Epitaxial La0.7Sr0.3MnO3 thin films with two in

JOURNAL OF APPLIED PHYSICS 97, 073905 共2005兲

Epitaxial La0.7Sr0.3MnO3 thin films with two in-plane orientations on silicon substrates with yttria-stabilized zirconia and YBa2Cu3O7−␦ as buffer layers Wei Chuan Goh Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

Kui Yaoa兲 Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602

C. K. Ong Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

共Received 19 July 2004; accepted 28 January 2005; published online 18 March 2005兲 Epitaxial La0.7Sr0.3MnO3 共LSMO兲 thin films were fabricated on silicon substrates by pulsed laser deposition utilizing yttria-stabilized zirconia 共YSZ兲 and YBa2Cu3O7−␦ 共YBCO兲 films as buffer layers. Structural characterization showed that the epitaxial LSMO films were 共001兲 oriented with two in-plane orientations and exhibited a columnar growth structure. In contrast, when LSMO was deposited directly on the YSZ/Si substrate without the YBCO template layer, it was characterized as a mixture of randomly oriented polycrystalline grains and 共001兲-oriented grains, without the columnar growth structure. The role of the YBCO layer for achieving the c-axis-oriented epitaxial LSMO film by introducing the dual-in-plane orientation mechanism on the YSZ/Si substrate has been analyzed. In addition, it was found that the LSMO film with the dual-in-plane orientations on the YBCO/YSZ/Si substrate exhibited a much lower resistivity compared to the LSMO film directly deposited on the YSZ/Si substrate. This effect is attributed to the existence of the randomly oriented grains in the latter, which resulted in more significant electron scattering at the grain boundaries. The higher density of grain boundaries in the LSMO film on YSZ/Si also led to a substantially higher magnetoresistance. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1876577兴 INTRODUCTION

The perovskite La0.7Sr0.3MnO3 共LSMO兲 has potential use in various magnetic applications, such as magnetic-field sensors,1 hard disk read heads,2 infrared devices,3 and microwave active components, to name a few, due to its large colossal magnetoresistance 共CMR兲. In addition, the high electrical conductivity of LSMO and its lattice matching with other perovskite ferroelectric oxides, such as lead zirconate titanate 共PZT兲 and barium strontium titanate 共BST兲, make LSMO an attractive candidate as the bottom electrode material for ferroelectric perovskite oxide thin films. As single-crystal materials often exhibit superior electrical and magnetic properties, epitaxial LSMO thin films are desirable for both electrical and magnetic applications. Although epitaxial LSMO thin films can be grown on single-crystal oxide substrates with lattice-matching parameters, including SrTiO3 and LaAlO3,4 reports of LSMO thin films grown on silicon substrates are very limited in the literature.5–7 There is no direct crystal lattice match between silicon and LSMO, and chemical reactions between LSMO and silicon could also occur to form interfacial phases. Thus, appropriate buffer layers need to be introduced in order to achieve high-quality epitaxial LSMO film growth. It is also of interest to investigate the epitaxial mechanism of LSMO a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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film growth on buffer-layered silicon substrates to clarify how the structure develops and how the buffer layers affect the film properties. In this paper, we describe the fabrication of LSMO films on an yttria-stabilized zirconia 共YSZ兲 buffer layer on silicon. The YSZ layer has good lattice matching with the LSMO and prevents the formation of an interfacial layer between LSMO and silicon. By introducing an additional buffer layer of YBa2Cu3O7−␦ 共YBCO兲 on the top of the YSZ layer, we can promote the epitaxial growth of 共001兲-oriented LSMO thin films. The epitaxial growth mechanism, crystal structures, surface morphology, resistivity, magnetoresistance, and the various interrelationships between these characteristics are discussed.

