Showing papers in "Vacuum in 1997"

The preparation of transparent electrically conducting indium oxide films by reactive vacuum evaporation

Abstract: Indium oxide thin films deposited by reactive thermal evaporation of indium in an oxygen atmosphere at different substrate temperatures and oxygen partial pressures are characterized by X-ray diffraction, scanning electron microscopy, optical transmission and electrical resistivity Preparation conditions are optimized to obtain highly transparent and conducting ln2O3 films

Growth of epitaxial (001) superlattice films

Abstract: A series of Fe V multilayers was grown on polished MgO (001) substrates by dual-target magnetron sputtering. The structural quality of the samples was investigated by reflection high energy electron diffraction, low- and high-angle X-ray diffraction, reciprocal space mapping, polar scans, cross-sectional transmission electron microscopy and resistivity measurements. The results show that epitaxial Fe V (001) superlattices with high crystallographic and interface quality can only be grown in a narrow substrate temperature region, 300–330 °C. The average interface roughness was estimated to be as low as ± 1 A .

Droplet formation on steel substrates during cathodic steered arc metal ion etching

Abstract: Cathodic arc discharges are commonly used as metal ion sources to carry out, in vacuo , the precleaning procedure in cathodic arc evaporation and combined cathodic arc/unbalanced magnetron deposition processes. During this very efficient etching step, droplets are generated, which give rise to growth defects in the subsequently deposited hard coating. The number and size of droplets depend on the melting point of the metals used during ion etching. The present paper investigates the generation of droplets by target materials Al, Cu, TiAl, Ti, Zr, Cr, Nb and Mo with melting points in the range 660–2650 °C. Under the process parameters used, the TiAl alloy target showed the highest number of droplets generated (100 × 10 3 mm −2 ) whereas the largest, up to 20 μm diameter and quite often ‘splash like’, formed during evaporation from Al targets. Both metals with melting points greater than 2000 °C show very similar droplet generation, with mean droplet deposition densities of 5−10 × 10 3 mm −2 and diameters up to 5 μm. A comparison between the as-etched and as-coated samples indicates that the number of droplets deposited during the etching phase with TiAl and Mo formed an identical number of growth defects. In the case of Cr the number of defects on top of the coated surface were less. In general, following a non droplet producing unbalanced magnetron deposition process, defects were observed to extend from the substrate/coating interface to the coating surface.

Studies on the preferred orientation changes and its influenced properties on ITO thin films

Abstract: Indium Tin Oxide thin films were deposited using reactive thermal deposition technique from the In-Sn alloy source. Deposition of either (400) or (222) predominant planes was obtained between (400) and (222) planes by changing the deposition rate. Low electrical resistivity of 3.32 × 10 −4 Ω cm and good polycrystalline structure was obtained for the films with (400) predominant planes, deposited at 170 °C substrate temperature. Decrease in the band gap value was observed during the change in the crystallization from (222) to (400) predominant planes using optical absorption and reflectance spectra measurements. High thermoelectric power of 69.99 μV/ °C was measured for the films with (400) predominant planes and 37.5 μV/ °C was obtained for (222) predominant planes. Annealing treatment on the (400) films under nitrogen atmosphere for 60 min at 310 °C resulted in realignment of atoms from (400) to (222) planes. Structural, optical and electrical characterization studies revealed that the films oriented with (400) planes were more oxygen deficient than the films with (222) preferred planes.

Understanding how the liquid-metal ion source works

Abstract: There is some interesting physics in how the liquid-metal ion source (LMIS) works. This short review describes our current understanding, covering the following topics. Distinction between the real Taylor-Gilbert cone and Taylor's mathematical cone. Thermodynamic origin of the pressure-difference formula. Shape of an operating LMIS: the cusp-on-a-cone as expected shape and as an electrically driven vena contracta. Onset behaviour and Taylor's slender-body formula. Ion energy distributions, and interpretation in terms of surface processes and space-charge. Swanson's proof that the emission mechanism is field evaporation. Predicting the low-current Ga LMIS apex radius as 1.5 nm. The low-current Ga LMIS as a device driven by a negative pressure of order 40 atm. Re-examination of Mair's formula for LMIS current/voltage characteristics, and the role of space-charge. Cusp length as a function of emission current: theory and modelling. Emitter stability and control mechanisms: space-charge stabilisation, inertial/geometrical stabilisation, pressure-change destabilisation. Secondary emission phenomena: electrons, neutrals and photons. The high-temperature anomaly in the ion energy distribution, and a possible explanation. Instability and the formation of microdroplets and nanodroplets. Numerical modelling of basic LMIS behaviour: current/voltage characteristics for sources with and without viscous drag; modelling of cusp length and limiting half-angle; modelling of time-dependent effects. Current research tasks.

