Ion beam effects on dust grains

Our experimental set-up allows us to keep a single dust grain trapped in a high-vacuum chamber for hours or days and to affect it by electron or ion beams. It apparently makes possible to study each particular charging process separately but, as our results have shown, the influence of emissions from the surfaces surrounding the experimental space cannot be neglected in interpretation of results.The paper is devoted to experimental and theoretical investigations of ion field emission from spherical non-conducting monodisperse melamine resin grains with diameters of , 4.97, and that were exposed by the ion beam with energies ranging from 0.3 to 5 keV. We present the methodology of the determination of different charging currents as well as the determination of a grain size. These quantities are then used for estimation of equilibrium potentials. Results obtained using He ions show that ion field emission from the investigated grains can be recorded at field intensities of the order of 10⁹ V/m and strongly depends on a history of the grain charging.     

Field emission properties of composite nano-Au/DLC films prepared by CVD technique

Nanocrystalline gold incorporated diamond-like carbon (nano-Au/DLC) films were deposited by capacitively coupled plasma (CCP) r.f. chemical vapour deposition (CVD) technique. Gold content in the DLC matrix was controlled by the amount of argon in the argon + methane mixture in the plasma. Field emission properties of these films were studied critically. Bonding environment (sp²/sp³ ratio) in these films was obtained from Raman measurements. Modification of the surface with the incorporation of gold nanocrystallites and associated modulation of sp²/sp³ ratio in the films culminated in improved field emission properties. Fowler-Nordheim model was used to ascertain the work function (?) which varied between 19 and 64 meV. The field factor (β) varied between 172 and 1050.     

Crystallinity and surface roughness dependent photoluminescence of Y1 − xGdxVO4:Eu thin films grown on Si (100) substrate

Gd-substitution dependency on the photoluminescence in YVO₄:Eu³⁺ films grown on Si (100) substrates have been investigated by analyzing the crystalline phase and surface morphology of the films. The substitution of Gd induced not only the change of crystallinity but also the surface roughness of the films. The change of the preferred orientation in the films can be explained on the basis of the lattice mismatch between the film and Si (100) substrate. Also, the surface roughness of the films shows the similar behavior to the grain size as a function of Gd amounts. The photoluminescence (PL) intensity obtained from the Y_1 − xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL intensity is more dependent on the surface roughness than the crystallinity of films. In particular, the incorporation of Gd into the YVO₄ lattice remarkably enhanced the intensity of PL and the highest emission intensity of Y_0.57Gd_0.40Eu_0.03VO₄ film was 3.3 times higher than that of YVO₄:Eu³⁺ film.     

Low-dimensional SiC nanostructures: Fabrication, luminescence, and electrical properties

Nanostructured silicon carbide has unique properties that make it useful in microelectronics, optoelectronics, and biomedical engineering. In this paper, the fabrication methods as well as optical and electrical characteristics of silicon carbide nanocrystals, nanowires, nanotubes, and nanosized films are reviewed. Silicon carbide nanocrystals are generally produced using two techniques, electrochemical etching of bulk materials to form porous SiC or embedding SiC crystallites in a matrix such as Si. Luminescence from SiC crystallites prepared by these two methods is generally believed to stem from surface or defect states. Stable colloidal 3C-SiC nanocrystals which exhibit intense visible photoluminescence arising from the quantum confinement effects have recently be produced. The field electron emission and photoluminescence characteristics of silicon carbide nanostructures as well as theoretical studies of the structural and electronic properties of the materials are described.     

Combined effects of heat treatment and seed layer materials on magnetic properties of CoCrPt perpendicular media

Samples of glass/Ti(20 nm)/CoCrPt (20 nm)/Ti (3.2 nm) and glass/seed (Cr or Cu)/Ti(20 nm)/CoCrPt(20 nm)/Ti (3.2 nm) were deposited by magnetron sputtering at ambient temperature or 200 °C. All samples were post-annealed at 400 °C for 15 min. For either Cu or Cr seed layer, post-annealing can enhance the out-of-plane coercivity. For post-annealed samples with either Cr or Cu seed layers, the ambient temperature growth induce an enhancement in the coercivity but a reduction in the slope of hysteresis loop at the coercivity, in comparison with that of the elevated temperature growth. With identical post-annealing and growth conditions, Cr or Cu seed layers can enhance the out-of-plane coercivity, in comparison with those grown on bare substrates. The nucleation field is negative for Cr seed layer and positive for the Cu seed layer although they have close coercivity. With the same growth and post-annealing conditions, the magnetic properties of CoCrPt layers depend on the seed layer thickness. Evolution of magnetic properties can be explained in terms of changes of structural properties of constituent layers.     

