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Electrical properties of copper-nickel manganite thin films prepared by metal-organic decomposition
Author’s Accepted Manuscript Electrical properties of copper-nickel manganite thin films prepared by metal-organic decomposition Chang Jun Jeon, Young Hun Jeong, Ji Sun Yun, Woon Ik Park, Jong Hoo Paik, Youn Woo Hong, Eung Soo Kim, Jeong Ho Cho www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)30694-6 http://dx.doi.org/10.1016/j.ceramint.2017.04.088 CERI15068
To appear in: Ceramics International Received date: 17 January 2017 Revised date: 16 March 2017 Accepted date: 13 April 2017 Cite this article as: Chang Jun Jeon, Young Hun Jeong, Ji Sun Yun, Woon Ik Park, Jong Hoo Paik, Youn Woo Hong, Eung Soo Kim and Jeong Ho Cho, Electrical properties of copper-nickel manganite thin films prepared by metalorganic decomposition, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrical properties of copper-nickel manganite thin films prepared by metal-organic decomposition
Chang Jun Jeona, Young Hun Jeonga, Ji Sun Yuna, Woon Ik Parka, Jong Hoo Paika, Youn Woo Honga, Eung Soo Kimb, and Jeong Ho Choa,* a
Electronic Materials & Component Center, Korea Institute of Ceramic Engineering & Technology, Jinju 52851, Korea b
Department of Materials Engineering, Kyonggi University, Suwon 16227, Korea
Abstract [(Ni0.2Mn0.8)1-xCux]3O4 (0.25 ≤ x ≤ 0.4) thin films were fabricated by metal-organic decomposition to develop new thermal imaging materials applicable to microbolometer. Effect of Cu content on the electrical properties of the annealed films was investigated. The microstructures of the annealed films were improved with an increase of Cu content. A single phase of cubic spinel structure was confirmed for the annealed films. With increasing Cu content, the resistivity (ρ) of the annealed films decreased due to the improved microstructures. The negative temperature coefficient of resistance (TCR) of the annealed films also decreased with an increase of Cu content. Good electrical properties with values of ρ = 30.9 Ω·cm and TCR = -3.253 %/K at room temperature were obtained in x = 0.275 films annealed at 450 °C for 15 h.
Keywords: copper-nickel manganite, metal-organic decomposition, microbolometer, temperature coefficient * Corresponding author. E-mail address: [email protected] (J.H. Cho).
1
1. Introduction
Since the acute epidemics such as SARS (2003), Influenza A (2009), Ebola (2014), MERS (2015), and ZIKA virus (2016) have spread throughout the world, the infrared imaging cameras have been widely used to prevent these viruses early. Recently, there is a growing interest in infrared detectors, which are classified into thermal types and photon types [1]. Thermal detectors do not require cooling because their operating temperature is room temperature (300 K). Therefore, they have advantages of miniaturization and low price compared with those of photon detectors [1]. These thermal detectors can be again divided into thermopile, pyroelectric and microbolometer according to the operating principle [2]. Among them, extensive studies on microbolometer have been conducted by optimization of microelectromechanical systems (MEMS) technology [1]. The temperature sensing materials in microbolometer require low resistivity (low 1/f noise) and high temperature coefficient of resistance (TCR) for high sensitivity of infrared sensors [1,2]. Also, they should have a low annealing temperature (≤ 450 °C) to prevent the damage in readout integrated circuits (ROIC) and to maintain the compatibility with complementary metal-oxide semiconductor (CMOS) [2]. Most sensing materials applicable to microbolometer include vanadium oxide (VOx), amorphous silicon (a-Si), silicon diodes [2]. Especially, the VOx is most commonly used because it is possible to process at low temperature. However, there are difficulties in controlling the stoichiometric composition and electrical properties by the deposition method and process variables, and in ensuring the reproducibility. In order to overcome these drawbacks, the nickel manganite of spinel structure was found to be promising
2
candidate for sensing materials in microbolometer [3-6]. Ko et al. reported that cubic spinel nickel manganite films with Mn/(Mn+Ni) = 0.80 showed the resistivity of 12 kΩ·cm and TCR of -3.9 %/K [5]. Although the TCR is high enough, it is not easy to apply these films in practical microbometer since the resistivity is relatively high. It has been reported that Copper doping reduces the resistivity of the nickel manganite films to < 1.0 kΩ·cm without remarkable degradation of TCR values (> -2 %/K), thus resulting in films suitable for microbolometer applications [4]. Among all the methods of preparing metal oxide thin films, the metal-organic decomposition (MOD) was selected in this study because of its simple preparation, precise control of chemical stoichiometry, low annealing temperature, no required vacuum system, low running cost, and ability to fabricate large-area films [7-9]. Therefore, the electrical properties of [(Ni0.2Mn0.8)1-xCux]3O4 thin films prepared by MOD were investigated as a function of Cu content (0.25 ≤ x ≤ 0.4). The TCR of the annealed films was discussed in relation to their thermal sensitivity for microbolometer applications.
