- Email: [email protected]
Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering
Thin Solid Films 410 (2002) 114–120
Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering Naoki Wakiya, Takaaki Azuma, Kazuo Shinozaki, Nobuyasu Mizutani* Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 25 September 2001; received in revised form 2 February 2002; accepted 25 February 2002
Abstract Epitaxial growth of LaNiO3 (LNO) thin films was successful on CeO2 yYSZySi(100), MgO(100) and SrTiO3 (STO)(100) substrates by RF magnetron sputtering at 300 8C, although pulsed laser deposition requires 600 8C to prepare epitaxial LNO films according to the literature. Epitaxial LNO films deposited on CeO2 yYSZySi(100) and STO(100) had single orientation of LNOw100x yyCeO2w110x yyYSZw110x yySiw110x) and LNOw100x yySTOw100x, respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNOw100x yyMgOw100x and LNOw100x yyMgOw110x. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with the resistivity in the literature. It was found that the temperature to achieve minimum resistivity was 200 8C, irrespective of the type of substrate. The surface of the LNO films was smooth and flat. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: RF sputtering; LaNiO3; Epitaxial; Resistivity
1. Introduction Recently, conductive oxides having perovskite-type structure, such as (La0.5Sr0.5)CoO3 (LSCO), SrRuO3 (SRO) and LaNiO3 (LNO), have been intensively studied for ferroelectric random access memory (FeRAM) electrodes because fatigue-free property of up to 1011 cycles can be realized using such oxide electrodes w1x. LSCO bulk shows low resistivity (10y6 V m) w2x, and therefore this material is very attractive for thin-film electrodes. However, from the viewpoint of preparation, it is not necessarily easy to control the composition, because LSCO consists of three cations. On the contrary, SRO and LNO consist of two cations, and therefore the difficulty in controlling the composition is small compared with LSCO. The resistivity of SRO and LNO bulk is 2.8=10y6 w3x and 10y5 w4x V m, respectively. Guerrero et al. w5x prepared epitaxial Pb(Zr,Ti)O3 (PZT) *Corresponding author. Tel.: q81-3-5734-2519; fax: q81-3-57343369. E-mail address: [email protected] (N. Mizutani).
thin films on SRO and LNO bottom electrodes and compared the ferroelectric properties. They reported that a remarkable improvement in the ferroelectric properties, i.e. remanent polarization increased from 7.5 to 37 mCy cm2, was observed when the bottom electrode material was changed from SRO to LNO. The reason for the improvement was attributed to a smoother ferroelectricy electrode interface and the formation of the tetragonal phase of PZT instead of the rhombohedral one. In addition, Cross et al. w6x fabricated PtySROy (Pb,La)(Zr,Ti)O3 yPt capacitors and evaluated the leakage current. They reported that the leakage current was greatly influenced by Sr interdiffusion and reaction with excess Pb to form a grain boundary conduction phase, SrPbO3. These results suggest that LNO is superior to SRO as the electrode for ferroelectric thin films. So far, several methods have been adopted to prepare LNO films, such as chemical solution method (CSD) w7–12x, pulsed laser deposition (PLD) w13–18x and RF magnetron sputtering w19,20x. The deposition or annealing temperature to obtain crystallized LNO films in the
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 2 3 8 - 9
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
115
Table 1 Comparison of minimum crystallization temperature of LNO in the literature Deposition method
Substrate
Minimum crystallization temperature (8C)
Reference
CSD (solvent: CSD (solvent: CSD (solvent: CSD (solvent: PLD PLD PLD RF magnetron
STO (100) SiO2 ySi, STO(100), fused quartz SiO2 ySi, SiO2 ySi, glass ceramics STO(100) STO(100), LAO(100) CeO2 yYSZySi(100) SiO2 ySi, PtyTiySiO2 ySi, glass
600 530 650 600 550 500 550 150
w7 x w8 x w11x w12x w14x w15x w18x w19x
organic) organic) water) water)
sputtering
STO, SrTiO3; LAO, LaAlO3; YSZ, Y2O3 –ZrO2.
literature is compared in Table 1. This table clearly shows that LNO films prepared by CSD and PLD require more than 600 8C to crystallize. On the other hand, it is reported that LNO films prepared by RF magnetron sputtering begin to crystallize at 150 8C w19x. This means that the substrate temperature required to crystallize LNO film is markedly decreased when the RF magnetron sputtering is used. However, in w19x, a detailed examination of changes in the composition and crystal structure with the substrate temperature was not clarified. In addition, the LNO films showed only (100)
Fig. 1. XRD patterns of LNO thin films deposited on SiO2ySi(100) at various substrate temperatures.
preferred orientation and epitaxial growth was not reported. The purpose of this work is to examine the possibility of epitaxial growth of LNO films and clarify changes in the composition, crystal structure and resistivity of the films with the substrate temperature. 2. Experimental The preparation of LNO thin films was carried out using RF magnetron sputtering. The deposition chamber was first evacuated to below 2=10y4 Pa to remove residual gases that might have been present. A mixture of Ar and O2 gases (AryO2s8.0:2.0, total flow rate 10
Fig. 2. Changes in composition wNiy(NiqLa)x and lattice parameter of LNO thin films with substrate temperature deposited on SiO2ySi(100).
