Orientation-Controlled Growth of Pt Films on SrTiO3 (001) by Atomic

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Orientation-controlled Growth of Pt Films on SrTiO3 (001) by Atomic Layer Deposition Jung Joon Pyeon, Jun-Yun Kang, Seung-Hyub Baek, ChongYun Kang, Jin-Sang Kim, Doo Seok Jeong, and Seong Keun Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03007 • Publication Date (Web): 11 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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Chemistry of Materials

Orientation-controlled Growth of Pt Films on SrTiO3 (001) by Atomic Layer Deposition Jung Joon Pyeon,†,‡ Jun-Yun Kang,§ Seung-Hyub Baek,†,⊥ Chong-Yun Kang,†, ‡ Jin-Sang Kim,† Doo Seok Jeong,†,⊥ and Seong Keun Kim†,⊥,* †

Center for Electronic Materials, Korea Institute of Science and Technology, Seoul 136-791, Korea

‡

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea

§

Ferrous Alloy Department, Korea Institute of Materials Science, Changwon, 642-831, Korea

⊥

Department of Nanomaterials Science and Engineering, Korea University of Science and Technology, Daejeon 305333, Korea

ABSTRACT: We grew Pt films on TiO2-terminated SrTiO3 (001) by atomic layer deposition, using trimethyl(methylcyclopentadienyl)-platinum(IV) as the Pt source and O2 and O3 as the oxidants. The orientation of the Pt films grown with O2 varied from (111) to (001) as the growth temperature was increased from 220 °C to 350 °C, while the Pt films grown with O3 have a strong preference for (111) orientation even at a high growth temperature of 350 °C. The difference in the orientation of the Pt films on SrTiO3 (001) was attributed to changes in the degree of chemical bonding across the Pt/SrTiO3 interface with respect to the oxidant. We observed an increase in Pt-O bonding at the interface between the Pt grown with O3 and the SrTiO3 substrate. The interfacial energy of Pt(111)||SrTiO3 (001) may have been significantly decreased by the increase in Pt-O bonding at the interface, which eventually led to the strong (111) preference of the Pt grown with O3. The findings provide the possibility of controlling the orientation of Pt without manipulating the kinetic energy of crystal growth.

INTRODUCTION Metal/oxide heterostructures have important applications in fields such as electronic devices and catalysis. The metal/oxide heterojunctions can form either ohmic contacts or Schottky barriers, depending on the work function of the metal and the electron affinity of the oxide. This permits the use of heterostructures in electronic devices such as diodes and transistors. Oxide-supported metal nanoparticles are also known to drastically enhance the catalytic activity of the metal.1-5 A particularly interesting example of these is the Pt/SrTiO3 heterostructure, which has drawn attention because of the epitaxial growth of Pt on SrTiO3. Epitaxial Pt layers on SrTiO3 have been widely used as stable bottom electrode materials for the use of functional perovskite oxides in applications in ferroelectrics and microwave capacitors.6-8 The orientation of the underlying Pt electrode governs the crystallographic orientation and, consequentially, the properties of the functional oxides. In addition, the Pt/SrTiO3 heter-

ostructure has been studied as a model photocatalyst for fuel production.9, 10 The catalytic activity of Pt is strongly affected by the orientation of the exposed faces of the metal. Therefore, controlling the orientation of Pt is imperative to attain desirable properties for the applications. Pt has a face-centered cubic structure with a lattice constant of 0.3924 nm, which is very close to that (a = 0.3905 nm) of SrTiO3. This excellent lattice match between Pt and SrTiO3 may enable the epitaxial growth of Pt (001) thin films on SrTiO3 (001) because of the low interfacial energy between the materials. The growth of Pt films on SrTiO3 (001) has been reported by several techniques such as pulsed laser deposition, molecular beam epitaxy, and sputtering.6, 7, 11-15 However, the growth of (001)-oriented Pt films occurred under very limited growth conditions; Pt films with (111) and (110) orientations were more frequently observed on SrTiO3 (001). One of the key parameters in controlling the orientation of Pt films on SrTiO3 (001) is growth temperature. Kahsay et al.12 reported that by a RF sputtering method, (001)-

