Scanning tunneling microscopy and photoemission studies of Ag films on metal/semiconductor surfaces
Abstract: The research presented in this thesis has been focused on the study of thin Ag films, grown on metal-reconstructed Si(111) and Ge(111) surfaces.The films have been grown at room temperature, and the morphologiesand electronic structures of the films have been investigated using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy(ARPES).On the Ga-, In-, and Sn/Si(111)-√3x√3 surfaces, the first layer of Ag forms a special interface which consists of short atomic rows, with slightly different appearances depending on the base surface. Starting from two monolayers (MLs), Ag grows as a thin film with bulk-like lattice parameters.The electronic structures of the films reveal the behavior of the intrinsic quantum well states (QWSs). STS data show peaks in the filled states which move towards the Fermi level with increased thicknesses. These peaks have been compared with ARPES spectra and linked to the QWSs. The evolution of the QWSs with film thicknesses has been examined within, and extending upon, the established theoretical framework. The results point towards metal-reconstructed Si(111) surfaces using group III and IV elements as strong candidates for uniform film growth, and open up new avenues for studying electronic coupling effects between film/substrate.On Sn/Si(111)-√3x√3, the Ag films grow in domains with two different lattice orientations, rotated 30° from each other. This is due to the interface consisting of two different structures, as revealed by the STM. One of the interfacial phases is a 3×3 honeycomb structure, and the other a line structure of short atomic rows with a three-fold symmetric 2√3×√3-R30° unit cell. Atomic models for the two interface phases have been proposed, based on two different spin configurations of the Sn/Si(111)-√3x√3 surface. The presence of two interfaces makes this system highly attractive for the study of interface related phenomena, and the difference in Ag filmson Sn/Ge(111) compared with Sn/Si(111) highlights the importance of electronic effects for the film growth.
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