Session V42: Metal Oxides: Growth, Structure, Interfaces

Characterization of Epitaxial Ag$_{2-x}$O Thin Films Grown on Sapphire

We have grown silver oxide films with a range of stoichiometry near Ag$_{2}$O by e-beam evaporation of silver in an oxygen electron cyclotron resonance (ECR) plasma. Films were deposited on r-cut sapphire substrates. A quartz crystal oscillator was used to monitor the film growth and to determine ECR oxygen flux by examining the rate of oxygen uptake on a silver film. This information was used to select the silver rate (0.1 or 1.0 {\AA}/s) and the oxygen flow rate (from 2 to 10 sccm). XRD and RHEED analysis reveals films grew with one-dimensional $<$111$>$ epitaxy, true three-dimensional $<$002$>$ epitaxy, or a mixed phase depending on the deposition conditions. XRD and XPS shows the composition varies with deposition conditions and can be a mixture of AgO and Ag$_{2}$O. UV-vis spectroscopy shows that the films have a single absorption edge between 3.1 and 3.5 eV. Optical transmission from 500 to 700 nm is between 70 and 80{\%}. Four-point van der Pauw conductivity and Hall effect measurements indicate that the Ag$_{2-x}$O films are p-type with a conductivity on the order of 3$\times $10$^{-3} \quad \Omega ^{-1}$cm$^{-1}$.   

Ultrathin CoO (100) films on Fe$_{3}$O$_{4}$ (100): a photoemission study

Using molecular beam epitaxy (MBE), 1 to 20 monolayer (ML) thick CoO (100) films were grown monolayer by monolayer on Fe$_{3}$O$_{4}$ (100) substrates. Auger measurements as a function of CoO film thickness indicated a layer-by-layer growth mode. Ultraviolet photoelectron spectroscopy (UPS) was used to monitor the evolution of the thin film electronic properties. To avoid oxidizing the Fe$_{3}$O$_{4}$ surface, Co was deposited in UHV and then oxidized for each monolayer. By comparing UPS spectra taken before and after oxidization of the Co, the separate contributions of Co and oxygen to the valence band electronic structure could be identified. Very thin ($e.g.$, 1 -- 3 ML) films exhibit valence band structures very different from those of bulk CoO. The interfacial electronic states were analyzed by comparing measured and model UPS spectra.   

Biaxial Texture Evolution in Ion-Beam Assisted Deposition of MgO

We examine the evolution of biaxial crystalline texture during ion-beam assisted deposition (IBAD) of MgO using reflection high-energy electron diffraction, \it in situ \rm ion scattering, and x-ray diffraction. The IBAD-MgO templates on metal tape are used for second generation high-temperature superconducting wire, also known as coated conductors. For MgO and some other materials with a rock salt crystalline structure, IBAD texturing can be achieved within the first few nanometers of deposited material. We find that the texture development is very sensitive to the nucleation surface conditions, both chemical species and surface morphology. In the best cases an in-plane texture of 2 degrees and an out-of-plane texture of 1 degree are attainable in a homoepitaxial MgO layer. We are utilizing a methodology of presenting data in terms of IBAD texture contour plots where we collect data as a function of ion-to-molecule ratios and film thickness. The striking conclusion from the data is that the texture development for different ion-to-molecule ratios can be scaled with the cumulative ion damage normalized to deposited MgO material. We discuss the results in terms of possible mechanisms for IBAD-MgO biaxial texturing and relationship to other IBAD texturing processes. This work is supported by the Department of Energy Office of Electricity Delivery \& Energy Reliability.   

Phase diagram for the Ni/Al$_{2}$O$_{3}$ interface and relationships to adhesion

First-principles$^{ }$calculations conducted over a broad range of atomic configurations have$^{ }$been used to determine the phase diagram and work of$^{ }$separation for Ni/Al$_{2}$O$_{3}$ interfaces[1]. Seven interfacial phases have been identified.$^{ }$The results reveal that the strongest (O-rich) phases derive their$^{ }$strength from ionic Ni-O bonds across the interface, reminiscent of$^{ }$NiO. The Al-rich phases are also strong, exhibiting a mix$^{ }$of Ni$_{3}$Al-like and Al$_{2}$O$_{3}$-like interfacial bonds. The stoichiometric interfaces are$^{ }$the weakest since they are formed from the ground-state Al$_{2}$O$_{3}$(0001)$^{ }$surface. [1] X.-G. Wang, J. Smith, A. G. Evans, Phys. Rev. B 74, 081403(2006).   

Interplay between structure and electronic properties in metal-oxide interfaces

Using first principles calculations we have investigated a broad spectrum of metal-insulator interfaces, including crystalline oxides and ferroelectrics. In our study we have focussed on the role of the interface phase in determining the properties of the composite system and, in particular, the relation between interface structure, charge transfer at the interface, and the associated interface dipole. For crystalline oxides such as BaO or SrO, our results demonstrate the possibility of tuning the Schottky barrier height by manipulating the interface phase [1]. For ferroelectric materials, such as BaTiO3, we have analyzed the interplay between the interface phase, the thickness of the ferroelectric layer and the residual polarization of the thin film. The polarization of the ferroelectric has been computed using modern theory of polarization via the displacements of the centers of the Wannier functions associated with the system. [1] M. Nunez and M. Buongiorno Nardelli, Phys. Rev. B, 73, 235422 (2006).   

