Jonathan L. Shaw

Analysis and design of microwave amplifiers employing field‐emitter arrays

An analysis of the microwave ( f≳1 GHz) properties of field‐emitter arrays (FEAs) and several representative medium power (10–100 W) microwave amplifiers employing FEAs is presented. The FEA analysis is limited to parallel‐plate structures having discrete pointlike vertical emitter tips and gate apertures aligned to each tip. A transmission line analysis of wave propagation in this structure is presented and used to evaluate the geometries and materials needed for microwave operation. This analysis is used to investigate the performance capabilities and emitter requirements of both modulated‐emission linear beam tubes and microdevices based on FEAs. Specific microtriode designs are used to investigate practical problems such as space charge and thermal effects. Competitive performance should be achievable in gated‐emission linear beam tubes by using FEAs that perform at levels previously reported by several laboratories. Existing FEA technology (currents of 10 μA per emitter, transconductances of 1 μS per...

Integrally gated carbon nanotube-on-post field emitter arrays

Multiwalled carbon nanotubes were grown using chemical vapor deposition on the tops of blunt vertical silicon posts in cells having a horizontal gate aperture of conventional field emitter design. We obtained over 1 mA total emission current from a single array, or 0.3 μA per cell at 40 V. In addition to the low voltage operation, the most distinctive differences from conventional field emitter arrays include their stability and the lack of catastrophic arcing without any special sample preparation.

Emittance of a field emission electron source

An analytical formula of the emittance of a field emitter is given. In contrast to thermal and photoemission, such a formula contains complexity due to the multidimensional nature of the source. A formulation of emittance is given for one- and three-dimensional (3D) field emitters. The 3D formulation makes use of the point charge model of a unit cell emitter coupled with a trajectory analysis to follow electrons to an evaluation plane where emittance is determined. The single tip theory is extended to an array and the resulting theory predicts the emittance of a Spindt-type square array of emitters 0.2cm on a side producing 2000A∕cm2 is 23mmmrad. Theory compares favorably with experimental measurements in the literature from ungated and gated sources. The impacts of several complications are estimated: the effects of a gate for modulating the emitter; the influence of space charge within the unit cell on the beam; and constraints imposed by modulation frequency, emitter dimensions, and rise/fall time requ...

Graded electron affinity electron source

We describe a new electron source using electric field and low electron affinity semiconductor materials to bring charge to potential energy levels near the vacuum level while still in the solid. The basic idea involves moving some of the potential barrier from the surface to the bulk, and distributing the barrier over a thin layer below the surface. In so doing, the emission physics is changed fundamentally from a quantum mechanical tunneling process largely controlled by surface properties to a classical transport process largely controlled by the band structure of a wide bandgap semiconductor. The composition of the thin layer below the surface would be graded such that the conduction band minimum changes from an energy close to the substrate Fermi level to an energy significantly closer to the vacuum level. Electrons from the substrate would be drawn into the graded composition layer with an electric field produced by a pointed emitter structure and extraction gate similar to that used in field emitte...

Effects of surface oxides on field emission from silicon

This report documents energy distribution measurements of field emission from single-tip Si field emitter arrays. The emission energy distributions are much broader than clean metal distributions, extending several volts below the Fermi level and often including multiple peak structure. The peak positions typically move to lower energy as the gate voltage is increased, however, the emission history as well as the emission current and/or gate voltage can change the energy as well as intensity of the energy distributions. Changes in the distribution including shifts to higher energy occur suddenly and spontaneously as well as slowly during emission. These results show that the emission comes from oxide and interface states, which refill at a finite rate, limiting the emission current. Changes in the local electric potential due to single charges becoming trapped in the oxide account for large and discrete changes in the emission distribution.

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.

1A∕cm2 current density from microgated carbon nanotube field-emitter arrays grown by dc plasma chemical-vapor deposition

We report improved emission performance of microgated carbon nanotube field-emitter arrays. The arrays with 0.1mm2 areas produced anode currents in excess of 1mA (1A∕cm2) and transconductance of up to 174μS (174mS∕cm2) at gate voltages of 75V or less. The carbon nanotubes were grown by dc plasma chemical-vapor deposition, but were not oriented vertically during inspection in the scanning electron microscope. Most of the nanotubes had diameters near 10nm. We suggest that the small tube diameter caused the relatively high emission current and also produced limber tubes, unable to support themselves vertically.

Regeneration of gated carbon nanotube field emission

We report large increases in field emission current when operating carbon nanotubes in substantial pressures of hydrogen, especially when the nanotubes were contaminated. We have previously demonstrated two different configurations of integrally gated carbon nanotube field emitter arrays (cNTFEAs), CNTs grown inside microfabricated gate apertures with and without silicon posts. Salient features of these in situ grown microgated cNTFEAs include the absence of electrical arcing, low operating voltage, and enhancing effect of some residual ambient gases. Operating both configurations of cNTFEAs without special precleaning in greater than 10−5Torr hydrogen produced orders of magnitude enhancement in emission. For a cNTFEA intentionally degraded by oxygen, the operation in hydrogen resulted in a 340-fold increase in emission current at constant gate voltage. The results suggested a dependence on atomic hydrogen produced from the interaction between emission electrons and molecular hydrogen. The observed emissi...

Electron emission from GaN n–p junctions

We report on electron emission from cesiated GaN n–p junctions in forward bias. Surface electric fields ∼3 V/μm caused a fivefold increase in emission current. Initial maximum currents in excess of 200 nA degrade to 50 nA due to charge trapping but are quickly recovered at zero bias. Energy spectra confirm negative electron affinity 80 h after cesiation, indicate resistive losses limit the emission current, and reveal significant emission at energies above the Fermi level of the injecting contact.