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A study on the thermostability of LaNiO3 films
Surface & Coatings Technology 192 (2005) 336 – 340 www.elsevier.com/locate/surfcoat
A study on the thermostability of LaNiO3 films Q. Zhao a,b,*, Z.M. Huang b, Z.G. Hu b, J.H. Chu b a
b
Nanotech Center, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, PR China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Yutian Road 500, Shanghai 200083, PR China Received 4 December 2003; accepted 7 May 2004 Available online 4 July 2004
Abstract Thin LaNiO3-x films with pseudocubic (100) preferred orientation were prepared by rf magnetron sputtering and in situ annealed at 265 jC. X-ray diffraction (XRD) results indicate that the annealing did not cause the lattice distortion of the films. The electric conductivity, the refractive index and the extinction coefficient decrease exponentially as annealing time increases. X-ray photoelectron spectroscopy (XPS) analysis manifested that the oxygen concentration in the LaNiO3-x film decreased 2.7% after 2 h annealing and the loss of the lattice oxygen in films led to the changes of the properties of the LaNiO3-x films. D 2004 Elsevier B.V. All rights reserved. Keywords: LaNiO3; Annealing; Conductivity; Optical properties
1. Introduction In recent years, increasing interests have been focused on metal oxides of oxygen octahedral structure due to metal – insulator (MI) transition, high-temperature superconductor, ferroelectric, colossal magnetoresistance and their heterostructures for future technological potentials [1 – 4]. Researches reveal that the microstructure of these films plays a critical role in determining their formidable physical properties, and usually, a chosen substrate or buffer layer with the lattice matching the films is expected [5,6]. In the past, although a metal-based film, such as Pt and Ir, is often used as bottom electrodes, it appears to be a problem for the material quality due to oxygen deficiency and interdiffusion [7 –9]. For this reason, oxide films with metallic conductivity have been studied as a substitute. Because of its good transport properties, composition simplicity, and pseudocubic perovskite crystal structure with a lattice parameter ˚ , LaNiO3 (LNO) is a most promising material a = 3.84 A of the few conductive oxides with a crystal structure suitable * Corresponding author. Nanotech Center, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, PR China. Tel.: +86-21-62233230; fax: +86-21-6223-2053. E-mail address: [email protected] (Q. Zhao). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.05.004
for these requests [10,11]. In most studies, LaNiO3 films are considered as being with good physical and chemical thermostabilities, and most metal oxide films, such as ferroelectric films, must be grown at a high temperature. Therefore, it becomes very necessary to study the thermostability of LaNiO3 films. LNO films have been grown on many materials by various methods [12 – 16], and are reported to begin to crystallize at 150 jC when prepared by rf magnetron sputtering [16]. In this paper, LNO films are deposited on Si substrates by a rf magnetron sputtering system, and their structure, electric conductivity, and optical property dependence on annealing time are investigated.
2. Experimental details The diameter and thickness of target are 100 and 5 mm, respectively. To make the target, La2O3 (99.9%) and Ni2O3 (99.9%) powders of 1:1 molar concentration ratio of La to Ni were well mixed, then were pressed and sintered at 1300 jC for 8 h. A radio frequency (13.6 MHz) sputtering system was employed, and a thermocouple was put directly on the substrate surface to detect the substrate temperature. Before deposition, the substrate was heated to a deposition temper-
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
337
Table 1 The major deposition conditions Target Substrate Target diameter Target – substrate spacing rf input power Sputtering gas Gas pressure Substrate temperature Deposition rate Annealing condition
Ceramic LaNiO3 (111)Si 100 mm 85 mm 80 W (13.6 MHz) Ar 2.0 Pa 265 jC 2.8 nm/min in situ
ature of 265 jC at a base pressure of 2 10-4 Pa, and its surface was cleaned by an ion beam. Then argon was introduced into the chamber to get a sputtering gas pressure of 2 Pa. The sputtering power is 80 W, and the film growth rate is estimated to be 2.8 nm/min. After deposition, the films were in situ annealed for 30, 60, and 120 min, respectively. Then, films were cooled down to room temperature naturally. Typical sputtering conditions are listed in Table 1. The concentration ratio of Ni to La in the films was obtained by an inductively coupled argon plasma atomic emission spectrometry (ICP-AES). The phase identification of the film was analyzed by X-ray diffraction (XRD) with ˚ ). The optic constants of films Cu Ka radiation (k = 1.0406 A were derived from the reflectance and transmittance spectra, measured by infrared Fourier transform spectrometer (BioRad FTS 65A), using commercial optical thin film software. The electrical resistivity was measured by a standard dc four-probe method at room temperature. The oxygen composition of the films was identified by X-ray photoelectron spectroscopy (XPS) using Al Ka radiation (1486.8 eV). The surface morphology of the films was observed by scanning electron microscopy (SEM, LEO1530VP), and the thickness was determined by the cross-section image.
