Defect formation of Au thin films on SiO2/Si upon annealing

We have studied structural changes of Au film surfaces grown on Si with native oxide layers. Using X-ray photoelectron spectroscopy (XPS), we found that annealing above 200°C can cause formation of defects (or cracks), which is most likely driven by interdiffusion of Au and Si accompanying strong Au–Si interactions at the interface regime. Scanning tunneling microscopy (STM) study is also in line with defect formation upon annealing. Interaction of O₂ with rough Au surfaces is discussed in connection with catalytic activities of Au surfaces.     

Improvement of interfacial adhesion in vertical GaN-based LEDs by introducing O2 plasma cleaning and intermediate layers

Interfacial adhesion between an indium tin oxide (ITO)/Ni/Ag/Ni/Au p-electrode, and Au and Ni/Au seeds in vertical GaN-based light emitting diodes (LEDs) was enhanced by O₂ plasma cleaning treatment of the Au surface in the p-electrode. However, AES and REELS analyses of the Au surface in the p-electrode detected surface damage to the p-electrode and photoresist (PR) passivation structure from O₂ plasma cleaning. W/Ni and Al/Ni adhesion layers were introduced in the Au seed to increase interfacial adhesion between Au seed and untreated PR passivation. Forward leakage current as low as 0.91 nA at 2 V was observed for the vertical LED with the Al/Ni/Au seed, for which adhesion strength to O₂ plasma-cleaned Au and untreated PR was 141.2 MPa and 62.8 MPa, respectively.     

Potential barriers to electron tunnelling in ultra-thin films of SiO2

The oxide potential barrier in metal-SiO₂-Si junctions is examined experimentally for oxide thickness 4 ? 50 Å using tunnel currents. The importance is emphasized of finding the electron concentration, n_s, at the Si surface before deducing the tunneling transmission coefficient of the oxide barrier. n_s increases rapidly with increasing δ, and is larger with Al than with Au electrodes.The SiO₂ layers give appreciably lower transmission coefficients (1) with Al than with Au electrodes, and (2) for SiO₂ grown in wet O₂ at 900°C than for SiO₂ grown in dry O₂ at 700°C.     

Preparation of highly oriented poly-diacetylene films with ion beam irradiation

Abstract

Substituting metallic or rare-earth atoms or removing O in the superconductor YBa₂Cu₃O₇ usually has a strong effect on the superconducting properties. We have made extensive XAFS studies on Co, Fe, Ni, Zn, Gd, and Pr substituted materials including some thin film samples. In nearly all cases, significant distortions are found. Although the lattice constant change is small (< 0.05 Å) the local distortions can be significant larger (～ 0.1 to 0.3 Å). The observed large local distortions of the lattice together with a change in the charge distribution at a site indicate that the interior of the unit cell is easily distorted. The correlation of a strong Tc suppression with large local distortions suggests that distortions of the nearby layers must play a significant role in models for superconductivity in high Tc materials.     

Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.     

Chemical state of the surface of mechanically activated Mn<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> oxides

X-ray photoelectron spectroscopy is used to study Mn₃O₄, Mn₂O₃, and MnO₂ manganese oxide surfaces subjected to mechanical activation by means of high intensity grinding. It is found that Mn₂O₃ is the most thermodynamically stable of these oxides; mechanical activation converts the surface layers of Mn₃O₄ and MnO₂ into this intermediate oxide. The chemical stability of activated Mn₂O₃ with respect to actions of the environment was considerably elevated. This result is explained in terms of features of the structural state of the mechanically activated surface.     

Auger electron spectroscopy of Au/NiOx contacts on p-GaN annealed in N2 and O2 + N2 ambients

