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Growth of epitaxial Pt thin films on (0 0 1) SrTiO3 by rf magnetron sputtering
Applied Surface Science 306 (2014) 23–26
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth of epitaxial Pt thin films on (0 0 1) SrTiO3 by rf magnetron sputtering A. Kahsay a , M.C. Polo a,∗ , C. Ferrater a , J. Ventura a , J.M. Rebled b , M. Varela a a b
Departament de Física Aplicada i Òptica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain Departament d’Electrònica, Universitat de Barcelona Institut de Nanociència i Nanotecnologia IN 2UB, 08028 Barcelona, Spain
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 20 January 2014 Accepted 20 January 2014 Available online 5 February 2014 Keywords: Thin films Metallic Sputtering Epitaxial growth Perovskite
a b s t r a c t The growth of platinum thin film by rf magnetron sputtering on SrTiO3 (0 0 1) substrates for oxide based devices was investigated. Platinum films grown at temperatures higher than 750 ◦ C were epitaxial ([1 0 0]Pt(0 0 1)//[1 0 0]STO(0 0 1)), whereas at lower temperatures Pt(1 1 1) films were obtained. The surface morphology of the Pt films showed a strong dependence on the deposition temperature as was revealed by atomic force microscopy (AFM). At elevated temperatures there is a three-dimensional (3D) growth of rectangular atomically flat islands with deep boundaries between them. On the other hand, at low deposition temperatures, a two-dimensional (2D) layered growth was observed. The transition from 2D to 3D growth modes was observed that occurs for temperatures around 450 ◦ C. The obtained epitaxial thin films also formed an atomically sharp interface with the SrTiO3 (0 0 1) substrate as confirmed by HRTEM. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Multifunctional devices based on heterostructures that include perovskite (ABO3 ) oxides have been widely investigated due to the possibility of exploiting their beneficial properties in a lot of technological applications. SrTiO3 (STO) highlights among all perovskites, exhibiting the largest mobility and a very low carrier density threshold for metallicity, and it is frequently used as substrate for different electronic devices [1]. Otherwise, integration of these devices requires stable electrodes and both, the choice of the contact material and the interface quality, are crucial for a proper operation. One of the most used electrode material is platinum (Pt) having lattice parameters very similar to STO, because of its low resistivity, high thermal stability, and high stability under an oxygen environment at high temperature [2]. On the other hand, the deposition method chosen for the growth of Pt electrodes determines their properties, especially in the case of pulsed laser deposition (PLD) and sputtering, for which the kinetic energy of the deposited species exceeds their thermal energy. The synthesis of oriented (1 1 1) Pt films on STO substrates has been reported either by PLD [3] or by magnetron sputtering [4]. In a previous paper we studied the in situ growth of Pt thin films onto (0 0 1) STO by means of PLD [5]. It was observed that the increment of the temperature up to 400 ◦ C promoted (1 1 1)
∗ Corresponding author. Tel.: +34934039222. E-mail address: [email protected] (M.C. Polo). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.123
Pt orientation. Indeed, despite the nearly matching between the lattice parameters of STO and Pt, the growth of epitaxial (0 0 1) oriented Pt films on (0 0 1) STO is still a challenge due to the three dimensional island growth with which metals tend to grow on oxide substrates [6]. In the literature there are only a few papers reporting such experimental results [7–9] and recent theoretical studies based on density functional theory suggested that the natural growth mode for Pt (0 0 1) on STO (0 0 1) is the Volmer-Weber island formation [10]. In the present work we investigate the growth of epitaxial Pt thin films on STO (0 0 1) substrates by rf magnetron sputtering. The study is focused on how the growth temperature affects the morphology and structure properties of the Pt films and on their evolution varying the film thicknesses. Furthermore, the electrical properties of the Pt/STO system have been characterized and discussed. 2. Experimental Platinum thin films were grown on (0 0 1) SrTiO3 substrates by sputtering of pure (99.999%) platinum target, supplied by Birmingham Metal, by Argon gas in rf magnetron sputtering system. The system base pressure was 10−5 Pa and throughout the deposition process an Argon partial pressure of 10−2 Pa was maintained. In order to remove the platinum target surface contamination, a presputtering cleaning process was done prior to the films growth on bare substrates (not previously annealed). During the deposition, the STO substrates were placed 8 cm apart from the Pt target in
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A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
Fig. 1. AFM topography images of 30 nm-thick Pt films grown on SrTiO3 (0 0 1) at different deposition temperatures: (a) 400 ◦ C and (b) 800 ◦ C.