3
2. Experimental
The enhanced MOD coating solutions of NiO (0.5 mol/L, SYM-NI05, Kojundo Chemical Laboratory, Japan), MnO1.5 (0.5 mol/L, SYM-MN05, Kojundo Chemical Laboratory, Japan), and CuO (0.4 mol/L, SYM-CU04, Kojundo Chemical Laboratory, Japan) were used as starting materials. Since the enhanced MOD solutions contain one metal-organic (carboxylate) compounds dissolved in organic solvent (xylene), the composition of solutions can be precisely controlled [9]. The precursor solutions were prepared according to the desired compositions of [(Ni0.2Mn0.8)1-xCux]3O4 and mixed with butyl acetate to improve wet adhesion of coating to substrates. The mixed solutions were spin-coated at 1000 rpm for 30 s on cleaned substrates of low-stress SiNx (200 nm) coated Si wafer (2.5 cm × 2.5 cm). In order to decompose the organic materials and obtain the amorphous metal oxide films, the deposited films were baked at 400 °C for 20 min. These films were annealed at 450 °C for 15 h in air for crystallization. The surface morphology and thickness of the annealed films were observed using a scanning electron microscopy (SEM, JSM-6700F, Jeol, Japan). The element mapping images of the annealed films were observed by transmission electron microscopy (TEM, Tecnai G2 F30 S-twin, FEI, Netherlands). The crystalline phase of the annealed films was identified using an X-ray diffractometer (XRD, D/Max-3C, Rigaku, Japan). The resistivity of the annealed films was measured by four-point probe method [10] in a temperature range from 25 °C to 80 °C. The measuring system is composed of probe station (VP75, DSF System, Korea), computer-controlled source meter, and purposebuilt automated data acquisition system [11].
4
3. Results and discussion
Fig. 1 shows the SEM micrographs of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. The nano-sized grains and puddle-shape defects (black area) were observed at x = 0.25. No grain growth has occurred in the puddle-shape defects. With increasing Cu content (x), the puddle-shape defects decreased and the annealed films become smooth and uniform, but the grain size decreased. These results could be explained that the heterogeneous precipitation of Cu in grain boundary was changed to homogeneous distribution by the decrease of puddle-shape defects with the Cu content. A similar tendency was confirmed for Mn-Co-Ni-Cu system reported by Xiong et al. [12]. The Cu played an important role in improving the morphology and uniformity of the annealed films. In order to further investigate the interfacial microstructure, the cross-sectional image and element mapping images of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h are shown in Fig. 2. The thickness of the annealed film was very thin (about 50 nm), uniform and compact. The annealed film and SiNx/Si substrate were well attached to each other without de-laminations (Fig. 2(a)) and distinct diffusions near the interface between film and substrate (Fig. 2(b)). A similar result of Fig. 2 was confirmed for the other compositions. Fig. 3 shows the XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. A single phase of the cubic spinel structure was confirmed in all of the compositions. Only two main peaks of (220) and (311) plans (JCPDS No. 32-0345) were detected at 2θ = 30° and 35°, respectively. However, their intensities were weak because the thickness of the annealed film was very thin, as confirmed in Fig. 2.
