PREPARATION OF NIO X FILMS BY POST-DEPOSITION THERMAL ANNEALING OF NI: ELECTROPHYSICAL AND STRUCTURAL PROPERTIES

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1 PREPARATION OF NIO X FILMS BY POST-DEPOSITION THERMAL ANNEALING OF NI: ELECTROPHYSICAL AND STRUCTURAL PROPERTIES Pavel HORÁK a,b, Václav BEJŠOVEC b, Vasyl LAVRENTIEV b, Jiří VACÍK b, Martin VRŇATA a, Josef KHUN a a Institute of Chemical Technology, Prague, Czech Republic, EU, Pavel.Horak@vscht.cz b Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez, Czech Republic, EU Abstract Nickel oxide thin films were prepared by thermal annealing of thin Ni films deposited by ion beam sputtering. The annealing was performed at 400 C with variable time (0.5-4 hours) in a quartz furnace opened to the air. During annealing the samples underwent structural changes, as well as changes of their electrical properties. The structural properties (surface morphology and crystalline phases) were analyzed by the AFM and XRD methods, O and Ni depth concentrations by the NRA method, and electrical properties by the Van der Pauw 4-point technique (sheet resistivity). Sheet resistivity of the as-deposited sample was found to be 23.0 Ω/ ; thermal annealing caused its rise up to Ω/ after 1.5 hours of treatment. After 2.5 hours of annealing, a sharp increase in resistivity (ρ S = 2.32 MΩ/ ) was observed. At this stage the layer was composed of almost completely oxidized Ni with a distinct polycrystalline structure. The sharp increase of sheet resitivity was related to the thorough oxidation of the layer, leaving only a small amount (ca 10%) of isolated Ni particles unoxidized. Further oxidation caused only a small increase in resistivity. Keywords: nickel oxide, ion beam sputtering, NRA, van der Pauw 1. INTRODUCTION Nickel oxide (NiO) is an extensively studied material with a wide range of possible applications, e.g., electrochromic display instruments [1,2], memory storage devices [3], spin-valve films [4] and chemical sensors [5,6]. NiO exhibits an FCC structure with Ni atoms occupying octahedral sites that are surrounded by 6 O atoms. The lattice parameter is Ǻ [7]. In its pure state, NiO is an insulator, however, due to the defects and Ni 2+ vacancies NiO shows semi conductive properties. NiO components are arranged, below the Néel temperature of 523 K, to ferromagnetic (111) planes. Adjacent planes have anti-parallel spin, thereby the overall behavior is antiferromagnetic [8,9]. Configuration of adjacent planes causes a magnetostriction effect (change of the substance size upon magnetization) contracting the cubic structure of NiO to a rhombohedral shape. This contraction occurs along one of its body-diagonals and decreases with temperature [10]. The NiO films can be fabricated by a number of chemical and physical techniques, e.g., electron beam evaporation [11], dc [12,13] and rf [14,15] magnetron sputtering and pulsed laser deposition [16] as an example of physical techniques, and citrate-gel [17] and co-precipitation [18] as an example of chemical methods. The most widely used are sputtering techniques. NiO films are deposited by the utilization of either a mixture of argon and oxygen ions [12,14], or by post-deposition thermal annealing of the Ni films [7,19]. Subsequent thermal annealing causes structural changes while accentuating several crystal orientation planes. This post-deposition processing can be used for controlled tuning of the desired structure that may exhibit interesting properties with strong potential applications. In this study, the method of ion beam sputtering and post-deposition thermal annealing (at 400 C for various times) was used for the preparation of thin Ni (NiOx) films. The aim of this study is to elucidate the structural changes of the deposited films in relation to the electrical resistivity of the samples annealed with various

