Surface Stoichiometries Research Articles

In situ MOCVD growth and band offsets of Al2O3 dielectric on β-Ga2O3 and β-(AlxGa1−x)2O3 thin films

The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al2O3 dielectrics on β-Ga2O3 and β-(AlxGa1−x)2O3 films is investigated as a function of crystal orientations and Al compositions of β-(AlxGa1−x)2O3 films. The interface and film qualities of Al2O3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al2O3 dielectrics with abrupt interfaces on (010), (100), and (2¯01) oriented β-(AlxGa1−x)2O3 films. The surface stoichiometries of Al2O3 deposited on all orientations of β-(AlxGa1−x)2O3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al2O3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al2O3/β-(AlxGa1−x)2O3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(AlxGa1−x)2O3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al2O3/β-(AlxGa1−x)2O3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the (2¯01) orientation. The results from this study on MOCVD growth and band offsets of amorphous Al2O3 deposited on differently oriented β-Ga2O3 and β-(AlxGa1−x)2O3 films will potentially contribute to the design and fabrication of future high-performance β-Ga2O3 and β-(AlxGa1−x)2O3 based transistors using MOCVD in situ deposited Al2O3 as a gate dielectric.

High-Mobility Toolkit for Quantum Dot Films

Semiconductor colloidal quantum dots (CQDs) are being increasingly exploited in electronics, optoelectronics, and solar energy harvesting, using a variety of different architectures, mostly based on ordered 2D or 3D arrays of these nanostructures. A crucial issue for optimizing the performance of such devices is the ability to predict and tune the transport properties of these assemblies. In this work we provide general guidelines to precisely that effect, indicating specific materials, crystal structures, lattice arrangements, surface stoichiometries, and morphologies that favor high electron mobilities in these systems and, conversely, materials that will exhibit low mobilities if nanostructured. At the same time our results evidence a surprising independence of the film’s transport properties from those of the bulk material from which the dots are made, highlighting the crucial role of theoretical modeling to guide device design.

Molecular dynamics simulations of Si etching in Cl- and Br-based plasmas: Cl+ and Br+ ion incidence in the presence of Cl and Br neutrals

Classical molecular dynamics (MD) simulations have been performed for Cl+ and Br+ ions incident on Si(100) surfaces with Cl and Br neutrals, respectively, to gain a better understanding of the ion-enhanced surface reaction kinetics during Si etching in Cl- and Br-based plasmas. The ions were incident normally on surfaces with translational energies in the range Ei = 20–500 eV, and low-energy neutrals of En = 0.01 eV were also incident normally thereon with the neutral-to-ion flux ratio in the range Γn0/Γi0 = 0–100, where an improved Stillinger--Weber potential form was employed for the interatomic potential concerned. The etch yields and thresholds presently simulated were in agreement with the experimental results previously reported for Si etching in Cl2 and Br2 plasmas as well as in Cl+, Cl2+, and Br+ beams, and the product stoichiometry simulated was consistent with that observed during Ar+ beam incidence on Si in Cl2. Moreover, the surface coverage of halogen atoms, halogenated layer thickness, surface stoichiometry, and depth profile of surface products simulated for Γn0/Γi0 = 100 were in excellent agreement with the observations depending on Ei reported for Si etching in Cl2 plasmas. The MD also indicated that the yield, coverage, and surface layer thickness are smaller in Si/Br than in Si/Cl system, while the percentage of higher halogenated species in product and surface stoichiometries is larger in Si/Br. The MD further indicated that in both systems, the translational energy distributions of products and halogen adsorbates desorbed from surfaces are approximated by two Maxwellians of temperature T1 ≈ 2500 K and T2 ≈ 7000–40 000 K. These energy distributions are discussed in terms of the desorption or evaporation from hot spots formed through chemically enhanced physical sputtering and physically enhanced chemical sputtering, which have so far been speculated to both occur in the ion-enhanced surface reaction kinetics of plasma etching.

