Ion Scattering Spectroscopy Spectra Research Articles

In situ measurement of surface potential developed on MgO thin film surface under ion irradiation using ion scattering spectroscopy

The application of ion scattering spectroscopy (ISS) to the in situ measurement of the surface potential developed on an insulator surface under positive ion irradiation was investigated. The ISS spectra measured for a MgO film of 600 nm thickness on a Si substrate by the irradiation of 950 eV He+ ions revealed that the surface is positively charged by approximately 180 V. For accurate measurement of the surface potential, a correction to take into account the angular deflection of primary ions induced by the high surface potential is required. The dependence of the surface potential on the sample temperature revealed that no charging is induced above 700 °C, indicating that accumulated charges can be removed by heating to 700 °C. From the measurement of the ion-induced secondary electron yield using a collector electrode located in front of the sample surface, the surface potential and ion-induced secondary electron yield were found to be strongly affected by the experimental setup. Secondary electrons produced by the impact of slow positive secondary ions, the maximum energy of which corresponds to the surface potential, play an important role when the bias voltage applied to the collector electrode is positively high for the present experimental setup. The surface potential developed on the surface of MgO films of 600 and 200 nm thickness was measured in situ, revealing that the amount of accumulated charges and the time required to attain the steady state of charging are slightly dependent on the beam current of primary ions and strongly dependent on the thickness of the MgO film. The present results confirmed that the application of ISS has high potential for investigating charging phenomena and the secondary electron emission from insulator surfaces under positive ion irradiation.

Application of ion scattering spectroscopy to measurement of surface potential of MgO thin film under ion irradiation

An experimental approach was proposed for the measurement of the surface potential (SP) induced on an insulator surface during ion irradiation by ion scattering spectroscopy (ISS). The resultant ISS spectra obtained for a MgO thin film of 600nm thickness on a Si substrate under 950eV He+ irradiation revealed that the surface is positively charged by approximately 230V. In addition, the onset energy of a secondary ion peak indicated a SP of approximately 205V. The present results confirmed that ISS is an effective technique for measuring the SP during ion irradiation.

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The ( 5 × 5 ) R27 o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O 2 TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.

ISS and AES studies of the initial oxidation of Dy, Tb and their silicides

Samples of Tb and Dy as well as their silicides were oxidized in high vacuum under controlled conditions for oxygen exposures ranging up to 50 L (1 L = 1 × 10 −6 Torr·s). The oxides were studied by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). Distinct low energy Auger peak shape changes occurred during the oxidation of the metals as well as during the oxidation of the metal silicides. These peak shape changes are very similar for Tb and Dy as expected due to the similarities in chemical properties of the lanthanide elements. When the silicides were exposed to oxygen, both the metal and the Si were oxidized. In both cases the oxide layers were enriched by the metal. The depletion of the Dy just below the surface suggested oxygen induced surface segregation of Dy. A model is proposed in which the low energy region of the ISS spectra were used to determine the Si peak height.

Surface characterization for sputter-cone formation on InP(100)

The dynamic processes of sputter-cone formation for the InP surface, which occur during the depth profiling of InP/InGaAs multilayers, have been studied by ion scattering spectroscopy (ISS), reflection electron microscopy (REM) and reflection high-energy election diffraction (RHEED) as a function of sputtering time. Results show preferential sputtering of phosphorus atoms and the formation of an altered amorphous layer which is followed by surface roughening as a transition stage to cone formation. After reaching a steady state, the ISS spectra remain almost the same despite ion sputtering, even though the surface becomes covered with a significant number of cones. RHEED and REM observations revealed that the shank of the cone is composed of crystalline InP, which is covered by an altered layer of InP with an In metal ball situated on the top of the cone. Monte Carlo simulation has predicted that preferential sputtering of phosphorus causes the InP surface to be In rich, suggesting that the excess In atoms should agglomerate to form the metallic In ball on the surface.

Adhesion and surface analysis of carbon fibres electrochemically oxidized in aqueous potassium nitrate