116
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
sccm) was then introduced to a fixed pressure of 4 Pa, and input RF power of 10 Wycm2 was applied between the targets and the substrates. Pre-sputtering was performed for 20 min, and the deposition of films was carried out at a prescribed substrate temperature between 20 and 600 8C. The substrate was directly heated by resistive heating using a Pt heater. The substrate tem-
perature was measured with a sheathed K-type (chromel–alumel) thermocouple positioned near the substrate. The substrate temperature was calibrated with another thin K-type thermocouple that was bonded by cement on the surface of the Si substrate. The distance between the targets and substrates was approximately 4.5 cm. In this work, a 2-inch-diameter target for LNO thin films was synthesized by the conventional solidstate reaction from starting materials of reagent grade La2O3 (after being calcined at 950 8C) and NiO powders. The sintering was carried out at 1050 8C for 5 h. The composition of the target was set to LayNis1:1.86, which was determined on the basis of results of a preliminary deposition using a LNO target of stoichiometric composition on MgO(100) substrates at 400 8C. The thickness of the LNO thin films was 200 nm. SiO2 ySi substrates were used for the comparison with the result of w19x. In this work, CeO2 yYSZySi(100), SrTiO3(100) and MgO(100) substrates were also selected as typical substrates on which epitaxial growth of LNO is expected to be realized. The lattice mismatch values of LNO with CeO2, STO and MgO are 0.89, y 1.13 and y8.34%, respectively. The CeO2 yYSZy Si(100) substrates were prepared by the deposition of 6-nm-thick YSZ and 10-nm-thick CeO2 layers on
Fig. 4. XRD patterns of LNO thin films deposited CeO2yYSZySi(100) at various substrate temperatures.
Fig. 5. (a–c) (111) X-ray pole figures of LNO thin films deposited on CeO2yYSZySi(100) at 250, 300 and 400 8C, respectively. (d–f) (111) X-ray pole figures of LNO thin films deposited on MgO(100) at 250, 300 and 400 8C, respectively.
Fig. 3. Change in resistivity of LNO thin films deposited on SiO2ySi(100) with substrate temperature.
on
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
117
Fig. 6. (a–d) RHEED patterns of LNO thin films deposited on SrTiO3(100) substrate at 20, 200, 300 and 400 8C, respectively.
Si(100) substrates using pulsed laser deposition (PLD) w21,22x. The YSZ and CeO2 layers were heteroepitaxially grown on Si(100) with a cube-on-cube relationship.
The constituent phase of the films was identified with a powder X-ray diffractometer (X’Pert-MPD (u–u), Philips, Netherlands). The texture measurement was carried out using an X-ray pole figure measurement apparatus wX’Pert MPD (open Eulerian cradle), Philips, Netherlandsx using CuKa radiation operating at 40 kV and 40 mA. The composition of the films was analyzed using X-ray fluorescence spectroscopy (XRF) wwavelength dispersive spectroscopy (WDS), (PW2404, Phillips, Netherlands)x. The microstructure of thin films was observed by FE-SEM (Hitachi, S-800). The resistivity of LNO films at room temperature was measured with a DC four-probe measurement unit (Keithley 236, 195A and 705). 3. Results and discussion 3.1. LNO thin films deposited on SiO2 ySi
Fig. 7. Changes in composition wNiy(NiqLa)x and lattice parameter of LNO thin films with substrate temperature deposited on SiO2ySi(100), CeO2yYSZySi(100), MgO(100) and SrTiO3(100).
Fig. 1 shows the change in XRD pattern of LNO thin films deposited on a SiO2 ySi(100) substrate with the deposition temperature. The film deposited at 20 8C was amorphous, and the film deposited at 100 8C showed quite weak XRD, indicating that this film was not fully crystallized. With increasing the substrate temperature to 200 and 300 8C, the XRD peaks increased. However, an increase in deposition temperature to 400 8C yielded decreasing XRD peaks. With increasing the substrate temperature to 500 and 600 8C, the XRD patterns showed that traces of La2NiO4 co-existed. This suggests that partial decomposition of LNO occurred above 400 8C, which tendency agrees well with the result reported by Yang et al. w19x (RF magnetron sputtering). Fig. 2
118
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
shows the changes in composition and lattice parameter of LNO thin films with substrate temperature deposited on SiO2 ySi substrates. This figure indicates that the Niy (NiqLa) ratio decreased with substrate temperature up to approximately 200 8C and increased above 250 8C. On the contrary, the lattice parameter of LNO thin films was constant between 200 and 400 8C. The lattice parameter of LNO thin films deposited between 200 and 400 8C was higher than that of LNO bulk. On the contrary, the lattice parameter of LNO thin films deposited at 500 and 600 8C (partially decomposed) was small and close to that of LNO bulk. These facts suggest that a LNO thin film having a large lattice parameter is tolerant against compositional deviation, otherwise amorphous La would co-exist in the LNO thin films. Another possibility to explain the change in lattice parameter with substrate temperature is that it is caused by the change in oxygen non-stoichiometry w23x. Fig. 3 shows the change in resistivity of LNO thin films deposited on SiO2 ySi with substrate temperature, with data reported in w19x also plotted. Comparing our data with the reported data, the change in resistivity with the substrate temperature agreed well, especially between 100 and 300 8C. These facts indicate that LNO thin films prepared in this work on SiO2 ySi substrates are very comparable with those reported by Yang et al. from the viewpoint of the crystal structure and resistivity. Fig. 3 also shows that the resistivity of partially decomposed LNO thin films was considerably higher. 