J. E. Yater

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...

Transmission of low-energy electrons in boron-doped nanocrystalline diamond films

Transmission electron spectroscopy is used to examine the low-energy electron transport and emission properties of nanocrystalline chemical-vapor-deposited diamond films. In particular, the intensity and energy distribution of transmitted electrons are measured as a function of film thickness and incident-beam parameters. Low-energy transmission is detected in measurements from two films of thickness 0.15 and 2.5 μm with similar boron concentrations. The transmitted energy distributions are very similar for the two samples and are nearly identical to those obtained in reflection measurements. The full width at half maximum of the transmitted distribution is slightly broader for the thinner film (∼0.8−0.9 eV) than for the thicker film (∼0.6−0.7 eV), and the maximum transmission yields are similar (∼3−5 emitted electrons per incident electron). However, different beam energies are required to produce the low-energy transmission. The energy-dependent data is interpreted using Monte Carlo simulations along wi...

Electron transport mechanisms in thin boron-doped diamond films

Electron transmission spectroscopy is used to examine the effect of transport distance, diamond nanostructure, and electron affinity on the cold emission characteristics of thin nanocrystalline diamond films. Energy distribution and intensity measurements are taken from films having different thicknesses (∼0.15, 2, and 4 μm) and surface properties (hydrogenated, cesiated), and two distinct transmission regimes are identified that exhibit fundamentally different characteristics. In measurements taken at sufficiently high beam energy Eo, electrons are transported through the conduction band of the diamond and emitted at a low-affinity surface, with transmission yields generally greater than 1. In this regime, the dependence on Eo results from the finite escape depth of the conduction-band electrons, which is determined to be ∼1 μm for these films based on a Monte Carlo analysis of the incident electron range. In measurements taken at lower values of Eo, electrons are generated outside of this escape range a...

Advanced emitters for next generation rf amplifiers

Next generation rf amplifiers, in particular the inductive output amplifiers (IOAs), will require electron sources characterized by high current density, high brightness, low emittance, and the ability to be emission gated. The strong interaction between the beam and the resonant or synchronous electromagnetic field may enable compact, highly efficient, and moderate gain X-band power booster amplifiers. An analysis of amplifier demands on generic emitter candidates is provided. Of the emitter candidates available, two (namely, field emitter arrays and wide-band-gap semiconductors) are amenable to an analysis predicated on a simple model of field emission from hyperbolas and ellipsoids. The simple model is used to investigate two problems of critical concern: for field emitter arrays (FEAs), we will investigate the conditions under which important space charge effects exist, and from the model predict optimum FEA performance characteristics for rf IOAs; for wide-band-gap materials, the simple model identif...

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. >

Electron transmission studies of diamond films

Diamond is an attractive cold emitter candidate because of specific negative-electron-affinity (NEA) surfaces and good electron transport properties. In this study, transmission electron spectroscopy is used to examine the low-energy electron transport and emission properties of thin CVD diamond films. After injecting electrons into diamond films using an external electron gun, the transmission of low-energy secondary electrons is examined. In particular, the intensity and energy distribution of the transmitted electrons are measured as a function of the film properties (thickness, doping concentration) and incident beam properties (energy, intensity). Electron transmission through a heavily-doped, 5 μm thick film exhibits a larger energy spread and lower intensity than electron transmission through a medium-doped, 2 μm thick film. However, the transmission yields are relatively low for both films compared to the reflected yield measurements. Furthermore, the incident beam parameters influence the transmitted electron distribution in ways that cannot be explained by our secondary emission model. On the other hand, the results of the study concerning the transmission of high-energy electrons confirm the theoretical predictions of electron penetration depths in diamond. These initial electron transmission measurements from thin diamond films indicate that the transport process is quite complex, and the study reveals several issues that need to be examined further before the cold emission capabilities of diamond can be assessed.

Bunch characteristics of an electron beam generated by a diamond secondary emitter amplifier

Electron bunches for high performance free electron lasers are subject to constraints on charge per bunch and pulse shape. A Diamond secondary emitter used in conjunction with a photocathode and drive laser has potential to enable a high brightness, high peak current photoinjector by increasing the effective quantum efficiency of the photocathode. A theoretical characterization of the bunches so produced has been heretofore absent. Using a combination of Monte Carlo and analytical models, the shape of the bunches, their transit time, and emission time constants are determined and shown to be sensitive to the accelerating field in the diamond flake, incident beam profile, doping, and surface conditions. Methods to allow for extension to regimes of technological interest in terms of diamond thickness, external field, and primary pulse shape are given.

Application of a general electron emission equation to surface nonuniformity and current density variation

Using a recently developed model of emission that includes field, thermal, and photoemission effects simultaneously for arbitrary magnitudes of field, temperature, and laser intensity, we perform a study of the consequences of emission site variation on the subsequent electron beam. The electron emission model incorporated into the particle-in-cell (PIC) code MICHELLE, which is a conformal mesh finite-element (FE) two-dimensional (2-D) and 3-D electrostatic PIC code for modeling steady-state electron guns (and collectors), is described in detail.The addition of the generalized emission model therefore allows for assessing the impact of local thermal, field, and work function variation on the resultant electron beam.