- Email: [email protected]
Preparation of LaNiO3 thin films by mist plasma evaporation from aqueous precursor
Materials Chemistry and Physics 87 (2004) 134–137
Preparation of LaNiO3 thin films by mist plasma evaporation from aqueous precursor Hui Huang∗ , Xi Yao Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an 710049, China Received 12 November 2003; received in revised form 28 April 2004; accepted 20 May 2004
Abstract LaNiO3 thin films were prepared by mist plasma evaporation on n-Si(1 1 1) wafers using lanthanum nitrate and nickel nitrate aqueous solutions as precursor. The obtained LaNiO3 thin films possessed a perovskite structure with rhombohedral distortion. No impurity phase was detected in the LaNiO3 film deposited at 630 ◦ C. The grain size of the films decreased with the decreasing substrate temperature, and increased with the increase of deposition time and atomizing rate of precursor. The resistivity of the LaNiO3 films decreased with the decreasing substrate temperature. After annealing at 400 ◦ C for 2 h, the room temperature resistivity of the LaNiO3 film deposited at 630 ◦ C for 20 min decreased from 5.60 to 0.68 m cm. © 2004 Elsevier B.V. All rights reserved. Keywords: Thin films; Plasma deposition; Electrical conductivity
1. Introduction Ferroelectric thin films have attracted much attention for a wide range of applications in pyroelectric and piezoelectric sensors, microwave devices and dynamic random access memories (DRAMs) [1,2]. Ferroelectric thin film capacitors with Pt metallic electrodes showed fatigue degradation due to charge segregation and decay on the non-ideal interface between films and Pt base electrodes. In recent years, ferroelectric films coated on perovskite related metallic oxide electrodes such as YBa2 Cu3 O7−x (YBCO), La0.5 Sr0.5 CoO3 (LSCO), SrRuO3 and LaNiO3 had good crystallizability and ferroelectric properties due to lattice matching, and were considered to be promising alternatives for solving the above problems in using conventional metal electrodes [3,4]. LaNiO3 thin films have a pseudocubic structure with a lattice parameter of 3.84 Å, and have been developed as a new metal oxide electrode. The resistivity of LaNiO3 at room temperature was about 10−5 m, and the temperature dependence indicated a good metallic behavior [5]. Since LaNiO3 films were first deposited by pulsed laser deposition (PLD) [6], they have been successfully fabricated by various methods such as RF sputtering [7], sol–gel method [8], metalorganic decomposition (MOD) [9] and metalorganic chemical vapor deposition (MOCVD) [10]. ∗ Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (H. Huang).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.017
Recently, a novel deposition technique, mist plasma evaporation (MPE), has been developed by us to prepare thin films [11–13]. In this technique, aqueous solution of metal nitrate or chloride was ultrasonically atomized into tiny droplets in several microns, and the droplets were introduced into radio frequency inductively coupled plasma torch generated at atmospheric pressure. Source materials in the droplets were decomposed to their components by the ultrahigh temperature (∼5000 ◦ C) of the plasma torch, and then reacted and deposited on substrate to form thin films during plasma cooling process. This method showed remarkable advantages such as high deposition rate, single precursor, wide choices of source materials and no subsequent annealing. In this work, LaNiO3 thin films were prepared by MPE, the effects of deposition parameters on structural and electrical properties of LaNiO3 films were investigated.
2. Experimental The precursor was prepared by dissolving equal molar of La(NO3 )3 ·6H2 O and Ni(NO3 )2 ·6H2 O in deionized water. Fig. 1 shows schematic illustration of MPE system. The torch tube was a set of triple-coaxial fused quartz tubes. Thermal plasma torch was ignited with a water-cooled Cu coil coupled to an RF oscillator (normally run at 3 kW and 30 MHz) at atmospheric pressure, and stabilized with Ar sheath gas flowing through the outer quartz tube. The
135
*
*
(312)
(024)
* La2O3 (113) (122) (104)
Intensity (a.u.)