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oriented Pt films were grown epitaxially on SrTiO3 (001) at temperatures above 750 °C, while (111)-oriented Pt films were obtained at lower temperatures. Zhao et al.13 also reported that Pt films grown at low temperatures from 300–500 °C had mixed orientations of (111) and (001) on SrTiO3 (001), while at 700 °C, (001)-oriented Pt films grew epitaxially on SrTiO3 (001). This temperature dependency of the Pt orientation indicates that the Pt (001) orientation is more energetically favorable on SrTiO3 (001) than the (111) orientation. (111)-oriented Pt grains appear under conditions of limited kinetic energy. Another decisive factor in determining the orientation of Pt on SrTiO3 (001) is the surface termination of the SrTiO3 substrate. The effect of the surface termination of SrTiO3 (001) on the orientation of Pt has been studied theoretically11, 16, 17 and experimentally.11, 14 It was reported that TiO2-terminated SrTiO3 (001) surfaces favor the epitaxial growth of Pt (001) more than SrO-terminated SrTiO3 (001) surfaces, despite the substrates possessing equal lattice mismatches. This is due to the difference in the surface energy. This indicates that differences in the degree of chemical bonding at the Pt/SrTiO3 interface induces a change in the interfacial energy, eventually influencing the orientation of the Pt film grown on SrTiO3.14 Atomic layer deposition (ALD) may be a good growth technique for Pt films in the examination of the effects of chemical bonding across the Pt/SrTiO3 interface on the orientation of the Pt film. In contrast to physical vapor deposition techniques, including sputtering and pulsed laser deposition, ALD relies on chemical reactions between the precursor and substrate, or reaction surface. Therefore, the degree of the chemical bonding across the interface could be controlled by the deposition parameters used for Pt ALD growth. From a practical point of view, the ALD growth of Pt nanoparticles on SrTiO3 has attracted great interest for catalytic applications, because the ALD process achieves efficient loading of Pt by forming well-dispersed Pt nanoparticles and drastically enhances the effective surface area of the Pt by using a support with a complex shape. Christensen et al.9 reported the ALD of Pt nanoparticles on SrTiO3 nanocubes with {001} faces for application as a heterogeneous catalyst. Although the size, dispersion, and chemical state of ALD Pt nanoparticles on SrTiO3 were investigated in previous studies,9, 18, 19 the orientation of ALD Pt on SrTiO3 (001) has not yet been examined, despite the significant influence of Pt film orientation on the catalytic activity of the metal. Here, we investigated the orientation of Pt films grown on single-crystal TiO2-terminated SrTiO3 (001) by ALD in a wide growth temperature range from 220 to 350 °C. We used two different oxidants, O2 and O3, to change the degree of chemical bonding across the Pt/SrTiO3 interface. The ALD growth of the Pt films was based on the oxidative reaction between the Pt precursor and the oxidant. The difference in oxidation potentials between O2 and O3 induced different initial growth behaviors. We demonstrated that Pt films grown with O2 were preferentially (001)-oriented on SrTiO3 (001), while Pt films grown with

