C. Hor

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 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. >

Electron emission from a single spindt-type field emitter: Comparison of theory with experiment

A simple analytic model of the electron emission from a single tip field emitter is correlated with experimental measurements made on a single Spindt-type molybdenum field emitter using a nanofabricated anode whose position from the emitter was determined using laser interferometry. It is shown how the model may be extended to find the trajectories needed for particle simulations. Methods used to correlate theory with experiment are explained, and the dependence of the beam profile on tip sharpness, gate diameter, anode distance, and tip work function are examined. A simple analysis of the effects of space charge on field emission is presented and correlated with experimental data. Analysis has shown that the rms spread angle is approximately 20°.

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.

Microwave triode amplifiers from 1 to 2 GHz using molybdenum thin-film-field-emission cathode devices

Experimental results are presented on microwave amplifiers using Molybdenum Thin Film-Field-Emission cathode devices, fabricated at Stanford Research Institute (SRT). A device having 250 tips, operating at 4.8 mA of current with g/sub m/=840 /spl mu/S is inferred to have an intrinsic power gain of 7 dB at 1.1 GHz. Other results are given for frequencies up to 1.7 GHz. For the first time, device and circuit modeling of sufficient accuracy has been performed that it is possible to confidently deduce the intrinsic performance parameters of the FE devices. The importance of integrated matching circuits for optimum power gain performance is exposed and quantified. >

Secondary electron emission properties of oxidized beryllium CFA cathodes

Heating an oxidized beryllium sample above 500/spl deg/C for eight hours or more establishes a stable surface composition that consists of about 35% carbon in carbide form, and Be and O in nearly one-to-one atomic ratio for the remainder. The secondary electron yield of this surface has a maximum yield, /spl delta//sub max/, of 2.8/spl plusmn/0.1 at the primary electron energy of 420/spl plusmn/20 eV. The secondary electron yield decreases slowly with increasing sample temperature. The energy of the emitted electrons is analyzed using a retarding potential method with the primary electron energy E/sub p/ ranging from 10 to 1600 eV. For E/sub p/>100 eV, most of the emitted electrons are the true secondary electrons (i.e., those electrons with energy less than 50 eV). The energy distribution of the true secondary electrons shows little change in functional form for E/sub p/ from 200 eV to 1600 eV, and for sample temperature from 20/spl deg/C to 530/spl deg/C. A small but steady change is observed in the narrowing of the peak width with increasing E/sub p/, or increasing sample temperature. The current practice in processing the crossed-field amplifier (CFA) tube with an oxidized beryllium cathode includes a bakeout between 500/spl deg/C and 550/spl deg/C for several hours. The present study suggests that this heating is sufficient to convert the oxidized beryllium CFA cathode surface to the stable composition with the large secondary electron yield. Heating to a much higher temperature will not reduce the carbide content, but rather will reduce the oxygen content and consequently the secondary electron yield. >

Comparison of reactivation following shelf-life poisoning of the impregnated and coated powder cathodes☆

Abstract A comparative study has been made of impregnated W matrix cathodes (type B) and coated powder cathodes (CPC) in their ability to rapidly turn on after extended periods of shelf life. Particular performance objectives were to attain pulsed emission levels in excess of 3 A/cm 2 within five seconds after shelf-life periods corresponding to several years. Preliminary measurements indicate that the type B cathode is more subject to poisoning by small concentrations of gas components in the tube ambient than the CPC. Mass spectrometer gas analysis of desorption products suggests that some of these gas components picked up by the cathode during shelf life appear to be methyl and ethyl hydrocarbon fragments. (These findings are consistent with earlier results that showed increases in work function of the type B cathode during shelf life, were correlated with increases in carbon Auger peak heights.) Studies were made of the kinetics of reactivation (i.e., work function reduction) as the temperature was increased during the time of cathode turn-on. These indicated that under actual turn-on procedures following shelf life, the CPC would reactivate to give 4–5 A/cm 2 pulsed current density in ≈ 5 s compared to ≈ 45 s for the type B. To reach these emission levels, the work functions were 1.85 eV and 1250 K for the CPC and 2.0 eV and 1350 K for the type B.

Design and construction of apparatus for characterization of gated field emitter array electron emission

An inchworm controlled microfabricated detector system with laser interferometric feedback has been developed to provide an absolute position characterization of the electron emission from gated field emitter arrays (FEAs) in an ultra‐high‐vacuum (UHV) environment with nanometric precision. The emission characteristics of single tip and multiple tip FEAs are required to provide comparison with theory and simulation in order for these to be predictably used as a reliable source of high current density for high frequency amplifiers and oscillators. In its present form, the instrumentation is used to determine the angular distribution of the field emitted electrons. The addition of a few components will allow the measurement of emittance of the FEAs for electron gun design. Angular distribution of the electrons is measured by means of a microfabricated detector which is manipulated within an UHV environment with nanometric precision. The absolute location of the detector (within 80 nm uncertainty) is determi...

Work function and gas desorption studies during turn-on of various shelf-stored cathodes

Abstract A comparative study has been made of the oxide-type coated powder cathode (CPC), and impregnated types B, M, tungstate and scandate cathodes in their ability to rapidly turn on after extended periods of shelf life. Particular performance objectives were to attain pulsed emission levels in the 4–5 A/cm2 region within a few seconds after shelf lives corresponding up to several years. Preliminary surface studies indicate that all cathodes significantly poison within a few hours or days after being shut down in a typical sealed-off tube environment. Therefore, apart from bringing the cathode up to thermionic emission temperature, the kinetics of the turn-on step must also involve the kinetics of reactivation. Comparative data are presented of the work function variation with reactivation temperature for the various cathodes, thus showing the degree of poisoning, the thermal reactivation characteristics and the work function-temperature relationship in their respective active states. The results show that in the active state, the CPC has the highest emission, the M and scandate impregnated cathodes are somewhat lower in emission, i.e. ∼50 K higher temperature for the same emission, while the type-B impregnated cathode requires ∼100 K more. The shelf-life poisoning among the impregnated cathodes, on the other hand, was greatest for the M cathodes, a bit less for the scandate and least for the type B. For typical fast turn-on reactivation schedules, the CPC reactivated below its normal operating temperature of ∼1250 K but all impregnated cathodes required activation temperatures of ∼1500 K and above (i.e., about 200 K above their operating temperatures). Studies were also made on the pick-up and desorption of various gases present in the shelf-life vacuum ambient. The data demonstrated a direct correlation between the chemical reactivity that the different cathodes had with the residual ambient gases and the corresponding extent to which these cathodes poisoned during shelf life. While a study of the shelf-life poisoning and reactivation mechanisms for the impregnated-type cathodes is still in progress, initial results have been completed for the oxide-type cathodes and these are presented elsewhere in this issue.