3.2. Preparation and properties of epitaxial LNO thin films Fig. 4 shows u–2u XRD patterns for a series of LNO thin films deposited on CeO2 yYSZySi(100) substrates at different temperature. Only (100) and (200) peaks of LNO were observed for thin films deposited at temperatures between 100 and 300 8C. As well as the LNO films deposited on SiO2 ySi substrate, the film deposited at 100 8C was not fully crystallized. Above 400 8C, the XRD peak intensity of LNO decreased and the presence of NiO was also observed. The same tendency was also observed for LNO films deposited on MgO(100) substrates. In the case of LNO thin films deposited on STO(100) substrates, it was difficult to determine the range of substrate temperatures for crystallization from the u–2u XRD measurements, because the 2u values of LNO are very close to those of STO. To examine the in-plane orientation of LNO thin films deposited on CeO2 yYSZySi(100) and MgO(100), X-ray pole figure measurements were carried out. For the measurement, the 2u value was fixed to approximately 40.48 wcorresponding to the 2u value of LaNiO3 (111)x and fscanning was repeated with changing w value between 0 and 908. Fig. 5a,b,c are (111) X-ray pole figures of LNO thin films deposited on CeO2 yYSZySi(100) at
250, 300 and 400 8C, respectively. These figures indicate that the lattice of LNO deposited at 250 8C is not aligned in the plane. With increasing the substrate temperature to 300 8C, epitaxial growth of LNO was achieved with a 908 rotating relationship on the CeO2 layer (LNOw100x yyCeO2 w110x yyYSZw110x yySiw110x). The film deposited at 400 8C on CeO2 yYSZySi(100) also showed poles at every 908 in f; however, this film was not epitaxially grown because the presence of NiO was detected, as shown in Fig. 4. Fig. 5d,e,f are (111) X-ray pole figures of LNO thin films deposited on MgO(100) at 250, 300 and 400 8C, respectively. Comparing these figures with Fig. 5a,b,c, it is apparent that epitaxial LNO thin films were also prepared on MgO(100) substrate at 300 8C; however, two kinds of in-plane orientations (cube-on-cube and 908 rotating relationship) were mixed together. Such two kinds of orientations are sometimes observed on MgO(100) w24,25x. For LNO thin films deposited on STO(100), the crystal structure was evaluated by reflection highenergy electron diffraction (RHEED) observation. The LNOySTO(100) thin films deposited by RF magnetron sputtering were taken out of the sputtering chamber and transferred to another vacuum chamber equipped with RHEED. Fig. 6a–d shows RHEED patterns of LNO thin films deposited on STO(100) at 20, 200, 300 and 400 8C, respectively. The film deposited at 20 8C showed no diffraction pattern, indicating that the LNO film was amorphous. With increasing the substrate temperature to 200 and 300 8C, streaky patterns with a regularly arranged spotty pattern were superimposed on the streaky pattern. This suggests that the surface of the LNOySTO(100) thin film was smooth but there was a trace, and a part of electron beam was diffracted through the mounds. As shown in these figures, the RHEED pattern observed at 300 8C (Fig. 6c) was much clearer than that observed at 200 8C (Fig. 6b). An increase in substrate temperature to 400 8C showed the RHEED pattern almost disappearing, suggesting decomposition of LNO, as well as the LNO deposited on other substrates mentioned above. These results indicate that epitaxial LNO thin films can be obtained at 300 8C, irrespective of the type of substrate wCeO2 yYSZy Si(100), MgO(100) and STO(100)x. So far, epitaxial LNO films have only been prepared by PLD; however, the minimum substrate temperature for epitaxial growth was above 500 8C. The result of this work indicates that the temperature for epitaxial growth is markedly lowered by the use of RF magnetron sputtering. Fig. 7 shows the changes in composition and lattice parameter of LNO thin films with substrate temperature deposited on SiO2 ySi, CeO2 yYSZySi(100), MgO(100) and STO(100) substrates. As was the case with Fig. 2, the Niy(NiqLa) ratio decreased with substrate temperature up to 200–300 8C and increased above this temperature. This figure indicates that the dependence of the com-
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
119
Fig. 8. (a–d) SEM photographs of LNO thin film deposited at 300 8C on SiO2ySi(100), CeO2 yYSZySi(100), MgO(100) and SrTiO3(100), respectively.
position and lattice parameter on the substrate temperature was almost identical, irrespective of the type of substrate and the in-plane alignment. Fig. 8a–d shows SEM photographs of LNO thin films deposited at 300 8C on SiO2 ySi, CeO2 yYSZySi(100), MgO(100) and STO(100), respectively. These photographs illustrate that LNO films were almost flat and smooth, indicating that these LNO films are good for bottom electrodes for ferroelectric materials. Such a flat and smooth surface was brought about the fact that the substrate temperature was as low as 300 8C. The change in resistivity of LNO thin films deposited on several substrates with substrate temperature is shown in Fig. 9. These data illustrate that LNO films with low resistivity can be obtained at a substrate temperature between 200 and 300 8C, irrespective of the type of substrate. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with the resistivity in the literature cited in Table 1. The fact that the resistivity of epitaxial LNO films deposited on CeO2 yYSZySi(100), MgO(100) and STO(100) is very close to that of polycrystalline LNO deposited on SiO2 ySi suggests that
Fig. 9. Change in resistivity of LNO thin film with substrate temperature deposited on SiO2ySi(100), CeO2yYSZySi(100), MgO(100) and SrTiO3(100) substrates.
