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In situ stress relaxation and diffraction studies across the metal–insulator transition in epitaxial and polycrystalline SmNiO3 thin films
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Scripta Materialia 66 (2012) 463–466 www.elsevier.com/locate/scriptamat
In situ stress relaxation and diffraction studies across the metal–insulator transition in epitaxial and polycrystalline SmNiO3 thin films B. Viswanath, G.H. Aydogdu, S.D. Ha and S. Ramanathan⇑ School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA Received 15 August 2011; accepted 16 December 2011 Available online 24 December 2011
We investigate structure and stress relaxation in situ across the metal–insulator transition in SmNiO3 thin films. An epitaxial thin film of SmNiO3 grown on LaAlO3 single crystal shows a metal–insulator transition at 155 °C based on electrical measurements. In situ electron diffraction experiments do not show any noticeable change with temperature. SmNiO3 thin films grown on silicon show a smoothly varying compressive stress across the transition boundary. The experimental observation of a metal–insulator transition without sharp stress changes is an encouraging preliminary result towards switching device applications. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Phase transformation; Stress relaxation; Metal–insulator transition; Thin films; Oxides
Issues related to structural distortion and stress relaxation in functional oxides are of great interest from the perspective of understanding phase transition mechanisms, and also have implications for electronic switch applications. Phase changes due to structural transitions often result in large and abrupt stress changes such those noted in vanadium oxide and chalcogenide thin films [1,2]. Repeated large stress fluctuations over multiple switchings may degrade the mechanical stability of the device through buckling or cracking of the thin film, leading to peel-off or void formation, respectively [3,4]. For example, a stress change of the order of 2 GPa is estimated across the amorphous–crystalline phase transition in chalcogenides (with a volume change of 5–7%) and has been identified as a potential issue for applications [5,6]. In this regard, it would be of interest to consider oxide materials that show metal–insulator transition without accompanying large structural or volume changes. Among the rare earth nickelates, SmNiO3 (SNO) is of particular interest as it exhibits a metal–insulator transition above room temperature (400 K) in bulk crystals [7]. Initial studies on this material revealed a metal–insulator transition while an orthorhombic perovskite structure was retained in both metallic and insulating phases [8,9]. In situ synchrotron X-ray and
⇑ Corresponding author; E-mail: [email protected]
neutron diffraction studies suggest no symmetry breaking across the metal–insulator transition in large rare earth members RNiO3 (R = Pr, Nd and Sm) [9]. However, this view has been challenged by a recent electron diffraction study carried out across the metal–insulator transition in NdNiO3 that revealed extra reflections at the forbidden (0 k l) positions for odd k, indicating breaking of the crystal symmetry in the orthorhombic cell parameter set [10]. Pronounced phonon softening observed in the temperature-dependent Raman scattering of NdNiO3, SmNiO3 and Sm0.6Nd0.4NiO3 suggest the possible existence of Pbnm–P21/n crystal symmetry breaking in large rare earth members RNiO3 (e.g. R = Nd and Sm) as a more subtle monoclinic distortion compared to conventional orthorhombic symmetry [11,12]. It is recognized that there is a volume change across the phase transition in rare earth nickelates due to the distortions in Ni–O bond length and Ni–O–Ni super-exchange bond angle. However, it has not yet been definitively determined whether or not SNO undergoes crystallographic symmetry breaking across the metal– insulator transition. It is important to note that most of the previous studies in this are have been carried out on polycrystalline SNO in bulk form. In addition to the atomic and bond distortions across the metal–insulator transition, the Ni+3 oxidation state itself undergoes instabilities in SNO thin films arising from growth conditions and lattice mismatch. For example, the in-plane
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.12.018
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B. Viswanath et al. / Scripta Materialia 66 (2012) 463–466
