T. W. Haas

Chemical Effects in Auger Electron Spectroscopy

The effects of a change in the chemical environment of an atom in a surface region are manifested in several subtle ways on the measured Auger electron spectra. The first, a shift in the energies of the Auger electrons, is the result of charge transfer and is a measure of the valence state of an atom. Examples of this effect are the oxidation results on refractory metals presented here. It is found that measurable changes are found even for submonolayer coverages of oxygen on an otherwise clean surface. The second main effect is a change in the shape of a complex spectrum, which occurs when some of the electrons involved in this Auger process are valence electrons. The best example of this behavior is that of carbon, and it is found that these complex spectra serve as a fingerprint for the identification of the form of the carbon at a surface. Problems and opportunities for the exploitation of both these effects are discussed, the data included here being illustrative of the usefulness of this type of mea...

ADSORPTION ON NIOBIUM(110), TANTALUM(110), AND VANADIUM(110) SURFACES.

Clean surfaces of Nb (110), Ta (110), and V (110) have been prepared by heating in an ultrahigh‐vacuum environment. Low‐energy, electron‐diffraction measurements indicated that all clean surfaces were the same as an ideal (110) plane in the bulk to within experimental error. Adsorption of CO and O2 on the clean surfaces of these metals led to similar patterns suggesting that the same structures formed on all three metals. The patterns obtained from adsorption of CO followed by warming to about 400°C were the same as those obtained by room‐temperature exposure to O2. This suggests decomposition of the adsorbed CO. At least three different structures of O2 on these metals are produced as coverage is increased. Unfortunately, dissolution of the O2 into the bulk prevents accurate determination of surface coverage and this, coupled with the inability to handle intensity‐distribution curves effectively, has made complete structure determinations of these oxides impossible.

Photoreflectance of AlxGa1−xAs and AlxGa1−xAs/GaAs interfaces and high‐electron‐mobility transistors

Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si‐doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation‐doped heterostructures is apparently associated with samples that show the presence of two‐dimensional electron gas.

Comparison of Auger spectra of Mg, Al, and Si excited by low−energy electron and low−energy argon−ion bombardment

Auger spectra of Mg, Al, and Si excited by low−energy electron and argon−ion bombardment have been measured and compared. The Auger spectra obtained under ion bombardment are interpreted as a combination of Auger production in excited sputtered species and bulk atoms. Under argon−ion bombardment at these low energies, L2,3 level ionization occurs in the target through an electron promotion mechanism. No L1 level ionization occurs (to first order), and the cross section for double L2,3 level ionization is also small at these ion energies, so no Auger transitions originating from such states result.

Some aspects of an AES and XPS study of the adsorption of O2 on Ni

Some aspects of the adsorption of O2 on Ni have been studied using electron‐excited Auger electron spectroscopy (AES) and x‐ray photoelectron spectroscopy (XPS). Changes in the oxygen Auger line shape were observed with increasing oxygen exposure, together with a broadening and energy shift of the main oxygen Auger peak. Measurements were also made of the broadening and energy shifts of the Ni L3VV Auger peak on oxidation. The variation in oxygen Auger signal strength with oxygen exposure shows that two distinct adsorption stages occur. Relative AES and XPS signal strengths from clean and heavily oxidized Ni were measured and compared with the atomic densities of Ni in pure Ni and NiO. To accurately measure the changes in signal strengths, the AES and XPS data had to be integrated to remove the effects of line shape changes.

Auger electron spectroscopy study of cathode surfaces during activation and poisoning. I. The barium‐on‐oxygen‐on‐tungsten dispenser cathode

Auger electron spectroscopic analyses of the chemical changes taking place on a commonly used dispenser cathode have been carried out during activation and poisoning by several reactive gases (O2, CO, CO2, H2, N2, Ar). The chemical composition and work‐function changes on the cathode were correlated at various stages of activation and poisoning, both at room temperature and at operating temperature. During activation, a process of considerable cleaning up of unwanted contaminants (chiefly carbon) occurs. Poisoning by reactive gases causes changes in the Ba–O ratio. A good agreement with the theoretical predictions for barium and oxygen concentrations of ∼1014 atoms/cm2 made by Zalm and our results was found. Over all, our results show that the excess barium adsorption model correctly explains electron emission from the dispenser cathode.

Some Problems in the Analysis of Auger Electron Spectra

Auger electron spectroscopy has been used in this study to determine common impurities found on the transition metals Sc, Ti, V, Cr, Fe, Co, Ni, Y, Zr, Nb, Mo, Ru, Rh, La, Hf, Ta, W, Re, Ir, Pt, and Au. Single crystal and polycrystal specimens were used, with both specimen types giving similar results. The most prevalent impurity on most of these surfaces is not carbon as had been previously supposed but rather is sulfur. Mild heat treatment causes sulfur to segregate at the surfaces of these materials, while more drastic heating, to 1000 °C or greater, will normally remove all the sulfur. Clean Mo surfaces exhibit a peak in the Auger spectrum at 150 V, the location of the sulfur peak, but arguments are given to suggest that this peak is characteristic of the clean rather than the contaminated surface. Observations of chemical shifts in Auger spectra have been made and are also discussed.

Photoreflectance measurements of unintentional impurity concentrations in undoped GaAs

Modulated photoreflectance is used to measure the unintentional impurity concentrations in undoped epitaxial GaAs. A photoreflectance signal above the band gap spreads with the unintentional impurity concentrations and shows well‐defined Franz–Keldysh peaks whose separation provide a good measure of the current carrier concentrations. In samples less than 3 μm thick, a photoreflectance signal at the band edge contains a substrate‐epilayer interface effect which precludes the analysis of the data by using the customary third derivative functional fits for low electric fields.

Scattering of Low‐Energy Electrons from Hydrogen‐Covered Mo(100) Surfaces

Hydrogen has been found to adsorb onto the Mo(100) surface at room temperatures in two states. The first gives rise to LEED patterns with extra‐order beams. The second structure arises from full monolayer coverage and gives only integral order spots. The full coverage structure is easily electron‐beam desorbed to give the lower coverage structure.

LEED Study of the Growth of Aluminum Films on the Ta(110) Surface

Growth of aluminum films on clean Ta(110) surfaces has been investigated by LEED techniques. Five phases of aluminum have been found, which are: (1) disordered Al(111), formed by depositing at T<600°C; (2) Al(111)c(2×2) formed by heating film, deposited at T<300°C, to 700°C; (3) Al(100)c(2×2), formed by heating film, deposited 300<T<600°C, to 700°C; (4) Epitaxial Al(111), formed by depositing film with substrate T in range 600<T<670°C; (5) Diffuse phase, formed by heating any of the preceding to 800°C. Two orientations of each structure have been observed; this results from the close match between primitive unit meshes of aluminum (111), (100) and Ta(110). A mechanism for the growth of Al(111) and Al(100) is suggested. Comparisons with some published LEED data on epitaxy are made.