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Epitaxial growth of Mn–Co–Ni–O thin films and thickness effects on the electrical properties
Materials Letters 130 (2014) 127–130
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Epitaxial growth of Mn–Co–Ni–O thin films and thickness effects on the electrical properties Guang Ji a,b,d, Aimin Chang a,d,n, Hongyi Li a,c,d, Yahong Xie a,d, Huimin Zhang a,d, Wenwen Kong a,b,d a Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, Urumqi 830011, China b University of Chinese Academy of Sciences, Beijing 100049, China c Quality of Products Supervision & Inspection Institute of Technology, Xinjiang Uygur Autonomous Region, Urumqi 830011, China d Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, Xinjiang University, Urumqi 830046, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 March 2014 Accepted 8 May 2014 Available online 23 May 2014
Mn1.56Co0.96Ni0.48O4 (MCN) thin films were grown on thermally oxidized Si(1 0 0) substrates using laser molecular beam epitaxy technique. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated an epitaxial structure with [1 0 0] growth direction and a lattice parameter of 8.28 Å. The resistivity-temperature relationships of the epitaxial MCN thin films exhibited negative temperature coefficient thermistor characteristics, and the electron conduction mechanism was found to be nearest-neighbor hopping. Interestingly, the room temperature resistivity ρ25, the characteristic temperature T0, the activation energy Ea, and the temperature coefficient of resistance αT were all highly dependent on the film thickness t, and as t increased all of them decreased. An approximately linear relation was further revealed between ρ25 and 1/t2, which could be approximately expressed as ρ25(t) ¼211.7þ 119.1 104/t2 (t in nm; ρ25 in Ω cm). The successful growth of epitaxial MCN thin films opens a door for studying the effects of thickness on the electrical properties of MCN thin films, and may consequently provide an alternative approach for controlling the properties. & 2014 Published by Elsevier B.V.
Keywords: Thin films Epitaxial growth Electrical properties Negative temperature coefficient thermistor Thickness effects
1. Introduction Spinel compound MnxCoyNi3 x yO4, with a general formula of AB2O4, is particularly attractive as a low resistivity and high sensitivity negative temperature coefficient (NTC) material for thermistor devices [1–3], and especially for uncooled microbolometers and infrared detectors in space satellites [4–8]. Moreover, MnxCoyNi3 x yO4 thin films exhibit a rich variety of electrical, optical, and magnetic properties, and are regarded as promising candidates for applications in integrated multifunctional spintronics devices [4,5,8,9]. The crystalline structures of MnxCoyNi3 x yO4 thin films reported previously showed varying results, from amorphous to polycrystalline, with some reports of oriented growth [6–10]. However, there has been no report for epitaxial growth of MnxCoyNi3 x yO4 thin films. Although single-[1 0 0]-oriented Mn1.56Co0.96Ni0.48O4 (MCN) thin films were successfully grown in our previous work [10], the films were still polycrystalline with
n
Corresponding author. Tel./fax: þ 86 99 1383 7510. E-mail address: [email protected] (A. Chang).
http://dx.doi.org/10.1016/j.matlet.2014.05.091 0167-577X/& 2014 Published by Elsevier B.V.
single preferred orientation rather than epitaxial, since the X-ray diffraction peaks of the films were quite broad and no epitaxial structure could be observed by microscopy. Nevertheless, epitaxial thin films with improved crystalline quality and homogeneity are essential to the reliability of electronics and spintronics devices, thus it’s of great technological importance to fabricate high-quality epitaxial MnxCoyNi3 x yO4 thin films. In this work, MCN thin films were epitaxially grown using laser molecular beam epitaxy (LMBE) technique which combines the advantages of both pulsed laser deposition and conventional MBE [11]. Moreover, we found that the film thickness had a significant influence on the electrical properties of epitaxial MCN thin films, although it was reported that polycrystalline nickel manganite thin films with different thicknesses showed little difference in electrical properties [12].
