Self-Template Growth of Orientation-Controlled Fe3O4 Thin Films

Apr 11, 2012 - Self-Template Growth of Orientation-Controlled Fe3O4 Thin Films. Ryota Takahashi* ... *E-mail: [email protected]. Cite this:Crys...
1 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/crystal

Self-Template Growth of Orientation-Controlled Fe3O4 Thin Films Ryota Takahashi,* Hikaru Misumi, and Mikk Lippmaa Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8581, Japan ABSTRACT: We have investigated self-template control of Fe3O4 thin film orientation on SrTiO3(001) substrates. The growth orientation of Fe3O4 films on SrTiO3(001) is dependent on the preparation temperature, with a crossover from the (001) to (111) grain orientation occurring at around 600 °C. In order to grow high-quality (001)-oriented Fe3O4 thin films at high temperature, a self-template technique was used, where an 8-nm-thick nucleation layer was deposited on a SrTiO3(001) substrate at 400 °C, followed by main film growth at 700 °C. This method achieved films that showed pure (001) grain orientation with bulk-like magnetic and transport behavior.

■

INTRODUCTION Crystallographic orientation control in heteroepitaxial thin films is critically important when the films are used for characterizing or optimizing anisotropic physical properties of materials, such as magnetization1−5 or ferroelectric polarization.6−10 In order to investigate the intrinsic material properties along certain crystallographic axes, bulk single crystals are usually preferred. However, bulk crystals cannot always be used due to crystal growth issues limiting maximum crystal size, twinning, domain formation, etc. From the point of view of materials characterization, thin films have two major advantages. First of all, the film growth process is strongly affected by the nonequilibrium kinetics that gives an additional degree of freedom for stabilizing unusual crystal phases, such as polar rock salts11,12 or natural superlattices.13 Another advantage is the ability to control the crystal orientation of an epitaxial film by a suitable choice of single-crystal substrates. This advantage has been demonstrated in measurements of both magnetic1−5 and ferroelectric6−10 properties of thin film materials. In cases where the film and substrate crystal structures are similar, the film orientation generally matches that of the substrate. However, in cases where the crystal structures or lattice parameters do not match, it is possible to nucleate grains with different orientations, even if all grains remain nominally epitaxial. Accurate characterization of anisotropic material properties is not possible in such a polycrystalline film due to the averaging over multiple crystallographic directions. A typical case where this problem appears is the growth of spinel films on perovskite substrates. While simple cube-on-cube growth of (001)oriented CoFe2O4 films on SrTiO3(001) substrates has been reported,4 InFeCoO4 and (Zn,Co)Fe2O4 films show random intermixing of grains with (001) and (111) plane directions.5,14 The complicated growth habit of spinel films on cubic perovskite substrates is caused by the large lattice mismatch, which is approximately +7.5% for a spinel Fe3O4 film on a SrTiO3(001) substrate.15,16 The spinel structure also has a © 2012 American Chemical Society

highly anisotropic surface energy that leads to a competition between growth directions, favoring either the (111) or (001) crystal planes.17 The use of a suitable buffer layer has been suggested by Suzuki et al. as a solution for obtaining spinel films with a welldefined and uniform grain orientation.2,3 They used a CoCr2O4 buffer layer with a thickness of up to 150 nm and subsequently deposited the desired spinel films, resulting in the formation of a film with a single grain orientation. While this type of buffering can be successful, it is not suitable for magnetic, ferroelectric, or dielectric characterization of materials because the response of the buffer layer would be included in the measurement. The heterostructure formed between the film and the buffer layer can have an additional effect on the characteristics of the film itself due to an exchange bias3 or due to crystal polarity discontinuities.18 In this work, we study the use of a thin self-template layer for orientation-controlled magnetite (Fe3O4) film growth on SrTiO3(001) substrates. Magnetite has a spinel structure and exhibits half-metallic ferrimagnetism (4.05 μB/formula unit (fu)) below 860 K.19 A unique feature of magnetite is the Verwey transition, which is related to the appearance of charge order at the iron sites below about 120 K. The crystal structure changes at the transition point from a high-temperature cubic phase to a low-temperature monoclinic symmetry.19−21 The reduction of crystal symmetry has been proposed to result in a spontaneous ferroelectric polarization in magnetite below the Verwey temperature.22,23 In a multiferroic material of this type, both the magnetic and dielectric properties are strongly anisotropic. It is important to develop a technique for growing orientation-controlled magnetite films that can be used for ferroelectric and magnetic characterization. Moreover, for the ferroelectric and dielectric characterization, SrTiO3 gives us the Received: February 28, 2012 Revised: April 4, 2012 Published: April 11, 2012 2679

