Session Y12: Oxide Surface and Interfaces

Capacitance Investigation of the Field-Induced Resistive Switching Interface

We investigate the capacitance associated with the field-induced resistive switch formed at the interface between metal and perovskite oxide, in particular Ag deposited on Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ thin film. The switch is dictated by the polarity of the applied voltage pulses, which drives the interface into either a nonvolatile high or low resistance state. Different models for this phenomenon have been proposed. We verify these models by examining the changes in the capacitance. The nature of the switching is therefore investigated through the measurement of the dielectric response with respect to frequency for both high and low resistance states. As a result, we can deduce information regarding the thickness of the switching interface, the density of the defects, and the distributions of the trap potentials. These parameters signal clear changes in both the defect density and the associated trapping potential. Lattice rearrangement, therefore, may play a major role in the switching.   

Strain Relaxation in Buried SrRuO$_{3}$ Thin Film under a Biaxial Compression: CaZrO$_{3}$/SrRuO$_{3}$/SrTiO$_{3}$ System

We have observed a novel strain relaxation phenomenon in the buried thin film that develops during the deposition of an overcoat layer. In SrRuO$_{3}$/SrTiO$_{3}$ (100) system, the SrRuO$_{3}$ film is initially in biaxial compression, but the strain relaxation develops after a CaZrO$_{3}$ overcoat is deposited, manifested as misfit dislocations at the CaZrO$_{3}$/SrRuO$_{3}$ interface and a cross-hatch pattern of surface corrugation on the CaZrO$_{3}$ surface. This arises because CaZrO$_{3}$ (0.4012 nm) has a larger lattice parameter than those of SrRuO$_{3}$ (0.393 nm) and SrTiO$_{3}$ (0.3905 nm), thus contributing to the strain energy. By increasing point defect population in CaZrO$_{3}$ to accommodate the misfit strain, this phenomenon can be avoided and atomically flat thin film stacks obtained.   

Atomically Flat SrRuO$_{3}$ Conductive Thin Films on SrTiO$_{3}$ (001) by Pulsed Laser Deposition

Atomically flat surfaces are important for thin film multilayers, superlattices, and heterostructures. For SrRuO$_{3}$ thin films grown on perovskite substrates, higher oxygen pressure is commonly used to achieve desired stoichiometry, crystallinity and conductivity, but it can also cause step bunching, pin holes and finger-like structures which destroy film/substrate coherency. We have found the finger-like structure is due to the slow 3-layer-nucleation process that occurs on the TiO$_{2}$ terminated SrTiO$_{3 }$substrate, whereas step bunching is due to excessive step mobility. Using a transition layer on SrTiO$_{3}$ that establishes the SrO termination, and adjusting deposition parameters and step spacing that reduce step migration time, we have obtained atomically flat SrRuO$_{3}$ films free of the above morphological defects. These films show a large enhancement of electrical conductivity, making them suitable for various applications.   

Growth Transient Scaling During Pulsed Laser Deposition of SrTiO3

Time resolved surface x-ray diffraction studies of pulsed laser deposition show that laser-pulse growth-transients nearly scale with the time between pulses, for dwell times varying by a factor of 250 (0.2s to 50s). AFM measurements show that shorter dwell times produce smoother surfaces than longer dwell times, and that the improved growth for short dwell times correlates with small but systematic changes in the observed growth-transients as the dwell time was varied from 0.2s to 50s. These results imply that the transverse length scale of surface structures in layer-by-layer growth is determined by the annealing time between laser pulses as well as the amount of material deposited per pulse. The impact of dwell time on layer-by-growth of will be discussed.   

Transport properties of SrTiO$_{3}$ / wide-gap insulator heterointerfaces

A field-effect transistor is a sensitive probe for the investigation of interfacial electronic properties. We have demonstrated the importance of an epitaxial interface, using SrTiO$_{3}$ (100) single crystal field-effect transistors with amorphous and epitaxial CaHfO$_{3}$ layers as gate insulators. The devices with amorphous insulator layers showed n-type transistor operation with a field-effect mobility of 0.4 to 0.5~cm$^{2}$~/~V~s at room temperature. A large threshold voltage shift was observed at low temperatures and the transistor performance was temperature independent when that shift was taken into account. The device properties were greatly affected by the interface between amorphous insulator and SrTiO$_{3}$. To improve the quality of the channel layer-insulator interface, an ultra-thin epitaxial CaHfO$_{3}$ layer was grown on the SrTiO$_{3}$ substrate surface at high temperature, followed by room-temperature deposition of an amorphous insulator layer. The devices with epitaxial interfaces exhibited a large improvement over the amorphous transistors. A field-effect mobility of around 2~cm$^{2}$~/~V~s was attained at room temperature and found to increase at low temperature, reaching 25~cm$^{2}$~/~V~s at 50~K. This result means that the carriers induced by the field effect behaved as would be expected for electron-doped SrTiO$_{3}$.   

