The behavior of hydrogen in crystalline silicon is examined with state-of-the-art theoretical techniques, based on the pseudopotential-density-functional method in a supercell geometry. Stable sites, migration paths, and barriers for different charge states are explored and displayed in totalenergy surfaces that provide immediate insight into these properties. The bond-center site is the global minimum for the neutral and positive charge states; in the negative charge state, the tetrahedral interstitial site is preferred. The positive charge state is energetically favorable in p-type material, providing a mechanism for passivation of shallow acceptors: electrons from the H atoms annihilate the free holes, and formation of H-acceptor pairs follows compensation. Also addressed are the issues of molecule formation and hydrogen-induced damage. A number of different mechanisms for defect formation are examined; hydrogen-assisted vacancy formation is found to be an exothermic process. ' ' have offered interpretations of the passivation data seeking to unravel the underlying mechanisms. Attempts to explain the observed phenomena led to a number of contradictory assumptions regarding the nature of the charge states of H along its diffusion path, and hence about the H-impurity reactions that can occur. Particular models were advanced for the structure of the hydrogen-impurity complexes that are a result of passivation. The electronic structure of these complexes is such that all impurity levels are removed from the band gap. A complete understanding of the passivation process can only be obtained, however, by considering 39 10 791

Protecting biodiversity involves preserving the maximum number and abundance of species while giving special attention to species with unique genetic or morphological characteristics. In balancing different priorities, conservation policymakers may consider quantitative measures that compare diversity across ecological communities. To serve this purpose, a measure should increase or decrease with changes in community composition in a way that reflects what is valued, including species richness, evenness, and distinctness. However, counterintuitively, studies have shown that established indices, including those that emphasize average interspecies phylogenetic distance, may increase with the elimination of species. We introduce a new diversity index, the phylogenetic entropy, which generalizes in a natural way the Shannon index to incorporate species relatedness. Phylogenetic entropy favors communities in which highly distinct species are more abundant, but it does not advocate decreasing any species proportion below a community structure-dependent threshold. We contrast the behavior of multiple indices on a community of phyllostomid bats in the Selva Lacandona. The optimal genus distribution for phylogenetic entropy populates all genera in a linear relationship to their total phylogenetic distance to other genera. Two other indices favor eliminating 12 out of the 23 genera.

No abstract

No abstract

The behavior of hydrogen in crystalline silicon is examined with state-of-the-art theoretical techniques, based on the pseudopotential-density-functional method in a supercell geometry. Stable sites, migration paths, and barriers for different charge states are explored and displayed in totalenergy surfaces that provide immediate insight into these properties. The bond-center site is the global minimum for the neutral and positive charge states; in the negative charge state, the tetrahedral interstitial site is preferred. The positive charge state is energetically favorable in p-type material, providing a mechanism for passivation of shallow acceptors: electrons from the H atoms annihilate the free holes, and formation of H-acceptor pairs follows compensation. Also addressed are the issues of molecule formation and hydrogen-induced damage. A number of different mechanisms for defect formation are examined; hydrogen-assisted vacancy formation is found to be an exothermic process. ' ' have offered interpretations of the passivation data seeking to unravel the underlying mechanisms. Attempts to explain the observed phenomena led to a number of contradictory assumptions regarding the nature of the charge states of H along its diffusion path, and hence about the H-impurity reactions that can occur. Particular models were advanced for the structure of the hydrogen-impurity complexes that are a result of passivation. The electronic structure of these complexes is such that all impurity levels are removed from the band gap. A complete understanding of the passivation process can only be obtained, however, by considering 39 10 791