epitaxial compressive stress (e.g. in SNO/LaAlO3) stabilizes the Ni+3 state by reducing the octahedral cavity in SNO thin films, while tensile stress (e.g. in SNO/SrTiO3) enhances the formation of oxygen vacancies and directly affects the phase transition characteristics [13]. Metastable SNO thin film stabilized by epitaxial compressive strain is prone to dissociate into Sm2O3 and NiO when the compressive strain is relaxed [14]. Hence, addressing the issues related to structural distortions and associated stress relaxation across the metal–insulator transition is important and has ramifications for the metal–insulator transition characteristics of SNO thin films. In this paper, we report on in situ transmission electron microscopy (TEM) diffraction studies and associated stress relaxation in radiofrequency (RF) sputtered SNO thin films spanning the phase transition boundary. Variable-temperature electron diffraction studies were carried out on epitaxial SNO films grown on LaAlO3 (LAO) single crystals and polycrystalline SNO films on Si by in situ TEM. Wafer curvature measurement studies were also carried out across the metal–insulator transition temperature on SNO films grown on 4 inch Si(1 0 0) single-crystal wafers. From these studies, we are able to qualitatively explain the observed smooth stress change across the phase transition in terms of two competing stress changes arising from volume shrinkage and thermal mismatch. SNO thin films on LAO and Si substrates were deposited by RF magnetron sputtering at 650 °C as described earlier [15]. TEM studies were carried out using JEOL 2010 microscope operated at 200 kV. A Gatan heating holder was used for in situ electron diffraction studies from room temperature to 300 °C. Cross-sectional TEM specimens were prepared by mechanical grinding from the substrate side followed by Ar ion milling. Thin-film stress measurements were carried out by the in situ wafer curvature method using a TENCOR FLX-2320 instrument. Slow temperature ramping of 2 °C min1 was used for the heating and cooling cycles. Resistance–temperature plots were obtained from current–voltage (I–V) measurements using a Keithley 2635 Source Measure Unit integrated with an MDC probe station. Figure 1a shows a representative high-resolution (HR) TEM image of a SNO–LAO interface, taken from a SNO thin film 80 nm thick grown on LAO (1 0 0) single-crystal substrate. It should be noted that SNO film grows epitaxially on LAO, as confirmed by X-ray diffraction [8] and also electron diffraction as discussed later. Fast Fourier transform (FFT) patterns corresponding to the LAO and SNO obtained from the HRTEM image show LAO(1 0 0)//SNO(1 0 0) epitaxial orientation. Figure 1b shows a bright-field image of polycrystalline SNO film of 200 nm thickness grown on Si(1 0 0) substrate. Resistance–temperature plots along with variabletemperature electron diffraction patterns recorded from epitaxial SNO film on LAO are shown in Figure 2a. Selected-area electron diffraction (SAED) patterns were recorded from the SNO–LAO interface after tilting the specimen to the [0 0 1] zone axis with respect to the LAO substrate. Room-temperature electron diffraction patterns recorded from LAO and SNO show the expected reflections of pseudo-cubic LAO and SNO crystals with a common zone axis along the [0 0 1] direction. This confirms the epitaxial growth of SNO film on LAO with the
Figure 1. Cross-sectional TEM micrographs of SmNiO3 films. (a) High-resolution image of epitaxial SNO film on LAO(1 0 0) substrate. FFT patterns obtained from SNO and LAO regions are shown as inset. (b) Bright-field image of 200 nm polycrystalline SNO film on Si(1 0 0) substrate. Inset shows an HRTEM image of SNO film on Si.
Figure 2. Normalized resistance–temperature plots along with in situ electron diffraction patterns are shown across the metal–insulator transition in (a) epitaxial SNO film and (b) polycrystalline SNO film. The metal–insulator transition is obtained from the derivative of the resistance–temperature plot. In situ electron diffraction patterns spanning the transition temperature are shown as an inset.
orientation relationship SNO(1 0 0)//LAO(1 0 0) and SNO[0 0 1]//LAO[0 0 1] in agreement with FFT patterns from the cross-section TEM (Fig. 1a) as well as previous reports [8,13,15]. The resistance–temperature plot of epitaxial SNO film on LAO reveals a metal–insulator transition at 155 °C (obtained from the derivative of the resistance–temperature plot) shown in Figure 2a.