Type II carbon fibres (PAN-based) have been electrochemically oxidized in aqueous potassium nitrate to varying electron charge densities (0–4294 Cg-1). The fibres were subsequently characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS) and the results were correlated to acid/base surface titrations and BET surface area measurements. Relative to the as-received fibres (commercially treated, but unsized), the electrochemical treatments increased the ARXPS O/C atomic ratios by approximately 50%–100% and the concentration of oxidized carbon became more uniform within the XPS sampling depth (≈10 nm). At the same time, the number of acidic functions titrated by sodium hydroxide rose from about 2.6 μeq g-1 to 1078 μeq g-1, and the BET surface area increased from 0.67 m2 g-1 to 2.9 m2 g-1. A large portion of the increase in acidic groups was due to the increased fibre oxidation below the XPS sampling depth. The surface densities of acidic functions (functions/nm2) determined from NaOH uptake and nitrogen BET surface area experiments were far larger than is physically possible. Thus, it is postulated that aqueous NaOH solutions are able to access a much larger surface region than can be measured by nitrogen BET. A model involving subsurface pores, voids, crevasses, etc., which become available via swelling during solvation in aqueous NaOH, but are at least partially closed off when dry (during BET measurements), was proposed. The quantity of acidic functions introduced (detected by NaOH) was directly proportional to the extent of oxidation as referenced to the electron charge density (C g-1). The ISS spectra suggest that the surface density of carbon/oxygen groups was also increased. Single-fibre fragmentation tests (using an epoxy resin matrix) revealed that in most cases the interfacial shear strengths (IFSS) increased with increasing ARXPS O/C atomic ratio probably due to enhanced fibre/matrix chemical bonding and/or mechanical interlocking. As the extent of the electrochemical oxidation progressed above 1500 C g-1 the IFSS of single filament specimens then began to decrease. This was due to a continuing decrease in filament tensile strength as the extent of electro-oxidation increased. The critical transfer length, Lc, also decreased from ∼0.36 mm to ∼0.18 mm as the extent of electro-oxidation proceeded. Electrochemically oxidized fibres were compared to nitric acid-oxidized fibres in terms of acidic groups generated, BET surface areas, acidic group surface densities, dye adsorption with methylene blue and the role of aqueous NaOH in exposing some of the microstructure created by oxidative processes. © 1998 Kluwer Academic Publishers

ISS Assessment of the Influence of Nonpore Surface in the XPS Analysis of Oil-Producing Reservoir Rocks

The application of X-ray photoelectron spectroscopy (XPS) to oil-producing reservoir rocks is new and has shown that pore surface concentrations can be related to rock wettability. In the preparation of fresh fractures of rocks, however, some nonpore surface corresponding to the connection regions in the rocks is created and exposed to XPS. To assess the potential influence of this nonpore surface in the XPS analysis of rocks here we use ion scattering spectroscopy (ISS), which has a resolution comparable to the size of the pores, higher than that of XPS, with an ion gun of He+at maximum focus. Sample charging effects are partially eliminated with a flood gun of low energy electrons. All the ISS signals are identified by means of a formula which corrects any residual charging on the samples. Three rock samples are analyzed by XPS and ISS. The almost unchanged ISS spectra obtained at different points of a given sample suggest that the nonpore surface created in the fracture process is negligibly small, indicating that XPS data, from a larger surface spot, represents the composition of true pore surfaces. The significant changes observed in ISS spectra from different samples indicate that ISS is sample specific.

Evidence of an implantation process in carbon deposition on Si(100) at high substrate temperature by laser ablation

The growth mode of carbon atoms on a clean and reconstructed Si(100) surface using the laser ablation deposition technique, with the substrate held at high temperature (≈500°C) during deposition, has been investigated by means of low energy ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and scanning tunneling microscopy (STM). In contrast with the room temperature case, where the mode of growth is layer-by-layer, evidence for implantation of carbon into the silicon surface is found. The implanted C forms a chemically bonded compound at the sub-surface. STM images of the resulting film show a compacted surface made of clusters with an homogeneous size of around 20 Å diameter. Moreover, analysis of the secondary ions of the ISS spectra reveals that the first arriving carbon species from the ablation process produce an important increase of the surface roughness.

A spectroscopic examination of carbon fiber cross sections using XPS and ISS

Multiple transverse sections of type II, PAN-based carbon fibers (unsized and commercially treated) were investigated using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). No previous XPS/ISS spectra of carbon fiber cross sections have been reported. The data are compared to angle-resolved XPS (ARXPS)/ISS spectra acquired from external (longitudinal) fiber surfaces in order to assess the experimental technique and aid spectral interpretation. The spectra are consistent with nitrogen and oxygen concentration profiles that decrease overall from the fiber surface inward to the fiber core. Oxygen is present primarily as COH within the fiber cores, with increasing amounts of CO and COOH groups near the fiber skin. The spectra also reveal the presence of zinc, sodium and calcium within the fiber cores.

Surface characterization of carbon fibers using angle-resolved XPS and ISS

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion-scattering spectroscopy (ISS) are utilized to investigate the surface composition of BASF type II carbon fibers as a function of progressive exposures to aqueous nitric acid. To the authors' knowledge this is the first such study of carbon fibers to incorporate both of these surface analytical techniques. Overall, the ARXPS and ISS spectra are shown to be highly synergistic and quite consistent with one another. The data indicate that the surface composition of the as-received fibers is heterogeneous and undergoes significant changes upon nitric acid exposure. The net effect of the nitric acid surface treatments appears to be the systematic and preferential formation of carbonyl and carboxyl groups, and the establishment of a more uniform surface composition as a function of depth. It is expected that these data will contribute toward a better understanding of the interphase in carbon fiber composites.

Distribution of fluorine in YBCO thin films prepared by BaF 2 technique

Thin YBa 2Cu 3O 7- x superconducting films were prepared by coevaporation of Y, BaF 2 and Cu components. An in-situ annealing process in dry oxygen was used to obtain high- T c films at substrate temperatures not exceeding 600°C for Al 2O 3, and 720°C for SrTiO 3. Analyses performed by X-ray diffraction, Auger electron spectroscopy (AES) and ion-scattering spectroscopy (ISS) revealed an escape of fluorine from the deposited films already during the coevaporation stage in vacuum and preannealing steps in oxygen. The ISS spectra indicate the main fluorine concentration right beneath the film surface. In addition, the AES analyses show both the presence of fluorine at the film-surface interface and rather limited diffusion of substrate constituents into the YBCO films apparently due to the low substrate temperatures used.