2. Experimental MCN thin films of different thicknesses were grown on thermally oxidized Si(1 0 0) substrates by LMBE utilizing a sintered ceramic target of MCN. A KrF pulsed excimer laser was employed,
128
G. Ji et al. / Materials Letters 130 (2014) 127–130
with a wavelength of 248 nm, a pulse duration of 25 ns, an energy of 350 mJ/pulse, and a repetition frequency of 1 Hz. The substrateto-target distance and the substrate temperature were 45 mm and 300 1C, respectively. The evacuated chamber was kept at around 10 6 Pa during the growth process. After the growth, each sample was in situ annealed under the growth condition for 60 min, and then annealed in air at 800 1C for 120 min. The epitaxial structure of the samples was analyzed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The resistivity-temperature (ρ-T) relationships were measured in the temperature range of 0–100 1C using a computer-controlled source meter and a purpose-built automated data acquisition system. To ensure the temperature deviation from the designated value to be o 7 0.01 1C, samples were immersed in a silicon oil bath during the electrical measurements.
3. Results and discussion Fig. 1(a) shows a typical XRD pattern of the MCN thin films, with the corresponding rocking curve in the inset. It can be seen that only the (4 0 0) diffraction peak of a cubic spinel structure appears and the full width at half maximum (FWHM) of the rocking curve is as less as 0.1331, indicating that the film was epitaxially grown with [1 0 0] orientation. From the d-spacing of the (4 0 0) diffraction peak, the lattice parameter of 8.28 Å can be determined for the epitaxial MCN thin film. The epitaxial structure was further confirmed by HRTEM as shown in Fig. 1(b), with the corresponding fast Fourier transform (FFT) pattern of the MCN thin film in the inset. The measured interplanar spacings marked by the arrows are 0.418 nm and 0.484 nm, which are consistent with the values of MCN (2 0 0) (0.414 nm) and (1 1 1) (0.478 nm). The measured angular separation between the two observed planes is 54.71, which is in good accordance with the standard calculated values of 54.71 between MCN (2 0 0) plane and (1 1 1) plane. The FFT pattern shows the equivalent diffraction spots of MCN (2 0 0) and (1 1 1) planes and an interplanar angle of 54.71, corresponding to the planes and angular separation observed in the HRTEM image. The HRTEM analysis of the MCN thin film shows a clear epitaxial structure with [1 0 0] growth direction, which is consistent with the XRD analyses. The relations between ρ and T of the epitaxial MCN thin films with different thicknesses are shown in Fig. 2(a). It can be seen that the ρ-T relationships of all the samples exhibit NTC thermistor characteristics described by the generalized expression for small polaron hopping model [13], p T ρðT Þ ¼ CT α exp 0 ; ð1Þ T where C is a constant, T is the absolute temperature, and T0 is the characteristic temperature in Kelvin. For nearest-neighbor hopping (NNH) in the quantum tunneling regime, α ¼p ¼1, while for variable-range hopping (VRH), 0.25o p¼ α/2 o1. In order to elucidate the electron conduction mechanism of epitaxial MCN thin films, lnW versus lnT plots for the samples are given in Fig. 2 (b), where W is defined as [13] p 1 dðln ρÞ T0 W¼ p : ð2Þ T dðT 1 Þ T For each sample in Fig. 2(b), a standard linear least square fit to the data points is made, and the negative slope of the fitting line approximately gives the value of the corresponding p. It’s indicated that the hopping type of the epitaxial MCN thin films is NNH, as the parameter p for each sample is found to be close to 1.
Fig. 1. (a) Typical XRD pattern of the MCN thin films, with the corresponding rocking curve in the inset. (b) Typical HRTEM image of the samples, with the corresponding FFT pattern of the MCN thin film in the inset.