120
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
the LNO film was isotropic. In this work, epitaxial LNO films were prepared as low as 300 8C on a CeO2 yYSZy Si(100) substrate as mentioned above. The preparation of epitaxial LNO thin films on a CeO2 yYSZySi(100) substrate was also reported by PLD w18x. The resistivity reported for epitaxial LNO films by PLD was also plotted in Fig. 9 for comparison. It is apparent that the resistivity of reported LNO films on CeO2 yYSZy Si(100) is approximately one order higher than that of our LNO films. 4. Conclusions The preparation of epitaxial LaNiO3 (LNO) films is one of the key technologies for FeRAM applications. In the literature, epitaxial LNO films were prepared by pulsed laser deposition (PLD); however, the crystallization temperature to realize epitaxial LNO was as high as 600 8C. In this work, the temperature to realize epitaxial LNO films was lowered to 300 8C using RF magnetron sputtering on CeO2 yYSZySi(100), MgO(100) and SrTiO3 (STO)(100) substrates. Epitaxial LNO films deposited on CeO2 yYSZySi(100) and STO(100) had a single orientation of LNOw100x yy CeO2w110x yyYSZw110x yySiw110x) and LNOw100x yy STOw100x, respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNOw100x yyMgOw100x and LNOw100x yy MgOw110x. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with that in the literature. The minimum resistivity of epitaxial LNO films deposited on CeO2 yYSZySi(100), MgO(100) and STO(100) is very close to that of polycrystalline LNO deposited on SiO2 ySi, suggesting the LNO film is isotropic. The surface of LNO films was smooth and flat. Acknowledgments This work was supported by the special co-ordination fund ‘Ceramics Integration’ from the Ministry of Education.
References w1x M.S. Chen, T.B. Wu, J.M. Wu, Appl. Phys. Lett. 68 (1996) 1430. w2x B.L. Chamberl, Solid-State Commun. 5 (1967) 663. w3x J.M. Longo, P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 39 (1968) 1327. w4x J.B. Goodenough, P.M. Raccah, J. Appl. Phys. 36 (1965) 1031. w5x C. Guerrero, F. Sanchez, C. Ferrater, J. Roldan, M.V. GarciaCuenca, M. Varela, Appl. Surf. Sci. 168 (2000) 219. w6x J.S. Cross, M. Fujiki, M. Tsukada, K. Matsuura, S. Otani, M. Tomotani, Y. Kataoka, Y. Kotaka, Y. Goto, Integrated Ferroelectr. 25 (1999) 265. w7x K. Hwang, Y. Lim, B. Kim, Mater. Res. Bull. 34 (1999) 2069. w8x A.D. Li, C.Z. Ge, D. Wu, P. Lu, Y.Q. Zuo, S.Z. Yang, N.B. Ming, Thin Solid Films 298 (1997) 165. w9x A. Li, C. Ge, P. Lu, N. Ming, Appl. Phys. Lett. 68 (1996) 1347. w10x K.S. Hwang, Y.M. Lim, B.H. Kim, J. Electroceram. 33 (1999) 269. w11x Y. Ling, W. Ren, X.Q. Wu, L.Y. Zhang, X. Yao, Thin Solid Films 311 (1997) 128. w12x Y. Liu, N. Xu, X.G. Zheng, T. Watanabe, O. Agyeman, M. Akiyama, J. Mater. Sci. 35 (2000) 937. w13x 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. w14x X. Guo, C. Li, Y. Zhou, Z. Chen, J. Vac. Sci. Technol. A 17 (1999) 917. w15x F. Sanchez, C. Ferrater, C. Guerrero, M.V. Garcia-Cuenca, M. Varela, Appl. Phys. A71 (2000) 59. w16x A.R. James, A. Pignolet, S. Senz, N.D. Zakharov, D. Hesse, Solid-State Commun. 114 (2000) 249. w17x D. Hesse, N.D. Zakharov, A. Pignolet, A.R. James, S. Senz, Cryst. Res. Technol. 35 (2000) 641. w18x F. Sanchez, C. Ferrater, X. Alcobe, J. Bassas, M.V. GarciaCuenca, M. Varela, Thin Solid Films 384 (2001) 200. w19x C.C. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. w20x T.B. Wu, J.M. Wu, C.M. Wu, M.J. Shyu, M.S. Chen, J.S. Dorng, C.C. Yang, Mater. Res. Soc. Symp. Proc. 433 (1996) 169. w21x N. Wakiya, T. Yamada, K. Shinozaki, N. Mizutani, Thin Solid Films 371 (2000) 211. w22x T. Yamada, N. Wakiya, K. Shinozaki, N. Mizutani, Jpn. J. Appl. Phys. 40 (2001) 281. w23x S. Rakshit, P.S. Gopalakrishnan, J. Solid-State Chem. 110 (1994) 28. w24x A. Masuda, K. Nashimoto, Jpn. J. Appl. Phys. 33 (1994) L793. w25x J. Senzaki, K. Kurihara, N. Nomura, O. Mitsunaga, Y. Iwasaki, T. Ueno, Jpn. J. Appl. Phys. 37 (1998) 5150.
Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering Naoki Wakiya, Takaaki Azuma, Kazuo Shinozaki, Nobuyasu Mizutani* Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 25 September 2001; received in revised form 2 February 2002; accepted 25 February 2002
Abstract Epitaxial growth of LaNiO3 (LNO) thin films was successful on CeO2 yYSZySi(100), MgO(100) and SrTiO3 (STO)(100) substrates by RF magnetron sputtering at 300 8C, although pulsed laser deposition requires 600 8C to prepare epitaxial LNO films according to the literature. Epitaxial LNO films deposited on CeO2 yYSZySi(100) and STO(100) had single orientation of LNOw100x yyCeO2w110x yyYSZw110x yySiw110x) and LNOw100x yySTOw100x, respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNOw100x yyMgOw100x and LNOw100x yyMgOw110x. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with the resistivity in the literature. It was found that the temperature to achieve minimum resistivity was 200 8C, irrespective of the type of substrate. The surface of the LNO films was smooth and flat. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: RF sputtering; LaNiO3; Epitaxial; Resistivity
1. Introduction Recently, conductive oxides having perovskite-type structure, such as (La0.5Sr0.5)CoO3 (LSCO), SrRuO3 (SRO) and LaNiO3 (LNO), have been intensively studied for ferroelectric random access memory (FeRAM) electrodes because fatigue-free property of up to 1011 cycles can be realized using such oxide electrodes w1x. LSCO bulk shows low resistivity (10y6 V m) w2x, and therefore this material is very attractive for thin-film electrodes. However, from the viewpoint of preparation, it is not necessarily easy to control the composition, because LSCO consists of three cations. On the contrary, SRO and LNO consist of two cations, and therefore the difficulty in controlling the composition is small compared with LSCO. The resistivity of SRO and LNO bulk is 2.8=10y6 w3x and 10y5 w4x V m, respectively. Guerrero et al. w5x prepared epitaxial Pb(Zr,Ti)O3 (PZT) *Corresponding author. Tel.: q81-3-5734-2519; fax: q81-3-57343369. E-mail address: [email protected] (N. Mizutani).
thin films on SRO and LNO bottom electrodes and compared the ferroelectric properties. They reported that a remarkable improvement in the ferroelectric properties, i.e. remanent polarization increased from 7.5 to 37 mCy cm2, was observed when the bottom electrode material was changed from SRO to LNO. The reason for the improvement was attributed to a smoother ferroelectricy electrode interface and the formation of the tetragonal phase of PZT instead of the rhombohedral one. In addition, Cross et al. w6x fabricated PtySROy (Pb,La)(Zr,Ti)O3 yPt capacitors and evaluated the leakage current. They reported that the leakage current was greatly influenced by Sr interdiffusion and reaction with excess Pb to form a grain boundary conduction phase, SrPbO3. These results suggest that LNO is superior to SRO as the electrode for ferroelectric thin films. So far, several methods have been adopted to prepare LNO films, such as chemical solution method (CSD) w7–12x, pulsed laser deposition (PLD) w13–18x and RF magnetron sputtering w19,20x. The deposition or annealing temperature to obtain crystallized LNO films in the
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 2 3 8 - 9
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
115
Table 1 Comparison of minimum crystallization temperature of LNO in the literature Deposition method
Substrate
Minimum crystallization temperature (8C)
Reference
CSD (solvent: CSD (solvent: CSD (solvent: CSD (solvent: PLD PLD PLD RF magnetron
STO (100) SiO2 ySi, STO(100), fused quartz SiO2 ySi, SiO2 ySi, glass ceramics STO(100) STO(100), LAO(100) CeO2 yYSZySi(100) SiO2 ySi, PtyTiySiO2 ySi, glass
600 530 650 600 550 500 550 150
w7 x w8 x w11x w12x w14x w15x w18x w19x
organic) organic) water) water)
sputtering
STO, SrTiO3; LAO, LaAlO3; YSZ, Y2O3 –ZrO2.
literature is compared in Table 1. This table clearly shows that LNO films prepared by CSD and PLD require more than 600 8C to crystallize. On the other hand, it is reported that LNO films prepared by RF magnetron sputtering begin to crystallize at 150 8C w19x. This means that the substrate temperature required to crystallize LNO film is markedly decreased when the RF magnetron sputtering is used. However, in w19x, a detailed examination of changes in the composition and crystal structure with the substrate temperature was not clarified. In addition, the LNO films showed only (100)
Fig. 1. XRD patterns of LNO thin films deposited on SiO2ySi(100) at various substrate temperatures.
preferred orientation and epitaxial growth was not reported. The purpose of this work is to examine the possibility of epitaxial growth of LNO films and clarify changes in the composition, crystal structure and resistivity of the films with the substrate temperature. 2. Experimental The preparation of LNO thin films was carried out using RF magnetron sputtering. The deposition chamber was first evacuated to below 2=10y4 Pa to remove residual gases that might have been present. A mixture of Ar and O2 gases (AryO2s8.0:2.0, total flow rate 10
Fig. 2. Changes in composition wNiy(NiqLa)x and lattice parameter of LNO thin films with substrate temperature deposited on SiO2ySi(100).