Using Thiol Adsorption on Supported Au Nanoparticle Catalysts To Evaluate Au Dispersion and the Number of Active Sites for Benzyl Alcohol Oxidation

Two techniques to study the surface chemistry of supported gold nanoparticles were developed. First, phenylethyl mercaptan (PEM) adsorption from hexane solution was followed with UV–vis spectroscopy to evaluate the total amount of surface Au available. Two catalysts, Au/Al2O3 and Au/TiO2, were found to have Au:S surface stoichiometries of ∼2:1, whereas a Au/SiO2 catalyst had a Au:S surface stoichiometry of ∼1:1. The room temperature equilibrium binding constants for PEM adsorption on the Au/Al2O3 and Au/TiO2 catalysts were similar (∼3 × 105 M–1; ΔG ≈ −31 kJ/mol); the PEM–Au/SiO2 binding constant was somewhat larger (∼2 × 106 M–1; ΔG ≈ −36 kJ/mol). XPS data for all of the catalysts showed no observable changes in the Au oxidation state upon adsorption of the thiol. Implications of these experiments regarding self-assembled monolayers and thiol-stabilized Au nanoparticles are discussed. Second, kinetic titrations (i.e., controlled thiol-poisoning experiments) were developed as a method for evaluating the nu...

Structure and stoichiometry prediction of surfaces reacting with multicomponent gases.

Reactive interactions of molecules with solid surfaces are of key interest for catalysis and surface functionalization. Here, conceptual shortcomings of previous theoretical methods for the prediction of steady-state surface structures and stoichiometries from first-principles thermodynamics are identified. An extension is then proposed, which now enables the unconstrained description of an arbitrary number of mutually reacting gas-phase species.

Nanocrystalline Sm0.5Sr0.5CoO3−δ synthesized using a chelating route for use in IT-SOFC cathodes: Microstructure, surface chemistry and electrical conductivity

Nanocrystalline Sm0.5Sr0.5CoO3−δ powders were synthesized by a chelating route using different polyfunctional HxAPC acids (APC=aminopolycarboxylate; x=3, 4, 5). Different homologous aminopolycarboxylic acids, namely nitrilotriacetic (H3nta), ethylenediaminetetraacetic (H4edta), 1,2-cyclohexanediaminetetracetic (H4cdta) and diethylenetriaminepentaacetic (H5dtpa) acid, were used as chelating agents to combine Sm, Sr, Co elements into a perovskite structure. The effects of the chelating agents on the crystalline structure, porosity, surface chemistry and electrical properties were investigated. The electrical properties of the perovskite-type materials emphasized that their conductivities in the temperature range of interest (600–800°C) depend on the nature of the precursors as well as on the presence of a residual Co oxide phase as shown by XRD and XPS analysis. The surface chemistry and the surface stoichiometries were determined by XPS revealing a complex chemical behavior of Sr that exhibits a peculiar “surface phase” and “bulk phase” chemistry within the detected volume (<10nm).

Determination of atomic Ni interaction with TiO2 by XPS

AbstractIt is impossible to directly measure the photoelectron spectrum of an atom using conventional XPS due to its low detection sensitivity. In this work, an indirect method has been developed to determine the binding energy (BE) of an atom. In this method, the BE of the atom can be obtained by extrapolation of the experimental BE curves with atomic deposition coverage to zero coverage. With this method, the Ni 2p3/2 BE's of atomic Ni on TiO2(001) and (110) surfaces are deduced to be 853.69 and 853.55 eV, respectively. Compared with atomic Ni in gas phase, relaxation shifts of 7.34 and 7.48 eV are obtained on TiO2(001) and (110) surfaces, respectively. These values are very close to the relaxation shift of 7.3 eV due to d electron screening, indicating d‐like screening effects from the TiO2 substrates after Ni 2p photoionization. This method is also used to determinate the Ni 2p3/2 BE's of atomic Ni on TiO2(001) with different surface stoichiometries. The difference of Ni 2p3/2 BE is due to different interfacial charge transfer. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Density functional theory study of clean, hydrated, and defective alumina(11¯02)surfaces