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O3 had a strong preference of (111) orientation, despite the same growth conditions on SrTiO3. EXPERIMENTAL SECTION Pt thin films were grown by ALD using a custom-built travelling-wave reactor. The film growth was performed at temperatures ranging from 220 to 350 °C. Singlecrystalline TiO2-terminated SrTiO3 (001) (MTI Corp.) was used as the substrate. To obtain atomically flat TiO2terminated SrTiO3 surfaces, the SrTiO3 substrates were etched in buffered HF and then annealed at 1000 °C for 2 h under flowing O2. Trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3, Strem Chemicals, Inc., purity 99%) was used as a Pt precursor. The canister containing the Pt precursor was maintained at 40 °C and the gas line for the delivery of the Pt precursor to the reactor was heated to 70 °C. The static exposure mode was applied for the injection of the Pt precursor to minimize the precursor consumption. A detailed report on the ALD growth of Pt films is located elsewhere.20 O2 and O3 were used as oxidant species. The concentration of O3 was fixed at 180 g/Nm3. The feeding and purging time of the oxidant were fixed to 3 and 5 s, respectively, for both types of oxidant. The preferred crystallographic orientation of the Pt films was examined by θ–2θ X-ray diffraction (XRD, Rigaku, ATX-G). Micro-texture analyses were also performed using electron backscatter diffraction (EBSD), using a NordlysNano camera and AZtec software provided by Oxford Instruments. The surface morphology was observed by atomic force microscopy (AFM, Veeco, Nanoscope Dimension 3100). X-ray photoelectron spectroscopy (XPS, Ulvac-PHI, PHI 5000 VersaProbe) was applied to examine the chemical binding states of the Pt films at very early growth stages. RESULTS AND DISCUSSION As described in the introduction, the preferred orientation of the Pt film on SrTiO3 (001) is known to depend strongly on the growth temperature, because of the limit of the kinetic energy. Therefore, the crystallographic orientations of Pt films grown at temperatures ranging from 220 to 350 °C were examined. Figure 1(a) shows the θ–2θ XRD patterns of Pt films grown at 220, 300, and 350 °C using O2 as the oxidant. The preferred orientation of the Pt films changes from (111) to (001) as the growth temperature increases. The Pt film grown at 220 °C shows only a Pt (111) peak, whereas a Pt (002) peak appears in the pattern from the film grown at 300 °C; the intensity of the Pt (002) peak is stronger while that of the Pt (111) peak is considerably weaker at 350 °C. This observation is consistent with previous examinations of the growth of Pt films by MBE21 and sputtering12 in that (111)-oriented Pt grains dominate at low growth temperatures, while Pt films grow epitaxially with (001) orientation on SrTiO3 (001) at high temperatures. A Pt (111) peak in the XRD


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Figure 2. θ–2θ XRD patterns of (a) the Pt film grown at 350 o C with O3 for 400 cycles after the ALD of the Pt layer with o O2 for 70 cycles and (b) the Pt film grown at 350 C with O2 for 400 cycles after the ALD of the Pt layer with O3 for 70 cycles.

Figure 1. θ – 2θ XRD patterns of the Pt films grown at 220, o 300, and 350 C on SrTiO3 with (a) O2 and (c) O3 as the oxidant species, respectively. The normal-direction EBSD mappings of the Pt grown at 350 °C on SrTiO3 with (b) O2 and (d) O3, respectively.

pattern of the Pt film grown at 350 °C may disappear in the XRD patterns from a film produced at a higher growth temperature. However, higher growth temperatures are out of the ALD window, as the thermal decomposition of the organometallic Pt precursor may disturb the growth of the film. An EBSD technique was also employed to characterize the Pt film with a stronger (001) orientation preference. The normal-to-substrate EBSD mapping of the Pt film grown at 350 °C with O2 shows a high degree of (001) orientation as shown in Fig. 1 (b), consistent with the XRD results in Fig. 1 (a). These findings imply that, in the ALD of Pt with O2, the Pt (001)||SrTiO3 (001) interface has a low interfacial energy and the kinetic energy is sufficient for Pt adatoms to diffuse to energetically favorable sites at 350 °C. Interestingly, the Pt films grown on SrTiO3 (001) with O3 have different orientation preferences than those grown with O2, despite having the same growth conditions. As shown in Fig. 1 (c), the XRD pattern of the Pt films grown with O3 shows only a Pt (111) peak throughout the growth temperature range. A Pt (002) peak does not appear even at a high temperature of 350 °C. The normaldirection EBSD mapping of the Pt grown at 350 °C in Fig. 1 (d) also shows a strong (111) preference, although the EBSD mapping also reveals some grains with (001) and (110) orientations. The growth per cycle in the ALD process of Pt using MeCpPtMe3 and O2 was reported to be almost identical to that in the ALD process of Pt using MeCpPtMe3 and O2 above 300 °C.22 This indicates that changes in the growth rate do not account for the change of the Pt orientation in this case. This finding indicates that the preferential orientation of Pt films may be determined by the oxidant in the ALD process as well as the growth temperature and surface termination of the SrTiO3 substrate.