Electron beam surface remelting and alloying of aluminium alloys

Abstract: This paper reports results of a study of structural and mechanical changes in layers of hypereutectic cast Al Si alloys as results electron beam remelting and alloying with FeCr powders and followed by electron beam remelting.

Exposure-resolved VLEED from the O-Cu(001) : bonding dynamics

Abstract: The appraisal of an association of the VLEED technique and the new models for oxygen chemisorption [Vacuum, J. Phys. Chem. Solids, in press] motivated and enabled us to seek physical insight into the dynamics of the O-Cu(001) surface reaction. It is shown that the discrete stages revealed by exposure-resolved VLEED can be quantified and interpreted by the process dominated by three bonding parameters: First, one Goldschmidt-contraction ionic bond forms between the O adsorbate and a Cu atom on the surface accompanying the vacating of the missing row. Second, another contracting ionic bond follows between the O and a Cu in the substrate; as a result, the oxygen buckles into the bulk. Then, the ionic bond angle increases leading to the relaxation, and finally, interaction develops between the non-bonding states of the oxygen and the lone-pair-induced metal dipoles.

The spinning rotor gauge: measurements of viscosity, velocity slip coefficients, and tangential momentum accommodation coefficients for N2 and CH4

Abstract: A selection of experimental measurements for N 2 and CH 4 made with a spinning rotor gauge (SRG) employing 4.00 mm and 4.50 mm diameter spherical steel rotors are reported. The experiments covered the continuum and slip regimes for both polyatomic gases. Theoretical results from a previous paper on the SRG are used to extract values of the viscosity, the velocity slip coefficient, and the tangential momentum accommodation coefficient from the experimentally obtained data for each of the gases. The measured viscosities are in excellent agreement with existing literature values. The question of whether there is an angular dependence (via the motor head alignment) in the measurements that was raised in our previous work [Tekasakul et al., JVSTA, 1996, 14 , 2946] is addressed.

Kinetics of reactions in CH4N2 afterglow plasma

Abstract: In this paper chemical reactions initiated by the decomposition of methane in the nitrogen flowing afterglow are studied by computer modelling with the input data from microwave plasma. The model consists of 61 reactions between 24 kinds of species. The simulation technique is based on a macroscopic kinetic approach which provides the final concentrations of stable products—C 2 H 6 , C 2 H 4 , C 2 H 2 , HCN, H 2 together with CH 4 and N 2 . It was found that the stiffness of the set of equations describing the decomposition process can be monitored by the C and H mass balance.

The effects of deposition parameters on the crystallographic orientation of AIN films prepared by RF reactive sputtering

Abstract: Aluminum nitride films with a preferred orientation of (002) have been successfully deposited on substrate Si(100) by radio-frequency (RF) magnetron reactive sputtering in an Ar-N2 gas mixture. The effects of sputtering parameters on the crystal orientation of AIN films were studied by X-ray diffraction. It was found that the c-axis Of the AIN grains was preferentially oriented perpendicular to the substrate surface when the sputtering gas pressure was decreased to 0.09 Pa. With increasing positive bias at the substrate the c-axis tended to be parallel with the surface and with increasing RF power the orientation of the AIN grains tended to random distribution.