an on-axis configuration. With the aim to study the temperature and thickness dependences of the growth of Pt on STO substrates two set of Pt/STO(0 0 1) samples were obtained under different deposition conditions. In the first series (temperature series) the growth temperature was varied over a range of 400–800 ◦ C keeping all other parameters constant. The second series of samples was obtained varying only the deposition time between 5 min and 60 min (thickness series), setting the growth temperature to 500 ◦ C. The films of thickness series were grown at a deposition rate of 1 nm/min by controlling of discharge electrical power. The morphology of the obtained films was characterized by atomic force microscopy (AFM) both in tapping mode and in contact mode. The windows scanning microscope software (WSXM) was used to analyze the distribution and morphology of platinum surfaces [11]. Moreover X-ray diffraction measurements of the Pt/SrTiO3 samples were performed in order to analyze their crystalline quality and calculate the lattice parameters. In order to further investigate the structure of the films, cross section Pt/STO(0 0 1) samples were prepared and studied by means of high resolution transmission electron microscopy (HRTEM). 3. Results and discussion Fig. 1 illustrates the observed growth modes of Pt thin films deposited on STO substrate at different deposition temperatures.
Whereas the Pt film obtained at 400 ◦ C shows a nearly flat and continuous surface, the film grown at 800 ◦ C exhibits a surface with deep holes and big squared grains with parallel edges suggesting that platinum crystals have been grown in the (0 0 1) direction. These features seem to indicate that there is a transition from two-dimensional (2D) to three-dimensional (3D) growth in the deposition of the Pt films with increasing the deposition temperature. In fact, there has been a decrease in the substrate coverage in the layers grown at increasing deposition temperatures presumably because of metal agglomeration on dielectric materials [12]. The inset of Fig. 1 shows stepped edges of around 4 nm at the surface of the films, confirming the layer-by-layer growth at low deposition temperatures. In our previous work, that mentioned before in which we reported the growth of Pt films on STO(0 0 1) by PLD [5], we had not explore the growth behavior at deposition temperatures higher than 400 ◦ C and the films showed granular continuous surfaces underneath this temperature value. However, a similar growth change from 2D to 3D was found by Galinski et al. [13] in the case of using PLD to grow Pt films onto single-crystal Y2 O3 -stabilized ZrO2 (0 0 1) (YSZ) when deposition temperature exceeded 400 ◦ C. These authors concluded that the capillary forces had a predominant impact on thin film growth at high deposition temperatures. They also predicted that this impact of capillary forces had to be present on the Pt thin film formation by sputtering. Our experimental results have confirmed such assumption
Fig. 2. AFM topography images of Pt thin films grown on SrTiO3 (0 0 1) at 700 ◦ C with different values of thickness: (a) 10 nm and (b) 50 nm.
A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
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Fig. 3. XRD /2 scans for Pt/STO(0 0 1) samples grown at different temperatures. (1 1 1), (1 0 0) and (1 1 0) Pt peaks are present for deposition temperatures lower than 750 ◦ C. Films grown at temperatures higher than 750 ◦ C are c-axis oriented, with no appreciable traces of other orientations.