B. Viswanath et al. / Scripta Materialia 66 (2012) 463–466
The in situ electron diffraction patterns recorded at the film–substrate interface from room temperature to 250 °C at intervals of 25 °C show no detectable changes in angular orientations of spots, indicating the absence of structural transformation. It is important to note that the subtle lattice parameter changes in SNO upon heating are too small to be resolved from the primary electron diffraction spot patterns. Additional reflections or superlattice reflections are sensitive enough to capture structural distortions and such extra reflections were indeed observed in NdNiO3 below the metal insulator transition temperature, confirming the change in symmetry from orthorhombic (Pbnm) to monoclinic (P21/n) across the transition [10]. Here, we do not see such extra reflections in the investigated temperature range from 25 to 250 °C for the epitaxial SNO thin film. Similar results have been observed over several thermal cycles and also in various regions of the sample. It should be noted that the extra reflections were observed at the forbidden positions in the low-temperature (93 K) insulating state in the case of NdNiO3 (accompanying volume change 0.23%) across the transition [9]. In situ electron diffraction studies were also carried out on polycrystalline SNO grown on Si and the corresponding selected-area diffraction patterns, along with the resistance–temperature plot are shown in Figure 2b. It is evident from the room-temperature SAED pattern that the thin film contains additional phases such as nickel oxide along with the SNO as seen in typical SNO growth on non-lattice-matched substrates [8]. No significant changes are seen in the diffraction pattern in the investigated temperature range (i.e. room temperature to 250 °C). The phase transition occurs at 185 °C estimated from the derivative of the resistance–temperature plot in Figure 2b. The exact transition temperature of SNO is different for the two substrates due to strainand defect-related phenomena as discussed earlier [15]. Further, in situ wafer curvature measurement was carried out on SNO film grown on Si(1 0 0) to investigate the stress relaxation across the metal–insulator transition. Figure 3a and b show respectively the radius of curvature vs. temperature and the corresponding stress–temperature plots of SNO film on Si(1 0 0). First, there is no abrupt stress change observed during the repeated thermal cycles. Secondly, the stress–temperature plots are linear and the measured stress changes are compressive in nature. SNO undergoes shrinkage along the b and c axes of the unit cell and in cell volume (0.15%) across the metal–insulator transition during heating [9]. A recent neutron scattering study with 154 Sm indicates smooth changes in the lattice parameters across the transition [16]. Nevertheless the volume shrinkage should have resulted in tensile stress during the transition. However, the absence of any discontinuous, phase-transition-induced stress change across the metal–insulator transition is evident from the plots shown in Figure 3a and b. It is important to note that the measured stress is an overall indicator of the stress state, whereas the phase-transition-induced tensile stress is estimated only from the volume change across the transition boundary. At least the volume change and thermal mismatch components have to be considered to obtain an estimate of the overall stress change across the phase transition; this is described below.
465
Figure 3. Stress relaxation in a polycrystalline SmNiO3 film 200 nm thick grown on silicon substrate as a function of temperature. The radius of curvature and the corresponding stress–temperature plots across the metal–insulator transition temperature are shown in (a) and (b), respectively. Opposing stresses from volume shrinkage and thermal mismatch at the transition are shown schematically in (c) for reference. While the compressive stress arises due to thermal expansion coefficient mismatch, the tensile stress arises due to abrupt volume change induced by Ni–O bond distortion at the transition temperature.
The estimated stress evolution of SNO considering the volume changes that accompany a metal–insulator transition as well as the thermal mismatch components is shown in Figure 3c. The transition-induced biaxial stress in SNO film is estimated to be +171 MPa during heating (from the volume shrinkage) based on the following expression. 1
rf ¼ Ef ð1 tf Þ ðDV=3VÞ;
ð1Þ
where Ef, tf,V and DV are the Young’s modulus, the Poisson ratio, the unit cell volume and the change in
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unit cell volume, respectively, of the film. Wafer curvature measurement by laser scanning is sensitive enough to detect curvature changes of the order of 104 m1 considering practical factors such as mechanical vibration, laser beam profile, noise, etc. [17]. The curvatures measured in the temperature range 25–250 °C are of the order of 102 m1. The scatter in the curvature measurement at the metal–insulator transition temperature (T = 185 °C) obtained from the width of the multiple thermal cycles is around 5 104 m1 and the corresponding scatter in the measured stress is 24 MPa, which is significantly lower than the phase-transitioninduced stress. Note that the phase-transition-induced tensile stress arises due to abrupt volume shrinkage that takes place within the transition boundary (6 °C) at 185 °C. This volume change is associated with the Ni–O–Ni bond distortions. The temperature-dependent thermal stress evolution of SNO on Si was estimated using the coefficient of thermal expansion of insulating and metallic SNO against Si substrate using the following expressions: 1 ð2Þ rf ðinsulatingÞ ¼ Ef ð1 tf Þ as afðInsulatingÞ DT 1 ð3Þ rf ðmetallicÞ ¼ Ef ð1 tf Þ as afðMetallicÞ DT where as, af and DT are the coefficient of thermal expansion and the temperature difference, respectively. Subscripts s and f denote Si substrate and SNO film in this case. The elastic modulus is estimated from the available bulk modulus (K = 167 GPa) based on the Poisson ratio for SNO (m = 0.24) [18]. The measured linearly increasing compressive stress trend of SNO film is comparable with the estimated thermal stress of SNO film that arises due to the thermal expansion mismatch between SNO and Si substrate. The slopes of the stress–temperature plots obtained from the linear regression analysis of experimentally measured SNO film are 0.87 and 0.78 during heating and cooling cycles, respectively. The estimated thermal stress shows considerable difference in slope between the insulating (0.97) and metallic (2.38) phases of SNO on Si. The higher slope (2.38) is due to the metallic nature of the SNO film that undergoes larger thermal expansion against the Si substrate. More importantly, considerable discontinuity is seen in the estimated thermal stress–temperature plot at the metal– insulator transition temperature (T = 185 °C) as the thermal expansion coefficient drops abruptly across the metal–insulator transition leading to a high compressive stress of 227 MPa. As a consequence, the volume shrinkage (0.15%) at the transition window generates a tensile stress of +171 MPa due to the subtle structural distortions described earlier. Since the thermal mismatch stress and phase-transition-induced stress are opposing each other and comparable in magnitude, as seen from the approximate analysis above, the measured overall thin film stress does not show any abrupt stress change at the metal–insulator transition temperature.
