Ultrahigh Vacuum Ion Research Articles

A new high parallel-field spectrometer at TRIUMF's β-NMR facility.

A new high field spectrometer has been built to extend the capabilities of the β-detected nuclear magnetic resonance (β-NMR) facility at TRIUMF. This new beamline extension allows β-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, and beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described.

Квантово-каскадный лазер с выводом излучения через текстурированный слой

The results of studies of ring quantum-cascade laser with a surface emission with radiation output through a textured layer formed in the layers of the top cladding of the waveguide by ultrahigh vacuum ion beam milling are presented. The far-field profile shows that the radiation is output through the windows with a textured layer in the range of angles of ~ 63–75 degrees to the normal.

A compact, ultra-high vacuum ion source for isotopically enriching and depositing 28Si thin films.

An ultrahigh vacuum (UHV) compatible Penning ion source for growing pure, highly enriched 28Si epitaxial thin films is presented. Enriched 28Si is a critical material for quantum information due to the elimination of nuclear spins. In some cases, the material must be grown by low temperature molecular beam epitaxy, e.g., scanning tunneling microscopy hydrogen lithography-based devices. Traditional high-purity physical vapor methods typically deliver a very small fraction of source material onto the target substrate, making the cost for use with highly enriched source materials very high. Thus, directed beam sources provide an efficient alternative. This UHV Penning source uses all metal or ceramic parts and a removable electromagnet to allow bake-out. The source gas is a commercial (natural isotope abundance) silane gas (SiH4), an inexpensive source material. High enrichment levels up to 99.999 87% (8.32 × 10-7 mol/mol 29Si) and high chemical purity of 99.965% are shown without postprocessing. We present and discuss the discharge properties of this new source, the ion mass spectrum when coupled to our mass filter, and the secondary ion mass spectroscopy of the grown films.

Development of a simple, low cost, indirect ion beam fluence measurement system for ion implanters, accelerators

We developed a simple, low cost user-friendly automated indirect ion beam fluence measurement system for ion irradiation and analysis experiments requiring indirect beam fluence measurements unperturbed by sample conditions like low temperature, high temperature, sample biasing as well as in regular ion implantation experiments in the ion implanters and electrostatic accelerators with continuous beam. The system, which uses simple, low cost, off-the-shelf components/systems and two distinct layers of in-house built softwarenot only eliminates the need for costly data acquisition systems but also overcomes difficulties in using properietry software. The hardware of the system is centered around a personal computer, a PIC16F887 based embedded system, a Faraday cup drive cum monitor circuit, a pair of Faraday Cups and a beam current integrator and the in-house developed software include C based microcontroller firmware and LABVIEW based virtual instrument automation software. The automatic fluence measurement involves two important phases, a current sampling phase lasting over 20–30 seconds during which the ion beam current is continuously measured by intercepting the ion beam and the averaged beam current value is computed. A subsequent charge computation phase lasting 700–900 seconds is executed making the ion beam to irradiate the samples and the incremental fluence received by the sampleis estimated usingthe latest averaged beam current value from the ion beam current sampling phase. The cycle of current sampling-charge computation is repeated till the required fluence is reached. Besides simplicity and cost-effectiveness, other important advantages of the developed system include easy reconfiguration of the system to suit customisation of experiments, scalability, easy debug and maintenance of the hardware/software, ability to work as a standalone system. The system was tested with different set of samples and ion fluences and the results were verified using Rutherford backscattering technique which showed the satisfactory functioning of the system. The accuracy of the fluence measurements is found to be less than 2% which meets the demands of the irradiation experiments undertaken using the developed set up. The system was incorporated for regular use at the existing ultra high vacuum (UHV) ion irradiation chamber of 1.7 MV Tandem accelerator and several ion implantation experiments on a variety of samples like SS304, D9, ODS alloys have been successfully carried out.

Investigation of the optical and the electrical characteristics of thin titanium dioxide films

This study dealt with the deposition of nanotitanium films with different thicknesses by using the ultrahigh vacuum ion beam sputtering system. It is found that the thickness of the titanium films was in the nanoscale thus the electric resistivity of the titanium films decreased. Moreover, the deposited titanium was transformed into titanium oxide in an oxygen atmosphere and by using a rapid annealing process. The photoelectronic properties of the nanotitanium thin film was enhanced, when the nanotitanium film was transformed to titanium dioxide.

