Sunghwan Jin

Thickness effect on secondary electron emission of MgO layers

Two series of MgO thin layers having various thicknesses were prepared on the Si substrate by electron-beam evaporation and by spin coating of MgO precursor solutions. We found that the magnitude of the secondary electron emission (SEE) yield of the MgO films strongly depends on the film thickness and the sample bias voltage. We ascribed it to the electric field through the insulating MgO layer, which allowed fast supply of electrons from the Si substrate to the surface. The mechanism of electron supply can be explained either as an acceleration through the MgO layer that becomes partially conductive upon primary electrons bombardment (radiation induced conductivity), or as a tunneling through the non-irradiated region of the insulating layer where the primary electrons cannot reach deeply into the sample with a certain penetration depth. The maximum SEE yield of the each MgO film on the Si substrate was observed when the penetration depth of primary electrons was close to the thickness of the MgO film, if the applied electric potential to the sample was low. Under a strong electric potential, the relationship between the penetration depth of primary electrons and the thickness of MgO films is not observed. It suggests the existence of the non-irradiated region, where electron supply is allowed by electron tunneling. Therefore, the magnitude of SEE yield for the thin insulating layer is strongly related to the detailed mechanism of electron supply, which is determined by the thickness of the insulating layer and the applied bias voltage to the sample during the SEE process.

Secondary electron emission yields from MgO deposited on carbon nanotubes

Enormously high secondary electron emission yields under electric field are observed from MgO deposited on carbon nanotubes. The yields reach a value as high as 15 000 and are strongly dependent upon the bias voltage applied to the sample. The creation of the electric field across the MgO film after bombardment of primary electrons is considered as one of key features, since positive charges are generated at the surface by departure of secondary electrons. Subsequent bombarding electrons produce other secondary electrons inside the MgO film, then the liberated secondaries are accelerated towards the surface under the strong field. Under this condition, the secondary electrons gain sufficient energy to create further electrons by impact ionization. The process continues until an equilibrium avalanche is established. To elucidate the earlier explanations, the kinetic energy spectra of secondary electrons are measured by an energy analyzer at various bias voltages in MgO/carbon nanotube samples. The analysis...

Energy distribution for undergate-type triode carbon nanotube field emitters

Field emission energy distribution (FEED) has been measured for undergate-type triode carbon nanotube (CNT) field emitters where the gate electrodes are located underneath the cathode electrodes. The diode-type emission for these CNT emitters was found to follow the Fowler–Nordheim relation, whereas the triode-type emission exhibited the deviation from this relation. The FEED peaks for the undergate CNT emitters under the triode-type emission shifted to lower energy as the gate voltage increased, indicating nonmetallic behavior for the CNT emitters. There exist two different characteristic FEED peaks, where their peak energy shifts as a function of the gate voltage belong to two different slopes. From the difference in the position and intensity of the peaks, it was found that one was field emission directly from CNTs and the other might be emitted from CNTs through glass powders which were added during the CNT field emitter fabrication process.

Microchannel plate for high-efficiency field emission display

The efficiency of a field emission display was improved significantly with a newly developed microchannel plate. The key features of this unit and its fabrication are summarized as follows: (a) bulk alumina is used as a substrate material, (b) channel location is defined by a programed-hole puncher, and (c) thin film deposition is conducted by electroless plating followed by a sol–gel process. With the microchannel plate between the cathode and the anode of a field emission display, the brightness of luminescent light increases three- to fourfold by electron multiplication through an array of pores in the device. In addition, the fabricated microchannel plate prevents spreading of electrons emitted from the cathode tips, thus improving both display resolution and picture quality.

Undergate-type Triode Carbon Nanotube Field Emission Display with a Microchannel Plate

The characteristics of a field emission display (FED), which is based on an undergate-type triode carbon nanotube (CNT), have been examined by incorporating an electron-multiplying microchannel plate (MCP) between the anode and cathode plates of a FED. The MCP was fabricated by electroless plating and the sol–gel process on punched alumina. By applying appropriate voltages between the two faces of an MCP within a FED, the current at the anode plate of a FED was found to be enhanced more than three to five times, leading to higher brightness. The focusing of field emitted electrons was also improved by adjusting the bottom voltage of the MCP, which resulted in a clear image. Therefore, the incorporation of the MCP improved the performance of an undergate-type CNT FED, which can now be considered as one of the key candidates for flat panel displays.

