Total Emission Current Research Articles

Investigation of emitter tips for scanning tunneling microscope-based microprobe systems

This paper reports the preparation and characterization of field emitter tips for use in a scanning tunneling microscope aligned field emission (SAFE) microprobe system. With the tip being at close proximity to the extraction electrode, new demands are imposed on the emitter tips: (1) a low extraction voltage, (2) a well-defined emission pattern, preferably a single lobe emission, and (3) a high angular emission density. A combined field ion–field electron emission microscope equipped with a special stage for mounting a small aperture in close proximity to the emitter tip, which was used to simulate the first element of the electro-optical system of the SAFE microprobe, was used to analyze different tip preparation techniques. A low-temperature field-assisted thermal annealing process has been developed to routinely produce sharp W 〈111〉 tips well suited for SAFE operation. Tips having an effective tip radius of less than 500 Å, an emission half cone angle of less than 10°, and a peak angular emission density of 7 μA/sr at a total emission current of 1 μA were successfully prepared and used in the SAFE microprobe system.

Liquid metal ion source for cluster ions of metals and alloys: design and characteristics

Currently liquid metal ion sources (LMISs) are of great interest for a wide variety of applications—ion implantation, ion microlithography, thrusters for electric space propulsion, etc. A novel application of the LMIS is for the production of metallic cluster ions. In our laboratory we have designed and optimized the performance of a LMIS for the production of cluster ions of alkali metals. Using liquid rubidium (Rb) we have observed copious production of singly charged cluster ions (Rb+N, N=1–100). As expected the largest fraction of the emission consists of atomic ions. For low source current (<5 μA) about 80% of the total emission current is that of Rb+. The remaining 20% consists of Rb+2 and Rb+3. However, for large emission currents (>80 μA) we observe cluster ions as large as Rb+100. We study the mass distribution using the time-of-flight technique.

A combination electron/ion field emission source

A combination electron/ion field emission source will be described. For ion emission the source was operated as a conventional indium or gallium liquid metal ion source (LMIS), thereby giving a beam of principally In+ or Ga+ ions with the usual range of angular intensities and total emission currents. In the case of indium it was possible to ‘‘freeze in’’ the field stabilized Taylor cone formed during LMIS operation by carefully reducing the voltage to the threshold value and then rapidly reducing both the voltage and the emitter temperature. Field electron emission was then obtained by reversing the voltage polarity. The dc electron emission angular distribution generally overlapped that observed in the LMIS mode. This process, which was determined to be reversible and reproducible, will be discussed in detail.

FACTORS INFLUENCING THE STABILITY OF COLD-CATHODES FORMED BY COATING A PLANAR ELECTRODE WITH A METAL-INSULATOR COMPOSITE

An investigation has been undertaken into the stability of populations of electron emission sites on extended-area, composite-coated cathodes for applied fields of ≤ 20 MV/m. These cathodes have been shown to give rise to total current instabilities (S) typically in the range 10 to 40 % for total emission currents of ≤ 100 µA. An optical imaging technique has revealed that the current instability is associated with temporal changes in the emission site distributions which appear to be influenced by the coating composition. Operation under increased pressure H2, N2, O2 and CO environments (P ≤ 10-4 mbar) has been shown to only slightly degrade the current stability. Furthermore, extended tests have revealed that emission sites on the composite cathodes are resistant to He-conditioning.

A comparison of boron emission characteristics for liquid metal ion sources of PtB, PdB, and NiB

The relative abundance, angular intensities, and energy distribution of B+ emitted from liquid metal ion sources of Ni4B6, Pd4B6, and Pt4B6 alloys on carbon are measured as functions of total emission current. Evidence is presented that shows the NiB system to possess the highest concentration of B in the liquid phase which, in turn, leads to the highest electric field at the emitting site for this system. While the NiB system exhibits the highest angular intensity for the B+ species, it also displays the highest value of the energy spread of B+.

Emission parameters of low-index surfaces by a combined field and photofield emission method

A new method is proposed to determine independently the work function and the field factor of selected low-index surface planes of a field emitter. The field dependences of the total emission current are measured both in field emission and in photofield emission, and the resulting Fowler-Nordheim plots are analyzed by means of an iterative procedure. Experimental measurements of the emission current from several low-index planes of tungsten show that reliable results are obtained so long as the surface is illuminated at a sufficiently large angle of incidence. Unexplained irreproducible behaviour is observed when the angle of incidence is small.

