First-principles studies of kinetics in epitaxial growth of III-V semiconductors

We demonstrate how first-principles calculations using density-functional theory (DFT) can be applied to gain insight into the molecular processes that rule the physics of materials processing. Specifically, we study the molecular beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs on GaAs(001), a growth model is presented that builds on results of DFT calculations for molecular processes on the beta2-reconstructed GaAs(001) surface, including adsorption, desorption, surface diffusion and nucleation. Kinetic Monte Carlo simulations on the basis of the calculated energetics enable us to model MBE growth of GaAs from beams of Ga and As_2 in atomistic detail. The simulations show that island nucleation is controlled by the reaction of As_2 molecules with Ga adatoms on the surface. The analysis reveals that the scaling laws of standard nucleation theory for the island density as a function of growth temperature are not applicable to GaAs epitaxy. We also discuss heteroepitaxy of InAs on GaAs(001), and report first-principles DFT calculations for In diffusion on the strained GaAs substrate. In particular we address the effect of heteroepitaxial strain on the growth kinetics of coherently strained InAs islands. The strain field around an island is found to cause a slowing-down of material transport from the substrate towards the island and thus helps to achieve more homogeneous island sizes.Comment: 12 pages, 7 figures, REVTeX, Final version to appear in Appl. Phys. A (2002). Other related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Effect of the cluster size in modeling the H_2 desorption and dissociative adsorption on Si(001)

Three different clusters, Si_9H_12, Si_15H_16, and Si_21H_20, are used in density-functional theory calculations in conjunction with ab initio pseudopotentials to study how the energetics of H_2 dissociativ e adsorption on and associative desorption from Si(001) depends on the cluster size. The results are compared to five-layer slab calculations using the same pseudopotentials and high qu ality plane-wave basis set. Several exchange-correlation functionals are employed. Our analysis suggests that the smaller clusters generally overestimate the activation barriers and reaction energy. The Si_21H_20 cluster, however, is found to predict reaction energetics, with E_{a}^{des}=56 +- 3 kcal/mol (2.4 +- 0.1 eV), reasonably close (though still different) to that obtained from the slab calculations. Differences in the calculated activation energies are discussed in relation to the efficiency of clusters to describe the properties of the clean Si(001)-2x1 surface.Comment: 10 pages, 6 figures, submitted to J. Chem. Phy

Bernoulli Potential, Hall Constant and Cooper Pairs Effective Masses in Disordered BCS Superconductors

It is analyzed what fundamental new information for the properties of the superconductors can be obtained by systematic investigation of the Bernoulli effect. It is shown that it is a tool to determine the effective mass of Cooper pairs, the volume density of charge carriers, the temperature dependence of the penetration depth and condensation energy. The theoretical results for disordered and anisotropic gap superconductors are systematized for this aim. For clean-anisotropic-gap superconductors is presented a simple derivation for the temperature dependence of the penetration depthComment: 13 pages, 3 figures, LaTeX 2e, New figure and reference

The data paper: a mechanism to incentivize data publishing in biodiversity science

<p/> <p>Background</p> <p>Free and open access to primary biodiversity data is essential for informed decision-making to achieve conservation of biodiversity and sustainable development. However, primary biodiversity data are neither easily accessible nor discoverable. Among several impediments, one is a lack of incentives to data publishers for publishing of their data resources. One such mechanism currently lacking is recognition through conventional scholarly publication of enriched metadata, which should ensure rapid discovery of 'fit-for-use' biodiversity data resources.</p> <p>Discussion</p> <p>We review the state of the art of data discovery options and the mechanisms in place for incentivizing data publishers efforts towards easy, efficient and enhanced publishing, dissemination, sharing and re-use of biodiversity data. We propose the establishment of the 'biodiversity data paper' as one possible mechanism to offer scholarly recognition for efforts and investment by data publishers in authoring rich metadata and publishing them as citable academic papers. While detailing the benefits to data publishers, we describe the objectives, work flow and outcomes of the pilot project commissioned by the Global Biodiversity Information Facility in collaboration with scholarly publishers and pioneered by Pensoft Publishers through its journals <it>Zookeys</it>, <it>PhytoKeys</it>, <it>MycoKeys</it>, <it>BioRisk</it>, <it>NeoBiota</it>, <it>Nature Conservation</it> and the forthcoming <it>Biodiversity Data Journal</it>. We then debate further enhancements of the data paper beyond the pilot project and attempt to forecast the future uptake of data papers as an incentivization mechanism by the stakeholder communities.</p> <p>Conclusions</p> <p>We believe that in addition to recognition for those involved in the data publishing enterprise, data papers will also expedite publishing of fit-for-use biodiversity data resources. However, uptake and establishment of the data paper as a potential mechanism of scholarly recognition requires a high degree of commitment and investment by the cross-sectional stakeholder communities.</p