Showing papers on "Field electron emission published in 1981"

Electric field enhanced emission from non‐Coulombic traps in semiconductors

Abstract: Electric field enhancement of emission from three non‐Coulombic traps has been calculated: the shielded Coulombic potential, the polarization potential, and the dipole potential. Both the Poole‐Frenkel effect and phonon‐assisted tunneling have been included, and both were found to be important. The field effect can be used to distinguish between these potentials on the basis of their long range character. This effect is most important in interpreting the results of capacitance transient studies of deep levels.

Oxygen‐related donor states in silicon

Abstract: Junction spectroscopy techniques have been applied to the study of defect states introduced by heat treatment of silicon containing ∼1018[O]/cm3. Two shallow electron traps are observed, whose concentrations increase with the electron concentration added during a 450 °C anneal. The zero field defect‐state activation energies are E(0.07 eV) and E(0.15 eV). Both states display a Poole‐Frenkel field emission process as expected for donors.

Determining the field emitter temperature during laser irradiation in the pulsed laser atom probe

Abstract: Three methods are discussed for determining the field emitter temperature during laser irradiation in the recently developed Pulsed Laser Atom Probe. A procedure based on the reduction of the lattice evaporation field with increasing emitter temperature is found to be the most convenient and reliable method between 60 and 500 K. Calibration curves (plots of the evaporation field versus temperature) are presented for dc and pulsed field evaporation of W, Mo, and Rh. These results show directly the important influence of the evaporation rate on the temperature dependence of the evaporation field. The possibility of a temperature calibration based on the ionic charge state distribution of field evaporated lattice atoms is also discussed. The shift in the charge state distributions which occurs when the emitter temperature is increased and the applied field strength is decreased at a constant rate of evaporation is shown to be due to the changing field and not the changing temperature. Nevertheless, the emitter temperature can be deduced from the charge state distribution for a specified evaporation rate. Charge state distributions as a function of field strength and temperature are presented for the same three materials. Finally, a preliminary experiment is reported which shows that the emitter temperature can be determined from field ion microscope observations of single atom surface diffusion over low index crystal planes. This last calibration procedure is shown to be very useful at higher temperatures (≳600 K) where the other two methods become unreliable.

Experimental evidence for multiple post-ionization of field-evaporated ions

Abstract: The charge-state distribution of field-evaporated tungsten ions has been measured as a function of electric field using the pulsed-laser atom probe. The results confirm the recent post-ionization model of Haydock and Kingham and give the first experimental evidence for multiple post-ionization. At low field strengths ( 3.0 V/\AA{}) and high surface temperatures (g 1000 K), field-evaporated triply charged dimers are detected, and built-up regions which significantly enhance the local electric field at the desorption site are observed.

Electroluminescence effects associated with the field emission of electrons from a carbon fibre micropoint emitter

Abstract: It has been observed that the field emission of electrons from a micropoint carbon fibre cathode is accompanied by the emission of optical photons. Image intensifier techniques have been used both to record the visible spectrum of this radiation and to show that the dependence of its intensity on the applied voltage is consistent with it being electroluminescent in origin. The phenomenon has been qualitatively interpreted in terms of an emission mechanism involving a composite microregime consisting of a metallic-like substrate overlayed by a thin insulating medium.

Optical emission: A probe of neutral atoms in liquid‐metal ion sources

Abstract: The optical emission close to the Taylor cone tips of liquid gallium and gold ion sources has been studied as a function of wavelength and ion current. The optical emission may be interpreted as due to excitations caused by binary collisions, among which Ga‐Ga+ collisions play a dominant role. The release rate of neutrals from the source inferred from the optical‐emission curves is consistent with the idea of contributions to the total ion current from field evaporation and field ionization, with the latter becoming increasingly important at higher currents. It is difficult to account for the origin of the neutrals by thermal evaporation at the tip.

