Ion-induced self-organized ripple patterns on graphite and diamond surfaces

In order to investigate the allotropic effect on ripple pattern formation, highly oriented pyrolytic graphite (HOPG) and single crystalline diamond were irradiated with 10-200 keV Xe⁺ at an incident angle of 60° with respective to the surface normal. The irradiation fluence was 2 × 10¹⁷ cm⁻² for all irradiations. Ripple patterns were observed on both HOPG and diamond surfaces. However, large differences in ripple wavelengths, amplitudes and surface roughnesses between HOPG and diamond were recognized. The reason for these differences is discussed.     

Enhanced diamond nucleation on copper substrates by graphite seeding and CO2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO₂ laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO₂ laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO₂ laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.     

Hard X-ray photoelectron spectroscopy from 5-14.5 keV

Photoemission spectroscopy at high energies can be used to probe bulk electronic states. We used a specially designed high-voltage retarding lens and a commercial Perkin-Elmer PHI 10-360 hemispherical electron analyzer to investigate the core and valence band region of Au, YBa₂Cu₃O_7−δ and highly oriented pyrolytic graphite samples with hard X-rays in the energy range 5-14.5 keV. The overall instrumental resolution obtained at 8 keV was 218 meV. The photo ionization cross-sections for Au 5d and 6s excitations were determined experimentally. In comparison with published calculations for atomic cross-sections neglecting corrections for angular anisotropy, the values we find are twice as large for the 5d and an order of magnitude larger for the 6s (conduction band) level. Our results demonstrate the feasibility of bulk sensitive valence band spectroscopy with high resolution at high brilliance X-ray sources such as the ESRF. The measured cross-sections provide important input for improving current theoretical models.     

Exit angle dependence of charge-state distribution of backscattered He ions

Exit angle and energy dependences of the charge-state distribution of backscattered He ions were investigated when 500 keV He⁺ ions were incident on SiO₂. The energy dependence of the He⁺ fraction was estimated by comparing the measured He⁺ spectra with the simulated spectra of He ions in all charge states at the exit angles of 5-25° with respect to the SiO₂ surface. We found that the He⁺ fraction is almost independent of the exit angle at energies higher than 250 keV and the observed energy dependence of the He⁺ fraction is in good agreement with that for the carbon-foil-transmission experiment. In the low energy region (<250 keV), however, the He⁺ fraction decreases as the exit angle decreases.     

Optical and electrical properties of different oriented CVD diamond films

Due to different oriented diamond films having different properties, in this paper optical and electrical properties of different oriented diamond films have been investigated. The measured results indicate diamond films are of high quality and the properties of the (0 0 1)-oriented diamond film are better than those of the (1 1 1)-oriented one. Refractive index and extinction coefficient of (0 0 1)-oriented diamond film in the wavelength range of 2.5-12.5 μm is 2.391 and in the order of 10⁻⁵, respectively. And for the (1 1 1)-oriented one it is 2.375 and in the order of 10⁻⁴. The dark current of the (0 0 1)-oriented diamond film is 33.7 nA under an applied electric field of 100 kV/cm. The resistivity of the (0 0 1)-oriented diamond film obtained is about 2.33 × 10¹⁰ Ω cm. The current of (0 0 1)-oriented diamond film is almost no change with the time testing.     

Combined X-ray-electron imaging techniques: Limitations on lateral resolution

Capabilities of the imaging techniques, in which X-rays are converted to electrons and then the emitted electrons are registered by means of an electron microscope, are analyzed, the focus being on the factors limiting lateral resolution at the stage of electron emission. Bearing in mind the tendency to use harder synchrotron X-rays for some combined X-ray-electron microscopy methods, calculations were made for two significantly different X-ray energies: E = 1.828 keV (K-edge of Si) and E = 11.923 keV (L₃-edge of Au). By using Monte Carlo simulations of the electron trajectories beneath the surface of the sample we show that the radius of the spot from which photoelectrons are emitted could be as small as 1 nm. However, when proper account is taken of an entire electron cascade associated with the re-building of electron shells after photoelectron emission, spots more than one order of magnitude larger result, limiting the best lateral resolution to 20–30 nm.     

Scattering effects and damping of electrons in Pt(1 1 1) and Cu(1 1 1)

In this work an analysis of experimental and theoretical data associated with the scattering and attenuation of electrons in the Pt(1 1 1) and Cu(1 1 1) crystalline samples is presented. The information about the crystalline structure of the first few atomic layers was obtained by the directional elastic peak electron spectroscopy (DEPES) at the primary electron beam energies E_p from 1.5 keV to 2.0 keV. The comparison of the experimental and theoretical DEPES distributions indicates a qualitative agreement between experiment and theory. The relative signal values associated with the intensity maxima were found to be different. The latter effect suggest that the electron attenuation in the crystalline samples can have an anisotropic character. A qualitative analysis of the characteristic pattern around the [1 1 1] direction concerning the calculation of the scattering factors was performed. The collective scattering of electrons by atoms located around the threefold symmetry axis resulting in the so called ring focusing effect is discussed.     