Hydrogen content and outgassing of air-baked and vacuum-fired stainless steel

Abstract: The hydrogen content of stainless steel samples, treated by 400 °C heat treatment for 2, 4, 8, 16 and 24 h in ambient air, was determined by the 1H(15N, αγ)12C nuclear resonance reaction, and a standard extraction method. Two reference samples were also analysed, one virgin sample which was not heat treated at all, and one which had been vacuum fired (950 °C, 1 h). The results show that the heat treatment in ambient air results in an outgassing rate comparable to vacuum fired material although the amount of hydrogen in the samples was only lowered by approximately 50%, by the air baking, compared to a reduction to

The first 50 years of electron stimulated desorption (1918-1968)

Abstract: Electron stimulated desorption (ESD) was first observed by Dempster in 1918 and, after investigation for technical reasons in the electron-tube industry and some sporadic research to understand the mechanism, it finally became a recognized subject for concerted research in the early 1960s when ultrahigh vacuum technology was becoming widely available and the production of well-characterized surfaces possible in several laboratories. In 1964 the Menzel-Gomer-Redhead (MGR) model of ESD was independently developed by Menzel and Gomer in Chicago and Redhead in Ottawa. A personal view is presented which surveys (i) the research (mainly for technical reasons) between 1918 and 1962, (ii) the origins of MGR in the period 1962–1964, and (iii) the early growth of the subject from 1964 to 1968. The further growth of research on ESD has been reviewed several times elsewhere. Emphasis is placed on the research on ESD by the group at the National Research Council, and on the role played by technological or applied science interests.

Effect of high energy ion irradiation on electrical and optical properties of organic nonlinear optical crystals

Abstract: Two new organic nonlinear single crystals viz., Methyl hydroxy benzoate (MHB) and p -hydroxy Acetophenone (HAP) are irradiated with swift heavy ions Ag 100 MeV and Si 50–100 MeV, from the 15 UD pelletron at NSC Delhi. Hydrogen depth profiling was carried out by using ERDA technique. The effect of irradiation on the dielectric, nonlinear optical properties have been investigated. A dramatic increase in the value of the dielectric constant was observed in both MHB and HAP. The dielectric constants increased by 10–13 times. The refractive index values were also increased. However the second harmonic generation efficiency of these crystals were only marginally affected.

Influence of the dopant concentration on the photoemission in NEA GaAs photocathodes

Abstract: The influence of impurity concentration on the photoemission from Negative Electron Affinity (NEA) GaAs photocathodes has been investigated. Experimental curves of photoemission spectral sensitivity are obtained for several dopant concentrations and when normalized to their peak value their spectral response shows three zones of well differentiated behaviour, in good agreement with theoretical predictions derived in this work. For illumination with wavelengths below ≈ 800 nm, photoemission is dominated by the escape probability. Higher concentration leads to higher normalized photoemission sensitivity. For wavelengths between ≈ 800 and 910 nm, the dominant role is played by the diffusion length of electrons in the bulk. Higher normalized sensitivity is obtained for lower concentration. For wavelengths above ≈ 910 nm, photoemission is dominated by the absorption coefficient, which depends on concentration through the value of the gap energy. The behaviour of sensitivity with concentration in this spectral zone is similar to that of wavelength shorter than 800 nm.

Determination of the inelastic mean free path (IMFP) of electrons in germanium and silicon by elastic peak electron spectroscopy (EPES) using an analyser of high resolution

Abstract: The IMFP of electrons is a fundamental material parameter of surface analysis by AES, XPS, EPES and EELS. In surface analysis calculated IMFP values are used. Their experimental determination is rather difficult. The IMFP of amorphous Ge and polycrystalline Si was determined by comparing the elastic peak intensity ratios with electrolytic Ni reference sample of 1 nm surface roughness, achieved by dedicated Ar+ ion bombardment cleaning and examined by STM. Experimental results obtained with a hemispherical analyser type ESA 31 developed by ATOMKI Debrecen have been evaluated by Monte Carlo analysis, based on Jablonski's differential elastic scattering cross sections and elaborated for the HSA analyser angular window. Due to the 5 × 10−5 energy resolution of the ESA 31 no spectrometer correction was needed. The ratio of the background to the elastic peak was

