No abstract

Using ob initio local densily calculalions, we consider lhe microscopic slruciures of two neutral pair deleas of B in Si. ?he s1111c1um are closely related IO propcued conligurations of the 'B-interstitial-related defect' in Si, with a B alom substituted for a Si atom. lhe B-split substitutional configuration has two empty, symmetrically equivalent, p orbitals, and may undergo (pseudo-) Jahn-Teller dirtorlions when negatively charged.supucell sire: the energy gained when relaxing the sccond-nearat-neighbour and funher away Si a t o m im Ihe larger 3Zalom cell is about 0.1 eV The band gap ermr due to the lml density appmimalion docs no1 mnlribule to the Lola1 energy, since, for lhe neutral defects a1 hand, all mcupied S t a h BR in the valence band. The p i l i o n s of the electmmie slates M accurate lo within about 05 cV, a s has been reported olher autho-, for a m p l e ,

We calculate the microscopic structure of the .B interstitial related defect. in Si.The charge-induced symmetry-lowering distortion seen in experiment is confirmed. The neutral and negatively charged structures consist of a novel combination of an SP2-hybridized B atom with an empty p z orbital and an SP3-hybridized Si atom with a lone pair. A new method for performing ab initio total energy calculations on small impurity atoms is also introduced.

Realistic ab initio total-energy calculations are performed to investigate the electronic and geometric structure of crystalline As2Se3. Results include the following: total energies for various distortions, the equilibrium geometric structure, intralayer and interlayer cohesive energies"charge densities, the density of states, the band structure, rigid-layer phonon frequencies, and effective masses. Spe(:ific attention is focused on studying the nature of bonding within and between layers, as well as elucidating the nature of the electron and hole wave functions in the vicinity of the fundamental gap. In particular it is found that the interactions between the layers are crucial in determining the properties of the wave functions at the band edges. Moreover, the electron and hole masses are predicted to exhibit a large and unusual anisotropy.

The atomic conhguralioii, electronic slates and a local vibrational mode of the Li-saturated vacancy ill Si are determined using ab bhio calculations. We find, in agreement with an earlier theoretical suggestion. that this defect consists of a tetrahedral combination of a negatively charged vacancy and four positively charged 1. i inlerstilials. A C, , configuration is found IO k higher in energy 'Chis conhnns recent experimenral suggestions that a symmetry lowering, which is found in the luminescence spectrum, is due to excitonic effects. The geometric simplicity of this defect complex suggesls that it may be universal for irradiated Si doped with column I and (from chemical syninirtry) column VI1 elements. As an example of the latter we consider the structure of the Br-saturated vacanq.