Double Pass Cylindrical Mirror Analyzer Research Articles

Mechanistic and Kinetic Measurements of Elementary Surface Reactions Using Temperature-Programmed X-ray Photoelectron Spectroscopy.

This paper describes a method by which surface-reaction kinetics can be measured by slowly and precisely ramping up the surface temperature at a constant rate while simultaneously collecting X-ray photoelectron spectra (XPS). This approach results in the collection of a large amount of data over relatively small temperature steps to produce quasi-continuous kinetic data. The method is illustrated for the desorption and reaction of diethyl disulfide (DEDS) on a Au(111) substrate in ultrahigh vacuum, where the results can be compared with previous conventional temperature-programmed desorption (TPD) data from Au(111). Experiments were carried out using a double-pass cylindrical-mirror analyzer with a channeltron detector to demonstrate how this approach can be implemented in a routine, multitechnique vacuum chamber. The approach will be even more effective in a more modern, specialized XPS apparatus with high-transmission hemispherical analyzers with multichannel array detectors, which will enable the spectra of several elements to be measured simultaneously. The results yielded an activation energy for multilayer desorption of DEDS of 41 ± 1 kJ/mol, with a pre-exponential factor of 8 ± 7 × 1012 s-1, an activation energy of 53 ± 6 kJ/mol and pre-exponential factor of 9 ± 8 × 1013 s-1 for monolayer desorption and an activation energy of 90 ± 6 kJ/mol with a prefactor of 1.0 ± 0.3 × 1015 s-1 for the reaction of adsorbed ethyl thiolate species to adsorbed DEDS. While these results were collected for a system for which the kinetic data could have been obtained using conventional TPD, this method can be more usefully applied to those surface reaction processes that do not rely on the formation of desorption products. This system, having been previously studied by TPD, facilitates a comparison with results obtained by conventional methods.

Employing a cylindrical single crystal in gas-surface dynamics

We describe the use of a polished, hollow cylindrical nickel single crystal to study effects of step edges on adsorption and desorption of gas phase molecules. The crystal is held in an ultra-high vacuum apparatus by a crystal holder that provides axial rotation about a [100] direction, and a crystal temperature range of 89 to 1100 K. A microchannel plate-based low energy electron diffraction/retarding field Auger electron spectrometer (AES) apparatus identifies surface structures present on the outer surface of the cylinder, while a separate double pass cylindrical mirror analyzer AES verifies surface cleanliness. A supersonic molecular beam, skimmed by a rectangular slot, impinges molecules on a narrow longitudinal strip of the surface. Here, we use the King and Wells technique to demonstrate how surface structure influences the dissociation probability of deuterium at various kinetic energies. Finally, we introduce spatially-resolved temperature programmed desorption from areas exposed to the supersonic molecular beam to show how surface structures influence desorption features.

Measured electron IMFPs for SiC

AbstractSilicon carbide (SiC) is a wideband semiconductor that is very promising for many applications in optoelectronics and microelectronics. Since information on the electron transport processes in SiC is scarce, its systematic study is highly desirable. The aim of this work is to determine experimentally by elastic peak electron spectroscopy (EPES), for the first time, the inelastic mean free paths (IMFPs) in bulk SiC with different structural properties (polycrystalline and monocrystalline materials) and surface concentrations of its constituents. The relative EPES measurements were carried out in the electron energy range 0.2–2.0 keV with two different analyzers (double‐pass cylindrical mirror analyzer (DCMA), spherical sector analyzer (SSA)) and the use of Ni standard. The measured IMFPs were uncorrected for surface excitations and compared with those calculated from the TPP‐2M and G‐1 predictive formulae using Fano plots. Good agreement was found between the measured and the calculated electron IMFPs in SiC exhibiting different structural properties. Copyright © 2006 John Wiley & Sons, Ltd.

Field emission from aCa nanotip grown on aMo⟨110⟩ microtip

We have observed field emission with an energy distribution of 68meV full width at half maximum (FWHM) from a thermally cleaned Mo⟨110⟩ single tip. The emission spectra are taken with the emitting surface at 295K, using a double-pass cylindrical mirror analyzer in ultrahigh vacuum. This narrow energy distribution is attributed to a nanotip spontaneously formed on the ∼75nm radius Mo tip from in situ buildup by field-induced surface diffusion. Auger electron spectroscopy showed that residual surface Ca segregated from the bulk during thermal cleaning and is likely the source of mobile atoms that formed the nanotip. The emission spectra show a discernible doublet that is attributed to a variation in the localized density of states of the nanotip. Sub-Langmuir oxygen exposure of the Mo tip immediately increased the energy distribution to a FWHM of >1eV.