New electron states that float on semiconductor and metal surfaces

We report first-principle electronic-structure calculations using the local density approximation in the density functional theory for Si(1 0 0) and Al(1 0 0) surfaces. We find a novel class of surface electron states that are distributed not near atomic arrays but float around with the maximum amplitudes being substantially separated from matters. We also find that the origin of the unusual character is quantum spill of electron density from matters and exchange-correlation effects among electrons.     

Crystalline structure of the Au(1 1 1) surface revealed by stereographic intensity DEPES maps

The intensity of elastically backscattered electrons at the primary electron beam energy 1.9 keV was used to obtain a stereographic map of Au(1 1 1) by means of the directional elastic peak electron spectroscopy (DEPES). An experimental result is compared with the theoretical data obtained by using multiple scattering calculations (MS) performed for both not-reconstructed and model-reconstructed clusters. The lateral lattice misfit of the first layer leads to quantitative changes of theoretical intensities showing a sensitivity of DEPES to the short atomic chain axial order. This comparison proves that a main contribution of the experimental contrast originates from a higher background level. Moreover an anisotropy of the inelastic mean free path is discussed in the paper.     

Characteristic and non-characteristic X-ray yields produced from thick Ti element by sub-relativistic electrons

Measurements are performed to study the electron impact energy dependence of doubly differential bremsstrahlung yields (DDBY) and of characteristic Ti K_α line yields produced from sub-relativistic electrons (10–25 keV) colliding with a thick Ti (Z = 22) target. The emitted radiation is detected by a Si-PIN photo-diode detector with energy resolution (FWHM) of 180 eV at 5.9 keV. The measured data of DDBY are compared with the results predicted by Monte-Carlo (MC) simulations using the general purpose PENELOPE code. A reasonable agreement is found between experimental and simulation results within the experimental uncertainty of measurements of 12%. Characteristic Ti K_α yields are obtained for the considered impact energy range and they are compared with the existing theoretical results. A good agreement is found between the present measurements and the theoretical calculations. Furthermore, data are presented for impact energy dependence of the ratio K_α/(K_α+ K_β) of a thick Ti target under impact of 10–25 keV electrons. The ratio shows a very weak dependence on impact energy in the studied range. The average value of the ratio is found to be 0.881 ± 0.003.     

Surface structure of cubic diamond nanowires

Presented are results of our ab initio study of the surface reconstruction and relaxation of (1 0 0) surfaces on diamond nanowires. We have used a density function theory within the generalized-gradient approximation using the Vienna ab initio simulation package, to consider dehydrogenated and hydrogenated surfaces. Edges of nanowires offer a new challenge in the determination of surface structure. We have applied the methodology for stepped diamond (1 0 0) surfaces to this problem, and consider it useful in describing diamond nanowire edges to first approximation. We have found that dimer lengths and atomic layer depths of the C(1 0 0)(2 × 1) and C(1 0 0)(2 × 1):H nanowire surfaces differ slightly from those of bulk diamond and nanodiamond surfaces. The aim of this study is provide a better understanding of the effects of nano-scale surfaces on the stability of diamond nanostructures.     

Response of chemical vapor deposition diamond detectors to X-ray

The outstanding properties of diamond, such as radiation hardness, high carrier mobility, high band gap and breakdown field, distinguish it as a good candidate for radiation detectors. The detector's performance is strongly limited by the concentration of defects (grain boundaries and/or impurities) in chemical vapor deposition (CVD) diamond. We report the response of free-standing CVD diamond with a thickness of 300 μm and area of 2×2 cm², synthesized by a hot filament chemical vapor deposition (HFCVD) technique, to 5.9 keV X-ray radiation from a ⁵⁵Fe source. The linear I-V characteristics indicate that CVD diamond has good ohmic contacts. This detector also shows good results such as dark-current of 10⁻⁸ A, photocurrent of 10⁻⁶ A, energy resolution <0.4%, and a high ratio of signal to noise.     

Characteristic energy band values and electron attenuation length of a chemical-vapor-deposition diamond (0 0 1)2 × 1 surface

The characteristic energy band values such as the Fermi-level position with respect to valence band top for a boron-doped p-type hydrogen-terminated chemical-vapor-deposition (CVD) diamond (0 0 1)2 × 1 surface and for a clean CVD diamond (0 0 1)2 × 1 surface have been determined by a new method with an accuracy of ±0.02 eV. The electron attenuation length for the clean diamond (0 0 1)2 × 1 surface for the electron kinetic energy of C 1s X-ray photoemission peak by Mg Kα excitation is experimentally determined to be 2.1-2.2 nm. These values are compared and discussed with the previously reported experimental and simulation values.     

