Focused electron beam induced deposition as a tool to create electron vortices

Focused electron beam induced deposition (FEBID) is a microscopic technique that allows geometrically controlled material deposition with very high spatial resolution. This technique was used to create a spiral aperture capable of generating electron vortex beams in a transmission electron microscope (TEM). The vortex was then fully characterized using different TEM techniques, estimating the average orbital angular momentum to be ∼0.8ℏ per electron with almost 60% of the beam ending up in the ℓ = 1 state.     

AFM study of a SiC film grown on Si(1 0 0) surface using a C2H4 beam

A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C₂H₄) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).     

Analysis of microscopic parameters of surface charging in polymer caused by defocused electron beam irradiation

The relationship between microscopic parameters and polymer charging caused by defocused electron beam irradiation is investigated using a dynamic scattering-transport model. The dynamic charging process of an irradiated polymer using a defocused 30 keV electron beam is conducted. In this study, the space charge distribution with a 30 keV non-penetrating e-beam is negative and supported by some existing experimental data. The internal potential is negative, but relatively high near the surface, and it decreases to a maximum negative value at z = 6 μm and finally tend to 0 at the bottom of film. The leakage current and the surface potential behave similarly, and the secondary electron and leakage currents follow the charging equilibrium condition. The surface potential decreases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. The total charge density increases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. This study shows a comprehensive analysis of microscopic factors of surface charging characteristics in an electron-based surface microscopy and analysis.     

Electron penetration depth in amorphous AlN exploiting the luminescence of AlN:Tm/AlN:Ho bilayers

The penetration depth of electron in amorphous aluminum nitride (AlN) is determined in terms of energy loss per unit length using electron beam in a cathodoluminescence (CL) apparatus. Thin films bilayers of holmium doped aluminum nitride (AlN:Ho) and thulium doped aluminum nitride (AlN:Tm) are deposited on silicon substrates by rf magnetron sputtering method at liquid nitrogen temperatures. The bilayers structure consisted of a 37.8 nm thick AlN:Tm film on the top of a 15.3 nm thick AlN:Ho film. Electron beam of different energies are allowed to penetrate the AlN:Tm/AlN:Ho bilayers film. The spectroscopic properties of AlN:Ho and AlN:Tm, the thickness of the film and the energies of electron beam are used to calculate the penetration depth of electron in amorphous AlN. Electron beam of 2.5 keV energy was able to pass through the 37.8 nm thick AlN:Tm film. The electron penetration depth for AlN is found to be 661.4 MeV/cm.     

Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy

We report on the optical characteristics of InAs quantum dots based on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [0 1 ?1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(1 0 0) quantum dots providing their emission wavelength in 1.55 μm are obtained, and good performances of quantum dots lasers are fabricated.     

Self-assembly of well-ordered and highly uniform nanoripples induced by focused ion beam

Well-ordered and highly uniform nanoripple structures on the surface of single crystal LaAlO₃ (1 0 0), SrTiO₃ (1 0 0) and Al₂O₃ (0 0 0 1) were formed via self-assembly (not by beam writing) by focused ion-beam bombardment. The morphology and topography of nanoripple structures were characterized using in-situ focused ion-beam/scanning electron microscope, as well as ex-situ atomic force microscopy. Under off-normal bombardment without sample rotation, the characteristic wavelength of nanoripples varying from 248 to 395 nm on the LaAlO₃ (1 0 0) surface can be obtained by changing ion fluence and incident angle. When all sputtering parameters except the ion fluence are constant, the wavelength of nanoripples is increased with the enhanced ion fluence. These results demonstrate the potential application of using ion sputtering method for fabricating the well-ordered and highly uniform nanoripples which can be used in nanodevices.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Novel bending sensor based on a meniscus shaped beam with LPG

A novel bending sensor based on a long period fiber grating (LPG) is presented. A LPG was glued into a V-shaped groove, which lies on the lower surface of a meniscus shaped beam. It is found that the transmission optical power of the LPG changed linearly with the variation of the bending of the beam. The bending applied on the beam can be measured by detecting the intensity variation of the LPG's resonant dip wavelength. Under a relative large bending measured range from 0 to 7.5 m^?1, the sensitivity of 3.003 dB m^?1 and curvature resolution of 0.001 m^?1 have been achieved for the proposed bending sensor.     