In conclusion, in situ wafer curvature experiments do not show a phase-transition-induced sharp stress change across the metal–insulator transition in SNO film on Si. The linear, temperature-dependent compressive stress observed in polycrystalline SNO film across the metal– insulator transition is primarily due to the thermal expansion mismatch between Si and SNO. A simple estimate of the stresses revealed two competing mechanisms involving volume shrinkage across the transition and thermal mismatch that minimize the overall stress changes. The results are correlated with in situ diffraction studies on both epitaxial and polycrystalline films that show similar patterns across the transition temperature. Financial support from ARO MURI (W911NF-09-1-0398) and the Focus Center Research Program in the Materials Structures and Devices Focus Center is acknowledged. We are very grateful to Prof. Frans Spaepen for valuable technical discussions. [1] M.J. Kang, S.Y. Choi, D. Wamwangi, K. Wang, C. Steimer, M. Wuttig, J. Appl. Phys. 98 (2005) 014904. [2] B. Viswanath, C. Ko, S. Ramanathan, Scripta Mater. 64 (2011) 490. [3] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution, Cambridge University Press, Cambridge, 2003. [4] W.D. Nix, Metal. Mater. Trans. A 20 (1989) 2217. [5] T.P. Leervad Pedersen, J. Kalb, W.K. Njoroge, D. Wamwangi, M. Wuttig, F. Spaepen, Appl. Phys. Lett. 79 (2001) 3597. [6] M. Zhong, Z. Song, B. Liu, Y. Chen, Y. Gong, F. Rao, S. Feng, F. Zhang, Y. Xiang, Scripta Mater. 60 (2009) 957. ˜ a Luisa, J. Phys. Cond. Mater. 9 (1997) 1679. [7] M. MarA [8] S.D. Ha, G.H. Aydogdu, S. Ramanathan, Appl. Phys. Lett. 98 (2011) 012105. [9] J.L. Garcia-Munoz, J. Rodriguez-Carvajal, P. Lacorre, J.B. Torrance, Phys. Rev. B 46 (1992) 4414. [10] M. Zaghrioui, A. Bulou, P. Lacorre, P. Laffez, Phys. Rev. B 64 (2001) 081102. [11] C. Girardot, J. Kreisel, S. Pignard, N. Caillault, F. Weiss, Phys. Rev. B 78 (2008) 104101. [12] F.P. de la Cruz, C. Piamonteze, N.E. Massa, H. Salva, J.A. Alonso, M.J. Martinez-Lope, T. Maria, Phys. Rev. B 66 (2002) 153104. [13] F. Conchon, A. Boulle, R. Guinebretiere, C. Girardot, S. Pignard, J. Kreisel, F. Weiss, E. Dooryhee, J.L. Hodeau, Appl. Phys. Lett. 91 (2007) 192110. [14] F. Conchon, A. Boulle, C. Girardot, S. Pignard, R. Guinebretie`re, E. Dooryhe´e, J.L. Hodeau, F. Weiss, J. Kreisel, Mat. Sci. Eng. B 144 (2007) 32. [15] G.H. Aydogdu, S.D. Ha, B. Viswanath, S. Ramanathan, J. Appl. Phys. 109 (2011) 124110. [16] P.F. Henry, M.T. Weller, C.C. Wilson, Chem. Mater. 14 (2002) 4104. [17] Z.B. Zhao, J. Hershberger, S.M. Yalisove, J.C. Bilello, Thin Solid Films 415 (2002) 21. [18] M. Amboage, M. Hanfland, J.A. Alonso, M.J. MartınezLope, J. Phys.: Cond. Mater. 17 (2005) S783.