116
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
sccm) was then introduced to a fixed pressure of 4 Pa, and input RF power of 10 Wycm2 was applied between the targets and the substrates. Pre-sputtering was performed for 20 min, and the deposition of films was carried out at a prescribed substrate temperature between 20 and 600 8C. The substrate was directly heated by resistive heating using a Pt heater. The substrate tem-
perature was measured with a sheathed K-type (chromel–alumel) thermocouple positioned near the substrate. The substrate temperature was calibrated with another thin K-type thermocouple that was bonded by cement on the surface of the Si substrate. The distance between the targets and substrates was approximately 4.5 cm. In this work, a 2-inch-diameter target for LNO thin films was synthesized by the conventional solidstate reaction from starting materials of reagent grade La2O3 (after being calcined at 950 8C) and NiO powders. The sintering was carried out at 1050 8C for 5 h. The composition of the target was set to LayNis1:1.86, which was determined on the basis of results of a preliminary deposition using a LNO target of stoichiometric composition on MgO(100) substrates at 400 8C. The thickness of the LNO thin films was 200 nm. SiO2 ySi substrates were used for the comparison with the result of w19x. In this work, CeO2 yYSZySi(100), SrTiO3(100) and MgO(100) substrates were also selected as typical substrates on which epitaxial growth of LNO is expected to be realized. The lattice mismatch values of LNO with CeO2, STO and MgO are 0.89, y 1.13 and y8.34%, respectively. The CeO2 yYSZy Si(100) substrates were prepared by the deposition of 6-nm-thick YSZ and 10-nm-thick CeO2 layers on
Fig. 4. XRD patterns of LNO thin films deposited CeO2yYSZySi(100) at various substrate temperatures.
Fig. 5. (a–c) (111) X-ray pole figures of LNO thin films deposited on CeO2yYSZySi(100) at 250, 300 and 400 8C, respectively. (d–f) (111) X-ray pole figures of LNO thin films deposited on MgO(100) at 250, 300 and 400 8C, respectively.
Fig. 3. Change in resistivity of LNO thin films deposited on SiO2ySi(100) with substrate temperature.
on
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
117
Fig. 6. (a–d) RHEED patterns of LNO thin films deposited on SrTiO3(100) substrate at 20, 200, 300 and 400 8C, respectively.
Si(100) substrates using pulsed laser deposition (PLD) w21,22x. The YSZ and CeO2 layers were heteroepitaxially grown on Si(100) with a cube-on-cube relationship.
The constituent phase of the films was identified with a powder X-ray diffractometer (X’Pert-MPD (u–u), Philips, Netherlands). The texture measurement was carried out using an X-ray pole figure measurement apparatus wX’Pert MPD (open Eulerian cradle), Philips, Netherlandsx using CuKa radiation operating at 40 kV and 40 mA. The composition of the films was analyzed using X-ray fluorescence spectroscopy (XRF) wwavelength dispersive spectroscopy (WDS), (PW2404, Phillips, Netherlands)x. The microstructure of thin films was observed by FE-SEM (Hitachi, S-800). The resistivity of LNO films at room temperature was measured with a DC four-probe measurement unit (Keithley 236, 195A and 705). 3. Results and discussion 3.1. LNO thin films deposited on SiO2 ySi
Fig. 7. Changes in composition wNiy(NiqLa)x and lattice parameter of LNO thin films with substrate temperature deposited on SiO2ySi(100), CeO2yYSZySi(100), MgO(100) and SrTiO3(100).
Fig. 1 shows the change in XRD pattern of LNO thin films deposited on a SiO2 ySi(100) substrate with the deposition temperature. The film deposited at 20 8C was amorphous, and the film deposited at 100 8C showed quite weak XRD, indicating that this film was not fully crystallized. With increasing the substrate temperature to 200 and 300 8C, the XRD peaks increased. However, an increase in deposition temperature to 400 8C yielded decreasing XRD peaks. With increasing the substrate temperature to 500 and 600 8C, the XRD patterns showed that traces of La2NiO4 co-existed. This suggests that partial decomposition of LNO occurred above 400 8C, which tendency agrees well with the result reported by Yang et al. w19x (RF magnetron sputtering). Fig. 2
118
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
shows the changes in composition and lattice parameter of LNO thin films with substrate temperature deposited on SiO2 ySi substrates. This figure indicates that the Niy (NiqLa) ratio decreased with substrate temperature up to approximately 200 8C and increased above 250 8C. On the contrary, the lattice parameter of LNO thin films was constant between 200 and 400 8C. The lattice parameter of LNO thin films deposited between 200 and 400 8C was higher than that of LNO bulk. On the contrary, the lattice parameter of LNO thin films deposited at 500 and 600 8C (partially decomposed) was small and close to that of LNO bulk. These facts suggest that a LNO thin film having a large lattice parameter is tolerant against compositional deviation, otherwise amorphous La would co-exist in the LNO thin films. Another possibility to explain the change in lattice parameter with substrate temperature is that it is caused by the change in oxygen non-stoichiometry w23x. Fig. 3 shows the change in resistivity of LNO thin films deposited on SiO2 ySi with substrate temperature, with data reported in w19x also plotted. Comparing our data with the reported data, the change in resistivity with the substrate temperature agreed well, especially between 100 and 300 8C. These facts indicate that LNO thin films prepared in this work on SiO2 ySi substrates are very comparable with those reported by Yang et al. from the viewpoint of the crystal structure and resistivity. Fig. 3 also shows that the resistivity of partially decomposed LNO thin films was considerably higher. 