Fig. 1. The X-ray spectrums of the films been annealed for different time. (A) 0 min, (B) 30 min, (C) 60 min, (D) 120 min. [Si(111) peak has been cut down].
estimated to be 230 nm. Typical surface and cross section images observed by SEM are shown in Fig. 2A and B, respectively. Fig. 3 shows the dependence of the sheet
3. Results and discussion The ICP-AES measurement shows that the concentration ratio of Ni to La in all films is of the same value as that of the target, 1:1. Fig. 1 presents the X-ray diffraction spectra of the films deposited for 80 min and were in situ annealed for various times. All films are identified to be pseudocubic perovskite crystal structure with a lattice ˚ , which is greater than 3.84 A ˚ of parameter a = 3.87 A powder material (JCPDS card, No. 33-0710). The patterns also indicate that all films consist of mainly (100) aligned grains, and the annealing does not affected the structure of the films. According to the surface and cross-section images observed by SEM, the grains are very small and uniform, and the surface is flat and smooth indicating that the films are good for bottom electrodes and buffer layers. There no changes of the film morphology and the film thickness are found, and the thickness of the film could be
Fig. 2. Typical SEM images of (A) film surface and (B) cross section.
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Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
where ne, e, me is the electron (conduction electron) density, charge and mass respectively. The root mean square fractional error r, defined by NR X 1 Rmod Rexp i i r ¼ NR M i¼1 rexp R;i
!2
2
NT X Tjmod Tjexp 1 þ NT M j¼1 rexp T; j
!2 ð4Þ
has been used to judge the quality of the fit between the measured (exp) and model (mod) data. Where NR and NT are the number of experimental intensity reflectance R and transmission T measurements, and M is the number of fit parameters. According to the results, the refractive index increases with increasing the wavelength, similar to that of metals in the same infrared region.
Fig. 3. The dependence of sheet resistance on the annealing time.
resistance on the annealing time. As annealing time increases, the sheet resistance increases exponentially, and could be simulated by equation R ¼ Rb þ R0 egt
ð1Þ
where R is the sheet resistance, Rb, R0 and B are three constants, and t is the annealing time. Fig. 4 gives the dependences of the refractive index and the extinction coefficient on the annealing time, which were derived by fitting the reflectance and transmittance spectra of the four samples simultaneously using commercial optical thin-film software. The multiple reflections inside substrate and the interface effect inside the LNO films were taken into account. Usually, Drude theory is the best to metallic oxides model [17], which gives the freecarrier contribution to the dielectric function e as a function of the high frequency lattice dielectric constant el, plasma frequency xp and electron scattering frequency m (or intraband relaxation frequency). Thus, in the calculation a Drude model in the following was used to describe the dielectric function e of the LNO films,
2
euðn ikÞ ¼ el 1
x2p EðE þ imÞ
! ð2Þ
where n and k are the refractive index and the extinction coefficient of the thin films. E is the wavelength energy, and xp is the plasma frequency which is defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pne e2 xp ¼ me
ð3Þ
Fig. 4. The dependences of (A) the refractive index and (B) the extinction coefficient on the annealing time. (Symbols are used to distinguish the samples.).