Coadsorption of cesium and oxygen on iridium and lanthanum hexaboride

Abstract: The coadsorption of cesium and oxygen on iridium and lanthanum hexaboride has been studied by measurements of elemental surface concentrations, work functions, and electron reflectivities The elemental surface concentrations were determined by Auger electron spectroscopy and the work functions and reflectivities by the field emission retarding potential technique Simultaneous deposition of cesium and oxygen on iridium or lanthanum hexaboride substrates produced a thick oxide layer For both substrates this oxide layer possessed a work function of 10±005 eV and an electron reflectivity at threshold of 045±010 eV Brief stepwise heatings of this layer on LaB6 showed stability to 390 K The threshold reflectivity for some higher work function surfaces depended strongly on whether the final deposition was cesium or oxygen

Large-area field emission diode for semiconductor annealing

Abstract: We report the characteristics of a low energy electron beam generator used for annealing of materials of interest as electron devices and photovoltaic cells. The high voltage pulser is a two-stage Marx circuit which supplies 10–50 kV pulses. A Blumlein line gives the pulses an almost square shape and a length of 50 ns. This short pulse is applied to a field emission diode. Various kinds of cathodes are used. They deliver electron pulses of up to 20 kA. Tungsten needles are used for small-area beams and blade arrays or graphite cathodes for large-area beams. Consequently, the beam diameter can vary from 2 to 120 mm with a good beam homogeneity. The anode is movable to vary the vacuum diode impedance and consequently the current-voltage characteristic of the electron beam. Energy densities varying from 0.1 to 100 J/cm 2 are obtained on the anode. The anode can be formed by a semiconductor monocrystalline wafer. We used crystalline silicon wafers covered by a thin (≈100 nm ) layer of vacuum deposited near-noble and refractory metals. Under electron beam bombardment with discharge energy densities ranging from 0.4 to 10 J/cm 2 , silicide formation is evident over all the annealed anodes. Metastable compounds are formed because of the fast heating and cooling rates. Silicon monocrystalline wafers, doped with 10 14 –10 15 ions/cm 2 of B or P ions exhibit a complete recovery of their crystalline structure after a single discharge.

Atom probe analysis of rf-sputtered a-Si:H films

Abstract: Using a computerized energy compensated time of flight atom probe (APFIM), we have obtained the average composition (Si,H,O) of two widely differing hydrogenated amorphous silicon rf‐sputtered films which were deposited onto W field emitter tips. Analysis of the evaporated ion species shows that the films prepared at the lower total gas pressure (5 mTorr) contain predominantly SiH and Si units while at the higher pressure (20 mTorr) the additional units of SiH2 and SiH3 are also present. These results are compared to films prepared and characterized on planar substrates and found to be in agreement.

Electron beam device

Abstract: PURPOSE:To control the scanning speed on the material and maintain the amount of the electron beam projected on the material at a fixed value, by detecting the quantity of the electron beam discharged from an electron gun, and controlling the deflection signal when the integrated amount of the detected electron beam has reached a fixed value CONSTITUTION:A field emission type electron gun is placed in the main part of vacuum box 1 Electron beam 4 moving out of emitter 2 of the electron gun is accelerated by anode 5 and is focused on material 8 A part of electron beam 4 is detected by current detector 6, which combines the function of a stop The detector signal is amplified (10) and integrated (11), and it increases as shown by a in the drawing When it reaches reference value B, the signal of comparator circuit 12 reaches deflection power source 13; and the signal of deflection power source 13 is moved by one step at a time, and the illumination point is moved to the next illumination point after a fixed amount of electron beam illumination has been done Then, the signal of comparator 12 returns the integration value to 0, and integration is started at the new illumination point By this it is possible to maintain the amount of the electron beam at a fixed value

Field emission of electrons from superconducting and normal electrodes

Abstract: Electron emission under the influence of a high electric field has been measured in ultra high vacuum for four superconducting electrodes. The temperature of the electrodes was controlled so that the emission was measured a few degrees above and below the superconducting transition temperatures. The data was analysed on the basis of the Fowler-Nordheim field emission theory, and the results indicated that a transition from the superconducting to the normal state did not affect the rate of emission. Any discontinuities in the Fowler-Nordheim graph were due to oxide layers, which could be removed by sparking, irrespective of the temperature. The work functions of the four metals were also measured for the same range of temperature, the metals being in the form of a thin film. Results showed that the value did not alter at the superconducting transition temperature, but that it was slightly less than that for the metal in bulk form.