5
Fig. 4 shows the resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. The resistivity of the annealed films decreased with an increase of measured temperature, which indicated typical negative temperature coefficient (NTC) thermistor characteristics. All the annealed films showed a low resistivity (below 40 Ω·cm) at room temperature. With increasing Cu content (x), the resistivity of the annealed films decreased at a given temperature. This result could be attributed to decrease of puddleshape defects and improvement of microstructure homogeneity in the annealed films (Fig. 1). The resistivity is actually the result of the combined factors such as grain size, crystal structure, defects and microstructure homogeneity [13,14]. In this work, the resistivity of the annealed films was affected by the defects and microstructure homogeneity, more so than by grain size and crystal structure. If no defects (black area (Fig. 1)) were found, the grain size (grain boundary) should be considered for the analysis of resistivity because the annealed films showed same crystal structure of spinel (Fig. 3). It has also been reported that the resistivity of thin films is affected by hopping probability of localized carriers and hopping distance as well as defects and grain size [15]. Microscopically, the electrical conductivity in spinel manganite thin films is dependent on the carrier hopping between neighboring Mn3+ and Mn4+ in octahedral sites because hopping distances of octahedral sites are shorter than those of tetrahedral positions in the spinel structure [16]. The substitution of transition metals such as nickel, cobalt and copper for Mn3+ ions induces formation of Mn4+ ions in octahedral sites resulting in the change of ordering and the increase of conductivity [13]. However, Mn2+ cannot participate in this process for the large on-site coulomb repulsion energy [17]. Jadhav and Puri reported that the decrease of resistivity with Cu content in Ni-Mn-Cu systems may also be due to the participation of Cu1+ and Cu2+ cations at
6
octahedral sites in the hopping process [14]. To reduce the resistivity of Ni-Mn-Cu systems, both Mn and Cu cations at octahedral sites can be considered as hopping ions [13]. The temperature dependent resistivity (ρ) of thin films is generally described by a nearest-neighbor hopping (NNH) model [15,16]: ρ(T) = CTexp(T0/T)
(1)
where C is a constant, T is the absolute temperature, and T0 is the characteristic temperature, which is proportional to the activation energy (Ea) for NNH. To derive the Ea of the annealed films, the plots of ln(ρ/T) versus 1/T are shown in Fig. 5. The T0 values were obtained from the slop of dash line by least squares fit of the data points. The validity of NNH was supported by the high values of adjusted R-square (≥ 0.9995) for all slops. The Ea was calculated from the following equation [18]: Ea = T0KB
(2)
where KB is Boltzmann constant. The T0 and Ea are listed in Table 1. The T0 is a measure of the sensitivity of NTC films and the Ea is the energy for the hopping process of localized carriers in octahedral sites [18]. With increasing Cu content (x), the T0 and Ea of the annealed films decreased. These values were lower than those (T0 = 3374-3910, Ea = 0.291-0.337) reported for Ni-Mn-Co thin films [11]. He et al. also reported that the T0 of Ni-Mn-Co-Cu thin films was lower than that of Ni-Mn-Co thin films [15]. The TCR at 298 K (T) was evaluated using the following equation [18]: TCR (%/K) = −100T0/T2
(3)
Like the T0 and Ea tendencies, the TCR of the annealed films decreased with an increase of Cu content (x), as shown in Table 1. Especially, the TCR values of x = 0.25 and 0.275 films were higher than those (< -3 %/K) of Cu-doped nickel manganite thin
7
films reported by Ko et al. [4]. Although many bulk ceramics, thick and thin films have mentioned various values of TCR until now, there were no reports of factors affecting TCR. For the copper-nickel manganite with a cubic spinel structure, the changes of TCR with Cu content may be related to the structural characteristics (octahedral distortion, bond length, bond strength, etc.) but are, as yet, unclear and under investigating in detail.
8
4. Conclusions
For [(Ni0.2Mn0.8)1-xCux]3O4 (0.25 ≤ x ≤ 0.4) thin films annealed at 450 °C for 15 h, a single phase of cubic spinel structure was confirmed. With increasing Cu content, the morphology and uniformity of the annealed films with a thickness of about 50 nm were improved. The annealed films showed the typical NTC thermistor characteristics. The resistivity of the annealed films decreased with an increase of Cu content. This result is due to the decrease of puddle-shape defects without grain growth. With decreasing Cu content, the T0 of the annealed films increased, which induced the increase of Ea and TCR. These tendencies may be used for predictive properties optimization of coppernickel manganite and other NTC materials for microbolometer applications.
Acknowledgements
This work was supported by the Industrial Strategic Technology Development Program (10045177, Development of Resistive Ceramic Thin Film using Solution Process and Low Temperature Thin Film Vacuum Getter).