2 times of treatment. Fabrication of well-defined NiOx films could then be directly utilized in gas-sensing applications. 2. EXPERIMENTAL The Ni thin layers were prepared by ion beam sputtering of the Ni target. The 99.99% pure, 1 mm thick, nickel foil (MaTecK GmbH, Germany) was bombarded by an intense Ar + (Ar gas %, SIAD Czech limited). The ion source, used for the sample preparation, was a classical Duoplasmatron of the Von Ardenne type (Fig. 1) equipped with an innovated power supply system with a high processing stability (ΔU/U ~ 10-6 ). Parameters of the deposition were following: external voltage (bias between anode and ground electrode) U E = 25 kv, background pressure p B = Pa, working pressure p = Pa, Ar + ion current I i = 380 µa, time of sputtering/deposition t = 50 minutes, distance between the Ni target and the substrate 5-6 cm. Thickness of the resulting (as-deposited) Ni films, estimated by RBS (Rutherford Back- Scattering), was ca 50 nm. The Ni films were deposited on the prefabricated (polished surface RMS roughness < 3 nm) Si (100) wafers (ca 10 x 10 x 0.5 mm). For the experiment, several sets of samples were prepared; each set was used for a specific analysis (see below). For measurement of the sheet resistivity, the Ni thin films were also deposited on insulating glass plates (15 x 15 x 0.15 mm). The deposited Ni films were subsequently annealed in air at 400 C for different times (0.5, 1, 1.5, 2.5, 4 hours). Thermal annealing was performed in a quartz tube with heating and cooling rates of 40 C/minute and 6 C/minute, respectively. For each annealing, a different sample was used. The prepared Ni and oxidized NiOx Fig. 1 Schematics of the low-energy ion beam sputtering system installed at NPI AS CR Řež where the depositions were carried out. C filament cathode, B z magnetic field induced by a coil, IE intermediate electrode, A anode, E extraction electrode, GND grounded electrode, TMP turbo-molecular pump. films were characterized by NRA (Nuclear Resonance Analysis) at the Tandetron 4130 MC accelerator at NPI ASCR Rez [20]. The NRA analysis of O was performed with a 3.04 MeV alpha particle probing beam (the energy 3.04 MeV corresponds to the 16 O(α,α) resonance [21]). The surface morphology and roughness of the Ni (NiOx) film was measured by AFM (NT-NDT NTEGRA) in a semi-contact mode. Crystallinity of the Ni (NiOx) films was analyzed by X-Ray Diffraction (PANalytical X'Pert Pro MRD) with a parallel-beam geometry, X-ray mirror in the primary beam and long Soller slits with a flat monochromator on the diffracted beam using the Cu Kα radiation. The electrical properties of the NiOx films were determined by the 4-point Van der Pauw method. Both as-deposited and thermally-annealed samples were analyzed.

3 3. RESULTS AND DISCUSSION 3.1 Electrical properties As mentioned above, the sheet resistivity of the Ni and oxidized NiOx films was measured on the insulating square glass substrates (15 x 15 x 0.15 mm). Measuring electrodes were set in the corners of the samples. As a result: in the as-deposited state, the value of the sheet resistivity was found to be 23.0 Ω/. Thermal annealing (at 400 C) of the Ni layers induced an interesting 3-stage behavior in their sheet resistivity (see Fig. 2). At first, the sheet resistivity was rising slightly and almost linearly from the initial value 23 Ω/ to 46.8 Ω/ after 0.5 hour, then 88.4 Ω/ after 1 hour and Ω/ after 1.5 hours. After 2.5 hours of isothermal annealing, however, a sharp increase was recorded with a value ρ S = 2.32 MΩ/, which is more than 4 orders of magnitude higher than the previous level. In a third stage (annealing longer than 2.5 hours), only a small increase in the sheet resistivity was registered. Based on this measurement and measurement of the NiOx film thickness, evaluated from the NRA data (using the SINMRA code [22] and assuming the bulk NiO density of 6.67 g.cm -3 this value can, however, differ from the thin film density), the resistivity of the fabricated NiOx films was found to be 23.1 Ω cm. Fig. 2 Dependence of sheet resistivity on annealing time at 400 C. Each value was determined as a mean value of 10 measurements. The estimated relative error is < 2%. 3.2 NRA studies NRA (composition and profiling) studies of the NiOx films were performed by NRA, using the α-particle probe with an energy of 3.04 MeV (i.e., utilizing the 16 O(α,α) resonance). The NRA spectra were analyzed by the SIMNRA code, where evolution of the O/Ni ratio was inspected (Fig. 3). The results showed that isothermal annealing at 400 C had a strong impact on the composition of the deposited Ni layer the intensity of the O peak was rapidly rising with the annealing time, which gives clear evidence to the intense process of (Ni) oxidation. Longer annealing, however, also caused oxidation of the Si substrate surface. This could interfere with the evaluation of the oxidation process in the Ni film. In order to exclude the amount of O in SiO2 from calculation of the O/Ni ratio, a careful SINMRA analysis had to be performed the composition of NiOx film was simulated by the multiple stacking layers with a different O, Ni and Si contents and thicknesses to get the optimal fit of the NRA curve. The obtained content of O in SiO2 (presenting oxidation of the Si substrate) was subtracted from the final summary of the NiOx film atomic concentration.