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
Both the refractive indexes and the extinction coefficients decrease as annealing time increases, and the gap of the refractive index or the extinction coefficient between two consecutive samples also increases as annealing time increases. Torrance et al. [1] studied the insulator-metal transitions of perovskites RNiO3 (R: rare earth elements) and their results manifested that the electric properties of RNiO3 strongly depend on the valence and conduction band overlap of 2p-O and 3d8-Ni, and the band overlap is determined by the angle of Ni –O – Ni bond and the distance between O2- and Ni3 + ions. Since no distortion is observed in the structure of the films after annealing according to the XRD spectra, the decrease of the conductivity of the films after annealing does not owe to the changes of the angle of Ni – O – Ni bond and the distance between O2- and Ni3 +. One reason is possible that the oxygen loss might take place in the lattice and caused the disappearance of the valence and conduction band overlap of 2p-O and 3d8-Ni; therefore, it degraded the electric conductivity of the films. To identify the oxygen composition, XPS analyses were performed and all of binding energies at various peaks were calibrated by using the binding energy of C1s (284.6 eV). Fig. 5 gives the narrow scan XPS spectrums of O1s, in which sample (A) is the film as deposited, and the sample (B) is the film annealed for 2 h. It can be seen that the O1s peak is composed of a doublet that could be ascribed to two oxygen states that are plotted with dash lines. The peak of lower binding energy near 528.1 eV is assigned to the lattice oxygen in LNO, while another peak of higher energy near 530.25 eV corresponds to the surface absorbed oxygen. The estimated percentages of the lattice oxygen and the surface absorbed oxygen from the spectra are mentioned in Table 2. The results demonstrated that the percentage of the lattice oxygen decreases 2.7% after annealing for 2 h. Because that the loss of the oxygen did not cause the lattice distortion, the
339
Table 2 The XPS analysis data of the oxygen in the sample (A) as deposited and the sample (B) annealed for 120 min Sample
Peak position (eV)
FWHM (eV)
Height (CPS)
Percentage (%)
(A)
528.07 530.29 528.15 530.24
2.96 2.01 2.92 2.31
11,508 5178 11,024 5180
81.9 18.1 79.18 20.82
(B)
oxygen vacancies would be formed in the LNO films after annealing even at a relative low temperature of 265 jC. When an oxygen atom escapes the lattice binding, the related band overlap of 2p-O and 3d8-Ni diminishes, and the electron localization then takes place; thus, the metallic conduction property disappears [1]. Suppose that the initial concentration of the lattice oxygen is I0, and that the possibility of an oxygen atom escaping the lattice is g. The relationship between the lattice oxygen concentration I and time t is IfI0 egt
Consequently, the sheet resistance r of the films could be deduced, rfr0 ebt
ð6Þ
where r0 is the initial sheet resistance, and b is a constant related to the material. Since the refractive index and extinction coefficient could be described as 8" 9 2 # 12 = < 1 r n2 ¼ e 1 þ þ1 ; 2 : xee0 8" 9 2 # 12 = < 1 r k2 ¼ e 1 þ 1 ; 2 : xee0
Fig. 5. The narrow scan XPS spectrums of O1s. (A) As deposited, (B) annealed for 2 h. (After annealing, the lattice oxygen percentage decreased 2.7%).
ð5Þ
ð7Þ
ð8Þ
where r is the optical conductivity, e0 is the permittivity in 2 vacuum. 2 The difference between n and k is small, thus e f xer 0 . Therefore, n is determined by r in the same extent as k is done. As described, after annealing, the lattice oxygen loss causes the metallic conduction diminishes in the areas of oxygen vacancies, and this lowers the conductivity r. Consequently, the refractive index and extinction coefficient would decrease as annealing time increases. On the other hand, although the lattice oxygen loss lowers the conductivity, refractive index and extinction coefficient of the LNO films, it does not lead to lattice distortion. This is useful to improve the properties of oxide layer on LNO by exchanging oxygen vacancies in the interface when other oxide films are grown upon [8,9,18,19].