9
References
[1] R.K. Bhan, R.S. Saxena, C.R. Jalwania, S.K. Lomash, Uncooled infrared microbolometer arrays and their characterization techniques, Def. Sci. J. 59 (2009) 580–589. [2] F. Niklaus, C. Vieider, H. Jakobsen, MEMS-based uncooled infrared bolometer arrays - a review, Proc. SPIE 6836 (2007) 68360D1–15. [3] H. Schulze, J. Li, E.C. Dickey, S. Trolier-McKinstry, Synthesis, phase characterization, and properties of chemical solution-deposited nickel manganite thermistor thin films, J. Am. Ceram. Soc. 92 (2009) 738–744. [4] S.W. Ko, J. Li, N.J. Podraza, E.C. Dickey, S. Trolier-McKinstry, Spin spraydeposited nickel manganite thermistor films for microbolometer applications, J. Am. Ceram. Soc. 94 (2011) 516–523. [5] S.W. Ko, H.M. Schulze, D.B.S. John, N.J. Podraza, E.C. Dickey, S.S. TrolierMcKinstry, Low temperature crystallization of metastable nickel manganite spinel thin films, J. Am. Ceram. Soc. 95 (2012) 2562–2567. [6] D.T. Le, C.J. Jeon, K.W. Lee, Y.H. Jeong, J.S. Yun, D.H. Yoon, J.H. Cho, Liquid flow deposited spinel (Ni,Mn)3O4 thin films for microbolometer applications, Appl. Surf. Sci. 330 (2015) 366–373. [7] K. Hamanaka, T. Tachiki, T. Uchida, Investigation of characteristics for different film thickness of Bi-2212/MgO fabricated by metal-organic decomposition, Physica C 470 (2010) 1457–1460. [8] F.J. Geng, C.H. Yang, P.P. Lv, C. Feng, Q. Yao, X.M. Jiang, Influence of precursor solution concentration on the microstructure, leakage current and dielectric tenability of Zn-doped Na0.5Bi0.5TiO3 thin films prepared by metal organic decomposition, Ceram. Int. 42 (2016) 12033–12037. [9] D.A. Wahid, J. Sato, M. Hosoda, H. Shimizu, Preparation and characterization of Bi substituted gadolinium iron garnet BixGd3-xFe5O12 films with x = 1 to 2.5 by enhanced metal organic decomposition, J. Magn. Soc. Jpn. 40 (2016) 107–114. [10] F.M. Smits, Measurement of sheet resistivities with the four-point probe, The Bell System Techn. J. 37 (1958) 711–718. 10
[11] G. Ji, A. Chang, H. Li, Y. Xie, H. Zhang, W. Kong, Epitaxial growth of Mn-Co-NiO thin films and thickness effects on the electrical properties, Mater. Lett. 130 (2014) 127–130. [12] Structure and electrical performance of Mn1.5-0.5xCo0.9-0.3xNi0.6-0.2xCuxO4 NTC ceramics prepared by heterogeneous precipitation, J. Alloys Compd. 606 (2014) 273–277. [13] R. Jadhav, D. Kulkarni, V. Puri, Structural and electrical properties of fritless Ni(1x)CuxMn2O4
(0 ≤ x ≤ 1) thick film NTC ceramic, J. Mater. Sci. Mater. Electron. 21
(2010) 503–508. [14] R.N. Jadhav, V. Puri, Influence of copper substitution on structural, electrical and dielectric properties of Ni(1-x)CuxMn2O4 (0 ≤ x ≤ 1) ceramics, J. Alloys Compd. 507 (2010) 151–156. [15] L. He, G. Zhang, Z.Y. Ling, Improvement of structural and electrical properties of Mn-based thin film thermistors by a bilayer structure, Mater. Lett. 128 (2014) 144– 147. [16] A. Feteira, Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective, J. Am. Ceram. Soc. 92 (2009) 967–983. [17] W. Kong, H. Bu, B. Gao, L. Chen, F. Cheng, P. Zhao, G. Ji, A. Chang, C. Jiang, Effects of preferred orientation on electrical properties of Mn1.56Co0.96Ni0.48O4±δ spinel films, Mater. Lett. 137 (2014) 36–40. [18] Y.Q. Gao, Z.M. Huang, Y. Hou, J. Wu, W. Zhou, C. OuYang, J.G. Huang, J.C. Tong, J.H. Chu, Structural and electrical properties of Mn1.56Co0.96Ni0.48O4 NTC thermistor films, Mater. Sci. Eng. B 185 (2014) 74–78.
11
List of Figures
Fig. 1. Surface morphology SEM images of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 2. Cross-sectional SEM image (a) and element mapping images (b-e) of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h.
Fig. 3. XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 4. Resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 5. Plots of ln(ρ/T) versus 1/T for [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
12
Fig. 1. Surface morphology SEM images of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
13
Fig. 2. Cross-sectional SEM image (a) and element mapping images (b-e) of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h.
