Energy Distribution Of Field-emitted Electrons Research Articles

The energy distribution of field-emitted electrons from GaAs crystals

Energy spectroscopy of electrons emitted from GaAs crystals of n- and p-type conductivityhas been studied for different values of the applied electric field as well as fordifferent surface conditions at the emitting surface of the crystals. Analysis of theelectron energy spectra for the n-GaAs emitter shows that the electrons forming thefield-emission current for the near degenerate n-type semiconductor originate from thevalence band. A physical mechanism that could possibly lead to predominantemission from the valence band of the n-type semiconductor has been proposed. Theelectron energy distribution has also been measured for the p-GaAs field emitters.The results show that it depends essentially on the external field applied to thesurface. Electron field emission from a p-type semiconductor has been discussedwith consideration of tunnel current from the surface self-consistent quantumwell.

Influence of nanocrystalline structure on work function of tungsten

The field emission investigations of nanocrystalline (NC) tungsten have revealed the essential difference in energy distribution of field-emitted electrons from NC tungsten as compared to the coarse-grained one. The measurement by the electron beam retarding potential technique also showed the reduction in work function of NC tungsten. The revealed features are explained using two phase model for NC metal.

Energy distributions of field emission electrons from silicon emitters

The energy distributions of field-emitted electrons from single-tip n type and p type Si field emitters have been analyzed. The energy distributions of both n type and p type Si emitters show a single peak, and the peak position shifts slightly towards the low energy side in the n type emitter as the gate voltage is increased. In the case of the p type emitter a considerably larger shift towards lower energies is observed. The distributions from the n type emitter have the same threshold energies near the Fermi level energy and become broader with the increase in the gate voltage. On the other hand, the threshold energies of the p type emitter are more than 1.5eV below the Fermi level and shift to the lower energy side as well as the peak position in the energy distribution with increasing the gate voltage. In addition, the energy distributions of the p type emitter are considerably broad in comparison with those of the n type emitter, but the full width half maximums (FWHMs) of the distributions do not significantly change with the increase in the gate voltage. These results suggest that for the n type Si field emitter electrons are basically supplied from the Si conduction band and are emitted from the neighborhood of the conduction band under the effect of the band bending and electron trapping in surface and oxide states at the interfaces of the emitter. On the other hand, for the p type Si field emitter, electrons are mainly emitted through the interface states supplied from the valence band of Si.

Characteristics of field emission from nanocrystalline metals

Nanocrystalline (NC) samples of tungsten have been investigated by transmission electron microscopy, field ion microscopy and field electron energy spectroscopy. It has been shown that energy distributions of field-emitted electrons from NC tungsten differ significantly from energy distributions of coarse-grained metal. Theoretical analysis shows that the revealed differences are caused by work function changes and Fermi's level shifts due to formation of NC structure in the metal.

Total energy distribution of field-emitted electrons from Al(100) surface with single-atom terminated protrusion.

We have analyzed field electron emission from Al(100) surfaces with a single-atom terminated protrusion on the basis of the density functional theory combined with real space scattering theory. We have obtained multiple peaks in total energy distribution of emission current from such a pure-metallic protrusion. Furthermore, we have also found that multiple peaks become pronounced as the size of a protrusion becomes larger, which explains the discrepancy among experiments. All the above results can be understood from two factors, the existence of localized states at the topmost Al atom and the potential barrier reduction in front of the topmost atom.

Electron emission mechanism from cubic boron nitride-coated molybdenum emitters

The energy distribution of field-emitted electrons from Mo tips coated with intrinsic cubic boron nitride (c-BN) was studied in an effort to determine the origin of the emitted electrons. Voltage-dependent field-emission energy distribution (V-FEED) spectra were collected from the Mo emitters under ultra-high-vacuum conditions both before and after being coated. Emission current at a given voltage increased by as much as two orders of magnitude for the c-BN-coated emitters relative to bare emitters. The energy of field-emitted electrons from the c-BN-coated emitters was linearly dependent upon the applied voltage. Extrapolation of V-FEED data from c-BN-coated emitters to the flatband condition evidenced that the electrons were emitted from the conduction-band minimum of the c-BN coating at the c-BN/vacuum interface.

Energy distribution of field-emitted electrons from p-type germanium

Experimental electron energy distributions on the Ge(111) surface are presented for different values of the applied field. The shift of the energy distribution peak position agrees with Modinos' theoretical findings. Numerical studies reveal fine structure in the energy distribution, which is assumed to be due to the existence of surface states in the valence band of germanium.