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Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
4. Conclusions
References
Pseudocubic perovskite LNO films with (100) preferred orientation have been grown on (111)Si substrates at temperature of 265 jC. Films present a flat and smooth surface and metallic behaviors. During the in situ annealing, the lattice oxygen loss causes the decreases of the conductivity, the refractive index, and the extinction coefficient. All results demonstrate that the electric and the optical properties of LNO films are unstable even at a relatively low temperature. But the annealing does not affect the lattice structure. Therefore, the LNO films are suitable for being utilized as the bottom and buffer layers for oxide films and their heterostructures. Nevertheless, the changes of electric and optical properties must be taken into account when they are applied in the optical and electrical devices.
[1] J.B. Torrance, P. Lacorre, A.I. Nazzal, Phys. Rev., B 45 (1992) 8209. [2] R. Von Helmolt, J. Wecker, R. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [3] T. Venkatesan, M. Rajeswari, Z.-W. Dong, S.B. Ogale, R. Ramesh, Philos. Trans. R. Soc. 356 (1998) 1661. [4] R. Ramesh, S. Aggarwal, O. Auciello, Mater. Sci. Eng. 32 (2001) 191. [5] R.S. Batzer, B. Yen, D. Liu, H. Chen, J. Appl. Phys. 80 (1996) 6235. [6] N. Floquet, J. Hector, P. Gaucher, J. Appl. Phys. 84 (1998) 3825. [7] R. Moazzami, Semicond. Sci. Technol. 10 (1995) 375. [8] M.C. Jiang, T.B. Wu, J. Mater. Res. 9 (1994) 1879. [9] J.F. Scott, C.A. Paz de Arujo, Science 246 (1989) 1400. [10] W. Wu, K.H. Wong, P.W. Chan, Physica C 297 (1998) 247. [11] M.S. Chen, T.B. Wu, J.M. Wu, Appl. Phys. Lett. 68 (1996) 1430. [12] F. Sa´nchez, C. Ferrater, X. Alcobee´, J. Bassas, M.V. Garcı´a-Cuenca, M. Varela, Thin Solid Films 384 (2001) 200. [13] C.C. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. [14] S. Miyake, S. Fujihara, T. Kimura, J. Eur. Ceram. Soc. 21 (2001) 1525. [15] K. Hwang, Y. Lim, B. Kim, Mater. Res. Bull. 34 (2000) 2069. [16] C.C. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. [17] F. Gervais, Mater. Sci. Eng. R39 (2002) 29. [18] J.F. Scott, M. Dawber, Appl. Phys. Lett. 76 (2000) 3801. [19] A.I. Kingon, S.K. Streiffer, C. Basceri, S.R. Summerfelt, MRS Bull. 16 (1996) 46.
Acknowledgements This work is partly supported by Science and Technology Commission of Shanghai Municipality P.R. China under the contracts 0352nm077.
