A. Shih

Secondary electron emission studies

Abstract Secondary-electron-emission processes under electron bombardment play an important role in the performance of a variety of electron devices. While in some devices, the anode and the grid require materials that suppress the secondary-electron-generation process, the crossed-field amplifier (CFA) is an example where the cathode requires an efficient secondary-electron-emission material. Secondary-electron-emission processes will be discussed by a three-step process: penetration of the primary electrons, transmission of the secondary electrons through the material, and final escape of the secondary electrons over the vacuum barrier. The transmission of the secondary electrons is one of the critical factors in determining the magnitude of the secondary-electron yield. The wide band-gap in an insulator prevents low-energy secondary electrons from losing energy through electron-electron collisions, thereby resulting in a large escape depth for the secondary electrons and a large secondary-electron yield. In general, insulating materials have high secondary-electron yields, but a provision to supply some level of electrical conductivity is necessary in order to replenish the electrons lost in the secondary-electron-emission process. Our secondary-emission study of diamond demonstrates that the vacuum barrier height can have a strong effect on the total yield. The combined effect of a large escape depth of the secondary electrons and a low vacuum-barrier height is responsible for the extraordinarily high secondary-electron yields observed on hydrogen-terminated diamond samples.

Secondary electron emission from diamond surfaces

Diamond exhibits very high, but widely varying, secondary-electron yields. In this study, we identified some of the factors that govern the secondary-electron yield from diamond by performing comparative studies on polycrystalline films with different dopants (boron or nitrogen), doping concentrations, and surface terminations. The total electron yield as a function of incident-electron energy and the energy distribution of the emitted secondary electrons showed that both bulk properties and surface chemistry are important in the secondary-electron-emission process. The dopant type and doping concentration affect the transport of secondary electrons through the sample bulk, as well as the electrical conductivity needed to replenish the emitted electrons. Surface adsorbates affect the electron transmission at the surface-vacuum interface because they change the vacuum barrier height. The presence of hydrogen termination at the diamond surface, the extent of the hydrogen coverage, and the coadsorption of hy...

Secondary electron emission characteristics of single-crystal and polycrystalline diamond

Secondary electron emission spectroscopy (SEES) is used to examine the transport and emission of low-energy electrons in diamond. In particular, SEES measurements from single-crystal (100) and (111) diamond and polycrystalline chemical vapor deposited (CVD) diamond are compared in order to examine the effect of crystallographic orientation on the emission characteristics. Crystal orientation is found to influence the surface properties of the samples but not the low-energy transport properties. Specifically, very high yields are obtained from negative-electron-affinity (NEA) surfaces of all three samples, indicating that low-energy electrons are transported and emitted very efficiently regardless of crystal orientation. However, the energy distributions measured from adsorbate-covered C(111) surfaces are broader and shifted lower in energy than those measured from corresponding C(100) surfaces. In fact, the energy distributions measured from polycrystalline CVD diamond surfaces appear to be a superpositio...

Secondary emission properties as a function of the electron incidence angle

Computer codes being developed to improve the understanding of crossed-field amplifier (CFA) performance require a more complete and reliable database of the secondary electron emission properties of the electrode materials than exists in the literature. The authors describe an experimental method and present results of secondary emission yield measurements on molybdenum surfaces, both clean and gas-exposed. The surface cleanliness was monitored by Auger electron spectroscopy (AES), and all measurements were made under ultrahigh-vacuum conditions (better than 1*10/sup -10/ torr). The results differ from the existing data for which the surface cleanliness was not determined. The secondary electron emission yields were measured as a function of the primary electron energy and also of the angle of incidence. The results were fitted with the analytical expressions of J.R.M. Vaughan (1989), with good overall agreement if Vaughans formulas are slightly modified. >

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.

A synchrotron radiation study of BaO films on W(001) and their interaction with H2O, CO2, and O2

The interaction of O2, CO2, and H2O with bulk BaO and BaO adlayers adsorbed on W(001) has been examined using ultraviolet photoelectron spectroscopy. H2O reacts with bulk BaO to form Ba(OH)2, while CO2 forms a surface layer of BaCO3. Water and carbon dioxide also react with a (√2×2)R45–BaO monolayer adsorbed on W(001) to produce adsorbed OH and CO3 species bound to the tungsten substrate. The interaction of O2 with W(001) is enhanced by the presence of a BaO monolayer on the substrate. The observations are compared with the results of previous studies.

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.

The characterization of the surfaces of tungsten-based dispenser cathodes

Abstract The “CD”, “M” and scandate cathodes all have a better thermionic emission performance than the “B” type of impregnated tungsten based dispenser cathode. Modern surface analysis techniques (AES, ISS, SIMS and SLEEP) have been used to examine the surfaces of the four cathode types. The cathode surfaces have been studied in their active state, during reactivation following a sputter cleaning of their surfaces and whilst poisoned by, and reactivating from, an exposure to 10 L O 2 . The results are discussed in terms of surface models which would be consistent with the observed data. The differences in the surfaces of the different cathode types are examined and the value of the various surface analysis techniques discussed.