Formation and local electronic structure of Ge clusters on Si（111）-7×7 surfaces

We report the formation and local electronic structure of Ge clusters on the Si(111)-7$times $7 surfacestudied by using variable temperature scanning tunnelling microscopy(VT-STM) and low-temperature scanning tunnelling spectroscopy (STS).Atom-resolved STM images reveal that the Ge atoms are prone toforming clusters with 1.0~nm in diameter for coverage up to 0.12~ML. Such Ge clusters preferentially nucleate at the centre of thefaulted-half unit cells, leading to the `dark sites'of Si centre adatoms from the surrounding three unfaulted-halfunit cells in filled-state images. Bias-dependent STM images show the charge transferfrom the neighbouring Si adatoms to Ge clusters.Low-temperature STS of the Ge clusters revealsthat there is a band gap on the Ge cluster and the large voltage threshold is about 0.9~V.