Interface excitation parameter from dielectric response theory

Abstract

We define the interface excitation parameter (IEP) as the change in excitation probability, caused by the presence of a medium‐medium interface crossed by an electron, in comparison with an electron for which only bulk excitations are considered. This definition is established by analogy with the definition of the surface excitation parameter for which one of the two media of the interface is the vacuum and which has already been extensively studied. To calculate the IEP (as well as the energy‐differential IEP or DIEP), we generalize the model developed by Tung, Chen, Kwei and Chou [C. J. Tung, Y. F. Chen, C. M. Kwei and T. L. Chou, Phys. Rev. B 49 (1994) 16684] from dielectric response theory for surface excitation parameter determination. We perform these calculations for angles between 0o and 60o, for electron energies between 200 and 3000eV and for various combinations of materials, chosen for their academic (as Al/In) or practical interest (as SiO2/Si for instance). We show that for materials with “similar” dielectric properties (metal/metal), the IEP is completely negligible. On the contrary when the materials of the interface are characterized by a large energy band gap difference, as metal/insulator or semiconductor/insulator, the IEP can reach a value of about 0.26 for the smallest electron energies considered here. Moreover, we show that for the SiO2/Si interface, the energy‐differential IEP obtained from our model is in good agreement with previous experimental data. Copyright © 2015 John Wiley & Sons, Ltd.

Local inverse inelastic mean free path at interface

“…In particular, for metal-insulator interfaces or for large crossing angles, IEP can reach a significant value. Finally, the general behaviour (but not the absolute values) of the IEP obtained in the present work is in good agreement with previous theoretical work results 11 in which the interface excitation was supposed to occur right at the interface. Moreover, our results well correspond to the few experimental results on this subject.…”

“…This behaviour corresponds to results observed in Vanbever et al 11 Looking now at the absolute values, for angles smaller than $20 , IEP is smaller than 0.1. This value is small but quite larger than results obtained in Vanbever et al 11 for metal-metal interface (typically $10 À3 ). This results tends to prove that, even for metal-metal interfaces, interface excitations cannot be completely neglected, as it was suggested in Vanbever et al 11 As a second example, Figure 13 shows the energy distribution of the IEP, calculated for a crossing interface angle α ¼ 0 , again for the Au/Si interface but now calculated for the two direction of propagation of the electron.…”

“…The IEP P 1!2 MM ðE, αÞ is a global parameter defined as the difference in excitation probability due to the presence of a MM interface by comparison with an infinite medium (see Equation 6). This definition was introduced in Vanbever et al 11 22 from dielectric response theory. The major difference between this previous model and the one presented here is that the interface excitation was supposed to occur right at the interface, without extension inside the two media, avoiding any local-IIMFP calculations.…”

“…However, it is possible to compare some IEP results obtained within the two models, in particular for the Si/SiO 2 interface. 11 As a first example, Figure 12 shows the IEP angle distribution calculated for a 500-eV electron crossing a Au/Si interface from Au to Si. In agreement with previous local-IIMFP results, IEP is increasing for an increasing angle to reach significant value for α ¼ 80 .…”

“…Vanbever et al, 11 the interface excitation was supposed to occur right at the interface, having thus no extension inside the two media themselves. Consequently, that model does not allow any local-IIMFP calculations.…”