Scripta Materialia 66 (2012) 463–466 www.elsevier.com/locate/scriptamat
In situ stress relaxation and diffraction studies across the metal–insulator transition in epitaxial and polycrystalline SmNiO3 thin films B. Viswanath, G.H. Aydogdu, S.D. Ha and S. Ramanathan⇑ School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA Received 15 August 2011; accepted 16 December 2011 Available online 24 December 2011
We investigate structure and stress relaxation in situ across the metal–insulator transition in SmNiO3 thin films. An epitaxial thin film of SmNiO3 grown on LaAlO3 single crystal shows a metal–insulator transition at 155 °C based on electrical measurements. In situ electron diffraction experiments do not show any noticeable change with temperature. SmNiO3 thin films grown on silicon show a smoothly varying compressive stress across the transition boundary. The experimental observation of a metal–insulator transition without sharp stress changes is an encouraging preliminary result towards switching device applications. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Phase transformation; Stress relaxation; Metal–insulator transition; Thin films; Oxides
Issues related to structural distortion and stress relaxation in functional oxides are of great interest from the perspective of understanding phase transition mechanisms, and also have implications for electronic switch applications. Phase changes due to structural transitions often result in large and abrupt stress changes such those noted in vanadium oxide and chalcogenide thin films [1,2]. Repeated large stress fluctuations over multiple switchings may degrade the mechanical stability of the device through buckling or cracking of the thin film, leading to peel-off or void formation, respectively [3,4]. For example, a stress change of the order of 2 GPa is estimated across the amorphous–crystalline phase transition in chalcogenides (with a volume change of 5–7%) and has been identified as a potential issue for applications [5,6]. In this regard, it would be of interest to consider oxide materials that show metal–insulator transition without accompanying large structural or volume changes. Among the rare earth nickelates, SmNiO3 (SNO) is of particular interest as it exhibits a metal–insulator transition above room temperature (400 K) in bulk crystals [7]. Initial studies on this material revealed a metal–insulator transition while an orthorhombic perovskite structure was retained in both metallic and insulating phases [8,9]. In situ synchrotron X-ray and
⇑ Corresponding author; E-mail: [email protected]
neutron diffraction studies suggest no symmetry breaking across the metal–insulator transition in large rare earth members RNiO3 (R = Pr, Nd and Sm) [9]. However, this view has been challenged by a recent electron diffraction study carried out across the metal–insulator transition in NdNiO3 that revealed extra reflections at the forbidden (0 k l) positions for odd k, indicating breaking of the crystal symmetry in the orthorhombic cell parameter set [10]. Pronounced phonon softening observed in the temperature-dependent Raman scattering of NdNiO3, SmNiO3 and Sm0.6Nd0.4NiO3 suggest the possible existence of Pbnm–P21/n crystal symmetry breaking in large rare earth members RNiO3 (e.g. R = Nd and Sm) as a more subtle monoclinic distortion compared to conventional orthorhombic symmetry [11,12]. It is recognized that there is a volume change across the phase transition in rare earth nickelates due to the distortions in Ni–O bond length and Ni–O–Ni super-exchange bond angle. However, it has not yet been definitively determined whether or not SNO undergoes crystallographic symmetry breaking across the metal– insulator transition. It is important to note that most of the previous studies in this are have been carried out on polycrystalline SNO in bulk form. In addition to the atomic and bond distortions across the metal–insulator transition, the Ni+3 oxidation state itself undergoes instabilities in SNO thin films arising from growth conditions and lattice mismatch. For example, the in-plane
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.12.018
464
B. Viswanath et al. / Scripta Materialia 66 (2012) 463–466