Transition of magnetic anisotropy in Ni/GaAs (0 0 1) observed by magnetization and ferromagnetic resonance

Polycrystalline thin Ni films deposited onto GaAs (0 0 1) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film ⩽12 nm. This becomes in-plane in the films having thicknesses ⩾15 nm. The films are deposited onto the n-type GaAs (0 0 1) substrate by the usual thermal evaporation method and also by the electron beam evaporation in ultra high vacuum onto a GaAs epilayer in the standard molecular beam epitaxy system. The magnetization and ferromagnetic resonance (FMR) are observed in the temperature range from 4.2 to 300 K. For the discussion of the microscopic origin of the anomalous properties in magnetization and FMR experiments, the experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization data. This calculated anisotropy is able to explain the temperature and angle dependency of the FMR spectra of the Ni films. Hence the magnetization and FMR spectra are in agreement with the type of the anisotropy and its temperature dependency. In addition to these, the temperature dependence of the in-plane magnetic anisotropy is able to explain the previously reported anomalous effect of reducing the squareness at low temperatures in Ni/GaAs.     

Wide-field imaging using a tunable terahertz free electron laser and a thermal image plate

The appearance of intense terahertz sources such as quantum cascade laser and free electron laser opens up new opportunities for 2D imaging. Though microbolometer and pyroelectric arrays are promising recorders, they are of small size and cannot be used when wide-field imaging in the longwave region is required. We applied for terahertz imaging 3″ × 3″ and 6″ × 6″ Macken Instruments Inc. “thermal image plates”, a set of thermal sensitive phosphor screens operating in a room temperature environment. The Novosibirsk free electron laser was used as a source of radiation. We have found that the response of thermal image plate is linear until the relative quenching is less than 60% of the initial luminescence intensity. The response curve follows the Seitz–Mott law. The threshold sensitivity was found to be 100 mW/cm² at 1.5 THz and 40 mW/cm² at 2.3 THz. Interferograms, holograms, and terahertz beam spatial distributions recorded in the spectral range of 1.2–2.5 THz are given as examples.     

Beam dynamics of 20 MeV MIRRORCLE-type tabletop storage ring

An experimental study on beam dynamics in MIRRORCLE-20, a tabletop storage ring of 15 cm orbit radius, was performed. Measurement of the infrared (IR) synchrotron light is the tool of this study. The IR emission is enhanced by a circular optics, named photon storage ring (PhSR), placed around the electron orbit, and is collected by a magic mirror associated with two plane mirrors in the storage ring. The measured average IR power in mid-IR region (λ < 50 μm) is ～59 mW. The observed stored beam current is about 1.2 A at maximum, which represents a record for a storage ring. The observed beam size is about 74 × 3 mm². We conclude that this very long beam size is due to the large betatron oscillation of 2/3 resonance injection.     

Growth of Nb thin films on SiO2

Nb thin films have been prepared by electron beam evaporation under ultrahigh vacuum conditions on fused silica substrates at various temperatures, and their structural and morphological evolutions have been investigated using X-ray diffraction and atomic force microscopy. The crystallographic texture of the Nb films is found to depend on the growth temperature. At room temperature, the [1 1 0] texture is dominant. However, at 200°C, the [3 1 0] oriented growth is favored, co-existing with [1 1 0] and [2 0 0] oriented grains. At 400–600°C, a completely [1 1 0] textured film is formed. At even higher temperature (800°C), a complex texture of [1 1 0] (dominant), [2 0 0] and [3 1 0] is observed again. It is also found that the single [1 1 0] textured Nb films have smooth surfaces, and the complex textured Nb films have rough surfaces.     