Coulomb scattering in field and photofield emission.

An anomalous high-energy tail has been observed in the measured total energy distribution (TED) in photofield emission from tungsten. The strength of this tail is proportional to the product of the photofield emission current and the total emission current. Similar high- and low-energy tails in the TED's in field emission, which have previously been reported by several workers, are also observed. In any given measurement, the fraction of the total photofield-emission current in the anomalous photofield-emission tail is approximately equal to the fraction of the total field-emission current in the anomalous field-emission tail. Measurements of both the absolute strengths and energy dependences of the anomalous tails are reported. The experimental observations are consistent with the predictions of a classical calculation of the energy transfer that results from the Coulomb interaction between electrons in the vacuum near the field emitter. The various internal mechanisms that have previously been invoked to account for the tails in field-emission TED's do not appear to contribute significantly to the anomalous distributions observed in the present work.

Gallium clusters from a liquid metal ion source

Gallium ion clusters Ga+n with n ranging up to 30 have been measured from liquid gallium ion sources using a time-of-flight spectrometric technique. The observations were made as a function of total ion emission current, angle of emission, and temperature of the liquid metal. Under all conditions Ga+ is the overwhelmingly dominate species emitted. The cluster data reveal several remarkable features. First, emission of clusters tends to be very low below 2 μA emission current, to rise steeply with current to 10 or 20 μA and decrease for higher current. Second, within the range examined, clusters with n=8 and 15 have particularly low abundances. Third, the energy distributions of the resolvable clusters are bimodal with characteristic peaks at approximately 30 and 120 eV below the source potential. Fourth, the angular distributions of the clusters are more nearly axial than that of the primary species. The temperature of the liquid metal has no noticeable effect on cluster emission over the range examined. A possible model for cluster formation will be discussed.

MICROPROCESSOR CONTROLLED REMOLDING OF FIELD EMITTERS

A fully automatic, microprocessor controlled remolding system is presented. I t has been tested with -oriented tip-shaped tungsten field electron emitters. Many of its features and capabilities wouldibe equally relevant for other types of field electron emitters or in the context of field ion emission. I t can produce with high reliability a single spot emission pattern, which is most frequently desired because i t normally represents highest brightness. Field emission tips that were in such bad conditions tha t they would normally have to be replaced could be restored to good emission by the system. Some results on the remolding process itself which were obtained through the use of the remolding system are presented. 1. I n t r o d u c t i o n Field emission electron guns with pointed emitters fabricated from single crystal tungsten wires electrolytically etched to a very fine tip can reach the highest brightness values (> 1 0 ~ ~ / c r n ~ s r ) known. However, a brightness as high as this is not normally achieved with a newly etched tip. Moreover, the brightness often deteriorates after a certain operation time. In these cases, the t ip can be restored to good emission conditions by a so-called remolding process, achieved by the simultaneous application of a high electric field and high temperature to the tip / I / . However, up to now remolding facilities are not used in commercial instruments due to the rather cumbersome and unreliable application of the process when performed manually. These difficulties have been described in detail in a previous paper / 2 / . In the same paper we presented a microprocessor aided remolding system. This system has now been developed into a fully automatic microprocessor controlled remolding system. It has been tested with -oriented tungsten t ip emitters. However, many of its features and capabilities would be equally relevant for other types of field electron emitters or in the context of field ion emission. 2. T h e m i c r o p r o c e s s o r r e m o l d i n g s y s t e m ( h a r d w a r e ) A block diagram of the system is shown in Fig. 1. All components are controlled by the microprocessor, which is a t present a 16 bit microprocessor Intel 8088. Any other suitable processor could be used as well. Two externally programmable high tension cascades provide the remolding and field emission voltages resp.. Both cascades are controlled by the microprocessor via 12 bit D/A-converters. The actual switching of the high voltages to the field emitter is performed by two high voltage Reed-relays. The total field emission current is measured by a sensing resistor in the extraction voltage lead. The resulting voltage drop is amplified and then Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986776 C7-454 JOURNAL DE PHYSIQUE

