THIN NICKEL OXIDE LAYERS PREPARED BY ION BEAM SPUTTERING: FABRICATION AND THE STUDY OF ELECTROPHYSICAL PARAMETERS

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1 THIN NICKEL OXIDE LAYERS PREPARED BY ION BEAM SPUTTERING: FABRICATION AND THE STUDY OF ELECTROPHYSICAL PARAMETERS Pavel HORÁK a,b, Václav BEJŠOVEC b, Vasyl LAVRENTIEV b, Jiří VACÍK b, Martin VRŇATA a, Josef KHUN a a Department of Physics and Measurements, Institute of Chemical Technology, Technická 5, Prague 6, Czech Republic b Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez, Czech Republic Pavel.Horak@vscht.cz Abstract Thin nickel oxide layers (thickness ca 50 and 100 nm) for sensorics were fabricated by ion beam sputtering method with subsequent annealing. Ion beam formed from a mixture of argon and oxygen was used to sputter the nickel foil. Different volume ratios of argon:oxygen in mixture were used, ranging from 1:0 to 1:4. Deposited layers were characterized in as-deposited state and after annealing at temperature of 400 C. The study of electrophysical properties (sheet resistance, mobility and concentration of charge carriers) was performed by four point Van der Pauw technique and Hall measurements respectively. Hall measurements revealed majority charge carriers to be electrons. For as-deposited layers electron surface concentration decreases with increasing amount of oxygen in ion beam and is in range (5 23) m -2 for above mentioned range of argon:oxygen ratios. According to this trend the sheet resistance of the layers increases with higher amount of oxygen in ion beam in the interval of values (40 420) Ω/. Keywords: Nickel oxide, Ion Beam Sputtering, van der Pauw 1. INTRODUCTION Nickel oxide is an extensively studied material with potential use in many applications like battery electrodes, electrochemical films, photo-electronic devices, catalysts, semiconducting gas sensors etc. [1]. In pure state NiO x is an insulator due to Ni 2+ vacancies, however, P type semiconductivity occurs. This makes nickel oxide especially interesting in gas sensing applications, when mainly N type semiconductors have been studied so far (SnO 2, α-fe 2 O 3, etc.). In nanoscale new behavior of compounds is found, the noise of sensor using thin layers is lower and by decreasing the crystal size potential barriers at the grain boundaries can be removed [2]. Also thin layer based sensors are faster and more sensitive than the sensors using thick bulk layers. Thin nickel oxide layers can be prepared by a number of deposition methods. Most common is magnetron sputtering (dc, rf) [3,4] as a well known technique widely used also in industry. Other techniques are e.g. EBE (Electron Beam Evaporation) [5,6], PLD (Pulsed Laser Deposition) [7,8]. Utilization of IBS (ion beam sputtering) for deposition of nickel oxide is a method used scarcely but its construction makes IBS a unique technique for preparation of thin layers. IBS has the advantage of easily controllable deposition process. It is more or less universal method; choice of the target material is almost not limited by its electrical and magnetic properties. Also demands on shape and composition of the target is lower (pressed targets can be used) compared to other techniques. Ion beam sputtering allows deposition of homogeneous layers qualitatively comparable only with magnetron sputtering [9].

2 Aim of this study is to clarify methodology of preparation and measurement of electro-physical parameters of NiO x thin layers with respect to application in sensorics. Thin films will serve as an active layer of the sensors. From this view, it is necessary to study the layer as a whole, rather than material parameters. 2. EXPERIMENTAL Thin layers of nickel oxide were prepared by ion beam sputtering. Experimental arrangement is shown in Fig. 1. Pure nickel foil (99.99%, Mateck) was bombarded by ion beam generated from the duoplasmatron ion source that was focused by a system of electrodes (unipotential lens) in the ion optics chamber. Ion beam was formed from a mixture of argon and oxygen. Different ratios of Ar : O 2 were used 1:0, 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 the only gas significantly reduces the lifetime of the cathode wire (Tungsten), from tens of hours to tens of minutes. The substrates used for the deposition were glass plates (15x15mm). Subsequently, samples were annealed in furnace at 400 C in air for a period ranging from 0.5 to 5 hours. Extraction voltage: Ion beam current: 25 kv 400 µa (without secondary emission) The layer thickness: ~ 50 and 100 nm (measured by RBS, Rutherford Back-scattering Spectrometry). Fig. 1 The Duoplasmatron ion source and deposition chamber. K-cathode, B-magnetic coil, A-anode. 3. RESULTS AND DISCUSSION The study of electrophysical properties, i.e. sheet resistance, mobility and concentration of charge carriers, was performed by four point van der Pauw technique [10] and Hall measurements [11] respectively. In case of as-deposited layers, different values of the measuring current (I = 0.1; 1 and 10 ma) were tested to find out its suitable value. In case of the current 0.1 ma, the obtained set of values of sheet resistance, charge concentration and mobility evinced standard deviation by one order higher comparing with the case of 1 and 10 ma measuring currents. In addition, at the measuring currents 1 and 10 ma, the layers showed to be reciprocal (the property called reciprocity is explained e.g. in [10]) while in case of 0.1 ma it was not achieved. Taking into consideration possible excessive heating of the layers at the current 10 ma, measuring current I = 1 ma was selected to be optimal.

