Residual strain and texture in strontium-doped lanthanum manganite ...

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FAMU-FSU College of Engineering and MARTECH, Center for Nonlinear and .... where Da is the difference between the thermal CTE of the film and the ...
J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 2 ( 2 0 0 1 ) 1 4 3 ± 1 4 6

Residual strain and texture in strontium-doped lanthanum manganite thin ®lms L. MEDA, C. BACALTCHUK, H. GARMESTANI FAMU-FSU College of Engineering and MARTECH, Center for Nonlinear and Non-equilibrium Aero-Sciences (CeNNas), National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310 K.-H. DAHMEN Department of Chemistry and MARTECH, The Florida State University Tallahassee, FL 32306 E-mail: [email protected] Thin ®lms of La0:67 Sr0:33 MnO3 (LSMO) have been deposited using liquid-delivery metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) 2y/y scans showed that all the ®lms has a cubic structure. Studies of the in-plane crystallographic orientations ( pole ®gure) revealed an (0 0 1) preferred growth structure on LAO, a weak (1 1 0) texture on Y-ZrO2 (YSZ), a random texture on sapphire (SAP) and silicon (Si). Our attention is focused on residual strain and its deviations from linearity for ej;C vs. sin2 C plots. The strain evolution from 05sin2 C50:8 showed a curvature and a ``snake-like'' pattern. These anomalies are attributed to texture and strain gradients. In-plane strain decreased as the lattice mismatch increased and varied from 0.05 to 3.03% depending on the substrate. An attempt is made to establish a relationship between lattice mismatch, growth process, and residual strain. # 2001 Kluwer Academic Publishers

1. Introduction

La1 x Srx MNO3 (LSMO) thin ®lms are attractive for applications in fuel cell, catalytic, sensors, and other devices [1, 2]. Interest in these perovskite-type materials has grown tremendously in the past few years due to their giant magnetoresistance effect (GMR). A plethora of research papers has been published in the past few years and mostly concentrated on the magnetic properties [1±4]. The GMR effect is related, in part, to the quality and the growth structure of the ®lms. The effect of strain on the properties of these ®lms has been the focused of numerous studies [5, 6]. LSMO thin ®lms have been deposited on a variety of substrates such as SrTiO3 , MgO, LaAlO3 silicon, sapphire, and Y-ZrO2 [1, 2, 4, 6] as a method of introducing different lattice strain and texture. Characterization of the mechanical behaviors of these ®lms is very important in order to understand their magnetic behaviors. Thus a comprehensive study of the residual strain/stress is needed because most deposited thin ®lms are under some kind of residual (internal) stresses [7] due to the growth process. Understanding the evolution of strain in thin ®lms is essential in order to grow better quality ®lms by controlling the deformation processes. Here, we present a study of the residual strain/stress in polycrystalline La0:67 Sr0:33 MnO3 thin ®lms. The relationship between the growth process, texture, and mechanical properties are discussed. 0957±4522

# 2001 Kluwer Academic Publishers

2. Methods of residual strain/stress determination

X-ray diffraction (XRD) is a very useful technique for measuring residual strain/stress. Stresses alter the spacing of crystallographic planes in ®lm/substrate by an amount that can be usually measured by X-ray diffraction [7, 8]. This method is unique because of its nondestructive attributes. The average strain can be measured along an arbitrary direction, L, as a function of the spherical coordinate system …j, C† with respect to the specimen coordinate system, S, Fig. 1. Assuming a biaxial stress state is present and the stress in the growth direction …s33 † as well as the shear stresses …sij † for i or j ˆ 1, 2, 3 where i=j) are zero, the strain can be calculated as follows: ej;C ˆ

hkl dC d0hkl ˆ …e11 hkl d0

e33 † sin2 C ‡ e33

…1†

hkl where dC is the interplanar spacing normal to the diffraction vector L, d0hkl is the unstressed d-spacing, C is the tilt angle (angle between diffracting plane and normal to the specimen surface), e11 and e33 are the in-plane and out-plane strain, respectively [8]. Residual strain normal to the surface …e33 † is obtained from the linear regression of ej;e vs. sin2 C and in-plane strain …e11 † can be calculated from the slope of the line …e11 e33 †. The internal stress is usually de®ned as a combination of intrinsic and thermal stresses. Intrinsic stress is largely

