Session X5: Structure and Formation of Oxide Surfaces and Interfaces

Quasi-long range ordered hole-adatoms pairs on SrTiO$_{3}$(110)-(4$\times $1) surface

The surface structure of transition metal oxides (TMOs) has been an important issue for chemistry and photocatalysis. We studied the surface of SrTiO$_{3}$, which is a wide-gap semiconductor and has been believed useful for photo-induced water splitting. Specifically we focused on the (110) surface that bears intrinsic instability of reconstruction due to the surface polarity. The monophased (4$\times $1)-reconstructed surface was obtained with the treatment of argon ion sputtering followed by annealing in ultra high vacuum. More interestingly, we observed a (4$\times $10) quasi-long range ordered hole-adatom structure. The atomic configuration was identified by both experimentally adsorbing additional Sr atoms and density functional calculations. The ordering of the hole-adatom pairs was robust since its formation effectively released the stress on (4$\times $1)-reconstructed SrTiO$_{3}$(110) surface. Such a surface with ordered defects served as a good template for the guided growth of noble metal nanoclusters with controlled size and density.   

Growth modes of multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ -- Real time X-ray synchrotron study

Real time X-ray scattering measurements during heteroepitaxial film deposition provide details of initial nucleation and growth, thus giving insight into atomic-scale processes and growth mechanisms. In this work the growth of pulsed laser deposited (PLD), and RF-magnetron sputter deposited multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ substrates are studied using \textit{in-situ} X-ray specular and diffuse scattering.~ Both the out-of-plane (height distribution) and lateral (surface mounds and correlations) information is obtained from these measurements. Specular X-ray intensity monitored at (00 $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ shows unit-cell growth oscillations during the first few monolayers, while both diffuse scattering width and intensity oscillate out-of-phase with specular, characteristic of 2D layer-by-layer growth for PLD deposited films. The coarsening process is consistent with growth and coalescence of islands - identified as key fundamental processes in epitaxial growth. Beyond several monolayers oscillations decay and the diffuse intensity increases indicating rapid growth of mounds.~ However, the mounds themselves subsequently merge after a few more deposited layers, leaving arrays of mesas with some holes due to incomplete coalescence. This behavior was corroborated with ex-situ Atomic Force Microscopy (AFM) at each stage. Results for sputter deposited films showed unusually strong specular and diffuse oscillations; implications for interface structure and formation will be discussed.   

Polarization Dependent Reconstructions of Ferroelectric Surfaces

We present an \emph{ab initio} density-functional study of the surface structures and the thermodynamic stability of ferroelectric tetragonal (001)-oriented slabs of BaTiO$_3$ and PbTiO$_3$, including both cases where the bulk polarization is directed towards (positive) and away (negative) from the surface. We analyzed systematically the total energies of surfaces with different reconstruction symmetries and stoichiometries. The thermodynamic stability diagrams show that the stable surface compositions for the positive and negative surfaces are different, which are in turn different from those of the paraelectric phase. We show that the prevalence of certain surface reconstructions can be understood in the light of the ability of ions to passivate ferroelectric surface charges.   

STM study of the Mn-dopants on the surface of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$(x=6{\%}, 16{\%})

The double-layered Sr$_{3}$Ru$_{2}$O$_{7 }$is a paramagnetic metal, but the substitution of Mn for Ru (Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7})$ results in a metal-to-insulator transition at T$_{MIT}$ and antiferromagnetic (AF) ordering at T$_{M}$ (the two transitions are closely coupled for x $<$ 6{\%}). STM measurements at 4.2 K and 100 K on the surface of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ ($x$ = 6{\%}, 16{\%}) reveal a ($\surd $2 $\times \surd $2)R45$^{o}$ unit cell, consistent with the orthorhombic bulk structure. The Mn dopant has been identified through bias-dependent STM topography and dI/dV mapping. The Mn dopant equally occupies two sites which are anti-phase - one sitting at the center and the other on the corner of the ($\surd $2 $\times \surd $2)R45$^{\circ}$ unit cell. We have directly imaged the chirality of MnO$_{6}$ rotation at the anti-phase sites. In contrast to the bulk measurements, the surface is always metallic for $x $= 16{\%} and insulating only for 4.2K measurements on the $x$ = 6{\%} sample. The surface apparently suppresses the Mn-induced insulating (AF) phase observed in the bulk.   