even although STO material instead of YSZ was employed as a substrate. It can be explained by the low values of deposition rate with whom the layers have been grown. The morphology-thickness dependence of the surface of Pt films deposited at 700 ◦ C >is illustrated in Fig. 2. From the AFM observations it has been deduced that as thickness increased, the grain size, roughness and surface coverage of the film were also increased. In fact, topography images showed that the films evolved from an early stage of nucleation of islands to a later stage of elongated islands and percolation. This is in good agreement with the experimental findings on the growth of metals on oxide substrates and with the published models [14]. From the point of view of the crystalline structure, the Pt films showed different type of X-ray diffractograms depending on the deposition temperature (Fig. 3). In increasing order of crystalline quality, the observed features range from the polycrystalline structure corresponding to the sample grown at 600 ◦ C, to the (0 0 l) textured Pt films deposited at temperatures higher than 750 ◦ C. This is in agreement with published density functional calculations which reveal that the interfacial energy of Pt(1 0 0) must be less than that of Pt(1 1 1) in order to, based on energy considerations alone, select the (1 0 0) orientation over (1 1 1) [7,15,16]. These (0 0 l) textured Pt films also showed a good out-of-plane alignment, as revealed by the narrow rocking curves of the (0 0 2) reflections, with full width at half maximum (FWHM) values of 0.22◦ (Fig. 4a), close to STO substrate value (0.08◦ ), and good inplane alignment with the substrate. This alignment is cube on
Fig. 4. (a) Rocking curves of (0 0 4) reflections of Pt thin film grown at 800 ◦ C for 30 min (FWHM of 0.22◦ ) and STO substrate (FWHM of 0.08◦ ), showing a good outof-plane film to substrate alignment. (b) Pt(1 1 1) and STO(1 1 1) PHI scans showing that the in-plane alignment of Pt crystallites onto STO substrate is cube on cube, (with no other in-plane alignments). (c) Reciprocal space map of (2 0 4) reflection of a 30 nm Pt film grown on STO(0 0 1) at 800 ◦ C, showing both, the epitaxial cube on cube growth, and the strain of the Pt crystallites, which adapted their in-plane cell parameter to that of substrate.
cube with low mosaicity, as the narrow Pt peaks showed in the phi-scans (Fig. 4b). For these films, the reciprocal space maps of (2 0 4) reflections (Fig. 4c), confirmed the film to substrate in-plane alignment, and revealed an in-plane strain of the crystals, which reduced their in-plane cell parameter to adapt it to the substrate (aSTO = 0.3905 nm). On the other hand, /2 scans obtained for the films of the thickness series (not shown here) did not show any significant effect of the thickness on the crystalline orientation of the films. However, as the thickness of the film increases, the peaks become slightly broadened showing that the crystalline quality decreases up on increasing thickness. The structure of the Pt films was also investigated by HRTEM. Fig. 5a shows a cross section image of the interface between platinum and STO substrate which corresponds to the Pt film deposited at 800 ◦ C for 30 min. It can be seen that the film has grown
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A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
Acknowledgments This work was financially supported by the Spanish Ministry of Science and Innovation projects NANOSELECT (Advanced Materials, devices and systems Nanotechnology for electronics and magnetoelectronics innovation project, under project CSD2007-00041) and MAT2011-29269-C03-03. XRD and HRTEM measurements were performed at the Scientific and Technological Centers of University of Barcelona (CCiTUB). The authors are also thankful to Pep Basas for technical assistant in XRD measurements. References
Fig. 5. (a) Cross section HRTEM image of epitaxial 20 nm Pt film grown on STO(0 0 1) deposited at 800 ◦ C. (b) and (c) Selected area electron diffraction patterns of the indicated regions. The film is formed by crystallites slightly misoriented from the substrate crystallographic directions.
epitaxially i.e. the lattice planes of the substrate and the film are parallel ((0 0 1)Pt//(0 0 1)STO). The Pt film forms an atomically sharp interface with the STO(0 0 1) substrate since a clearly defined interface is observed. The crystal structure features of the epitaxial films are in good agreement with, both, the morphology obtained in the AFM images (Fig. 1b) and the XRD results (Fig. 3). 4. Conclusions Epitaxial c-axis oriented Pt films on (0 0 1) SrTiO3 substrates were successfully grown by rf magnetron sputtering. Essentially, the results obtained indicate that Pt thin films undergo a 2D to 3D growth transition as a function of the deposition temperature, being the temperature threshold for such a transition of about 450 ◦ C. The Pt films deposited at low temperatures were polycrystalline whereas those grown at high temperatures (≥700 ◦ C) were highly crystalline and oriented in the (00l) direction, i.e. Pt(0 0 1)||STO(0 0 1). Reciprocal space map shows a strained Pt lattice for 800 ◦ C sample. This investigation provides a milestone rf sputtering condition for forming either platinum nanocubes or continuous platinum film on SrTiO3 (0 0 1), thus presenting potentially useful substrates for catalysis and microelectronics applications. The choice of the appropriate deposition temperature thus allows for an application-specific tailoring of the thin film structure and hence its properties.