(101) Si (110) (012) (021) (003) (202)
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
(c)
(b)
*
(a) 20
30
40
50
60
70
80
2 theta (deg.) Fig. 2. XRD patterns of the films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min.
3. Results and discussion
Fig. 1. Schematic illustration of the MPE system.
precursor solution was ultrasonically atomized, and was introduced into the center of the plasma torch through inner quartz tube by Ar carrier gas. Oxygen was added into Ar carried gas to enhance the oxidation of source materials. Substrate was fixed on fused quartz substrate holder, and was heated by tail flame of the plasma. Substrate temperature Ts was detected by a shielded thermocouple pressed on the back of substrate holder, and was controlled by adjusting substrate distance (Dsn ) from substrate to the spraying nozzle of inner quartz tube with a motor-driven mechanism. Summary of typical deposition parameters was listed in Table 1. Phase compositions of the films were characterized by a Rigaku D/max-2400 X-ray diffractometer (XRD) using Cu K␣ radiation. Morphology of the films was observed by a Philips XL20 scanning electron microscopy (SEM). Thickness and refractive index of the films were measured by a Filmetric F20 Thin-Film Measurement System. Electrical resistivity at room temperature of the films was measured by the standard four-probe technique.
Table 1 Summary of typical deposition conditions of MPE RF power Pressure Sheath gas (Ar) Plasma gas (Ar) Carrier gas Concentration of precursor Atomizing rate Dsn Ts Deposition time
3 kW at 30 MHz Atmospheric pressure 17 L/min 1 L/min Ar: 0.5 L/min, O2 : 0.25 L/min 0.2 mol/L 0.06 mL/min 10–14 cm 710–630 ◦ C 20, 40 min
Fig. 2 shows XRD patterns of the LaNiO3 films prepared at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min, respectively. Both LaNiO3 and La2 NiO4 were detected in the film deposited at 710 ◦ C, and La2 O3 was found in the film deposited at 660 ◦ C. LaNiO3 is unstable at high temperature, and will decompose into NiO and LaNiO2 , which irreversibly change into La2 NiO4 . La2 NiO4 and other compositions in the films deposited at high Ts were caused by decomposition of LaNiO3 at high temperature. As the Ts decreased to 630 ◦ C, only pure LaNiO3 was detected in the film, which possessed a perovskite structure with rhombohedral distortion. Cell data of rhombohedrally distorted perovskite is a = b = c = 3.8375 Å, α = β = γ = 90.64◦ , and can be considered as pseudocubic [14]. Fig. 3 shows SEM photographs of the LaNiO3 films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min, respectively. The LaNiO3 films were very dense, and grains in the films were regular and in uniform size. The grain size of the films deposited at 710 and 660 ◦ C was about 370 and 200 nm, respectively, and decreased to 50 nm in the film deposited at 630 ◦ C. As Ts decreased from 710 to 630 ◦ C, the grain size of the film decreased remarkably. Some large cubic-like grains with a size of ∼125 nm were observed on the LaNiO3 film deposited at 630 ◦ C (Fig. 3c), which was accordant with the rhombohedrally distorted perovskite structure detected by XRD. Thickness and refractive index of the LaNiO3 film deposited at 630 ◦ C for 20 min was 200 nm and 1.93, respectively. Fig. 4 shows the SEM micrograph of the LaNiO3 films deposited at 660 ◦ C for 40 min. Cubic-like crystals were also observed in the film. The grain size of the film was about 500 nm, which was much larger than that of the film deposited at 660 ◦ C for 20 min. The effect of atomizing rate on film morphology was also investigated. When the atomizing rate of the precursor increased from 0.08 to 0.12 mL/min, the gain size was increased from 200 nm (Fig. 3b) to about 300 nm (as shown in Fig. 5). Fig. 6 shows the resistivity of the LaNiO3 films deposited at various Ts for 20 min. As Ts increased from 630 to 710 ◦ C,
136
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
Fig. 4. SEM micrograph of the LaNiO3 film deposited at 660 ◦ C for 40 min.