Time-resolved VLEED from the O-Cu(001): atomic processes of oxidation

Abstract: Decoding of time-resolved VLEED data for oxidation processes for Cu reveals that, apart from long-term ageing, the reaction upon ageing and annealing differs slightly from that upon increasing exposure of the O-Cu (001) surface. It is realised that the annealing supplies disturbance other than a driving force for the processes of oxidation. Besides the tetrahedral Cu 2 O bond geometry, pyramid c (2 × 2)- O structures were examined in detail for the initial stage of the reaction. The off-centred pyramid with oxygen higher than 0.4 A above the top layer or the centred pyramid with four identical O-Cu bonds were excluded. It is suggested that oxygen reacts with the Cu(001) surface by evolving a CuO 2 ( Cu +2 + 2 O -1 ) pairing pyramid with a single contracting ionic bond into a Cu 3 O 2 ( Cu +2 + 2 Cu +1 + 2 O −2 ) pairing tetrahedra with sp 3 -hybridisation of oxygen. This process transfers the nanometer-scale c (2 x 2)- O domains into the ordered missing-row type (√2 × 2√2) R 45 °- O reconstruction. Calculations also reveal that the O -chemisorption affects the energy states dominantly in the upper of valence bands. As a result of the O-Cu bonding, the O −2 -hybrid with nonbonding states and the Cu-dipole induced by either the O −1 or the O −2 as well as the vacating atoms play key roles in the atomic processes of oxidation and corrosion.

Threshold switching in Te46 − xAs32+xGe10Si12 chalcogenide glass system

Abstract: Five different compositions of Te46 − xAs32+xGe10Si12 chalcogenide glass system were prepared with the atomic percentages 0, 1, 2, 4 and 5. I–V characteristic curves for thin film samples of the investigated compositions were typical for threshold switching behaviour. It was found that the threshold voltage increased linearly with film thickness in the investigated range (100–800 nm) for all compositions. However, threshold voltage Vth decreased exponentially with temperature. The threshold voltage activation energy (e) was obtained for the investigated compositions. The electrical conduction activation energy (Eσ) for the investigated compositions was obtained from the corresponding temperature dependence of film sample resistance. The ratio ( e E σ ) was obtained theoretically on the basis of an electrothermal breakdown process. A good agreement was obtained between theoretical and experimental values of ( e E σ ). The lifetime for threshold switch devices on the basis of the investigated compositions was measured also. The compositional dependence of Vth, e and the lifetime was also investigated.

Influence of oxygen pressure on the physical properties of dc magnetron reactive sputtered cadmium oxide films

Abstract: Thin films of cadmium oxide were deposited onto glass substrates by dc magnetron reactive sputtering from a metallic cadmium target. A systematic study has been made on the influence of partial pressure of oxygen ranging from 8 × 10 −5 mbar to 5 × 10 −3 mbar on the film structure, electrical and optical properties. The dependence of electrical resistivity, Hall mobility and carrier concentration on the deposition temperature in the range 348–523 K was reported. At an optimum partial pressure of oxygen 1 × 10 −3 mbar and substrate temperature 473 K, the films exhibited a resistivity 4.6 × 10 −3 ohm-cm, Hall mobility 53cm 2 /V.s, carrier concentration 3.5 × 10 19 cm -3 , with an optical transmission of 85% in the wavelength range 600–1600nm with a band gap of 2.46 eV.

Physical model of dynamic phenomena in impulse plasma coaxial accelerator

Abstract: Impulse plasma is used in surface engineering (Impulse Plasma Deposition 1) as an efficient source of mass and energy in the synthesis and deposition of various materials in the form of layers. The plasma is generated and accelerated in a coaxial accelerator. 2,3 In the present work, on the basis of previous studies4 and a snow plow model of the current sheet motion, we propose and discuss a physical model of dynamic phenomena in such device.

Adsorption of gadolinium on the Mo(112) crystal face

Abstract: Structure and electron-adsorptive properties of Gd films adsorbed on Mo(112) surface have been investigated using LEED and CPD techniques. For the coverage range 0 0.6 the structures of type c(2 × 1 θ ) occur jointly with the type p(1 × 1 θ ) (at a very small region of θ〉 and show features of one-dimensional compression of the adlayers with increasing coverage. During the process of further Gd deposition the adsorbed layer undergoes one-dimensional compression in the 〈111〉 direction and incommensurate structures (1 × 4) and (1 × 5) are formed. The one-dimensional compression vanishes at θ = 1.25. With increasing θ, the work function change and binding energy are correlated with the phase transition in the adlayer structure. The peculiarities of indirect interaction between adsorbed Gd atoms have been revealed at low coverage.