We have designed a promising contact scheme to p-GaN. Au/NiO_x layers with a low concentration of O in NiO_x are deposited on p-GaN by reactive dc magnetron sputtering and annealed in N₂ and in a mixture of O₂ + N₂ to produce low resistivity ohmic contacts. Annealing has been studied of NiO_x layers with various contents of oxygen upon the electrical properties of Au/NiO_x/p-GaN. It has been found that the Au/NiO_x/p-GaN structure with a low content of oxygen in NiO_x layer provides a low resistivity ohmic contact even after subsequent annealing in N₂ or O₂ + N₂ ambient at 500 °C for 2 min.Auger depth profiles and transmission electron microscopy (TEM) micrographs reveal that while annealing in O₂ + N₂ ambient results in reconstruction of the initial deposited Au/NiO_x/p-GaN contact structure into a Au/p-NiO/p-GaN structure, annealing in N₂ brings about reconstruction into Au/p-NiO/p-GaN and Ni/p-NiO/p-GaN structures. Hence, in both cases, after annealing in N₂ as well as in O₂ + N₂ ambient, the ohmic properties of the contacts are determined by creation of a thin oxide layer (p-NiO) on the metal/p-GaN interface. Higher contact resistivities in the samples annealed in O₂ + N₂ ambient are most likely caused by a smaller effective area of the contact due to creation of voids.     

Effect of thermal annealing sequence on the crystal phase of HfO2 and charge trap property of Al2O3/HfO2/SiO2 stacks

In this study, we investigated the effect of a post annealing sequence on the HfO₂ crystal phase and the memory window of charge trap devices with TiN-Al₂O₃-HfO₂-SiO₂-Si stacks. The charge trap dielectrics of HfO₂ were deposited by atomic layer deposition and were annealed in an oxygen environment with or without Al₂O₃ blocking oxides. X-ray diffraction analysis showed that, after thermal annealing, the predominant crystal phase of HfO₂ is divided into tetragonal and monoclinic phase depending on the presence or absence of Al₂O₃ blocking oxide. In addition, deconvolution of X-ray diffraction spectra showed that, with increasing annealing temperature, the fraction of the tetragonal phase in the HfO₂ film was enhanced with the Al₂O₃ blocking oxide, while it was reduced without the Al₂O₃ blocking oxide. Finally, measurements of program/erase and increase-step-pulse programming showed that the charge trap efficiency and the memory window of the charge trap devices increased with decreasing fraction of tetragonal HfO₂.     

Ion-beam reduction of the surface of higher niobium oxide

X-ray photoelectron spectroscopy is used to study the process of reduction of the surface of the higher oxide Nb₂O₅ upon bombardment with inert gas ions (Ar⁺) and reactive gas ions (O₂⁺) with an energy of 1 and 3 keV in high vacuum at room temperature. It is found that, upon bombardment with Ar⁺ ions, the lower oxide NbO and the intermediate oxide NbO₂ are formed in the surface layers of the oxide Nb₂O₅. Bombardment with O₂⁺ ions leads to the formation of an extremely insignificant amount of the intermediate oxide NbO₂ in the surface layers of the oxide Nb₂O₅. It is revealed that the process of ion-beam reduction of the surface of the oxide Nb₂O₅ depends on the ion type, dose, and energy of exposure.     

Memory characteristics of platinum nanoparticle-embedded MOS capacitors

The capacitance characteristics of platinum nanoparticle (NP)-embedded metal–oxide–semiconductor (MOS) capacitors with gate Al₂O₃ layers are studied in this work. The capacitance versus voltage (C–V) curves obtained for a representative MOS capacitor exhibit flat-band voltage shifts, demonstrating the presence of charge storages in the platinum NPs. The counterclockwise hysteresis and flat-band voltage shift, observed from the C–V curves imply that electrons are stored in a floating gate layer consisting of the platinum NPs present between the tunneling and control oxide layers in the MOS capacitor and that these stored electrons originate from the Si substrate. Moreover, the charge remains versus time curve for the platinum NP-embedded MOS capacitor is investigated in this work.     

Ion-beam reduction of the surface of tantalum higher oxide

The process of reduction of the surface of higher oxide Ta₂O₅ under irradiation by inert gas (Ar⁺) and chemically active gas (O₂⁺) ions with an energy of 3 keV in high vacuum is investigated by X-ray photoelectron spectroscopy at room temperature. It is found that intermediate oxide TaO₂, lower oxide TaO, and metallic Ta form in the surface layers of Ta₂O₅ under Ar⁺ ion bombardment. An insignificant amount of intermediate oxide TaO₂ forms in the surface layers of Ta₂O₅ under O₂⁺ ion bombardment. Ion-beam-induced reduction of the Ta₂O₅ surface is shown to depend on the type of ion and irradiation dose.     