Intercomparison of methods for separation of REELS elastic peak intensities for determination of IMFP

AbstractThe consistency and accuracy of inelastic electron mean free paths (IMFPs) determined from comparison of the intensity of elastically reflected electrons with theoretical calculations were studied. The variation with experimental geometry, spectrometer energy resolution and the procedure for background subtraction was studied. Four different types of spectrometers— double pass cylindrical mirror analyser (CMA) pre‐retarded CMA, hemispherical analyser (HSA) and retarding field analyser (RFA)— with widely different geometries and energy resolutions, placed in four different laboratories in three countries, were used. Four background subtraction methods (Shirley, linear, Tougaard and ELPSEP) were applied to isolate the elastic peak intensity from the reflected electron spectra. It was found that the IMFPs determined with data from the double pass CMA deviated considerably more (by a factor of 2) from the average values as well as from theoretical IMFP values than IMFPs determined with data from the other spectrometers. The data from the double pass CMA were therefore excluded from the statistical analysis. The root mean square (RMS) deviation of the IMFP from a function fitted to the data was 1.6–2.8 Å, depending on the background subtraction method, and it is smallest for the Tougaard method. The RMS deviation from IMFP values calculated by Tanuma et al. is 2.1–3.2 Å, again with the smallest value for the Tougaard method. The percentage deviation of the results using a Tougaard background from Tanuma et al. values is 12.6%. The results point to the conclusion that the major contribution to the inaccuracies in IMFPs determined with this method is not the background subtraction procedure but rather the lack of accuracy of the presently available models for elastic electron scattering, i.e. atomic elastic scattering cross‐sections and effects of crystallinity that are not included in the presently applied models. Copyright © 2001 John Wiley & Sons, Ltd.

Retractable miniature double pass cylindrical mirror analyzers

The construction and performance of a double pass cylindrical mirror analyzer (CMA) is described. This electron energy analyzer is retractable and has a diameter of less than 1.5 in. (30 mm). This analyzer is superior in performance, for gas phase photoemission experiments, to the single pass CMA of similar diameter.

Electron energy distributions of ferroelectric emission from plzt 8/65/35

Strong pulsed electron emission from ferroelectrics due to fast polarization switching (i.e., ferroelectric emission) has attracted much interest recently. In this paper, electron energy distributions of PLZT 8/65/35 were measured using a double pass cylindrical mirror analyzer (DPCMA) operating in Auger mode. The results showed a very narrow energy distribution (FWHM: 10eV∼20eV), and the emission energy was on the order of the applied pulse potential. In addition, electron emission energy was relatively independent of driving pulse frequency, but increased with driving pulse amplitude, and decreased with sample temperature. At a temperature (200°c) far above the phase transition temperature (≈130°C), strong electron emission was still observed. These results suggest that ferroelectric emission is primarily related to the material polarization switching rate.

Comparison of TiO2(110) Surfaces by XPS: Effects of UV Exposure, Electron Beam and Ion Beam Damage

Many studies have been carried out on rutile TiO2 since it was discovered to be a photocatalyst with relatively high efficiency for the decomposition of water. Surface defects are believed to play an important role in dissociation of absorbed water and other small molecules. Defects created on a TiO2(110) surface by thermal annealing in vacuum, or by low-density electron beam irradiation, ultraviolet illumination, and Ar+ ion bombardment, were examined by x-ray photoelectron spectroscopy (XPS). The XPS multiplex spectra of O 1s and Ti 2p peaks were collected. The experiment was carried out in a Perkin-Elmer Physical Electronics 560 Multiprobe surface analysis system with a double pass cylindrical mirror analyzer. Nonmonochromated Mg Kα x-rays were used to generate the spectra.

Energy transfers in the quasielastic scattering of 70-1250-eV electrons by surfaces.

We have observed energy transfers of up to about 200 meV in the quasielastic scattering of 70--1250-eV electrons by sputtered surfaces of polycrystalline copper, silver, and gold. The experiment consisted of measuring shifts in positions of elastic peaks with a double-pass cylindrical mirror analyzer for different voltages applied to the cathode of the electron gun. The shifts are believed due to changing cross sections for phonon excitations with electron energy. The results are significant in the use of the elastic peak technique for high-accuracy calibrations of the energy scales of electron spectrometers.