epitaxial compressive stress (e.g. in SNO/LaAlO3) stabilizes the Ni+3 state by reducing the octahedral cavity in SNO thin films, while tensile stress (e.g. in SNO/SrTiO3) enhances the formation of oxygen vacancies and directly affects the phase transition characteristics [13]. Metastable SNO thin film stabilized by epitaxial compressive strain is prone to dissociate into Sm2O3 and NiO when the compressive strain is relaxed [14]. Hence, addressing the issues related to structural distortions and associated stress relaxation across the metal–insulator transition is important and has ramifications for the metal–insulator transition characteristics of SNO thin films. In this paper, we report on in situ transmission electron microscopy (TEM) diffraction studies and associated stress relaxation in radiofrequency (RF) sputtered SNO thin films spanning the phase transition boundary. Variable-temperature electron diffraction studies were carried out on epitaxial SNO films grown on LaAlO3 (LAO) single crystals and polycrystalline SNO films on Si by in situ TEM. Wafer curvature measurement studies were also carried out across the metal–insulator transition temperature on SNO films grown on 4 inch Si(1 0 0) single-crystal wafers. From these studies, we are able to qualitatively explain the observed smooth stress change across the phase transition in terms of two competing stress changes arising from volume shrinkage and thermal mismatch. SNO thin films on LAO and Si substrates were deposited by RF magnetron sputtering at 650 °C as described earlier [15]. TEM studies were carried out using JEOL 2010 microscope operated at 200 kV. A Gatan heating holder was used for in situ electron diffraction studies from room temperature to 300 °C. Cross-sectional TEM specimens were prepared by mechanical grinding from the substrate side followed by Ar ion milling. Thin-film stress measurements were carried out by the in situ wafer curvature method using a TENCOR FLX-2320 instrument. Slow temperature ramping of 2 °C min1 was used for the heating and cooling cycles. Resistance–temperature plots were obtained from current–voltage (I–V) measurements using a Keithley 2635 Source Measure Unit integrated with an MDC probe station. Figure 1a shows a representative high-resolution (HR) TEM image of a SNO–LAO interface, taken from a SNO thin film 80 nm thick grown on LAO (1 0 0) single-crystal substrate. It should be noted that SNO film grows epitaxially on LAO, as confirmed by X-ray diffraction [8] and also electron diffraction as discussed later. Fast Fourier transform (FFT) patterns corresponding to the LAO and SNO obtained from the HRTEM image show LAO(1 0 0)//SNO(1 0 0) epitaxial orientation. Figure 1b shows a bright-field image of polycrystalline SNO film of 200 nm thickness grown on Si(1 0 0) substrate. Resistance–temperature plots along with variabletemperature electron diffraction patterns recorded from epitaxial SNO film on LAO are shown in Figure 2a. Selected-area electron diffraction (SAED) patterns were recorded from the SNO–LAO interface after tilting the specimen to the [0 0 1] zone axis with respect to the LAO substrate. Room-temperature electron diffraction patterns recorded from LAO and SNO show the expected reflections of pseudo-cubic LAO and SNO crystals with a common zone axis along the [0 0 1] direction. This confirms the epitaxial growth of SNO film on LAO with the
Figure 1. Cross-sectional TEM micrographs of SmNiO3 films. (a) High-resolution image of epitaxial SNO film on LAO(1 0 0) substrate. FFT patterns obtained from SNO and LAO regions are shown as inset. (b) Bright-field image of 200 nm polycrystalline SNO film on Si(1 0 0) substrate. Inset shows an HRTEM image of SNO film on Si.
Figure 2. Normalized resistance–temperature plots along with in situ electron diffraction patterns are shown across the metal–insulator transition in (a) epitaxial SNO film and (b) polycrystalline SNO film. The metal–insulator transition is obtained from the derivative of the resistance–temperature plot. In situ electron diffraction patterns spanning the transition temperature are shown as an inset.
orientation relationship SNO(1 0 0)//LAO(1 0 0) and SNO[0 0 1]//LAO[0 0 1] in agreement with FFT patterns from the cross-section TEM (Fig. 1a) as well as previous reports [8,13,15]. The resistance–temperature plot of epitaxial SNO film on LAO reveals a metal–insulator transition at 155 °C (obtained from the derivative of the resistance–temperature plot) shown in Figure 2a.