Electron energy-loss spectroscopy investigation of Y2O3 films on Si (001) substrate

Electron energy -loss spectroscopy has been used to investigate the interface between a Y₂O₃ film and the silicon substrate. The chemical composition of the interface layer is revealed to be nearly pure amorphous SiO₂. Yttrium silicates are found at the Y₂O₃/SiO₂ interface region. The formation of the interfacial yttrium silicates has been interpreted by the direct chemical reaction between the deposited Y₂O₃ film and the SiO₂ interface layer. The Si L₂₃ and O K edges of yttrium silicates (Y₂SiO₅ and Y₂Si₂O₇) have been calculated by the first-principle full multiple - scattering method. The theoretical results are consistent with the experimental spectra, which confirms the formation of yttrium silicates.     

Oxidation kinetics of Ni metallic films: Formation of NiO-based resistive switching structures

Resistive switching controlled by external voltage has been reported in many Metal/Resistive oxide/Metal (MRM) structures in which the resistive oxide was simple transition metal oxide thin films such as NiO or TiO₂ deposited by reactive sputtering. In this paper, we have explored the possibility to form NiO-based MRM structures from the partial oxidation of a blanket Ni metallic film using a Rapid Thermal Annealing route, the remaining Ni layer being used as bottom electrode. X-ray diffraction was used to apprehend the Ni oxidation kinetics while transmission electron microscopy enabled investigating local microstructure and film interfaces. These analyses have especially emphasized the predominant role of the as-deposited Ni metallic film microstructure (size and orientation of crystallites) on (i) oxidation kinetics, (ii) NiO film microstructural characteristics (crystallite size, texture and interface roughness) and (iii) subsequent electrical behavior. On this latter point, the as-grown NiO films were initially in the low resistance ON state without the electro-forming step usually required for sputtered films. Above the threshold voltage varying from 2 to 5 V depending on oxidation conditions, the Pt/NiO/Ni MRM structures irreversibly switched into the high resistance OFF state. This irreversibility is thought to originate in the microstructure of the NiO films that would cause the difficulty to re-form conductive paths.     

Formation of Ni2Si silicide in Ni/Si bilayers by ion beam mixing

We have studied ion mixing in Ni-Si(1 1 1) bilayers using noble gas ions. Thin Ni films of 45 nm thickness, deposited on a Si (1 1 1) substrate, were irradiated with 175 keV Kr and 110 keV Ar ions at the same fluence of 4×10¹⁶ ions/cm² at room temperature. The formation of the mixing and the elemental depth profile were investigated by Rutherford backscattering spectrometry. In the Ar irradiated sample, there was no structural change. On the other hand, we have noted the formation of Ni₂Si for the sample irradiated with Kr ions. X-ray diffraction measurements confirmed the formation of the Ni₂Si phase. The surface morphology of the Kr irradiated sample was also studied by scanning electron microscopy.     

The effect of the trapping of dust grains on the sheath structure and the charging in a plasma

A model of the trapping of dust grains is shown in a plasma for the first time. The multiple sheath potential, the space charge density and the multiple electric field associated with our model are simulated. Our result explains the confinement of the dust grains observed in the experiment.     

The effect of the trapping of dust grains on the sheath structure and the charging in a plasma

A model of the trapping of dust grains is shown in a plasma for the first time. The multiple sheath potential, the space charge density and the multiple electric field associated with our model are simulated. Our result explains the confinement of the dust grains observed in the experiment.     

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaO_x thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaO_x films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.     