A competition between (0 0 1) and (0 1 1) alignments in yttria stabilized zirconia thin films fabricated by ion beam assisted deposition

Biaxially textured yttria stabilized zirconia (YSZ) thin films, were deposited on glass substrates by ion beam assisted deposition method with different deposition time. As contrasts, films were also fabricated without assisting ion beam. The orientation properties of the films were characterized by X-ray diffraction. A comparative study shows that there is a competition between (0 0 1) and (0 1 1) alignments during the growth process. Assisting ions make the films (0 0 1)-advantaged and biaxially textured. The competitive growth and the orientation development are explained by selective resputtering and anisotropic damage on growing films induced by assisting ions.     

Ion beam as a probe to study the behavior of hydrogen on silicon surfaces

Hydrogen adsorption and its behavior on Si surfaces is studied by ion beam techniques in the energy range of MeV–keV. Elastic recoil detection analysis employing MeV ion beams is one of the most reliable experimental techniques for direct determination of absolute hydrogen coverages on Si surfaces. Results of its application to Si(1 0 0) and Si(1 1 1) clean surfaces are described. Important new results of the role of adsorbed hydrogen on the growth process or structures of metallic thin films on Si(1 1 1) surfaces also are described. Characterization of the growth process or structure of the thin films, as well as the characterization of hydrogen, is performed by ion beam techniques.     

Irradiation induced modifications in morphology and magnetic property of Mn/Si structure

Mn films of ∼50 nm has been deposited by electron beam evaporation technique on cleaned and etched Si [(1 0 0), 8–10 Ω cm] substrates to realize a Mn/Si interfacial structures. The structures have been irradiated from energetic (∼100 MeV) ion beam from Mn side. The irradiated and unirradiated structures have been characterized from atomic force microscopy, X-ray diffractometry, magnetic force microscopy, and vibrating sample magnetometer facilities. It has been found that surface/interfacial granular silicide phases (of Mn_xSi_y) are formed before and after the irradiation with a irradiation induced modifications of surface morphology and magnetic property. The surface/interface roughness has been found to increase on the irradiation from the atomic force microscopy data. The magnetic property on the irradiation shows an interesting and significant feature of an increased coercivity and a ferromagnetic like behavior in the Mn–Si structure. The observed increased coercivity has been related to the increased roughness on the irradiation. The ferromagnetism after the irradiation is a curious phenomenon which seems due to the formation of Mn–C–Si compound from the carbon dissolved in silicon.     

Optimal ablation of fluorine-doped tin oxide (FTO) thin film layers adopting a simple pulsed Nd:YAG laser with TEM00 mode

In material processing, a laser system with optimal laser parameters has been considered to be significant. Especially, the laser ablation technology is thought to be very important for fabricating a dye-sensitized solar cell (DSSC) module with good quality. Moreover, the TEM₀₀ mode laser beam is the most dominant factor to decide the incident photon to current conversion efficiency (IPCE) characteristics. In order to get the TEM₀₀ mode, a pin-hole is inserted within a simple pulsed Nd:YAG laser resonator. And the spatial field distribution is measured by using three pin-hole diameters of 1.6, 2.0 and 4.0 mm, respectively. At that moment, each case has the same laser beam energy by adjusting the discharge voltage and pulse per second (pps). From those results, it is known that the pin-hole size of 1.6 mm has the perfect TEM₀₀ mode. In addition, at the charging voltage of 1000 V, 10 pps, the feeding speed of 6.08 mm/s and the overlapping rate (OL) of 62%, the scanning electron microscope (SEM) photograph of fluorine-doped tin oxide (FTO) thin film layers shows the best ablation trace.     

Practical aspects of strain measurement in thin SiGe layers by (0 0 4) dark-field electron holography in Lorentz mode

