Thin Pt films on the polar SrTiO3(111) surface: an experimental and theoretical study (original) (raw)
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SrTiO 3 (STO) thin films were grown on Pt(1 1 1)/Ti/˛-Al 2 O 3 (0 0 0 1) substrates by ion beam sputter deposition. Growth temperature was varied between 600 and 750 • C, while Ti layer thickness was adjusted from 3 to 36 nm. Experimental results demonstrate that both the crystalline orientation and surface morphology are significantly dependent on change in the growth temperature and Ti layer thickness. From X-ray structural analysis, the STO thin films grown at 600 • C are perfectly (1 1 0)-oriented in spite of the change in Ti layer thickness. The STO thin films with a perfect (1 1 1) orientation are achievable at a sufficiently high growth temperature (≥700 • C) and a thin Ti layer (3-6 nm). Each crystalline orientation of STO thin films has a specific in-plane orientation relationship with respect to the Pt(1 1 1) layer. Flat-, rod-, and triangle-shaped grains are clearly observed from atomic force microscopy investigation, which are associated with the (1 1 0)-, (1 0 0)-, and (1 1 1)-oriented grains, respectively. Their grain orientations also show a good correlation with the X-ray structural analysis. The growth mechanism of each crystalline orientation can be described in term of surface diffusion, surface energy anisotropy, electrostatic, and role of TiO x seeds.
Journal of Applied Physics, 2011
Platinum grown by atomic layer deposition (ALD) on SrTiO 3 (001) surfaces was studied as a function of Pt coverage and post-deposition thermal treatment. The combination of atomic-force microscopy, scanning electron microscopy, x-ray fluorescence, x-ray reflectivity, and grazing-incidence small-angle x-ray scattering reveals significant changes in the nanoscale surface morphology and crystallinity for the differently prepared films. Surfaces with Pt coverages from 1 to 40 monolayers (ML) show different initial morphologies that evolve into different final Pt microstructures after annealing to 800 C. These distinct nanoscale structural changes are explained by surface diffusion and the surface and interface energies for noble metals on oxides.
Solid State Communications, 2013
SrTiO 3 (STO) thin films were directly grown on Pt(111)/a-Al 2 O 3 (0001) substrates without Ti buffer layer using electron cyclotron resonance ion beam sputter deposition. Highly (110)-and (111)-textured STO films were obtained at 600 and 750 1C, respectively. The structural, morphological and electrical properties of both textured films were studied and compared. From high-resolution X-ray diffraction data and in-plane pole figure analysis, highly (110)-textured STO film was found to contain three orientation variants related by a 1201 in-plane rotation, while highly (111)-textured STO film had two orientation variants related by a 1801 in-plane rotation. The presence of rod-and triangular-shaped grains was observed for the (110)-and (111)-textured film surfaces, respectively. Their grain orientations were also consistent with the structural analysis. The leakage current characteristics of Pt/STO/Pt capacitors were found to follow the Schottky emission and Poole-Frenkel conduction mechanisms. A larger dielectric constant of (111)-textured film (320 at 100 kHz) was obtained in comparison with that of (110)-textured film (238 at 100 kHz) due to the effect of biaxial in-plane tensile strain and larger grain size.
Interface of Pt with SrTiO3(001); A combined theoretical and experimental study
Surface Science
Density Functional Theory and X-ray standing wave atomic imaging are combined to develop models of platinum overlayers, patches and small particles on (001) surfaces of SrTiO 3. The higher coverage (0.3 ML nominal Pt) experimental result matches best to a simple 1x1 surface model. For lower coverage (0.1 ML) a Ti deficient TiO 2 double layer termination is identified as most favorable. A simple model is developed by which XSW amplitudes can be used to infer the aspect ratio of surface structures.
Ultrathin TiO x Films on Pt(111): A LEED, XPS, and STM Investigation
Journal of Physical Chemistry B, 2005
Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO x , revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO 2 ), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO x phases wet the Pt surface, in contrast to rect-TiO 2 . This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO x phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z-and z′-TiO x phases), but with distinct rectangular unit cells. Among them, z′-TiO x , which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO 2 is found The symmetry and lattice dimensions are almost identical with rect-VO 2 , observed in the system VO x /Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w-[( 43 × 43) R 7.6°] and w′-TiO x phase [(7 × 7) R 21.8°]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO 2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO x surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO 2 , represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO x /Pd , VO x /Rh(111), TiO x / Pd(111), TiO x /Pt(111), and TiO x /Ru(0001). Similar and distinct features are observed in the TiO x /Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate. * gaetano.granozzi@unipd.it.
Growth of epitaxial Pt thin films on (001) SrTiO3 by rf magnetron sputtering
Applied Surface Science, 2014
The growth of platinum thin film by rf magnetron sputtering on SrTiO 3 (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 SrTiO 3 (0 0 1) substrate as confirmed by HRTEM.