Heteroepitaxy of zinc-blende SiC nano-dots on Si substrate by organometallic ion beam

The self-assembled SiC nano-dots were fabricated on Si(111) substrate at low-temperatures using the organometallic ion beam deposition technique. The single precursor of methylsilicenium ions (SiCH₃⁺) with the energy of 100 eV was deposited on Si(111) substrate at 500, 550 and 600 °C. The characteristics of the self-assembled SiC nano-dots were analyzed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy and atomic force microscope (AFM). The RHEED patterns showed that the crystal structure of the SiC nano-dots formed on Si(111) substrate was zinc-blende SiC (3C-SiC) and it was heteroepitaxy. The self-assembled SiC nano-dots were like a dome in shape, and their sizes were the length of 200-300 nm and the height of 10-15 nm. Despite the low-temperature of 500 °C as SiC crystallization the heteroepitaxial SiC nano-dots were fabricated on Si(111) substrate using the organometallic ion beam.     

Thermal oxidation properties of titanium nitride and titanium-aluminum nitride materials — A perspective for high temperature air-stable solar selective absorber applications

Air-stable high temperature solar selective surfaces have the advantages of simplifying the design, and reducing the cost of solar thermal energy conversion systems. Previous studies on the properties of titanium nitride (TiN) or titanium-aluminum nitride (TiAlN) films suggested that these materials could be a candidate for solar energy applications. In this paper, we report that oxidation occurs at 450 °C, and an oxide layer of about 20-30 nm was formed after only a few minutes of heat treatment with oxygen. The thickness of the oxide layer is comparable to the thickness of the absorbing layer of the solar thermal selective absorbers, which can affect significantly the solar thermal performance. TiN produced at higher nitrogen pressure (2.1 Pa with 40% nitrogen in argon) could absorb oxygen more easily into bulk and was less oxidation resistant during the heat treatment than that produced at 0.4 Pa of 40% nitrogen in argon. The hardness after the oxidation treatment was slightly increased by approximately 10%, consistent with reported oxidation resistant properties of this material for mechanical protection applications. As a result of this study, TiN or TiAlN as an element may not be suitable candidates for use as solar selective absorbers in air-stable high temperature applications.     

Ion beam mixing of Co/Fe multilayers: Magnetic and structural properties

[Co(7 nm)/Fe(7 nm)]₆ multilayers were electron-beam evaporated onto Si(100) substrates in ultrahigh vacuum and irradiated at room temperature with 200-keV Xe ions, leading to ion beam mixing within the Co/Fe multilayer, but not with the Si substrate. Irradiation-induced changes in structural and magnetic properties were characterized by means of Rutherford backscattering spectroscopy, X-ray diffraction and in-plane magneto-optical Kerr effect. Irradiation with 1 × 10¹⁶ Xe ions/cm² induced Co/Fe intermixing to a 1 : 1 atomic concentration ratio (RBS) and the formation of the Fe₅₀Co₅₀ permendur phase in the intermixed zone (XRD). For lower ion fluences, the coercivity decreased strongly, but then increased slowly for higher fluences. The angular pattern of the relative remanence showed a perfect uniaxial anisotropy. The magnetic energy density was parametrized with the expression E_s / M_s ∝ (K_u1 / M_s) sin²(φ − φ₀) + (K_u2 / M_s) sin⁴(φ − φ₀), M_s being the saturation magnetization and φ₀ the symmetry angle. The second-order term K_u2 / M_s was found to decrease strongly with increasing Xe fluence.     

Influence of hydrogen annealing on the properties of hafnium oxide thin films

Thin films of hafnium oxide were deposited by electron beam evaporation, and were subsequently annealed in hydrogen. X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, photoluminescence, spectrophotometry, and current–voltage measurements were performed to investigate the structural, chemical, optical, and electrical properties of the films. As-deposited films were amorphous and nearly stoichiometric. Annealing led to crystallization of the films, and reduction of stoichiometry. Photoluminescence measurements revealed the presence of oxygen-related defects. Optically, the films were transparent with a wide band gap, and this was not affected by hydrogen annealing. Moreover, the films were suitable as anti-reflection coatings on silicon. The electrical resistivity of the films was significantly reduced as a result of annealing.     