4 Fig. 3 Variation in the layer composition during annealing. The values of the O/Ni were determined with about 5% relative error The O/Ni ratio was then evaluated from the summary concentration of O and Ni (at/cm 2 ) in individual layers of NiOx. As expected, O concentration grew according to the annealing time. The O/Ni ratio quickly increased from the initial negligible level to 0.19 after 0.5 hour, then to 0.68 after 1.5 hours and finally to 0.87 after 2.5 hours. After that the O concentration was rising slowly, approaching the saturation state around 0.9 (the lower O/Ni ratio value observed for 4-hour annealing was due to greater thickness of the NiOx layer in this particular measurement a sample from another set was used). From the O/Ni ratio data one can see that a small amount of Ni still remained unoxidized. It is obvious, however, that further annealing would finally lead to full oxidation of the Ni layer. Detailed SIMNRA analysis of the O peak shape also enabled estimation of the O depth profile. Fig. 4 shows the evaluated O concentration depth distributions: with increasing annealing time, the O concentration was found to be higher and more uniform along the depth. Though annealing for 0.5 hour left the Ni layer poorly oxidized in an inhomogeneous manner (with a rapid decline of O with a depth), annealing for 2.5 hours caused almost perfect oxidation throughout the Ni film, so the O concentration appeared almost constant in depth. Fig. 4 The depth profile of oxygen in the produced layer for various annealing time. The data were evaluated with a relative error of about 5% 3.3 XRD and AFM studies The XRD analysis was performed for the Θ scans (with a constant incident angle ω - 1 ). The crystalline structure of the as-deposited sample and samples annealed at 400 C for 1 hour and 2.5 hours are shown in Fig. 5. There are presented spectra before (1-hour annealing) and after (2.5-hour annealing) the

5 Fig. 5 XRD spectra of as-deposited and annealed films sharp increase in resistivity (Fig. 2). After 1-hour annealing, diffraction peaks from the polycrystalline NiO appeared. An ordering of a small fraction of Ni atoms to a hexagonal structure was also registered. From x- ray diffraction pattern one can deduce that short (1-hour) annealing leads to the formation of unstable hexagonal crystallographic structures. However, longer annealing results in gradual relaxation of the accumulated stress (i.e., the hexagonal structure disappear). Annealing for 2.5 hours caused further accentuating of several dominant crystalline planes and the layer became highly significantly polycrystalline. The XRD data pointed out the presence of (111), (200), (220), (113) and (222) NiO diffraction peaks. The high intensity peak was mainly (200) reflection with a ratio (111)/(200) of Similar results were mentioned by Hotovy et al. who used magnetron sputtering for NiO film preparation [12]. The hexagonal Ni structure disappeared, but a small amount of unoxidized Ni [(111) and (200) crystallites] was still present. During thermal annealing, the crystalline structure of the formed NiO film changed only slightly. After 1 hour the lattice parameter was 4.174(2) Å, after 2.5 hours 4.176(2) Å and after 5 hours remained still 4.176(1) Å. The magnetostriction effect was not observed; however, if it was present then it was lower than Fig. 6 AFM images of a: as-deposited sample; sample annealed for b: 1h; c: 1.5 h and d: 2.5 h at 400 C. Dimensions of images are 500 x 500 nm The AFM surface morphology images of the as-deposited and annealed samples are shown in Fig. 6. In the as-deposited state, the Ni layer is composed of small Ni clusters agglomerated into large irregular surface