3.2. Preparation and properties of epitaxial LNO thin films Fig. 4 shows u–2u XRD patterns for a series of LNO thin films deposited on CeO2 yYSZySi(100) substrates at different temperature. Only (100) and (200) peaks of LNO were observed for thin films deposited at temperatures between 100 and 300 8C. As well as the LNO films deposited on SiO2 ySi substrate, the film deposited at 100 8C was not fully crystallized. Above 400 8C, the XRD peak intensity of LNO decreased and the presence of NiO was also observed. The same tendency was also observed for LNO films deposited on MgO(100) substrates. In the case of LNO thin films deposited on STO(100) substrates, it was difficult to determine the range of substrate temperatures for crystallization from the u–2u XRD measurements, because the 2u values of LNO are very close to those of STO. To examine the in-plane orientation of LNO thin films deposited on CeO2 yYSZySi(100) and MgO(100), X-ray pole figure measurements were carried out. For the measurement, the 2u value was fixed to approximately 40.48 wcorresponding to the 2u value of LaNiO3 (111)x and fscanning was repeated with changing w value between 0 and 908. Fig. 5a,b,c are (111) X-ray pole figures of LNO thin films deposited on CeO2 yYSZySi(100) at
250, 300 and 400 8C, respectively. These figures indicate that the lattice of LNO deposited at 250 8C is not aligned in the plane. With increasing the substrate temperature to 300 8C, epitaxial growth of LNO was achieved with a 908 rotating relationship on the CeO2 layer (LNOw100x yyCeO2 w110x yyYSZw110x yySiw110x). The film deposited at 400 8C on CeO2 yYSZySi(100) also showed poles at every 908 in f; however, this film was not epitaxially grown because the presence of NiO was detected, as shown in Fig. 4. Fig. 5d,e,f are (111) X-ray pole figures of LNO thin films deposited on MgO(100) at 250, 300 and 400 8C, respectively. Comparing these figures with Fig. 5a,b,c, it is apparent that epitaxial LNO thin films were also prepared on MgO(100) substrate at 300 8C; however, two kinds of in-plane orientations (cube-on-cube and 908 rotating relationship) were mixed together. Such two kinds of orientations are sometimes observed on MgO(100) w24,25x. For LNO thin films deposited on STO(100), the crystal structure was evaluated by reflection highenergy electron diffraction (RHEED) observation. The LNOySTO(100) thin films deposited by RF magnetron sputtering were taken out of the sputtering chamber and transferred to another vacuum chamber equipped with RHEED. Fig. 6a–d shows RHEED patterns of LNO thin films deposited on STO(100) at 20, 200, 300 and 400 8C, respectively. The film deposited at 20 8C showed no diffraction pattern, indicating that the LNO film was amorphous. With increasing the substrate temperature to 200 and 300 8C, streaky patterns with a regularly arranged spotty pattern were superimposed on the streaky pattern. This suggests that the surface of the LNOySTO(100) thin film was smooth but there was a trace, and a part of electron beam was diffracted through the mounds. As shown in these figures, the RHEED pattern observed at 300 8C (Fig. 6c) was much clearer than that observed at 200 8C (Fig. 6b). An increase in substrate temperature to 400 8C showed the RHEED pattern almost disappearing, suggesting decomposition of LNO, as well as the LNO deposited on other substrates mentioned above. These results indicate that epitaxial LNO thin films can be obtained at 300 8C, irrespective of the type of substrate wCeO2 yYSZy Si(100), MgO(100) and STO(100)x. So far, epitaxial LNO films have only been prepared by PLD; however, the minimum substrate temperature for epitaxial growth was above 500 8C. The result of this work indicates that the temperature for epitaxial growth is markedly lowered by the use of RF magnetron sputtering. Fig. 7 shows the changes in composition and lattice parameter of LNO thin films with substrate temperature deposited on SiO2 ySi, CeO2 yYSZySi(100), MgO(100) and STO(100) substrates. As was the case with Fig. 2, the Niy(NiqLa) ratio decreased with substrate temperature up to 200–300 8C and increased above this temperature. This figure indicates that the dependence of the com-
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
119
Fig. 8. (a–d) SEM photographs of LNO thin film deposited at 300 8C on SiO2ySi(100), CeO2 yYSZySi(100), MgO(100) and SrTiO3(100), respectively.
position and lattice parameter on the substrate temperature was almost identical, irrespective of the type of substrate and the in-plane alignment. Fig. 8a–d shows SEM photographs of LNO thin films deposited at 300 8C on SiO2 ySi, CeO2 yYSZySi(100), MgO(100) and STO(100), respectively. These photographs illustrate that LNO films were almost flat and smooth, indicating that these LNO films are good for bottom electrodes for ferroelectric materials. Such a flat and smooth surface was brought about the fact that the substrate temperature was as low as 300 8C. The change in resistivity of LNO thin films deposited on several substrates with substrate temperature is shown in Fig. 9. These data illustrate that LNO films with low resistivity can be obtained at a substrate temperature between 200 and 300 8C, irrespective of the type of substrate. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with the resistivity in the literature cited in Table 1. The fact that the resistivity of epitaxial LNO films deposited on CeO2 yYSZySi(100), MgO(100) and STO(100) is very close to that of polycrystalline LNO deposited on SiO2 ySi suggests that
Fig. 9. Change in resistivity of LNO thin film with substrate temperature deposited on SiO2ySi(100), CeO2yYSZySi(100), MgO(100) and SrTiO3(100) substrates.