Quantitative depth profiling with AES: application to oxide layers of NiCrFe alloys

To achieve an understanding of alloy oxidation at room temperature, AES sputter depth profiling was used to disclose the elemental distribution, the existence of a sequence of different oxide layers and their respective thickness. A polycrystalline, homogenized, and polished sample of Ni21 at % Cr-12 at % Fe was oxidized in air at room temperature and atmospheric pressure for 20 min after sputter cleaning. Additionally, a polycrystalline sample of Fe18 a t% Cr was passivated for 24 h in deaerated 0.5 tool/1 H2SO 4 at 1 VN~E. Then, photoelectron spectra (2p of the alloy elements and Ols) were taken using an Mg K~ X-ray source to characterize the chemical state of the oxides and to determine the oxide thickness. The depth profile analysis was performed with a rastered Ar + ion beam of 3 keV with 34 ~ incidence angle to the surface normal and an Ar pressure of 4104 Pa in the analysis chamber. After each sputtering cycle the Auger peaks of the elements Ni (848 eV), Cr (529 eV), Fe (650 eV) and O (510 eV) were recorded in the derivative mode with 2 Vpp modulation voltage using a double pass cylindrical mirror analyser with 0.6% energy resolution. Additionally, the M23VV transitions of the alloy elements were measured in the low energy region between 20 eV and 70 eV. For the quantitative evaluation of the composition in the oxide layers and the oxide thicknesses the sequential layer sputtering model (SLS) [ / 3] was applied to the measured intensity/sputtering time profiles. This model assumes a monoatomic layer structure and a Poisson distribution as resolution function in combination with the exponential escape depth correction and is appropriate for very thin layers if the information depth of the electrons used for the analysis is comparable or larger than the thickness of the film [1]. It is suitable for an automated data evaluation by computers. The sputtering time scale is first converted into a depth scale by calibration with the total oxide layer thickness obtained from XPS measurements [4, 5]. Assuming constant oxide density and sputtering rate and neglecting a possible preferential sputtering, an intensity corresponding to each removed monolayer equivalent is obtained by a spline interpolation to the measured intensities. The functional dependence of these elemental intensities on the unknown concentrations in each atomic layer is given by Eq. (13) in ref. [6] which can be modified into a linear equation system

The Adsorption of Gallium on the Cleaved Surface of InP,InAs and InSb

Studies of chemical trends may provide additional insights in complicated reactions between adsorbates and substrates. Here, the interaction of gallium with InP, InAs and InSb cleaved surfaces was investigated by using photoemission spectroscopy (UPS).Ga was evaporated from a Knudsen-type cell which was placed at a distance of approximately 15 cm in front of the surfaces cleaved in situ. After the evaporation rate, which was monitored by using a quartz oscillator, had stabilized the amount of Ga deposited was determined from the exposure time. The film thicknesses given in A are nominal values since Ga was found to exhibit island growth. The energy distribution curves (EDC-s) of the photoemitted electrons, which were excited by HeI and HeII radiation provided by a home-built discharge lamp, were recorded by using a single or a double-pass cylindrical mirror analyzer.

A technique for performing angle‐resolved electron energy loss spectroscopy: A study of polycrystalline tin

AbstractA method is presented for performing angle‐resolved electron energy loss spectroscopy (ARELS) using a doublepass cylindrical mirror analyzer (CMA) which contains an angle‐resolving movable aperture. Both the electron gun contained in the CMA and an off‐axis electron gun are used in these studies. The technique is illustrated by ARELS spectra taken from a cleaned polycrystalline tin surface because observing the behavior of the surface and bulk plasmon features provides a direct and simple way of assessing surface sensitivity. The spectra demonstrate that the incidence angle of the primary beam, the collection angle with respect to the sample normal and the primary beam energy all have a very large effect on surface senstivity.

Local structure of silicon films and silicon carbide by surface extended energy loss fine structure

Surface extended energy loss fine structure (SEELFS) has been shown to yield nearest neighbor coordination and bond length for carbon in different structural forms. A standard Auger electron spectrometer(double pass cylindrical mirror analyzer) is used to obtain energy loss spectra in the laboratory. Preliminary work on diamond, graphite and ‘‘Aquadag’’ using spectra in the second derivative mode is now extended to silicon and α- and β-silicon carbide. Silicon has been used as reference for phase and amplitude in the bond length and coordination determination of β-silicon carbide. β-SiC has been used as a standard for α-SiC. The coordination and radius determined agree with known values to within uncertainties estimated. The uncertainties in the determination of bond length and coordination due to the choice of reference phase and amplitude are discussed.

