Scanning tunnelling spectroscopy of atomic clusters deposited on oxidized silicon surfaces: induced surface dipole and resonant electron injection

We investigated the tunnelling conductance of atomic clusters (C60, Si6Hx, AsSi2Hx) deposited on oxidized p-type Si(100) surfaces using scanning tunnelling microscopy where the metal probe/vacuum barrier/cluster/oxide/silicon structures form an asymmetric double-barrier tunnel (ADBT) nanoscale junction. Atomic clusters were assembled onto ultra-thin (~0.3 nm) silicon oxide by either C60 sublimation or ion beam deposition of Si-clusters generated in an ion trap. Electron transfer to the clusters induced surface dipole and reduced junction current under reverse biasing conditions (negatively biased substrate) depending on cluster structure and composition, where the AsSi2Hx clusters created the strongest dipole. Conductance enhancement was observed for forward bias originating in resonance-like electron injection through the unoccupied orbital of the clusters, which was spatially localized within ~1 nm diameter for C60. The resonance peak positions and the weak surface dipole indicated that the orbital energies of C60 and Si6Hx were beyond the forbidden energy gap of Si and shifted with respect to the silicon Fermi energy for heavily doped substrates. In contrast, the orbital energy of doped AsSi2Hx clusters was below the silicon Fermi level. These results demonstrate that the ADBT junction configuration reveals electronic coupling of the clusters to the semiconductor surfaces.