Secondary Electron Emission Studies of Diamond and GaN Materials
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mechanism. Therefore, we can use SEES to study the transport and emission processes that are common to both secondary electron emission and cold electron emission. Another important aspect of SEES is the variable incident beam energy Eo. We can take advantage of the variable Eo to influence the secondary emission process. First, the number of secondary electrons generated in the material increases proportionally with Eo. Second, as Eo increases, the incident electrons penetrate deeper into the material where most of the secondary electrons are generated. As a result, the secondary electrons must travel greater distances to reach the surface, and there is an increased probability of energy and intensity loss due to inelastic scattering and electron capture in the material. By examining changes in the emitted electron intensity and energy distribution as a function of Eo, we can deduce information about the transport and emission processes in the material. For example, the secondary yield measurements are plotted as a function of Eo, and the resulting yield curve typically has a bell shape that can be understood by considering the competing electron generation and escape processes. At low values of E., the yield increases with increasing Eo as more secondary electrons are generated. However, as Eo increases, the electron penetration depth, Dp, increases and the secondary electrons are generated deeper in the material. The secondary electrons can still escape as long as they are generated within the electron escape depth Desc, where Desc is independent of Eo and depends only on the material properties. As a result, the yield reaches a maximum at a value of Eo for which Dp = Dese [7]. As Eo increases further, Dp > Desc and the yield decreases as increasing numbers of electrons are captured in the material. Hence, the shape of the yield curve can be analyzed to deduce information about the electron escape process. In addition, the magnitude of the yield provides information about the size and uniformity of the surface energy barrier. Information is obtained from the energy distribution measurements by plotting the data in energy distribution curves (EDCs). The energy distribution of the emitted electrons depends on the energy distribution of electrons reaching the surface and on the position of Evac relative to this distribution. Hence, the EDCs are affected by the electron transport process and by the surface properties of the material. Since it is difficult to directly probe the secondary electron transport process using SEES measurements, information is instead deduced from the analysis. However, the surface electronic properties can be characterized in a straightforward manner from the energy distribution measurements. While SEES measurements cannot be used to measure primary emission currents from cold emitter materials, they can be used to evaluate the capabilities of promising emitter materials. In particular, the technique provides information about both the electron transport and emission processes in a materia
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