B. Viswanath et al. / Scripta Materialia 66 (2012) 463–466
The in situ electron diffraction patterns recorded at the film–substrate interface from room temperature to 250 °C at intervals of 25 °C show no detectable changes in angular orientations of spots, indicating the absence of structural transformation. It is important to note that the subtle lattice parameter changes in SNO upon heating are too small to be resolved from the primary electron diffraction spot patterns. Additional reflections or superlattice reflections are sensitive enough to capture structural distortions and such extra reflections were indeed observed in NdNiO3 below the metal insulator transition temperature, confirming the change in symmetry from orthorhombic (Pbnm) to monoclinic (P21/n) across the transition [10]. Here, we do not see such extra reflections in the investigated temperature range from 25 to 250 °C for the epitaxial SNO thin film. Similar results have been observed over several thermal cycles and also in various regions of the sample. It should be noted that the extra reflections were observed at the forbidden positions in the low-temperature (93 K) insulating state in the case of NdNiO3 (accompanying volume change 0.23%) across the transition [9]. In situ electron diffraction studies were also carried out on polycrystalline SNO grown on Si and the corresponding selected-area diffraction patterns, along with the resistance–temperature plot are shown in Figure 2b. It is evident from the room-temperature SAED pattern that the thin film contains additional phases such as nickel oxide along with the SNO as seen in typical SNO growth on non-lattice-matched substrates [8]. No significant changes are seen in the diffraction pattern in the investigated temperature range (i.e. room temperature to 250 °C). The phase transition occurs at 185 °C estimated from the derivative of the resistance–temperature plot in Figure 2b. The exact transition temperature of SNO is different for the two substrates due to strainand defect-related phenomena as discussed earlier [15]. Further, in situ wafer curvature measurement was carried out on SNO film grown on Si(1 0 0) to investigate the stress relaxation across the metal–insulator transition. Figure 3a and b show respectively the radius of curvature vs. temperature and the corresponding stress–temperature plots of SNO film on Si(1 0 0). First, there is no abrupt stress change observed during the repeated thermal cycles. Secondly, the stress–temperature plots are linear and the measured stress changes are compressive in nature. SNO undergoes shrinkage along the b and c axes of the unit cell and in cell volume (0.15%) across the metal–insulator transition during heating [9]. A recent neutron scattering study with 154 Sm indicates smooth changes in the lattice parameters across the transition [16]. Nevertheless the volume shrinkage should have resulted in tensile stress during the transition. However, the absence of any discontinuous, phase-transition-induced stress change across the metal–insulator transition is evident from the plots shown in Figure 3a and b. It is important to note that the measured stress is an overall indicator of the stress state, whereas the phase-transition-induced tensile stress is estimated only from the volume change across the transition boundary. At least the volume change and thermal mismatch components have to be considered to obtain an estimate of the overall stress change across the phase transition; this is described below.
465
Figure 3. Stress relaxation in a polycrystalline SmNiO3 film 200 nm thick grown on silicon substrate as a function of temperature. The radius of curvature and the corresponding stress–temperature plots across the metal–insulator transition temperature are shown in (a) and (b), respectively. Opposing stresses from volume shrinkage and thermal mismatch at the transition are shown schematically in (c) for reference. While the compressive stress arises due to thermal expansion coefficient mismatch, the tensile stress arises due to abrupt volume change induced by Ni–O bond distortion at the transition temperature.
The estimated stress evolution of SNO considering the volume changes that accompany a metal–insulator transition as well as the thermal mismatch components is shown in Figure 3c. The transition-induced biaxial stress in SNO film is estimated to be +171 MPa during heating (from the volume shrinkage) based on the following expression. 1
rf ¼ Ef ð1 tf Þ ðDV=3VÞ;
ð1Þ
where Ef, tf,V and DV are the Young’s modulus, the Poisson ratio, the unit cell volume and the change in
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unit cell volume, respectively, of the film. Wafer curvature measurement by laser scanning is sensitive enough to detect curvature changes of the order of 104 m1 considering practical factors such as mechanical vibration, laser beam profile, noise, etc. [17]. The curvatures measured in the temperature range 25–250 °C are of the order of 102 m1. The scatter in the curvature measurement at the metal–insulator transition temperature (T = 185 °C) obtained from the width of the multiple thermal cycles is around 5 104 m1 and the corresponding scatter in the measured stress is 24 MPa, which is significantly lower than the phase-transitioninduced stress. Note that the phase-transition-induced tensile stress arises due to abrupt volume shrinkage that takes place within the transition boundary (6 °C) at 185 °C. This volume change is associated with the Ni–O–Ni bond distortions. The temperature-dependent thermal stress evolution of SNO on Si was estimated using the coefficient of thermal expansion of insulating and metallic SNO against Si substrate using the following expressions: 1 ð2Þ rf ðinsulatingÞ ¼ Ef ð1 tf Þ as afðInsulatingÞ DT 1 ð3Þ rf ðmetallicÞ ¼ Ef ð1 tf Þ as afðMetallicÞ DT where as, af and DT are the coefficient of thermal expansion and the temperature difference, respectively. Subscripts s and f denote Si substrate and SNO film in this case. The elastic modulus is estimated from the available bulk modulus (K = 167 GPa) based on the Poisson ratio for SNO (m = 0.24) [18]. The measured linearly increasing compressive stress trend of SNO film is comparable with the estimated thermal stress of SNO film that arises due to the thermal expansion mismatch between SNO and Si substrate. The slopes of the stress–temperature plots obtained from the linear regression analysis of experimentally measured SNO film are 0.87 and 0.78 during heating and cooling cycles, respectively. The estimated thermal stress shows considerable difference in slope between the insulating (0.97) and metallic (2.38) phases of SNO on Si. The higher slope (2.38) is due to the metallic nature of the SNO film that undergoes larger thermal expansion against the Si substrate. More importantly, considerable discontinuity is seen in the estimated thermal stress–temperature plot at the metal– insulator transition temperature (T = 185 °C) as the thermal expansion coefficient drops abruptly across the metal–insulator transition leading to a high compressive stress of 227 MPa. As a consequence, the volume shrinkage (0.15%) at the transition window generates a tensile stress of +171 MPa due to the subtle structural distortions described earlier. Since the thermal mismatch stress and phase-transition-induced stress are opposing each other and comparable in magnitude, as seen from the approximate analysis above, the measured overall thin film stress does not show any abrupt stress change at the metal–insulator transition temperature.