A study on the thermostability of LaNiO3 films Q. Zhao a,b,*, Z.M. Huang b, Z.G. Hu b, J.H. Chu b a
b
Nanotech Center, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, PR China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Yutian Road 500, Shanghai 200083, PR China Received 4 December 2003; accepted 7 May 2004 Available online 4 July 2004
Abstract Thin LaNiO3-x films with pseudocubic (100) preferred orientation were prepared by rf magnetron sputtering and in situ annealed at 265 jC. X-ray diffraction (XRD) results indicate that the annealing did not cause the lattice distortion of the films. The electric conductivity, the refractive index and the extinction coefficient decrease exponentially as annealing time increases. X-ray photoelectron spectroscopy (XPS) analysis manifested that the oxygen concentration in the LaNiO3-x film decreased 2.7% after 2 h annealing and the loss of the lattice oxygen in films led to the changes of the properties of the LaNiO3-x films. D 2004 Elsevier B.V. All rights reserved. Keywords: LaNiO3; Annealing; Conductivity; Optical properties
1. Introduction In recent years, increasing interests have been focused on metal oxides of oxygen octahedral structure due to metal – insulator (MI) transition, high-temperature superconductor, ferroelectric, colossal magnetoresistance and their heterostructures for future technological potentials [1 – 4]. Researches reveal that the microstructure of these films plays a critical role in determining their formidable physical properties, and usually, a chosen substrate or buffer layer with the lattice matching the films is expected [5,6]. In the past, although a metal-based film, such as Pt and Ir, is often used as bottom electrodes, it appears to be a problem for the material quality due to oxygen deficiency and interdiffusion [7 –9]. For this reason, oxide films with metallic conductivity have been studied as a substitute. Because of its good transport properties, composition simplicity, and pseudocubic perovskite crystal structure with a lattice parameter ˚ , LaNiO3 (LNO) is a most promising material a = 3.84 A of the few conductive oxides with a crystal structure suitable * Corresponding author. Nanotech Center, East China Normal University, North Zhongshan Road 3663, Shanghai 200062, PR China. Tel.: +86-21-62233230; fax: +86-21-6223-2053. E-mail address: [email protected] (Q. Zhao). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.05.004
for these requests [10,11]. In most studies, LaNiO3 films are considered as being with good physical and chemical thermostabilities, and most metal oxide films, such as ferroelectric films, must be grown at a high temperature. Therefore, it becomes very necessary to study the thermostability of LaNiO3 films. LNO films have been grown on many materials by various methods [12 – 16], and are reported to begin to crystallize at 150 jC when prepared by rf magnetron sputtering [16]. In this paper, LNO films are deposited on Si substrates by a rf magnetron sputtering system, and their structure, electric conductivity, and optical property dependence on annealing time are investigated.
2. Experimental details The diameter and thickness of target are 100 and 5 mm, respectively. To make the target, La2O3 (99.9%) and Ni2O3 (99.9%) powders of 1:1 molar concentration ratio of La to Ni were well mixed, then were pressed and sintered at 1300 jC for 8 h. A radio frequency (13.6 MHz) sputtering system was employed, and a thermocouple was put directly on the substrate surface to detect the substrate temperature. Before deposition, the substrate was heated to a deposition temper-
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
337
Table 1 The major deposition conditions Target Substrate Target diameter Target – substrate spacing rf input power Sputtering gas Gas pressure Substrate temperature Deposition rate Annealing condition
Ceramic LaNiO3 (111)Si 100 mm 85 mm 80 W (13.6 MHz) Ar 2.0 Pa 265 jC 2.8 nm/min in situ
ature of 265 jC at a base pressure of 2 10-4 Pa, and its surface was cleaned by an ion beam. Then argon was introduced into the chamber to get a sputtering gas pressure of 2 Pa. The sputtering power is 80 W, and the film growth rate is estimated to be 2.8 nm/min. After deposition, the films were in situ annealed for 30, 60, and 120 min, respectively. Then, films were cooled down to room temperature naturally. Typical sputtering conditions are listed in Table 1. The concentration ratio of Ni to La in the films was obtained by an inductively coupled argon plasma atomic emission spectrometry (ICP-AES). The phase identification of the film was analyzed by X-ray diffraction (XRD) with ˚ ). The optic constants of films Cu Ka radiation (k = 1.0406 A were derived from the reflectance and transmittance spectra, measured by infrared Fourier transform spectrometer (BioRad FTS 65A), using commercial optical thin film software. The electrical resistivity was measured by a standard dc four-probe method at room temperature. The oxygen composition of the films was identified by X-ray photoelectron spectroscopy (XPS) using Al Ka radiation (1486.8 eV). The surface morphology of the films was observed by scanning electron microscopy (SEM, LEO1530VP), and the thickness was determined by the cross-section image.