dx.doi.org/10.1021/cg300287r | Cryst. Growth Des. 2012, 12, 2679−2683

Crystal Growth & Design

Article

and Fe3O4(202) reflections, as shown in Figure 1c and Figure 1d, respectively. The polar scan peak alignment indicates a Fe3O4[100]/[010]∥SrTiO3[100]/[010] in-plane orientation relationship. The cube-on-cube growth of Fe3O4/SrTiO3 at 400 °C agrees with a previous report on Fe3O4 films grown on a SrTiO3(001) substrate.23 Reciprocal space mapping around the Fe3O4(206) reflection yielded in-plane and out-of-plane lattice parameters of 8.33 and 8.44 Å, consistent with the bulk lattice parameter of a = 8.396 Å.16 As expected, the 150 nm thick (001)-oriented Fe3O4 films were almost fully relaxed on the SrTiO3(001) substrate. The crystal quality was evaluated from the rocking curve width of the Fe3O4(004) reflection. The full width at half-maximum (fwhm) value was estimated at 1.41°, indicating low crystallinity of Fe3O4 films grown on a SrTiO3(001) substrate at a low temperature. In order to improve the crystallinity, Fe3O4 films were grown at various higher temperatures. As shown in Figure 2 for a film

possibility of using a metallic Nb:SrTiO3 substrate as a bottom electrode of a planar capacitor structure,23 even though the lattice mismatch is much larger than that with MgO or MgAl2O4 substrates. We show that thermodynamic nucleation control in a self-template layer at the initial stage of film growth is effective at imposing a pure cube-on-cube growth mode where only a single crystal orientation appears. The biggest advantage of using a self-template layer is that the film consists only of iron oxide without having to introduce other cations in a buffer layer.

■

EXPERIMENTAL SECTION

Fe3O4 films were grown on 0.2° miscut and wet-etched nondoped SrTiO3(001) substrates24,25 by pulsed laser deposition. A polycrystalline Fe2O3 target was used for ablation at a laser fluence of 3 J/cm2 and an oxygen background pressure of 1 × 10−6 Torr. The ablation laser (KrF, λ = 248 nm) operated at 10 Hz. The film thickness was fixed at 150 nm. The sample holder was heated by an infrared laser, giving a maximum growth temperature of about 1400 °C.26 After growth, the films were quickly cooled below 200 °C in about 5 min in order to suppress the oxidization of Fe and the formation of a secondary hematite Fe2O3 phase. The surface morphology was studied by noncontact atomic force microscopy (AFM). The film orientation and crystallinity were characterized by X-ray diffraction (XRD). All AFM and XRD measurements were done at room temperature. The transport measurements were performed for Fe3O4 films grown on nondoped SrTiO3 substrates by evaporating Ti (50 nm) and Au (100 nm) bilayer electrodes on the Fe3O4 film surface and attaching aluminum wires by ultrasonic wire bonding to the Au electrode pads. Resistivity measurements were performed in a four-point geometry over an electrode gap of 0.4 mm. The magnetic properties were characterized by magnetization measurements in a superconducting quantum interference device (SQUID) magnetometer.

Figure 2. An XRD pattern (a) and an AFM image (b) of an Fe3O4 film grown at 700 °C. The triangular (111)-oriented grains assume one of four equivalent in-plane orientations, rotated by about 15° from the SrTiO3[100]/[010] directions.

