A new electron source which consists of microsize field emitter arrays attract strong attention for realizing new applications such as flat panel display, super-high frequency vacuum devices etc. Although the conventional field emitter that is usually a single crystal tungsten needle is operated under an ultra-high vacuum condition, new microsize field emitter arrays are required to be prepared by deposition methods and to be operated under a normal vacuum condition. Therefore, the researches on the new materials and structures for the development of microsize field emitter arrays have been needed. The selection of a low work function material as an emitter material is a prospective method to realize a stable operation of microsize field emitter array.
Atomistic mechanism of field emission is discussed in relation to the concept of local work function, which plays an important role for understanding materials properties of nano-structures on surfaces. First, the concept of local work function is clarified on the basis of the nano-scale potential distribution over the fringe of surface. Next, present status is described on the first-principles theory of the work function and field emission properties on metal surfaces. Further, various fundamental issues of field emission still to be resolved are discussed. The topics include various many body effects on the field emission, such as emission from superconductors or strongly correlated materials as well as from nano-particles weakly coupled with the surface. Finally recent developments of approaches probing local work function by SPM are reviewed and further problems are introduced.
Emission characteristics of individual micro emitters on a FEA have been measured by use of a laboratory made emission microscope. It was found that the total emission current is apparently stable, but the individual emitter current is unstable. The fluctuating modes can be classified into two categories: apparent dependence on the IP product, where I is total emission current and P is the residual gas pressure in the vacuum, and no such dependency. The former type of fluctuation is successfully interpreted by our model that ionic hitting from the gate electrode causes the fluctuation.
Field emission from individual tips in n- and p-type silicon field emitter arrays (FEAs) has been directly evaluated by using an electrostatic lens projector. The number of the emission spots in the p-type FEA increased with increasing total emission current, and the p-type FEA exhibited better uniformity of the emission than the n-type FEA. In order to improve the emission uniformity, the operation of the FEAs under an atmosphere of ambient gas was executed. The emission uniformity was improved dramatically by exposing C2H4 gas. Also, surface modification of Si field emitters was investigated for damageless vacuum sealing. The vacuum sealing process reduced the emission currents from the nontreated Si FEA by a factor of 10. On the contrary, the emission current from the CHF3-plasma-treated FEA did not change before and after the vacuum sealing process.
Recent developments of highly efficient electron emissions from chemical-vapor-deposited (CVD) single-crystalline diamond under high fields of ≈ 106 to ≈ 107 V/cm have been described in relation to those of surface structure analysis using a scanning tunneling microscope, photoemission threshold-energy measurements and Monte Carlo (MC) simulations of carrier transports under high electric fields. Photoemission data indicate that the electron affinity of as-grown CVD diamond (100) changes from negative to positive values with increasing oxygen coverages, being well reproduced by the Topping model. From MC calculations, it has been verified that valence electrons of diamond can be excited to the conduction band through impact ionization events under the high fields concerned. Very high (current) efficiencies of ≈ 100% have been observed for a diode-type electron emitter having a buried electrode layer, an insulating diamond layer and a p-type diamond emission surface with a negative electron affinity. A model has been proposed for the high emission efficiency attained, based upon the impact carrier excitation process.
Carbon nanotubes (CNTs) possess unique geometrical, electrical, mechanical and chemical properties that make them excellent field emitters. We have investigated the field emission of electrons from CNTs by using field emission microscopy (FEM). In this article, our recent observations of adsorption and desorption of residual gas molecules on clean CNT surfaces and their effects on the emission current are first reported, and then the electron energy distributions are described. Adsorbed gas molecules were observed as bright spots in FEM patterns, giving rise to an abrupt increase in the emission current. The energy spectra clearly exhibited a shoulder at about 500 meV below the main peak, and the full width at half-maximum (FWHM) was about 330 meV. Finally, the methods for preparing CNT cathodes for field emission displays (FEDs) are described and recent development of FEDs with triode-type structures are reviewed.
We have found that heavy Si doping is efficient for large field emission (FE) from AlN. For Si-doped AlN and AlxGa1−xN (0.38≤x<1), from a linear relation between the applied bias and the anode-sample distance, we clarified the band-gap (the Al content) and Si-dopant-density dependences of the electric field necessary for FE, EFE. As the band gap (the Al content) increases, EFE decreases due to the lower electron affinity. As the Si-dopant density increases, EFE decreases due to spontaneous formation of ridges with nanometer-order sharpness. Heavily Si-doped AlN showed a large FE current density of 0.22 A/cm2 and high stability in FE current over time (fluctuation: 3%).
Electrically conductive diamond-like amorphous carbon (DAC) films with nitrogen (DAC:N) were deposited, and studied with respect to their potential application to field emitters. In the deposition of electrically conductive DAC:N films, a supermagnetron plasma CVD apparatus with multi-function was used. By using the supermagnetron plasma CVD apparatus, hard and thick DAC:N films of very low electrical-resistivity (0.034 Ωcm) were deposited. Cone-shaped Si field emitters were fabricated, and the surfaces were coated with 50 nm-thick DAC:N films. Characteristics of the emitters revealed that large reductions of turn-on voltage were achieved by the coating.
Electromagnetic grand anomalies prior to large earthquake occurrence have been paid attention especially after the 1995 Hyogo-ken Nanbu earthquake, though there is much of the debate whether the grand electromagnetic anomalies rest on sound scientific bases or not. Recent laboratory experiments conduced by Ikeya and his group using a Van de Graaff electrostatic generator suggested that such anomalies should be attributed to electrification of the ground level. In this “popular science” note, similar pre-seismic magnetic anomaly that happened at Edo age about 150 years ago is highlighted. Ansei-kenmon-shi published in 1856 noted that at the time about 2 h before destructive Ansei-Edo earth-quake in 1855, a natural magnetic stone at the Ohsumi's spectacle shop in Asakusa, Edo (Tokyo) dropped some iron nails, which had been attached to it. This observation led to the invention of a magnetic seismo-scope for prediction of earthquake occurrence. It is of interest to note that a scientist of ‘elektriciteit (electricity)’ at Edo-age, Sohkichi Hashimoto (1743−1836) had already demonstrated about 190 years ago that electrification of a natural magnetic stone was able to drop iron nails attached to it. The electromagnetic anomalies that accompanied to the Ansei-Edo earthquake were discussed in terms of the ‘Evaluation of proposed earthquake precursors’ given by American Geophysical Union.