Fig. 1. The X-ray spectrums of the films been annealed for different time. (A) 0 min, (B) 30 min, (C) 60 min, (D) 120 min. [Si(111) peak has been cut down].
estimated to be 230 nm. Typical surface and cross section images observed by SEM are shown in Fig. 2A and B, respectively. Fig. 3 shows the dependence of the sheet
3. Results and discussion The ICP-AES measurement shows that the concentration ratio of Ni to La in all films is of the same value as that of the target, 1:1. Fig. 1 presents the X-ray diffraction spectra of the films deposited for 80 min and were in situ annealed for various times. All films are identified to be pseudocubic perovskite crystal structure with a lattice ˚ , which is greater than 3.84 A ˚ of parameter a = 3.87 A powder material (JCPDS card, No. 33-0710). The patterns also indicate that all films consist of mainly (100) aligned grains, and the annealing does not affected the structure of the films. According to the surface and cross-section images observed by SEM, the grains are very small and uniform, and the surface is flat and smooth indicating that the films are good for bottom electrodes and buffer layers. There no changes of the film morphology and the film thickness are found, and the thickness of the film could be
Fig. 2. Typical SEM images of (A) film surface and (B) cross section.
338
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
where ne, e, me is the electron (conduction electron) density, charge and mass respectively. The root mean square fractional error r, defined by NR X 1 Rmod Rexp i i r ¼ NR M i¼1 rexp R;i
!2
2
NT X Tjmod Tjexp 1 þ NT M j¼1 rexp T; j
!2 ð4Þ
has been used to judge the quality of the fit between the measured (exp) and model (mod) data. Where NR and NT are the number of experimental intensity reflectance R and transmission T measurements, and M is the number of fit parameters. According to the results, the refractive index increases with increasing the wavelength, similar to that of metals in the same infrared region.
Fig. 3. The dependence of sheet resistance on the annealing time.
resistance on the annealing time. As annealing time increases, the sheet resistance increases exponentially, and could be simulated by equation R ¼ Rb þ R0 egt
ð1Þ
where R is the sheet resistance, Rb, R0 and B are three constants, and t is the annealing time. Fig. 4 gives the dependences of the refractive index and the extinction coefficient on the annealing time, which were derived by fitting the reflectance and transmittance spectra of the four samples simultaneously using commercial optical thin-film software. The multiple reflections inside substrate and the interface effect inside the LNO films were taken into account. Usually, Drude theory is the best to metallic oxides model [17], which gives the freecarrier contribution to the dielectric function e as a function of the high frequency lattice dielectric constant el, plasma frequency xp and electron scattering frequency m (or intraband relaxation frequency). Thus, in the calculation a Drude model in the following was used to describe the dielectric function e of the LNO films,
2
euðn ikÞ ¼ el 1
x2p EðE þ imÞ
! ð2Þ
where n and k are the refractive index and the extinction coefficient of the thin films. E is the wavelength energy, and xp is the plasma frequency which is defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pne e2 xp ¼ me
ð3Þ
Fig. 4. The dependences of (A) the refractive index and (B) the extinction coefficient on the annealing time. (Symbols are used to distinguish the samples.).
