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Microstructure and metal–insulator transition of NdNiO3 thin films on various substrates
Thin Solid Films 354 (1999) 50±54 www.elsevier.com/locate/tsf
Microstructure and metal±insulator transition of NdNiO3 thin ®lms on various substrates P. Laffez a,*, M. Zaghrioui a, I. Monot b, T. Brousse c, P. Lacorre d a
Laboratoire de Physique de l'Etat CondenseÂ, Faculte des Sciences UPRES-A CNRS 6087, Universite du Maine 72085, Le Mans Cedex 9, France b Laboratoire de Cristallographie et des MateÂriaux, UMR CNRS 6508 ISMRA, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France c Laboratoire de GeÂnie des MateÂriaux, ISITEM, BP 90604, 44306 Nantes Cedex France d Laboratoire des Fluorures, Faculte des Sciences UPRES-A CNRS 6010, Universite du Maine, 72085 Le Mans Cedex 9, France Received 11 February 1999; received in revised form 11 June 1999; accepted 20 July 1999
Abstract We succeeded in preparing single phase NdNiO3 thin ®lms with a thermally driven metal±insulator transition by RF sputtering and subsequent annealing under oxygen pressure. The ®lms were strongly oriented and their electrical properties have been studied between 80 and 300 K. The in¯uence of the substrate on the transport properties was studied. The ®lms exhibit a metal±insulator transition around 160 K when the bulk shows a transition around 200 K. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Thin ®lms; Perovskite; NdNiO3; Metal±insulator transition; RF sputtering
1. Introduction Rare earth nickelates RNiO3 which adopt a pseudo-cubic perovskite structure have peculiar magnetic and electrical properties. In particular, the appearance of a metal to insulator (MI) transition as the temperature decreases is of great interest for physics as well as for device applications. The lanthanum compound shows a metallic behavior along the whole temperature range, while the other rare earth nickelates have an MI transition at a temperature (TMI) which increases as the size of the rare earth R decreases [1]. An unusual antiferromagnetic structure in the insulating regime was also evidenced [2,3]. The origin of the MI transition has been discussed in numerous papers and it can be understood as the opening of a charge transfer gap coupled with Jahn± Teller polarons (see refs. [4,5] for details). The change of TMI with the rare earth is related to the variation of the Ni± O±Ni angle in the perovskite cell as the size of the rare earth is reduced. From a technological point of view, such materials are of great interest if one is able to control the transition temperature. This would open numerous applications such as temperature modulated switches, sensors, or thermochromic coatings if deposited as thin ®lms over various substrates [6]. The synthesis of bulk ceramic samples neces* Corresponding author. Tel.: 1 33-2-43-833268; fax: 1 33-2-43833518. E-mail address: [email protected] (P. Laffez)
sitates high temperature solid state reaction under oxygen pressure. Although the initial condition of 50 kbar of oxygen has been considerably lowered by using several chemical routes for the synthesis [7], an oxidizing atmosphere around 150 bar is still needed (when R ± La) to reach the right oxidation state of Ni. Films of LaNiO3 were previously grown by laser ablation, [8] and spray pyrolysis [9]. In a previous work, DeNatale and Kobrin [10] demonstrated the feasibility of electrical switching in NdNiO3 ®lms deposited on LaAlO3 single-crystal substrates. These authors reported a transition temperature at 150 K in ®lms of NdNiO3 although TMI in the corresponding bulk material is 200 K. They suggested that the epitaxial nature of the ®lm plays an important role in the variation of TMI, if one considers that the stress generated by the LaAlO3 substrate is similar to what is observed when the bulk material is submitted to high pressure. In fact it has been previously observed [11] that the transition temperature TMI decreases under pressure with a rate of dT MI =dP 24:2 K/kbar. This behavior suggests that the nature of the substrate should strongly in¯uence the temperature of the metal±insulator transition. In order to address this issue, we have grown NdNiO3 ®lms by magnetron sputtering on LaAlO3, SrTiO3, and NdGaO3 substrates and subsequent annealing. These various substrates have been chosen in agreement with their reduced cubic parameter on both sides of the NdNiO3 cell parameters. The crystallinity, the composition, the microstructure and trans-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0055 7-X
P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
51
pressure of 170 bar for two days. X-ray diffraction analysis was performed using a Philips diffractometer with Cu Ka radiation by step scanning over an angular range 108 , 2u , 1108, with an increment of 0.048. DC resistance was measured upon heating, using the four-probe method with silver paste electrodes. The composition of the ®lms was controlled using energy dispersive X-ray analysis in a Philips FEG XL30 scanning microscope at 20 kV. The thickness of the ®lms was measured in situ using a quartz microbalance and was ®xed at 400 nm. 3. Results 3.1. X-ray diffraction experiment Fig. 1. u ±2u X-ray diffraction patterns of ®lms grown on an LaAlO3 substrate by sputtering, then annealed at 8008C in 170 bar O2. Inset is the detail of the diffraction peak of the (006)/(330) plane, showing that the intensity of this latter is higher than the (020)/(200) one (see texts for details). Indexation is in the Pbnm space group.