Study of the interaction of SO2 with TiO2 (110) surface

Abstract: The adsorption of SO2 on TiO2(110) surfaces at 100 K has been studied by means of the X-ray photoemission and ultra-violet photoemission spectroscopy, and photon stimulated desorption techniques using Synchrotron radiation. For a saturation dose at 100 K, the SO2 is molecularly adsorbed and a chemisorbed interface is formed by SO42− and SO32− species on the surface. Annealing of the adsorbed layer to room temperature leads to the formation of sulfur derived species.

Carbon nitride thin films formation by N2+ ion implantation

Abstract: Low energy (1–5 keV) N2+ ion irradiation of graphite and diamond surfaces has been investigated by angle resolved X-ray photoelectron spectroscopy (ARXPS). Analysis of the N 1s band indicates the existence of three bands that could be assigned to CN sp3, CN sp2 types of bonding and molecular nitrogen, respectively. Such an explanation is consistent with the analysis of C 1s band. The depth profiles of implanted nitrogen, recovered by inversion of the angle-dependent XPS data, revealed that ion-dose, ion-energy and angle between the ion beam and the surface normal are relevant parameters to control the composition and the thickness of the produced films.

Spectral sensitivity of the VLEED to the bonding geometry and the potential barrier of the O-Cu(001) surface

Abstract: The significance and reliability of very-LEED (VLEED) in revealing the details of a surface is demonstrated. This was done through the association of high resolution data, a suitable calculation code and reasonable modelling, as well as appropriate approaches in the calculations. This association enables us to classify the factors dominating the fine-structure features appearing on the VLEED spectra. Critical examination of the spectral sensitivity to the Cu(001)-O surface provides evidence that the VLEED are more highly sensitive to the bonding geometry than individual atomic-shift. The shapes of the fine-structure features are dominated by the elastic potential that determines the phase change of the diffracted beams. The inelastic potential has influence on the absolute intensity other than the shape of the I-V curve. It is justified that the inelastic damping is predominated by the outermost layer. Therefore, VLEED is such a technique that it is able to reveal nondestructive information from the overlayer integrating the bond forming and the behaviour of surface electrons.

The physical properties and the chemical bond approach for Se-Ge-As amorphous chalcogenide glasses

Abstract: There is considered interest among experimenters and theorists in the study of the electrical conductivity σ in structurally disordered state as a function of various external factors: structure, composition, temperature, effect of illumination and the effect of chemical bonding on the physical behaviour of the investigated amorphous thin films Se0.75 Ge0.25 − y Asy (y = 0.05, 0.10, 0.15 and 0.20) system. X-ray diffration patterns indicate the amorphous structure of thin film samples and DTA thermograms showed an decrease of Tg with increase of the As content in the system under test. The dependence of the electrical conductivity, measured either in darkness or after exposure to different durations of light, on temperature and exposure time have been studied. The pronounced glass-forming tendencies of alloys of Se and Ge with As were discussed topologically in terms of the chemical bonds expected to be present in these materials. Using simple considerations based on coordination number 〈m〉 and bond energies, the average number of near neighbors of each type expected to surround atom has been estimated. These average numbers of bonds have been used to estimated the cohesive energies (CE) of these glasses assuming simple additivity of bond energies. On the other hand in random networks, with high average coordination (amorphous solids) the rigid regions have percolated to form a rigid solid with a few floppy or spongy inclusions. A trial was made to correlate our results with available published experimental data.