Ultra-thin titanium nanolayers for plasmon-assisted enhancement of bioluminescence of chloroplast in biological light emitting devices

Ultra-thin titanium films were deposited via ultra-high vacuum ion beam sputter deposition. Since the asymmetric electric field of the metal foil plane matches the B-band absorption of chlorophyll a, the ultra-thin titanium nanolayers were able to generate surface plasmon resonance, thus enhancing the photoluminescence of chlorophyll a. Because the density of the states of plasmon resonance increases, the enhancement of photoluminescence also rises. Due to the biocompatibility and inexpensiveness of titanium, it can be utilized to enhance the bioluminescence of chloroplast in biological light emitting devices, bio-laser, and biophotonics.

Size evolution of nanomagnetic particles and magnetotransport properties of (Co90Fe10)20Ag80 nanogranular films: influence of Cu80Ag15Au5 underlayer

The influence of (Co90Fe10)20Ag80 film thickness and of Cu80Ag15Au5 underlayer on nanomagnetic, magnetotransport and structural properties of (Co90Fe10)20Ag80 nanogranular films prepared by ultra-high vacuum ion beam sputtering technique have been investigated using X-ray diffraction, X-ray reflectivity and vibrating sample magnetometer. Films indicate a superparamagnetic behavior. The evolution of the magnetic particle size in (Co90Fe10)20Ag80 film with different thicknesses was calculated by fit of magnetization curves using Langevin equation. A parallel resistance model has been used to determine particle size distribution. The average (mean) particle sizes range from 1.6 to 2.1 nm. A linear dependence of giant magnetoresistance (GMR) on the particle size has been found. This result is also supported by the observed linear correlation between the square root of the effective magnetization and the particle size. Furthermore, a linear correlation between the square of GMR and the distance between particles has been found. This remains unchanged also in (Co90Fe10)20Ag80 films with Cu80Ag15Au5 underlayer. Using of the underlayer leads to an enhancement of the mean particle size (1.8–2.5 nm) and GMR; however, to a reduction of distances between particles accompanied by the destruction of the 〈111〉 texture in the Ag matrix structure. The reason of the GMR effect in the nanogranular films is discussed by means of the obtained results.

Formation of microcrystalline silicon films using rapid crystal aluminum induced crystallization under low-temperature rapid thermal annealing

The process of obtaining thin film solar cells using the method of aluminum-induced crystallization under rapid thermal annealing (RTA) was investigated. 200-nm-thick amorphous Si (a-Si) film was deposited on a glass substrate using an ultra-high vacuum ion beam sputtering system. A 50-nm-thick crystal aluminum layer was then evaporated and deposited onto the a-Si film. In contrast to conventional furnace annealing, RTA can supply rapid thermal energy so that a-Si can be induced into microcrystalline-Si (μc-Si) in a short time at low temperatures. The crystal Al may promote the crystallization reaction because its surface energy is higher than 0.89 N/m, which is the minimum energy required to produce the (111) orientation. Free Si atoms are induced at the interface of the Al and Si sub-layers by the diffusion of Al along the grain boundaries. The Raman spectrum shows that the sample could be induced to crystallize at 350 °C. After the aluminum was etched, the maximum grain size was 4 μm. The carrier mobility was between 6.2 cm 2/Vs and 18.8 cm 2/Vs. The proposed method can be used to obtain μc-Si with reduced energy and time during the thermal annealing.

Current-confined-path (CCP) giant magnetoresistive (GMR) sensors

Current-perpendicular-to the plane (CPP) giant magnetoresistive (GMR) sensors with a current-confined-path (CCP) layer inserted within the Cu spacer have been manufactured using ultrahigh vacuum PVD sputtering, photolithography, and ion milling processes. Compared with a pure metallic CPP system, the CCP insertion layer enables a substantial increase in sensor resistance with an equivalent or better GMR ratio, and thus a significant improvement in the ΔRA amplitude. Heads with such a sensor have been tested under various bias currents, both quasistatically and on a spinstand. It was found that the resistance of the sensors increases with increasing bias current and voltage, following typical metallic behavior. Also, the CCP insertion layer enables operation at higher bias currents compared with the pure metallic sensors (without the insertion layer) and thus a higher output signal. This effect is attributed to less magnetic instability due to the reduced Ampere fields around the narrow (∼6 nm) current-confined paths. Finally, the CPP-GMR heads with CCP layer were tested under high-density recording conditions using the perpendicular recording media. Bit error rate (BER) as a function of linear density is reported. Microtrack profiles were also recorded to determine track density capability.