14
(311)
(220)
Intensity (arb. unit)
Spinel Substrate
x = 0.4 x = 0.375 x = 0.35 x = 0.325 x = 0.3 x = 0.275 x = 0.25
10
20
30
40
50
60
2 (degree)
Fig. 3. XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
15
45
x = 0.25 x = 0.275 x = 0.3 x = 0.325 x = 0.35 x = 0.375 x = 0.4
40
Resistivity (cm)
35 30 25 20 15 10 5 0 295
305
315
325
335
345
355
Temperature (K)
Fig. 4. Resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
16
1
x = 0.25 x = 0.275 x = 0.3 x = 0.325 x = 0.35 x = 0.375 x = 0.4
ln(/T) (cm/K)
0 -1 -2 -3 -4 -5 -6 -7 0.0028
0.0030
0.0032
0.0034
1/T (1/K)
Fig. 5. Plots of ln(ρ/T) versus 1/T for [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
17
List of Tables
Table 1 The characteristic temperature (T0), activation energy (Ea) and temperature coefficient of resistance (TCR) of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
18
Table 1 The characteristic temperature (T0), activation energy (Ea) and temperature coefficient of resistance (TCR) of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
x (mol)
T0 (K)
Ea (eV)
TCR (%/K)
0.25
3154
0.2718
-3.436
0.275
2987
0.2574
-3.253
0.3
2684
0.2313
-2.923
0.325
2671
0.2302
-2.910
0.35
2293
0.1976
-2.498
0.375
1709
0.1473
-1.862
0.4
1433
0.1234
-1.560
19
PII: DOI: Reference:
S0272-8842(17)30694-6 http://dx.doi.org/10.1016/j.ceramint.2017.04.088 CERI15068
To appear in: Ceramics International Received date: 17 January 2017 Revised date: 16 March 2017 Accepted date: 13 April 2017 Cite this article as: Chang Jun Jeon, Young Hun Jeong, Ji Sun Yun, Woon Ik Park, Jong Hoo Paik, Youn Woo Hong, Eung Soo Kim and Jeong Ho Cho, Electrical properties of copper-nickel manganite thin films prepared by metalorganic decomposition, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrical properties of copper-nickel manganite thin films prepared by metal-organic decomposition
Chang Jun Jeona, Young Hun Jeonga, Ji Sun Yuna, Woon Ik Parka, Jong Hoo Paika, Youn Woo Honga, Eung Soo Kimb, and Jeong Ho Choa,* a
Electronic Materials & Component Center, Korea Institute of Ceramic Engineering & Technology, Jinju 52851, Korea b
Department of Materials Engineering, Kyonggi University, Suwon 16227, Korea
Abstract [(Ni0.2Mn0.8)1-xCux]3O4 (0.25 ≤ x ≤ 0.4) thin films were fabricated by metal-organic decomposition to develop new thermal imaging materials applicable to microbolometer. Effect of Cu content on the electrical properties of the annealed films was investigated. The microstructures of the annealed films were improved with an increase of Cu content. A single phase of cubic spinel structure was confirmed for the annealed films. With increasing Cu content, the resistivity (ρ) of the annealed films decreased due to the improved microstructures. The negative temperature coefficient of resistance (TCR) of the annealed films also decreased with an increase of Cu content. Good electrical properties with values of ρ = 30.9 Ω·cm and TCR = -3.253 %/K at room temperature were obtained in x = 0.275 films annealed at 450 °C for 15 h.
Keywords: copper-nickel manganite, metal-organic decomposition, microbolometer, temperature coefficient * Corresponding author. E-mail address: [email protected] (J.H. Cho).