[1] P. Zubko, S. Gariglio, M. Gabay, P. Ghosez, J.M. Triscone, Interface physics in complex oxide heterosructures, Annu. Rev. Condens. Matter Phys. 2 (2011) 141–165. [2] P. Ekkels, X. Rottenberg, R. Puers, H.A.C. Tilmans, Evaluation of platinum as a structural thin film material for RF-MEMS devices, J. Micromech. Microeng. 19 (2009) 65010–65018. [3] A.J. Francis, Y. Cao, P.A. Salvador, Epitaxial growth of Cu(1 0 0) and Pt(1 0 0) thin films on perovskite substrates, Thin Solid Films 496 (2006) 317–325. [4] R. Bachelet, R. Muralidharan, F. Rigato, N. Dix, X. Martí, J. Santiso, F. Sánchez, J. Fontcuberta, Enhanced thermal stability of Pt electrodes for flat epitaxial biferroic-YMnO3 /Pt heterostructures, Appl. Phys. Lett. 95 (2009) 181907–181910. [5] L.E. Coy, J. Ventura, C. Ferrater, E. Langenberg, M.C. Polo, M.V. García-Cuenca, M. Varela, Synthesis and characterization of platinum thin film as top electrodes for multifunctional layer devices by PLD, Thin Solid Films 518 (2010) 4705–4709. [6] A.D. Polli, T. Wagner, T. Gemming, M. Rühle, Growth of platinum on TiO2 - and SrO-terminated SrTiO3 (1 0 0), Surf. Sci. 448 (2000) 279–289. [7] A.J. Francis, P.A. Salvador, Crystal orientation and surface morphology of face-centered-cubic metal thin films deposited upon single-crystal ceramic substrates using pulsed laser deposition, J. Mater. Res. 22 (2007) 89–102. [8] X.M. Xu, J. Liu, Z. Yuan, J. Weaver, C.L. Chen, Y.R. Li, H. Gao, N. Shi, Single crystalline highly epitaxial Pt thin films on (0 0 1) SrTiO3 , Appl. Phys. Lett. 92 (2008) 102102–102105. [9] H. Seo, A.B. Posadas, A.A. Demkov, First-principles study of the growth thermodynamics of Pt on SrTiO3 (0 0 1), J. Vac. Sci. Technol. B 30 (2012), 04E1081-04E108-4. [10] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705–013713. [11] H. Galinski, T. Ryll, P. Elser, J.L.M. Rupp, A. Bieberle-Hütter, L.J. Gauckler, Agglomeration of Pt thin films on dielectric substrates, Phys. Rev. B 82 (2010) 235415–235426. [12] H. Galinski, T. Ryll, P. Reibisch, L. Schlagenhauf, I. Schenker, L.J. Gauckler, Temperature-dependent 2D to 3D growth transition of ultra-thin Pt films deposited by PLD, Acta Mater. 61 (2013) 3297–3303. [13] G. Jeffers, M.A. Dubson, P.M. Duxbury, Island to percolation transition during growth of metal films, J Appl. Phys. 10 (1994) 5016–5020. [14] A. Asthagiri, D.S. Sholl, DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3 (1 0 0), SrTiO3 (1 1 1) and SrTiO3 (1 1 0), Surf. Sci. 581 (2005) 66–87. [15] T. Ning, Q. Yu, Y. Ye, Multilayer relaxation at the surface of fcc metals: Cu, Ag, Au, Ni, Pd, Pt, Al, Surf. Sci. 206 (1988) L857–L863.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth of epitaxial Pt thin films on (0 0 1) SrTiO3 by rf magnetron sputtering A. Kahsay a , M.C. Polo a,∗ , C. Ferrater a , J. Ventura a , J.M. Rebled b , M. Varela a a b
Departament de Física Aplicada i Òptica, Universitat de Barcelona, Martí i Franquès 1
a r t i c l e
i n f o
Article history: Received 31 October 2013 Received in revised form 20 January 2014 Accepted 20 January 2014 Available online 5 February 2014 Keywords: Thin films Metallic Sputtering Epitaxial growth Perovskite
a b s t r a c t The growth of platinum thin film by rf magnetron sputtering on SrTiO3 (0 0 1) substrates for oxide based devices was investigated. Platinum films grown at temperatures higher than 750 ◦ C were epitaxial ([1 0 0]Pt(0 0 1)//[1 0 0]STO(0 0 1)), whereas at lower temperatures Pt(1 1 1) films were obtained. The surface morphology of the Pt films showed a strong dependence on the deposition temperature as was revealed by atomic force microscopy (AFM). At elevated temperatures there is a three-dimensional (3D) growth of rectangular atomically flat islands with deep boundaries between them. On the other hand, at low deposition temperatures, a two-dimensional (2D) layered growth was observed. The transition from 2D to 3D growth modes was observed that occurs for temperatures around 450 ◦ C. The obtained epitaxial thin films also formed an atomically sharp interface with the SrTiO3 (0 0 1) substrate as confirmed by HRTEM. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Multifunctional devices based on heterostructures that include perovskite (ABO3 ) oxides have been widely investigated due to the possibility of exploiting their beneficial properties in a lot of technological applications. SrTiO3 (STO) highlights among all perovskites, exhibiting the largest mobility and a very low carrier density threshold for metallicity, and it is frequently used as substrate for different electronic devices [1]. Otherwise, integration of these devices requires stable electrodes and both, the choice of the contact material and the interface quality, are crucial for a proper operation. One of the most used electrode material is platinum (Pt) having lattice parameters very similar to STO, because of its low resistivity, high thermal stability, and high stability under an oxygen environment at high temperature [2]. On the other hand, the deposition method chosen for the growth of Pt electrodes determines their properties, especially in the case of pulsed laser deposition (PLD) and sputtering, for which the kinetic energy of the deposited species exceeds their thermal energy. The synthesis of oriented (1 1 1) Pt films on STO substrates has been reported either by PLD [3] or by magnetron sputtering [4]. In a previous paper we studied the in situ growth of Pt thin films onto (0 0 1) STO by means of PLD [5]. It was observed that the increment of the temperature up to 400 ◦ C promoted (1 1 1)
∗ Corresponding author. Tel.: +34934039222. E-mail address: [email protected] (M.C. Polo). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.123
Pt orientation. Indeed, despite the nearly matching between the lattice parameters of STO and Pt, the growth of epitaxial (0 0 1) oriented Pt films on (0 0 1) STO is still a challenge due to the three dimensional island growth with which metals tend to grow on oxide substrates [6]. In the literature there are only a few papers reporting such experimental results [7–9] and recent theoretical studies based on density functional theory suggested that the natural growth mode for Pt (0 0 1) on STO (0 0 1) is the Volmer-Weber island formation [10]. In the present work we investigate the growth of epitaxial Pt thin films on STO (0 0 1) substrates by rf magnetron sputtering. The study is focused on how the growth temperature affects the morphology and structure properties of the Pt films and on their evolution varying the film thicknesses. Furthermore, the electrical properties of the Pt/STO system have been characterized and discussed. 2. Experimental Platinum thin films were grown on (0 0 1) SrTiO3 substrates by sputtering of pure (99.999%) platinum target, supplied by Birmingham Metal, by Argon gas in rf magnetron sputtering system. The system base pressure was 10−5 Pa and throughout the deposition process an Argon partial pressure of 10−2 Pa was maintained. In order to remove the platinum target surface contamination, a presputtering cleaning process was done prior to the films growth on bare substrates (not previously annealed). During the deposition, the STO substrates were placed 8 cm apart from the Pt target in
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A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
Fig. 1. AFM topography images of 30 nm-thick Pt films grown on SrTiO3 (0 0 1) at different deposition temperatures: (a) 400 ◦ C and (b) 800 ◦ C.