Influence of postdeposition annealing on structural properties and electrical characteristics of thin Tm2O3 and Tm2Ti2O7 dielectrics

We describe the structural properties and electrical characteristics of thin thulium oxide (Tm₂O₃) and thulium titanium oxide (Tm₂Ti₂O₇) as gate dielectrics deposited on silicon substrates through reactive sputtering. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy, measurements. It is found that the Tm₂Ti₂O₇ film annealed at 800 °C exhibited a thinner capacitance equivalent thickness of 19.8 Å, a lower interface trap density of 8.37 × 10¹¹ eV⁻¹ cm⁻², and a smaller hysteresis voltage of ∼4 mV than the other conditions. We attribute this behavior to the Ti incorporated into the Tm₂O₃ film improving the interfacial layer and the surface roughness. This film also shows negligible degrees of charge trapping at high electric field stress.     

Electrochemical performance of La2O3-coated layered LiNiO2 cathode materials for rechargeable lithium-ion batteries

Uncoated and La₂O₃-coated LiNiO₂ cathode materials were synthesized by polymeric sol gel process using metal nitrate precursors at 600 °C for 10 h. The structure and electrochemical properties of the surface-coated LiNiO₂ materials were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry, charge/discharge and electrochemical impedance spectroscopy techniques. X-ray powder diffraction and SEM result show that no significant bulk structural differences were observed between the lanthanum oxide coated and pristine LiMn₂O₄. The galvanostatic charge/discharge studies on the uncoated and lanthanum oxide-coated LiNiO₂-positive material at 0.5-C rate in the potential range between 3 and 4.5 V revealed that lanthanum oxide-coated positive electrode material has enhanced charge/discharge capacities; 2.0 wt.% of lanthanum oxide-coated LiNiO₂-positive material has satisfied the structural stability, high reversible capacity and high electrochemical performances.     

Epitaxial growth of Au and Pt on Fe3O4(1 1 1) surface

We have studied the epitaxial growth of Au and Pt layers on Fe₃O₄(1 1 1) as a function of the deposition temperature and thickness. The layers were deposited by UHV sputtering and the structural properties were investigated by reflection high energy electron diffraction (RHEED), X-ray experiments and transmission electron microscopy (TEM). The epitaxial growth of both metals was obtained whatever the deposition conditions but the wetting is however different for the two metals. Comparison between the coverage ratios of Au and Pt is correlated with their surface and interfaces energies. The optimum conditions to achieve a 2D flat epitaxial metallic layer are determined.     

Electric-field gradients at Ta impurities in Sc2O3 semiconductor

In this work we present an ab initio study of Ta-doped Sc₂O₃ semiconductor. Calculations were performed at dilute Ta impurities located at both cationic sites of the host structure, using the Augmented Plane Wave plus Local Orbitals (APW+lo) method. The structural atomic relaxations and the electric-field gradients (EFG) were studied for different charge states of the cell in order to simulate different ionization states of the double-donor Ta impurity. From the results for the EFG tensor at Ta impurity sites and the comparison with experimental results obtained using the Time-Differential γ–γ Perturbed-Angular-Correlations technique we could determined the structural distortions induced by the Ta impurity and the electronic structure of the doped-semiconductor.     

Micro‐Raman characterization of laser‐induced local thermo‐oxidation of thin chromium films

Micro‐Raman spectroscopy was applied to the characterization of the chemical composition and topography of protective oxide layers formed under atmospheric conditions on the surface of thin chromium films. Strips of the layers were produced by local thermal heating using focused sub‐picosecond pulsed laser radiation. It is shown that a CrO₂ layer is initially formed on the chromium surface at low light exposures. Increasing the exposure results in the transformation of the CrO₂ layer to Cr₂O₃. The influence of the etching conditions on the composition and thickness of the oxide layers is investigated. The topography of the CrO₂ and Cr₂O₃ oxide layers in transverse sections of the strips is demonstrated by the Raman mapping. Copyright © 2011 John Wiley & Sons, Ltd.     