Fig. 5. SEM micrograph of the LaNiO3 film deposited at 660 ◦ C for 20 min with increased atomizing rate.
Resistivity (mΩ cm)
250 200
Before annealing After annealing
150 100 50 0 620
640
660
680
700
720
o
Ts ( C) Fig. 3. SEM micrographs of the LaNiO3 films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min.
resistivity of the LaNiO3 films increased sharply from 5.60 to 225.78 m cm. After annealed at 400 ◦ C for 2 h in air, the resistivity of the films deposited at 710 ◦ C decreased sharply from 225.78 to 3.28 m cm, and that of the film deposited at 630 ◦ C also decreased from 5.60 to 0.68 m cm, which was lower than the resistivity of LaNiO3 films prepared by wet chemical solution process [8,15]. High resistivity of the film deposited at 710 ◦ C resulted from the decomposition of LaNiO3 at high deposition temperature, which was proved by XRD. When the films were annealed at 400 ◦ C for 2 h
Fig. 6. Resistivity of the LaNiO3 films deposited at various Ts for 20 min.
in air, the decomposed components of the LaNiO3 films reacted again, so the resistivity of the films decreased after annealing.
4. Conclusions Lanthanum nitrate and nickel nitrate aqueous solution were used as precursor to deposit LaNiO3 thin films by MPE. The films were rhombohedral LaNiO3 . The grain size of the films decreased with the decreasing substrate temperature,
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
and increased with the increase of deposition time and atomizing rate of precursor. The resistivity of the LaNiO3 films decreased with the decreasing substrate temperature. It was proved that MPE was a new efficient way to deposit thin films with simple metal nitrate aqueous solutions. Acknowledgements This work was sponsored by the 973 National High Technology Program of China under Contract No. 2002CB613305 and International Cooperation Research Program between China and Israel (time: 2002–2003). References [1] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] R. Ramesh, B. Dutta, T.S. Ravi, J. Lee, T. Sands, V.G. Keramidas, Appl. Phys. Lett. 64 (1994) 1588.
137
[3] Q.X. Jia, X.D. Wu, S.R. Foltyn, P. Tiwari, Appl. Phys. Lett. 66 (1995) 2197. [4] C. Guerrero, F. Sánchez, C. Ferrater, J. Roldán, M.V. Garc´ıa-Cuenca, M. Varela, Appl. Surf. Sci. 168 (2000) 219–222. [5] K.P. Rajeev, G.V. Shivashankar, A.K. Raychaudhuri, Solid State Commun. 99 (1991) 591. [6] K.M. Satyalakshmi, R.M. Mallya, K.V. Ramanathan, X.D. Wu, B. Brainard, D.C. Gautier, N.Y. Vasanthacharya, M.S. Hegde, Appl. Phys. Lett. 62 (1993) 1233. [7] H.Y. Lee, T.B. Wu, J.F. Lee, J. Appl. Phys. 80 (1996) 2175. [8] K.S. Hwang, S.S. Min, Y.J. Park, Surf. Coat. Technol. 137 (2001) 205. [9] A.D. Li, C.Z. Ge, P. Lu, N.B. Ming, Appl. Phys. Lett. 68 (1996) 1347. [10] O.Yu. Gorbenko, A.A. Bosak, J. Cryst. Growth 186 (1998) 181. [11] H. Huang, X. Yao, M.Q. Wang, X.Q. Wu, J. Cryst. Growth 263 (2004) 406. [12] H. Huang, X. Yao, Ceram. Int., in press. [13] H. Huang, X. Yao, Mater. Res. Innov., in press. [14] H. Ichinose, M. Nagano, J. Mater. Sci. 29 (1994) 5115. [15] A.D. Li, D. Wu, C.Z. Ge, H. Wang, M. Wang, Z.G. Liu, N.B. Ming, Thin Solid Films 375 (2000) 220.
