R.E. Thomas

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.

Interference Effects in the Reflection of Low‐Energy Electrons from Thin Films of Au on Ir

Retarding field measurements were taken on thin films (10–−‐100 A) of Au evaporated on an Ir (111) surface. In the range of 0–5 eV the current‐vs‐voltage plots exhibited a decaying periodic variation in amplitude. The amplitude and period of the variations both decreased with film thickness. This interference phenomenon is interpreted as being due to electron reflection occurring at both the film‐vacuum and the film‐substrate interfaces. Measurements of the period as a function of film thickness agree well with a simple free‐electron picture for the phase shift experienced by electrons traversing the film, giving an effective inner potential of 16 eV for Au. From the temperature dependence of the energy at which maxima and minima occur the thermal expansion coefficient for the films is determined. Also, estimates of the mean‐free path for electrons in the film are obtained from the decay of the variation amplitude with thickness. Values obtained for both these quantities are consistent with previously pub...

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. >

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.

Effects of S, Ba, and C on impregnated cathode surfaces

Auger electron spectroscopy (AES) and scanning low‐energy electron probe (SLEEP) techniques have been used simultaneously for correlating electronic and surface chemical behavior of impregnated matrix cathodes. The role of S, Ba, and C has been related to emission poisoning, decay with life, and delayed response after cathode shutdown.

Electronic and chemical surface studies in oxide cathodes

Abstract Modern surface electronic and chemical analysis techniques are used to study the behavior of an oxide cathode during various states of activation and life. Compositional changes are noted at the interface with the substrate and at the vacuum surface, and these are correlated with changes in the work function. The existence of surface monolayers was not observed and it was noted that both chemical and electronic behavior were analoguous to that seen in BaO single crystal films.

Thermionic properties of BaO on iridium

Abstract The thermionic properties of BaO films on both single and polycrystalline Ir substrates have been measured using Auger, LEED, and thermionic emission microscopy to determine the chemical and structural effects that contribute to these thermionic properties. Measurements were made of the work function (φ) as a function of film thickness, and the temperature dependence of φ for different film thicknesses and substrate orientations. In addition, the effects of nucleation, carbon contamination, as well as evaporation and diffusion on the electron emission from the BaO-Ir system were examined, and quantitative data were obtained on the steady state coverage of BaO on an Ir surface at cathode operating temperatures. A comparison is made between the thermionic properties of the BaO-Ir system and other Ba/BaO-metal systems that have been used in cathodes.