an on-axis configuration. With the aim to study the temperature and thickness dependences of the growth of Pt on STO substrates two set of Pt/STO(0 0 1) samples were obtained under different deposition conditions. In the first series (temperature series) the growth temperature was varied over a range of 400–800 ◦ C keeping all other parameters constant. The second series of samples was obtained varying only the deposition time between 5 min and 60 min (thickness series), setting the growth temperature to 500 ◦ C. The films of thickness series were grown at a deposition rate of 1 nm/min by controlling of discharge electrical power. The morphology of the obtained films was characterized by atomic force microscopy (AFM) both in tapping mode and in contact mode. The windows scanning microscope software (WSXM) was used to analyze the distribution and morphology of platinum surfaces [11]. Moreover X-ray diffraction measurements of the Pt/SrTiO3 samples were performed in order to analyze their crystalline quality and calculate the lattice parameters. In order to further investigate the structure of the films, cross section Pt/STO(0 0 1) samples were prepared and studied by means of high resolution transmission electron microscopy (HRTEM). 3. Results and discussion Fig. 1 illustrates the observed growth modes of Pt thin films deposited on STO substrate at different deposition temperatures.
Whereas the Pt film obtained at 400 ◦ C shows a nearly flat and continuous surface, the film grown at 800 ◦ C exhibits a surface with deep holes and big squared grains with parallel edges suggesting that platinum crystals have been grown in the (0 0 1) direction. These features seem to indicate that there is a transition from two-dimensional (2D) to three-dimensional (3D) growth in the deposition of the Pt films with increasing the deposition temperature. In fact, there has been a decrease in the substrate coverage in the layers grown at increasing deposition temperatures presumably because of metal agglomeration on dielectric materials [12]. The inset of Fig. 1 shows stepped edges of around 4 nm at the surface of the films, confirming the layer-by-layer growth at low deposition temperatures. In our previous work, that mentioned before in which we reported the growth of Pt films on STO(0 0 1) by PLD [5], we had not explore the growth behavior at deposition temperatures higher than 400 ◦ C and the films showed granular continuous surfaces underneath this temperature value. However, a similar growth change from 2D to 3D was found by Galinski et al. [13] in the case of using PLD to grow Pt films onto single-crystal Y2 O3 -stabilized ZrO2 (0 0 1) (YSZ) when deposition temperature exceeded 400 ◦ C. These authors concluded that the capillary forces had a predominant impact on thin film growth at high deposition temperatures. They also predicted that this impact of capillary forces had to be present on the Pt thin film formation by sputtering. Our experimental results have confirmed such assumption
Fig. 2. AFM topography images of Pt thin films grown on SrTiO3 (0 0 1) at 700 ◦ C with different values of thickness: (a) 10 nm and (b) 50 nm.
A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
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Fig. 3. XRD /2 scans for Pt/STO(0 0 1) samples grown at different temperatures. (1 1 1), (1 0 0) and (1 1 0) Pt peaks are present for deposition temperatures lower than 750 ◦ C. Films grown at temperatures higher than 750 ◦ C are c-axis oriented, with no appreciable traces of other orientations.