Preparation of LaNiO3 thin films by mist plasma evaporation from aqueous precursor Hui Huang∗ , Xi Yao Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an 710049, China Received 12 November 2003; received in revised form 28 April 2004; accepted 20 May 2004
Abstract LaNiO3 thin films were prepared by mist plasma evaporation on n-Si(1 1 1) wafers using lanthanum nitrate and nickel nitrate aqueous solutions as precursor. The obtained LaNiO3 thin films possessed a perovskite structure with rhombohedral distortion. No impurity phase was detected in the LaNiO3 film deposited at 630 ◦ C. The grain size of the films decreased with the decreasing substrate temperature, and increased with the increase of deposition time and atomizing rate of precursor. The resistivity of the LaNiO3 films decreased with the decreasing substrate temperature. After annealing at 400 ◦ C for 2 h, the room temperature resistivity of the LaNiO3 film deposited at 630 ◦ C for 20 min decreased from 5.60 to 0.68 m cm. © 2004 Elsevier B.V. All rights reserved. Keywords: Thin films; Plasma deposition; Electrical conductivity
1. Introduction Ferroelectric thin films have attracted much attention for a wide range of applications in pyroelectric and piezoelectric sensors, microwave devices and dynamic random access memories (DRAMs) [1,2]. Ferroelectric thin film capacitors with Pt metallic electrodes showed fatigue degradation due to charge segregation and decay on the non-ideal interface between films and Pt base electrodes. In recent years, ferroelectric films coated on perovskite related metallic oxide electrodes such as YBa2 Cu3 O7−x (YBCO), La0.5 Sr0.5 CoO3 (LSCO), SrRuO3 and LaNiO3 had good crystallizability and ferroelectric properties due to lattice matching, and were considered to be promising alternatives for solving the above problems in using conventional metal electrodes [3,4]. LaNiO3 thin films have a pseudocubic structure with a lattice parameter of 3.84 Å, and have been developed as a new metal oxide electrode. The resistivity of LaNiO3 at room temperature was about 10−5 m, and the temperature dependence indicated a good metallic behavior [5]. Since LaNiO3 films were first deposited by pulsed laser deposition (PLD) [6], they have been successfully fabricated by various methods such as RF sputtering [7], sol–gel method [8], metalorganic decomposition (MOD) [9] and metalorganic chemical vapor deposition (MOCVD) [10]. ∗ Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: [email protected] (H. Huang).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.017
Recently, a novel deposition technique, mist plasma evaporation (MPE), has been developed by us to prepare thin films [11–13]. In this technique, aqueous solution of metal nitrate or chloride was ultrasonically atomized into tiny droplets in several microns, and the droplets were introduced into radio frequency inductively coupled plasma torch generated at atmospheric pressure. Source materials in the droplets were decomposed to their components by the ultrahigh temperature (∼5000 ◦ C) of the plasma torch, and then reacted and deposited on substrate to form thin films during plasma cooling process. This method showed remarkable advantages such as high deposition rate, single precursor, wide choices of source materials and no subsequent annealing. In this work, LaNiO3 thin films were prepared by MPE, the effects of deposition parameters on structural and electrical properties of LaNiO3 films were investigated.
2. Experimental The precursor was prepared by dissolving equal molar of La(NO3 )3 ·6H2 O and Ni(NO3 )2 ·6H2 O in deionized water. Fig. 1 shows schematic illustration of MPE system. The torch tube was a set of triple-coaxial fused quartz tubes. Thermal plasma torch was ignited with a water-cooled Cu coil coupled to an RF oscillator (normally run at 3 kW and 30 MHz) at atmospheric pressure, and stabilized with Ar sheath gas flowing through the outer quartz tube. The
135
*
*
(312)
(024)
* La2O3 (113) (122) (104)
Intensity (a.u.)
(101) Si (110) (012) (021) (003) (202)
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
(c)
(b)
*
(a) 20
30
40
50
60
70
80
2 theta (deg.) Fig. 2. XRD patterns of the films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min.