■

RESULTS AND DISCUSSION Conventional Film Growth. Figure 1a shows an XRD pattern of a Fe3O4 thin film grown at 400 °C on a SrTiO3

grown at 700 °C, the Fe3O4(00l) reflections had disappeared, and it is clear that a (111)-oriented Fe3O4 film had crystallized on the SrTiO3(001) substrate. An AFM image in Figure 2b shows that the film consisted of large triangular grains, characteristic of the (111) facets of Fe3O4. A closer analysis of the triangular domains showed that the in-plane orientation of the grains was not random and that the grain edges followed certain well-defined directions, marked with the red dotted lines in Figure 2b. Four distinct orientations of the triangles could be identified from the microscopy images. The base of each triangle appeared to be tilted by approximately 15° from the SrTiO3[100]/[010] axes. In order to quantitatively evaluate the in-plane orientation of the triangular Fe3O4 domains, XRD Φ scans of the SrTiO3(101) and Fe3O4(311) reflections were performed, as shown in Figure 3a,b, respectively. There are 12 peaks visible along the Φ angle for the Fe3O4(311) reflection. This suggests that the Fe3O4 films had four types of (111)oriented domains and each domain was twisted by 15° from the SrTiO3[100]/[010] crystal directions, consistent with the AFM analysis result in Figure 2b. A similar growth habit of forming twisted triangular domains in a (111)-oriented ferrite film has been reported for NiFe2O 4 films grown on Pt(001)/ MgO(001).27 The well-defined in-plane orientations show that despite having the (111) orientation and hexagonal symmetry, the grains still have an in-plane relationship with the cubic substrate. In order to consider the reasons for the appearance of the (111) grains, the atomic arrangements at the surfaces of a

Figure 1. An XRD pattern (a) and an AFM image (b) of a Fe3O4 thin film grown at 400 °C. Closed circles mark the SrTiO3 substrate XRD peaks. Polar XRD scans for (c) SrTiO3 101 and (d) Fe3O4 202 reflections indicate cube-on-cube growth.

substrate. Only (00l) reflections of the Fe3O4 phase were observed besides the SrTiO3(00l) substrate reflections, indicating that the film had a pure [001] orientation. An AFM image of the surface of this film is shown in Figure 1b. The image is dominated by small square grains, rotated 45° relative to the SrTiO3[100]/[010] axes. Cube-on-cube growth was verified by recording polar Φ scans for the SrTiO3(101) 2680

dx.doi.org/10.1021/cg300287r | Cryst. Growth Des. 2012, 12, 2679−2683

Crystal Growth & Design

Article

isosceles triangle are close to 60.00°, matching the observed grain morphology in Figure 2b. The lattice parameter of Fe3O4(111) is 11.874 Å, when calculated from the magnetite bulk lattice parameter of a = 8.396 Å.16 As marked by the red solid line in Figure 4b, higherorder lattice matching can be obtained for a distance of 32.653 Å, giving a mismatch of +1.40% for Fe3O4[11̅0]//SrTiO3[410] and −1.45% for Fe3O4[11̅0]//SrTiO3[110], which are small enough to grow a (111)-oriented Fe3O4 film on a SrTiO3(001) surface. There are four types of equivalent isosceles triangle domains on a SrTiO3(001) surface, as marked by the red dotted and solid lines in Figure 4a, explaining the 12 diffraction peaks in the polar scan of Figure 3b. The well-defined in-plane orientation of the (111)-oriented grains shows that the appearance of multiple grain orientations in a spinel film is related to substrate lattice matching, rather than being an effect of faceting caused by an anisotropic surface energy. Film growth experiments showed that the probability of nucleating (111) domains increases gradually with growth temperature, starting at about 500 °C. When grown at 600 °C, for example, the XRD patterns of the films showed a mixture of (111) and (001) orientations. Self-Template Film Growth. In order to stabilize the (001)-oriented growth of Fe3O4 films at higher temperature and thus improve the crystallinity of the films, a self-template method was employed, as schematically illustrated in Figure 5a.