port properties of the various ®lms have been characterized. We discuss here the in¯uence of the nature of the substrate on the physical properties. 2. Experimental Targets for thin ®lm sputtering were synthesized by classical solid state reaction. Stoichiometric mixtures of Nd2O3 and NiO were ground and calcined twice in air at 10008C for 12 h with an intermediate grinding. The resulting powders were then pressed into pellets and sintered in air at 12008C for 12 h. The target was analyzed by X-ray diffraction and showed a mixture of Nd2NiO4 and NiO. A mixture of argon and oxygen was used during the sputtering process. The variation of the deposition temperature leads to a monotonic decrease of the Nd/Ni ratio versus temperature. The deposition temperature was ®xed at 2508C, which was the optimum in our process for reaching stoichiometric ®lms with an Nd/Ni ratio equal to one. Before annealing the ®lms were amorphous. These ®lms were then kept at a temperature of 8008C under an oxygen
Fig. 1 shows a u ±2u X-ray diffraction pattern of ®lms grown on LaAlO3. No impurity is visible. The line of the perovskite axis subcell appears as the strongest line, overlapped with the line of the substrate. In Fig. 1 one can observe a diffraction peak at 33.318 (2u ), corresponding to Ê . This line can be indexed as (020), a distance d 2:687 A (200) or (112) in the orthorhombic cell, ((110) in the cubic perovsite) and indicates that unoriented domains are present in the ®lms grown by sputtering. This diffraction peak corresponds to the strongest line usually found in bulk samples. This result thus indicates that a part of the sample is polycrystalline. However, we think that the ratio of polycrystalline ®lm is small compared to the oriented one. If we Ê ) line with compare the intensity of the (020) (d 2:687 A Ê the (006) one (d 1:2691 A), (inset of Fig. 1) we ®nd a ratio I020 =I006 0:19. This ratio is usually I020 =I006 150 in a bulk sample. This shows the relatively low amount of unoriented phase in this sample. The results of the X-ray measurement are listed in Table 1. The ®lm grown on SrTiO3 exhibits a similar orientation as shown in Fig. 2a. The strongest line of the ®lm still corresponds to a preferential orientation along the perovskite Ê also appears. subcell, although the line at d 2:687 A The quality of the orientation was checked by performing a rocking curve (omega scan) on the (004) (220) peak at 47.98. The result is displayed in Fig. 2b (inset of Fig. 2). We found for this diagram a full width at half maximum FWHM
Table 1 Cell parameters and TMI after annealing for samples deposited by sputtering at 2508C and annealed at 8008C under 170 bar O2 Type of substrate
Substrate Ê) parameter (A
Pseudo-cubic parameter of the ®lm after annealing
Ratio I020/I006
Temperature of the metal± insulator transition (K)
LaAlO3 NdGaO3 SrTiO3 Bulk: average reduced cell parameter a
3.79 3.86 3.90 a 3:813, b 3:8084, c 3:8081
3.807 3.793 3.793
0.19 1.64 0.49 150
167 162 126 200
a
Ê , b 5:3859 A Ê , c 7:6162 A Ê , using the space group Pbnm. Orthorhombic cell is a 5:3934 A
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P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
Finally the X-ray diffraction pattern for the ®lm deposited on NdGaO3 is displayed in Fig. 4. Although the diffraction peak corresponding to the (002) (200) or (112) planes is still high compared to that of a bulk sample, the intensity of the Ê line at 33.318 (2u ), corresponding to a distance d 2:687 A is also high, suggesting that in this sample the part of the polycrystalline phase seems to be more important than in the previous ones. Moreover we found an unidenti®ed phase labelled with an asterisk in Fig. 4. 3.2. Microstructure investigation
of 0.348. This value is to be compared with FWHM 0:2388 for the substrate in the same crystalline orientation. These two values are of the same magnitude and suggest a strong orientation of the ®lm along the perovskite subcell. However, as observed in the sample grown on LaAlO3, a part of the sample is polycrystalline. This was con®rmed by recording an X-ray diffraction pattern, using a ®xed omega angle (88) (Fig. 3). This con®guration avoids the contribution of the substrate and of the epitaxial part of the ®lm. Fig. 3 shows an X-ray pattern where all the lines and the intensities correspond to that of a powder pattern. If the sample was fully epitaxied, we should not have observed any diffraction peaks using this con®guration. A quantitative texture analysis is now in progress in order to estimate the ratio of polycrystalline part versus the oriented one in these samples.