EXPERIMENTAL PROCEDURE

共100兲-oriented silicon substrates were cleaned using nitric acid in an ultrasonic cleaner for 5 min to remove any natural oxide layer or oxide contaminant on the surface. The substrates were subsequently cleaned with de-ionized water, acetone, and ethanol. All the YSZ, YBCO, and LSMO thin films were prepared by the pulsed laser deposition 共PLD兲 method, in which a KrF excimer laser was used 共pulse duration 30 ns, wavelength 248 nm, Lambda Physik Lextra 200兲. The detailed processing parameters for all the three films are summarized in Table I. Two kinds of multilayer samples were prepared, namely, LSMO/YSZ/Si and LSMO/YBCO/

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Goh, Yao, and Ong TABLE I. Summary of PLD processing parameters for the YSZ, YBCO, and LSMO thin films.

Layer

Deposition temperature 共°C兲

Oxygen ambience 共mbar兲

Laser fluence 共J / cm2兲

Repetition rate 共Hz兲

Deposition time 共min兲

Thickness 共⫻10−9 m兲

YSZ YBCO LSMO

670 650 670

5 ⫻ 10−4 5 ⫻ 10−1 2 ⫻ 10−1

5 3.5 5

5 3 5

13 8 45

10 200 1000

YSZ/Si. In addition, a LSMO film grown on a latticematching single-crystal LaAlO3 共LAO兲 substrate was fabricated for comparison purposes. Both ␪-2␪ and ␾-scan x-ray diffraction 共XRD, Bruker, D8-ADVANCE兲 studies were conducted to determine the crystalline phase and the crystal orientation of the films. Scanning electron microscopy 共SEM, JEOL 6700F兲 was carried out to investigate the morphology of the films. A fourprobe resistance measurement system 共Keithley 220 current source, Keithley 182 voltmeter, and Lakeshore 330 temperature controller兲 was used to analyze the electrical properties of the samples from room temperature to 77 K. Magnetoresistance 共MR兲 experiments with magnetic field up to 3000 G 共homemade electromagnet兲 were also carried out at 77 K to assess the magnetic properties of the LSMO films.

for both samples, as revealed by the matched fourfold symmetry for both YSZ and silicon in Figs. 2共a兲 and 2共b兲. The LSMO film on the YSZ layer, which shows the incomplete 共001兲 orientation in the ␪-2␪ scan in Fig. 1, exhibits a weak four-fold in-plane symmetry with broad diffraction peaks of plane 兵113其, as shown in Fig. 2共b兲. From these results we can conclude that the LSMO film on the YSZ consists mostly of randomly oriented grains with a small number of epitaxial grains. For the LSMO/YBCO/YSZ/Si sample, the ␾ scan of the LSMO 兵103其 plane exhibits strong eight-fold diffraction peaks, as shown in Fig. 2共a兲. This confirms that there are two

RESULTS A. Crystalline structure

XRD ␪-2␪ scans for the LSMO/YSZ/Si and LSMO/ YBCO/YSZ/Si samples are shown in Fig. 1. An incomplete 共001兲-orientation preference and some unidentified phases are observed in the LSMO film grown on YSZ/Si, whereas the LSMO film grown on YBCO/YSZ/Si exhibits a complete 共001兲 orientation and no unidentified phase is present. We did not observe strong XRD peaks of the YBCO layers because the power of the x ray was low 共20 kV, 5 mA兲. XRD ␾ scans for the LSMO/YBCO/YSZ/Si and LSMO/ YSZ/Si samples are presented in Figs. 2共a兲 and 2共b兲, respectively. The YSZ layers exhibit a cube-on-cube epitaxial relationship to the 共100兲-oriented single-crystal silicon substrates

FIG. 1. X-ray diffraction patterns 共␪-2␪兲 for the LSMO thin films deposited on YBCO/YSZ/Si and YSZ/Si substrates.

FIG. 2. 共a兲 X-ray diffraction patterns 共␾ scan兲 for 共I兲 Si 兵113其, 共II兲 YSZ 兵113其, and 共III兲 LSMO 兵113其 peaks for the LSMO/YBCO/YSZ/Si multilayer. The inset figure shows the relative ␾ angle relationship between YBCO 兵103其 and LSMO 兵103其. 共b兲 X-ray diffraction results 共␾ scan兲 for 共I兲 Si 兵113其, 共II兲 YSZ 兵113其, and 共III兲 LSMO 兵113其 peaks for the LSMO/YSZ/Si multilayer.