Figure 3. Polar XRD scans of (a) SrTiO3 101 and (b) Fe3O4 311 reflections. The 12 Fe3O4 reflections every 30° imply the presence of four kinds of Fe3O4(111) domains, with each domain rotated by 15° relative to SrTiO3[100]/[010].

SrTiO3(001) substrate and the Fe3O4(111) film are schematically illustrated in Figure 4. Possible lattice-matching in-plane alignments that are close to the observed 15° twist of the (111)-oriented Fe3O4 grains on the SrTiO3(001) surface are shown in Figure 4a. The red solid line marks the most probable in-plane alignment that optimizes the twist angle, which is 14.04° for the model arrangement. The angles of the Fe3O4

Figure 5. (a) Illustration of the self-template process for growing (001)-oriented Fe3O4 films on a SrTiO3(001) substrate. (b) The temperature history of self-template Fe3O4 film growth, with gray shading denoting the deposition periods of the 8 nm template layer at 400 °C and the bulk of the 150 nm film at 700 °C. Upward and downward arrows denote the start and end of each deposition, respectively. The dotted line represents the pyrometer temperature measurement limit.

At first, an 8-nm-thick magnetite film, corresponding to 10 unit cells of Fe3O4, was deposited at 400 °C as a self-template layer. AFM images of the self-template layer confirmed the presence of an atomically smooth surface with 4 Å steps, originating from the SrTiO3 substrate surface steps. The low growth temperature ensured that the template layer had the (001) orientation. After the template layer growth, the substrate was heated at 50 °C/min to 700 °C, followed by the deposition of a 150 nm Fe3O4 layer. After growth, the films were rapidly cooled by

Figure 4. (a) Schematic illustration of the in-plane relationship between the SrTiO3(001) and Fe3O4(111) planes at the interface. The red solid line marks the most probable in-plane relation that minimizes the lattice mismatch between Fe3O4(111) and SrTiO3(001) at a twist angle of ∼15° relative to the SrTiO3(001) substrate. Four equivalent domain structures of a Fe3O4 film are marked by the red solid and dotted lines. (b) A surface view of an Fe3O4 crystal, showing a triangle that obtains the best higher-order lattice match with SrTiO3. The black solid lines denote single crystal unit cells. 2681

dx.doi.org/10.1021/cg300287r | Cryst. Growth Des. 2012, 12, 2679−2683

Crystal Growth & Design

Article

switching off the sample heating laser. Figure 5b shows a typical temperature history for a Fe3O4 film grown by the self-template method at 700 °C. Upward and downward arrows denote the start and end of film depositions, respectively. The effectiveness of the self-template method is clearly demonstrated by the XRD pattern in Figure 6a. The film shows

Figure 7. Magnetization curves at 5 K for Fe3O4 thin films prepared by the conventional (400 °C) and self-template (700 °C) methods. The saturated magnetization of 4 μB/fu is consistent with the bulk Fe3O4 magnetization.

strongly affected by the presence of antiphase boundaries (APB), which have been reported to induce anomalous magnetization effects in Fe3O4 films.1 It is likely that a similar change in the APB structure occurs here due to the crystallinity change. Figure 8 shows the resistivity behavior of Fe3O4 films grown conventionally at 400 and 700 °C and by the 700 °C self-

Figure 6. An XRD pattern (a) of an Fe3O4 thin film grown by the selftemplate method at 700 °C. (b) Comparison of XRD rocking curves of Fe3O4 004 reflections for a film grown conventionally at 400 °C and by the self-template method at 700 °C. (c) AFM image of a Fe3O4 thin film grown by the self-template method at 700 °C.