A hyperthermal energy ion beamline for probing hot electron chemistry at surfaces

An ultrahigh vacuum ion beamline and chamber have been assembled to produce hyperthermal (<400 eV) energy ions for studying hot electron chemistry at surfaces. The specific design requirements for this modified instrument were chosen to enable the exposure of a metal-oxide-semiconductor (MOS) device to monoenergtic, well-collimated beams of alkali ions while monitoring both the scattered beam flux and the device characteristics. Our goal is to explore the role that hot electrons injected toward the MOS device surface play in the neutralization of scattered ions. To illustrate the functionality of our system, we present energy-resolved spectra for Na+, K+, and Cs+ ions scattered from the surface of a Ag(001) single crystal for a range of incident energies. In addition, we show MOS device current-voltage characteristics measured in situ in a new rapid-turnaround load lock and sample translation stage.

Spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces

Recently, the spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces was observed. In the present work, the emission of these ions in ultra-high vacuum has been studied and the long-term stability of the emission efficiency has been investigated.

High trap density and long retention time from self-assembled amorphous Si nanocluster floating gate nonvolatile memory

The memory performance of floating gate nonvolatile memory based on amorphous Si (a-Si) nanoclusters self-assembled during low-temperature oxidation is investigated. A 2nm thick a-Si layer was grown on a top of a 5.6nm thick thermal oxide tunneling layer by ultrahigh vacuum ion beam sputter deposition and subsequently oxidized by annealing in flowing N2∕O2 (9:1) environment for 0–540s at 900°C. After oxidation, a 14nm thick Al2O3 control oxide layer was grown by atomic layer deposition. The authors find that the a-Si layer breaks up upon oxidation, self-assembling into a dense array of 1000s at at 150°C.

Combined EELS, LEED and SR-XPS study of ultra-thin crystalline layers of indium nitride on InP(1 0 0)—Effect of annealing at 450 °C

In this study, InP(1 0 0) surfaces were bombarded by argon ions in ultra high vacuum. Indium metallic droplets were created in well controlled quantities and played the role of precursors for the nitridation process. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. X-ray photoelectron spectroscopy (XPS) studies indicated that the nitrogen combined with indium surface atoms to create InN thin films (two monolayers) on an In rich-InP(1 0 0) surface. This process occurred at low temperature: 250 °C. Synchrotron radiation photoemission (SR-XPS) studies of the valence band spectra, LEED and EELS measurements show an evolution of surface species and the effect of a 450 °C annealing of the InN/InP structures. The results reveal that annealing allows the crystallization of the thin InN layers, while the LEED pattern shows a (4 × 1) reconstruction. As a consequence, InN related structures in EELS and valence bands spectra are different before and after the annealing. According to SR-XPS measurements, the Fermi level is found to be pinned at 1.6 eV above the valence band maximum (VBM).

Electron transport through individual Ge self-assembled quantum dots on Si

Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here the authors report on the fabrication of Ge quantum dots (QDs) onto Si (100) by ultrahigh-vacuum ion beam sputtering and the electrical properties of individual QDs. Transmission electron microscopy images show that samples with completely incoherent or coherent semispherical islands can be produced under different ion energies. The current-voltage (I-V) characteristics with conductive atomic force microscopy at room temperature. exhibit linear behavior at low bias and nonlinear behavior at large bias from coherent islands, whereas the staircase structures are clearly observed in the I-V curve from incoherent islands, which are attributed to electron tunneling through the quantized energy levels of a single Ge QD.

Surface modification of polystyrene by low energy hydrogen ion beam

Polystyrene was modified by low energy hydrogen ions in ultrahigh vacuum. The modified surfaces were analyzed in situ by x-photoelectron spectroscopy and ex situ by atomic force microscopy. Results revealed that 10 eV H + ion bombardment on polystyrene produces a cross-linked network film, while the polymer backbone remains intact and undesirable impurity (such as nitrogen, oxygen, etc) incorporation is eliminated. The as-prepared network film is just about 5–6 nm thick, but is highly resistant to chemical solvent. However, when 100 eV H + ion bombardment is used, the aromatic rings are seriously damaged and the surface of organic film is etched. This work suggests a new route to the preparation of ultrathin polymer network films.