In conclusion, in situ wafer curvature experiments do not show a phase-transition-induced sharp stress change across the metal–insulator transition in SNO film on Si. The linear, temperature-dependent compressive stress observed in polycrystalline SNO film across the metal– insulator transition is primarily due to the thermal expansion mismatch between Si and SNO. A simple estimate of the stresses revealed two competing mechanisms involving volume shrinkage across the transition and thermal mismatch that minimize the overall stress changes. The results are correlated with in situ diffraction studies on both epitaxial and polycrystalline films that show similar patterns across the transition temperature. Financial support from ARO MURI (W911NF-09-1-0398) and the Focus Center Research Program in the Materials Structures and Devices Focus Center is acknowledged. We are very grateful to Prof. Frans Spaepen for valuable technical discussions. [1] M.J. Kang, S.Y. Choi, D. Wamwangi, K. Wang, C. Steimer, M. Wuttig, J. Appl. Phys. 98 (2005) 014904. [2] B. Viswanath, C. Ko, S. Ramanathan, Scripta Mater. 64 (2011) 490. [3] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution, Cambridge University Press, Cambridge, 2003. [4] W.D. Nix, Metal. Mater. Trans. A 20 (1989) 2217. [5] T.P. Leervad Pedersen, J. Kalb, W.K. Njoroge, D. Wamwangi, M. Wuttig, F. Spaepen, Appl. Phys. Lett. 79 (2001) 3597. [6] M. Zhong, Z. Song, B. Liu, Y. Chen, Y. Gong, F. Rao, S. Feng, F. Zhang, Y. Xiang, Scripta Mater. 60 (2009) 957. ˜ a Luisa, J. Phys. Cond. Mater. 9 (1997) 1679. [7] M. MarA [8] S.D. Ha, G.H. Aydogdu, S. Ramanathan, Appl. Phys. Lett. 98 (2011) 012105. [9] J.L. Garcia-Munoz, J. Rodriguez-Carvajal, P. Lacorre, J.B. Torrance, Phys. Rev. B 46 (1992) 4414. [10] M. Zaghrioui, A. Bulou, P. Lacorre, P. Laffez, Phys. Rev. B 64 (2001) 081102. [11] C. Girardot, J. Kreisel, S. Pignard, N. Caillault, F. Weiss, Phys. Rev. B 78 (2008) 104101. [12] F.P. de la Cruz, C. Piamonteze, N.E. Massa, H. Salva, J.A. Alonso, M.J. Martinez-Lope, T. Maria, Phys. Rev. B 66 (2002) 153104. [13] F. Conchon, A. Boulle, R. Guinebretiere, C. Girardot, S. Pignard, J. Kreisel, F. Weiss, E. Dooryhee, J.L. Hodeau, Appl. Phys. Lett. 91 (2007) 192110. [14] F. Conchon, A. Boulle, C. Girardot, S. Pignard, R. Guinebretie`re, E. Dooryhe´e, J.L. Hodeau, F. Weiss, J. Kreisel, Mat. Sci. Eng. B 144 (2007) 32. [15] G.H. Aydogdu, S.D. Ha, B. Viswanath, S. Ramanathan, J. Appl. Phys. 109 (2011) 124110. [16] P.F. Henry, M.T. Weller, C.C. Wilson, Chem. Mater. 14 (2002) 4104. [17] Z.B. Zhao, J. Hershberger, S.M. Yalisove, J.C. Bilello, Thin Solid Films 415 (2002) 21. [18] M. Amboage, M. Hanfland, J.A. Alonso, M.J. MartınezLope, J. Phys.: Cond. Mater. 17 (2005) S783.