Microstructures of the sputtered samples after annealing are displayed in Fig. 5. To observe them, the samples were simply inserted in the microscope without any metallization; the conductivity of the samples was high enough to avoid charge accumulation. Observation of the ®lms after oxygen pressure annealing clearly indicates that the grain growth is highly sensitive to the substrate. Fig. 5a shows the surface of a sample grown on LaAlO3. The grain boundaries are not well de®ned, suggesting the growth of large single-crystal domains in agreement with the X-ray diffraction experiments. This low contrast of the grain boundary makes the average grain size hard to estimate. For this sample we observed extreme coalescence of porosity into macrovoids even after the high temperature annealing under oxygen pressure. This remaining porosity explains the absence of shrinkage that is sometimes observed after the annealing process. The sample deposited on SrTiO3 is displayed in Fig. 5b. This shows a microstructure which is quite different from the previous one. In that case, one can observe a very homogeneous surface with grain size up to 0.5 mm (this kind of highly dense microstructure allows grain boundaries to be evidenced: the grain size was estimated around 0.5 mm). Little porosity remains; this appears in some parts, only randomly dispersed. We noticed from time to time the
Fig. 3. 2u X-ray scan for omega ®xed at 88. This con®guration avoids the contribution of the substrate and so shows the polycrystalline part of the samples. All the peaks are indexed in the orthorhombic cell of NdNiO3.
Fig. 4. u ±2u X-ray diffraction patterns of a ®lm grown on NdGaO3 substrate by sputtering, then annealing at 8008C in 170 bar O2. The indexation refers to the orthorhombic NdNiO3. Peaks corresponding to unreacted products are labeled with *.
Fig. 2. u ±2u X-ray diffraction patterns of ®lms grown on a SrTiO3 substrate by sputtering, then annealing at 8008C in 170 bar O2, showing the strong orientation of the ®lm along the perovskite subcell. Inset is the detail of the rocking curve measured on the 004/220 re¯ection showing a full width at half maximum of 0.348.
P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
Fig. 5. Scanning electron microscopy image of annealed ®lms deposited by sputtering; the deposition temperature was 2508C, and the annealing temperature was 8008C, under 170 bar O2. The substrates are LaAlO3 (a), and SrTiO3 (b).
presence of small, needle-like particles. However, neither X-ray diffraction nor EDX analysis has revealed traces of secondary phase; thus these inclusions sometimes observed on the surface of samples grown on SrTiO3 do not appear to correspond to any second phase. Finally, the sample grown on NdGaO3 shows a microstructure with aspects similar to the two previous ones. Fig. 6a shows a microstructure similar to the one observed on the LaAlO3 sample, with even less porosity. The grain size is estimated at about 100 nm. On the other hand, on the same sample, a microstructure very similar to the one on the SrTiO3 sample was found. (Fig. 6b). The reason why these two kinds of microstructure was evidenced in the same sample is still unclear and requires further investigation.
53
Fig. 6. Scanning electron microscopy image of annealed ®lms deposited by sputtering; the deposition temperature was 2508C, and the annealing temperature was 8008C, under 170 bar O2. The substrate is NdGaO3.
resistance is around 3 V. In each sample the 2R=2T in the metallic state is the same, 8.02 mV/K. However, below 150 K, the resistance thermal evolution changes. The samples grown on LaAlO3 and NdGaO3 show the most abrupt transition. The metallic behavior of resistance disap-
3.3. Transport properties Resistivity versus temperature curves for the samples grown by sputtering are displayed in Fig. 7. A common feature of these samples is the metallic behavior observed between 300 and 160 K. The metallic state of the three samples has nearly the same thermal evolution of resistance. The samples deposited on SrTiO3 and NdGaO3 have a metallic state with a room temperature (RT) resistance around 8 V, while for the sample grown on LaAlO3, RT
Fig. 7. Resistance versus temperature for ®lms deposited by sputtering at 2508C and annealed at 8008C under 170 bar O2 for 2 days. The substrates are LaAlO3 (a), NdGaO3 (b) and SaTiO3 (c). Inset is the log(R) versus 1/T curve for a sample grown on LaAlO3, showing the activated regime below 130 K.