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FIG. 3. SEM images of cross sections through 共a兲 LSMO/YBCO/YSZ/Si and 共b兲 LSMO/YSZ/Si.

sets of in-plane orientations in the LSMO layer with an inplane shift of 45° from each other. The two sets of in-plane orientation in the LSMO layer are found to result from the corresponding two sets of the in-plane orientation in the YBCO template layer, as indicated by the ␾ scan results of the 兵103其 planes for both LSMO and YBCO layers in the inset figure in Fig. 2共a兲. As Fig. 1 shows a complete 共001兲 orientation, the LSMO film on the YBCO/YSZ/Si substrate is an epitaxial film but has two sets of in-plane orientation, with a shift of 45° from each other.

on the YBCO/YSZ/Si substrate 共⬃2 % 兲. The results are consistent with the report by Gu et al.8 who found that LSMO films with a higher density of grain boundaries showed a larger MR.

B. Morphology

DISCUSSION

SEM images of cross sections through the LSMO/ YBCO/YSZ/Si and LSMO/YSZ/Si multilayer structures are presented in Figs. 3共a兲 and 3共b兲, respectively. The LSMO film on the YBCO layer exhibits a columnar growth structure, as shown in Fig. 3共a兲, while the LSMO film grown on the YSZ/Si substrate does not show a columnar structure, as apparent from Fig. 3共b兲. The columnar grains in the LSMO film on the YBCO/YSZ/Si substrate are around 100– 175 nm in width across the film. The LSMO film on the YSZ/Si substrate has more grain boundaries than the LSMO film on the YBCO/YSZ/Si substrate, which is also apparent from SEM images of the film surfaces 共not shown兲. The SEM images indicate better epitaxial quality for the LSMO film on the YBCO/YSZ/Si substrate than that on the YSZ/Si, which is consistent with the XRD results. Both of the LSMO films show a root-mean-square surface roughness of around 10 nm by atomic force microscopy 共AFM兲. Our LSMO films were always rather granular due to the nature of substantial structural mismatch and imperfect epitaxial growth by the pulsed laser deposition.

The growth of YSZ on 共100兲 silicon is based on a 关100兴储关100兴 cube-on-cube mechanism, which is the same as reported previously by our group.9 When the YBCO film was subsequently deposited onto the YSZ film, the YBCO film exhibited a strong c-axis orientation along the thickness direction and two in-plane orientations with a 45° shift from each other, i.e., YBCO关100兴共001兲储 YSZ关100兴共001兲 and YBCO关110兴共001兲储 YSZ关100兴共001兲, as indicated in Fig. 2共a兲. A similar dual-in-plane orientation for YBCO thin films was also reported in the literature.10,11 We also observed that the orientation of LSMO关100兴共001兲 is more preferred when the LSMO film is grown on the YBCO layer with the dual-inplane orientation, i.e., YBCO关100兴共001兲储 YSZ关100兴共001兲 and YBCO关110兴共001兲储 YSZ关100兴共001兲, as its XRD intensity is much stronger than the orientation of LSMO关110兴共100兲 in Fig. 2共a兲. Without introducing the YBCO film, the LSMO

FIG. 4. Temperature dependence of LSMO thin films deposited on 共a兲 LAO, 共b兲 YBCO/YSZ/Si, and 共c兲 YSZ/Si substrates.

C. Resistivity and magnetoresistance

Figure 4 shows the temperature dependence of the LSMO film resistivity deposited on LAO, YBCO/YSZ/Si, and YSZ/Si substrates. The resistivity of the LSMO film on the YBCO/YSZ/Si substrate is much lower than that of the LSMO film on the YSZ/Si, which has randomly oriented grains, but is still higher than the resistivity of the epitaxial LSMO film grown on the lattice-matched substrate LAO. The MR of the LSMO films deposited on YBCO/YSZ/Si and YSZ/Si substrates is given in Fig. 5. A magnetic field of up to 3000 G was applied perpendicular to the surface of the samples at 77 K. In Fig. 5, the LSMO film on the YSZ/Si substrate has much larger MR 共⬃16% 兲 than the LSMO film

FIG. 5. Magnetoresistance 共MR兲 vs magnetic field for LSMO thin films deposited on 共a兲 YBCO/YSZ/Si and 共b兲 YSZ/Si substrates.