143

Figure 1 Coordinate system …S† and the diffraction vector Lj;C . The sample can be rotated in both angles …j, C†.

due to process and thermal stress is due to differences in the coef®cient of thermal expansion (CTE). Thermal mismatch stress …sT † is obtained straightforwardly from the following equation by Z TS E dT …2† sT ˆ …Da† …1 n† TRT where Da is the difference between the thermal CTE of the ®lm and the substrate, E…1=S11 † and n… S12 =S11 † are the elastic constants, TS and TRT are the substrate and room temperature (25  C), respectively [9, 10].

3. Experimental

Details of the deposition procedures were previously described elsewhere [1, 2]. LSMO thin ®lms were deposited on LAO(0 0 1), YSZ(0 0 1), SAP(0 0 1) and SiO2 =Si…0 0 1†. All X-ray diffraction (XRD) experiments were conducted on a four-circle goniometer …2y, o, j, C†, Philips X'Pert-3040 MRD diffractometer, operated at 40 kV and 45 mA using CuKa radiation. A thin ®lm attachment equipped with a ¯at crystal monochromator and a parallel plate collimator was used. A diffraction pattern was obtained in the range of 2y ˆ 20 140 . The (1 1 0) re¯ection was used for the analysis. A ®tting pro®le analysis software (ProFit) from Philips with the Pseudo-Voigt method was used to obtain the centroid of the peak position, intensity, and full width at half maximum (FWHM) [11]. The strain free interplanar spacing was obtained from a powder, which was synthesized from La2 O3 ; SrCO3 and MnCO3 using the appropriate molar ratio. Because the elastic constant for LSMO is unavailable, those for La0:87 Sr0:13 MnO3 were used [12].

showed that the ®lms have columnar grain structure. The XRD 2y/y scans revealed a pseudo-cubic structure with an out-of-plane lattice parameter …af † of 0.3860, 0.3865, 0.3863, and 0.3871 nm for Si, LAO, SAP, and YSZ, respectively, which are very close to the bulk value of 0.3873 nm. The LSMO ®lms on these substrates are expected to be under compressive strain based on the lattice mismatch. Films deposited on LAO, …a ˆ 0.3821 nm†, developed a preferred (0 0 1) crystallographic orientation normal to the surface (Fig. 2) and Fig. 3a shows the cube on cube orientation LSMO [0 0 1]==LAO [0 0 1]. Fig. 3b shows a weakly (1 1 0) textured ®lm on YSZ with a random in-plane orientation. Pole ®gures for ®lms grown on SAP, and Si (not shown) revealed a random texture. Fig. 4 shows the plots of ej;C vs. sin2 C for the (1 1 0) plane. For ®lms on LAO (Fig. 4d) C-splitting occurs due to the effects of shear stresses [13, 14]. The C-splitting phenomenon was not observed for the randomly/weakly oriented ®lms on YSZ, SAP, and Si. The (1 1 0) sin2 C plots for these ®lms are shown in Fig. 4a±c. However, a straight line was not obtained as predicted for isotropic materials in Equation 1 [8, 13]. For the randomly textured ®lms, this deviation from linearity is usually attributed to inhomogeneous strain distribution, i.e., strain gradient, normal to the interface [8, 13]. As the tilt angle increased (Fig. 1), i.e., incident beam becomes parallel to the surface, the strain becomes larger (Table I). A straight line is observed when the plots are broken into regions (Fig. 4). Region I for SAP and YSZ …sin2 C ˆ 0 0:5 or C ˆ 0 45 † yield a straight line and lower strain, while in region II …sin2 C ˆ 0:5 0:8† the strain is higher (Table I). For ®lms grown on Si with an oxidized SiO2 layer, an oscillating behavior is obtained. This trend is usually due to texture, although the pole ®gure reveals a random orientation. In this curve, three different regions are labeled (Fig. 4c). Avery high initial strain * 3.03% is observed in region I. The strain relaxed in region II and a high positive slope resurface in region III. It is not unusual, however, for thin ®lms to have a very high initial strain [15]. Since both positive and negative C-tilting plots are about the same for the randomly textured ®lms, therefore it may be reasonable to conclude that the plane stress model was valid. The same cannot be said for LAO (Fig. 4d). The model was not valid because the splitting of C 5 0 and C 4 0 suggests the presence of shear stresses.