Preparation of proton conducting BaCe0.8Gd0.2O3 thin films

Thin films of BaCe_0.8Gd_0.2O₃ were prepared by solid state reaction of two screen-printed layers over porous substrates. The first layer consists of the oxygen ion conductor Ce_0.8Gd_0.2O₂ with a fluorite structure, whereas the top layer consists of BaCO₃. After decomposition of the carbonate, BaO reacts with Ce_0.8Gd_0.2O₂ forming the perovskite oxide BaCe_0.8Gd_0.2O_3−δ with protonic conductivity. The in-situ reaction and densification on the porous substrates results in gastight thin layers of 10 to 50 μm and allows overcoming the problems due to the poor sinterability of the proton conductor. Two different porous substrates prepared by warm-pressing were studied as membrane supports, i.e., (i) porous composite NiO–Zr_0.85Y_0.15O₂, commonly employed as solid oxide fuel cell anode and (ii) porous Ce_0.8Gd_0.2O₂ oxide. The structural properties of the layer, compositional gradients and occurring phases are described, as well as water uptake, gastightness (He leaking rate) and emf measurement. Protonic conducting membranes are particularly suited not only for hydrogen separation combined with reforming and water–gas-shift converters but also as a protonic fuel cell electrolyte.     

Tailoring oxide properties: An impact on adsorption characteristics of molecules and metals

Both density functional theory calculations and numerous experimental studies demonstrate a variety of unique features in metal supported oxide films and transition metal doped simple oxides, which are markedly different from their unmodified counterparts. This review highlights, from the computational perspective, recent literature on the properties of the above mentioned surfaces and how they adsorb and activate different species, support metal aggregates, and even catalyse reactions. The adsorption of Au atoms and clusters on metal-supported MgO films are reviewed together with the cluster׳s theoretically predicted ability to activate and dissociate O₂ at the Au–MgO(100)/Ag(100) interface, as well as the impact of an interface vacancy to the binding of an Au atom. In contrast to a bulk MgO surface, an Au atom binds strongly on a metal-supported ultra-thin MgO film and becomes negatively charged. Similarly, Au clusters bind strongly on a supported MgO(100) film and are negatively charged favouring 2D planar structures. The adsorption of other metal atoms is briefly considered and compared to that of Au. Existing computational literature of adsorption and reactivity of simple molecules including O₂, CO, NO₂, and H₂O on mainly metal-supported MgO(100) films is discussed. Chemical reactions such as CO oxidation and O₂ dissociation are discussed on the bare thin MgO film and on selected Au clusters supported on MgO(100)/metal surfaces. The Au atoms at the perimeter of the cluster are responsible for catalytic activity and calculations predict that they facilitate dissociative adsorption of oxygen even at ambient conditions. The interaction of H₂O with a flat and stepped Ag-supported MgO film is summarized and compared to bulk MgO. The computational results highlight spontaneous dissociation on MgO steps. Furthermore, the impact of water coverage on adsorption and dissociation is addressed. The modifications, such as oxygen vacancies and dopants, at the oxide–metal interface and their effect on the adsorption characteristics of water and Au are summarized. Finally, more limited computational literature on transition metal (TM) doped CaO(100) and MgO(100) surfaces is presented. Again, Au is used as a probe species. Similar to metal-supported MgO films, Au binds more strongly than on undoped CaO(100) and becomes negatively charged. The discussion focuses on rationalization of Au adsorption with the help of Born–Haber cycle, which reveals that the so-called redox energy including the electron transfer from the dopant to the Au atom together with the simultaneous structural relaxation of lattice atoms is responsible for enhanced binding. In addition, adsorption energy dependence on the position and type of the dopant is summarized.     

Interaction of NO with Au nanoparticles supported on (100) terraces and topological defects of MgO

The adsorption of nitric oxide (NO) on Au_n (n = 1–3) particles deposited on anionic (O^2?) sites of MgO has been studied using the DFT (Density Functional Theory) approach. The regular O^2? sites of MgO(100) and the sites in edge and corner topological defects with high symmetry of MgO were considered. The adhesion energy of Au_n to MgO is larger for Au₂ and Au₃ due to higher polarization effects. On the other hand, the interaction strength of NO with supported Au_n particles depends mainly on the electronic configuration (open or closed shell) of the particle; the Au particles with odd number of atoms show larger NO binding energies. A comparison was performed with the reactivity of free Au_n particles. From this, it is possible to conclude that the support enhances the NO–Au_n bonding strength for the monomer, weakens this interaction in the case of the dimer, and does not have an effect in the trimers. Besides, the NO–Au_n bonding is essentially insensitive to the coordination of the anionic site where the Au_n particle is linked. A large red-shift of the N–O stretching frequency was obtained, particularly for the Au particles with odd number of atoms, due to a negative charge transfer from Au to NO.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.