We report an ab initio thermodynamic analysis of the $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}\text{ }(1\overline{1}02)$ surface aimed at understanding the experimentally observed terminations over a range of surface preparation conditions as well as a stoichiometric model for the $(2\ifmmode\times\else\texttimes\fi{}1)$ surface reconstruction observed after high-temperature annealing. As temperature is increased under both ultrahigh vacuum and ambient hydrated conditions, the predicted minimum-energy structural model goes through the same series of changes: from the hydroxylated ``missing-Al'' surface model (or half-layer model in which the topmost Al site of the stoichiometric surface has zero occupancy), to the hydroxylated stoichiometric model, to another hydroxylated missing-Al surface model with tetrahedral coordinated surface Al, and finally to the clean $(1\ifmmode\times\else\texttimes\fi{}1)$ stoichiometric model. These results are in agreement with observations of both missing-Al and bulklike stoichiometries under wet conditions and in agreement with similar trends reported for isostructural hematite. However, we observe that the models with excess oxygen have a relatively higher surface-free energy and distinct surface relaxations in the case of alumina as compared to hematite. At very high temperatures where oxygen defects are generated, we find that a stoichiometric, charge-neutral $(2\ifmmode\times\else\texttimes\fi{}1)$ structure becomes the most thermodynamically stable. This is consistent with the observation of a $(2\ifmmode\times\else\texttimes\fi{}1)$ electron diffraction pattern when the surface is annealed at 2000 K while a $(1\ifmmode\times\else\texttimes\fi{}1)$ pattern persists at lower annealing temperatures. A general rule that emerges from our modeling results is that while the full phase space of hydrated and defective surfaces is expansive, model stoichiometries that can be made charge neutral through either hydration or defects offer the greatest thermodynamic stability. However, the unique trends in structure and relative energies of alumina surface stoichiometries as compared to hematite can be understood based on the difference in electronic structure of the substrate.

XPS, AES, and Electrochemical Study of Iridium Oxide Coating Materials for Cardiovascular Stent Application

The surface composition and morphology of iridium oxide (IrO2) coatings prepared by a multi-step physical vapor deposition process on 316L stainless steel stent substrates has been investigated by XPS, AES, and FESEM. The obtained data show that even though the morphologies of iridium oxide films produced within the examined range of PVD process parameters are almost identical, there is a difference in their surface composition, in particular in the ratios of Ir-O σ and Ir=O π bonds. A correlation between the surface stoichiometries of the IrO2 films and their electrochemical CV and EIS responses has been established. The observed differences in the O 1s photoemission line shape of the investigated iridium oxide films were strikingly reflected in their voltammetric curve features. The implication of these finding on the mechanism of the catalytic decomposition of hydrogen peroxide by IrO2 is discussed.

A comparison of liquid and gas phase surface preparation of III–V compound semiconductors for atomic layer deposition

Native oxide removal and surface termination of InAs(1 0 0) and InSb(1 0 0) using liquid and gas phase HF chemistries were studied using X-ray photoelectron spectroscopy. Aqueous HF etching removed the native oxides on InAs and produced elemental As, which reoxidized when exposed to air. On InSb the native oxides were not completely removed due to F-termination, which passivated the surface. Gas phase HF etching of InSb native oxide completely removed Sb 2O 5 producing a stoichiometric semiconductor surface terminated by F atoms on primarily In surface sites. On InAs gas phase HF completely removed As 2O 3 producing two surface stoichiometries. For the majority of HF to water molar ratios studied, a stoichiometric bulk metal and an As-rich overlayer were produced. For a lean HF composition, an As-rich bulk metal and In-rich overlayer were produced. Deposition of Al 2O 3 by atomic layer deposition (ALD) at 170 °C directly onto F-terminated InSb produced a chemically sharp Al 2O 3/InSb interface. ALD of Al 2O 3 on an In-rich overlayer on InAs resulted in an interfacial layer containing As-oxide.

Surface Verwey Transition in Magnetite

We report density functional studies of the (001) surface of magnetite that account for local Coulomb interactions. Iron cations in the surface layers exhibit charge and t2g orbital ordering that is coupled with the lattice strains. Orbital ordering is present for various surface stoichiometries and causes opening of the band gap Eg approximately 0.3 eV at the surface, such that the (001) surface of Fe3O4 remains insulating also in the high temperature cubic phase. The (radical 2 x radical 2)R45 degrees surface reconstruction is related to orbital ordering.