no (111)-oriented Fe3O4 reflections, despite being grown at 700 °C. The thin self-template layer thus stabilized the (001)oriented growth of a Fe3O4 film. The high-temperature growth resulted in greatly improved crystallinity, as shown by the comparison of the Fe3O4(004) reflection rocking curves in Figure 6b. When a magnetite film was grown at 400 °C, the rocking curve width was 1.41°, as mentioned before. In contrast, the self-template film grown at 700 °C had a rocking curve width of only 0.63° and a much higher intensity, although the total film thickness was the same. Furthermore, the reciprocal space mapping analysis around SrTiO3(103) exhibited a sharper Fe3O4(206) peak when the self-template method was used. The in-plane coherence length28 of a Fe3O4 film grown by the self-template method was estimated at 35 nm, much larger than for a film grown by the conventional method, which had a coherence length of 12.7 nm. The crystallinity improvement can also be seen indirectly in AFM images because the grain size of the self-template film in Figure 6c is significantly larger than for the film shown in Figure 1b. A thin self-template layer can thus be concluded to play an essential role in forcing an Fe3O4 film to grow along the (001) direction on a SrTiO3 substrate, even at high growth temperatures. Figure 7 shows a comparison of magnetization curves measured at 5 K for Fe3O4 films prepared by conventional growth at 400 °C and by the self-template method at 700 °C. At applied fields of over 0.5 T, both samples reach a saturation magnetization of approximately 4 μB/fu, consistent with the bulk Fe3O4 magnetization.19 However, there is a difference in the hysteresis curve shapes. Both films have the same crystal orientation and the change in the remanent field values appears to be related to the crystallinity or grain size change. It is known for the Fe3O4/MgO case that the hysteresis loop shape is

Figure 8. Temperature dependence of resistivity for Fe3O4 thin films grown by the conventional (400 and 700 °C) and the self-template (700 °C) methods. The Verwey temperature is visible only in two (001)-oriented films at the expected bulk temperature.

template method. The Fe3O4 film conventionally grown at 700 °C exhibited higher resistance than the other samples and a gradual increase of resistivity with the decrease of the measurement temperature. The degraded behavior is a manifestation of the intermixing of the four triangle domain structures in Fe3O4 films grown conventionally at 700 °C, as was previously discussed in reference to Figures 3 and 2. In contrast, the other two samples exhibit a metallic state with lower resistance at room temperature and a discontinuous transition to an insulating state at 120 K due to the Verwey transition, consistent with the bulk Fe3O4 behavior.19−21 No hysteresis was observed between the cooling and heating over the Verwey temperature. In general, the Verwey transition temperature is known to be very sensitive to deviations from the ideal oxygen stoichiometry in magnetite single crystals.21 As shown by the resistivity data, there was no shift of the transition temperature even as the growth temperature was increased, showing that the oxygen stoichiometry of the films was correct, regardless of the growth temperature. 2682

dx.doi.org/10.1021/cg300287r | Cryst. Growth Des. 2012, 12, 2679−2683

Crystal Growth & Design

■

Article

(25) Ohnishi, T.; Shibuya, K.; Lippmaa, M.; Kobayashi, D.; Kumigashira, H.; Oshima, M.; Koinuma, H. Appl. Phys. Lett. 2004, 85, 272−274. (26) Ohashi, S.; Lippmaa, M.; Nakagawa, N.; Nagasawa, H.; Koinuma, H.; Kawasaki, M. Rev. Sci. Instrum. 1999, 70, 178−183. (27) Lüdders, U.; Bibes, M.; Bobo, J. F.; Fontcuberta, J. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 427−431. (28) Ohtomo, A.; Kimura, H.; Saito, K.; Makino, T.; Segawa, Y.; Koinuma, H.; Kawasaki, M. J. Cryst. Growth 2000, 214/215, 284−288.

CONCLUSIONS We have demonstrated self-template control of crystallographic orientation in Fe3O4 spinel thin films grown on SrTiO3(001) substrates. The appearance of multiple grain orientations in spinel films was shown to be related to lattice matching at the initial growth stage. A thin self-template layer deposited at 400 °C stabilized the (001)-oriented growth of Fe3O4 thin films at higher temperatures, greatly improving the film crystallinity and grain size. This growth technique is particularly useful for preparing thin film samples for the characterization of magnetic, transport, and dielectric properties where the introduction of a buffer layer with a different chemical composition may strongly affect the measurement results.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the Asahi Glass Foundation, Futaba Electronics Memorial Foundation, and the Murata Science Foundation for funding.

