No abstract

The neutral vacancy (V) snd the (V,H") (n = 1, . . . , 4) complexes in silicon are studied in various molecular clusters at the approximate ab initio and ab initio Hartree-Fock levels, with post-HartreeFock corrections in electron correlation. The quantities calculated are the equilibrium con6gurations and electronic structures, dissociation energies, diffusion paths, and activation energies. The dissociation energies are compared to those of other traps for H calculated at the same level of theory, such as substitutional C, interstitial Ti, or the (B,H) pair. The calculations predict that the (V,Hq) pair in Si should be mobile above room temperature. A part of the barrier for diffusion of (V,Hq) is lower than that of V, due to a mechanism analogous to the H-enhanced diffusion of interstitial 0 in Si. Another part of this barrier is higher than that for V. The consequences of this process include the possibility of enhanced diffusion of H in polycrystalline versus crystalline Si and leads us to propose a mechanism for the nucleation of platelets in the subsurface region of plasma-exposed Si.

The relative stabilities of hydrogen at (or near) the tetrahedral interstitial and bond-centered sites as well as that of hydrogen dimers in the molecular and bond-centeredantibonding configurations are calculated at the ab initio level in molecular clusters for c-C, Si, Ge, and o.-Sn. The trends show that the lowest-energy configurations change as one goes down the Periodic Table. The relative stability of the possible equilibrium sites aKects which charge states of H are likely to be realized in a given host. This in turn a8'ects the di8'usion properties of H and its interactions with dopants and other defect centers. The trends in equilibrium geometries and relative stabilities show that silicon is a particular case among group IV hosts in which both isolated interstitials and both dimer states are close to each other in energy. We also examine some properties of two charge states of molecular hydrogen in Si in order to determine the key features of their electron paramagnetic resonance spectra.

The potential-energy surfaces and electronic structures of neutral interstitial hydrogen in various polytypes of SiC are calculated at the approximate ab initio Hartree-Fock level using the method of partial retention of diatomic differential overlap. The host crystals are represented by a variety of hydrogen-saturated clusters containing about 50 host atoms. The stable interstitial site for I in the 3C, 2H, and 6H polytypes of SiC are predicted. The results can be generalized to other hexagonal poly types.

The equilibrium structure, electronic properties and potential energy surfaces of interstitial oxygen (Oi) in c-C, Si, BP, AlP, c-SiC and c-BN are calculated in small and large molecular clusters. The theoretical level ranges from the 'approximate ab initio' Hartree-Fock method of partial retention of diatomic differential overlap to large-basis-set ab initio Hartree-Fock followed by second-order corrections for electron correlation (MP2). The equilibrium site is a puckered bridged bond in all hosts. In compound semiconductors, Oi has a larger degree of bonding with the most electronegative of the two host atoms (P, C or N) than with the least electronegative one and puckers in a direction that maximizes the overlap with its second-nearest neighbour. The dipole moment of the defect and the barrier for reorientation of Oi around and through the (111) axis are calculated. In order to estimate the relative stability of Oi in the various hosts, we determine the energies involved in inserting molecular O2 into the lattice and dissociating it into two isolated Ois. Finally, we calculate the barriers for migration of Oi between adjacent equilibrium sites. There are two such barriers in compound semiconductors. Whenever possible, we correlate the properties of Oi with various properties of the host, such as its and length and its ionic character, in order to gain predictive insight into the fundamental properties of interstitial oxygen in semiconductors.