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Comparison between silicon and germanium quantum sheets has been made regarding their electronic and optical properties. Ab initio calculations have been applied for this purpose by mean of the linearized augmented plane wave method. Quantum confinement is found to shift the band gap of both Si and Ge bulk to the blue, however this upshift depends strongly on the surface orientation. Moreover also the nature of the band gap is related to the surface symmetry: for (1 0 0)-oriented films the band gap becomes direct for both semiconductors, whereas the (1 1 0)- and (1 1 1)-oriented films show a different behavior. In the first case the band gap is direct for Si and indirect for Ge, in the second it is direct for Ge and indirect for Si. Concerning the optical properties, since in the Si films the folded bulk band energies are found to retain their original indirect character, all the Si films have an intense absorption peak only at high energies corresponding to the blueshifted direct band gap of Si bulk. In Ge films the conduction band minimum retains a strong Γ component. Therefore, dielectric function calculations clearly show that Ge films have a strong optical absorption in the visual energy region. Analysis of the squared optical matrix elements is also presented and the data are compared to the results for GaAs.

We study using first-principle calculations the electronic and optical properties of In0.5Ga0.5As/InP and In0.5Al0.5As/InP superlattices, where the InGaAs and InAlAs alloys are described through an appropriate ordered ternary structure. The calculated electronic properties show that the substitution of Ga with Al originate an opening of the band gap from the infrared to the near visible and a transformation of the band alignment from type I to type II. Through the analysis of the optical properties we discuss successfully the giant polarization anisotropy observed in these systems.

The Si layer thickness dependence of the optical properties of silicon/silicon dioxide (Si/SiO 2 ) superlattices has been, for the first time, theoretically investigated. In our first principle calculation we consider both fully passivated interfaces and the presence of oxygen vacancy at the interface. Our results show the key role played both by the quantum confined states and interface states in the experimentally observed visible luminescence in Si/SiO 2 confined systems.

The electronic properties of ultra-thin Si layers have been investigated. Using the linear muffin tin orbital (LMT0) method, we have studied band gap and interface states for supercells formed by Si(111) layers of variable thickness (1-7 double layers) embedded in a CaF 2 host crystal, a system with interesting analogies to porous silicon. Our results show that the band gap opening is dominated by the valence band which follows the effective mass confinement picture, while the conduction band is found to flatten out and shift very modestly. The latter behaviour is due to hybridization effects between Si and Ca states which lead to bonding-antibonding interface states in the gap

The surface linear muffin-tin method in the atomic-sphere approximation (SLMTO-ASA) is applied to study the electronic properties of the CaF 2 Si(111) interface. We investigate the growth of CaF 2 on Si(111) following the interface formation from low to high coverages. The early stages of the growth are simulated by an interface formed by a monolayer of Ca deposited on Si(111) followed by a F layer, i.e., a FCaSi(111) system. Both valence- and core-electron states have been calculated. Their analysis gives important information about the nature of the CaSi and CaF bonds. We explore the dependence of the electronic properties on the number of deposited layers showing that the main contribution to the interface properties is established at monolayer coverage and that the transfer of charge between the two crystals is localized in their last layers.

A method of describing the non-locality of exchange and correlation energy and potential. used by the authors in a density-functional calculation of atomic structure, is now adopted in a self-consistent calculation of a solid surface in the framework of the jellium model. The exchange-correlation potential VXC(z), within the approximation proposed by Gunnarsson and Jones, is written for a system with translational invariance in the plane parallel to the surface. The electron density n(z) and the total effective potential Veff(z) seen by an electron are self-consistently calculated for different electron densities (rs varying from 2 to 6). The behaviour of the different contributions to the surface potential, the intensity of the Friedel oscillations, the surface energies, and the image-plane position z0 are presented, discussed and compared with the results of the local density approximation.

We review the theoretical studies on the reactive interfaces between silicon and transition metals. Much information on the interface compounds can be obtained through an analysis of the electron states of bulk compounds of different structure and stoichiometry. A more detailed analysis of the interface reaction and typical surface information, like the interface geometry or the Schottky barrier height B , can be obtained only through direct investigations of the interface structure. The difficulty of interface calculations can be overcome by using the linear methods devised by Andersen, like the linear augmented plane wave (LAPW) and the linear muffin tin orbitals (LMTO) methods. The LMTO method has been adapted to perform a self-consistent calculation of the electronic properties of the Si(111)-NiSi 2 (111) epitaxial interface. This interface has been investigated for both the experimentally observed type-A and type-B orientations. The densities of states projected on the various sites provide, for the first time, a detailed analysis of the interface electronic states. The perturbation due to the interface is shown to be very strong and various interface features are found on the silicon and silicide layers. The computed B is different for the two orientations, with a value of 1.1 eV for the type-A orientation, and 0.8 eV for the staking-fault type-B interface.

We present a selfconsistent calculation of the electron distribution at metal surfaces in the presence of a static electric field in the semi-infinite jellium model. The calculation is based on the Hohenberg-Kohn-Sham theory with a nonlocal approximation for the exchange-correlation energy and potential. For different metallic density rs=2, 3, 4 we present numerical results for the mean position and the spread of the induced charge density. The results for the centroid of the charge induced by a small field are discussed and compared with the image plane position values obtained from the exchange-correlation potential.