Upper bound of the beam energy broadening in acceleration region

Using an expression of the local space-charge force derived in a previous paper, we analyze the broadening of axial energy distribution of beam particles in the acceleration region and calculate an upper bound of the energy broadening 〈‖ΔE‖〉UB. Assuming that the emitter is a sphere of radius R with potential V, we obtain the expression 〈‖ΔE‖〉UB ∼∝ m1/4 I1/2e V−1/4 T1/2eff α−1e R0, where m is the particle mass, Ie is the total emission current, V is the acceleration voltage, Teff is an effective temperature showing the kinetic energy of particles at the emitter surface, and αe is the beam semiangle at the emitter. Theory gives 〈‖ΔE‖〉UB ∼several eV for typical electron guns operating in space-charge unlimited conditions. The result is compared with experiments and a discussion is given of the problem we have to solve to calculate the magnitude of 〈‖ΔE‖〉.

A new and simple method to determine the temperature coefficient of effective work function for thermal positive ion production on a clean polycrystalline metal surface

The total emission current of Li + ions produced from LiI molecules incident upon a polycrystalline rhenium filament heated to a constant temperature ( T) was measured as a function of the time ( t) immediately after the filament was flashed at ∼ 2200 K for ∼ 3 min. This measurement was made at various temperatures, and each emission curve including a small peak at t ≲ 1 s was analyzed by our original theory on dissociative thermal ionization. Even under an easily attained residual-gas pressure (∼ 2×10 −7 torr), this new and simple method made it possible to determine the temperature coefficient of the effective work function (φ 0 +) for thermal positive ionization on a clean surface of the filament, yielding the following equation in the range of ∼ 1150–2150 K. φ 0 + = (5.247±0.008)+(1.47±0.05)×10 −4× T where φ 0 + and T are expressed in eV and K, respectively.

Photoemission from activated gallium arsenide. I. Very-high-resolution energy distribution curves.

The energy distribution curves (EDC's) of the photoelectrons emitted from the (100) face of a p-type doped (\ensuremath{\sim}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) GaAs crystal, activated to negative electron affinity in ultrahigh-vacuum conditions, is investigated. The study is performed at 300 and 120 K under well-focused ${\mathrm{Kr}}^{+}$-laser excitation and with a very-high-energy resolution (20 meV). The analysis of the EDC's as a function of the photon energy, mainly at low temperature, is shown to provide a very direct picture of the GaAs band structure away from the Brillouin-zone center. The experimental results are well fitted by a spherical, nonparabolic k\ensuremath{\rightarrow}\ensuremath{\cdot}p\ensuremath{\rightarrow} perturbation calculation of the coupled conduction and valence bands, for electron kinetic energies up to 1 eV in the central \ensuremath{\Gamma} valley. The essential role played by the subsidiary L and X minima in the energy relaxation and photoemission processes is evidenced. The main contribution to the total emitted current is due to electrons which were thermalized in the bulk \ensuremath{\Gamma} minimum and have lost an average energy \ensuremath{\simeq}130 meV in the band-bending region prior to emission into vacuum. The band-bending value is shown to be \ensuremath{\ge}0.5 eV. The yield and time evolution of GaAs photocathodes are discussed. This detailed study leads to a reexamination of the pioneer work of L. W. James and J. L. Moll [Phys. Rev. 183, 740 (1969)] and to a good understanding of the photoemission properties of activated GaAs.

Field electron emission properties of TiC single crystals

Very stable field electron emission from TiC single crystals was observed. No current fluctuations were included in an emission current of 10 μA in a vacuum of 10−8 Pa. Step- and spike-like current fluctuations were slightly included when the emission current was large. The frequency and the amplitude of the current fluctuations were largely dependent on the sorts of the residual gas molecules. For instance, when the total emission current was 10 μA, no current fluctuation was included in 10−6 Pa of H2; on the other hand, fluctuations of several percents were included in 10−7 Pa of O2.

