Photoemission and scanning tunneling microscopy investigation of elemental-semiconductor surfaces

The clean and reacted surfaces of Si(111)-(7x7), Si(100)-(2x1), Ge(111)-c(2x8), and Ge(100)-(2x1) have been studied to better understand the nature of these elemental-semiconductor surfaces. By carefully monitoring the adsorbate-induced changes in the electronic properties at the surface via spectroscopic methods such as photoemission, and by studying the resulting surface structures by scanning tunneling microscopy (STM), a quantitative description of the interaction and reaction between adsorbate and surface can be obtained. The photoemission method allows a distinction between atoms in different layers and in inequivalent sites by their binding energy shifts. By comparison with structural models and reference samples the number of atoms in each distinct chemical configuration can be determined. An adsorbate-induced chemical shift can be correlated with electronegativity differences between substrate and adsorbate atoms. In particular, studies of noble-metal interfaces with the (100) faces of Si and Ge demonstrate this novel combined application of photoemission and STM.The (111) faces of Si and Ge reconstruct to exhibit rather complex chemisorption geometries which are generally not well understood. For clean Si(111)-(7x7), there are three distinct surface sites giving rise to three different chemical environments. To establish the correlation between various surface-shifted components of the core levels and the surface sites, several experiments were designed and performed. The main idea behind these experiments has been to selectively replace atoms or saturate the dangling bonds of a certain surface site by adsorbate atoms, using the noble metal-semiconductor interface as the model system to test this basic approach. Results for other interface systems such as Sb, Sn and NH$\sb3$ with these semiconductors are also presented for comparison. These studies indicate that the "adatoms" on the clean Si(111)-(7x7) surface are directly responsible for the metallic surface state in the valence band. Additionally these adatoms exhibit a core level shift of $-$0.77 eV relative to the bulk atoms.U of I OnlyETDs are only available to UIUC Users without author permissio