Victor E. Henrich

Thermal faceting of (110) and (111) surfaces of MgO

Abstract Using LEED, we have observed the thermal faceting of MgO (110) and (111) surfaces into sets of (100) faces. Some faceting occurs under ion bombardment at room temperature, and annealing at 900–1400 K by means of electron bombardment produces complete faceting, with facets up to 1 μm across. These observations are consistent with theories of the stability of ionic crystal surfaces.

Chemisorbed phases of H2O on TiO2 and SrTiO3

Abstract Ultraviolet photoemission spectroscopy in ultrahigh vacuum has been used to study the interaction of H2O with argon-ion-bombarded and annealed TiO2(110) and SrTiO3 (100) surfaces. At low exposure, evidence is found for the presence of the OH free radical on both bombarded and annealed TiO2. At high exposure, a spectrum of slightly perturbed adsorbed H2O is observed, with the a1 molecular orbital shifted 0.5–1 eV toward tighter binding. This suggests that H2O binds to the surface of TiO2 via its a1, in-plane O lone-pair orbital. No evidence is found for OH on the surface of SrTiO3. The spectrum of H2O adsorbed on bombarded SrTiO3 shows both the a1 and b2 molecular orbitals shifted 0.4 toward tighter binding, indicating a more complicated bonding to the surface. For SrTiO3 that has been bombarded and then annealed, the spectrum of adsorbed H2O shows no significant bonding effects.

Fast, Accurate Secondary‐Electron Yield Measurements at Low Primary Energies

We describe a method of accurately measuring the secondary electron yield of solids over the entire incident energy range of 0–1000 eV in less than 30 msec using a commercial LEED/Auger system. Particular attention is paid to the region of low‐energy incident electrons (Ep=0 to 100 eV), and the spatial and energy resolution there are considered. The method is fast enough to see temporal changes in yield on the scale of hundreds of milliseconds. Criteria for measuring the yield of semi‐insulators are considered, as well as measurements of fine structure in the yield near Ep=0 eV.

Preparation and properties of sputtered MgO/Au, MgO/Ag, and MgO/Ni cermet films

Finely grained films of three cermets‐MgO/Au, MgO/Ag, and MgO/Ni‐have been grown by rf sputtering from composite targets. Electron‐microscopic studies show that MgO/Au and MgO/Ag films consist of small crystallites (usually <150 A) of both MgO and Au or Ag. In MgO/Ni films, it appears that amorphous Ni particles are embedded in a polycrystalline MgO matrix. These cermet films are good secondary‐electron emitters, especially in cases where differential‐sputtering effects are large. In MgO/Au films, the Au particles sputter much faster than MgO resulting in a MgO‐rich surface layer. In MgO/Ag films, the differential‐sputtering effect is smaller, probably due to stronger interaction between MgO and Ag crystallites. In MgO/Ni, the Ni particles sputter slower than MgO resulting in a slightly Ni‐rich surface (and hence in poorer electron‐emission properties). The small particle sizes and the presence of metallic particles in the bulk of the films greatly reduce surface charging.

Differential sputtering of MgO/Au cermet films and its application to high-yield secondary-electron emitters

Very large differential-sputtering effects in fine-grained (particle size < 50 A) MgO/Au cermet films have been observed by Auger electron spectroscopy. Under Ar-ion bombardment, the surface Au content was found to decrease exponentially with time by up to a factor of 20. A theoretical model for differential sputtering of granular systems is presented that gives good agreement with the experimental data. The differential sputtering produces a MgO-rich surface layer and therefore results in excellent secondary-electron-emission properties for low-energy incident electrons.

Surface states on n-type SrTiO3

Abstract We have used a model based on local density of states functions to calculate the surface electronic structure for both perfect SrTiO3(100) surfaces and for surfaces containing O-vacancy defects. The calculations are in excellent agreement with ultraviolet photoemission spectra for both types of surfaces. A magnetic layer with μ ≈ 2.3 μB per surface unit cell is predicted for the O-vacancy SrTiO3(100) surface.

Auger spectroscopy studies of the oxidation of amorphous and crystalline germanium

We have used Auger−electron spectroscopy and ion−beam etching to study the room−temperature oxidation of sputtered and electron−beam−evaporated Ge films. Both amorphous and polycrystalline films were examined, as well as single−crystal Ge. Electron−escape−depth effects were removed by a deconvolution procedure in order to obtain the O distribution function. Large differences of O distribution were found between sputtered and e−beam amorphous films. Sputtered amorphous films oxidize in the same manner as single−crystal Ge, with the O confined to the first 5−10 A of the surface. In e−beam amorphous films, the depth of O penetration is more than 200 A, although the heavily oxidized region is still only 10−12 A thick. Polycrystalline films have heavily oxidized layers 6−9 A thick, with some O present to a depth of 100−200 A.

Oxidation studies of amorphous and crystalline germanium films by Auger spectroscopy

Using Auger spectroscopy and ion‐beam etching, we have studied the room‐temperature oxidation of sputtered and electron‐beam‐evaporated germanium films. Large differences are observed between the oxygen profiles of sputtered and e‐beam amorphous films. Sputtered amorphous films have only a very thin layer of oxide on the surface (4–5 A), possibly because the presence of argon in the films inhibits further oxidation. In e‐beam amorphous films, the heavily oxidized layer is also thin (7–8 A), but some oxygen is present as far as 200 A into the samples.

Effects of cesiation on secondary‐electron emission from MgO/Au cermets

The effect of Cs adsorption on secondary‐electron emission from MgO/Au cermets has been studied by both a rapid yield measurement technique and Auger electron spectroscopy. The presence of Cs enhances the emission from these materials, but the Cs is weakly bound and is easily removed by electron bombardment. The emission is not adversely affected by the possible alloying of Cs with Au.

Use of cylindrical Auger spectrometers for retarding‐potential secondary‐electron yield measurements

The retarding‐potential method of secondary‐electron yield measurement described previously for use with hemispherical retarding‐grid LEED optics has been extended to use with cylindrical mirror Auger spectrometers. The basic operation of the system was found to be comparable in the two cases, but a serious problem with the design of some electron guns was encountered. This problem could occur with either LEED or Auger spectrometers.