Dark-field electron holography (DFEH) is a powerful transmission electron microscopy technique for mapping strain with nanometer resolution and high precision. However the technique can be difficult to set up if some practical steps are not respected. In this article, several measurements were performed on thin Si_(1−x)Ge_x layers using (0 0 4) DFEH in Lorentz mode. Different practical aspects are discussed such as sample preparation, reconstruction of the holograms and interpretation of the strain maps in terms of sensitivity and accuracy. It was shown that the measurements are not significantly dependent on the preparation tool. Good results can be obtained using both FIB and mechanical polishing. Usually the most important aspect is a precise control of the thickness of the sample. A problem when reconstructing (0 0 4) dark-field holograms is the relatively high phase gradient that characterises the strained regions. It can be difficult to perform reconstructions with high sensitivity in both strained and unstrained regions. Here we introduce simple methods to minimise the noise in the different regions using a specific mask shape in Fourier space or by combining several reconstructions. As a test, DFEH was applied to the characterization of eight Si_(1−x)Ge_x samples with different Ge concentrations. The sensitivity of the strain measured in the layers varies between 0.08% and 0.03% for spatial resolutions of 3.5–7 nm. The results were also compared to finite element mechanical simulations. A good accuracy of ±0.1% between experiment and simulation was obtained for strains up to 1.5% and ±0.25% for strains up to 2.5%.     

Exploring the role of samarium in the modification of rhodium catalysts through surface science approach

In this paper we review the preparation and reaction properties of ordered SmRh surface alloys and SmO_x/Rh(1 0 0) model catalyst which have been systematically investigated by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and temperature desorption spectroscopy (TDS). The growth of Sm on Rh(1 0 0) at room temperature follows the Stranski-Krastanov mode. Thermal treatment of the Sm films on Rh(1 0 0) leads to the formation of ordered SmRh surface alloys. An “inverse” SmO_x/Rh(1 0 0) model catalyst is produced under controlled oxidation of the SmRh surface alloy. CO adsorption on the SmRh alloy and SmO_x/Rh(1 0 0) surfaces gives rise to five TDS characteristic features originating from different adsorption sites. Both the site blocking of SmO_x and the electron transfer between SmO_x and Rh substrate significantly affect the CO adsorption. Acetate decomposition on both Rh(1 0 0) and the SmO_x/Rh(1 0 0) surfaces are found to undergo two competitive pathways that yields either (i) CO(a) and O(a) or (ii) CO₂(g) and H₂(g) at high temperature. The reactive desorption of acetic acid on SmO_x/Rh(1 0 0) is dramatically different from that on Rh(1 0 0). A rapid decomposition of acetic acid to produce CO(g) and CO₂(g) can be observed only on SmO_x/Rh(1 0 0), where CO(g) becomes the predominant product at 225 K, indicating a strong promotional effect of SmO_x in the selective decomposition of acetate. Finally, we briefly describe the future outlook of research on rare earth oxide/metal model catalysts.     

18.2 nm Schwarzschild microscope for diagnostics of hot electron transport

Schwarzschild microscope at 18.2 nm for diagnostics of hot electron transport in femtosecond laser-plasma interaction has been developed. Based on the third-order aberration theory the microscope is designed for numerical aperture of 0.1 and magnification of 10. Mo/Si multilayer films with peak throughput at 18.2 nm is designed and deposited by magnetron sputtering method. The 24 lp/mm copper mesh is imaged via Schwarzschild microscope, and resolutions of less than 3 μm are measured in 1.2 mm field. The diagnostics experiment of hot electron transport is performed on 286 TW SILEX-I laser facilities, and the spatial distribution of radiation caused by hot electron is imaged by Schwarzschild microscope.     

Si-delta doping and spacer thickness effects on the electronic properties in Si-delta-doped AlGaAs/GaAs HEMT structures

In this work, we investigate the effect of the δ-Si doping on the barrier and the spacer thickness on the electronic properties of AlGaAs/GaAs HEMTs structures grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. Photoluminescence measurements as function of the temperature are used to determine the relaxation processes of the electron and the hole in the channel. The photoluminescence characterizations of Si-delta-doped AlGaAs/GaAs HEMTs structures have been studied in the 10–300 K temperature range. Low temperature PL spectra show the optical transition (E_e–h) that occurs between the fundamental states of electrons to holes in the GaAs channel. Increase of the Si-δ-doping density and decrease of the spacer width improve the two-dimensional electron gas confinement and decrease defects densities in the canal. The band structure of Si-delta-doped AlGaAs/GaAs HEMTs structures at T = 10 K has been studied theoretically using the finite differences method to self-consistently and simultaneously solve Schrödinger and Poisson equations written within the Hartree approximation.