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P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
pears around 162±167 K. Between 150 and 100 K the resistance increases by three orders of magnitude, from a few ohms up to almost 10 4 V. (TMI was estimated at the change of sign of the derivative curve 2R=2T). In the sample deposited on LaAlO3, it is suggested that the metallic phase coexists with the insulating phase, at least down to 130 K. This phenomenon is usual in bulk rare earth nickelates RNiO3 and is related to the ®rst-order character of the transition. (See ref. [12] for details). This explains the two-step-like transition that is observed in the resistivity versus temperature curve of this sample. If the data are plotted in a log(R) versus 1/T scale as in the inset of Fig. 7, the low temperature part (T , 120 K) displays an activated behavior, which allows us to extract an activation energy of about 19 meV. This value is in agreement with what is usually observed in bulk material (25 meV) [12]. For the sample grown on NdGaO3 the metal±insulator transition occurs at 162 K. Below this temperature the resistance increases monotonically when temperature decreases. It is suggested that, in this sample, metallic and insulating phases coexist down to 50 K, with no activated behavior being observed. Finally, the resistance versus temperature of the SrTiO3 sample shows only a small kink of the resistance at 126 K, without drastic increase of the resistance as in the two other substrates.
process. Both the absolute value of TMI and the resistivity change at the transition are probably correlated with many different parameters such as the substrate nature, the ®lm texture and microstructure, the deposition and annealing conditions. However no clear relationship between these parameters has been found yet and further work is needed in order to elucidate it. We will focus mainly on the incidence of the annealing temperature and on texture analysis. De Natale and Kobrin have recently reported TMI values as high as 200 K for ®lms on LaAlO3 also annealed at 9508C, but for a lesser time at higher oxygen pressure [13]. The incidence of the annealing time and oxygen pressure on TMI will be studied for different types of substrates. In addition, we will carry out quantitative texture analysis in order to estimate the in¯uence of the orientation on the properties of the ®lms. This technique also allows a structural re®nement using the Rietveld method which should be useful in order to get a higher precision on the ®lm cell parameters.
Acknowledgements The authors are grateful to G. Ripault and G. Niessron from LPEC for their technical support in the sample preparations, and Dr Laligant for his assistance during transport measurements.
4. Concluding remarks In this work we succeeded in preparing single-phase NdNiO3 thin ®lms with the right stoichiometric composition. All the studied ®lms exhibit a metal to insulator transition with temperature. Moreover, a strong preferential orientation of the ®lms along the axis of the perovskite subcell was observed whatever the substrate, in spite of the disparity in the substrate cell parameters. However, the in¯uence of the nature of the substrate on the MI transition does not appear to be straightforward. In a previous work [10], DeNatale and Kobrin evidenced a MI transition at 150 K on a sample deposited on an LaAlO3 substrate and annealed at 9508C under oxygen pressure. Our results con®rm such a large decrease of TMI compared to the bulk, where T MI 200 K. De Natale and Kobrin attributed such a decrease to the stress induced by the substrate in the ®lm, which is similar to an over-pressure given the smaller cell parameter of LaAlO3. Our results do not support this assertion, since no detectable difference in the ®lm cell parameters is observed within the measurement accuracy (see Table 1). In all 3 samples, an important decrease of TMI is observed compared to the bulk transition temperature. The reactivity of the ®lms under high pressure is thought to be highly related to their thermal history in the sputtering
References [1] P. Lacorre, J.B. Torrance, J. Pannetier, A.I. Nazzal, P.W. Wang, T. Huang, J. Solid State Chem. 91 (1991) 225. [2] J.L. Garcia-Munoz, J. Rodrigues-Carvajal, P. Lacorre, Phys. Rev. B 50 (1994) 978. [3] J. Rodriguez Carvajal, S. Rosenkranz, M. Medarde, P. Lacorre, M.T. Fernadez-Diaz, F. Fauth, V. Trounov, Phys. Rev. B 57 (1998) 456. [4] J.B. Torrance, P. Lacorre, Physica C 182 (1991) 351. [5] M. Medarde, P. Lacorre, K. Conder, F. Fauth, A. Furrer, Phys. Rev. Lett. 80 (1998) 2397. [6] N. Massa, J.A. Alonso, M.J. Martinez-Lopes, I. Rasines, Phys. Rev. B 56 (1997) 986. [7] J.K. Vassiliou, M. Hornbostel, R. Ziebarth, F.J. Disalvo, J. Solid State Chem. 81 (1989) 208. [8] K.M. Satyalakshmi, R. Mallya, K.V. Ramanathan, X.D. Wu, B. Brainard, D.C. Gautier, N.T. Vasanthacharya, M.S. Hegde, Appl. Phys. Lett. 62 (1993) 1233. [9] A.R. Raju, N.A. Hemantkumar, C.N.R. Rao, Chem. Mater. 7 (1995) 225. [10] J.F. Denatale, P.H. Kobrin, J. Mater. Res. 10 (1995) 2992. [11] X. Obradors, L.M. Paulius, M.B. Maple, J.B. Torrance, A.I. Nazzal, J. Fontcuberta, X. Granados, Phys. Rev. B 47 (1993) 12353. [12] X. Granados, J. Fontcuberta, X. Obradors, Ll. ManÄosa, J.B. Torrance, Phys. Rev. B 48 (1993) 11666. [13] J.F. Denatal, P.H. Kobrin, Mater. Res. Soc. Symp. Proc. 479 (1997) 147.
