Mn-Co-Ni-O thin films prepared by sputtering with alloy target

The thin film of heat-sensitive materials has been widely concerned with the current trend of miniaturization and integration of sensors. In this work, Mn1.56Co0.96Ni0.48O4 (MCNO) thin films were prepared on SiO2/Si substrates by sputtering with Mn–Co–Ni alloy target and then annealing in air at different temperatures (650–900 °C). The X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis indicated that the main crystalline phase of MCNO thin films was spinel crystal structure; the surface of the thin films was very dense and uniform. The electrical properties of the thin films were studied in the temperature range of–5–50 °C. The MCNO thin film with a low room temperature resistance R25 of 71.1 kΩ and a high thermosensitive constant B value of 3305 K was obtained at 750 °C. X-ray photoelectron spectroscopy (XPS) analysis showed that the concentration of Mn3+ and Mn4+ cations in MCNO thin films is the highest when annealing temperature is 750 °C. The complex impedance analysis revealed internal conduction mechanism of the MCNO thin film and the resistance of the thin film was dominated by grain boundary resistance.


Introduction
Negative temperature coefficient (NTC) thermistors are widely used in temperature sensors, household appliances, infrared detection, and other fields due to their high sensitivity, good stability, and low price [1,2]. The Mn-Co-Ni-O system materials with spinel structure (AB 2 O 4 ) has become one of the most commonly used NTC thermistor materials because of its excellent negative temperature coefficient and good stability [3]. Compared with traditional bulk and thick film thermistors, the thin film materials can meet the requirements of miniaturization and integration in the semiconductor industry, and have the advantages of high sensitivity, fast response, etc. It has broad application prospects in the fields of MEMS, integrated circuits, and micro-nano devices, and has become the current research hotspots of NTC thermistors [4][5][6].
At present, the main preparation methods of NTC thin films are radio frequency magnetron sputtering, evaporation coating, metal organic thermal decomposition, pulsed laser deposition, and so on [7][8][9][10][11]. The metal organic thermal decomposition method is characterized by good controllability and simple operation, but the composition and thickness of the thin film may be uneven. Pulsed laser deposition has the advantages of fast deposition speed, low deposition temperature, and uniform film preparation, but the equipment is very expensive. The thin films prepared by the magnetron sputtering have good crystallinity, high purity, good www.springer.com/journal/40145 uniformity, and are easy to be mass-produced. However, the traditional ceramic target is easily fragmented during sputtering, resulting in the surface unevenness of the target, which affects the quality of the film, and the utilization rate of the target is low. In addition, sputtering alloy target and annealing in air can make the grain on the film surface compact and reduce defects such as voids. Therefore, based on the Mn 1.56 Co 0.96 Ni 0.48 O 4 system with the lowest resistivity in bulk materials, Mn 1.56 Co 0.96 Ni 0.48 O 4 (MCNO) thin films were successfully prepared by sputtering Mn 1.56 Co 0.96 Ni 0.96 alloy target and annealing in air. The thin film also has spinel structure, and its surface is very compact, which is conducive to reducing room temperature resistance. The effects of annealing temperature on the microstructure and electrical properties of MCNO thin films prepared by this method were discussed, and the conductive mechanism in MCNO thin films was analyzed.

1 Material preparation
The thin film was firstly deposited on a 2 inch SiO 2 /Si substrate by sputtering an Mn 1.56 Co 0.96 Ni 0.48 alloy target. The deposition conditions of the NTC thin films are shown in Table 1. The 2 inch thin film was then cut into 10 mm×10 mm chips and annealed in air at 650-900 ℃ for 120 min. The heating and cooling rates are both 5 ℃/min. Silver finger electrodes were deposited on the thin films by evaporation. Then the thin films were placed in high and low temperature box for electrical performance testing in the range of -5-50 ℃. The overall structure and plane diagram of the MCNO thin film is shown in Fig. 1.

2 Characterization methods
The crystal structure of the MCNO thin films were identified by X-ray diffraction (Empyrean, PANalytical B.V.) with Cu Kα radiation. The surface morphology and thickness of MCNO thin films were analyzed by field emission scanning electron microscopy (GeminiSEM 300-71-12). Complex impedance spectroscopy was measured over the range of 20 Hz-30 MHz using a WK6500B impedance analyzer. The chemical states were examined by X-ray photoelectron spectroscopy (AXIS-ULTRA DLD-600W). The resistivity-temperature (R-T) curves of the MCNO thin films were measured between -5 and 50 ℃ using high-low temperature box (G-DJS-50A) and digital multimeter (FLUKE,8808A 5-1/2).

