Quantum Electron Transport through Ultrathin Si Films: Effects of Interface Passivation on Fermi-Level Pinning

We report first-principles calculations on electron transport through ultrathin silicon films between aluminum electrodes. The passivation of interface Si atoms at one side of the film with hydrogen makes the current-voltage characteristics asymmetric with quasirectifying properties. The low conductivity in this case can be explained by the weakened metal-induced gap states due to the passivation. We also demonstrate that the applied bias changes the strength of Fermi-level pinning for the passivated interface.

Figure 1

(a) Calculated $I\mathrm{\text{−}}V$ characteristics of ultrathin Si films. The values of the current is per Si atoms at the interface. (b) Atomic structure of the unpassivated interface between the Si film and the right Al electrode. (c) Atomic structure of the H-passivated interface. The interface between the Si film and the left Al electrode is unpassivated for both cases of AlSiAl and AlSiHAl. (d) Local density of states for the region between third and fourth Si atoms from the right side of the thin films in the zero-bias case [Region II in Fig. 2a]. Empty states are also shown in this figure.

Figure 2

(a) Effective potential of AlSiHAl for $V=0$ (dashed line) and its shifts due to applied biases of $V=1.8$ (solid line) and $-1.8\mathrm{V}$ (dotted line). (b) Local density of states for the region between the Si atom at the passivated interface and the outermost H atom of AlSiHAl [Region I in (a)]. The hatched LDOS represents nonequilibrium states in the transport window.

Figure 3

Shift of the potential $\Delta V$ for the Si atom at the right interfaces relative to the right electrode. The lines for $\Delta V=0$ and $\Delta V=V$ are also shown.