Tantalum induced butterfly-like clusters on Si (111)-7× 7 surface: STM/STS study at low coverage

摘要

The adsorption of the small amounts of tantalum on Si (111 )-7× 7 reconstructed surface is investigated sys tematically using scanning tunneling microscopy and tunneling spectroscopy combined with first-principles density functional theory calculations. We find out that the moderate annealing of the Ta covered surface re sults in the formation of clusters of the butterfly-like shape. The clusters are sporadically distributed over the surface and their density is metal coverage dependent. Filled and empty state STM images of the clusters dif fer strongly suggesting the existence of covalent bonds within the cluster. Tunneling spectroscopy measure ments reveal small energy gap, showing semiconductor-like behavior of the constituent atoms. The cluster model based on experimental images and theoretical calculations has been proposed and discussed. Pre sented results show that Ta joins the family of adsorbates, that are known to form magic clusters on Si (111)-7× 7 , but its magic cluster has the structural and electronic properties that are different from those reported before.