Strain effects on the magnetic properties of epitaxial SrRuO$_{3}$ thin films

We study the effects of lattice strain on the magnetic behavior of epitaxial SrRuO$_{3}$ thin films grown by pulsed laser deposition. Most films have been grown on SrTiO$_{3}$ and KTaO$_{3}$ substrates that provide compressive and tensile strain, respectively. The Curie temperature (Tc) is reduced compared to the corresponding bulk for compressively strained SrRuO$_{3}$ films. This is consistent with previous results. Tc is enhanced an almost equal amount for films under tension. The transition temperature shows a fairly large change between the films with different types of strain (compressive and tensile), but further variation of strain produces only small changes in Tc. The rise in Tc with tensile strain is unique as far as we know. We present the temperature-strain phase diagram of SrRuO$_{3}$ thin films and discuss possible mechanisms for the strain dependent changes in magnetic properties. This work is supported through NSF DMR-0239667. Some data was taken at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.   

Structural Analysis of La$_{x}$ Mn O$_{3}$ Films

Local and long-range structure have been correlated with the magnetic and transport properties in La$_{x}$MnO$_{3}$ films (2000$\AA$) with La:Mn content (x) ranging from 0.83 to 1.09. The local structure has been investigated by Mn K-edge X-Ray Absorption spectroscopy as a function of x. The long range structure has been evaluated by detailed measurements of the in-plane and out-of-plane lattice parameters via high-resolution synchrotron x-ray diffraction. The detailed local distortions of the MnO$_{6}$ octahedra are related to the low temperature magnetization. This research is supported by NSF DMR-0209243 and DMR-0512196.   

WITHDRAWN

This abstract was withdrawn.   

Structure of Hydrated $\alpha$-Fe$_2$O$_3$ (0001) and (1$\bar{1}$02)

The structure and reactivity of $\alpha$-Fe$_2$O$_3$ depends on several factors, including the composition of the bulk material, crystallographic orientation, and local coordination of the surface atoms. As an example, the adsorption and dissociation of water on $\alpha$-Fe$_2$O$_3$ and other metal oxide surfaces is not well understood, since the oxide surface has largely been characterized under ultrahigh vacuum or clean conditions. However, interactions at the solid-liquid and solid-solid interface play major roles in environmental processes, including contaminant sequestration, mobility, and bioavailability. In this work, we present density functional theory results on the structure of clean and hydrated $\alpha$-Fe$_2$O$_3$ (0001) and (1$\bar{1}$02), and show the changes in surface structure upon heterolytic water dissociation and water physisorption.   

Dopant induced surface reconstruction in N-doped rutile TiO$_{2}$(110)

Recently N-doping of TiO$_{2}$ has attracted some attention because it has been demonstrated to shift the photocatalytic activity of TiO$_{2}$ from the UV-region to the visible light. Here we examine the impact of N-doping on the electronic structure and surface morphology of single crystal TiO$_{2}$ surfaces.* Often it is assumed that bulk dopants have little influence on the surface properties. In the case of rutile TiO$_{2}$(110) it is, however, well established that intrinsic O-vacancies cause the surface to reconstruct to form a 1x2 superstructure. We find that N-doping reduces the formation energy of O-vacancies in TiO$_{2}$ and as a consequence destabilizes the surface to cause a 1x2 reconstruction. Similar surface effects may be expected for other bulk dopants that are know to induce oxygen vacancies in TiO$_{2}$. * M. Batzill, E.H. Morales, U. Diebold, Phys. Rev. Lett. accepted   

Electron-stimulated desorption ion angular distribution (ESDIAD) investigations of the rutile TiO$_{2}$(011)-(2x1) surface*

A wide variety of potential applications have stimulated investigations of the atomic-scale properties of TiO$_{2 }$surfaces. In a combined experimental and theoretical study it was shown recently that the rutile TiO$_{2}$(011)-(2x1) reconstruction is distinct from other TiO$_{2}$ surfaces: a model was proposed based on onefold coordinated (titanyl) oxygen atoms, giving rise to double-bonded Ti=O species at the surface [T. J. Beck et al., PRL 93 (2004) 036104]. These species may play a significant role in the enhanced photocatalytic activity of TiO$_{2}$(011). The present work is an attempt to provide a direct experimental test of the model. The ESDIAD method combined with LEED is used to determine the orientation of Ti-O bonds relative to the (2x1) surface. The ESDIAD data for O$^{+}$ exhibit two beams along [100] azimuths, each tilted $>$ 20 degrees from the surface normal; the data provide supporting evidence for the proposed model. *supported in part by NSF   

