31
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Characterization of the nanostructured nickel oxide layer prepared