even although STO material instead of YSZ was employed as a substrate. It can be explained by the low values of deposition rate with whom the layers have been grown. The morphology-thickness dependence of the surface of Pt films deposited at 700 ◦ C >is illustrated in Fig. 2. From the AFM observations it has been deduced that as thickness increased, the grain size, roughness and surface coverage of the film were also increased. In fact, topography images showed that the films evolved from an early stage of nucleation of islands to a later stage of elongated islands and percolation. This is in good agreement with the experimental findings on the growth of metals on oxide substrates and with the published models [14]. From the point of view of the crystalline structure, the Pt films showed different type of X-ray diffractograms depending on the deposition temperature (Fig. 3). In increasing order of crystalline quality, the observed features range from the polycrystalline structure corresponding to the sample grown at 600 ◦ C, to the (0 0 l) textured Pt films deposited at temperatures higher than 750 ◦ C. This is in agreement with published density functional calculations which reveal that the interfacial energy of Pt(1 0 0) must be less than that of Pt(1 1 1) in order to, based on energy considerations alone, select the (1 0 0) orientation over (1 1 1) [7,15,16]. These (0 0 l) textured Pt films also showed a good out-of-plane alignment, as revealed by the narrow rocking curves of the (0 0 2) reflections, with full width at half maximum (FWHM) values of 0.22◦ (Fig. 4a), close to STO substrate value (0.08◦ ), and good inplane alignment with the substrate. This alignment is cube on
Fig. 4. (a) Rocking curves of (0 0 4) reflections of Pt thin film grown at 800 ◦ C for 30 min (FWHM of 0.22◦ ) and STO substrate (FWHM of 0.08◦ ), showing a good outof-plane film to substrate alignment. (b) Pt(1 1 1) and STO(1 1 1) PHI scans showing that the in-plane alignment of Pt crystallites onto STO substrate is cube on cube, (with no other in-plane alignments). (c) Reciprocal space map of (2 0 4) reflection of a 30 nm Pt film grown on STO(0 0 1) at 800 ◦ C, showing both, the epitaxial cube on cube growth, and the strain of the Pt crystallites, which adapted their in-plane cell parameter to that of substrate.
cube with low mosaicity, as the narrow Pt peaks showed in the phi-scans (Fig. 4b). For these films, the reciprocal space maps of (2 0 4) reflections (Fig. 4c), confirmed the film to substrate in-plane alignment, and revealed an in-plane strain of the crystals, which reduced their in-plane cell parameter to adapt it to the substrate (aSTO = 0.3905 nm). On the other hand, /2 scans obtained for the films of the thickness series (not shown here) did not show any significant effect of the thickness on the crystalline orientation of the films. However, as the thickness of the film increases, the peaks become slightly broadened showing that the crystalline quality decreases up on increasing thickness. The structure of the Pt films was also investigated by HRTEM. Fig. 5a shows a cross section image of the interface between platinum and STO substrate which corresponds to the Pt film deposited at 800 ◦ C for 30 min. It can be seen that the film has grown
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A. Kahsay et al. / Applied Surface Science 306 (2014) 23–26
Acknowledgments This work was financially supported by the Spanish Ministry of Science and Innovation projects NANOSELECT (Advanced Materials, devices and systems Nanotechnology for electronics and magnetoelectronics innovation project, under project CSD2007-00041) and MAT2011-29269-C03-03. XRD and HRTEM measurements were performed at the Scientific and Technological Centers of University of Barcelona (CCiTUB). The authors are also thankful to Pep Basas for technical assistant in XRD measurements. References
Fig. 5. (a) Cross section HRTEM image of epitaxial 20 nm Pt film grown on STO(0 0 1) deposited at 800 ◦ C. (b) and (c) Selected area electron diffraction patterns of the indicated regions. The film is formed by crystallites slightly misoriented from the substrate crystallographic directions.
epitaxially i.e. the lattice planes of the substrate and the film are parallel ((0 0 1)Pt//(0 0 1)STO). The Pt film forms an atomically sharp interface with the STO(0 0 1) substrate since a clearly defined interface is observed. The crystal structure features of the epitaxial films are in good agreement with, both, the morphology obtained in the AFM images (Fig. 1b) and the XRD results (Fig. 3). 4. Conclusions Epitaxial c-axis oriented Pt films on (0 0 1) SrTiO3 substrates were successfully grown by rf magnetron sputtering. Essentially, the results obtained indicate that Pt thin films undergo a 2D to 3D growth transition as a function of the deposition temperature, being the temperature threshold for such a transition of about 450 ◦ C. The Pt films deposited at low temperatures were polycrystalline whereas those grown at high temperatures (≥700 ◦ C) were highly crystalline and oriented in the (00l) direction, i.e. Pt(0 0 1)||STO(0 0 1). Reciprocal space map shows a strained Pt lattice for 800 ◦ C sample. This investigation provides a milestone rf sputtering condition for forming either platinum nanocubes or continuous platinum film on SrTiO3 (0 0 1), thus presenting potentially useful substrates for catalysis and microelectronics applications. The choice of the appropriate deposition temperature thus allows for an application-specific tailoring of the thin film structure and hence its properties.
