We have studied the electronic properties of Zintl phase KSi, by the ab initio density functional pseudopotential method. Our interest in this Zintl compound is in its current use as a reagent in the synthesis of Si nanoclusters. The structure consists of isolated Si 4 tetrahedra with K atoms situated above each face. The crystal system is cubic with the symmetry of the P4 h3m space group. Band structure calculations show a band gap of 1.3 eV. The presence of K atoms has widened the band gap over that found between occupied and unoccupied energy levels in the Si 4 cluster. The valence bandwidth lies between the valence bandwidth of crystalline Si with diamond structure and the width of the occupied energy levels of the Si 4 cluster. The density of states shows four major structures for the occupied bands. The lowest energy band of conduction states is also given in the density of states plot. The nature of the bonding in the crystal is revealed by an examination of the charge density associated with each of the structures in the density of states. It is found that the dominant bonding between Si atoms is not the covalent bonding of sp 3 hybridized orbitals, as found in diamond structure Si, but is rather a mutual overlap of s-and p-like mixed atomic states from each Si atom. These overlapped states form a bonding state located at the center of the tetrahedron. Furthermore, each K atom is ionized by the nearest Si 4 tetrahedra, allowing the Si atom to fill its 3s and 3p shells.

Using first-principles calculations and experimental evidence concerning the essential environment for surfactant-mediated epitaxial growth on the GaAs/Te͑100͒ surface, we determine a short-ranged reaction path for the As↔Te exchange that is energetically favorable and prepares the surface for continued layer-by-layer growth. Furthermore, we explain the required partial coverage of the surfactant atoms as well as the required presence of both As and Ga adatoms.

Based on the facts that: (a) the transverse acoustic vibrational branch frequency is softened at the Brillouin zone boundaries of crystalline GaAs; (b) at the surface, the Ga Ã/As bond is stronger than Ga Ã/Te bond; and (c) the requirement that the final bond orientation of the Te surfactant should be rotated by 908 with respect to its initial orientation, we carried out a model study of an exchange process in epitaxial growth of GaAs (100). Even with very restrictive conditions imposed on the atomic movements, this study explains why Te is an effective surfactant for this type of growth. #