Characterizations of Co/Al 2O 3/Co/NiFe multilayers elaborated by ultra-high vacuum ion beam sputtering

Ion beam sputtering is used for elaborating ferromagnetic Co/Al 2O 3/Co/NiFe multilayers. The interface state and the roughness of the multilayer are examined by TEM and AFM. The insulating layer is found to be homogenous with a low RMS roughness. Magnetic properties of each electrode and of the full stack are studied by P-MOKE and AGFM. The magnetic behaviour of the full stack is characteristic of two switching fields with a Néel coupling.

Investigation of the deformation behavior in nanoindented metal/nitride multilayers by coupling FIB-TEM and AFM observations

AbstractEpitaxial TiN/Cu bilayers and multilayers with periods L between 5 and 50 nm have been grown by ultrahigh vacuum ion beam sputtering on Si and MgO(001) substrates at room temperature. The deformation modes induced by a Berkovich nanoindent have been imaged using Focused Ion Beam – Transmission Electron Microscopy (FIB-TEM) and Atomic Force Microscopy (AFM). The observations suggest that the mechanical response of the multilayers is essentially governed by an extensive plastic flow inside the Cu layers, which is confined by a bending of the more rigid TiN layers. This specific deformation behavior, with no contribution of the interfaces as a barrier for dislocation motion could explain the absence of significant hardness enhancement in this system.

Effect of nitride passivation on the visible photoluminescence from Si-nanocrystals

The effect of nitride passivation on the visible photoluminescence from nanocrystal Si (nc-Si) is investigated. Silicon-rich silicon nitride (SRSN) and silicon-rich silicon oxide (SRSO), which consist of nc-Si embedded in silicon nitride and silicon oxide, respectively, were prepared by reactive ultrahigh vacuum ion beam sputter deposition followed by a high temperature anneal. Both SRSN and SRSO display photoluminescence peaks after high temperature annealing, coincident with the formation of Si nanocrystals, and similar changes in the peak luminescence position with the excess Si content. However, the luminescence peak positions from SRSN are blueshifted by about 0.6eV over that of comparable SRSO such that its luminescence peaks in the visible range. The results demonstrate that control of the surface passivation is critical in controlling the nc-Si luminescence, and indicate the possibility of using nitride-passivated nc-Si for visible luminescence applications including white luminescence.

Magnetic properties of Co/Al2O3/Co junctions deposited by ultra high vacuum ion beam sputtering

Cobalt and alumina layers are deposited by UHV ion beam sputtering to elaborate Co/Al2O3/Co junctions. To form the Al2O3 insulator barrier, oxygen is introduced into the chamber during the aluminum target sputtering. The experimental conditions to obtain stoichiometric Al2O3 are determined using auger electron spectrometer (AES) and Rutherford backscattering spectroscopy (RBS). Stoichiometric Al2O3 is obtained with an oxygen pressure of 6×10−7 mbar, i.e. a deposition rate of 0.5Å/mn, and a beam intensity of 55μA. Magnetic properties of the Co bottom electrode and the mutilayers are studied by magneto-optical Kerr effect (MOKE) and by alternating gradient field magnetometry (AGFM). The easy axis of magnetization is in-plane and the Co layers present an in-plane anisotropy. Moreover, the Co bottom electrode is paramagnetic for a thickness below 40Å, at room temperature.

Structural change of radiation defects in graphite crystals induced by STM probing

It was found that STM (scanning tunneling microscopy) images of defects in highly oriented pyrolytic graphite introduced by bombardment of 400 eV Ar+ ions in ultra-high vacuum exhibit substantial changes in the course of STM probing. Detailed examination of abrupt changes in the tunneling current measured at defect sites during voltage scans shows that the primary cause of the defect-image change was found to be neither the injected current nor the injected power but the absolute value of the voltage applied between the probe tip and the sample. We propose that an electric polarization induced force attracting the sample surface toward the probe tip widens the layer spacing of the graphite surface, leading to an acceleration of the lateral diffusion of interstitial atoms introduced by the ion irradiation, which results in a change in the defect structures and the accompanying electronic structures sensible in the STMimaging.