Subsurface excitations in a metal

We investigate internal hot carrier excitations in a Au thin film bombarded by hyperthermal and low energy alkali and noble gas ions. Excitations within the thin film of a metal-oxide-semiconductor device are measured revealing that ions whose velocities fall below the classical threshold given by the free-electron model of a metal still excite hot carriers. Excellent agreement between these results and a nonadiabatic model that accounts for the time-varying ion-surface interaction indicates that the measured excitations are due to semilocalized electrons near the metal surface.

Figure 1

(Color online) Cross section of MOS device under ion bombardment. The plot shows the hot-electron current measured through the MOS device in response to three periods of bombardment by 600 eV ${\text{He}}^{+}$. The currents $I_{\text{f}}$ and $I_{\text{b}}$ were used to determine the kinetically induced hot-electron yield ${\Gamma }_{\text{ie}}=\left|I_b/\left(I_f-I_b\right)\right|$. Shaded regions represent 30 s intervals when the device was exposed to the beam.

Figure 2

(Color online) The upper panel shows current yield as a function of incident ion energy for four different ion species. The lower panel shows the same data plotted as a function of incident velocity. The solid lines are theoretical results from a model that describes charge-carrier excitations caused by ion-surface interactions. The vertical dashed lines represent the threshold for charge-carrier excitations using a free-electron model of a metal below which there should be no excitations.

Figure 3

Hot charge-carrier response during three periods of exposure for 1 keV ${\text{Li}}^{+}$ incident on an MOS device. The time-varying response is indicative of hot-hole interaction with oxide trap states.