Fig. 2(c) shows the plots of ln(ρ/T) versus 1/T, and for each sample a linear line has been fitted through the data points. The slope of each fitting line gives the corresponding characteristic temperature T0, which is a measure of the sensitivity of the MCN thin film thermistor. Table 1 lists the values of room temperature resistivity (25 1C, ρ25) and T0 of the epitaxial MCN thin films with different thicknesses, together with the values of activation energy Ea (Ea ¼kT0, where k is the Boltzmann’s constant) for the NNH process of the films. The values of temperature coefficient of resistance αT, defined by
αT ¼
1 dρ ; ρ dT
ð3Þ
are also calculated and listed in Table 1. It’s interesting to notice that ρ25, T0, Ea and αT are all highly dependent on the film thickness t, and as t increases all of them decreases. In contrast, for polycrystalline nickel manganite thin films with different thicknesses it was reported that there was little difference in their electrical properties [12]. Hence, the fabrication of epitaxial MCN thin films is of great technological significance, since it can provide an alternative approach for controlling the electrical properties by
G. Ji et al. / Materials Letters 130 (2014) 127–130
129
Fig. 3. Plot of ρ25 vs 1/t2 for the epitaxial MCN thin films.
when film thickness is large enough. Thus, the thickness dependence of ρ25 can be approximately expressed as
ρ25 ðtÞ ¼ ρ25B þ D=t 2 ;
ð4Þ
where ρ25B is the bulk resistivity at 25 1C, and D is a constant. Through a linear fit to the data points in Fig. 3, the values of ρ25B and D are derived to be 211.7 Ω cm and 119.1 104 respectively, when the unit of t is nm.
4. Conclusions Using LMBE technique, epitaxial MCN thin films were successfully grown with [1 0 0] orientation. The ρ-T relationships exhibited NTC thermistor characteristics, and the electron transport was found to be by NNH with p 1. The film thickness t had a great influence on the electrical parameters of epitaxial MCN thin films. ρ25, T0, Ea and αT all decreased as t increased, and the thickness dependence of ρ25 could be approximately expressed as ρ25(t) ¼ 211.7 þ119.1 104/t2 (t in nm; ρ25 in Ω cm). The successful growth of epitaxial MCN thin films opens a door for studying the effects of thickness on the electrical properties of MCN thin films, and may consequently provide an alternative approach for controlling the properties. Nevertheless, further researches on the mechanisms of the thickness effects will be necessary in order to better utilize the thickness effects in the design of NTC thermistor devices.
Acknowledgments
Fig. 2. Plots of (a) ρ vs T, (b) lnW vs lnT, and (c) ln(ρ/T) vs 1/T for the epitaxial MCN thin films with different thicknesses.
Table 1 Electrical parameters of the epitaxial MCN thin films with different thicknesses. t (nm)
ρ25 (Ω cm)
T0 (K)
Ea (eV)
αT at 300 K (K
65 88 115 143 173
492.6 368.3 300.7 267.9 252.6
3910 3726 3564 3445 3374
0.337 0.321 0.307 0.297 0.291
4.0 3.8 3.6 3.5 3.4
1
) (%)
virtue of the thickness effects. It’s further found that an approximately linear relation exists between ρ25 and 1/t2, as shown in Fig. 3. It’s known that film resistivity should tend to bulk resistivity
This work was supported by National Natural Science Foundation of China (21201145), Postdoctoral Science Foundation of China (2013M532103), and Natural Science Foundation of Xinjiang (2014211B038). References [1] Feteira A. Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective. J Am Ceram Soc 2009;92:967–83. [2] He L, Ling ZY, Huang YT, Liu YS. Effects of annealing temperature on microstructure and electrical properties of Mn–Co–Ni–O thin films. Mater Lett 2011;65:1632–5. [3] He L, Ling ZY. Studies of temperature dependent ac impedance of a negative temperature coefficient Mn–Co–Ni–O thin film thermistor. Appl Phys Lett 2011;98:242112. [4] Schulze H, Li J, Dickey EC, Trolier-McKinstry S. Synthesis, phase characterization, and properties of chemical solution-deposited nickel manganite thermistor thin films. J Am Ceram Soc 2009;92:738–44. [5] Ko SW, Schulze HM, Saint John DB, Podraza NJ, Dickey EC, Trolier-McKinstry SS. Low temperature crystallization of metastable nickel manganite spinel thin films. J Am Ceram Soc 2012;95:2562–7.