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FIG. 6. A schematic diagram of the proposed epitaxial growth mechanisms of the YBCO thin film for the two observed in-plane orientations on the YSZ buffer layer: 共a兲 YBCO 关100兴共001兲储 YSZ关100兴共001兲, in which the Cu–O plane of the YBCO is grown on the O–O plane of the YSZ 共3 / 4 height of the unit cell of YSZ兲; 共b兲 YBCO 关110兴共001兲储 YSZ关100兴共001兲, the Cu–O plane of the YBCO is turned 45° in plane on the Zr–Zr plane of YSZ.

film on the YSZ did not exhibit the same in-plane orientation relationship with the underlying YSZ layer, as shown in Fig. 2共b兲. The epitaxial growth of YBCO关110兴共001兲储 YSZ关100兴 ⫻共001兲 is obviously due to the small misfit of 6.24% for the 共110兲 lattice of YBCO and 共100兲 lattice of YSZ. However, it is interesting to note that YBCO关100兴共001兲储 YSZ关100兴共001兲 epitaxial growth occurred while LSMO关100兴 ⫻共001兲储 YSZ关100兴共001兲 did not. Schematic diagrams to illustrate both of the in-plane orientations of YBCO on YSZ are shown in Fig. 6. The Cu–O plane layers are depicted to be in contact with YSZ as a proposed growth mechanism. In Fig. 6共a兲, the Cu–O plane of the YBCO is grown onto the O–O plane of the YSZ 共at 3 / 4 of an YSZ unit cell兲 for the YBCO关100兴储 YSZ关100兴 orientation. In Fig. 6共b兲, the Cu–O plane of the YBCO is turned 45° in plane on the Zr–Zr plane of YSZ to realize the YBCO关110兴储 YSZ关100兴 epitaxial growth. The structures of YBCO and LSMO need to be analyzed to understand the reason why YBCO exhibits YBCO关100兴储 YSZ关100兴-type in-plane orientation while LSMO, which apparently has similar lattice parameters, does not grow with the same in-plane orientation. The misfits for and LSMO关100兴储 YSZ关100兴 YBCO关100兴储 YSZ关100兴 growths are 24.90% and 24.71%, respectively. In the orthorhombic YBCO crystal structure, as shown in Fig. 7共a兲, the Cu–O bond at the second and third planes is slightly bent, whereas the Mn–O plane in the perovskite LSMO structure is flat, as shown in Fig. 7共b兲. The bending of the Cu–O plane in YBCO is better suited for the epitaxial growth, as depicted in Fig. 6共a兲, because it better tolerates the lattice misfit and provides a more angular flexibility for forming chemical bonds between copper and oxygen at the interface. In addition, the electron configuration 3d104s1 of copper could also form Cu+ that electrically neutralizes oxygen vacancies at the interface of the YSZ and YBCO, and provides more structural flexibility than LSMO. It is therefore essential to have the additional YBCO template layer in order to grow com-

FIG. 7. Structure of 共a兲 orthorhombic YBCO crystal and 共b兲 perovskite LSMO crystal.