Morphology of cerium oxide surfaces in an oxidzing enviroment:a first-principles investigation

A good understanding of the stability and chemistry of CeO$_2$ surfaces is crucial for a better designing of solid oxide fuel cells. As the first step, we use DFT [1] to study the structural and electronic ground state properties of bulk CeO$_2 $. various surface termination of the low-index surface of CeO$_2$ are then investigated, namely the stoichiometric, metal- and oxygen- rich terminations, and defected surfaces. Using the concept of \textquotedblleft{$ab$ $initio$ atomistic thermodynamics}\textquotedblright~[2], we calculate the surface free energy phase diagram. This allows us to identify and predict stable, and potentally catalytically important, structures. There is an evidence to suggest an interesting morphological change in the surface structures with varying oxygen concentration. Reaction pathways for methane oxidation on low energy cerium oxide surfaces are being investigated and will be reported.\newline [1] Formulated in the DMol$^3$ code; B. Delley, J. Chem. Phys. 92, 508 (1990);ibid. 113, 7756 (2000).\newline [2] K. Reuter, C. Stampfl and M. Scheffler, in Handbook of Materials Modeling, Volume 1, Fundamental Models and Methods, Sidney Yip (Ed)(2005).   

Stabilization mechanisms of polar ZnO surfaces revisited

The polar (0001) surfaces of ZnO exhibit a variety of different morphologies. The mechanisms underlying this diversity have not been definitively identified. Here we evaluate the role of several possible candidates. We show that electrostatics does not play a significant role. Instead, we argue that surface morphology is determined by a competition between two other mechanisms. The first is the electron counting rule, which leads to semiconducting surfaces. The second arises from the large cohesive energy of ionic crystals such as ZnO, which tends to preserve the surface stoichiometry at its bulk value, leading to metallic surfaces. First-principles calculations show a crossover in the relative stability of semiconducting and metallic surfaces as the chemical potential of oxygen is varied. This behavior accounts for the many observed surface morphologies on ZnO(0001), including triangular islands and pits.   

Crystalline Metal Oxide Nanoparticle Films for Renewable Energy Technologies

Hot-wire chemical vapor deposition (HWCVD) has been employed as a scalable method for the deposition of crystalline tungsten oxide nanorods and nanoparticles. Under optimal synthesis conditions, only crystalline WO$_{3}$ nanostructures with a smallest dimension of $\sim $ 10 - 50 nm are observed with extensive transmission electron microscopy (TEM) analyses. X-ray diffraction (XRD), Raman spectroscopy and electron diffraction confirm that the crystalline phases of the nanostructures may be tuned by varying the synthesis conditions such that a single phase is obtained. HWCVD has also been employed to produce crystalline molybdenum oxide nanoparticles at high density. TEM analyses show that the smallest dimension of these nanostructures is $\sim $ 5 -- 30 nm. XRD and Raman analyses reveal that the materials are highly crystalline and consist of Mo, MoO$_{2}$ and MoO$_{3}$ phases. It is also possible to fabricate large area porous films of either the tungsten or molybdenum oxide nanoparticles using a novel electrophoresis deposition technique. Furthermore, WO$_{3}$ nanoparticle films have led to profound advancement in state-of-the--art electrochromic technologies, and MoO$_{x}$ films are promising for new lithium-ion batteries.   

Nanostructured Molybdenum Oxides for Lithium-Ion Batteries

Lithium-ion batteries are the current power sources of choice for portable electronics. Although such batteries are commercially successful, they are not keeping pace with the rapid advances in computing technologies. Also, further improvement of performance and simultaneous reduction in cost as well as material toxicity remain the subject of intensive research. Here we report the synthesis and electrochemical performance of a novel molybdenum oxide nanoparticle anode that dramatically improves current Li-ion battery technologies. Crystalline MoO$_{x}$ nanoparticles have been grown by an economical hot-wire chemical-vapor-deposition (HWCVD) technique and a recently developed electrophoresis technique is employed for the fabrication of porous nanoparticle anodes. Our material exhibits a high reversible capacity of $\sim $600 mAh/g in the range 0.005-3.0 V with excellent cycling characteristics as well as high-rate capability. Both cycling stability and rate capability issues are addressed by employing these porous molybdenum oxide films that consist of nanoscale active particles. These materials will impact the next generations of rechargeable lithium batteries, not only for applications in consumer electronics, but also for clean energy storage and use in hybrid electric vehicles.   

The Onset of Ordered Vacancy Configurations on the TiO$_{2}$ (110) Surface.