Structure and energetics of step edges on anatase TiO$_{2}$(101)

Defects, such as step edges, are of paramount importance to the physical and chemical properties of metal and metal oxide surfaces. However, while steps on metal surfaces have been extensively investigated both experimentally and theoretically, much less is known about the structure and properties of steps on metal oxide surfaces. Recent STM work examined the step structure of the (101) surface of anatase TiO$_{2}$ , which is the most stable and widely exposed surface of this interesting TiO$_{2}$ polymorph. Steps were found to exhibit a few preferred orientations, giving rise to islands with identical shape on the surface. Motivated by these observations, we have carried out extensive density functional theory calculations to determine the formation energies of steps along various orientations and with different structures. A procedure based on systematic calculations of related vicinal anatase TiO$_{2}$ surfaces has been used, which yields step edge energies with remarkable accuracy. The electronic structures of the stepped surfaces and adsorption of prototype molecules have been also investigated. The present results allow us to obtain a very detailed and complete understanding of the experimental observations.   

Atomic structure of a Ultrathin TiO$_{2}$ film on Mo(112) surface

Materials with reduced dimensions attract much interest because their properties are often significantly different from the properties of bulk materials. Additionally, thin oxide films represent model systems well suited for studying structure reactivity relationships on oxides and oxide supported metal particles considered as models for heterogeneous catalysts. Recently, gold clusters on titania/Mo(112) surface are found to be high catalytic activity, attributing to the structural effects. Despite extensive experimental and theoretical studies the detailed atomic structure of TiO$_{2}$ films on Mo(112) substrate remain unknown. The geometrical and electronic structures of a ultrathin TiO$_{2}$ film expitaxially grown on a Mo(112) substrate have been determined by first principles density-functional theory calculations. The results show that the TiO$_{2}$ perfer to the 8$\times $2 surface structure on the Mo(112) surface, which is well agreement with the experiments.   

Thin oxide films: analysis of finite-size effects

Oxide surfaces have attracted considerable interest over the last years. In order to employ surface science tools, oxides are often grown epitaxially on metals. Recent findings for SiO$_2$ on Mo(112) indicate that well-ordered films are only 2-3 atomic layers thick -- less than previously thought% \footnote{L. Giordano {\it et al.}, Surf.\ Sci.\ 584, 225 (2005).}$^,$\footnote{J. Weissenrieder {\it et al.}, Phys.\ Rev.Lett. 95, 076103 (2005).}. But is the surface of the films characteristic for the surface of a bulk oxide? Not only the structure may differ from any known bulk structure, but also the metal might alter the electronic structure. Also, sizeable quantum effects are expected at these dimensions. We present DFT simulations for oxide films of various thicknesses for SiO$_2$, Al$_2$O$_3$, and HfO$_2$. In order to separate the thickness from the substrate dependence, free-standing films are considered. We find that the electronic structure at the DFT level becomes bulk-like for very few atomic layers. The case of silica is discussed in detail. The film structure on Mo(112) corresponds to the most stable quartz(0001) surface. The electronic structure is essentially bulk-like and independent of the film thickness. The presence of the substrate does not alter these findings. For the electronic response of the thin film, however, the quasiparticle picture predicts a thickness-dependent band gap due to the dielectric discontinuity at the interface. Likewise, the chemistry and stability of defects is expected to be influenced by the substrate.   

Surface and Interface Structures of Crystalline Oxides on Silicon (COS)

The structure of the Sr-covered Si(001) surface (the precursor to COS heteroepitaxy) is a matter of on-going scientific debate with experiment and theory casting opposing views. Real space Z-contrast imaging shows that half of the silicon atoms on the (001) surface are absent, and this has presented us with the intriguing question -- ``Where does the silicon go?''. A metallic surface termination of silicon was required in the layer-sequenced heteroepitaxy of COS. What has been common to all experimental realizations of the layer-sequenced COS structure, is the presence of hydrogen evolving from the metal sources during the film growth. We report here that this hydrogen has inadvertently played a pivotal role in defining the silicon termination and thus the evolution of the interface structure for COS. We will describe the thermodynamic basis for this hydrogen effect along with experimental and theoretical characterization of the structural details. Research sponsored jointly by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory under contract DE-AC05-00OR22725 with UT-Battelle, LLC and at the University of Tennessee under contract DE-FG02-01ER45937. Calculations have been performed on CCS supercomputers at Oak Ridge National Laboratory.