The difference in the oxidant species in the ALD processing of Pt films may lead to different reaction mechanisms for the formation of the films. The orientation of the Pt films may be determined by the ALD reaction pathway, regardless of the substrate. For the verification of this possibility, we examined the orientation of Pt films grown by two-step ALD processes, composed of MeCpPtMe3-O2 and MeCpPtMe3-O3 steps. Figure 2 (a) shows the θ–2θ XRD pattern of the Pt film grown at 350 °C with O3 for 400 cycles after the ALD of a Pt layer with O2 for 70 cycles. The θ–2θ XRD pattern of the Pt film grown at the same temperature by a single-step reaction with O3 for 400 cycles is also displayed in Fig. 2 (a). Considering the orientation of the Pt films in Fig. 1 (a), the thin Pt layer grown with O2 for 70 cycles should be strongly (001)-oriented. If the ALD reaction and oxidant species determines the orientation of the Pt films, the upper Pt layer grown with O3 should have a strong preferential (111) orientation rather than (001) orientation, regardless of the orientation of the Pt underlayer. However, the Pt film grown by the two-step process has a strong (001) orientation, indicating that the ALD chemical reaction between MeCpPtMe3 and O3 does not determine the orientation of the Pt film. Although the XRD pattern of the Pt film shows a weak Pt (111) peak, the pattern of the Pt film is comparable to that of the Pt film grown by the single-step process with O2. We examined the opposite case as well. In Fig. 2 (b), the Pt film grown at 350 °C with O2 for 400 cycles after the ALD of Pt with O3 for 70 cycles shows a very strong (111) orientation and no (002) peak, although the Pt film grown by a single step with O2 has a strong preferential (001) orientation. These findings imply that the orientation of the Pt film on SrTiO3 (001) is governed by the properties of the Pt/SrTiO3 interface, rather than the ALD reaction conditions. Herein, a possible mechanism is suggested for the change in the interfacial properties by the use of different oxidant species in the ALD of Pt films. The surface of SrTiO3 may be reconstructed by the flow of O3, which has a strong oxidation potential, during the ALD process of Pt. If the surface reconstruction of SrTiO3 occurs by the flow of O3, the change in the interfacial energy caused by the surface reconstruction of the SrTiO3 may influence the orientation of the Pt film formed on the substrate. To understand the effect of the O3 flow on the SrTiO3, a



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of O3 does not cause the change in the orientation of Pt observed in Fig. 1.

Figure 4. AFM images of Pt layers on SrTiO3 (001) using (a) O2 for 20 cycles and (b) O3 for 15 cycles. XPS spectra of Pt 4f core level in the Pt on SrTiO3 (001) using (c) O2 for 20 cycles and (d) O for 15 cycles. The XPS spectra of (e) Sr 3d and (f) Ti 2p core levels in the Pt layers on SrTiO3 (001) using O2 and O3, respectively.

Although the mechanism for the ALD of Pt with MeCpPtMe3 and O3 is almost identical to that with MeCpPtMe3 and O2 in the steady-state growth stage, because of the recombination of most of the O3 to O2 on the Pt surface,23-26 the two ALD processes may differ in the very early growth stages as the reaction dominating initial growth in ALD generally deviates from that in the steadystate growth.27 In particular, O3 recombination in Pt ALD does not occur early in the growth process.26 To further elucidate the difference in the degree of the chemical bonding across the Pt/SrTiO3 interface with respect to the oxidant species, we examined the chemical states of Pt in the very early ALD growth stage on SrTiO3, using both O2 and O3 as oxidant species. Prior to the examination of the chemical binding states of the Pt, the surface morphologies of the Pt layers on SrTiO3 were observed. In the ALD of Pt, Pt grows on oxides in the Volmer-Weber growth mode with a long incubation cycle.28 Therefore, a lower distribution density of the Pt islands may obstruct the analysis of the chemical bonding across the Pt/SrTiO3 interface. Figures 4 (a) and (b) show AFM images of the Pt films on SrTiO3 at 350 °C using O2 and O3 for 20 and 15 cycles, respectively. Considering the growth per cycle, the thickness of each film is less than 2 nm. Small Pt grains are observed to cover most of the SrTiO3 surfaces, despite the short ALD cycles in both cases. Therefore, we examined the chemical binding states of both Pt/SrTiO3 heterostructures. Figure 4 (c) and (d) show the XPS spectra of the Pt 4f core level in the ALD Pt on SrTiO3 using O2 and O3, respectively. The XPS spectra reveal that, at the interface with SrTiO3, the Pt grown with O2 is mainly composed of metallic Pt with small amounts of PtO and PtO2 phases. The Pt grown with O3 consists mainly of PtO and PtO2 phases. This indicates that the degree of chemical bonding at the Pt/SrTiO3 interface is significantly changed by the oxidant species. The chemical binding states of the Pt-coated SrTiO3 substrates were examined as well. Figure 4 (e) and (f) show the XPS spectra of Sr 3d and Ti 2p core levels, respectively, in the SrTiO3 substrates. The intensity of the Sr 3d and Ti 2p peaks is almost identical in the spectra of samples formed using O2 and O3. This supports the similarity in Pt thickness in both Pt/SrTiO3 heterostructures. Each ion of Sr and Ti in the platinized SrTiO3 has a single binding state corresponding to SrTiO3, regardless of the oxidant species. This indicates that the Pt/SrTiO3 heterostructures have a sharp interface without interdiffusion of Pt and SrTiO3 during the ALD of Pt.