3. Results and discussion
Fig. 1. Schematic illustration of the MPE system.
precursor solution was ultrasonically atomized, and was introduced into the center of the plasma torch through inner quartz tube by Ar carrier gas. Oxygen was added into Ar carried gas to enhance the oxidation of source materials. Substrate was fixed on fused quartz substrate holder, and was heated by tail flame of the plasma. Substrate temperature Ts was detected by a shielded thermocouple pressed on the back of substrate holder, and was controlled by adjusting substrate distance (Dsn ) from substrate to the spraying nozzle of inner quartz tube with a motor-driven mechanism. Summary of typical deposition parameters was listed in Table 1. Phase compositions of the films were characterized by a Rigaku D/max-2400 X-ray diffractometer (XRD) using Cu K␣ radiation. Morphology of the films was observed by a Philips XL20 scanning electron microscopy (SEM). Thickness and refractive index of the films were measured by a Filmetric F20 Thin-Film Measurement System. Electrical resistivity at room temperature of the films was measured by the standard four-probe technique.
Table 1 Summary of typical deposition conditions of MPE RF power Pressure Sheath gas (Ar) Plasma gas (Ar) Carrier gas Concentration of precursor Atomizing rate Dsn Ts Deposition time
3 kW at 30 MHz Atmospheric pressure 17 L/min 1 L/min Ar: 0.5 L/min, O2 : 0.25 L/min 0.2 mol/L 0.06 mL/min 10–14 cm 710–630 ◦ C 20, 40 min
Fig. 2 shows XRD patterns of the LaNiO3 films prepared at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min, respectively. Both LaNiO3 and La2 NiO4 were detected in the film deposited at 710 ◦ C, and La2 O3 was found in the film deposited at 660 ◦ C. LaNiO3 is unstable at high temperature, and will decompose into NiO and LaNiO2 , which irreversibly change into La2 NiO4 . La2 NiO4 and other compositions in the films deposited at high Ts were caused by decomposition of LaNiO3 at high temperature. As the Ts decreased to 630 ◦ C, only pure LaNiO3 was detected in the film, which possessed a perovskite structure with rhombohedral distortion. Cell data of rhombohedrally distorted perovskite is a = b = c = 3.8375 Å, α = β = γ = 90.64◦ , and can be considered as pseudocubic [14]. Fig. 3 shows SEM photographs of the LaNiO3 films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min, respectively. The LaNiO3 films were very dense, and grains in the films were regular and in uniform size. The grain size of the films deposited at 710 and 660 ◦ C was about 370 and 200 nm, respectively, and decreased to 50 nm in the film deposited at 630 ◦ C. As Ts decreased from 710 to 630 ◦ C, the grain size of the film decreased remarkably. Some large cubic-like grains with a size of ∼125 nm were observed on the LaNiO3 film deposited at 630 ◦ C (Fig. 3c), which was accordant with the rhombohedrally distorted perovskite structure detected by XRD. Thickness and refractive index of the LaNiO3 film deposited at 630 ◦ C for 20 min was 200 nm and 1.93, respectively. Fig. 4 shows the SEM micrograph of the LaNiO3 films deposited at 660 ◦ C for 40 min. Cubic-like crystals were also observed in the film. The grain size of the film was about 500 nm, which was much larger than that of the film deposited at 660 ◦ C for 20 min. The effect of atomizing rate on film morphology was also investigated. When the atomizing rate of the precursor increased from 0.08 to 0.12 mL/min, the gain size was increased from 200 nm (Fig. 3b) to about 300 nm (as shown in Fig. 5). Fig. 6 shows the resistivity of the LaNiO3 films deposited at various Ts for 20 min. As Ts increased from 630 to 710 ◦ C,
136
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
Fig. 4. SEM micrograph of the LaNiO3 film deposited at 660 ◦ C for 40 min.
Fig. 5. SEM micrograph of the LaNiO3 film deposited at 660 ◦ C for 20 min with increased atomizing rate.