Morphology and Magnetism of Atomically Thin Layers of Chromia -- An STM Investigation

A low temperature scanning tunneling microscopy (LT-STM) study of ultrathin chromium oxide films on Cu(111) is presented. The (0001) surface of Cr$_{2}$O$_{3}$ (chromia) exhibits long-range antiferromagnetic ordering, and its usefulness for electric field control of exchange bias has been recently established [Xi He, Ch. Binek, et al., Natuer Materials 9, 579 -- 585 (2010)]. We deposited ultrathin chromium layers on Cu(111), followed by post-annealing in oxygen partial pressures to promote oxidation of the chromium films. We find that chromium grows on Cu(111) in the Volmer-Weber mode, resulting in small 3D islands. During annealing, the small islands coalesce to form large, flat terraces and the most stable oxide of chromium, Cr$_{2}$O$_{3}$, is formed at 630\r{ }C. Spin-polarized scanning tunneling microscopy has been performed to image the layer-wise antiferromagnetism in different structural layers of the chromia films. Thin layers of cobalt have been deposited on the chromia so that the magnetism of the Co couples to that of the chromia. Imaging the magnetism of the Co islands on the Cr$_{2}$O$_{3}$ terraces with spin-polarized STM helped us distinguishing between magnetic and electronic contrast in the complex dI/dV maps of the chromia surface.   

Structures of Stoichiometric and Oxygen Deficient Sapphire Surfaces

Sapphire, $\alpha $-Al$_{2}$O$_{3}$ is one of widely used oxide substrates. Its (0001) surface is presumed quite stable. However, when it is heat-treated under vacuum, the surface shows the variety of different surface structures, mostly due to oxygen deficiency. The initial surface structure could be also modified by covering the substrate with another sapphire substrate during air annealing above 1500\r{ } C. During the course of the heat-treatment under UHV condition, the x-ray reflectivity and crystal truncation rods were measured in addition to observing reflection high energy electron diffraction patterns. Metal films such as Pb, Ag, and Pd were deposited onto these various sapphire surfaces. Contrary to common belief that metal films would not grow well on insulating oxide substrates, metal films could be formed on some of the sapphire surfaces. The influence of the sapphire surface structure to the morphology of the metal films will be discussed.   

Crystallographic Orientation(s) of Vanadium Dioxide Nano-Grains on Various Single-Crystal Sapphire Substrates

Vanadium dioxide (VO$_{2}$) is a material of particular interest due to its reversible structural semiconductor to metal phase transition near room temperature (\~ 68 $^{\circ}$C) and its accompanied hysteresis. Electron Back-Scattered Diffraction (EBSD) was used to study the orientation of the crystalline VO$_{2}$ grains deposited on three cuts of sapphire (a-, c-, and r-cuts) by pulsed laser deposition. EBSD showed a predominant family of crystallographic relationships present in all cuts of sapphire wherein the rutile VO$_{2}$ \{001\} planes tend to lie parallel to the substrate's \{10-10\} and the rutile VO$_{2}$ \{100\} planes lie parallel to the substrate's \{1-210\} and \{0001\}. This family of relationships accounts for the majority of the VO$_{2}$ grains observed on all the studied sapphire substrates. However, due to the symmetry of the substrate, there were variations of these same relationships that prevent a single epitaxy from taking place in these cuts as the VO$_{2}$ grains did orient themselves with equivalent out-of plane directions in the substrate.   

Growth of Vanadium Dioxide Thin Films Using Magnetron Sputtering

In this work, we present our experimental investigation of vanadium oxide thin film deposition. RF and DC magnetron sputtering are used for thin film deposition. Post deposition anneal are introduced to stabilize the vanadium dioxide phase. The impact of deposition conditions and anneal conditions on the structural and morphological properties of the thin films, as determined by x-ray diffraction and scanning electron microscopy, will be discussed. Results indicate that on the technologically relevant silicon dioxide surface, the transitional phase of vanadium dioxide can be stabilized by post deposition anneal at 550$^{\circ}$C and oxygen partial pressures between $3.6 \times 10^{-4}$ torr and $10^{-2}$ torr. The films obtained show a resistivity change of a factor of 200 at 350k. The shift in transition temperature is attributed to thin film stress.   

Studies of high coverage oxidation of the Cu(100) surface using low energy positrons

The study of oxidation of single crystal metal surfaces is important in understanding the corrosive and catalytic processes associated with thin film metal oxides. The structures formed on oxidized transition metal surfaces vary from simple adlayers of chemisorbed oxygen to more complex structures which result from the diffusion of oxygen into subsurface regions. In this work we present the results of theoretical studies of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the oxidized Cu(100) surface under conditions of high oxygen coverage. Calculations are performed for various high coverage missing row structures ranging between 0.50 and 1.50 ML oxygen coverage. The results of calculations of positron binding energy, positron work function, and annihilation characteristics of surface trapped positrons with relevant core electrons as function of oxygen coverage are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy (PAES).   

\textit{In situ} TEM Studies of the Initial Oxidation stage of Cu and Cu Alloy Thin Films

The fundamental understanding of oxidation at the nanoscale is important for the environmental stability of coating materials as well as processing of oxide nanostructures. Our previous studies show the epitaxial growth of Cu$_{2}$O islands during the initial stages of oxidation of Cu thin films, where surface diffusion and strain impact the oxide development and morphologies. The addition of secondary elements changes the oxidation mechanism. If the secondary element is non-oxidizing, such as Au, it will limit the Cu$_{2}$O island growth due to the depletion of Cu near the oxide islands. When the secondary element is oxidizing, for example Ni, the alloy will show more complex behaviour, where duplex oxide islands were observed. Nucleation density and growth rate of oxide islands are observed under various temperatures and oxygen partial pressures (pO$_{2})$ as a function of time by \textit{in situ }ultra high vacuum (UHV)-transmission electron microscopy (TEM). Our initial results of Cu-Ni(001) oxidation is that the oxide epitaxy and morphologies change as function of Ni concentration. For higher spatial resolution, we are examining the atomic scale oxidation by aberration-corrected ETEM with 1{\AA} resolution.   