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G. Ji et al. / Materials Letters 130 (2014) 127–130
[6] Baliga S, Jain AL. Hopping conduction in sputtered Ni–Co–Mn–O spinel films. Mater Lett 1991;11:226–8. [7] Karanth S, Sumesh MA, Shobha V, Shanbhogue HG, Nagendra CL. Infrared detectors based on thin film thermistor of ternary Mn–Ni–Co–O on micromachined thermal isolation structure. Sens Actuators, A 2009;153:69–75. [8] Hou Y, Huang ZM, Gao YQ, Ge YJ, Wu J, Chu JH. Characterization of Mn1.56Co0.96Ni0.48O4 films for infrared detection. Appl Phys Lett 2008;92: 202115. [9] Gao YQ, Huang ZM, Hou Y, Wu J, Ge YJ, Chu JH. Optical properties of Mn1.56Co0.96Ni0.48O4 films studied by spectroscopic ellipsometry. Appl Phys Lett 2009;94:011106.
[10] Ji G, Chang AM, Xu JB, Zhang HM, Hou J, Zhang B, et al. Low-temperature ( o300 1C) growth and characterization of single-[1 0 0]-oriented Mn–Co–Ni– O thin films. Mater Lett 2013;107:103–6. [11] Opel M. Spintronic oxides grown by laser-MBE. J Phys D: Appl Phys 2012;45: 033001. [12] Schulze HM. Synthesis and characterization of nickel manganite thin films for application in uncooled microbolometers [dissertation]. The Pennsylvania State University; 2008. [13] Schmidt R, Basu A, Brinkman AW, Klusek Z, Datta PK. Electron-hopping modes in NiMn2O4 þ δ materials. Appl Phys Lett 2005;86:073501.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Epitaxial growth of Mn–Co–Ni–O thin films and thickness effects on the electrical properties Guang Ji a,b,d, Aimin Chang a,d,n, Hongyi Li a,c,d, Yahong Xie a,d, Huimin Zhang a,d, Wenwen Kong a,b,d a Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, Urumqi 830011, China b University of Chinese Academy of Sciences, Beijing 100049, China c Quality of Products Supervision & Inspection Institute of Technology, Xinjiang Uygur Autonomous Region, Urumqi 830011, China d Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, Xinjiang University, Urumqi 830046, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 March 2014 Accepted 8 May 2014 Available online 23 May 2014
Mn1.56Co0.96Ni0.48O4 (MCN) thin films were grown on thermally oxidized Si(1 0 0) substrates using laser molecular beam epitaxy technique. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated an epitaxial structure with [1 0 0] growth direction and a lattice parameter of 8.28 Å. The resistivity-temperature relationships of the epitaxial MCN thin films exhibited negative temperature coefficient thermistor characteristics, and the electron conduction mechanism was found to be nearest-neighbor hopping. Interestingly, the room temperature resistivity ρ25, the characteristic temperature T0, the activation energy Ea, and the temperature coefficient of resistance αT were all highly dependent on the film thickness t, and as t increased all of them decreased. An approximately linear relation was further revealed between ρ25 and 1/t2, which could be approximately expressed as ρ25(t) ¼211.7þ 119.1 104/t2 (t in nm; ρ25 in Ω cm). The successful growth of epitaxial MCN thin films opens a door for studying the effects of thickness on the electrical properties of MCN thin films, and may consequently provide an alternative approach for controlling the properties. & 2014 Published by Elsevier B.V.