Microstructure and metal±insulator transition of NdNiO3 thin ®lms on various substrates P. Laffez a,*, M. Zaghrioui a, I. Monot b, T. Brousse c, P. Lacorre d a
Laboratoire de Physique de l'Etat CondenseÂ, Faculte des Sciences UPRES-A CNRS 6087, Universite du Maine 72085, Le Mans Cedex 9, France b Laboratoire de Cristallographie et des MateÂriaux, UMR CNRS 6508 ISMRA, 6 boulevard du MareÂchal Juin, 14050 Caen Cedex, France c Laboratoire de GeÂnie des MateÂriaux, ISITEM, BP 90604, 44306 Nantes Cedex France d Laboratoire des Fluorures, Faculte des Sciences UPRES-A CNRS 6010, Universite du Maine, 72085 Le Mans Cedex 9, France Received 11 February 1999; received in revised form 11 June 1999; accepted 20 July 1999
Abstract We succeeded in preparing single phase NdNiO3 thin ®lms with a thermally driven metal±insulator transition by RF sputtering and subsequent annealing under oxygen pressure. The ®lms were strongly oriented and their electrical properties have been studied between 80 and 300 K. The in¯uence of the substrate on the transport properties was studied. The ®lms exhibit a metal±insulator transition around 160 K when the bulk shows a transition around 200 K. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Thin ®lms; Perovskite; NdNiO3; Metal±insulator transition; RF sputtering
1. Introduction Rare earth nickelates RNiO3 which adopt a pseudo-cubic perovskite structure have peculiar magnetic and electrical properties. In particular, the appearance of a metal to insulator (MI) transition as the temperature decreases is of great interest for physics as well as for device applications. The lanthanum compound shows a metallic behavior along the whole temperature range, while the other rare earth nickelates have an MI transition at a temperature (TMI) which increases as the size of the rare earth R decreases [1]. An unusual antiferromagnetic structure in the insulating regime was also evidenced [2,3]. The origin of the MI transition has been discussed in numerous papers and it can be understood as the opening of a charge transfer gap coupled with Jahn± Teller polarons (see refs. [4,5] for details). The change of TMI with the rare earth is related to the variation of the Ni± O±Ni angle in the perovskite cell as the size of the rare earth is reduced. From a technological point of view, such materials are of great interest if one is able to control the transition temperature. This would open numerous applications such as temperature modulated switches, sensors, or thermochromic coatings if deposited as thin ®lms over various substrates [6]. The synthesis of bulk ceramic samples neces* Corresponding author. Tel.: 1 33-2-43-833268; fax: 1 33-2-43833518. E-mail address: [email protected] (P. Laffez)
sitates high temperature solid state reaction under oxygen pressure. Although the initial condition of 50 kbar of oxygen has been considerably lowered by using several chemical routes for the synthesis [7], an oxidizing atmosphere around 150 bar is still needed (when R ± La) to reach the right oxidation state of Ni. Films of LaNiO3 were previously grown by laser ablation, [8] and spray pyrolysis [9]. In a previous work, DeNatale and Kobrin [10] demonstrated the feasibility of electrical switching in NdNiO3 ®lms deposited on LaAlO3 single-crystal substrates. These authors reported a transition temperature at 150 K in ®lms of NdNiO3 although TMI in the corresponding bulk material is 200 K. They suggested that the epitaxial nature of the ®lm plays an important role in the variation of TMI, if one considers that the stress generated by the LaAlO3 substrate is similar to what is observed when the bulk material is submitted to high pressure. In fact it has been previously observed [11] that the transition temperature TMI decreases under pressure with a rate of dT MI =dP 24:2 K/kbar. This behavior suggests that the nature of the substrate should strongly in¯uence the temperature of the metal±insulator transition. In order to address this issue, we have grown NdNiO3 ®lms by magnetron sputtering on LaAlO3, SrTiO3, and NdGaO3 substrates and subsequent annealing. These various substrates have been chosen in agreement with their reduced cubic parameter on both sides of the NdNiO3 cell parameters. The crystallinity, the composition, the microstructure and trans-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0055 7-X