Field‐emission liquid‐metal ion source and triode ion gun

Abstract: A pointed‐filament‐type field‐emission liquid‐metal ion source is designed and employed as a gold ion source. By adding a crossbar across a hairpin bend, the amount of the gold adhering on the filament is increased. The lifetime is estimated to be over 200 h at 10‐μA emission current. The emission current increases with increasing extraction voltage up to a saturation value which is ascribed to a limitation of the supply of liquid gold to the needle apex. The value of current density per unit solid angle is 30 μA/sr at a total current of 30 μA, which is of the same order as that obtained from a gallium ion source previously reported. Emission current fluctuations of a few tens of percent of the dc component are observed. In order to regulate the emission current and suppress current fluctuations, a bias electrode in addition to a counterelectrode is placed close to the needle apex. With such a triode structure, the emission current is regulated by a bias voltage of several hundred volts and stabilized to ...

The Zr-O-W(100) emitter and coadsorption of zirconium and carbon monoxide on W(100)

Abstract: The properties of the Zr‐O‐W(100) emitter and the coadsorption of zirconium and carbon monoxide on W(100) have been studied by Auger electron spectroscopy and work function measurements. The thermionic work function of the Zr‐O‐W(100) emitter from 1450 to 1800 K is 2.54 eV with a pre‐exponential value of 4. The field‐emission retarding potential work function is 2.75–2.80 eV. A Zr‐O‐W(100) emitter free of bulk carbon possesses a longer lifetime than an emitter containing bulk carbon. Heating the emitter in carbon monoxide causes diffusion of zirconium into the bulk, and vacuum annealing restores the zirconium to the surface. The coadsorption of zirconium and carbon monoxide on W(100) followed by vacuum annealing produces a minimum work function of 2.3 eV. The adsorption sequence is important and lower work functions correlate with higher carbon concentrations. The stability of the 2.3 eV surface under background gas adsorption and high‐temperature heating is discussed.

Electrical breakdown properties of stainless steel and titanium electrodes in ultra-high vacuum

Abstract: The breakdown voltage in ultra-high vacuum of stainless steel and titanium electrodes is measured as a function of the electrode separation in the range 0.05-0.80 mm. This relationship can be described by the simple equation V b = Cd a . Values for c and a are given for all electrode combinations. For titanium electrodes the current just before breakdown was about three times that for stainless steel electrodes. Comparative measurements with all combinations of stainless steel and titanium show that the cause of this difference must be sought in the anode. The microscopic breakdown field strength was a function of the emitting cathode area, as derived from Fowler-Nordheim plots. This dependence suggests that the breakdown was initiated by thermal instabilities at the anode due to field emission electrons. This is consistent with a theoretical analysis. Measurements of the pre-breakdown current allow an estimate of the number and dimensions of emitting sites.

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

Abstract: 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.

Field emission cold cathode devices based on eutectic systems

Abstract: : A survey has been made of the performance as field emission cold cathodes of selected refractory materials fabricated as needle arrays by unidirectional solidification. Cathodes based upon refractory metal carbides have been fabricated and tested. Performance data has been obtained using niobium, tantalum and vanadium carbides with various cathode geometries. Life times of over 1,000 hours and current densities of up to 1 A/sq. cm. have been obtained. Preliminary experiments indicated that LaB6 can be formed into platelets by conventional cooling in an aluminum matrix and that directional solidification should produce suitable rod-like morphology.

Non-equilibrium between electrons and field in a gas breakdown ionising wave. II. Structure of the leading edge

Abstract: For pt.I see ibid., vol.14, p.649 (1981). The authors present here the results of the numerical solution to the macroscopic model of a gas discharge described in part I (preceding paper). This model considers the non-equilibrium between the electron energy and field in the electron shock leading the ionising wave in the discharge. It is possible to analyse the structure of this shock wave. It is found to be mainly composed of two unequal parts separated by the position of the electron pressure maximum. It is also possible through this model to calculate the principal physical quantities (density of charge carriers, field and electron temperature) in a non-equilibrium electron field discharge. The authors have elucidated the role of the different physical processes, or the roles of the terms of the model equation, in determining the difference between the static equilibrium temperature and the electron temperature. It is also shown that a steady-state shock solution to the nonlinear hydrodynamic model of the gas discharge can exist in the sense that an initially strong relative electron density gradient is maintained in the first zone of the electron shock, whereas the shock upstream can possibly produce a precursor effect due to the combined effects of thermal diffusion and overmultiplication of these hot electrons.

