Molecular beam epitaxy

The structure and properties of metal-semiconductor interfaces

Diffraction patterns obtained from atomically clean germanium surfaces contained half‐integral order beams in (110) azimuths for both (100) and (110) surfaces and in all azimuths for the (111) surface. These results are considered to be due to displacements of surface atoms from their normal bulk lattice positions in the surface plane. Adsorption of oxygen on all of these surfaces extinguished all of the diffraction beams which were not integral order.In addition to the normal surface lattice spacings of clean (111) and (100) surfaces of silicon, there were surface structures with larger spacings, most of which depended on the conditions of ion bombardment and/or subsequent heat treatment. Two such structures have been observed for the (100) surface of silicon; one is a double‐spaced lattice in the (110) azimuth, similar to that for germanium, and the other has a spacing about 8% greater than that of normal silicon and was obtained only after radiation quenching of the crystal from 1000°C. Two large‐spaced structures were observed for the (111) surface. All of these structures were extinguished by exposure to oxygen. Evidence is presented which indicates that these structures were not due to contamination but to the silicon itself.The method of determining the kinetics of gas adsorption from the low‐energy electron diffraction beams is outlined and the calculations of the fractional coverage and sticking probability are presented for oxygen. For silicon crystals, the calculations of surface coverages and sticking coefficients for oxygen were found to depend on the preceding treatments of the crystals. The rate of adsorption of oxygen was proportional to the pressure, at least for pressures below 10‐6 mm Hg, and depended on the preceding treatments of the crystals. After oxygen adsorption, the clean germanium surface could be regenerated by heating at 500°C for 30 min and for silicon it was regenerated by heating at 900°C for a few minutes. A comparison with results of other observers is given.

Structure and Adsorption Characteristics of Clean Surfaces of Germanium and Silicon

The adsorption of CO on Ni(100) has been studied using LEED, Auger, and work function measurements and was found to be completely reversible. The isosteric heat of adsorption was approximately 1.30 eV/molecule independent of coverage up to 0.61 monolayers. Three surface structures were observed: (1) a disordered phase at high temperature and/or low coverage, (2) a c(2× 2) phase at coverages up to and including 0.5 monolayers at sufficiently low temperature, and (3) a compressible hexagonal phase between 0.61 and 0.69 monolayers. Evidence is presented which supports an attractive CO–CO interaction at coverages of less than 0.5 monolayers. A number of two‐dimensional phase transitions were observed and suggest an interesting T—θ phase diagram with a eutecticlike triple point. The importance of scrupulous cleaning procedures in allowing all possible adsorbed phases to be observed has again been demonstrated.

Structural Influences on Adsorption Energy. III. CO on Cu(100)

The energy spectra of Auger electrons emitted from many elements and alloys have been observed. Differentiation of the secondary electron‐energy distribution by perturbation and synchronous detection techniques, and suppression of the effects of low‐energy secondaries, were used to obtain well‐defined spectra. The Auger lines afford a sensitive indicator for light elements, though spectra for elements as heavy as gold have been obtained. Because the method examines only a superficial layer of the sample, it is useful for detecting contamination, surface migration, or segregation, and for diffusion studies. Cleaning of the surfaces is critical for useful analysis. Quantitative analysis is hindered by noise and the lack of known surface conditions for calibration.

Analysis of Materials by Electron‐Excited Auger Electrons

The ion‐bombardment cleaning method has been successfully applied to the (100) faces of germanium, silicon, and nickel, and to the (0001) face of titanium. Conditions and precautions necessary for the production of clean surfaces are described. Tests have been made for contamination from the ambient during the cleaning procedure for germanium. It has been shown that contamination approximating one‐half monolayer does not occur under the conditions which were obtained, and that the method is capable of producing surfaces which are atomically clean.Results for clean (100), (111), and (110) germanium faces and the (100) silicon face indicate that the atomic positions in the surface planes are not the same as the corresponding positions in the bulk structure. For (100) nickel and (0001) titanium, the positions of the atoms in the surface planes are in agreement with x‐ray data. In the case of titanium, an unidentified surface structure was present until after several cycles of alternate heating and ion bombardment. In the case of nickel, a double‐spaced surface monolayer and in some cases a single‐spaced, simple‐square structure, attributed to carbon, was present until after several cycles of alternate heating and ion bombardment.

Application of the Ion Bombardment Cleaning Method to Titanium, Germanium, Silicon, and Nickel as Determined by Low‐Energy Electron Diffraction

Ion Bombardment‐Cleaning of Germanium and Titanium as Determined by Low‐Energy Electron Diffraction

Both nitrogen and oxygen chemisorb on the clean (0001) surface of titanium in crystalline monolayers having the same hexagonal symmetry and lattice constant as that of the titanium surface, but with different distances between the surface titanium plane and the gas planes. Adsorption as a function of exposure was determined for both gases. An average sticking coefficient for nitrogen at room temperature of about 1.8×10—5 was obtained.

R. E. Schlier

Papers

Application of the Ion Bombardment Cleaning Method to Titanium, Germanium, Silicon, and Nickel as Determined by Low‐Energy Electron Diffraction

Ion Bombardment‐Cleaning of Germanium and Titanium as Determined by Low‐Energy Electron Diffraction

Some Measurements of Adsorption of Nitrogen and Oxygen on a (0001) Titanium Surface Using Low-Energy Electron Diffraction

CORROSION AND PASSIVITY STUDIES OF TITANIUM. Final Technical Report for July 1, 1952-September 30, 1955. Dept. of Physics

Corrosion and Passivity Studies with Titanium