Paul R. Davis

Transition metal carbides for use as field emission cathodes

Field emission characteristics of clean ZrC, HfC, and TaC cathodes are reported. High current density emission, greater than 1×108 A/cm2, is discussed, and a method for determining the cathode changes leading to this high current emission condition is proposed. A close‐spaced triode designed for testing individual emitters is described, and results are reported. The effective thermionic work functions of clean and partially oxygen covered surfaces of ZrCx specimens of two different bulk compositions are reported and discussed. Clean values of 3.5 and 3.4 eV were observed for ZrC0.92 and ZrC0.86 specimens, respectively, at 1500 K. With adsorbed oxygen, values as low as 3.4 and 3.2 eV, respectively, were observed for these surfaces at 1500 K.

Field emission from hafnium carbide

An electron source for a high resolution SEM/TEM application should produce a high brightness, have a minimal energy distribution, and should be highly stable. In an application in which the resolution is limited by chromatic aberrations, one can improve the performance over a commonly used thermal field emission source, such as a ZrOW Schottky emitter, by using a cold field emission source (CFE).

Comparison of thermionic cathode parameters of low index single crystal faces of LaB6, CeB6 and PrB6

Abstract The work functions and activation energies for vaporization of low index crystal faces of LaB 6 , CeB 6 and PrB 6 have been studied. The effect of bulk stoichiometry on these properties has been determined for LaB 6 over the range 5.74 6.0 and CeB 6.2 . The only PrB 6 specimen investigated was PrB 5.87 (100). Work functions were determined by the field emission retarding potential (FERP) technique at room temperature and by pulsed thermionic diode measurements at 1600–1700 K. Surface compositions of the specimens were studied by Auger electron spectroscopy, and line-of-sight mass spectrometry was used to determine vaporization energies. Results for the (100) faces of all the compounds and stoichiometries examined are compared on the basis of a cathode figure of merit equal to the ratio of electron emission rate to the rate of material evaporation at 1600 K. The best cathode determined in this study, based upon this figure of merit criterion, is the LaB 6.09 (100) surface, although both CeB 6 and PrB 6 may be viable cathode materials.

High current density field emission from transition metal carbides

Abstract We report on fabrication, performance and modeling of field emission cathodes made from refractory transition metal carbides. The primary carbides under study include HfC, TaC, and ZrC. Single crystal carbide specimens were prepared by arc floating zone refinement from sintered stock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means of field emission microscopy is reported and comparisons are made with results from thermionic projection microscopy. Typically, the carbides show that ∅210,310 2400 h are reported. Stable pulse field emission of > mA for a single emitter at pulse lengths of 10 μs and duty factors of 0.01-0.001 are reported. Single-pulse currents as high as 48 mA were observed from a single emitter tip. Current densities have been calculated to be in the high 108 A/cm2 range.

Cesium and oxygen coadsorption on LaB6 single crystal surfaces: II. Thermal desorption of cesium from LaB6 (100)

Thermal desorption of neutral Cs from LaB6 (100) with and without preadsorbed oxygen occurs from discrete binding states at high Cs coverage and a continuum of states at low coverage. The two discrete binding states have activation energies of 1.2 and 1.5 eV on clean LaB6(100) which shift to 1.4 and 1.7 eV with oxygen preadsorption and are independent of Cs coverage. The terminal coverage Cs desorption energy increases from 2.3 to 3.0 eV as a result of an oxygen underlayer, consistent with the formation of Cs-O bonds between the two chemisorbed layers. The adsorption of Cs on either surface is apparently limited to a single layer at room temperature. Cs ion desorption is observed from the oxygen covered surface at low (θCs ⩽ 0.08) coverage with an evaporation energy of 2.5 eV. All Cs desorption reactions occurring on these surfaces are shown to be first-order processes.

A study of oxygen interaction with a LaB6(100) single crystal surface

The desorption of boron and lanthanum oxides from a LaB6(100) single crystal surface has been studied by standard surface analysis techniques. Measurements were made both on preadsorbed oxygen layers in vacuum and under steady-state conditions in fixed oxygen pressures. The observed oxide desorption products were BO, B2O2, B2O3 and LaO. Temperature and pressure dependencies of the boron oxide species have been studied in detail. The findings are discussed from the point of view of cathode applications.