Development of boron liquid–metal–ion source

A boron liquid–metal–ion source is described that uses a combination of a glassy carbon or carbide emitter and a Ni–B base alloy as its source material. The B+ ion emission current is 25%–35% of the total emission current, and the energy spread for B+ ions is 12 eV at a total current of 30 μA. A source lifetime of more than 250 h was achieved with a total current of 30–50 μA. This source mounted on a mass-separated focusing column has led to B+ submicron beams with maximum energies of 20 keV for preliminary experiments on maskless implantation.

Broadening of the energy distribution of thermal-field emitted electrons from carbon fibres

The energy distribution of electrons thermal-field emitted from carbon fibres and post-accelerated up to 25 keV has been measured. At low total emission currents (of the order of nano-amperes) the energy distribution of the emitted electrons agrees, at room temperature, with the theoretical prediction for the free-electron model, and at elevated temperatures (up to 1520 K) the energy spread is about 0.1 eV higher than predicted. This difference may be explained by assuming that the emission occurs from sharp microfibrils. With increasing current the energy distribution becomes broadened. It is shown that this broadening is independent of the tip temperature and most probably due to Coulomb interaction of the beam electrons in regions of high current density within the electron optical system.

Current stability of single-crystal and sintered LaB6 cathodes

The specimen current stability of single-crystal and of sintered LaB6 cathodes are compared from the viewpoint of their dependence on the cathode tip temperature and the vacuum pressure. The specimen current of the single-crystal LaB6 cathode is very stable during degradation of the vacuum in the gun chamber, while in the sintered LaB6 cathode it becomes very unstable in poor vacuum even though the total emission current is stable. The cause of this remarkable difference between the single-crystal and sintered LaB6 cathodes is discussed.

Influence of energy spread of field-emitted electrons on resolution in the Scanning Transmission Electron Microscope (STEM)

Resolution in a 50 kV STEM with a field-emission gun is determined by the energy spread of the field-emitted electrons. We have shown that the (100)-oriented tungsten tip provided the theoretically expected resolution limit of 0.5 nm only when emitting at 300 K and with a total emission current of 1 μA after a remolding process. When operating the same tip in an oxygen environment at 1100 K and with emission currents of 10 μA, the resolution limit increases to 0.9 nm.

Energy spectra of electrons field emitted from a broad area composite cathode of tantalum carbide

Details are given of the field electron emission (FEE) characteristics of a cold cathode consisting of a multi-array of tantalum carbide needles prepared from a directionally solidified eutectic. It is shown that, whilst these needles have similar dimensions, only a small fraction of these potential sources actually contribute to the total emission current. From electron spectroscopy studies it has been established that the characteristic half-width of the emission from individual areas is about ~ 260 meV and that the emission mechanism is based on a semiconductor regime. The Fowler-Nordheim (F-N) plots of the total emission from the cathode and that from small emitter areas are both linear over a wide field range.

IGFET hot electron emission model

A new model for hot electron emission into the oxide layer of IGFETs is presented. When IGFETs operate under high gate and drain bias conditions, electrons are accelerated by the high drain field and injected into the oxide layer at a certain probability. The present model takes into account the following: (1) the energy dependence of hot electron scattering probability, (2) the three-dimensional angle at which electrons are injected into the oxide layer, (3) the quantum effect at the SiSiO 2 interface that gives the reflection probability, and (4) the scattering and reaching probabilities without energy loss. The total emission current is obtained by a five-fold integral over the energy, position and solid angle elements. The current thus predicted is expected to be two or three orders of magnitude smaller than that from Phillips' model previously proposed for IGFET emission current and hence closer to experimintal data. The present model can give the position dependence of emitted current and is suitable for incorporation into two-dimensional numerical analysis.

Thermionic emission from single-crystal LaB6 tips with [100], [110], [111], and [210] orientations

Thermionic emission patterns from the single-crystal LaB6 [100], [110], [111], and [210] tips have been observed under the Schottky condition. The observed emission patterns are interpreted on the basis of a model involving an anisotropic work function of LaB6; the regions around the [111] crystal orientation have a high-work function, while those around the [210] crystal orientation have a low work function. This model also explains qualitatively the order of the total emission currents from those tips. The observed patterns predominantly consist of the emission from the conical face rather than the top of the tips under this experimental condition. Changes in the emission pattern and cathode surface in operation at 10−4 Torr are also discussed.