Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

Stability of ZnO{0 0 0 1} against low energy ion bombardment

The steady-state surface compositions of the polar (O and Zn terminated) faces of ZnO{0 0 0 1} produced by low energy (0.3-2 keV) Ar⁺ ion bombardment were studied by Auger electron spectroscopy and electron energy loss spectroscopy. The alterations produced by the ion bombardment using different ion energies were monitored by calculating the intensity ratios of the low and high energy Zn Auger peaks (59 eV and 994 eV, respectively); Zn and O Auger peaks (59 eV and 510 eV, respectively). Based on the dependence of these ratios on the ion energy and termination of the surface, we could conclude that the stability of the Zn face is higher against the low energy argon ion bombardment-induced compositional changes than that of the O face.     

Study of S ion-assisted sulfurization of n-GaAs (1 0 0) surface

The chemical structure and site location of sulfur atoms on n-GaAs (1 0 0) surface treated by bombardment of S⁺ ions over their energy range from 10 to 100 eV have been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. The formation of Ga-S and As-S species on the S⁺ ion bombarded n-GaAs surface is observed. An apparent donor doping effect is observed for the n-GaAs by the 100 eV S⁺ ion bombardment. It is found that the S⁺ ions with higher energy are more effective in the formation of Ga-S species, which assists the n-GaAs (1 0 0) surface in reconstruction into an ordered (1 × 1) structure upon subsequent annealing. The treatment is further extended to repair Ar⁺ ion damaged n-GaAs (1 0 0) surface. It is found that after a n-GaAs (1 0 0) sample is damaged by 150 eV Ar⁺ ion bombardment, and followed by 50 eV S⁺ ion treatment and subsequent annealing process, finally an (1 × 1) ordering GaAs (1 0 0) surface with low surface states is obtained.     

Ga-induced superstructures on the Si(1 1 1) 7 × 7 surface

Monolayer Ga adsorption on Si surfaces has been studied with the aim of forming p-delta doped nanostructures. Ga surface phases on Si can be nitrided by N₂⁺ ion bombardment to form GaN nanostructures with exotic electron confinement properties for novel optoelectronic devices. In this study, we report the adsorption of Ga in the submonolayer regime on 7 × 7 reconstructed Si(1 1 1) surface at room temperature, under controlled ultrahigh vacuum conditions. We use in-situ Auger electron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction to monitor the growth and determine the properties. We observe that Ga grows in the Stranski-Krastanov growth mode, where islands begin to form on two flat monolayers. The variation in the dangling bond density is observed during the interface evolution by monitoring the Si (LVV) line shape. The Ga adsorbed system is subjected to thermal annealing and the residual thermal desorption studied. The difference in the adsorption kinetics and desorption dynamics on the surface morphology is explained in terms of strain relaxation routes and bonding configurations. Due to the presence of an energetic hierarchy of residence sites of adatoms, site we also plot a 2D phase diagram consisting of several surface phases. Our EELS results show that the electronic properties of the surface phases are unique to their respective structural arrangement.     

Desorption of chemisorbed Carbon on Mo(1 0 0) by noble gas ion sputtering: Validation of ground test measurements of ion engine lifetimes

We report desorption cross section measurements for one monolayer of chemisorbed carbon on a Mo(1 0 0) surface induced by sputtering with noble gas ions (Ne⁺, Ar⁺, Xe⁺) at different incident angles, ion energies, and substrate temperatures. Desorption cross sections were determined by using low-energy ion scattering (LEIS) to monitor the increase of the signal from the Mo substrate. A monolayer of p(1 × 1) carbon adatoms on the Mo(1 0 0) surface was created by dosing ethylene (C₂H₄) to the substrate at 800 K, and characterized by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). We find that the carbon desorption cross section increases with increasing mass and energy of the impinging ions, and there is a maximum value for the desorption cross section at an incident angle for the ions of 30° from the surface plane. The desorption cross section also increases up to a substrate temperature of 300 °C. Values for the carbon desorption cross section for carbon adatoms on Mo(1 0 0) by 400-eV Xe⁺ ion sputtering are about 2 × 10⁻¹⁵ cm², which is one order of magnitude higher than those for bulk carbon samples. This information is particularly important for evaluation of ion-engine lifetimes from ground-test measurements in which contaminant carbon is deposited on Mo accelerator grids, potentially altering the sputtering rate of the Mo. Our measurements show that monolayer amounts of carbon on Mo have desorption cross sections that are two orders of magnitude higher than estimates of what would be required to reduce the Mo erosion rate, and thus ground-test measurements can be used with confidence to predict ion-engine wear in space, from this perspective.     

FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

Neutralization of Li ions scattered by the Cu (1 0 0) and (1 1 1) surfaces: A localized picture of the atom-surface interaction

Large and face dependent neutral fractions have been found recently in the scattering of Li⁺ by Cu(1 0 0) and Cu(1 1 1) surfaces. These results for high work function surfaces are unexpected within the ‘traditional’ picture of a Li⁺ ion departing from a jellium surface model. In the present work the Li⁺/Cu(1 0 0) and Li⁺/Cu(1 1 1) interacting systems are described by a previously developed bond-pair model based on the localized interactions between the projectile ion and the atoms of the surface, and on the extended features of the electronic band structure through the surface local density of states. By only including the resonant neutralization to the Li atom ground state we explained the face and energy dependences of the measured neutral fractions for large outgoing energy values. We found that the downward shift of the Li ionization level below the Fermi level caused by the short range chemical interactions, is the main responsible of a high neutralization by the resonant mechanism. The remaining differences between theory and experiment values can be explained in terms of the energy gaps and image potential states appearing in these surfaces. The calculated distance behaviours of the energy levels corresponding to the first excited (Li-1s²2p) and the negative (Li-1s²2s²) atomic configurations indicate that they can also participate in the ion-surface charge exchange process.     

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar⁺ ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10¹³ ions/cm² to 1 × 10¹⁸ ions/cm². Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N₂⁺ ions and 7.2 × 10¹⁷ ions/cm² are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar⁺ ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.     

Electrolytical production of Ni + Mo + Si composite coatings with enhanced content of Si

Ni + Mo + Si composite coatings were prepared by co-deposition of nickel with molybdenum and silicon powders from a nickel solution in which Mo and Si particles were suspended by stirring. The layers have been deposited on a carbon steel substrate (St3S) under galvanostatic conditions. The content of Si in deposited layers was about 2-5 wt.% depending on deposition current density and the value of electric charge. For comparison Ni + Mo composite coatings were obtained under analogous current conditions. Composite coatings of enhanced Si content (15 wt.%) were deposited from an electrolyte in which 40 g/dm³ of Si covered with electroless plated nickel was dispersed. Deposition current density was equal 0.1 A/cm² and the value of electric charge Q = 500 C/cm². The thickness of the coatings was about 100-300 μm depending on their kind, electric charge and the deposition current density. Surface and cross-section morphology were investigated by scanning electron microscope (SEM). All deposited coatings are characterized by great, developed surface area. No internal stresses causing their cracking were observed. Chemical composition of the layers was determined by X-ray fluorescence spectroscopy (XRF) method and quantitative X-ray analysis (QXRD). It was stated, that the content of molybdenum and silicon in Ni + Mo + Si coatings depends on deposition current density and the amount of the powder in bath. The results of structural investigation of the obtained layers by the X-ray diffraction (XRD) method show, that they consist in crystalline Mo or Mo and Si phases built into Ni matrix. Moreover, Ni + Mo + Si composite coatings were modified by thermal treatment. It has been found that the thermal treatment of Ni + Mo + Si composite coatings caused that the new phases (NiSi, Mo₂Ni₃Si and Ni₆Mo₆C_1.06) were obtained.     

Growth of Ag nanostructures on TiO2(1 1 0)

We present a study of the growth of silver nanoparticles or clusters on a TiO₂(1 1 0) substrate in ultra-high vacuum. The growth is monitored in situ by ion and neutral scattering spectroscopy using He⁺ scattering and Auger spectroscopy. The scattering measurements show that only part of the surface is covered by Ag suggesting formation of clusters. Additionally an ex-situ study was performed by scanning electron microscopy and atomic force microscopy to determine the size distribution of these clusters. The average size distributions were found to range from about 5 to about 20 nm as a function of the evaporation flux. At the higher evaporation flux we observe formation of the smaller sized clusters.