Our laboratory is developing new chemical precursors for the growth of electropositive metal and element thin films by atomic layer deposition (ALD). We are also interested in processes that exhibit area selective growth. ALD has many current applications in copper metallization, diffusion barriers, liners, and transistor fabrication. Thermal ALD is often preferred because plasmas can afford low conformal coverage due to radical recombination on the walls of deep and narrow features. There has been extensive progress in the thermal ALD of copper and noble metal films in recent years, because the positive electrochemical potentials allow relatively easy reduction of precursor ions to the metals. Thermal ALD approaches to most other metals and elements in the periodic table are not well developed, due to the negative electrochemical potentials of the ions and a current lack of ALD co-reagents that can convert the ions to the metals or elements. Herein, we will describe the thermal ALD growth of electropositive metals such as nickel, cobalt, aluminium, and others. The ALD of nickel and cobalt metal films has been achieved using precursors containing diazadienyl ligands (precursors 1 and 2). These precursors enable the deposition of cobalt and nickel metal films at temperatures below 200 °C and use alkylamines as benign co-reagents. Growth rates are high (0.60 Å/cycle for nickel, 0.98 Å/cycle for cobalt), high purity, low resistivity metal films are obtained, and the films have low rms roughnesses. The processes exhibit inherent selective growth on metal substrates such as platinum, ruthenium, and copper. By contrast, no growth is observed on insulating substrates. We will also overview a new thermal ALD process for the growth of aluminum metal films. This process entails treatment of surface-bound AlCl3 with a thermally stable, volatile aluminium hydride co-reagent. The growth rate for the aluminium metal ALD process is high, and high purity, low resistivity aluminium metal films are obtained. Prospects for the area selective growth of aluminum metal films will be presented. These examples demonstrate that ALD processes can be enabled for electropositive metals through careful design of precursors and chemistry. Moreover, many of the metal ALD processes exhibit area selective growth. Figure 1

We present an ultrahigh vacuum setup for ion spectroscopy of freestanding two-dimensional solid targets. An ion beam of different ion species (e.g., Xe with charge states from 1 to 44 and Ar with charge states from 1 to 18) and kinetic energies ranging from a few 10 eV to 400 keV is produced in an electron beam ion source. Ions are detected after their transmission through the 2D target with a position sensitive microchannel plate detector allowing the determination of the ion's exit charge state as well as the scattering angle with a resolution of approximately 0.04°. Furthermore, the spectrometer is mounted on a swiveling frame covering a scattering angle of ±8° with respect to the incoming beam direction. By utilizing a beam chopper, we measure the time-of-flight of the projectiles and determine the energy loss when passing a 2D target with an energy uncertainty of about 2%. Additional detectors are mounted close to the target to observe emitted secondary particles and are read-out in coincidence with the position and time information of the ion detector. A signal in these detectors can also be used as a start trigger for time-of-flight measurements, which then yield an energy resolution of 1% and an approximately 1000-fold larger duty cycle. First results on the interaction of slow Xe30+ ions with a freestanding single layer of graphene obtained with the new setup are compared to recently published data where charge exchange and energy were measured by means of an electrostatic analyzer.

In a recent paper [A. J. Window et al., Phys. Rev. Lett. 107, 016105 (2011)], it was proposed that V_{2}O_{3}(0001) is terminated by the so-called O_{3} termination, a reconstruction with a terminating distorted hexagonal oxygen layer. We show that the surface is terminated by vanadyl (V═O) groups instead. This conclusion is based on quantitative low-energy electron diffraction combined with scanning tunneling microscopy, fast atom scattering, and density functional theory employing the Heyd-Scuseria-Ernzerhof functional. New insights into the subsurface sensitivity of ion beam triangulation show that results previously interpreted in favor of the O_{3} termination are reconcilable with vanadyl termination as well.

A few years ago, quantum effects were observed for the scattering of fast atoms from surfaces under a grazing angle of incidence. We discuss basic features of Fast Atom Diffraction (FAD) which adds a further powerful method to the established tools in ion beam analysis and surface science. Attractive features of FAD in studies on the structure of surfaces comprise negligible radiation damage, cost effective operation of the complete setup, no charging effects in studies with insulators, and an extreme sensitivity to the topmost layer of surface atoms. The observation of diffraction patterns is based on the quantum coherence which is preserved during the scattering process with the target surface. We will discuss basic features of the coherence phenomena and its role for the observation of quantum effects in the angular distributions for grazingly scattered atoms. In a comparison of data obtained for H and He atoms we demonstrate that two different mechanisms for decoherence are important.