Transmission electron microscopy study of the initial growth stage of GaSb grown on Si (001) substrate by molecular beam epitaxy method

The microstructural properties at the initial growth stage of the GaSb heteroepitaxial growth on a silicon (Si) substrate were investigated using transmission electron microscopy. Well-separated and tall GaSb islands were observed when GaSb was directly grown on a Si substrate (sample A). On the other hand, GaSb was grown to the coalesced and flat islands when a low-temperature AlSb buffer (sample B) was introduced. The different morphologies of the GaSb islands were related to the microstructural properties of the interface between the GaSb and the Si substrate. The GaSb/Si interface was rough, and disordered atomic arrangements were observed at the interface in sample A. On the other hand, the GaSb/Si interface was flat, and well-ordered atomic arrangements appeared at the interface in sample B. Entirely different mechanisms for the relaxation of a misfit strain were demonstrated from a microstructural viewpoint.     

Charge photogeneration and recombination in poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene composite films studied by photocurrent response

The photocurrent spectral responses of poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene (C60) composites are measured as a function of C60 concentration. At low concentration, the relationship between the external quantum efficiency (EQE) and absorption spectra is exhibited as the strengthened antibatic effect, and the EQE of the composite devices declines with increasing concentration of C60. At higher concentration, however, the maximum EQE gradually coincides with the absorption peak (symbatic response) and the EQE of composite devices begins to increase with increasing C60 concentration. It is proposed that at low concentration, dopant C60 increases the self-absorption rate of composite films, and charge trapping by C60 molecules causes the loss of efficiency. At high C60 concentration, the large-scale aggregations in composite films build pathways for charge carrier transport to respective electrodes, inhibiting the self-absorption effect and charge recombination on C60.     

Grain boundary effects on the electrical and magnetic properties of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 manganites

Electrical and magnetic properties of orthorhombic Pr_2/3Ba_1/3MnO₃ (PBMO) and La_2/3Ca_1/3MnO₃ (LCMO) manganites with considerable difference in variance factors (σ²) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower σ² LCMO system. Low field magnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott's 3-D variable range hopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field. Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron-magnon scattering process. LFMR at 77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO.     

Investigation of electronic structure of Si nanocrystals and their interface with host matrix in P-doped SiO2:Si and Al2O3:Si nanocomposites

The system of the nanoinclusions of Si in the SiO₂ and Al₂O₃ matrix (SiO₂:Si, Al₂O₃:Si) attracts great attention due to its ability of the luminescence in visible and near-IR range of spectrum. The influence of the P ion alloying on the electronic structure of nanocomposites was investigated. The P ion doping and post-annealed at T = 1000 °C (2 h) results in the enhancement of the photoluminescence (PL) peak connected with the Si nanocrystals. The electronic structure was investigated by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy losses spectroscopy (HREELS) methods. Ion surface modification and annealing forms the special nanostructure with Si nanocrystals in SiO₂ and Al₂O₃ matrix having high density of interfaces with special atomic structure and various degree of oxidation of Si atoms on the boundaries. HREELS investigations show that the P ion doping increases the probability of interband transitions in SiO₂:Si and Al₂O₃:Si composites.     

Oxidation resistance of thin boron carbo-nitride films on Ge(100) and Ge nanowires

Chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BC_0.7N_0.08, or BCN) on Ge(100) and Ge nanowire (GeNW) surfaces was studied to determine the ability of BCN to prevent oxidation of Ge. X-ray photoelectron spectroscopy was used to track Ge oxidation of BCN-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO₂ process. BCN overlayers incorporate O immediately upon ambient or water exposure, but it is limited to 15% O uptake. If the BCN layer is continuous, the underlying Ge(100) surface is not oxidized despite the incorporation of O into BCN. The minimum continuous BCN film thickness that prevents Ge(100) oxidation is ~ 4 nm. Thinner films (≤ 3.2 nm) permitted Ge(100) oxidation in each of the oxidizing environments studied. GeNWs with a 5.7 nm BCN coating were resistant to oxidation for at least 5 months of ambient exposure. High resolution transmission electron microscopy images of HfO₂/BCN/Ge(100) cross-sections and BCN-coated GeNWs reveal clean, abrupt BCN-Ge(100) interfaces.