G.A. Haas

Thermionic sources for high-brightness electron beams

The capabilities and limitations of modern thermionic electron sources for producing high-emission-density (>10 A/cm/sup 2/) high-brightness beams are surveyed. The emphasis is on dispenser cathodes. The fabrication techniques and operating mechanisms that determine the operating limitations of existing commercial cathodes are discussed. In addition, cathode improvements that have been demonstrated in various prototype structures are described. >

Work function and secondary emission studies of various Cu crystal faces

The work function and its temperature dependence has been measured for the 〈100〉, 〈110〉, 〈111〉, 〈112〉 as well as 13 other Cu crystal planes. The results, obtained with two types of incident electron beam techniques in this experiment, are in good agreement with recent photoelectric and contact potential difference results reported on ion‐sputter‐cleaned surfaces. Variation in secondary emission for different crystal faces was found to be determined primarily by the work function changes of the different crystals and not so much by the bulk lattice orientation. It was also observed that the secondary electrons are not as dependent on the velocity constraints imposed by the Bragg condition as are the electrons having thermal energies.

Diffusion measurements in thin films utilizing work function changes: Cr into Au

The role of diffusion in limiting the reliability of substrate bonding for bimetallic Cr–Au thin films was investigated using work function, Auger electron spectroscopy, and LEED measurements to characterize the structure and chemical composition of these films. The diffusion rates for Cr into Au films are measured by means of a new technique utilizing the change in electronic work function as a measure of the surface concentration change. It was found that the diffusion coefficient (D) depended on the deposition rate of the Au film, with a twofold increase in deposition rate giving rise to an order of magnitude increase in D. The presence of sulfur as an impurity in the Cr also had a significant effect on D. Comparison of these results with values of D obtained on Cr–Au films by other workers give similar activation energies, but much lower values of D0 (temperature independent part of D) were obtained in this work. The effect on the lifetimes of devices using thin bimetallic films is discussed in terms ...

Electron beam scanning technique for measuring surface work function variations

Abstract A technique has been developed which utilizes a scanning beam of electrons in the retarding field region for the purpose of analyzing the work function variations (patchiness) of surfaces. Since the surface potential barrier is probed with electrons from an external source, either hot or cold surfaces can be investigated. Characteristics such as adsorption, migration, effusion and other problems of surface kinetics can be monitored from a continuously presented television display of the surface work function topography with a resolution of a few microns. A 400 channel analyzer which is incorporated in the equipment permits the determination of the distribution in work function by an analysis of the point-by-point retarding potential characteristics of the scanned surface. Thirty thousand work function samples are obtained during the three seconds required for one scan of the surface. Similar techniques also afford a plot of the distribution of patch fields on the surface. It is furthermore possible to determine the nature of the patch fields encountered just in front of the low work function patches. An analysis of these type patch field plots provides a determination of the average low work function patch diameter.

Interatomic Auger analysis of the oxidation of thin Ba films: II. Applications to impregnated cathodes

Abstract The oxidation and thermal desorption of evaporated Ba films on different substrates have been characterized using low energy interatomic AES techniques. The films, which were deposited on W〈100〉 (as well as Ir and Os-W) varied from a fractional layer to ∼ 10 layers. Oxidation states, i.e. Ba: O x , ranged from x =0 to x ≈2. Comparison of these data to those from actual impregnated cathodes allows one to infer the thickness and state of oxidation of the cathodes. The results indicate that the active state for “B-type” impregnated cathodes can be reproduced by a near monolayer of the stoichiometric BaO on the W surface. The density of Ba atoms (or O atoms) in this “monolayer” should be ∼ one half that of the W atoms based on size consideration of the Ba compared to the W. This was confirmed by LEED. Measurements of the effusion of Ba compounds from the pores, the substrate interaction and the thermal evaporation indicate that on the W substrate, the monolayer BaO stoichiometric ratio is not only a stable configuration, but also is the one that has the lowest work function (i.e. ∼ 2 eV). The substrate work function is lowered because of the dipole formed from the adsorbed BaO. Studies of the electron interactions between the Ba and O indicate that (in agreement also with surface plasmon results) a more complete electron transfer exists between the Ba and O for the BaO layer on the Ir substrate than on the W substrate. These results are based on the observation that O 2p states on the W substrate appear to be filled not only by Ba 6s but also by W valence electrons.