We have scattered He+ and Ar+ ions with energies of 10 and 20keV from solid surfaces and investigated by means of a quadrupole mass spectrometer the emission of secondary ions. Compared to the established method of secondary ion mass spectroscopy (SIMS), the impact of ions proceeds under a grazing angle of incidence of about 2°. In experiments with a Cu(100) target covered with an ultrathin Fe3O4 film as well as ZnO and ZnMgO surfaces we have explored some basic features of this variant of SIMS concerning the potential application as surface analytical tool.

Recently, quantum effects were observed for the scattering of fast atoms from surfaces under a grazing angle of incidence. We discuss basic features of Fast Atom Diffraction (FAD) which adds a further powerful method to the established tools in ion beam analysis. Attractive features of FAD in studies on the structure of surfaces comprise negligible radiation damage, no charging effects, and an extreme sensitivity to the topmost layer of surface atoms. Scanning the quantum wavelength associated with the motion of a massive particle by the variation of its kinetic energy allows one to apply interferometric concepts to surface analysis based on the scattering of fast atoms.

Fast H, He atoms, and H2 molecules with projectile energies ranging from 200 eV up to 3 keV were scattered under a grazing angle of incidence from a clean and flat β-Ga2O3(100) surface. The bulk single crystal was grown by the Czochralski method and prepared via annealing under ultra-high vacuum conditions. For scattering along low-index directions, we observed defined diffraction patterns in the angular distributions for scattered projectiles. From the analysis of diffraction patterns, we derive the surface unit cell in good accord with the parameters b and c for the lattice of the bulk crystal and derive information on the termination of the surface.

We have studied structures of the chiral amino acid alanine adsorbed on Cu(110) via low-energy electron diffraction (LEED) as well as scattering of fast light atoms and molecules. The adsorption process was controlled in situ by the intensity of specularly reflected 2-keV He atoms. For projectile energies less than 1 keV, we applied the method of fast atom diffraction for studies on the structure of adsorbed alanine molecules on an atomically flat Cu(110) surface with focus on a p(3 x 2) adsorbate phase. The results are consistent with LEED and explain distortions in LEED patterns via an elongated surface unit cell with incommensurate c(3.16 x 2) symmetry of parts of the adsorbate. From triangulation using fast atoms via the azimuthal rotation of the target surface, the positions of protruding methyl groups are derived.

The enhanced electron screening effect in nuclear reactions taking place in dense astrophysical plasmas is extremely important for determination of stellar reaction rates in terrestrial laboratories as well as in prediction of cross sections enhancement in interiors of stars such as White and Brown Dwarfs or Giant Planets. This effect resulting in reduction of the nuclear Coulomb potential by the atomic electrons has been confirmed in many laboratory experiments. Unfortunately, experimental screening energies are much higher than the theoretical predictions and the reason for that remains unknown. Here, we present absorbing results of the experiment studying d + d nuclear reactions in different deuterized metallic targets under ultra high vacuum conditions. The total cross sections and angular distributions of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by the electron cyclotron ion source. The atomic cleanness of the target surface has been secured by combining Ar sputtering of the target and Auger electrons spectroscopy. Due to application of an on-line analysis method, the homogeneity of the implanted deuteron densities could be continuously monitored. We will discuss probable causes of the large discrepancy between theoretical and experimental data.

Angular distributions for scattering of 12keV He atoms from a LiF(001) surface under a grazing angle of incidence were recorded in coincidence with the projectile energy loss and the number of electrons emitted from the target surface during the collision. For scattering along the low indexed 〈110〉 and 〈100〉 directions of the crystal surface collisional rainbow peaks were observed. For scattering along a 〈110〉 direction the resulting rainbow peaks can be attributed to scattering from strings of anions which form active sites for charge exchange and emission of electrons. The data can be interpreted by trajectory computer simulations where charge transfer takes place from F− sites.

In studies on titanium oxide thin films we demonstrate the potential of Fast Atom Diffraction (FAD) and triangulation methods to derive the surface unit cell with enhanced surface sensitivity. Helium atoms with energies of 1–2keV are scattered from the surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. For low TiOx coverages a well-ordered c(2×4) superstructure and for higher coverage a p(8×2) film is observed. Based on FAD and triangulation methods for azimuthal rotation of the target the arrangement of topmost atoms in smaller sub-unit cells is revealed.

