M. Liehr

Electron spectroscopic characterization of oxygen adsorption on gold surfaces: II. Production of gold oxide in oxygen DC reactive sputtering

Abstract Gold oxide is produced by oxygen DC reactive sputtering in a UHV compatible chamber. It is subsequently characterized by High Resolution Electron Energy Loss, Auger and X-ray Photoelectron Spectroscopies. It is demonstrated that the oxide is of the Au 2 O 3 type (auric oxide) and that it decomposes under thermal treatment. Au 2 O (aurous oxide) is a possible intermediate of this reduction.

Electron spectroscopic characterization of oxygen adsorption on gold surfaces

Tentative adsorption on clean gold (110) and (111) crystals of molecular oxygen in the pressure range 10 −10 to 10 −5 Torr, at a temperature varying between 100 and 800 K is reported together with the subsequent characterization of the surfaces by High Resolution Electron Energy Loss, Auger and X-ray Photoelectron Spectroscopies. It is found that oxygen does not adsorb in these UHV conditions, except when a contaminant is present on the surface. Such an interaction with a low level silicon impurity is described.

High resolution electron energy loss spectroscopy of an anisotropic insulator surface: A test for the dielectric theory

Electron energy loss measurements were performed on the α‐Al2O3(0001) surface at temperatures between 120 and 350 K. Spectra of the Fuchs–Kliewer phonons were recorded with high energy resolution (26 cm−1). Numerical fits revealed that the spectra are quantitatively described by the macroscopic dielectric theory for anisotropic crystals. Comparisons are made with infrared data.

Vibrational study of the SiO2/Si interface by high resolution electron energy loss spectroscopy

By using high resolution electron energy loss spectroscopy (HREELS) the surface phonon spectra of SiO2(0001) and of thick amorphous SiO2 layers have been measured. The spectra recorded on the crystalline surface are characterized by three first order peaks at 498, 798, and 1176 cm−1. The HREELS data could be interpreted by the dielectric theory applied to anisotropic material, and the optical constants were compared to infrared spectroscopy values. HREELS spectra as a function of oxide thickness could be obtained from oxidized Si(100) wafers by sputter erosion. Peak positions shifted to lower frequencies and intensities decreased regularly with decreasing oxide thickness. At the interface a transition layer of about 25 A was observed and carbon contamination was identified. The dielectric theory applied to a thin homogeneous supported film cannot account for the observed frequency shift.

Hydrogen interaction with Al(110) studied by high resolution electron energy loss spectroscopy

The interaction of atomic hydrogen with the Al(110) surface in a glow discharge results in the formation of an Al–H species identified by its stretching vibrational band at 1900 cm−1. This band is also detected during early oxidation of Al single crystal surfaces. It is demonstrated that this species is localized at the interface between the metallic substrate and the surface oxide layer. The HREELS results are in total agreement with recent IETS work on Al/Al–oxide/metal tunneling junctions.

High resolution electron energy loss study of AlxGa1−xAs mixed crystals

The surface optical phonon spectra of AlxGa1−xAs mixed crystals have been measured by high resolution electron energy loss spectroscopy (HREELS). Two modes (Al–As and Ga–As types, respectively) were observed whose frequencies shifted almost linearly with aluminum concentration. The relative intensities of these modes are analyzed. Comparison is made with Raman scattering measurements and theoretical calculations.

Surface and interface optical phonons of a GaAs–AlGaAs superlattice measured by high resolution electron energy loss spectroscopy

High resolution electron energy loss spectra have been measured on a GaAs–Al0.3Ga0.7As superlattice. Fuchs–Kliewer‐like surface and interface optical phonons were observed with frequencies and intensities in excellent agreement with calculations based on the dielectric theory. The spectra obtained after sputter erosion of the superlattice are discussed in terms of induced defects or segregation of Al at the interface.

Investigation of InAs and GaSb by high‐resolution electron energy‐loss spectroscopy

Crystalline InAs and GaSb layers grown by molecular‐beam epitaxy have been studied with high‐resolution electron energy‐loss spectroscopy (HREELS). From the observation of optical surface phonons, resonance frequencies ωTO and oscillator strengths Δe could be determined. The values found for InAs(ωTO=219 cm−1 and Δe=2.9) are in agreement with optically derived data from the literature. As for GaSb, the resonance frequency and the oscillator strength determined by HREELS are at variance with published values. Preliminary results on InAs–GaSb superlattices are presented.Crystalline InAs and GaSb layers grown by molecular‐beam epitaxy have been studied with high‐resolution electron energy‐loss spectroscopy (HREELS). From the observation of optical surface phonons, resonance frequencies ωTO and oscillator strengths Δe could be determined. The values found for InAs(ωTO=219 cm−1 and Δe=2.9) are in agreement with optically derived data from the literature. As for GaSb, the resonance frequency and the oscillator strength determined by HREELS are at variance with published values. Preliminary results on InAs–GaSb superlattices are presented.

Investigation of interfaces by high resolution electron energy loss spectroscopy

Recent measurements performed on epitaxial layers clearly demonstrate the capability of HREELS to observe interface optical phonons in dielectric layered materials. Two examples are presented: First, the case of epitaxial CaF 2 on Si(111) where the CaF 2 interface phonon is detected for a whole range of CaF 2 layer thicknesses. Discrepancies with theoretical predictions are interpreted as resulting from the residual strain in the CaF 2 layers. The second example is a III–V semiconductor superlattice GaAs−AlGaAs in which an Al−As type interface mode is clearly identified and quantitatively interpreted by the dielectric theory.

High Resolution Electron Energy Loss Spectra from Surfaces and Interfaces of Semiconductor Oxides

In recent years, Electron Energy Loss Spectroscopy (EELS) has emerged as a highly reliable tool for the study of clean or contaminated surfaces. In these experiments a beam of monoenergetic electrons is scattered by a surface and analyzed in energy to detect losses characteristic of surface electronic or vibrational excitations. In spite of numerous successes on other materials, these experiments could not easily be carried out on insulators or poorly conducting materials mainly because of surface charging effects. Recently, however, the state of the art drastically evolved as it was discovered that these undesirable potential instabilities could be avoided by an adequate experimental setup1. The new procedure requires an auxiliary gun delivering electrons in the keV energy range to continuously flood the surface as the monoenergetic electron beam is operated. The resolution obtained in these experiments is quite impressive (sometimes as good as 2.5 meV) and an extremely broad range of materials can now be studied by EELS.