Field-Dependent Secondary Emission

Abstract

Experiments have been conducted with magnesium oxide surfaces in which high dc fields were applied to the surface, while at the same time secondary emission measurements were being made. Very high secondary emission ratios were obtained reproducibly (100 to 1 and greater). Under static conditions it was found that the ratios would increase exponentially with increasing fields. In addition, for constant fields, the ratio of liberated electrons to bombarding electrons was constant, over a wide range of bombardment energy. Measurements were made of secondary emission with a square wave field applied to the surface and time lag effects were noted. Decay time in the order of 30 microseconds were measured. The rise time was found to be dependent upon field and bombarding currents. The mechanism of field dependent secondary emission was shown to be related to an avalanche effect, triggered by the bombarding electrons. beam currents he used were 3 to 4 microamperes, under which conditions he sometimes observed a blue fluorescence of his target upon electron bombardment.Malter found experimentally that the yield from his target increased as a power of his collector voltage:where I c = collected current, V c -collector voltage, and B,m = constants. His collector current was also found to rise as a power of the beam current. In addition he found that the surface exhibited long time lag characteristics. That is, after the primary beam was cut off, the secondary emission first decayed rapidly and then more slowly, persisting for many hours in some cases.The theory that Malter and later Koller, 7 developed was that the high secondary emission ratios observed (i.e., 1000 to 1) were due to positive ions formed at the surface of the secondary emission material by electron bombardment. These positive charges were thought to reside on the surface for long lengths of time, due to the high resistivity of the film. During this surface ionization period, high fields could exist in the material and electrons could be liberated in greater quantities due to the field.Pensak 5 did some work too, which may be pertinent, although he studied conductivity rather than secondary emission. In his experiments, steady bombarding currents, penetrating a thickness of amorphous silica film, induced a conductivity producing a current of one hundred times the bombarding beam. He used a doublebeam technique, to measure the potential at the surface.The beam used for inducing conductivity was kept at about 10,000 volts and 0.02 jua. He concluded that the induced currents were proportional to the energy absorbed by the beam, and that the beam in question must have sufficient energy to penetrate the film for induced conductivity. It was found too, that an increase in induced currents could be obtained with an increase in field across the insulator. No time lag measurements were specifically referred to in this article. 7 L. R. Koller and R. P. Johnson, Phys. Rev. 52, 519-523 (1937). 877

Emissions From Nonconducting Negatively Charged Dust Grains

“…• An enhancement of the secondary emission yield was observed. The yield increases by a factor of 5 for the field intensity 6 10 V/m, whereas [4], [5] reported a factor of 10 for thin films and similar field intensities. However, the estimation of field intensity is rather crude in these papers and a dependence of the yield on the field is exponential.…”

“…The negative grain potential would increase in order to reach a stable point where . However, it should be taken into account that a highelectric field at the grain surface would increase the secondary emission yield [4], [5]. This increase would lead to a shift of the point toward higher impact energies.…”

“…If we make a plot of versus , the work function can be derived from the slope of this plot (4) where the function weakly depends on and and varies in most cases within the range of 0.83 -1, i.e., its change is smaller than the scatter of experimental points. All three functions , , and , are described in [6], and their values are tabulated.…”

Secondary electron emission of MgO thin layers prepared by the spin coating method

“…The high SEE yields observed at V b ϭϪ1000 V is understood as the field dependent secondary electron emission suggested by Jacobs. [20][21][22] It is known that the SEE yield of a polycrystalline insulator decreases due to the reduced mean free path of the SEs through scattering and trapping by defects. 4 However, the crystallinity and surface roughness of the three MgO layers in this report do not reflect their subtle differences on the SEE behavior.…”

Electron emission from carbon nanotube-dispersed MgO layer

“…Also, a very high SEE yield could be obtained from a porous insulator, which is attributed to the growth of strong electric fields inside small pores and it produces a field enhanced SEE. 11,12 However, virtually limited attempts have been made to utilize SEs as an electron source, due mainly to the low SEE yield and the requirement of a primary source. And, we focus on this SEE characteristic in this letter.…”