120
N. Wakiya et al. / Thin Solid Films 410 (2002) 114–120
the LNO film was isotropic. In this work, epitaxial LNO films were prepared as low as 300 8C on a CeO2 yYSZy Si(100) substrate as mentioned above. The preparation of epitaxial LNO thin films on a CeO2 yYSZySi(100) substrate was also reported by PLD w18x. The resistivity reported for epitaxial LNO films by PLD was also plotted in Fig. 9 for comparison. It is apparent that the resistivity of reported LNO films on CeO2 yYSZy Si(100) is approximately one order higher than that of our LNO films. 4. Conclusions The preparation of epitaxial LaNiO3 (LNO) films is one of the key technologies for FeRAM applications. In the literature, epitaxial LNO films were prepared by pulsed laser deposition (PLD); however, the crystallization temperature to realize epitaxial LNO was as high as 600 8C. In this work, the temperature to realize epitaxial LNO films was lowered to 300 8C using RF magnetron sputtering on CeO2 yYSZySi(100), MgO(100) and SrTiO3 (STO)(100) substrates. Epitaxial LNO films deposited on CeO2 yYSZySi(100) and STO(100) had a single orientation of LNOw100x yy CeO2w110x yyYSZw110x yySiw110x) and LNOw100x yy STOw100x, respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNOw100x yyMgOw100x and LNOw100x yy MgOw110x. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5=10y6 V m, which value almost agrees with that in the literature. The minimum resistivity of epitaxial LNO films deposited on CeO2 yYSZySi(100), MgO(100) and STO(100) is very close to that of polycrystalline LNO deposited on SiO2 ySi, suggesting the LNO film is isotropic. The surface of LNO films was smooth and flat. Acknowledgments This work was supported by the special co-ordination fund ‘Ceramics Integration’ from the Ministry of Education.
References w1x M.S. Chen, T.B. Wu, J.M. Wu, Appl. Phys. Lett. 68 (1996) 1430. w2x B.L. Chamberl, Solid-State Commun. 5 (1967) 663. w3x J.M. Longo, P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 39 (1968) 1327. w4x J.B. Goodenough, P.M. Raccah, J. Appl. Phys. 36 (1965) 1031. w5x C. Guerrero, F. Sanchez, C. Ferrater, J. Roldan, M.V. GarciaCuenca, M. Varela, Appl. Surf. Sci. 168 (2000) 219. w6x J.S. Cross, M. Fujiki, M. Tsukada, K. Matsuura, S. Otani, M. Tomotani, Y. Kataoka, Y. Kotaka, Y. Goto, Integrated Ferroelectr. 25 (1999) 265. w7x K. Hwang, Y. Lim, B. Kim, Mater. Res. Bull. 34 (1999) 2069. w8x A.D. Li, C.Z. Ge, D. Wu, P. Lu, Y.Q. Zuo, S.Z. Yang, N.B. Ming, Thin Solid Films 298 (1997) 165. w9x A. Li, C. Ge, P. Lu, N. Ming, Appl. Phys. Lett. 68 (1996) 1347. w10x K.S. Hwang, Y.M. Lim, B.H. Kim, J. Electroceram. 33 (1999) 269. w11x Y. Ling, W. Ren, X.Q. Wu, L.Y. Zhang, X. Yao, Thin Solid Films 311 (1997) 128. w12x Y. Liu, N. Xu, X.G. Zheng, T. Watanabe, O. Agyeman, M. Akiyama, J. Mater. Sci. 35 (2000) 937. w13x 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. w14x X. Guo, C. Li, Y. Zhou, Z. Chen, J. Vac. Sci. Technol. A 17 (1999) 917. w15x F. Sanchez, C. Ferrater, C. Guerrero, M.V. Garcia-Cuenca, M. Varela, Appl. Phys. A71 (2000) 59. w16x A.R. James, A. Pignolet, S. Senz, N.D. Zakharov, D. Hesse, Solid-State Commun. 114 (2000) 249. w17x D. Hesse, N.D. Zakharov, A. Pignolet, A.R. James, S. Senz, Cryst. Res. Technol. 35 (2000) 641. w18x F. Sanchez, C. Ferrater, X. Alcobe, J. Bassas, M.V. GarciaCuenca, M. Varela, Thin Solid Films 384 (2001) 200. w19x C.C. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. w20x T.B. Wu, J.M. Wu, C.M. Wu, M.J. Shyu, M.S. Chen, J.S. Dorng, C.C. Yang, Mater. Res. Soc. Symp. Proc. 433 (1996) 169. w21x N. Wakiya, T. Yamada, K. Shinozaki, N. Mizutani, Thin Solid Films 371 (2000) 211. w22x T. Yamada, N. Wakiya, K. Shinozaki, N. Mizutani, Jpn. J. Appl. Phys. 40 (2001) 281. w23x S. Rakshit, P.S. Gopalakrishnan, J. Solid-State Chem. 110 (1994) 28. w24x A. Masuda, K. Nashimoto, Jpn. J. Appl. Phys. 33 (1994) L793. w25x J. Senzaki, K. Kurihara, N. Nomura, O. Mitsunaga, Y. Iwasaki, T. Ueno, Jpn. J. Appl. Phys. 37 (1998) 5150.