Electron concentration in YBa 2Cu 3O 6.9 single crystals determined from field electron emission spectroscopic measurements

Results of field electron emission spectroscopic investigations of YBa 2Cu 3O 6.9 single crystal specimens oriented perpendicular to the c-axis are presented. It is shown that the shift of the total energy distribution of field-emitted electrons from YBa 2Cu 3O 6.9 along the energy scale with increasing electric-field strength at the surface of the specimen is due to the penetration of the electric field into the subsurface region of the YBa 2Cu 3O 6.9 specimen. The electron concentration in YBa 2Cu 3O 6.9 found from spectroscopic measurements is n=(2−4) × 10 21 cm −3. Field electron emission spectroscopy and microscopy of YBa 2Cu 3O 6.9 allows one to draw the conclusion that the electron concentration in the bulk of the material does not change with cooling of specimens from 300 K to 115 K.

Local heating of single-atom protrusion tips during field electron emission

The total energy distributions of field-emitted electrons from a single-atom on top of nano-scale protrusions show the existence of a band-structure at the tip apex. The energy exchange at the tip apex during electron emission must take into account this band-structure and this can lead to an important heating of the nano-scale cathode even at very low emission currents. We report here experimental measurements at the atomic scale of temperature increases during field emission in the 10 −13–10 −9A range. These increases can reach several hundred degrees.

Energy-broadening of field-emitted electrons due to Coulomb collisions

Abstract We present a theory of energy-broadening of field emitted beams of electrons or ions due to Coulomb interaction between the particles of the beam. The theory is applied to the analysis of experimental data on total energy distributions of field-emitted electrons at high current density. It explains the observed variation of the full width at half maximum of the distribution with emitter radius and with the angular intensity of the emitted current and its independence of temperature.

Energy-broadening of field-emitted electrons due to Coulomb collisions

We present a theory of energy-broadening of field emitted beams of electrons or ions due to Coulomb interaction between the particles of the beam. The theory is applied to the analysis of experimental data on total energy distributions of field-emitted electrons at high current density. It explains the observed variation of the full width at half maximum of the distribution with emitter radius and with the angular intensity of the emitted current and its independence of temperature.

Total energy distribution of field-emitted electrons from polyacrylonitrile-based high-modulus carbon fibres

Abstract The total energy distribution of field-emitted electrons from polyacrylonitrile carbon fibres and reactor graphite was investigated. Two peaks appeared in the total energy-distribution spectrum of the carbon fibre pyrolysed at 2500°C. In contrast, only one energy peak resulted from the carbon fibre re-pyrolysed at 3000°C and from the reactor graphite. The peak with a high energy is attributed to π electrons of graphite, while the other is attributed to σ electrons. This finding corresponds well with the results obtained by soft X-ray spectroscopy of carbon fibres. Re-pyrolysation of carbon fibres at 3000°C provides high graphitization which results in a disappearance of a electrons. Field-ion microscopical observations of highly graphitized carbon fibres showed the agglomeration of fibrils even at the central area of the fibre where electrons were field-emitted.

Total energy distributions of field-emitted electrons at high current density

Measurements of the total energy distributions (TED) have been carried out at high current densities ($J={10}^{6} \mathrm{to} {10}^{8}$ A/${\mathrm{cm}}^{2}$) for a high-work-function ($\ensuremath{\varphi}=4.5$ eV) and low-work-function ($\ensuremath{\varphi}=2.5$ eV) field emitter. At high values of $J$, both emitters give values of the full width at half maximum (FWHM) of the TED which exceed the values based on Fowler-Nordheim theory by a few eV. This deviation of the FWHM values increases with $J$, is independent of temperature, and decreases with emitter radius for a specified beam acceptance angle. The study of these anomalous values of FWHM values extends from 84 to 1975 K. It is believed that Coulomb interaction in the vacuum space in front of the emitter can account for these results, although internal electron energy-broadening mechanisms cannot be ruled out.

Total energy distributions of field-emitted electrons at high current density

Measurements of the total energy distributions (TED) have been carried out at high current densities ($J={10}^{6} \mathrm{to} {10}^{8}$ A/${\mathrm{cm}}^{2}$) for a high-work-function ($\ensuremath{\varphi}=4.5$ eV) and low-work-function ($\ensuremath{\varphi}=2.5$ eV) field emitter. At high values of $J$, both emitters give values of the full width at half maximum (FWHM) of the TED which exceed the values based on Fowler-Nordheim theory by a few eV. This deviation of the FWHM values increases with $J$, is independent of temperature, and decreases with emitter radius for a specified beam acceptance angle. The study of these anomalous values of FWHM values extends from 84 to 1975 K. It is believed that Coulomb interaction in the vacuum space in front of the emitter can account for these results, although internal electron energy-broadening mechanisms cannot be ruled out.