Resistivity (mΩ cm)
250 200
Before annealing After annealing
150 100 50 0 620
640
660
680
700
720
o
Ts ( C) Fig. 3. SEM micrographs of the LaNiO3 films deposited at (a) 710 ◦ C, (b) 660 ◦ C and (c) 630 ◦ C for 20 min.
resistivity of the LaNiO3 films increased sharply from 5.60 to 225.78 m cm. After annealed at 400 ◦ C for 2 h in air, the resistivity of the films deposited at 710 ◦ C decreased sharply from 225.78 to 3.28 m cm, and that of the film deposited at 630 ◦ C also decreased from 5.60 to 0.68 m cm, which was lower than the resistivity of LaNiO3 films prepared by wet chemical solution process [8,15]. High resistivity of the film deposited at 710 ◦ C resulted from the decomposition of LaNiO3 at high deposition temperature, which was proved by XRD. When the films were annealed at 400 ◦ C for 2 h
Fig. 6. Resistivity of the LaNiO3 films deposited at various Ts for 20 min.
in air, the decomposed components of the LaNiO3 films reacted again, so the resistivity of the films decreased after annealing.
4. Conclusions Lanthanum nitrate and nickel nitrate aqueous solution were used as precursor to deposit LaNiO3 thin films by MPE. The films were rhombohedral LaNiO3 . The grain size of the films decreased with the decreasing substrate temperature,
H. Huang, X. Yao / Materials Chemistry and Physics 87 (2004) 134–137
and increased with the increase of deposition time and atomizing rate of precursor. The resistivity of the LaNiO3 films decreased with the decreasing substrate temperature. It was proved that MPE was a new efficient way to deposit thin films with simple metal nitrate aqueous solutions. Acknowledgements This work was sponsored by the 973 National High Technology Program of China under Contract No. 2002CB613305 and International Cooperation Research Program between China and Israel (time: 2002–2003). References [1] J.F. Scott, C.A. Paz de Araujo, Science 246 (1989) 1400. [2] R. Ramesh, B. Dutta, T.S. Ravi, J. Lee, T. Sands, V.G. Keramidas, Appl. Phys. Lett. 64 (1994) 1588.
137
[3] Q.X. Jia, X.D. Wu, S.R. Foltyn, P. Tiwari, Appl. Phys. Lett. 66 (1995) 2197. [4] C. Guerrero, F. Sánchez, C. Ferrater, J. Roldán, M.V. Garc´ıa-Cuenca, M. Varela, Appl. Surf. Sci. 168 (2000) 219–222. [5] K.P. Rajeev, G.V. Shivashankar, A.K. Raychaudhuri, Solid State Commun. 99 (1991) 591. [6] K.M. Satyalakshmi, R.M. Mallya, K.V. Ramanathan, X.D. Wu, B. Brainard, D.C. Gautier, N.Y. Vasanthacharya, M.S. Hegde, Appl. Phys. Lett. 62 (1993) 1233. [7] H.Y. Lee, T.B. Wu, J.F. Lee, J. Appl. Phys. 80 (1996) 2175. [8] K.S. Hwang, S.S. Min, Y.J. Park, Surf. Coat. Technol. 137 (2001) 205. [9] A.D. Li, C.Z. Ge, P. Lu, N.B. Ming, Appl. Phys. Lett. 68 (1996) 1347. [10] O.Yu. Gorbenko, A.A. Bosak, J. Cryst. Growth 186 (1998) 181. [11] H. Huang, X. Yao, M.Q. Wang, X.Q. Wu, J. Cryst. Growth 263 (2004) 406. [12] H. Huang, X. Yao, Ceram. Int., in press. [13] H. Huang, X. Yao, Mater. Res. Innov., in press. [14] H. Ichinose, M. Nagano, J. Mater. Sci. 29 (1994) 5115. [15] A.D. Li, D. Wu, C.Z. Ge, H. Wang, M. Wang, Z.G. Liu, N.B. Ming, Thin Solid Films 375 (2000) 220.