pletely c-axis-oriented LSMO films through the dual-inplane orientation mechanisms. With that being said, we do understand that there could be other oxide candidates to be used as a buffer layer instead of YBCO. However, our results show that the relatively complicated and more tolerant structure of YBCO may promote the epitaxial growth between two materials with substantial difference in structure, such as LSMO and YSZ described in this paper. The difference in the resistivities of the LSMO/YBCO/ YSZ/Si and LSMO/YSZ/Si films can be understood by consideration of electron polarization and scattering mechanisms. Inside the magnetic domain, the conduction electrons are spin polarized, i.e., having the same magnetic dipole direction, and electrons are easily transferred between pairs of Mn3+ and Mn4+ ions. However, when these electrons travel across the grain boundaries, they experience strong spindependent scattering.11 A large density of grain boundaries would lead to an increase in scattering and hence result in an increase of the resistivity. The lower resistivity of the LSMO film grown on YBCO/YSZ/Si as compared to the LSMO film on YSZ/Si is due to its complete c-axis orientation with the columnar grain structure. The smaller, randomly oriented grains in the LSMO/YSZ/Si form many more grain boundaries. It is therefore understandable that the LSMO films grown on YBCO/YSZ/Si show a larger resistivity when compared to the epitaxial LSMO film on LAO substrate, which has only one in-plane orientation. With the application of a magnetic field, the randomly oriented magnetic domains in the LSMO film on the YSZ buffer layer are reoriented, which leads to a significant decrease in the resistivity. Therefore, a larger MR was observed in the LSMO film deposited on the YSZ/Si substrate than that on the YBCO/YSZ/Si substrate, which has no substantial amount of randomly oriented grains. CONCLUSION

共001兲-oriented epitaxial LSMO thin films with two inplane orientations were fabricated on silicon substrates by


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PLD with YSZ and YBCO films as buffer layers. The epitaxial LSMO films on the YBCO layers had a cross-sectional columnar growth structure. The dual-in-plane orientations of the LSMO film were formed by epitaxial growth from dual orientations in the YBCO template layer. The crystal orientation relationship among the LSMO, YBCO, and YSZ films can be expressed as, LSMO/ YBCO关110兴共001兲储 YSZ关100兴 ⫻共001兲 and LSMO/ YBCO关100兴共001兲储 YSZ关100兴共001兲. In contrast, when LSMO films were deposited directly on YSZ/Si without using the YBCO template layer, it contained a mixture of randomly oriented polycrystalline grains and 共001兲-oriented grains. Therefore, introduction of the YBCO layer was essential to obtain c-axis-oriented epitaxial LSMO films through the dual-in-plane orientation growth mechanism of YBCO on the YSZ/Si substrate. The difference between the in-plane orientations of the LSMO and YBCO films on the YSZ layer is mainly attributed to the disparity in their crystallographical structure. The LSMO film with the dual-in-plane orientations on the YBCO/YSZ/Si substrate exhibited a much lower resistivity as compared to the LSMO

film on the YSZ/Si substrate, which had more randomly oriented grains and hence more electron scattering at the grain boundaries. The greater density of grain boundaries in the LSMO film on the YSZ/Si substrate also led to a substantially higher magnetoresistance. 1

S. Jin, M. McCormack, T. H. Tiefel, and R. Ramesh, J. Appl. Phys. 76, 6929 共1994兲. 2 K. Derbyshire and E. Korczynski, Solid State Technol. 38, 57 共1995兲. 3 A. Lisauskas, S. I. Khartsev, and A. Grishin, Appl. Phys. Lett. 77, 756 共2000兲. 4 J. Li, C. K. Ong, Q. Zhan, and D. X. Li, J. Phys.: Condens. Matter 14, 6341 共2002兲. 5 Z. Tranjanovic et al., Appl. Phys. Lett. 69, 1005 共1996兲. 6 J. Y. Gu et al., Appl. Phys. Lett. 70, 1763 共1997兲. 7 J.-H. Kim, S. I. Khartsev, and A. M. Grishin, Appl. Phys. Lett. 82, 4295 共2003兲. 8 J. Y. Gu et al., Appl. Phys. Lett. 72, 1113 共1998兲. 9 S. J. Wang, S. Y. Xu, L. P. You, S. L. Lim, and C. K. Ong, Supercond. Sci. Technol. 13, 362 共2000兲. 10 S. M. Garrison, N. Newman, B. F. Cole, K. Char, and R. W. Barton, Appl. Phys. Lett. 58, 2168 共1991兲. 11 A. Gupta et al., Phys. Rev. B 54, 015629 共1996兲.