Quantum-Mechanical Investigations for the Oxidation Mechanism of Ti$_{3}$Al

First-principles density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the $\alpha _{2}$-Ti$_{3}$Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the $\alpha _{2}$-Ti$_{3}$Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the $\alpha _{2}$-Ti$_{3}$Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e., the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O/$\alpha _{2}$-Ti$_{3}$Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.   

Site Specific Molecular Chemisorption of O$_{2}$ on TiO$_{2}$(110): A Scanning Tunneling Microscopy Study

The investigation of O$_{2}$ adsorption on TiO$_{2}$ is critical since it can help us to better understand the photooxidation mechanism of TiO$_{2}$. In our work, molecularly chemisorbed O$_{2}$ were directly imaged on reduced TiO$_{2}$(110) at 50 K with scanning tunneling microscopy (STM). Two O$_{2}$ adsorption channels, one at bridging oxygen vacancies (V$_{O})$ and another at five-fold coordinated Ti atoms (Ti$_{5c})$, have been identified. While O$_{2}$ at Ti$_{5c}$ appears as a single protrusion, the O$_{2}$ at V$_{O}$ manifests itself by a disappearance of the V$_{O}$ feature. It is found that STM tip can easily dissociate O$_{2}$ and the dissociation details strongly depend on the tunneling conditions and the type of the O$_{2}$ adsorption site. The chemisorbed O$_{2}$ at these two distinctive sites are the most likely precursors for the two previously established O$_{2}$ dissociation channels, observed at temperatures above 150 and 230 K at the V$_{O}$ and Ti$_{5c}$ sites, respectively   

Hard x-ray photoemission studied and band alignment in TiO$_{2}$/HfO$_{2}$/Ge heterojunctions

Novel high K oxides such as hafnium oxide and zirconium oxide have replaced silicon dioxide as gate oxide. Although titanium oxide has a high dielectric constant, the poor conduction band offset between titanium oxide and Si/Ge makes it a poor choice for gate oxide. One way to address this issue is to have a thin intermediate layer with appropriate band alignment between titanium oxide and the semiconductor. Here we present hard x-ray photoelectron spectroscopy (HAXPES) study on the band alignment between atomic layer deposited (ALD) TiO$_{2}$/HfO$_{2}$/Ge heterojunctions. The exact position of the valence band maximum was determined by convoluting theoretical calculated density of states from first-principles calculations and comparing with experimental valence band data. We will also discuss the dependence of the band alignment on the thickness of the intermediate layer.   

Synchrotron Photoemission Characterization of Process Dependent Oxidation Control in InGaAs/High-k Film Systems

To continue CMOS scaling, higher mobility substrates, such as III-V semiconductors, are being investigated. However, certain technological barriers must be overcome before high-mobility devices can be employed; notably, defects at the semiconductor/high-k interface need to be well controlled. Here, we investigate the role of an AlN interlayer between InGaAs and ALD ozone deposited ZrO2 using synchrotron source X-ray photoemission spectroscopy (XPS). XPS is able to quantify the relative levels of oxidation from the In, Ga and As signals. Also, by comparing the relative As 3d3/2: As 3d5/2 peak intensities, it is possible to quantify the level of As-As bond formation, which is common to coarse As-O reduction processing and consistent with mobility degradation. To elucidate the role of the AlN interlayer, XPS spectra of InGaAs/AlN/ZrO2, InGaAs/ZrO2 and InGaAs/AlN films will be presented and compared. We will demonstrate that an AlN interlayer is effective in reducing In, Ga and As interfacial oxides, as well as As-As bonding, yet is not sufficient to passivate the interface against further oxidation from ozone based ALD processing.   

Engineering an epitaxial dielectric/metal/dielectric trilayer system

Ag is a favorable material platform for plasmonic applications. Most plasmonic devices to date have been based on granular polycrystalline metal films, which suffer from scattering due to surface roughness. Recently, superior plasmonic properties have been demonstrated by using atomically smooth epitaxial Ag(111) films grown on Si(111), illustrating the advantages of an epitaxial approach. For further device applications of the above example, it is desirable to create epitaxially grown dielectric/metal/dielectric multi-layer systems. Here we report successful epitaxial growth of a MgO(001)/Ag(001)/MgO(001) trilayer system which can be used as a material platform for creating multi-layered (dielectric/metal)$_{N}$ heterostructures with any desired number, N. This type of new structure can be utilized for plasmonic devices with a wider wavelength range due to the transparency of MgO substrates in visible light. Moreover, it can open a door to new integrated devices with high quality atomically smooth films.