Metals, electron emission current

Table 4. Electron Emission Current from Metals at Elevated Temperatures...

To further substantiate the proposed model, they have carried out some investigations connected with modification of semiconductor electron subsystem [174, 175]. Temperature is one of the important factors. Having no effect on the electron emission from the metal under the action of RGMAs, temperature strongly affects the current-transfer processes at the metal - semiconductor contacts. The impact of temperature on the interaction of RGMAs with Au/ZnO structures can be evaluated as follows. 

In the first model, the mnneling electron mainly interacts with the electronic polarization of water ( = 1.88) since tunneling was assumed to be fast in comparison with the orientational response of the dipolar molecules of the liquid. Considering water as a dielectric continuum between a jellium spherical tip and planar substrate yields an effective barrier for tunneling that is about 1 eV lower than that for the vacuum case [95]. This result is consistent with photoemission studies of metal/aqueous interfaces, which reveal electron emission into water at 1 eV below the vacuum level [95-97]. Similar models have been employed to examine the effect of thermal fluctuations on the tunneling current [98-100]. Likewise, a related model assessing the noise associated with the reorientation of adsorbed molecules has been presented [101]. 

Fig. 4.8. Field-emission spectrum of Mo(lOO). The quantity displayed, Jf, is the ratio between the observed field-emission current and the prediction based on a free-electron model, Eq. (4.20). As shown, the field-emission spectrum of Mo(lOO) near the Fermi level is substantially different from a free-electron-metal behavior. (After Weng, 1977.)...

Fig. 14,4. Tip treatment for tunneling spectroscopy. (A) By applying a relatively large positive bias on the sample, a sharp tip generates a field-emission current. (B) When the field-emission current is very high, the tip end melts. (C) The tip end recrystallizes to form facets with low surface energy. In the case of tungsten, the W(llO) facets are preferred. Its surface DOS resembles a free electron metal. (After Feenstra et al., 1987a.)...

The frequency dependence of SHG at simple metal surface has been the focus of a recent theoretical study of Liebsch [100]. Time-dependent density functional theory was used in these calculations. The results suggest that the perpendicular surface contribution to the second harmonic current is found to be significantly larger than had been assumed previously. He also concludes that for 2 a> close to the threshold for electron emission, the self-consistently screened nonlinear electronic response becomes resonantly enhanced, analogous to local field enhancement in the linear response near the bulk plasma frequency. 

Thermionic emission. The number of electrons which escape from the metal surface increases rapidly with temperature (thermionic emission). In general, the higher the temperature and the lower the work function, the higher is the electron emissivity. The current density can be calculated by the Richardson-Dushman equation (in the absence of an external electrical field), according to i — AT exp(—rp/kT), where A is the Richardson constant (A cm K ), T is the temperature (K), and

work function (eV). For pure tungsten A — 60.2 (A cm K ) [1.91]. The thermionic current (A cm ) can then be calculated as i — 60.2r exp(—52230/T) [1.37]. 

Fig. 30. Electron emission from metals with differently shaped surface barriers. j—current density in amp/em1, F—field in volts/A. Calculations for cf> = tf>2 = 4.5 ev, tj> j = 5.5 ev width of conduction band = 0.4 ev d = 2.5A. Slopes of F-N curves identical to better than 1%.

The compounds formed often have higher secondary electron emission than metals, so that more of the energy transferred by the ions is used to produce and to accelerate secondary electrons. The increased secondary electron emission, in the case of constant-current power supplies, automatically decreases the cathode voltage for a fixed power setting. It is therefore -better to maintain a constant voltage. In this case the abrupt rate decrease becomes more smoothed out. 

Arcs with hot cathode spots. If the cathode is made from lower-melting-point metals like copper, iron, silver, or mercury, the high temperatme required for emission caimot be sustained permanently. Electric current flows in this case through hot spots that appear, move fast, and disappear on the cathode surface. Current density in the spots is extremely high (10" -10 A/cm ), which leads to intensive but local and short heating and evaporation of the cathode material while the rest of the cathode actually stays cold. The mechanism of electron emission from the hot spots is thermionic field emission. Cathode spots appear not only on the low-melting-point cathodes but also on refractory metals at low currents and low pressures. 

The cathodes spots are the localized current centers, which appear on the cathode when significant current should be provided but the cathode carmot be heated enough as a whole. The most typical cause of cathode spots is the application of metals with relatively low melting points. The cathode spots can also be caused by low arc currents, which are only able to provide the necessary electron emission when concentrated to a small area. The cathode spots also appear at low gas pressures (<1 Torr), when metal vapor from the cathode provides atoms to generate positive ions bringing their energy to the cathode to sustain the electron emission. To provide the required evaporation, cmrent is concentrated in spots at pressures <1 Torr and currents 1-10 A, such spots occur even on refractory metals. 

An electron multiplier can amplify an ion current up to a factor of 10 . Each ion which impinges on the metal surface of the first dynode of the multiplier causes the emission of a number of secondary electrons. These are accelerated to another electrode inducing consecutive, additional electron emission thus producing a cascade of electrons. 

Thermal electron emission from a solid (dark current in a photocathode) can be modeled in various degrees of complexity and full models for either classical or NEA emitters [5.139,140] require a fair amount of detail. In simplified form we can consider initially the limit of treating the substance as a perfect electron source (an electron black body ). Here all electrons which are thermally excited to energy greater than the surface barrier (work function or electron affinity) are emitted, and each emitted electron is assumed to be instantly replaced by an electron from the bulk. This is roughly the Richardson model, which applies fairly well to a metal. The dark current computed from this model is an upper limit to emission of a real device. 

Field emission potential diagram. Large electric fields induce barrier narrowing, which increases the number of electrons tunnelling from the Fermi level in the metallic, electron-rich, surface into the vacuum. Variations in the density of occupied states, N E), and current density, J[E), as a function of electron energy. Surface contamination and charging effects (Schottky rounding) can be seen to drastically alter the potential profile. 

The eiectronic properties of carbon nanotubes are therefore a function of the diameter, helicity and kinks which can be observed in their cyiindrical structure, and their electronic properties vary in a periodic way between those of a metal and those of a semiconductor. Accordingiy, of all the possible tubes one could make, one third would be either metallic or narrow band semiconductors, and two thirds would be moderate gap semiconductors [17]. Metailic carbon nanotubes have recently been achieved in very high yields [12], and field emission currents of 0.1 microampere at 80 volts have been achieved for individualiy mounted nanotubes [54],... 

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