Threshold behavior in kinetic electron emission from oxide insulators

We have measured the yields $\gamma$ of the secondary electrons emitted from MgO, BaO, and $\mathrm{Si}{\mathrm{O}}_2$ surfaces by impact of slow noble gas ions and identified the apparent threshold ion energies $\left(E_0\right)$ of kinetic electron emission from the discontinuity points in the $\gamma \text{−}E$ curves. We found that the center-of-mass threshold energy $\left(E_{\mathit{th}}\right)$ in the binary collision with an oxygen anion is projectile independent and equal to the sum of the band-gap energy and the electron affinity. Based on this threshold behavior, we propose that kinetic emission from oxide insulators occurs by direct collisional excitation of $\mathrm{O}2p$ electron to a continuum state via repulsive interactions with the electrons of a projectile.

Figure 1

The secondary electron emission yields $\gamma$ of BaO, MgO, and $\mathrm{Si}{\mathrm{O}}_2$ as a function of ion energies for ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ at normal incidence.

Figure 2

The $\gamma$ vs $E$ replotted against the center-of-mass collision energy $\left(E_{\mathrm{c.m.}}\right)$ of the projectile-oxygen binary collision from the data in Fig. 1. The threshold ion energies $\left(E_{\mathit{th}}\right)$ of kinetic emission are indicated by the vertical dashed lines.

Figure 3

The apparent threshold ion energy $\left(E_0\right)$ of kinetic emission plotted against the mass ratio $m_1∕m_2$. From the relation $E_0=(1+m_1∕m_2)E_{\mathit{th}}$, the center-of-mass threshold energy $\left(E_{\mathit{th}}\right)$ values, obtained from the slope and the intercept of the linear plots, are also shown.

Figure 4

A schematic energy diagram for kinetic secondary electron emission from oxide surfaces. An electron is directly excited to the continuum state by kinetic energy transfer. $E_k\left(i\right)$ and $E_k\left(f\right)$ represent the initial and final collision energies, respectively, and $\epsilon$ the electron excitation energy.