A study on bacterial attachment on titanium and hydroxyapatite based films

The attachment of the oral bacteria Porphyromonas gingivalis on titanium (Ti), titanium nitride (TiN) and modified hydroxyapatite (HA) films was studied. Direct current magnetron sputtering method was used to deposit various Ti and TiN films with similar smooth topography but different surface composition as investigated by using atomic force microscope and X-ray photoelectron spectroscope. Native oxidation was predominant in all the films, however, the nitrogen percentage varied from 2.6–14.1% and 3.1–19.3% in the surface of Ti and TiN films respectively. Fluorine and zinc modified HA films were prepared by solution gel method. Replacement of a percentage of OH− group by F and 3.2 at.% of Ca by Zn in hydroxyapatite was confirmed by Fourier transform infrared spectroscope and energy dispersive analysis using X-rays measurements respectively. All the film samples were sterilized by gamma radiation or pressurized steam prior to bacterial culture experiments. The bacterial culture was performed at anaerobic condition and the bacteria treated samples were examined by means of scanning electron microscope measurements. A few of the Ti films with typical surface stoichiometries of combinations of oxygen and nitrogen exhibited minimum bacterial attachment, and on the modified HA films relatively less attached bacteria were recorded. The TiN films did not show any inhibiting action against the attachment of the bacteria, where more number of bacteria were found attached on the TiN films independent of the surface chemical composition of the films.

X-ray photoelectron spectroscopy of LiM 0.05Mn 1.95O 4 (M = Ni, Fe and Ti)

LiMn 2O 4 and LiM 0.05Mn 1.95O 4 (M = Ni, Fe and Ti) were synthesized by using solid-state reactions and their surface stoichiometries were confirmed by XPS data. The crystal and electronic structures were investigated by using X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). XRD data suggested that LiM 0.05Mn 1.95O 4 possesses nearly no any variations in lattice parameters compared with LiMn 2O 4 for slight substitution of Ni, Fe and Ti; the substituted Ni, Fe and Ti ions were located on the 16d octahedral sites in the spinel crystal lattice. The XPS results suggested that Fe and Ti ions were at + 3 and + 4 oxidation states, respectively; while Ni ions are mixed with + 2 and + 3 oxidation states. The normal oxidation state of Mn ions in the above four materials is almost the same and calculated as + 3.55 according to the splitting energies of Mn3s states.

The influence of annealing temperature on the structure and properties of TiO2 films prepared by sputtering

The TiO2 films were prepared on slides by dc reactive magnetron sputtering, then the samples were annealed at 300°C, 350°C, 400°C, 450°C, 500°C and 550°C, respectively. X-ray diffraction (XRD) was used to obtain the TiO2 film crystalline structure; X-ray photoelectron spectroscopy (XPS) was used to study the film surface stoichiometries; surface morphologies were studied by scanning electron microscopy (SEM); the contact angle was tested to indicate the TiO2 film wettability; and the photocatalytic activity testing was conducted to evaluate the photocatalysis properties. The photocatalytic activity and contact angle testing results were correlated with the crystallinity, surface morphologies and surface ·OH concentration of TiO2 films. The samples with a higher polycrystalline anatase structure, rough surface and high ·OH concentration displayed a better photoinduced hydrophilicity and a stronger photocatalysis.

The effect of surface composition of titanium films on bacterial adhesion

We show that bacterial adhesion on titanium (Ti) films could be radically minimized by tailoring the surface chemical stoichiometry of the films. Using a dc magnetron sputtering system, Ti films with various surface compositions, such as oxide and nitride combinations, were prepared by controlling processing parameters such as cathode power, sputtering pressure and base vacuum. The surface topography of the films was observed to be smooth and similar in all the films prepared under different conditions. The order of adhesion of the oral bacterial Porphyromonas gingivalis varied with the surface chemical stoichiometry of the Ti films. Few surface stoichiometries of typical oxide nitride combination resulted in nearly nil bacterial adhesion.