1 Microstructure and surface morphology
The XRD patterns of the MCNO thin films annealing at different temperatures of 650-900 ℃ are shown in Fig. 2 has no obvious change. The main peak (311) of thin film is the strongest at 750 ℃, which indicates that the MCNO thin film is highly crystalline and oriented at annealing temperature of 750 ℃ [12]. When the annealing temperature reaches 850 ℃, the impurity phases of Mn 2 O 3 and MnO 2 occur due to the decomposition of the spinel structure at excessive temperature, which affects the electrical properties of the thin film [13].
The SEM surface images of MCNO thin films at different annealing temperatures are shown in Fig. 3. The surface of thin films at different annealing temperatures is relatively dense and has no obvious defects. There is no pore at the grain boundary of the thin films, which facilitates the transmission of electrons and reduces the electrical resistivity of the material. As the annealing temperature increases, the grain size of the thin films gradually increases. The average grain size of the MCNO thin films annealed at 650, 700, 750, 800, 850, and 900 ℃ are 52.55, 62.74, 84.09, 107.11, 126.12, and 184.15 nm, respectively. This indicates that the increase of annealing temperature can promote the growth of grains [14,15]. Figure 4 is the SEM image of the thickness of 2 inch thin film measuring four boundary points (as shown in Fig. 4(e)) annealed at 750 ℃. The average thickness is 712, 713, 726, and 719 nm. The variance of the film thickness is 1.05%, 0.71%, 2.52%, and 0.08% respectively, which indicates that the prepared film has uniform thickness and good quality. It can also be seen from Figs. 4(a)-4(d) that the thin film is tightly bound to the substrate, which is conducive to improving the electrical properties and stability of the film.

2 X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) can identify  the structure, elements, and chemical states of the sample surface, which can be quantitatively or qualitatively analyzed [16,17]. The conductive properties of the Mn-Co-Ni-O system spinel structure material are mainly affected by the concentration of Mn 3+ Table  2. As the annealing temperature increases, the concentration of Mn 3+ and Mn 4+ cation pairs increased first and then decreased, and the highest concentration was obtained at 750 ℃.
The XPS spectra of the Co 2p and Ni 2p levels are shown in Figs. 6(a) and 6(b). The Co 2p spectra are dominated by two peaks located at 780.2 and 795.9 eV in binding energy, corresponding to the Co 2p 3/2 and Co 2p 1/2 spin-orbit components, respectively. The satellite peak at 6.3 eV above the main Co 2p 3/2 peak marked by an arrow in Fig. 6(a) is attributed to the contribution from Co 2+ species [18]. Besides, the satellite structure related to Co 3+ species was hardly detected, which means that Co 3+ has not been found in the thin films. In the Ni 2p spectra, Ni 2p 3/2 and Ni 2p 1/2 peaks are located at 854.6 and 872.7 eV, respectively. In the nickel-containing oxides, the Ni 2p 3/2 always shows two obvious peaks labeled as c3d9L and c3d10L2. It demonstrates that the Ni element exists in the form of +2 valence in all films.
In the Mn-Co-Ni-O system spinel structure, the divalent cation first occupies the tetrahedral A site, and there is an empirical rule for the priority order of Mn 2+ > Co 2+ > Ni 2+ [19]. Based on this empirical rule, the surface valence state distribution of the MCNO thin films is shown in Table 3. The ratio of elements in the   MCNO thin film at different annealing temperature did not change much, but the ratio of Mn 3+ and Mn 4+ in the positive octahedral position changed, which laid the foundation for the analysis of electrical properties.

3 Electrical properties
The electrons in the MCNO system of spinel are in the form of small polarons, and the resistance temperature relationship of the materials is obtained by the following equation [20]: Constant T 0 is the characteristic temperature of the material, and its product with the Boltzmann constant k B is the activation energy. T 0 is an electrical parameter corresponding to the constant B value of industrial thermistors [21,22]. α and p are temperature dependence indices of the material. When 0.25 < p = α/2 < 1, electrons mainly jump between electronic states (intermediate states) with similar energies, which is the variable range hopping (VRH) model. When α = p = 1, the hopping of electrons are mainly between nearest neighbor states, which is the nearest neighbor hopping (NNH) model.