Effects of electronic energy-band structure on the energy distribution of field-emitted electrons

Effects of electronic energy-band structure on the energy distribution of field-emitted electrons

A Green function technique for calculating total energy distributions of field-emitted electrons

A formalism is developed which effectively reduces the problem of evaluating the escape probability due to field-assisted tunnelling, of an electron with a given energy and a given wavevector parallel to the surface, to that of evaluating the reflection matrix for an electron with the same energy and wavevector incident on the metal surface from outside. The possibility then arises of using in field-emission calculations some of the approximate methods developed in relation to the theory of low-energy electron diffraction.

Surface density of states and field emission

The relation between the total energy distribution of electrons field emitted from metals and the surface density of states is analyzed in general terms. Numerical reslts on the total energy distribution of field-emitted electrons from the (100) and (110) planes of tungsten obtained previously by the authors and extended here to a wider energy region are interpreted in terms of surface density-of-states effects. It is shown in particular that the peak in the total energy distribution from the (100) plane at 0.29 eV below the Fermi level is due to virtual surface states which give rise to enhanced tunneling at that energy. For the (100) plane of tungsten virtual surface states are shown to exist in an energy region extending from 0.20 to 0.95 eV below the Fermi level, but only the states at 0.29 eV below ${E}_{F}$ give rise to peaks in the total energy distribution of the emitted electrons. Another broad peak in the total energy distribution from the (100) plane observed at about 1.3 eV below the Fermi level is also reproduced by our calculation. This peak is the cumulative effect of a number of factors which cannot be disentangled in a simple way, but is not a result of surface states.

Band-structure effects in field-emission energy distributions in tungsten

A calculation of the total energy distribution of field-emitted electrons from the (100) and (110) planes of tungsten is presented. The potential inside the metal is represented by a superposition of atomic potentials in the muffin-tin approximation as evaluated by Mattheiss. The crystal is terminated abruptly at the surface and the potential barrier outside the surface is represented by a straight line. Using the mathematical apparatus developed by low-energy-electron-diffraction theorists, the band structure of tungsten is reproduced and the tunneling probability calculated by matching the wave function outside the metal to a superposition of Bloch waves, including a number of evanescent waves, inside the metal. The total energy distribution from the (100) plane exhibits a sharp peak at 0.29 eV below the Fermi level which suggests that the experimentally observed hump at about the same energy is due to band- structure effects. It is shown that this peak in the total-energy-distribution curve is due to electron states associated with certain sections of the constant- energy surface and it is argued that spin-orbit interaction is not likely to remove this peak. The calculated distribution from the (110) plane does not show any marked deviation from the free-electron behavior in agreement withmore » the experimental observations. For the (100) plane the ratio of the exact total energy distribution to its corresponding value evaluated in the free-electron approximation for the (100) plane is considerably smaller than unity away from the peak in the total-energy-distribution curve. For the (110) plane it is of the order of unity.« less

Band-structure effects in field-emission energy distributions in tungsten

A calculation of the total energy distribution of field-emitted electrons from the (100) and (110) planes of tungsten is presented. The potential inside the metal is represented by a superposition of atomic potentials in the muffin-tin approximation as evaluated by Mattheiss. The crystal is terminated abruptly at the surface and the potential barrier outside the surface is represented by a straight line. Using the mathematical apparatus developed by low-energy-electron-diffraction theorists we reproduce the band structure of tungsten and calculate the tunneling probability by matching the wave function outside the metal to a superposition of Bloch waves, including a number of evanescent waves, inside the metal. The total energy distribution from the (100) plane exhibits a sharp peak at 0.29 eV below the Fermi level which suggests that the experimentally observed hump at about the same energy is due to band-structure effects. We show that this peak in the total-energy-distribution curve is due to electron states associated with certain sections of the constant-energy surface and we argue that spin-orbit interaction is not likely to remove this peak. The calculated distribution from the (110) plane does not show any marked deviation from the free-electron behavior in agreement with the experimental observations. For the (100) plane the ratio of the exact total energy distribution to its corresponding value evaluated in the free-electron approximation for the (100) plane is considerably smaller than unity away from the peak in the total-energy-distribution curve. For the (110) plane it is of the order of unity.

Energy distributions of field-emitted electrons from silicon: Evidence for surface states

Energy distributions of field-emitted electrons were obtained from field evaporated and annealed silicon tips. Electrons are emitted from levels below the Fermi level but above the valence band. The shape of the distributions depends on the crystallographic orientation of the emitting surface. The results are intepreted in terms of emission from a band of surface states which overlaps with the band gap.