P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
51
pressure of 170 bar for two days. X-ray diffraction analysis was performed using a Philips diffractometer with Cu Ka radiation by step scanning over an angular range 108 , 2u , 1108, with an increment of 0.048. DC resistance was measured upon heating, using the four-probe method with silver paste electrodes. The composition of the ®lms was controlled using energy dispersive X-ray analysis in a Philips FEG XL30 scanning microscope at 20 kV. The thickness of the ®lms was measured in situ using a quartz microbalance and was ®xed at 400 nm. 3. Results 3.1. X-ray diffraction experiment Fig. 1. u ±2u X-ray diffraction patterns of ®lms grown on an LaAlO3 substrate by sputtering, then annealed at 8008C in 170 bar O2. Inset is the detail of the diffraction peak of the (006)/(330) plane, showing that the intensity of this latter is higher than the (020)/(200) one (see texts for details). Indexation is in the Pbnm space group.
port properties of the various ®lms have been characterized. We discuss here the in¯uence of the nature of the substrate on the physical properties. 2. Experimental Targets for thin ®lm sputtering were synthesized by classical solid state reaction. Stoichiometric mixtures of Nd2O3 and NiO were ground and calcined twice in air at 10008C for 12 h with an intermediate grinding. The resulting powders were then pressed into pellets and sintered in air at 12008C for 12 h. The target was analyzed by X-ray diffraction and showed a mixture of Nd2NiO4 and NiO. A mixture of argon and oxygen was used during the sputtering process. The variation of the deposition temperature leads to a monotonic decrease of the Nd/Ni ratio versus temperature. The deposition temperature was ®xed at 2508C, which was the optimum in our process for reaching stoichiometric ®lms with an Nd/Ni ratio equal to one. Before annealing the ®lms were amorphous. These ®lms were then kept at a temperature of 8008C under an oxygen
Fig. 1 shows a u ±2u X-ray diffraction pattern of ®lms grown on LaAlO3. No impurity is visible. The line of the perovskite axis subcell appears as the strongest line, overlapped with the line of the substrate. In Fig. 1 one can observe a diffraction peak at 33.318 (2u ), corresponding to Ê . This line can be indexed as (020), a distance d 2:687 A (200) or (112) in the orthorhombic cell, ((110) in the cubic perovsite) and indicates that unoriented domains are present in the ®lms grown by sputtering. This diffraction peak corresponds to the strongest line usually found in bulk samples. This result thus indicates that a part of the sample is polycrystalline. However, we think that the ratio of polycrystalline ®lm is small compared to the oriented one. If we Ê ) line with compare the intensity of the (020) (d 2:687 A Ê the (006) one (d 1:2691 A), (inset of Fig. 1) we ®nd a ratio I020 =I006 0:19. This ratio is usually I020 =I006 150 in a bulk sample. This shows the relatively low amount of unoriented phase in this sample. The results of the X-ray measurement are listed in Table 1. The ®lm grown on SrTiO3 exhibits a similar orientation as shown in Fig. 2a. The strongest line of the ®lm still corresponds to a preferential orientation along the perovskite Ê also appears. subcell, although the line at d 2:687 A The quality of the orientation was checked by performing a rocking curve (omega scan) on the (004) (220) peak at 47.98. The result is displayed in Fig. 2b (inset of Fig. 2). We found for this diagram a full width at half maximum FWHM
Table 1 Cell parameters and TMI after annealing for samples deposited by sputtering at 2508C and annealed at 8008C under 170 bar O2 Type of substrate
Substrate Ê) parameter (A
Pseudo-cubic parameter of the ®lm after annealing
Ratio I020/I006
Temperature of the metal± insulator transition (K)
LaAlO3 NdGaO3 SrTiO3 Bulk: average reduced cell parameter a
3.79 3.86 3.90 a 3:813, b 3:8084, c 3:8081
3.807 3.793 3.793
0.19 1.64 0.49 150
167 162 126 200
a
Ê , b 5:3859 A Ê , c 7:6162 A Ê , using the space group Pbnm. Orthorhombic cell is a 5:3934 A
52
P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
Finally the X-ray diffraction pattern for the ®lm deposited on NdGaO3 is displayed in Fig. 4. Although the diffraction peak corresponding to the (002) (200) or (112) planes is still high compared to that of a bulk sample, the intensity of the Ê line at 33.318 (2u ), corresponding to a distance d 2:687 A is also high, suggesting that in this sample the part of the polycrystalline phase seems to be more important than in the previous ones. Moreover we found an unidenti®ed phase labelled with an asterisk in Fig. 4. 3.2. Microstructure investigation