A numerical and experimental study of the electric field distribution within field emission systems

Abstract: The design of field electron and field ion sources which are useful in device fabrication systems, electron or ion microscopes requires an accurate and complete knowledge of the electric field distribution both within the gun and near the field emitter itself. Analysis has not been possible by conventional analytical methods due to the intractable nature of the equations. An analysis is presented in the present work which is obtained using the method of finite elements. The numerical approach allows the consideration of the geometric detail common to field ion and field electron microscopes. In these microscopes the ion or electron source serves as the actual specimen to be observed. It is shown that instrument features as far away as 105 tip diameters can substantially affect the electric field at the emitter. The dependence of the electric field at the apex is found to vary as k1 exp(−k2ϑ) where ϑ is the emitter shank angle. The constant k2 describes the strength of the shank angle dependence. These con...

Field emission from tungsten (110)

Abstract: The total energy distribution of the field-emitted electrons from tungsten (110) is calculated for two different bulk potentials and three different surface barriers. The authors find that structure due to surface resonance may exist in the energy distribution at energies immediately above the Fermi level depending on the choice of potentials. The results are consistent with the existing experimental data for energies below the Fermi level.

Application of a thermal field emission source for scanning Auger microscopy

Abstract: Application of a Thermal Field Emission Source for Scanning Auger Microscopy David W. Tuggle, M.S. Oregon Graduate Center, 1981 Supervising Professor: Lynwood W. Swanson A thermal field emission (TFE) electron source has been incorporated into a microprobe with two magnetic lenses to produce a 0.10 pm beam spot with 0.11 PA current at 12 kV and a working distance of 13 cm, in agreement with calculated performance assuming a source angular intensity of 1 mA sr-l. Experience with cold field emitters has discouraged their use in many applications because of noise problems, instability, short life, and extreme vacuum requirements. The TFE mode of operation allows a relaxation of vacuum requirements by rapid thermal annealing of sputter-induced surface deformation, thereby minimizing the probability of emitter destruction by a regenerative vacuum arc. Additionally, the high emitter temperature maintains a low and constant coverage of adsorbed gases on the emitter surface thereby eliminating time dependent work function change. The emitter used was oriented W coated with Zr, operated at 1800 K. The microprobe was used to construct a scanning Auger microscope which produced submicron reso lu t ion Auger elemental maps wi th scan times of 5 min o r less. The high vol tage Zr/W<100> TF emi t t e r not only e x h i b i t s long l i f e (5000 h on severa l t e s t s ) , but s u f f i c i e n t l y low noise Ce.g., 0.23%) t o be used i n SAM app l i ca t ions without t h e need f o r sophis t ica ted beam current s t a b i l i z a t i o n schemes. Specimen current d e n s i t i e s which f a r exceed SAM requirements can be obtained a t 0.1 pm spot s i z e with r e l a t i v e l y simple gun op t i cs . A t present a current densi ty of 1300 ~ / c m ~ o r power densi ty of 1.6 x lo7 ~ / c m ~ has been achieved i n a 0.1 pm beam spot . This corresponds t o an image plane br ightness of 5.5 x 10' A c G 2 s r l a t 12 kV. The geometric s t a b i l i t y of t h e emit ter s t r u c t u r e i s excel lent once i t achieves equil ibrium temperatu re . Measurements show the emit ter d r i f t over 16 h periods t o be < 0.05 vm/h f o r a TFE emi t t e r operat ing a t 1800 K.

Field emission type electron gun

Abstract: PURPOSE:To obtain a stable electron emission even at a lower temperature below 1,000 degK by using specific materials as cathode material CONSTITUTION:An electron gun is constituted of a needle-shaped cathode 1 for field emission, a filament 2 for holding the needle cathode 1 and temporarily heating it, the first anode 3 for pulling out a field emission electron from the cathode 1, and the second anode 5 for accelerating and focusing the electrons which passed through the center hole 4 of the anode 3 As a material for the cathode 1, cerium sulfide, lanthanum sulfide, or gadolinium sulfide is used

Interaction of Nitrogen, Hydrogen and Ammonia with Iridium Surfaces

Abstract: Hydrogen, nitrogen and ammonia adsorption on iridium surfaces has been studied by thermal desorption and field emission techniques. The steady state rate of ammonia decomposition has been measured. The results of the present study are compared with those obtained on iron.