Potential electron emission induced by multiply charged ions in thin film tunnel junctions

Thin film metal-insulator-metal tunnel junctions are used to investigate the electronic excitation process induced by the impact of multiply charged ions onto a metallic surface. Hot charge carriers (electrons and holes) generated by the dissipation of the kinetic and potential energies of the projectiles are detected as an ion induced internal emission current from the bombarded “top” metal film into the “bottom” substrate electrode. Results are presented for ${\text{Ar}}^{q+}$ ions with a kinetic impact energy of 1 keV and charge states $q=1–8$ impinging onto an ${\text{Ag-AlO}}_x\text{-Al}$ junction. It is shown that the internal emission yield exhibits an approximately linear dependence on the potential energy of the projectile. At low potential energy, a bias voltage applied between the two metal films is found to strongly influence the internal emission current, whereas this influence becomes much weaker with increasing projectile charge state. The results are shown to be qualitatively well described in the framework of a thermodynamical free-electron model.

Figure 1

(Color online) Schematic drawing of the ${\text{Ag-AlO}}_x\text{-Al}$ thin film tunnel junction, showing the silver face being bombarded by multiply charged argon ions.

Figure 2

Tunneling yield $\gamma$, induced by ${\text{Ar}}^{q+}$ multiply charged ions in ${\text{Ag-AlO}}_x\text{-Al}$, plotted against the total potential energy $E_{\text{pot}}$. The solid line is a linear fit to the experimental data (squares). The kinetic energy of the ions was 1 keV.

Figure 3

Bias dependence of the normalized tunneling yield, induced by ${\text{Ar}}^{q+}$ in a ${\text{Ag-AlO}}_x\text{-Al}$ junction, for selected charge states $q$. The kinetic energy of the ions was 1 keV. The connecting lines are drawn to guide the eye.

Figure 4

Temperature of the electronic excitation induced by bombardment with ${\text{Ar}}^{q+}$ of 720 eV, plotted against the potential energy of the ${\text{Ar}}^{q+}$ ions. The data (squares) are derived from Fig. 1 of Ref. 30. The solid line represents a linear fit to the data.

Figure 5

Temperature dependences of the (a) chemical potential, (b) internal energy per unit volume, and (c) specific heat per unit volume of a Fermi-Dirac (connected stars) and a Maxwell-Boltzmann electron gases (solid line), respectively.

Figure 6

Calculated electron (squares), hole (circles), and total (stars) current density flowing from the top to the bottom layer of the ${\text{Ag-AlO}}_x\text{-Al}$ junction. Following parameters have been used: $n_e^{\text{Ag}}=5.86\times {10}^{28}{\text{m}}^{-3}$, ${\Phi }_e=3.0\text{eV}$, and ${\Phi }_h=4.0\text{eV}$.

Figure 7

Energy scheme of a metal-insulator-metal tunnel junction: (a) without a bias voltage and (b) with a (positive) bias voltage $U$ applied between the two metal electrodes (1) and (2). Nomenclature: (a) ${\Phi }_e^{\left(1\right)}$ and ${\Phi }_h^{\left(1\right)}$: barrier heights for electrons and holes, respectively, at the top-metal-oxide interface; ${\Phi }_e^{\left(2\right)}$, ${\Phi }_h^2$: barrier heights for electrons and holes, respectively, at the bottom-metal-oxide interface; $E_g$: band gap of the oxide; $E_F^{\left(1\right)}$, $E_F^{\left(2\right)}$: Fermi level of metal (1) and metal (2), respectively; (b) $E$: electron energy; ${\Phi }_e\left(z\right)$: electron barrier potential at a distance $z$ from the top-metal-oxide interface; $U$: applied bias voltage; and $d$: thickness of the oxide layer.

Figure 8

Calculated bias voltage dependence of the barrier heights ${\Phi }_e$ and ${\Phi }_h$ for electron and hole emission, respectively, for a trapezoidal barrier with ${\Phi }_e^{\left(1\right)}={\Phi }_e^{\left(2\right)}=3.0\text{eV}$, ${\Phi }_h^{\left(1\right)}={\Phi }_h^{\left(2\right)}=4.0\text{eV}$, $d=4\text{nm}$, and ${ϵ}_r=8$.

Figure 9

Total tunneling current density in a ${\text{Ag-AlO}}_x\text{-Al}$ junction flowing from Ag into Al plotted as a function of the applied bias voltage at $1\times {10}^4\text{K}$ (circles), $3\times {10}^4\text{K}$ (triangles), and $15\times {10}^4\text{K}$ (squares), respectively. The current is normalized to its value at 0 V. The following parameters have been used: $n_e^{\text{Ag}}=5.86\times {10}^{28}{\text{m}}^{-3}$, ${\Phi }_e=3.0\text{eV}$, and ${\Phi }_h=4.0\text{eV}$.