4. Results and discussion

Polycrystalline LSMO thin ®lms with grain sizes ranged from 20 to 100 nm and a thickness of 80±400 nm were grown on LaAlO3 (LAO), sapphire (SAP), Y-ZrO2 (YSZ), and Si. Scanning electron microscopy (SEM) 144

Figure 2 XRD 2y/y scans of as-deposited LSMO ®lms on (a) LAO, (b) YSZ, and (c) SAP.

Figure 3 Calculated pole ®gures of LSMO thin ®lms deposited on (a) LAO, (b) YSZ.

Figure 4 ej;C vs. sin2 C plots for (1 1 0) LSMO ®lms grown on (a) YSZ, (b) SAP, (c) Si, and (d) LAO.

145

T A B L E I Lattice plane, paparmeters, mismatch, % strain, and stress of as-deposited LSMO thin ®lms. The lattice parameter for LMSO …bulk† ˆ 0:3873 nm, LAO ˆ 0:3821 nm, SAP a ˆ 0:4758 nm, c ˆ 12:99 nm; YSZ ˆ 0:5125 nm, and Si ˆ 0:5404 nm. Coef®cient of thermal expansion …CTE†: LSMO ˆ 13:4610 6 K 1 , SAP ˆ 7:5610 6 K 1 , LAO ˆ 1610 6 K 1 , YSZ ˆ 10:3610 6 K 1 , and Si ˆ 10:3610 6 K 1 . Young's modulus ˆ 1=S11 ˆ 96:71 GPa and Poisson's ratio ˆ v ˆ S12 =S11 Lattice

% Strain

Substrate

Plane

Parameters (nm)

LAO Si

(110) (110)

0.3865 0.3865

1:2 1:2

YSZ

(110)

0.3872

6:9

SAP

(110)

0.3864

15:1

a

Average strain and

Mismatch (%)

Slope …e11 Ð 0:71a 0:34I 0:19II 1:11III 0:44a 0:24I 1:33II 0:37a 0:14I 0:80II

e33

e11

Thermal

In-plane

Ð 0:23 0:37 0:03 0:56 0:18 0:19 0:78 0:19 0:14 0:03

Ð 0.48 3.03 0.16 0.55 0.26 0.05 0.55 0.18 0.00 0.77

Ð 137 Ð Ð Ð 137 Ð Ð 267 Ð Ð

Ð 464 2930 155 532 251 48 532 174 Ð 745

I, II, III

Average strain taking over a particular region (see Fig. 3).

A conclusion on the propagation of strain in the ®lms is dif®cult to establish at this time. There could be a number of reasons for the occurrence of the patterns in Fig. 4 such as, dislocations and grain growth (densi®cation). When the patterns are broken into different regions, the effect of strain gradient is obvious. Stresses generated by cooling the MOCVD reactor from the deposition temperature (685  C) to room temperature seem to be more or less lower than the calculated in-plane stress. Both of these measurements are in agreement with the theoretical prediction that a tensile strain should result because the CTE of the ®lms is higher than the substrate. Lattice mismatches calculated with the lowest energy plane predict a compressive strain at the interface. The in-plane stress was estimated using the young modulus from a closely related structure as mentioned above. As shown in Table I, the in-plane strain increases as the lattice mismatch increases. For SAP, it is evident that the total average stress generated is due to thermal stress. However, a contribution from internal stress is noticeable when we compare the average thermal stress to the stress calculated from region II. For the other substrates, internal strain is the dominant factor for the average stress. A correlation between the growth structure and residual strain/stress cannot be established without further investigations. Strain evaluation of the epitaxial ®lms on LAO, STO, and MgO will be published later.