Interaction of cesium and barium on partially oxygen covered Nb(110)

We present results of investigations of the adsorption and desorption of Cs and Ba on clean and oxygenated Nb(110) surfaces. The experiments were conducted under ultrahigh vacuum conditions using the techniques of line‐of‐sight thermal desorption mass spectrometry, Auger electron spectroscopy, and retarding potential work function measurement. The room temperature work function was found to increase from 4.62 eV for clean Nb(110) to 5.63 eV for Nb(110) with a saturated chemisorbed layer of oxygen. For a 4.8 L (1 L=1 Langmuir=1×10−6 Torr s) oxygen exposure, corresponding to about 70% of saturated oxygen coverage, minimum work functions of 2.21 and 1.42 eV were found with the adsorption of Ba and Cs, respectively. The minimum work function tends to increase with further adsorption of oxygen. The adsorption of Cs on the Ba/Nb(110) surface yielded a minimum work function of 1.47 eV. The results of the thermal desorption studies showed an initial (low coverages) binding energy of 3.72 eV for Ba on Nb(110) and ...

The effect of oxygen pressure on volatility and morphology of LaB6 single crystal cathodes

Single crystal LaB6 has become an important cathode material for a variety of electron beam systems. At the typical operating pressures of many such systems interaction of the cathode with the residual gases present can cause radical enhancement of the cathode volatility with concomitant change in the usual conical shape, oxygen‐bearing gases being the most active in promoting such effects. In this paper we discuss a systematic investigation of the variation of volatility and morphology of heated LaB6 cathodes with oxygen pressure. The results of this study show that at a low operating temperature of 1600 K the volatility enhancement due to PO2=1×10−6 Torr is almost a factor of 100 relative to operation at 1×10−8 Torr or less. In contrast, at a cathode operating temperature of 1900 K the enhancement due to the same oxygen pressure is negligible. The volatility enhancement of LaB6 for PO2=1×10−7 Torr and T=1700 K is only a factor of 2. The volatility enhancement was observed to vary with crystallographic d...

Three dimensional axisymmetric space charge simulation via boundary elements and emitted particles

The boundary element (i.e., charge density) method can often be used to reliably and efficiently compute field strengths and field emission currents where space charge is not a factor. In order to approximate space charge effects, launching and tracking of simulated particles are added to a basic boundary element scheme. Quantities of most interest include the emitted current and gate current once the startup transients die out. Simplistic initial attempts at such a simulation reveal difficulties in the following areas: (1) choosing a single time step suitable for all emitted particles; (2) computing the effect of newly emitted particles on the tip field controlling the next emission step; and (3) stabilizing the computed result. Methods for combating these problems are implemented and tested in the case of concentric spheres where one dimensional potentials can be reliably numerically approximated by use of an ordinary differential equation solver. Finally, the resulting code is applied to laboratory geo...

Preparation and characterization of zirconium carbide field emitters

The authors report on experiments to determine the feasibility of using the refractory transition metal carbide ZrC as a stable field-emission cathode. Emphasis is on the preparation and analysis of methods needed to obtain stable electron emission from a cold field-emission cathode. ZrC single-crystal specimens were prepared by arc floating zone refinement from sintered stock, yielding an average bulk stoichiometry of C/Zr=0.913. Due to its brittle nature and the high temperatures required for cleaning of this carbide, new mounting methods were developed. Emitter etching procedures are reported for ZrC, as well as in situ tip sharpening techniques of neon-ion bombardment and temperatures required for thermal cleaning. A temperature of 1500 degrees C is required to remove adsorbates including oxygen. A clean ZrC field-emission pattern is shown. Ordering of work functions of various crystal planes is reported through field-emission microscopy, and comparisons are made with thermionic projection microscopy. Effective thermionic work functions are presented for clean surfaces to support further the ordering obtained. The ability of ZrC field emitters to operate at pressures far above those commonly found for field-emission cathodes is demonstrated. >