[1] P. Zubko, S. Gariglio, M. Gabay, P. Ghosez, J.M. Triscone, Interface physics in complex oxide heterosructures, Annu. Rev. Condens. Matter Phys. 2 (2011) 141–165. [2] P. Ekkels, X. Rottenberg, R. Puers, H.A.C. Tilmans, Evaluation of platinum as a structural thin film material for RF-MEMS devices, J. Micromech. Microeng. 19 (2009) 65010–65018. [3] A.J. Francis, Y. Cao, P.A. Salvador, Epitaxial growth of Cu(1 0 0) and Pt(1 0 0) thin films on perovskite substrates, Thin Solid Films 496 (2006) 317–325. [4] R. Bachelet, R. Muralidharan, F. Rigato, N. Dix, X. Martí, J. Santiso, F. Sánchez, J. Fontcuberta, Enhanced thermal stability of Pt electrodes for flat epitaxial biferroic-YMnO3 /Pt heterostructures, Appl. Phys. Lett. 95 (2009) 181907–181910. [5] L.E. Coy, J. Ventura, C. Ferrater, E. Langenberg, M.C. Polo, M.V. García-Cuenca, M. Varela, Synthesis and characterization of platinum thin film as top electrodes for multifunctional layer devices by PLD, Thin Solid Films 518 (2010) 4705–4709. [6] A.D. Polli, T. Wagner, T. Gemming, M. Rühle, Growth of platinum on TiO2 - and SrO-terminated SrTiO3 (1 0 0), Surf. Sci. 448 (2000) 279–289. [7] A.J. Francis, P.A. Salvador, Crystal orientation and surface morphology of face-centered-cubic metal thin films deposited upon single-crystal ceramic substrates using pulsed laser deposition, J. Mater. Res. 22 (2007) 89–102. [8] X.M. Xu, J. Liu, Z. Yuan, J. Weaver, C.L. Chen, Y.R. Li, H. Gao, N. Shi, Single crystalline highly epitaxial Pt thin films on (0 0 1) SrTiO3 , Appl. Phys. Lett. 92 (2008) 102102–102105. [9] H. Seo, A.B. Posadas, A.A. Demkov, First-principles study of the growth thermodynamics of Pt on SrTiO3 (0 0 1), J. Vac. Sci. Technol. B 30 (2012), 04E1081-04E108-4. [10] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705–013713. [11] H. Galinski, T. Ryll, P. Elser, J.L.M. Rupp, A. Bieberle-Hütter, L.J. Gauckler, Agglomeration of Pt thin films on dielectric substrates, Phys. Rev. B 82 (2010) 235415–235426. [12] H. Galinski, T. Ryll, P. Reibisch, L. Schlagenhauf, I. Schenker, L.J. Gauckler, Temperature-dependent 2D to 3D growth transition of ultra-thin Pt films deposited by PLD, Acta Mater. 61 (2013) 3297–3303. [13] G. Jeffers, M.A. Dubson, P.M. Duxbury, Island to percolation transition during growth of metal films, J Appl. Phys. 10 (1994) 5016–5020. [14] A. Asthagiri, D.S. Sholl, DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3 (1 0 0), SrTiO3 (1 1 1) and SrTiO3 (1 1 0), Surf. Sci. 581 (2005) 66–87. [15] T. Ning, Q. Yu, Y. Ye, Multilayer relaxation at the surface of fcc metals: Cu, Ag, Au, Ni, Pd, Pt, Al, Surf. Sci. 206 (1988) L857–L863.