H atoms with energies up to 1 keV were scattered under a grazing angle of incidence from an Al2O3 (11 (2) over bar0) surface. After preparation of the target by annealing at a temperature of 1700 degrees C well defined angular distributions for scattered projectiles are observed showing for scattering in the regime of axial channeling rich diffraction patterns. For scattering along the low indexed [(1) over bar 100] direction diffraction spots are arranged on a circle which is attributed to full coherence preserved during the collision with the surface. By an azimuthal rotation of the target surface by some degrees Laue circles of higher orders can be observed which is accompanied with a substantial enhancement of resolution for periodicity intervals of the target surface. This is demonstrated by results for the (12 x 4) phase of the Al2O3(11 (2) over bar0) surface which indicate an overall transfer width for fast atom diffraction of several 100 angstrom. (C) 2013 Elsevier B.V. All rights reserved.

The adsorption of oxygen on a Mo(112) surface is studied via scattering of fast He atoms under a grazing angle of incidence. We show that, in addition to the intensity for reflected atoms, diffraction effects for fast atoms can be used to monitor adsorption and to obtain also information on the unit cell of adsorbed atoms. The new method is demonstrated by studies on oxygen exposure where as a function of O2 dose and target temperature a variety of superstructures can be found. Our conclusions are supported by work performed with LEED.

We demonstrate that quantum scattering of fast atoms and molecules under grazing angles of incidence can be exploited to study the structure of organic molecules on metal surfaces. Making use of keV H and He atoms as well as H2 molecules, the surface structures of the chiral amino acid alanine adsorbed on a Cu(110) surface is studied. We present a detailed investigation on the (3×2) phase of a monolayer of enantiopure and racemic alanine on Cu(110), revealing the formation of an elongated surface unit cell of c(n×2) symmetry with n=3.16±0.04 for the sticking out methyl groups of the alanine molecules.

Pairwise interaction potentials for multi-electron atoms moving in front of a LiF (001) surface are investigated theoretically and experimentally. From angular distributions of fast He, N, S, Cl and Kr atoms grazingly scattered under axial surface channeling conditions, rainbow angles are experimentally determined for a wide range of energies for the motion normal to the surface plane. These angles are used as a benchmark to probe the pairwise potential model. In the simulations the scattering process is described by means of the surface eikonal approximation, while the atom-surface interaction is derived by adding binary interatomic potentials that include the proper asymptotic limit. (C) 2013 Elsevier B.V. All rights reserved.

The structure of the topmost layer of thin V 2 O 3 films on a Au(111) substrate is studied via Ion Beam Triangulation. From electron emission induced by fast H atoms scattered from the film surface under a grazing angle of incidence as function of azimuthal rotation of the target, we find evidence for a reconstructed O 3 termination as proposed from DFT-calculations and recent experimental work using methods based on large angle impact of fast ions. From our studies we derive detailed lateral positions for the topmost O atoms.

In studies on two structures of oxygen adsorbates on Mo(112), we demonstrate the potential of fast atom diffraction to derive the surface unit cell size and its symmetry. Helium atoms with energies of 1-2 keV are scattered from an adsorbate covered Mo(112) surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. In addition to the periodic lattice, information on the structure within the unit cell can be obtained from double slit type of interference.

The influence of decoherence on the diffraction during grazing scattering of fast hydrogen and helium atoms from a LiF(001) single crystal surface with projectile energies of some keV, is investigated by two-dimensional angular distributions for scattered projectiles in coincidence with their energy loss and emitted electrons from the target surface. For keV hydrogen atoms, we identify the excitations of electrons and excitons of the target surface as the dominant mechanisms for decoherence, whereas for keV helium atoms these contributions are negligibly small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.

Recently pronounced diffraction effects for grazing scattering of fast light atoms and molecules with energies up to some keV under axial surface channeling were observed. The rich diffraction patterns provide information on the interatomic spacings between axial surface channels and on the corrugation of the interaction potential. The latter effect can be used to study the structure of surfaces with fast atoms via an interferometric technique. The new method shows similarities to thermal He atom scattering (HAS), but has a number of advantages as simple tuning of the projectile energy (de Broglie wavelength) and, in particular, an orders of magnitude more efficient detection of scattered projectiles. As an example for the application of Fast Atom Diffraction (FAD) for studies on the structure of surfaces, we present results for the rumpling of the LiF(001) surface. The quantum coherence in the scattering process is preserved by specific features of surface channeling which is investigated in detail via the coincident detection of the diffraction patterns with the energy loss of scattered atoms. It turns out that the suppression of electronic excitations owing to the band gap of insulator surfaces play a key role for coherent scattering and the application of FAD in surface science.