An in situ study of the reactive ion beam etching of tungsten with tetrafluoromethane/argon mixtures using ion scattering spectroscopy and secondary ion mass spectrometry

The reactive ion beam etching (RIBE) of tungsten with carbon tetrafluoride (CF4))/Ar mixtures has been studied. The beam energy was kept constant at 550 eV. Helium or neon was added to the gas mixture to give about 5% He+ or Ne+ in the beam. Ions ejected from the sample region were energy and mass analyzed. The atomic composition of the bombarded surface was determined via ion scattering spectroscopy (ISS). Mass spectra of the secondary ions (SIMS) ejected from the surface were obtained. As the mole fraction of CF4 in the CF4/Ar mixture was increased, the ISS spectra showed that the W surface became covered with F atoms. The SIMS spectra had peaks corresponding to W+, WF+, and WF+2 and WF+3. The ratio WF+/W+ was proportional to the CF4 to argon flow ratio. In contrast, it was observed that threshold mole fractions of CF4 are required for the formation of WF+2 and WF+3.

Surface composition analyses of Co–Ni alloys under Ar+ ion bombardment by AES and ISS: A possible standard sample for surface chemical analysis

AbstractThe change in surface composition of Co(50.4 at.%)–Ni alloy under Ar+ ion bombardment was investigated by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). The surface concentrations were estimated by employing a spectrum synthesis method, since the Co and Ni signals in both AES and ISS spectra overlap each other. The results based on the quantifications by AES and ISS have indicated that the composition of the Co–Ni alloy surface under Ar+ ion bombardment is the same as the bulk composition within the accuracy in quantification, leading to the conclusion that ion bombardment does not cause surface composition change in the Co–Ni alloy. This supports the proposal that the Co–Ni alloy system is an appropriate reference material for surface chemical analysis.

Oberflächensegregation an Au-Pd-Legierungen

Measurements of the surface composition of Au-Pd binary alloys performed by Ion Scattering Spectroscopy (ISS) and X-ray Photoelectron-Spectroscopy (XPS) are presented. A set of 11 alloys with different bulk composition was used for these studies. Argon bombardment induced alternation of the surface composition due to preferential sputtering effects and surface segregation due to temperature excursions up to 500 °C have been studied. For quantitative evaluation of the ISS spectra pure metal standards and also calculated scattering cross sections were used. In general, little gold enrichment due to preferential sputtering and strong surface segregation of gold at elevated temperature was found by ISS measurements. The thickness of the altered layer is about 1 or 2 monolayers.

Analysis of solid surfaces by Neutral Beam Incidence Ion Scattering Spectroscopy.

Low-energy rare-gas Ion Scattering Spectroscopy (ISS) has been improved by the use of neutral beams as projectiles, which we call Neutral Beam Incidence Ion Scattering Spectroscopy (NBISS).Experiments on ISS and NBISS are compared on the basis of electron exchange between the projectiles and solid surfaces.In ISS, the scattered ions consist of two elements: One is “surviving ions” characterized by the ions which survived neutralization, and the other is “reionized ions” characterized by those which experienced neutralization on the incoming trajectory and subsequent reionization during the violent collision. The shadowing and focusing effects are often weakened in the intensity of surviving ions, whereas these effects appear intensively in the intensity of reionized ions. Since the threshold energy for the reionization is different for the combination of projectiles and targets, these effects are often not observed at all in the ISS spectra. On the other hand, these effects appear explicitly in the NBISS spectra. It is also found that NBISS is valid for analyzing insulator surfaces since charging effects are minimized by the use of neutral beams.

Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes

In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.

A Comparative Study of Solid Surface Analyses between Low Energy Ion Scattering Spectroscopy(ISS)and Secondary Ion Mass Spectroscopy(SIMS)

Experimental studies of solid surface analyses are carried out by means of low energy ion scattering spectroscopy(ISS)and secondary ion mass spectroscopy(SIMS) . The instrument is composed of a HITACHI IMA-2 ion probe and a stigmatic second-order double focusing mass spectrometer(electric toroidal sector: re=212mm, Φe=85°, c=0.5; uniform magnetic sector: rm=200mm, Φm=90°, ε1=32°, ε2=-10°). ISS analyses are carried out using the electric toroidal sector, while SIMS analyses employ both the electric and magnetic sectors. Primary 5 keV argon and helium ion beams are used for ISS analyses, and 8 keV oxygen ion beams for SIMS analyses. An aluminium plate, an Au-Ag-Cu alloy and a GaP crystal are analyzed. The obtained ISS and SIMS spectra are compared. The element discrimination characteristics of ISS are much more accurate than those of SIMS. Maximum mass resolving powers are obtained at30(50% valley separation)for ISS, and 11, 000(10% valley separation)for SIMS.