3 Fig. 2 The sheet resistance of the as-deposited samples with different mixture ratios (thickness ca 50 nm) The sheet resistance of the as-deposited samples (Fig. 2) for different Ar:O 2 ratios shows that the oxygen concentration in the gas mixture leads to an increase in sheet resistance of the NiO x layer. Lower oxygen concentration, however, does not significantly influence NiO x resistivity. Majority charge carriers were found to be electrons with surface carrier concentration ( ) x m -2. It suggests prevailing metallic behavior of the deposited NiO x layers. For sensoric purposes, above presented values of sheet resistance (Fig. 2) are too low; the influence of detected gas on the layer resistance would be negligible. In addition, the as-deposited layers would not be thermodynamically stable in the long term period. For these reasons, the layers were subsequently treated by annealing which was performed in the air, at temperature 400 C, for different times. The as-deposited layers were in this case prepared only by Ar ions due to the negative influence of O 2 on the cathode in Duoplasmatron (O 2 decreases the working time of cathode filament) with subsequent thermal oxidation. Van der Pauw measurement of samples with thickness 50 nm revealed their non-reciprocity, asymmetry and the set of sheet resistance values featured standard deviation two orders higher comparing with the case of as-deposited samples at measuring current of 1 ma. A possible explanation is that the layers became discontinuous after annealing. However approximate value of sheet resistance is in order of magnitude 10 6 Ω/ after 2.5 hours of annealing. Due to this fact, subsequently prepared layers had two times greater thickness ca 100 nm. In Fig. 3 and Fig. 4 the results of van der Pauw and Hall measurements are presented.

4 mobility (m^2 / V s) sheet resistance (Ω/ ) 250,0 200,0 150,0 100,0 50,0 0, annealing time (hour) 100 nm 50 nm Fig. 3 The dependence of sheet resistance on annealing time for layer thicknesses 50 and 100 nm, respectively. Ion sputtering deposition - Ar ions only, annealing in air at t = 400 C. 1,6E-02 1,4E-02 1,2E-02 1,0E-02 8,0E-03 6,0E-03 4,0E-03 2,0E-03 0,0E annealing time (hour) Fig. 4 The dependence of charge mobility on annealing time; ion sputtering deposition - Ar ions only, annealing in air at t = 400 C, thickness ca 100 nm. From Fig. 3 it is seen that, in case of 100 nm layers, after 5 hours annealing, the sheet resistance increases by about two orders of magnitude and charge carrier mobility (Fig. 4), decreases accordingly. Oxidation of the whole layer requires longer annealing times, the preliminary results showed, however, that the sheet resistance will reach order of 10 6 Ω/. In Fig. 3, the comparison of sheet resistance for different layers thicknesses 50 nm and 100 nm is presented.

5 Fig. 5 Layers after annealing, from left to right annealing times 0, 2.5, 4 a 5 hours. In Fig. 5, the dependence of layer transparency on the annealing time is presented. It could be concluded that the oxidation degree of layers is proportional to the layer transparency. In Fig. 6, the photo of the NiO x layer deposited on the Al 2 O 3 sensor substrate can be seen. Fig. 6 NiO x layer deposited on the Al 2 O 3 sensor substrate (dimensions 2.5 x 2.5 mm) 4. CONCLUSION The main goal of this study was to prepare methodical procedure of the deposition of thin nickel oxide layers by ion beam sputtering with subsequent use in sensorics. Due to the necessity of thermodynamical stability as the most feasible approach appears subsequent annealing of layers. Also working life of cathode filament of duoplasmatron ion source is longer without oxygen and therefore nonreactive ion beam sputtering (Ar only) was finally used to prepare as-deposited layers. Measurement of the layer sheet resistance in as-deposited state is most feasibly performed by current of 1 ma. Sample heating is then limited and results show suitable accuracy. During annealing, the 50 nm layer reaches sheet resistance of 10 2 Ω/ order after ca 1.5 hour, whereas in case of 100 nm layer, this change takes place after ca 4.5 hours. ACKNOWLEDGEMENT Financial support from specific university research (MSMT no. 21/2012) is gratefully acknowledged.

6 REFERENCES [1] WEI Z. et al. Characterization of NiO nanoparticles by anodic arc plasma method. Journal of Alloys and Compounds 479 (2009) [2] VRŇATA, M. Chemical sensors. Proceeding. Praha: VŠCHT, [3] Mallikarjuna Reddy A., Sivasankar Reddy A., Sreedhara Reddy P. Thickness dependent properties of nickel oxide thin films deposited by dc reactive magnetron sputtering. Vacuum 85 (2011) [4] CHEN H.-L., LU Y.-M., HWANG W.-S. Characterization of sputtered NiO thin films. Surface & Coatings Technology 198 (2005) [5] HAKIM A., HOSSAIN J., KHAN K. A. Temperature effect on the electrical properties of undoped NiO thin films. Renewable Energy 34 (2009) [6] WISITSORAAT A. et al. Characterization of n-type and p-type semiconductor gas sensors based on NiO x doped TiO 2 thin films. Thin Solid Films 517 (2009) [7] BRILIS N. et al. Development of NiO-based thin film structures as efficient H 2 gas sensors operating at room temperatures. Thin Solid Films 515 (2007) [8] GUPTA R. K., GHOSH K., KAHOL P. K. Fabrication and characterization of NiO/ZnO p n junctions by pulsed laser deposition. Physica E: Low-dimensional Systems and Nanostructures 41 (2009) [9] NISHIZAWA S. et al. Structural changes in ZnO/NiO artificial superlattices made by ion beam sputtering. Thin Solid Films 302 (1997) [10] NÁHLÍK J., KAŠPÁRKOVÁ I., FITL P. Study of quantitative influence of sample defects on measurements of resistivity of thin films using van der Pauw method. Measurement 44 (2011) [11] Van der PAUW L. J. A method of measuring specific resistivity and Hall effect of discs of arbitrary shape. Philips Research Reports 13 (1958) 1 9.