Keywords: Thin films Epitaxial growth Electrical properties Negative temperature coefficient thermistor Thickness effects
1. Introduction Spinel compound MnxCoyNi3 x yO4, with a general formula of AB2O4, is particularly attractive as a low resistivity and high sensitivity negative temperature coefficient (NTC) material for thermistor devices [1–3], and especially for uncooled microbolometers and infrared detectors in space satellites [4–8]. Moreover, MnxCoyNi3 x yO4 thin films exhibit a rich variety of electrical, optical, and magnetic properties, and are regarded as promising candidates for applications in integrated multifunctional spintronics devices [4,5,8,9]. The crystalline structures of MnxCoyNi3 x yO4 thin films reported previously showed varying results, from amorphous to polycrystalline, with some reports of oriented growth [6–10]. However, there has been no report for epitaxial growth of MnxCoyNi3 x yO4 thin films. Although single-[1 0 0]-oriented Mn1.56Co0.96Ni0.48O4 (MCN) thin films were successfully grown in our previous work [10], the films were still polycrystalline with
n
Corresponding author. Tel./fax: þ 86 99 1383 7510. E-mail address: [email protected] (A. Chang).
http://dx.doi.org/10.1016/j.matlet.2014.05.091 0167-577X/& 2014 Published by Elsevier B.V.
single preferred orientation rather than epitaxial, since the X-ray diffraction peaks of the films were quite broad and no epitaxial structure could be observed by microscopy. Nevertheless, epitaxial thin films with improved crystalline quality and homogeneity are essential to the reliability of electronics and spintronics devices, thus it’s of great technological importance to fabricate high-quality epitaxial MnxCoyNi3 x yO4 thin films. In this work, MCN thin films were epitaxially grown using laser molecular beam epitaxy (LMBE) technique which combines the advantages of both pulsed laser deposition and conventional MBE [11]. Moreover, we found that the film thickness had a significant influence on the electrical properties of epitaxial MCN thin films, although it was reported that polycrystalline nickel manganite thin films with different thicknesses showed little difference in electrical properties [12].
2. Experimental MCN thin films of different thicknesses were grown on thermally oxidized Si(1 0 0) substrates by LMBE utilizing a sintered ceramic target of MCN. A KrF pulsed excimer laser was employed,
128
G. Ji et al. / Materials Letters 130 (2014) 127–130
with a wavelength of 248 nm, a pulse duration of 25 ns, an energy of 350 mJ/pulse, and a repetition frequency of 1 Hz. The substrateto-target distance and the substrate temperature were 45 mm and 300 1C, respectively. The evacuated chamber was kept at around 10 6 Pa during the growth process. After the growth, each sample was in situ annealed under the growth condition for 60 min, and then annealed in air at 800 1C for 120 min. The epitaxial structure of the samples was analyzed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The resistivity-temperature (ρ-T) relationships were measured in the temperature range of 0–100 1C using a computer-controlled source meter and a purpose-built automated data acquisition system. To ensure the temperature deviation from the designated value to be o 7 0.01 1C, samples were immersed in a silicon oil bath during the electrical measurements.