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
Both the refractive indexes and the extinction coefficients decrease as annealing time increases, and the gap of the refractive index or the extinction coefficient between two consecutive samples also increases as annealing time increases. Torrance et al. [1] studied the insulator-metal transitions of perovskites RNiO3 (R: rare earth elements) and their results manifested that the electric properties of RNiO3 strongly depend on the valence and conduction band overlap of 2p-O and 3d8-Ni, and the band overlap is determined by the angle of Ni –O – Ni bond and the distance between O2- and Ni3 + ions. Since no distortion is observed in the structure of the films after annealing according to the XRD spectra, the decrease of the conductivity of the films after annealing does not owe to the changes of the angle of Ni – O – Ni bond and the distance between O2- and Ni3 +. One reason is possible that the oxygen loss might take place in the lattice and caused the disappearance of the valence and conduction band overlap of 2p-O and 3d8-Ni; therefore, it degraded the electric conductivity of the films. To identify the oxygen composition, XPS analyses were performed and all of binding energies at various peaks were calibrated by using the binding energy of C1s (284.6 eV). Fig. 5 gives the narrow scan XPS spectrums of O1s, in which sample (A) is the film as deposited, and the sample (B) is the film annealed for 2 h. It can be seen that the O1s peak is composed of a doublet that could be ascribed to two oxygen states that are plotted with dash lines. The peak of lower binding energy near 528.1 eV is assigned to the lattice oxygen in LNO, while another peak of higher energy near 530.25 eV corresponds to the surface absorbed oxygen. The estimated percentages of the lattice oxygen and the surface absorbed oxygen from the spectra are mentioned in Table 2. The results demonstrated that the percentage of the lattice oxygen decreases 2.7% after annealing for 2 h. Because that the loss of the oxygen did not cause the lattice distortion, the
339
Table 2 The XPS analysis data of the oxygen in the sample (A) as deposited and the sample (B) annealed for 120 min Sample
Peak position (eV)
FWHM (eV)
Height (CPS)
Percentage (%)
(A)
528.07 530.29 528.15 530.24
2.96 2.01 2.92 2.31
11,508 5178 11,024 5180
81.9 18.1 79.18 20.82
(B)
oxygen vacancies would be formed in the LNO films after annealing even at a relative low temperature of 265 jC. When an oxygen atom escapes the lattice binding, the related band overlap of 2p-O and 3d8-Ni diminishes, and the electron localization then takes place; thus, the metallic conduction property disappears [1]. Suppose that the initial concentration of the lattice oxygen is I0, and that the possibility of an oxygen atom escaping the lattice is g. The relationship between the lattice oxygen concentration I and time t is IfI0 egt
Consequently, the sheet resistance r of the films could be deduced, rfr0 ebt
ð6Þ
where r0 is the initial sheet resistance, and b is a constant related to the material. Since the refractive index and extinction coefficient could be described as 8" 9 2 # 12 = < 1 r n2 ¼ e 1 þ þ1 ; 2 : xee0 8" 9 2 # 12 = < 1 r k2 ¼ e 1 þ 1 ; 2 : xee0
Fig. 5. The narrow scan XPS spectrums of O1s. (A) As deposited, (B) annealed for 2 h. (After annealing, the lattice oxygen percentage decreased 2.7%).
ð5Þ
ð7Þ
ð8Þ
where r is the optical conductivity, e0 is the permittivity in 2 vacuum. 2 The difference between n and k is small, thus e f xer 0 . Therefore, n is determined by r in the same extent as k is done. As described, after annealing, the lattice oxygen loss causes the metallic conduction diminishes in the areas of oxygen vacancies, and this lowers the conductivity r. Consequently, the refractive index and extinction coefficient would decrease as annealing time increases. On the other hand, although the lattice oxygen loss lowers the conductivity, refractive index and extinction coefficient of the LNO films, it does not lead to lattice distortion. This is useful to improve the properties of oxide layer on LNO by exchanging oxygen vacancies in the interface when other oxide films are grown upon [8,9,18,19].
340
Q. Zhao et al. / Surface & Coatings Technology 192 (2005) 336–340
4. Conclusions
References
Pseudocubic perovskite LNO films with (100) preferred orientation have been grown on (111)Si substrates at temperature of 265 jC. Films present a flat and smooth surface and metallic behaviors. During the in situ annealing, the lattice oxygen loss causes the decreases of the conductivity, the refractive index, and the extinction coefficient. All results demonstrate that the electric and the optical properties of LNO films are unstable even at a relatively low temperature. But the annealing does not affect the lattice structure. Therefore, the LNO films are suitable for being utilized as the bottom and buffer layers for oxide films and their heterostructures. Nevertheless, the changes of electric and optical properties must be taken into account when they are applied in the optical and electrical devices.
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Acknowledgements This work is partly supported by Science and Technology Commission of Shanghai Municipality P.R. China under the contracts 0352nm077.