The (110) surface of TiO$_{2}$ is a prototypical metal-oxide surface system that has numerous important industrial applications relating to solar energy, gas sensors, and the decomposition of harmful organic compounds. Extensive experimental and theoretical studies have shown that the most common surface defects, bridging O vacancies, play an important role in the desired oxidation processes of this model photocatalyst. Additionally, experimentally observed surfaces have shown the tendency for these defects to arrange themselves in both isolated and highly ordered configurations at low and high vacancy concentrations, respectively. Through Monte Carlo simulations of a converged cluster-expansion model parameterized by density functional calculations, we have observed the onset of multiple ordered configurations of bridging oxygen vacancies at different concentrations. In this talk, we will present our results that show two long-ranged ordered configurations over a relatively wide range of chemical potentials in addition to two semi-ordered configurations, all of which are in good qualitative agreement with experimental and theoretical results.   

One-dimensional Au chains on TiO$_{2}$(110)

One-dimensional (1D) Au chains on the 1$\times $2 missing row (mr) defected, and the added row (ar) reconstructed, TiO$_{2}$(110) surfaces are calculated from first principles. Single, dimer, and triple Au rows were considered. The single Au row binds strongly to the mr (2.83 eV) but much weaker to the ar surface (1.56 eV). On the mr surface the bonding of Au is mainly to the Ti atoms below, and to the neighboring Au atoms. In the mr surface the binding energy is decreasing with the number of Au atoms per row, while in the ar surface converse is observed. Even for triple Au rows the binding per atom (2.42 eV) is by 0.17 eV stronger for the mr than for the ar, the latter being suggested as the most favored structure of the clean surface. Thus, Au forms on the 1$\times $2 missing row TiO$_{2}$(110) surface strongly adsorbed 1D chains. The charge density distribution and the increased density of occupied states around the Fermi edge suggest metallic behavior of the Au rows. The bonding to the substrate is predominantly covalent. Intra- and inter-chain spacings are determined by the substrate periodicity. The large inter-chain distance (13 {\AA}) on the more or less insulating substrate makes this system ideal for studies of 1D phenomena.   

Time Resolved Surface Diffuse Scattering During Oxide Growth

The time dependence of surface x-ray diffraction during pulsed laser deposition provides detailed information about transverse surface structure and interlayer transport during layer-by-layer growth. To investigate the nature of growth during homoepitaxy of SrTiO$_3$, we measured the time-dependent evolution of surface diffuse scattering around the specular crystal truncation rod during deposition from a single laser shot (0.1 monolayer/pulse) on a pristine surface of (001) SrTiO$_3$. We observed the nucleation of very small islands and measured the time dependence of ripening into larger structures for the temperature range of 600 to 760C. We will relate these measurements to measurements of time-resolved diffuse scattering from multiple pulses during layer-by-layer growth and previous measuremens of time-resolved truncation rod intensities.   

Leakage current reduction and magneto-electric coupling studies in BiFeO$_{3}$ thin films

The sol-gel derived BiFeO$_{3}$ thin films were prepared on Pt/Si substrate with less than 20nm of interfacial layer of Ba$_{0.25}$Sr$_{0.75}$TiO$_{3}$ (BST). The XRD data revealed a single-phase compound having crystallite size of 25-50 nm. Surface morphology was characterized utilizing atomic force microscope and the surface roughness and the particle size were found to be reduced compared to BFO films grown without the BST sheet layer. Current voltage characteristic graphs indicated a significant reduction in leakage current of 2-3 orders of magnitude. Anomalies in the dielectric constant as a function of temperature were observed near the Neel's temperature $\sim $600K which are indicative of the so called magneto-electric coupling in this compounds. Tangent loss spectra as a function of temperature and frequencies indicated the dielectric relaxation near the Neel temperature. Micro Raman spectroscopy was carried out as a function of temperature shows the disappearance of normal modes in the range of 300-600 cm$^{-1}$ and there was a shift towards the low frequency side with increase in half width. The anomalies in Raman spectra also support the spin-phonon coupling near the Neel temperature.   

Ab initio Study of Metal Interfaces with HfO$_{2}$ and SiO$_{2}$: Work Function Modulation

For the next generation of metal-oxide-semiconductor field-effect transistors (MOSFETs), the suitable metal candidate has to be identified. The choice of a specific metal electrode on high-k gate dielectric oxide films is strongly influenced by the possible interface chemical reactions and defects. In this study, \textit{ab initio} calculations are employed to investigate and analyze a number of possible interface structures between gate dielectric oxides, HfO$_{2}$ and SiO$_{2}$, and metal electrodes. The structural stability and electronic structure of the interfaces with implications to metal work functions are discussed. The work function of metals on oxides is significantly influenced by the interface configurations and by the particular bonding pattern at the interface. For work function modulation, model interface systems of metal bi-layers are constructed based on structural and compositional heterogeneity. It is found that a few atomic layers of the underlying metal shift the work function of bi-layers to that of underlying metal.