3. Results and discussion Fig. 1(a) shows a typical XRD pattern of the MCN thin films, with the corresponding rocking curve in the inset. It can be seen that only the (4 0 0) diffraction peak of a cubic spinel structure appears and the full width at half maximum (FWHM) of the rocking curve is as less as 0.1331, indicating that the film was epitaxially grown with [1 0 0] orientation. From the d-spacing of the (4 0 0) diffraction peak, the lattice parameter of 8.28 Å can be determined for the epitaxial MCN thin film. The epitaxial structure was further confirmed by HRTEM as shown in Fig. 1(b), with the corresponding fast Fourier transform (FFT) pattern of the MCN thin film in the inset. The measured interplanar spacings marked by the arrows are 0.418 nm and 0.484 nm, which are consistent with the values of MCN (2 0 0) (0.414 nm) and (1 1 1) (0.478 nm). The measured angular separation between the two observed planes is 54.71, which is in good accordance with the standard calculated values of 54.71 between MCN (2 0 0) plane and (1 1 1) plane. The FFT pattern shows the equivalent diffraction spots of MCN (2 0 0) and (1 1 1) planes and an interplanar angle of 54.71, corresponding to the planes and angular separation observed in the HRTEM image. The HRTEM analysis of the MCN thin film shows a clear epitaxial structure with [1 0 0] growth direction, which is consistent with the XRD analyses. The relations between ρ and T of the epitaxial MCN thin films with different thicknesses are shown in Fig. 2(a). It can be seen that the ρ-T relationships of all the samples exhibit NTC thermistor characteristics described by the generalized expression for small polaron hopping model [13], p T ρðT Þ ¼ CT α exp 0 ; ð1Þ T where C is a constant, T is the absolute temperature, and T0 is the characteristic temperature in Kelvin. For nearest-neighbor hopping (NNH) in the quantum tunneling regime, α ¼p ¼1, while for variable-range hopping (VRH), 0.25o p¼ α/2 o1. In order to elucidate the electron conduction mechanism of epitaxial MCN thin films, lnW versus lnT plots for the samples are given in Fig. 2 (b), where W is defined as [13] p 1 dðln ρÞ T0 W¼ p : ð2Þ T dðT 1 Þ T For each sample in Fig. 2(b), a standard linear least square fit to the data points is made, and the negative slope of the fitting line approximately gives the value of the corresponding p. It’s indicated that the hopping type of the epitaxial MCN thin films is NNH, as the parameter p for each sample is found to be close to 1.
Fig. 1. (a) Typical XRD pattern of the MCN thin films, with the corresponding rocking curve in the inset. (b) Typical HRTEM image of the samples, with the corresponding FFT pattern of the MCN thin film in the inset.
Fig. 2(c) shows the plots of ln(ρ/T) versus 1/T, and for each sample a linear line has been fitted through the data points. The slope of each fitting line gives the corresponding characteristic temperature T0, which is a measure of the sensitivity of the MCN thin film thermistor. Table 1 lists the values of room temperature resistivity (25 1C, ρ25) and T0 of the epitaxial MCN thin films with different thicknesses, together with the values of activation energy Ea (Ea ¼kT0, where k is the Boltzmann’s constant) for the NNH process of the films. The values of temperature coefficient of resistance αT, defined by
αT ¼
1 dρ ; ρ dT
ð3Þ
are also calculated and listed in Table 1. It’s interesting to notice that ρ25, T0, Ea and αT are all highly dependent on the film thickness t, and as t increases all of them decreases. In contrast, for polycrystalline nickel manganite thin films with different thicknesses it was reported that there was little difference in their electrical properties [12]. Hence, the fabrication of epitaxial MCN thin films is of great technological significance, since it can provide an alternative approach for controlling the electrical properties by
G. Ji et al. / Materials Letters 130 (2014) 127–130
129
Fig. 3. Plot of ρ25 vs 1/t2 for the epitaxial MCN thin films.
when film thickness is large enough. Thus, the thickness dependence of ρ25 can be approximately expressed as
ρ25 ðtÞ ¼ ρ25B þ D=t 2 ;
ð4Þ
where ρ25B is the bulk resistivity at 25 1C, and D is a constant. Through a linear fit to the data points in Fig. 3, the values of ρ25B and D are derived to be 211.7 Ω cm and 119.1 104 respectively, when the unit of t is nm.
4. Conclusions Using LMBE technique, epitaxial MCN thin films were successfully grown with [1 0 0] orientation. The ρ-T relationships exhibited NTC thermistor characteristics, and the electron transport was found to be by NNH with p 1. The film thickness t had a great influence on the electrical parameters of epitaxial MCN thin films. ρ25, T0, Ea and αT all decreased as t increased, and the thickness dependence of ρ25 could be approximately expressed as ρ25(t) ¼ 211.7 þ119.1 104/t2 (t in nm; ρ25 in Ω cm). The successful growth of epitaxial MCN thin films opens a door for studying the effects of thickness on the electrical properties of MCN thin films, and may consequently provide an alternative approach for controlling the properties. Nevertheless, further researches on the mechanisms of the thickness effects will be necessary in order to better utilize the thickness effects in the design of NTC thermistor devices.