is strictly related to thin ®lms growth formation. A direct relationship between the growth structure and the residual strain cannot be drawn without further study.

Acknowledgments

This research is supported in parts by DARPA and the Of®ce of Naval Research under contract ONR-N0001496-1-0767. One of the authors acknowledges support from NASA through CeNNas.

References 1.

2. 3. 4. 5. 6. 7. 8. 9.

5. Summary

Residual strains in polycrystalline LSMO thin ®lms were measured using X-ray diffraction method. This method gives average strain value irrespective of microstructural details. Errors in the strain measurements due to low angle re¯ections are minimized with the use of parallel beam optics. These results indicate that strain gradient plays a major role in determining the average strain. A correlation between the lattice mismatch and the average residual strain shows that in-plane strain decreases as the lattice mismatch increasing. It is clear that the average strain is misleading if a straight line is not obtained. Different regions in the ®lms give different strain, which 146

e33 †

Stress (MPa)

10. 11. 12. 13. 14. 15.

K . - H . D A H M E N and M . W. H A R R I S , Chemical Vapor Deposition 3 (1997) 27. M . I . W E AV E R , L . P. M . B R A N D AO , A . M O R RO N E , E . S . G I L L M A N , K . - H . D A H M E N and H . G A R M E S TA N I , J. Mater. Res. 14 (1999) 2007. J . J . H E R E M A N S , M . CA R R I S , S . WAT T S , X . Y U , K . H . Â R , J. Appl. Phys. 81 (1997) 4967. D A H M E N and S . VO N M O L N A H . Y. H WA N G , S . - W. C H E O N G , R . B . VA N D OV E R and Y. S U Z U K I , Appl. Phys. Lett. 71 (1997) 140. S . J I N , T. H . T I E F E L , M . M C C O R M AC K , H . M . O B R Y A N , L . H . C H E N , R . R A M E S H and D . S C H U R I G , Appl. Phys. Lett. 67 (1995) 557. R . A . R AO , D . L AV R I C , T. K . N AT H , L . W U , F. T S U I and C . B . E O M Appl. Phys. Lett. 73 (1998) 3294. È L L E R and M . M U R A K A M I , in ``Analytical A. SEGMU Techniques for Thin Films'', edited by K. N. Tu and R. Rosenberg (Academic Press, Inc., Boston, 1988) p. 143. I . C . N O Y A N and J . B . CO H E N , ``Residual Stress: Measurement by Diffraction and Interpretation,'' (Springer-Verlag, New York, 1987). M . VA N L E E U W E N , J . - D . K A M M I N G A and E . J . M I T T E M E I J E R , J. Appl. Phys. 86 (1999) 1904. M . F. D O E R N E R and W. D . N I X , Critical Reviews in Solid State and Materials Science 14 (1988) 225. W. N . S C H R E I N E R and R . J E N K I N S , Adv. In X-ray Anal. 26 (1983) 141. T. W. D A R L I N D , A . M I G L I O R I , E . G . M O S H O P O U LO U , S . A . T R U G M A N , J . J . N E U M E I E R , J . L . S A R R AO , A . R . B I S H O P and J . D . T H O M P S O N , Phys. Rev. B 57 (1998) 5093. H . D O L L E , J. Appl. Cryst. 12 (1979) 489. V. M . H A U K , Adv. X-Ray Anal. 27 (1984) 10. D . S A N D E R , Rep Prog. Phys. 62 (1999) 808.

Received 24 October 2000