of 0.348. This value is to be compared with FWHM 0:2388 for the substrate in the same crystalline orientation. These two values are of the same magnitude and suggest a strong orientation of the ®lm along the perovskite subcell. However, as observed in the sample grown on LaAlO3, a part of the sample is polycrystalline. This was con®rmed by recording an X-ray diffraction pattern, using a ®xed omega angle (88) (Fig. 3). This con®guration avoids the contribution of the substrate and of the epitaxial part of the ®lm. Fig. 3 shows an X-ray pattern where all the lines and the intensities correspond to that of a powder pattern. If the sample was fully epitaxied, we should not have observed any diffraction peaks using this con®guration. A quantitative texture analysis is now in progress in order to estimate the ratio of polycrystalline part versus the oriented one in these samples.
Microstructures of the sputtered samples after annealing are displayed in Fig. 5. To observe them, the samples were simply inserted in the microscope without any metallization; the conductivity of the samples was high enough to avoid charge accumulation. Observation of the ®lms after oxygen pressure annealing clearly indicates that the grain growth is highly sensitive to the substrate. Fig. 5a shows the surface of a sample grown on LaAlO3. The grain boundaries are not well de®ned, suggesting the growth of large single-crystal domains in agreement with the X-ray diffraction experiments. This low contrast of the grain boundary makes the average grain size hard to estimate. For this sample we observed extreme coalescence of porosity into macrovoids even after the high temperature annealing under oxygen pressure. This remaining porosity explains the absence of shrinkage that is sometimes observed after the annealing process. The sample deposited on SrTiO3 is displayed in Fig. 5b. This shows a microstructure which is quite different from the previous one. In that case, one can observe a very homogeneous surface with grain size up to 0.5 mm (this kind of highly dense microstructure allows grain boundaries to be evidenced: the grain size was estimated around 0.5 mm). Little porosity remains; this appears in some parts, only randomly dispersed. We noticed from time to time the
Fig. 3. 2u X-ray scan for omega ®xed at 88. This con®guration avoids the contribution of the substrate and so shows the polycrystalline part of the samples. All the peaks are indexed in the orthorhombic cell of NdNiO3.
Fig. 4. u ±2u X-ray diffraction patterns of a ®lm grown on NdGaO3 substrate by sputtering, then annealing at 8008C in 170 bar O2. The indexation refers to the orthorhombic NdNiO3. Peaks corresponding to unreacted products are labeled with *.
Fig. 2. u ±2u X-ray diffraction patterns of ®lms grown on a SrTiO3 substrate by sputtering, then annealing at 8008C in 170 bar O2, showing the strong orientation of the ®lm along the perovskite subcell. Inset is the detail of the rocking curve measured on the 004/220 re¯ection showing a full width at half maximum of 0.348.
P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
Fig. 5. Scanning electron microscopy image of annealed ®lms deposited by sputtering; the deposition temperature was 2508C, and the annealing temperature was 8008C, under 170 bar O2. The substrates are LaAlO3 (a), and SrTiO3 (b).
presence of small, needle-like particles. However, neither X-ray diffraction nor EDX analysis has revealed traces of secondary phase; thus these inclusions sometimes observed on the surface of samples grown on SrTiO3 do not appear to correspond to any second phase. Finally, the sample grown on NdGaO3 shows a microstructure with aspects similar to the two previous ones. Fig. 6a shows a microstructure similar to the one observed on the LaAlO3 sample, with even less porosity. The grain size is estimated at about 100 nm. On the other hand, on the same sample, a microstructure very similar to the one on the SrTiO3 sample was found. (Fig. 6b). The reason why these two kinds of microstructure was evidenced in the same sample is still unclear and requires further investigation.