Interatomic Auger analysis of the oxidation of thin Ba films: I. Characterization of the low energy Auger spectrum

The oxidation states of thin Ba films on Ir have been studied using an analysis of interatomic Auger lines. From published XPS data, some of the low energy AES lines were described in terms of various possible core, core, valence transitions where the core states were Ba states but the valence state could be either due to Ba or O. The magnitude of all AES lines, whose energy indicated an O valence state, was observed to increase with oxidation while those ascribed to Ba valence states decreased. The Ba 4d, 5p, valence transitions in the 65–75 V range appeared relatively free from clutter of other Ba and substrate lines and were the ones primarly used in determining the Ba-O bonding. The increase with oxidation of the interatomic Ba 4d, 5p, O 2p line at ∼ 68 V was found to be correlated with the increase in the O 2p loss peak. Furthermore, shifts in energy of this peak for various film thicknesses varied in accordance with corresponding shifts in O 2p binding energy with respect to the Fermi level. Also, the decrease with oxidation of the Ba 4d, 5p, 6s line at ∼ 73 V was found to be correlated with the decrease in conduction electrons contributing to the surface plasmon peak. The manner in which these lines can be used to determine the extent of electron transfer in the formation of compounds other than BaO (such as BaC2) is also given.

Investigation of scandate cathodes: emission, fabrication, and activation processes

Top-layer cathodes have been shown to exhibit work function values in the 1.5-eV range near 1100 K. Emission from cathodes made by sputter deposition of a Sc/sub 2/O/sub 3//W layer on top of an impregnated cathode were limited by the deposition conditions that cause dissociation of Sc/sub 2/O/sub 3/ and/or formation of volatile oxides of W. Pure Sc/sub 2/O/sub 3/ layers were also activated by an external Ba source to the value of the work function reported above. These results and activation mechanisms and characteristics that are peculiar to the Ba-O-Sc surface are discussed. >

Preparation and oxidation of a thin Ba film

A method of determining the thickness of vapor-deposited thin Ba films is described. The thickness is expressed in terms of numbers of Ba monolayers which were defined and characterized on W(100) and Ir(100) substrates using Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). The changes in the electronic properties of a 10-layer Ba film on Ir(100) during oxidation were studied using Low Energy Electron Reflectivity (LEER) measurements. When the oxidation of Ba was nearly complete (as shown by AES), the LEER patterns were similar to that found for a randomly oriented BaO layer with perhaps a partial layer of Ba on the surface. Additional O2 exposure, however, resulted in the formation of a BaO crystal whose characteristic LEER patterns indicated a Fermi level position which suggested a high density of oxygen-vacancy donors and whose large electron affinity indicated non-stoichiometry at the surface. Further O2 exposure resulted in a BaO-type surface dipole and oxygen vacancy donor densities more typical of BaO lattices. At this level of oxidation, a minimum work function of 1.4 eV was attained, which was identical to that of an active thin BaO film. Furthermore, AES indicated a Ba to O ratio identical to that of a BaO film. Any change from this stoichiometry through the addition of either oxygen or barium resulted in an increase in the work function.

Auger peak height calibrations of Ba, Sr and Ca in the oxide form

Abstract Clean alkaline earth oxides are prepared in an UHV system in order to obtain the relative Auger sensitivity factors of the alkaline earths as they exist in the oxide forms. These relative sensitivity factors with respect to Ag are determined to be 0.12, 0.091, 0.014 and 0.28 for Ba (584 eV), Sr (103 eV), Sr (1649 eV) and Ca (291 eV), respectively.

Electronic and chemical behavior of oxygen in BaO films on Ir(100)

Abstract A new surface analysis technique involving low energy electron reflection (LEER) has been used to separate the electron affinity and position of the conduction band edge for BaO single crystal films epitaxially grown on Ir(100) substrates. AES and LEED were used in the study for characterizing chemical and structural effects. Active films have a work function of 1.3 eV with an electron affinity of 0.6 eV. Thermal activation was principally the result of the generation of O-vacancy donors by O diffusion into the substrate and not by evaporation of O from the coating. Donor generation could be retarded by the formation of an oxide diffusion barrier at the substrate interface or by pre-saturating the substrate surface region with oxygen. Electron bombardment desorption caused changes in surface stoichiometry which increased the worl function by increasing the electron affinity. Poisoning by oxygen took place by neutralization of O- vacancy donors until near intrinsic conditions occurred. Additional oxygen accumulates at the surface increasing the electron affinity. Since the Fermi level does not go below the mid-gap position, the formation of Ba-vacancies does not seem likely. Reactivation is possible but the initial donor density cannot be achieved due to O saturation of the substrate.