Study of the secondary-electron emission from thermally grown SiO2 films on Si

Abstract The secondary-electron emission (SEE) coefficient, δ, was measured for thermally grown SiO 2 films on a Si wafer. As the thickness of SiO 2 film becomes greater than 40∼50 nm, the SEE curve changes to the double-humped shape having a low δ value from the typical ‘universal curve’ shown at ordinary SEE measurements. We conclude, from a comparison of the SiO 2 thickness and the penetration depth of primary electrons, that this is due to the surface charging effect of an insulating SiO 2 film. To overcome the charging effect, an electric field is introduced inside a thick SiO 2 film (55 nm) by applying a negative bias potential to the sample. When the bias potential is increased, δ of thick SiO 2 is increases constantly up to value similar to that for thin SiO 2 (2 nm), and the SEE curve recovers the original, universal curve form. The role of the bias potential is believed to increase the tunneling probability between the bulk Si and the SiO 2 surface, and thus electrons from Si can easily be supplied to the SiO 2 surface, where holes are generated upon the departure of secondary electrons.

Novel fabrication method of microchannel plates

We have developed a novel microchannel plate (MCP) by introducing new materials and process technologies. The key features of our MCP are summarized as follows: (i) bulk alumina as a substrate, (ii) the channel location defined by a programmed-hole puncher, (iii) thin film deposition by electroless plating and/or sol–gel process, and (iv) an easy fabrication process suitable for mass production and a large-sized MCP. The characteristics of the resulting MCP have been evaluated with a high input current source such as a continuous electron beam from an electron gun and Spindt-type field emitters to obtain information on electron multiplication. In the case of a 0.28 μA incident beam, the output current enhances ∼170 times, which is equal to 1% of the total bias current of the MCP at a given bias voltage of 2600 V. When we insert a MCP between the cathode and the anode of a field emission display panel, the brightness of luminescent light increases 3–4 times by multiplying the emitted electrons through pore...

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

Three series of MgO thin films were prepared by the spin coating of MgO precursor solutions (aqueous and organic based solutions) and by electron-beam evaporation. The quality of the films coated on the Si (100) substrate was characterized by observing crystallinity and surface roughness of the films. The measurement of the secondary electron emission (SEE) yield of the MgO films does not reveal any significant dependence on the MgO film fabrication process. However, it was found that the magnitude of the SEE yield is strongly dependent on the sample bias voltage. The maximum SEE yield of over 6 was obtained for the films prepared by both aqueous and organic based solutions. MgO layer formation by precursor solutions is a promising method considering the fact of its easiness and convenience, which also gives a relatively large SEE yield comparable to the MgO layer prepared by electron-beam evaporation.

Double- to single-hump shape change of secondary electron emission curve for thermal SiO2 layers

Secondary electron emission yields (SEEYs) were measured for silicon oxides which were thermally grown on doped silicon substrates. Generally, SEEY curves can be described by the so-called universal curve, i.e., one hump with a monotonic increase (decrease) before (after) the hump. However, we found that our thick oxide layers exhibited double-hump shaped SEEY curves instead of single-hump shaped curves. Additionally, we were able to change the shape of a SEEY curve with two humps to a curve with one hump, or vice versa, by varying the experimental parameters. This change in curve shape can be explained if we consider the competition between the oxide layer thickness and the electron’s penetration depth, the charge accumulation due to emission of secondary electrons, and charge traps created during thermal oxidation at the same time.

Secondary electron emission characteristics for sol–gel based SiO2 thin films

We have fabricated six SiO2 thin films by the spin coating of tetraethyl orthosilicate (TEOS) sol–gel solutions of different concentrations. It was found that the thickness of SiO2 film decreased as the concentration of TEOS decreased. Among six samples, 9 nm thick SiO2 film exhibited the highest secondary electron emission (SEE) yield. Moreover, SEE yields for sol–gel based SiO2 films were found to be higher than those for thermal SiO2 films. In order to confirm the applicability of this method, electron-amplifying microchannel plates (MCPs) with a sol–gel based SiO2 layer as an electron emissive layer were fabricated and their amplifying characteristics were investigated. Reasonably good current gains were obtained for those MCPs suggesting that this SiO2 layer formed by the sol–gel method was a good candidate for a SEE layer.