Correlation between processing parameters and mechanical properties as a function of substrate polarisation and depth in a nitrided 316 L stainless steel using nanoindentation and scanning force microscopy

Abstract: The effects of substrate polarisation in a nitrided 316 L stainless steel have been investigated in an attempt to accurately correlate processing parameters with surface mechanical properties. Nanoindentation allows the Vickers hardness to be measured at precise depths, meaning that the variation in properties with nitriding depth can be evaluated and correlated with the process parameters. By combining such measurements with surface imaging techniques (scanning force microscopy and scanning electron microscopy) and electron probe micro-analysis, it is possible to explain both the mechanical property and microstructural variations of such layers, having been produced in a low pressure arc plasma discharge at 680 K with a mixed Ar-N2 gas. In this study the nanoindentation technique is presented as a new and valid method for the characterisation of nitrided layers, shown by hardness measurements on four nitrided layers produced with different substrate polarisation potentials. The net advantages of such an approach over conventional methods (e.g. microhardness testing) and the possibility of analysing microstructural phases previously not well detected by X-ray diffraction, make nanoindentation an attractive tool for a more complete understanding of the nitriding process.

Water adsorption properties of amorphous BaTiO3 thin films

Abstract: Amorphous barium titanate films were flash evaporated on silicon and studied with X-ray photoelectron spectroscopy. After water adsorption, the surface oxygen 1s photoelectron peak reveals the presence of hydroxyl groups. A thermal desorption experiment on a similar thin film evaporated on rhenium shows one peak of water partial pressure at about 480 K, which decreases after repeated cycles of adsorption and thermal desorption, suggesting that oxygen vacancies are filled up during desorption.

On the wear of TiN (PVD) coated cermet cutting tools

Abstract: The last decade was characterised by the introduction of PVD coated cutting tools, which presently take a large share of the cutting tool market. Recently, in contrast to predictions, TiC-based cermets appeared on the market exhibiting cutting behaviour which makes them competitive with coated WC-based hardmetals. The present paper discusses the results of a study devoted to understanding wear mechanisms of a TiN(PVD) coated Ti(C,N)-based cermets. Static diffusion experiments were conducted in order to simulate the diffusion process across the tool-work piece interface at high temperature and pressure. Experimental coated and uncoated tool materials were tested in machining Ck 45 steel under different cutting conditions. The diffusion couples and the worn cutting tools were analysed by SEM, EDS and AES in order to identify the dominant wear mechanisms.

Hydrogen intake capacity of ZrVFe alloy bulk getters

Abstract: The technique of elastic recoil detection analysis was used to study hydrogen absorption by a non-evaporable getter (NEG) material st 707, an alloy of Zr, V and Fe. The depth distribution and intake of hydrogen in the samples activated at different heating temperatures showed that the hydrogen intake was optimum at about 300 °C and tended to fall at higher temperatures from desorption. The maximum hydrogen storage capacity of the material was studied by loading hydrogen in the sample by electrocharging technique. It was shown that it was possible to obtain hydrogen concentrations up to 1.0 × 10 22 at cm −3 i.e. about 26 at%.

Neural network methodologies for mass spectra recognition

Abstract: The purpose of this work was to establish the methodology for automated mass spectra recognition using neural networks. Four different neural networks techniques were tested (backpropagation, improved Kohonen network, ART2 and multilayered perceptron) and compared on simulated mass spectra samples. The testing environment set for all four neural networks spectra recognition systems showed almost the same efficiency and robustness for improved Kohonen type network as well as for the ART2 system. According to test results, both systems can be recommended for practical use in mass spectra recognition. The stage of development of neural network methodologies is gaining on maturity. It is evident that their use is especially powerful in applications dealing with numerous parameters and their correlations (some of them unknown) such as mass spectrometry, Auger electron spectroscopy (AES), X-ray diffraction (XRD) etc.

Surface defects and platinum overlayers on TiO2(110) surfaces: STM and photoemission studies

Abstract: The TiO 2 (110) surface has been studied with scanning tunneling microscopy (STM). Different types of defects such as point defects, “line defects”, and single steps were characterized with atomic resolution. The interaction of platinum at submonolayer and monolayer coverages with nearly stoichiometric as well as defective (110) surfaces was studied by means of ultraviolet and X-ray photoemission spectroscopies (UPS and XPS), electron stimulated desorption/ion energy distribution measurements (ESD/IED), and resonant photoemission spectroscopy using synchrotron radiation (ResPES). We find that Pt atoms adsorb preferably on top of the five-fold coordinated Ti without significant electronic interaction on the “ideal” surface. On defective surfaces which show a Ti 3d emission of Ti 3+ in the band gap we observed a charge transfer from these Ti 3d states to Pt atoms.