SrTiO3 (001) substrate was exposed to an O3 flow at 350 °C for 5 min followed by the ALD of a Pt film with O2 as the oxidant species. Figure 3 shows the θ–2θ XRD pattern of the Pt films grown on SrTiO3 (001) with O2 as the oxidant both with and without the O3 pretreatment. Despite the O3 pretreatment of the SrTiO3 substrate, the intensity of the Pt (002) peak is not suppressed, which demonstrates that the surface reconstruction of SrTiO3 (001) by the flow

It was reported that γPt{111}/γPt{001} (where γ is the surface energy) is 0.84.29 In general, therefore, the preferential growth of Pt (111) is more favorable than the growth of Pt (001) on a lattice-mismatched substrate. However, the small interfacial energy induced by the low degree of lattice mismatch between the Pt and SrTiO3 reduces the total energy, inducing the growth of Pt (001) on SrTiO3 (001). Neglecting the energy contribution from the internal strain of the Pt films for the purpose of simplification,

Figure 3. θ–2θ XRD pattern of the Pt films grown on SrTiO3 (001) with O2 as the oxidant with and without O3 pretreatment, respectively.


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the following inequality is satisfied for the growth of Pt (001) on SrTiO3 (001):30 γPt(111) + γi(111) > γPt(001) + γi(100)

0082471) and by the Korea Institute of Science and Technology (KIST through 2E25440).

(1)

where γi(111) and γi(001) are the interfacial energy of Pt (111)/SrTiO3 (001) and Pt (001)/SrTiO3 (001), respectively. In the Pt ALD using O3, however, Pt grains with a strong (111) preference are predominant. This indicates that, in the ALD of Pt using O3 on SrTiO3 (001), the inequality (1) is not satisfied and the sign of the inequality should be reversed. The γi(111) requires a large reduction to meet the condition in inequality (1). From this perspective, the difference in the chemical bonding states of Pt grown with O2 and O3, as observed in Fig 4. (c) and (d), is believed to cause the change in the preferential orientation of Pt films on SrTiO3 (001). In particular, an increase in the amount of Pt-O bonding at the interface of the Pt film grown with O3 on SrTiO3 could significantly decrease γi(111) compared to that in the case of direct contact between Pt and SrTiO3. CONCLUSION Controlling the orientation of Pt films grown on singlecrystal TiO2-terminated SrTiO3 (001) by ALD using O2 and O3 was investigated. In general, Pt(001)||SrTiO3(001) is more energetically favorable than Pt(111)||SrTiO3(001), because of the small lattice mismatch between Pt and SrTiO3. In the Pt films grown with O2, Pt (001) orientation is strongly preferred at a high growth temperature of 350 °C, at which the kinetic energy is sufficient for Pt adatoms to diffuse to energetically favorable sites. Meanwhile, Pt films grown with O3 have strong preferential (111) orientations, despite the films having the same growth temperatures. This change in preferred orientation is attributed to the increase in Pt-O bonding at the Pt/SrTiO3 interface in ALD with O3, which reduces the interfacial energy of Pt(111)||SrTiO3(001). This finding provides the possibility of controlling the orientation of Pt films for applications such as catalysts and microelectronics.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT This work was supported by the Future Semiconductor Device Technology Development Program (10047231) funded by MOTIE (Ministry of Trade, Industry & Energy) and KSRC (Korea Semiconductor Research Consortium) and by the Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2009-

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A TOC graph


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Orientation-Controlled Growth of Pt Films on SrTiO3 (001) by Atomic

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