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Fig. 6. Scanning electron microscopy image of annealed ®lms deposited by sputtering; the deposition temperature was 2508C, and the annealing temperature was 8008C, under 170 bar O2. The substrate is NdGaO3.
resistance is around 3 V. In each sample the 2R=2T in the metallic state is the same, 8.02 mV/K. However, below 150 K, the resistance thermal evolution changes. The samples grown on LaAlO3 and NdGaO3 show the most abrupt transition. The metallic behavior of resistance disap-
3.3. Transport properties Resistivity versus temperature curves for the samples grown by sputtering are displayed in Fig. 7. A common feature of these samples is the metallic behavior observed between 300 and 160 K. The metallic state of the three samples has nearly the same thermal evolution of resistance. The samples deposited on SrTiO3 and NdGaO3 have a metallic state with a room temperature (RT) resistance around 8 V, while for the sample grown on LaAlO3, RT
Fig. 7. Resistance versus temperature for ®lms deposited by sputtering at 2508C and annealed at 8008C under 170 bar O2 for 2 days. The substrates are LaAlO3 (a), NdGaO3 (b) and SaTiO3 (c). Inset is the log(R) versus 1/T curve for a sample grown on LaAlO3, showing the activated regime below 130 K.
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P. Laffez et al. / Thin Solid Films 354 (1999) 50±54
pears around 162±167 K. Between 150 and 100 K the resistance increases by three orders of magnitude, from a few ohms up to almost 10 4 V. (TMI was estimated at the change of sign of the derivative curve 2R=2T). In the sample deposited on LaAlO3, it is suggested that the metallic phase coexists with the insulating phase, at least down to 130 K. This phenomenon is usual in bulk rare earth nickelates RNiO3 and is related to the ®rst-order character of the transition. (See ref. [12] for details). This explains the two-step-like transition that is observed in the resistivity versus temperature curve of this sample. If the data are plotted in a log(R) versus 1/T scale as in the inset of Fig. 7, the low temperature part (T , 120 K) displays an activated behavior, which allows us to extract an activation energy of about 19 meV. This value is in agreement with what is usually observed in bulk material (25 meV) [12]. For the sample grown on NdGaO3 the metal±insulator transition occurs at 162 K. Below this temperature the resistance increases monotonically when temperature decreases. It is suggested that, in this sample, metallic and insulating phases coexist down to 50 K, with no activated behavior being observed. Finally, the resistance versus temperature of the SrTiO3 sample shows only a small kink of the resistance at 126 K, without drastic increase of the resistance as in the two other substrates.
process. Both the absolute value of TMI and the resistivity change at the transition are probably correlated with many different parameters such as the substrate nature, the ®lm texture and microstructure, the deposition and annealing conditions. However no clear relationship between these parameters has been found yet and further work is needed in order to elucidate it. We will focus mainly on the incidence of the annealing temperature and on texture analysis. De Natale and Kobrin have recently reported TMI values as high as 200 K for ®lms on LaAlO3 also annealed at 9508C, but for a lesser time at higher oxygen pressure [13]. The incidence of the annealing time and oxygen pressure on TMI will be studied for different types of substrates. In addition, we will carry out quantitative texture analysis in order to estimate the in¯uence of the orientation on the properties of the ®lms. This technique also allows a structural re®nement using the Rietveld method which should be useful in order to get a higher precision on the ®lm cell parameters.
Acknowledgements The authors are grateful to G. Ripault and G. Niessron from LPEC for their technical support in the sample preparations, and Dr Laligant for his assistance during transport measurements.
4. Concluding remarks In this work we succeeded in preparing single-phase NdNiO3 thin ®lms with the right stoichiometric composition. All the studied ®lms exhibit a metal to insulator transition with temperature. Moreover, a strong preferential orientation of the ®lms along the axis of the perovskite subcell was observed whatever the substrate, in spite of the disparity in the substrate cell parameters. However, the in¯uence of the nature of the substrate on the MI transition does not appear to be straightforward. In a previous work [10], DeNatale and Kobrin evidenced a MI transition at 150 K on a sample deposited on an LaAlO3 substrate and annealed at 9508C under oxygen pressure. Our results con®rm such a large decrease of TMI compared to the bulk, where T MI 200 K. De Natale and Kobrin attributed such a decrease to the stress induced by the substrate in the ®lm, which is similar to an over-pressure given the smaller cell parameter of LaAlO3. Our results do not support this assertion, since no detectable difference in the ®lm cell parameters is observed within the measurement accuracy (see Table 1). In all 3 samples, an important decrease of TMI is observed compared to the bulk transition temperature. The reactivity of the ®lms under high pressure is thought to be highly related to their thermal history in the sputtering
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