Structure of Reduced Ultrathin TiO x Polar Films on Pt(111
Journal of Physical Chemistry C, 2009
The structure of well-ordered reduced TiO x nanolayers (NLs) on Pt(111), prepared by reactive evaporation of Ti in an oxygen background and postannealed in different conditions, is discussed on the basis of experimental and theoretical data. All of the observed NLs are formed by a wetting 2D Ti-O bilayer where the Ti atoms are at the interface and the O atoms form the topmost layer. Different structures and stoichiometries depend on the actual Ti coverage and on the conditions of the postannealing. They represent the final products of a self-assembling process where the Ti atoms tend to organize in pseudoepitaxial regions, whereas the O atoms in the topmost layer (more abundant due to stoichiometry constraints) solve the crowding problem by creating pseudoepitaxial regions that exhibit dislocation lines and alternate with regions where Ti vacancies (defects or holes) occur. Given the inherent similarity among such reduced phases, as confirmed by an analysis of their electronic structure, it turns out that subtle effects are at the origin of the large variety of possible structures and that kinetic effects during their synthesis can play a role in driving toward a specific phase.
Nanoscale structure and morphology of Atomic layer deposition platinum on SrTiO3 (001)
2009
The early stages of nucleation and growth of atomic layer deposition (ALD) platinum on SrTiO 3 (001) have been studied. Scanning electron microscopy reveals the ALD Pt deposits as discrete nanoparticles that grow and coalesce with increasing number of ALD cycles, ultimately resulting in a continuous film after ∼40 cycles. Atomic force microscopy shows the films to be fine-grained and highly conformal such that the 0.4 nm atomic steps of the underlying SrTiO 3 (001) surface remain visible even after 80 Pt ALD cycles. Grazing-incidence small-angle X-ray scattering (GISAXS) studies demonstrate that the early stages of Pt ALD yields nanoparticles that are well approximated as cylinders with a height to radius ratio that is nearly unity. Consistent with nanoparticle coalescence, GISAXS also reveals an interparticle spacing that increases with the number of ALD cycles. X-ray fluorescence measurements of the Pt coverage reveal growth dynamics in which the Pt deposition is initially faster than the steady-state growth rate that emerges after 40-70 ALD cycles. These experimental results are understood through the application of a model that suggests that the SrTiO 3 surface is more reactive than the Pt species and that Pt diffusion is operative in nanoparticle formation. Overall, this study delineates ALD growth conditions for forming either Pt nanoparticles or continuous Pt thin films on SrTiO 3 (001), thus presenting potentially useful substrates for catalysis and microelectronics, respectively.
Surface Science, 2009
XPS and LEED have been used to characterize the interaction of sputter-deposited Pt (maximum coverage <5 ML) with Nb-doped SrTiO 3 (0 0 1) surfaces prepared either by annealing in O 2 and then UHV, or by chemical-etching in aqua regia. The annealed surface exhibits an ordered (1 Â 1) LEED pattern, with additional diffraction spots and streaks indicating the presence of oxygen vacancies. Increasing Pt coverage results in the decrease of the observed Pt(4f 7/2) binding energy and the uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to smaller binding energies, as expected for Pt cluster growth and surface-to-Pt charge donation on an n-type semiconductor. The etched surface is disordered, and exhibits a hydroxylated surface with a contaminant C film of 23A˚A0averagethickness.PtdepositionontheetchedsurfaceresultsinanimmediatedecreaseintheintensityoftheOHfeatureintheO(1s)spectrum,andauniformshiftoftheSr(3d),Ti(2p)andO(1s)levelstolargerbindingenergieswithincreasingPtcoverage.TheobservedPt(4f7/2)bindingenergyontheetchedsurface(23 Å A 0 average thickness. Pt deposition on the etched surface results in an immediate decrease in the intensity of the OH feature in the O(1s) spectrum, and a uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to larger binding energies with increasing Pt coverage. The observed Pt(4f 7/2) binding energy on the etched surface (23A˚A0averagethickness.PtdepositionontheetchedsurfaceresultsinanimmediatedecreaseintheintensityoftheOHfeatureintheO(1s)spectrum,andauniformshiftoftheSr(3d),Ti(2p)andO(1s)levelstolargerbindingenergieswithincreasingPtcoverage.TheobservedPt(4f7/2)bindingenergyontheetchedsurface(72 eV) is independent of Pt coverage, and indicates substantial electronic charge donation from the Pt to surface hydroxyl species. The observation of band bending towards higher binding energies upon Pt deposition (behavior normally associated with ptype semiconductors) demonstrates that sub-monolayer quantities of adsorbates can alter metal/oxide interfacial charge transfer and reverse the direction of band bending, with important consequences for Schottky barrier heights and device applications.
The growth of thin Ti and TiOx films on Pt(111): Morphology and oxidation states
Surface Science, 1992
The initial stages of the growth of Ti and TiO, films on Pttlll), grown by evaporation in ultra-high vacuum or under controlled pressure of oxygen, has been studied using X-ray absorption fine structure &IFS) taken above the Ti K-edge, and by ion scattering and X-ray photoelectron spectroscopy. Extended XAFS spectra of the metallic films reveal that growth occurs from the earliest stages as islands. Analysis of the EMS amplitudes and the low-energy ion scattering data indicate that the islands retain a morphology in which the thickness and lateral extent are similar as they coarsen through continued growth. Room-temperature oxidation of these metallic films leads to mixed oxidation states, but evaporation in oxygen atmospheres allows films of higher, and more homogeneous, oxidation state to be prepared, as judged by near-edge XAFS and Ti 2p level photoemission.