Local structure investigations of carbon by surface extended energy loss fine structure

Surface extended energy loss fine structure (SEELFS) spectroscopy has been used to determine structural parameters for several different forms of carbon. A standard Auger electron spectrometer (double pass cylindrical mirror analyser) was used to obtain energy loss spectra. First shell coordination numbers and bond lengths were obtained for single crystal diamond, pyrolytic graphite, and “Aquadag” (graphite particle suspension) samples. The study of pure carbon surfaces allowed us to probe the limitations on the determination of bond lengths and coordination. The validity of some of the theoretical assumptions is also discussed.

Local structure investigations of carbon by surface extended energy loss fine structure

Surface extended energy loss fine structure (SEELFS) spectroscopy has been used to determine structural parameters for several different forms of carbon. A standard Auger electron spectrometer (double pass cylindrical mirror analyser) was used to obtain energy loss spectra. First shell coordination numbers and bond lengths were obtained for single crystal diamond, pyrolytic graphite, and “Aquadag” (graphite particle suspension) samples. The study of pure carbon surfaces allowed us to probe the limitations on the determination of bond lengths and coordination. The validity of some of the theoretical assumptions is also discussed.

A simultaneous angle-resolved photoelectron spectrometer

The authors show how a double pass cylindrical mirror analyser can be modified for use as a simultaneous angle resolved photoelectron spectrometer. The full acceptance cone of the analyser is imaged on a two-dimensional position sensitive detector. The position information is stored in a two-dimensional multichannel analyser. The spectrometer accumulates multiple angle-resolved spectra simultaneously. Its energy resolution is determined as 0.9% FWHM and the angle-resolving capability is demonstrated with valence band spectra from Ag(111), giving an angular resolution in the present set-up of about two degrees.

Two-stage electron energy analyzer for angle-resolved photoemission spectroscopy

A two-stage energy analyzer is described having a 127° cylindrical deflection analyzer as an injection stage followed by a 37° plane mirror analyzer with a resolution function ΔE/E∼1%. The design is very compact about the axis of analyzer rotation. The entire instrument including turntable and magnetic shields fits within a 150-mm tube, and is physically interchangeable with a commercial double-pass cylindrical mirror analyzer (CMA). The advantage it has over an angle-resolving CMA is that the directions of collected electrons all lie in a common plane. Construction details are described, and the performance of the analyzer is illustrated with some angle-resolved photoemission data on Ag(100) using synchrotron radiation.

Adsorption of O2 and CO on cleaved GaAs(110) at low temperatures

We have studied the low-temperature (∼50 K) adsorption of O2 and CO on GaAs (110) with UV photoemission. Spectra were taken using a He discharge lamp and a double pass cylindrical mirror analyzer (CMA) in the angle-integrated mode. Spectra showing peaks characteristic of gas-phase O2 and CO were observed after dosing GaAs (110) surfaces cleaved at a temperature T ≂50 K with <5 L of O2 or CO. The molecular CO peaks disappeared on warming the sample to 100 K leaving a spectrum characteristic of the clean surface. In the case of O2, the O2 1πg orbital was 5.4 eV below the valence band maximum (VBM) and 13.2 eV above the Ga 3d core, which implies a relaxation shift towards smaller binding energies of ∼1.4 eV for adsorbed O2 as compared to the gas phase. On warming the O2-dosed sample the molecular peaks disappeared by ∼60 K and by 300 K the spectrum was transformed to one characteristic of chemisorbed oxygen. Auger analysis indicated that oxygen concentrations of a few percent remained at 300 K after O2 dosing with only 10–40 L at 50 K. The 300 K spectra are attributed to atomic rather than molecular oxygen, which implies a transition from the physisorbed molecule to the chemisorbed atom.

Modification of a cylindrical mirror analyzer for angle-resolved electron spectroscopy

A commercial double-pass cylindrical mirror analyzer has been modified for angle-resolved electron spectroscopy by insertion of an aperture-containing drum within the inner cylinder of the second stage. Three operating modes are possible; high angular resolution (±2°), low angular resolution (±6°), and complete retraction of the drum to permit normal angle integration. The gearing mechanism for rotation and translation of the drum is described, and the performance of the analyzer is discussed.

Time-of-flight measurements with a CMA for simultaneous energy and mass determinations of desorbed ions

Using a commercial double-pass cylindrical mirror analyzer (CMA) we have made mass, energy, and angle-resolved measurements of positively charged ions emitted from a surface by electron-stimulated desorption. Because the dispersive character of the CMA selects the ion energy, the mass can be uniquely and simultaneously determined from the flight time in the analyzer. We show that this can be achieved with relatively simple additional electronics, and unity mass resolution is possible up to at least mass 20. Some of the principles, limitations, and capabilities of the technique are described and illustrated by measurements of H+, O+, F+ and Cl+ desorption from a W{100} surface.