6 structures. The roughness was, however, very low (see Table 1) which also suggested low porosity of the deposited film. With increasing annealing time, however, the roughness of the layer rose and the porous structure became clearly visible. Thermal annealing caused portioning effect (the large structural objects began to split) and homogenization (the annealed samples became composed of more uniform grains). The grain size appeared to be lower with higher annealing time; after 2.5 hours the grains became very uniform with a size range of ca 8 15 nm. Table 1 Parameters of the deposited films resulting from AFM analysis. Time of annealing at 400 C As-deposited 1 hour 1.5 hours 2.5 hours RMS roughness (nm) Smallest particle (nm) ~ 20 ~ 12 ~ 10 ~ 8 Largest particle (nm) ~ 30 ~ 28 ~ 30 ~ 15 As observed by thermal annealing, a sheet resistivity of the as-deposited films increased. Obviously, the increase of the sheet resistivity is related to the formation of the NiOx phases. The XRD and AFM data showed that the Ni films, prepared by ion beam sputtering, were largely amorphous and composed of randomly agglomerated Ni clusters. The clusters differed in size, but the roughness of the Ni surface was found to be very low. This may demonstrate the formation of the Ni films by sputtering of only small Ni clusters from the target that can result, even at room temperature, in a compact film with a rather smooth surface morphology. The original structure, composed of stacked small Ni clusters with a large surface area, is very accessible to thermal oxidation. As a result, after 0.5 hour of annealing, the O content in the film was found to already be about 17 % of relative concentration, but the concentration decreased sharply with the film depth (see Fig. 4). With a longer annealing time, the O concentration was higher and more uniformly distributed along the depth. Obviously, thermal oxidation caused restructuralization in the entire Ni layer. The still unoxidized Ni atoms formed crystallites of various orientations; among them the hexagonal structure was also observed. This structure, however, disappeared with longer annealing time. One can conclude that when the O atoms penetrate the whole Ni layer and oxidize most of the Ni atoms, the resistivity of the film sharply increases. This was observed in the present paper - after 2.5 hours of annealing, the value of resistivity achieved ca 30 Ω cm which is well within the semiconducting area. As only a small amount of unoxidized Ni atoms still existed in the film after annealing, it can be assumed that further annealing may change (increase) the resistivity only slightly. It is supposed, however, that in such a case the resistivity may achieve a Ω cm nominal value. 4. CONCLUSION In conclusion, thin Ni films with an almost amorphous structure were prepared by ion beam sputtering. Thermal annealing at 400 C for various times (from 0.5 to 4 hours) was applied in order to achieve controlled oxidation of the Ni films. By oxidation, the polycrystalline structure, with a prominent NiO (200) peak, was obtained. The Ni film restructuring resulted in the alteration of the film sheet resistivity. It was found that the increase of the sheet resistivity is related to the incorporation of the O atoms into the Ni film and formation of the NiOx phase. After 2.5 hours of isothermal annealing, a sharp increase of resistivity was observed, achieving the value ρ S = 2.32 MΩ/ (that is ca 23 Ω cm). Thin NiO films, with such a resistivity, can be used as a gas sensing material. ACKNOWLEDGEMENT This study received financial support from GACR - project GAP108/11/1298. LITERATURE [1] K. Yoshimura, T. Miki, S. Tanemura: Jpn. J. Appl. Phys. 34 (1995) 2440

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