T T T T
As the annealing temperature increases, the R 25 of the sample decreases first and then increases, while B 25/50 monotonously increases with increasing annealing temperature. When the annealing temperature is 750 ℃, the sample reaches its minimum electrical resistance of 71.1 kΩ and its B value is 3305 K. Generally, in the same volume of the MCNO thin film, the larger the grain size, the less the number of insulating grain boundaries and the lower the resistance of the film. However, if the annealing temperature is too high, the grain growth will be excessively stacked, and the number of grain boundaries will increase and meanwhile the spinel phase in the MCNO thin films is decomposed into Mn 2 O 3 and MnO 2 , which affects the conductivity of the film. Then the resistance will increase. On the other hand, the concentration of Mn 3+ and Mn 4+ cations in the octahedral position participating in conduction is the highest at 750 ℃. Therefore, when the annealing temperature is 750 ℃, the R 25 turning point of the thin film appears.
www.springer.com/journal/40145  In order to further clarify the electron conduction mechanism in the MCNO thin films, the curve of lnW-lnT is plotted as shown in Fig. 9. Shklovskii and Efros proposed a method for determining p [11,23]: The slope of fitting is p value. For MCNO film samples with different annealing temperatures, the p values are 1.12, 1.14, 1.11, 0.99, 1.10, and 0.95. All p values are close to 1, which indicates that films with different annealing temperatures are consistent with the nearest neighbor hopping (NNH) model [2,24]. The MCNO thin films at different annealing temperatures were placed in a high-low temperature test box and aged at 80 ℃ for 240 h. The relative resistance change was shown in Fig. 10   oxidation, and the cation at the grain boundary is more easily oxidized than in the grain [25]. As the annealing temperature increases, the grain size gradually increases, and the cation at the grain boundary decreases, so the aging property of the MCNO thin film becomes good.

4 Complex impedance spectrum
AC impedance spectroscopy is a powerful technique for explaining the conduction and polarization processes in materials. It can analyze the grain and grain boundary conductivity of NTC materials separately [26,27]. The complex impedance spectrum of MCNO films at different annealing temperatures measured at 25 ℃ is shown in Fig. 11. The equivalent circuit is shown in the inset of Fig. 11, which is obtained by fitting the measurement data by software Zview 2. R g and R gb represent grain resistance and grain boundary resistance, and CPE is a constant phase element. The complex impedance spectra of thin films at different annealing temperatures are all shown as flattened semicircles. When the annealing temperature is 750 ℃, the half circle area is the smallest, indicating that the impedance is obviously smaller than other samples. And the semicircle has little change in the left intercept of the Z′ axis, and the right intercept changes significantly, indicating that the grain resistance is very small and grain boundary resistance plays a dominant role [9,28].
In NTC materials, electrons are excited by heat, and activation energy can be expressed by the following equation: The curves of ln(R g /T) and ln(R gb /T)-1000/T of film annealed at 750 ℃ are shown in Fig. 12. The activation of the fit shows that the conductance activation energy of the grains and grain boundaries is much lower than the long-range mobility activation  energy of the oxygen vacancies, so the conductance of the grains and grain boundaries is caused by electron hopping [29]. Understanding the conductive mechanism inside the NTC material will greatly help the application of NTC thin film.

Conclusions
MCNO thin films were prepared by sputtering using Mn 1.56 Co 0.96 Ni 0.48 alloy target and then annealing in air. It can be seen from the XRD pattern that the MCNO thin films at different annealing temperature all have cubic spinel structures. The SEM image shows that the thin films were very dense and had no obvious defects, and the grain size gradually increased as the annealing temperature increases. The MCNO films also had good thickness uniformity. When the annealing temperature is 750 ℃, the MCNO film has the lowest R 25 (71.1 kΩ) and a higher B value (3305 K). XPS analysis shows that the concentration of Mn 3+ and Mn 4+ cations in MCNO thin films is the highest when annealing temperature is 750 ℃. Through impedance analysis, it can be known that the influence of annealing temperature on room temperature resistance is mainly the effect on grain boundary resistance.
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