Local electronic edge states of graphene layer deposited on Ir(1 1 1) surface studied by STM/CITS

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to observe electronic structure of the edges of monolayer graphite film deposited on the Ir(1 1 1) surface. The electronic structure derived from the tunnelling spectra revealed peak in electron local density of states very close to the Fermi level. This electronic state was interpreted in terms of localised edge state caused by the topology of the π electrons networks typical for the zig-zag edges. The observed maximum of local density of states at about 0.2 eV above the Fermi level was ascribed to the presence of resonant state caused by the appearance of disclinations centres in the vicinity of the graphite edges.     

Substrate induced enhancement of atomic layer growth on Al(1 1 1): The effect of the mass anisotropy

The evolution of the thin-film morphology is studied by molecular dynamics simulations and we find a strong tendency of adatom island growth on the (1 1 1) surface of a thin Al overlayer placed on a heavy substrate (Pt(1 1 1)) when the system is subjected to low-energy Xe⁺ irradiation. The large adatom yield of 10²−10³ is found for 5-10 keV rare gas ion impacts. We also find that the mass effect due to the small atomic mass ratio (large mass anisotropy) in the bilayer has a direct effect on the atomic layer growth on the surface. A mass anisotropy induced scattering of the light overlayer atoms from the heavy substrate contributes to the enhancement of adatom production. It has been found that the volume increase (density decrease) of the amorphous intermixed phase keeps the adatoms on the surface. The competition between cratering and atomic layer growth can also be seen: three events out of 10 leads to cratering (erosion) morphology at 6 keV ion energy. The substrate induced enhancement of atomic layer growth might be a promising tool for making arrays of nanodots as nanotemplates for nanofabrication.     

Experimental and theoretical study of charge transfer in hydrogen ion scattering from a graphite surface

Ion scattering spectrometry (ISS) with time of flight (TOF) analysis is employed to measure the ion fraction of positively charged hydrogen (H⁺) projectiles scattered from a well characterized highly oriented pyrolitc graphite (HOPG) surface at a 45° scattering angle, various ingoing/outgoing angles and two different incoming energies (4 and 5 keV). In the theoretical approach, the negative ionization probability is calculated by employing a Green's function formalism to solve the dynamic collisional process. Both theoretical and experimental results are analyzed and contrasted. The theoretical negative ion fraction evolution during the collisional process is described in detail.     

Formation of Ti and TiN ultra-thin films on Si by ion beam sputter deposition

Ultra-thin titanium and titanium nitride films on silicon substrate were obtained by ion beam sputtering of titanium target in vacuum and nitrogen atmosphere, using argon ions with energy of 5 keV and 15 μA target current. Elemental composition and chemical state of obtained films were investigated by X-ray photoelectron spectroscopy with using Mg-Kα X-ray radiation (photon energy 1253.6 eV). It was shown that it is possible to form both ultra-thin titanium films (sputtering in vacuum) and ultra-thin titanium nitride films (sputtering in nitrogen atmosphere) in the same temperature conditions. Photoelectron spectra of samples surface, obtained in different steps of films synthesis, detailed spectra of photoelectron emission from Si 2p, Ti 2p, N 1s core levels and also X-ray photoelectron spectra of Auger electrons emission are presented.     

Energieanalyse im Elektronenstreudiagramm einer dünnen Silizium-Folie

The energy distribution of electrons (47 keV) scattered by thin foils of silicon was measured by means of an electrical retarding field. Laue spots, Kikuchi lines and the region of the background near the primary spot from where the Kikuchi line originates, were investigated. The spectra show characteristic differences indicating a high portion of multiple energy-losses (of 16,9 eV) in the Kikuchi lines and in the background region, and a small portion in the Laue spots, indicating the different ways by which electrons are scattered into the various scattering angles.     

Characterization of diamond-like carbon films by SEM, XRD and Raman spectroscopy

Diamond-like carbon films were deposited by electrolysis of a water-ethanol solution on Cu at low voltages (60-100 V) at 2 mm interelectrode separation. The films were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and Raman spectroscopy. The films were found to be continuous and compact with uniform grain distribution. Raman spectroscopy analysis revealed two broad bands at ∼1350 and ∼1580 cm⁻¹. The downshift of the G band of graphite is indicative of the presence of DLC. For XRD analysis, the three strong peaks located at 2θ values of 43.2°, 74.06° and 89.9° can be identified with reflections form (1 1 1), (2 2 0) and (3 1 1) plane of diamond.     

Surface plasmon excitation at a Au surface by 150-40,000 eV electrons

Spectra of electrons with energies between 5 and 40 keV reflected from a homogeneous Au surface have been measured and analyzed to give the normalized distribution of energy losses in a single surface and volume excitation, as well as the total probability for excitation of surface plasmons. The resulting single scattering loss distributions compare excellently in (absolute units) with data from previous work taken at lower energies (150-3400 eV). An empirical relationship is derived for the total surface excitation probability as a function of the energy. For high energies the surface scattering zone represents only a small fraction of a typical electron trajectory and hence interference effects should be small at these energies. Since we find that both the energy dependence of the surface plasmon excitation probability and the shape of the single scattering loss distributions are the same at high and low electron energies, we conclude that there is no evidence for interference effects in the entire energy range studied.