1
1. Introduction
Since the acute epidemics such as SARS (2003), Influenza A (2009), Ebola (2014), MERS (2015), and ZIKA virus (2016) have spread throughout the world, the infrared imaging cameras have been widely used to prevent these viruses early. Recently, there is a growing interest in infrared detectors, which are classified into thermal types and photon types [1]. Thermal detectors do not require cooling because their operating temperature is room temperature (300 K). Therefore, they have advantages of miniaturization and low price compared with those of photon detectors [1]. These thermal detectors can be again divided into thermopile, pyroelectric and microbolometer according to the operating principle [2]. Among them, extensive studies on microbolometer have been conducted by optimization of microelectromechanical systems (MEMS) technology [1]. The temperature sensing materials in microbolometer require low resistivity (low 1/f noise) and high temperature coefficient of resistance (TCR) for high sensitivity of infrared sensors [1,2]. Also, they should have a low annealing temperature (≤ 450 °C) to prevent the damage in readout integrated circuits (ROIC) and to maintain the compatibility with complementary metal-oxide semiconductor (CMOS) [2]. Most sensing materials applicable to microbolometer include vanadium oxide (VOx), amorphous silicon (a-Si), silicon diodes [2]. Especially, the VOx is most commonly used because it is possible to process at low temperature. However, there are difficulties in controlling the stoichiometric composition and electrical properties by the deposition method and process variables, and in ensuring the reproducibility. In order to overcome these drawbacks, the nickel manganite of spinel structure was found to be promising
2
candidate for sensing materials in microbolometer [3-6]. Ko et al. reported that cubic spinel nickel manganite films with Mn/(Mn+Ni) = 0.80 showed the resistivity of 12 kΩ·cm and TCR of -3.9 %/K [5]. Although the TCR is high enough, it is not easy to apply these films in practical microbometer since the resistivity is relatively high. It has been reported that Copper doping reduces the resistivity of the nickel manganite films to < 1.0 kΩ·cm without remarkable degradation of TCR values (> -2 %/K), thus resulting in films suitable for microbolometer applications [4]. Among all the methods of preparing metal oxide thin films, the metal-organic decomposition (MOD) was selected in this study because of its simple preparation, precise control of chemical stoichiometry, low annealing temperature, no required vacuum system, low running cost, and ability to fabricate large-area films [7-9]. Therefore, the electrical properties of [(Ni0.2Mn0.8)1-xCux]3O4 thin films prepared by MOD were investigated as a function of Cu content (0.25 ≤ x ≤ 0.4). The TCR of the annealed films was discussed in relation to their thermal sensitivity for microbolometer applications.
3
2. Experimental
The enhanced MOD coating solutions of NiO (0.5 mol/L, SYM-NI05, Kojundo Chemical Laboratory, Japan), MnO1.5 (0.5 mol/L, SYM-MN05, Kojundo Chemical Laboratory, Japan), and CuO (0.4 mol/L, SYM-CU04, Kojundo Chemical Laboratory, Japan) were used as starting materials. Since the enhanced MOD solutions contain one metal-organic (carboxylate) compounds dissolved in organic solvent (xylene), the composition of solutions can be precisely controlled [9]. The precursor solutions were prepared according to the desired compositions of [(Ni0.2Mn0.8)1-xCux]3O4 and mixed with butyl acetate to improve wet adhesion of coating to substrates. The mixed solutions were spin-coated at 1000 rpm for 30 s on cleaned substrates of low-stress SiNx (200 nm) coated Si wafer (2.5 cm × 2.5 cm). In order to decompose the organic materials and obtain the amorphous metal oxide films, the deposited films were baked at 400 °C for 20 min. These films were annealed at 450 °C for 15 h in air for crystallization. The surface morphology and thickness of the annealed films were observed using a scanning electron microscopy (SEM, JSM-6700F, Jeol, Japan). The element mapping images of the annealed films were observed by transmission electron microscopy (TEM, Tecnai G2 F30 S-twin, FEI, Netherlands). The crystalline phase of the annealed films was identified using an X-ray diffractometer (XRD, D/Max-3C, Rigaku, Japan). The resistivity of the annealed films was measured by four-point probe method [10] in a temperature range from 25 °C to 80 °C. The measuring system is composed of probe station (VP75, DSF System, Korea), computer-controlled source meter, and purposebuilt automated data acquisition system [11].
4
3. Results and discussion
Fig. 1 shows the SEM micrographs of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. The nano-sized grains and puddle-shape defects (black area) were observed at x = 0.25. No grain growth has occurred in the puddle-shape defects. With increasing Cu content (x), the puddle-shape defects decreased and the annealed films become smooth and uniform, but the grain size decreased. These results could be explained that the heterogeneous precipitation of Cu in grain boundary was changed to homogeneous distribution by the decrease of puddle-shape defects with the Cu content. A similar tendency was confirmed for Mn-Co-Ni-Cu system reported by Xiong et al. [12]. The Cu played an important role in improving the morphology and uniformity of the annealed films. In order to further investigate the interfacial microstructure, the cross-sectional image and element mapping images of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h are shown in Fig. 2. The thickness of the annealed film was very thin (about 50 nm), uniform and compact. The annealed film and SiNx/Si substrate were well attached to each other without de-laminations (Fig. 2(a)) and distinct diffusions near the interface between film and substrate (Fig. 2(b)). A similar result of Fig. 2 was confirmed for the other compositions. Fig. 3 shows the XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. A single phase of the cubic spinel structure was confirmed in all of the compositions. Only two main peaks of (220) and (311) plans (JCPDS No. 32-0345) were detected at 2θ = 30° and 35°, respectively. However, their intensities were weak because the thickness of the annealed film was very thin, as confirmed in Fig. 2.