In situ spectroscopic ellipsometry in plasma-assisted molecular beam epitaxy of InN under different surface stoichiometries

InN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy under different surface stoichiometries were characterized by in situ spectroscopic ellipsometry in the range from 0.731eV (1697nm) to 4.95eV (250nm). Nitrogen polarity InN epilayers were grown at 600°C on GaN-underlayer/sapphire substrate. The surface stoichiometry during growth was changed by varying the indium-beam flux under the same nitrogen-beam flux. It was found that the pseudodielectric functions were drastically affected by the surface stoichiometry. The dielectric functions of InN grown under different stoichiometries were obtained. Both real and imaginary parts of the dielectric functions tended to be larger with increasing In-beam flux.

Direct Measurement of Surface Complex Loading and Surface Dipole and Their Effect on Simple Device Behavior

Tin complexes of phenoxide ligands having a range of dipole moments were prepared on the surface of indium-tin oxide (ITO). Surface complex loadings and stoichiometries were measured by quartz crystal microgravimetry. Work functions of ITO substrates treated with these various surface complexes were measured using a Kelvin probe. Surface complex dipole moments were then calculated based on measured surface loadings. Changes in the ITO work function effected by surface phenoxide complex introduction correlate with these surface complex dipole moments and with total surface dipole per unit area, and current densities in simple hole-only diode devices also correlate with these total surface dipoles.

Room temperature study of a strain-induced electronic superstructure on a magnetite (111) surface

A magnetite (Fe3O4) single crystal (111) surface has been studied at various oxygen-iron surface stoichiometries. The stoichiometry was modified by controlling the in-situ sample anneal conditions. We have found the conditions that lead to the formation of an oxygen-rich surface that forms a quasi-hexagonal superstructure with a 42A periodicity. The superstructure is highly regular and was observed by both LEED and STM. The superstructure consists of three regions, two of which have identical atomic scale structures with a periodicity of 2.8A, and a third having a periodicity that is about 10% larger (3.1A). The subtle difference in the atomic periodicities between the three areas results from the modulation of intrinsic strain developed along the surface. The superstructure results from electronic effects rather than being a mosaic of different iron oxide terminations. The onset of the superstructure is sensitive to the surface stoichiometry. From our results we could estimate the critical density of defects leading to the disappearance of the superstructure. We have modelled the experimental results and calculated the electron density using a DFT algorithm. The model clearly shows the development of strain along the surface.

First-principles study of adhesion atCu/SiO2interfaces

The structural, electronic, and adhesive properties of ${\mathrm{C}\mathrm{u}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interfaces are investigated using first-principles density-functional theory within the local density approximation. Interfaces between fcc Cu (001) and \ensuremath{\alpha}-cristobalite (001) slabs with different surface stoichiometries are considered. Interfacial properties are found to be sensitive to the choice of the termination and the interfacial oxygen density is the most important factor influencing the strength of adhesion. For oxygen-rich interfaces, the O atoms at the interface substantially rearrange after the deposition of Cu layers, suggesting the formation of Cu-O bonds. The large structural rearrangement, site-projected local densities of states, and changes in electron density indicate hybridization between $\mathrm{Cu}\ensuremath{-}d$ and $\mathrm{O}\ensuremath{-}p$ states at the interface. As oxygen is systematically removed from the interface, less rearrangement is observed, reflecting less hybridization and weaker adhesion. Computed adhesion energies for each of the interfaces are consistent with the observed structural and bonding trends, leading to the largest adhesion energy in the oxygen rich cases. The adhesion energy is also calculated between Cu and ${\mathrm{SiO}}_{2}$ substrates terminated with hydroxyl groups, and adhesion of Cu to these substrates is found to be considerably reduced. This work supports the notion that Cu films can adhere well to hydroxyl-free ${\mathrm{SiO}}_{2}$ substrates should oxygen be present in sufficient amounts at the interface.

Review of Structure of Bare and Adsorbate-Covered GaN(0001) Surfaces

A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.