■

REFERENCES

(1) Margulies, D. T.; Parker, F. T.; Spada, F. E.; Goldman, R. S.; Li, J.; Sinclair, R.; Berkowitz, A. E. Phys. Rev. B 1996, 53, 9175−9187. (2) Suzuki, Y.; van Dover, R. B.; Gyogy, E. M.; Phillips, J. M.; Korenivski, V.; Werder, D. J.; Chen, C. H.; Cava, R. J.; Krajewski, J. J.; Peck, W. F.; Do, K. B. Appl. Phys. Lett. 1996, 68, 714−716. (3) Suzuki, Y. Annu. Rev. Mater. Res. 2001, 31, 265−289. (4) Thang, P. D.; Rijnders, G.; Blank, D. H. A. J. Magn. Magn. Mater. 2007, 310, 2621−2623. (5) Wang, Y.; Li, D. F.; Dai, J. Y. J. Cryst. Growth 2010, 313, 26−29. (6) Lee, H. N.; Hesse, D.; Zakharov, N.; Gosele, U. Science 2002, 296, 2006−2009. (7) Lee, H. N.; Hesse, D. Appl. Phys. Lett. 2002, 80, 1040−1042. (8) Takahashi, R.; Yonezawa, Y.; Nakajima, K.; Chikyow, T.; Koinuma, H.; Matsumoto, Y. Appl. Phys. Lett. 2006, 88, No. 152904. (9) Imai, A.; Nagarajan, V.; Takahashi, R.; Lippmaa, M.; Matsumoto, Y. Cryst. Growth Des. 2010, 10, 5233−5237. (10) Kan, D.; Takeuchi, I. J. Appl. Phys. 2010, 108, No. 014104. (11) Noguera, C. J. Phys.: Condens. Matter 2000, 12, R367−R410. (12) Susaki, T.; Kumada, S.; Katase, T.; Matsuzaki, K.; Miyakawa, M.; Hosono, H. Appl. Phys. Express 2009, 2, No. 091403. (13) Takahashi, R.; Valset, K.; Folven, E.; Eberg, E.; Grepstad, J. K.; Tybell, T. Appl. Phys. Lett. 2010, 97, No. 081906. (14) Matvejeff, M.; Lippmaa, M. J. Cryst. Growth 2010, 312, 2386− 2392. (15) JCPDS Card 35-0734. (16) JCPDS Card 19-0629. (17) Mishra, R. J.; Thomas, G. J. Appl. Phys. 1977, 48, 4576−4580. (18) Ohtomo, A.; Hwang, H. Y. Nature 2004, 427, 423−426. (19) Walz, F. J. Phys.: Condens. Matter 2002, 14, R285−R340. (20) Verwey, E. J. W. Nature 1939, 144, 327−328. (21) Aragon, R.; Buttrey, D. J.; Shepherd, J. P.; Honig, J. M. Phys. Rev. B 1985, 31, 430−436. (22) Kato, K.; Iida, S. J. Phys. Soc. Jpn. 1982, 51, 1335−1336. (23) Alexe, M.; Ziese, M.; Hesse, D.; Esquinazi, P.; Yamauchi, K.; Fukushima, T.; Picozzi, S.; Gösele, U. Adv. Mater. 2009, 21, 4452− 4455. (24) Kawasaki, M.; Takahashi, K.; Maeda, T.; Tsuchiya, R.; Shinohara, M.; Ishiyama, O.; Yonezawa, T.; Yoshimoto, M.; Koinuma, H. Science 1994, 266, 1540−1542. 2683

dx.doi.org/10.1021/cg300287r | Cryst. Growth Des. 2012, 12, 2679−2683