5
Fig. 4 shows the resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h. The resistivity of the annealed films decreased with an increase of measured temperature, which indicated typical negative temperature coefficient (NTC) thermistor characteristics. All the annealed films showed a low resistivity (below 40 Ω·cm) at room temperature. With increasing Cu content (x), the resistivity of the annealed films decreased at a given temperature. This result could be attributed to decrease of puddleshape defects and improvement of microstructure homogeneity in the annealed films (Fig. 1). The resistivity is actually the result of the combined factors such as grain size, crystal structure, defects and microstructure homogeneity [13,14]. In this work, the resistivity of the annealed films was affected by the defects and microstructure homogeneity, more so than by grain size and crystal structure. If no defects (black area (Fig. 1)) were found, the grain size (grain boundary) should be considered for the analysis of resistivity because the annealed films showed same crystal structure of spinel (Fig. 3). It has also been reported that the resistivity of thin films is affected by hopping probability of localized carriers and hopping distance as well as defects and grain size [15]. Microscopically, the electrical conductivity in spinel manganite thin films is dependent on the carrier hopping between neighboring Mn3+ and Mn4+ in octahedral sites because hopping distances of octahedral sites are shorter than those of tetrahedral positions in the spinel structure [16]. The substitution of transition metals such as nickel, cobalt and copper for Mn3+ ions induces formation of Mn4+ ions in octahedral sites resulting in the change of ordering and the increase of conductivity [13]. However, Mn2+ cannot participate in this process for the large on-site coulomb repulsion energy [17]. Jadhav and Puri reported that the decrease of resistivity with Cu content in Ni-Mn-Cu systems may also be due to the participation of Cu1+ and Cu2+ cations at
6
octahedral sites in the hopping process [14]. To reduce the resistivity of Ni-Mn-Cu systems, both Mn and Cu cations at octahedral sites can be considered as hopping ions [13]. The temperature dependent resistivity (ρ) of thin films is generally described by a nearest-neighbor hopping (NNH) model [15,16]: ρ(T) = CTexp(T0/T)
(1)
where C is a constant, T is the absolute temperature, and T0 is the characteristic temperature, which is proportional to the activation energy (Ea) for NNH. To derive the Ea of the annealed films, the plots of ln(ρ/T) versus 1/T are shown in Fig. 5. The T0 values were obtained from the slop of dash line by least squares fit of the data points. The validity of NNH was supported by the high values of adjusted R-square (≥ 0.9995) for all slops. The Ea was calculated from the following equation [18]: Ea = T0KB
(2)
where KB is Boltzmann constant. The T0 and Ea are listed in Table 1. The T0 is a measure of the sensitivity of NTC films and the Ea is the energy for the hopping process of localized carriers in octahedral sites [18]. With increasing Cu content (x), the T0 and Ea of the annealed films decreased. These values were lower than those (T0 = 3374-3910, Ea = 0.291-0.337) reported for Ni-Mn-Co thin films [11]. He et al. also reported that the T0 of Ni-Mn-Co-Cu thin films was lower than that of Ni-Mn-Co thin films [15]. The TCR at 298 K (T) was evaluated using the following equation [18]: TCR (%/K) = −100T0/T2
(3)
Like the T0 and Ea tendencies, the TCR of the annealed films decreased with an increase of Cu content (x), as shown in Table 1. Especially, the TCR values of x = 0.25 and 0.275 films were higher than those (< -3 %/K) of Cu-doped nickel manganite thin
7
films reported by Ko et al. [4]. Although many bulk ceramics, thick and thin films have mentioned various values of TCR until now, there were no reports of factors affecting TCR. For the copper-nickel manganite with a cubic spinel structure, the changes of TCR with Cu content may be related to the structural characteristics (octahedral distortion, bond length, bond strength, etc.) but are, as yet, unclear and under investigating in detail.
8
4. Conclusions
For [(Ni0.2Mn0.8)1-xCux]3O4 (0.25 ≤ x ≤ 0.4) thin films annealed at 450 °C for 15 h, a single phase of cubic spinel structure was confirmed. With increasing Cu content, the morphology and uniformity of the annealed films with a thickness of about 50 nm were improved. The annealed films showed the typical NTC thermistor characteristics. The resistivity of the annealed films decreased with an increase of Cu content. This result is due to the decrease of puddle-shape defects without grain growth. With decreasing Cu content, the T0 of the annealed films increased, which induced the increase of Ea and TCR. These tendencies may be used for predictive properties optimization of coppernickel manganite and other NTC materials for microbolometer applications.