Acknowledgments
Fig. 2. Plots of (a) ρ vs T, (b) lnW vs lnT, and (c) ln(ρ/T) vs 1/T for the epitaxial MCN thin films with different thicknesses.
Table 1 Electrical parameters of the epitaxial MCN thin films with different thicknesses. t (nm)
ρ25 (Ω cm)
T0 (K)
Ea (eV)
αT at 300 K (K
65 88 115 143 173
492.6 368.3 300.7 267.9 252.6
3910 3726 3564 3445 3374
0.337 0.321 0.307 0.297 0.291
4.0 3.8 3.6 3.5 3.4
1
) (%)
virtue of the thickness effects. It’s further found that an approximately linear relation exists between ρ25 and 1/t2, as shown in Fig. 3. It’s known that film resistivity should tend to bulk resistivity
This work was supported by National Natural Science Foundation of China (21201145), Postdoctoral Science Foundation of China (2013M532103), and Natural Science Foundation of Xinjiang (2014211B038). References [1] Feteira A. Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective. J Am Ceram Soc 2009;92:967–83. [2] He L, Ling ZY, Huang YT, Liu YS. Effects of annealing temperature on microstructure and electrical properties of Mn–Co–Ni–O thin films. Mater Lett 2011;65:1632–5. [3] He L, Ling ZY. Studies of temperature dependent ac impedance of a negative temperature coefficient Mn–Co–Ni–O thin film thermistor. Appl Phys Lett 2011;98:242112. [4] Schulze H, Li J, Dickey EC, Trolier-McKinstry S. Synthesis, phase characterization, and properties of chemical solution-deposited nickel manganite thermistor thin films. J Am Ceram Soc 2009;92:738–44. [5] Ko SW, Schulze HM, Saint John DB, Podraza NJ, Dickey EC, Trolier-McKinstry SS. Low temperature crystallization of metastable nickel manganite spinel thin films. J Am Ceram Soc 2012;95:2562–7.
130
G. Ji et al. / Materials Letters 130 (2014) 127–130
[6] Baliga S, Jain AL. Hopping conduction in sputtered Ni–Co–Mn–O spinel films. Mater Lett 1991;11:226–8. [7] Karanth S, Sumesh MA, Shobha V, Shanbhogue HG, Nagendra CL. Infrared detectors based on thin film thermistor of ternary Mn–Ni–Co–O on micromachined thermal isolation structure. Sens Actuators, A 2009;153:69–75. [8] Hou Y, Huang ZM, Gao YQ, Ge YJ, Wu J, Chu JH. Characterization of Mn1.56Co0.96Ni0.48O4 films for infrared detection. Appl Phys Lett 2008;92: 202115. [9] Gao YQ, Huang ZM, Hou Y, Wu J, Ge YJ, Chu JH. Optical properties of Mn1.56Co0.96Ni0.48O4 films studied by spectroscopic ellipsometry. Appl Phys Lett 2009;94:011106.
[10] Ji G, Chang AM, Xu JB, Zhang HM, Hou J, Zhang B, et al. Low-temperature ( o300 1C) growth and characterization of single-[1 0 0]-oriented Mn–Co–Ni– O thin films. Mater Lett 2013;107:103–6. [11] Opel M. Spintronic oxides grown by laser-MBE. J Phys D: Appl Phys 2012;45: 033001. [12] Schulze HM. Synthesis and characterization of nickel manganite thin films for application in uncooled microbolometers [dissertation]. The Pennsylvania State University; 2008. [13] Schmidt R, Basu A, Brinkman AW, Klusek Z, Datta PK. Electron-hopping modes in NiMn2O4 þ δ materials. Appl Phys Lett 2005;86:073501.