Acknowledgements
This work was supported by the Industrial Strategic Technology Development Program (10045177, Development of Resistive Ceramic Thin Film using Solution Process and Low Temperature Thin Film Vacuum Getter).
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References
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[11] G. Ji, A. Chang, H. Li, Y. Xie, H. Zhang, W. Kong, Epitaxial growth of Mn-Co-NiO thin films and thickness effects on the electrical properties, Mater. Lett. 130 (2014) 127–130. [12] Structure and electrical performance of Mn1.5-0.5xCo0.9-0.3xNi0.6-0.2xCuxO4 NTC ceramics prepared by heterogeneous precipitation, J. Alloys Compd. 606 (2014) 273–277. [13] R. Jadhav, D. Kulkarni, V. Puri, Structural and electrical properties of fritless Ni(1x)CuxMn2O4
(0 ≤ x ≤ 1) thick film NTC ceramic, J. Mater. Sci. Mater. Electron. 21
(2010) 503–508. [14] R.N. Jadhav, V. Puri, Influence of copper substitution on structural, electrical and dielectric properties of Ni(1-x)CuxMn2O4 (0 ≤ x ≤ 1) ceramics, J. Alloys Compd. 507 (2010) 151–156. [15] L. He, G. Zhang, Z.Y. Ling, Improvement of structural and electrical properties of Mn-based thin film thermistors by a bilayer structure, Mater. Lett. 128 (2014) 144– 147. [16] A. Feteira, Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective, J. Am. Ceram. Soc. 92 (2009) 967–983. [17] W. Kong, H. Bu, B. Gao, L. Chen, F. Cheng, P. Zhao, G. Ji, A. Chang, C. Jiang, Effects of preferred orientation on electrical properties of Mn1.56Co0.96Ni0.48O4±δ spinel films, Mater. Lett. 137 (2014) 36–40. [18] Y.Q. Gao, Z.M. Huang, Y. Hou, J. Wu, W. Zhou, C. OuYang, J.G. Huang, J.C. Tong, J.H. Chu, Structural and electrical properties of Mn1.56Co0.96Ni0.48O4 NTC thermistor films, Mater. Sci. Eng. B 185 (2014) 74–78.
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List of Figures
Fig. 1. Surface morphology SEM images of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 2. Cross-sectional SEM image (a) and element mapping images (b-e) of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h.
Fig. 3. XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 4. Resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
Fig. 5. Plots of ln(ρ/T) versus 1/T for [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
12
Fig. 1. Surface morphology SEM images of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
13
Fig. 2. Cross-sectional SEM image (a) and element mapping images (b-e) of [(Ni0.2Mn0.8)1-xCux]3O4 (x = 0.3) thin film annealed at 450 °C for 15 h.
14
(311)
(220)
Intensity (arb. unit)
Spinel Substrate
x = 0.4 x = 0.375 x = 0.35 x = 0.325 x = 0.3 x = 0.275 x = 0.25
10
20
30
40
50
60
2 (degree)
Fig. 3. XRD patterns of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
15
45
x = 0.25 x = 0.275 x = 0.3 x = 0.325 x = 0.35 x = 0.375 x = 0.4
40
Resistivity (cm)
35 30 25 20 15 10 5 0 295
305
315
325
335
345
355
Temperature (K)
Fig. 4. Resistivity of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
16
1
x = 0.25 x = 0.275 x = 0.3 x = 0.325 x = 0.35 x = 0.375 x = 0.4
ln(/T) (cm/K)
0 -1 -2 -3 -4 -5 -6 -7 0.0028
0.0030
0.0032
0.0034
1/T (1/K)
Fig. 5. Plots of ln(ρ/T) versus 1/T for [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
17
List of Tables
Table 1 The characteristic temperature (T0), activation energy (Ea) and temperature coefficient of resistance (TCR) of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
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Table 1 The characteristic temperature (T0), activation energy (Ea) and temperature coefficient of resistance (TCR) of [(Ni0.2Mn0.8)1-xCux]3O4 thin films annealed at 450 °C for 15 h.
x (mol)
T0 (K)
Ea (eV)
TCR (%/K)
0.25
3154
0.2718
-3.436
0.275
2987
0.2574
-3.253
0.3
2684
0.2313
-2.923
0.325
2671
0.2302
-2.910
0.35
2293
0.1976
-2.498
0.375
1709
0.1473
-1.862
0.4
1433
0.1234
-1.560
19