C. Mailhiot

Ab initio pseudopotential calculations of point defects and boron impurity in silicon

Abstract Ab initio pseudopotential calculations were performed to obtain the formation energies of point defects in crystalline Si. A supercell of 64 Si atoms was used for the calculations. For the self-interstitial defect, a weak bond is formed between two defect Si atoms. However, no evidence of extra bond formation is found in the vacancy study. The formation energies of both defects are comparable, i.e. 3.7 eV for the vacancy and 3.25 eV for the self-interstitials, respectively. Our self-interstitial formation energy agrees well with other theoretical results, while the vacancy formation energy is much smaller. We also present the results of boron substitutional impurities in Si where the charge density plot indicates an ionic character of the BSi bonding.

New surface atomic structures for column V overlayers on the (110) surfaces of III–V compound semiconductors

Two new minimum‐energy surface structures have been identified for p(1×1) overlayers of Sb on the (110) surface of III–V compound semiconductors using a tight‐binding total‐energy formalism previously developed for these systems. The first is the ‘‘epitaxical on top structure’’ (EOTS) in which Sb zig–zag chains are commensurate with, and on top of, the Ga–As unreconstructed surface zig–zag chains. This structure differs from the previously found ‘‘epitaxical continued layer structure’’ (ECLS) by virtue of the registry of the Sb chains ‘‘on top of ’’ rather than ‘‘in between’’ the substrate Ga–As chains. Like the ECLS, the EOTS is compatible with both scanning tunneling microscopy (STM) and photoemission data. The second new structure is the ‘‘epitaxical overlapping chain structure’’ (EOCS) in which the Sb chains are 180° out‐of‐phase with, and on top of, the Ga–As substrate chains. This structure is, however, incompatible with both low‐energy electron diffraction and STM data for GaAs(110)–p(1×1)‐Sb. Comp...

In situ monitoring of surface postprocessing in large-aperture fused silica optics with optical coherence tomography

Optical coherence tomography (OCT) is explored as a method to image laser-damage sites located on the surface of large aperture fused silica optics during postprocessing via CO2 laser ablation. The signal analysis for image acquisition was adapted to meet the sensitivity requirements for this application. A long-working-distance geometry was employed to allow imaging through the opposite surface of the 5 cm thick optic. The experimental results demonstrate the potential of OCT for remote monitoring of transparent material processing applications.

Role of ionicity in the determination of surface atomic geometries: GaP, ZnS, and CuCl (110) surfaces

First‐principles total energy calculations were carried out in order to determine the local atomic geometry of the (110) surfaces for the isoelectronic series GaP, ZnS, and CuCl. Despite the large change in ionicity across the series (Phillips ionicities: fGaP=0.37, fZnS=0.62, and fCuCl=0.75), it was found that all three surfaces exhibit an activationless, bond‐rotation relaxation with a rotation angle of ∼25° or greater. While this result is well known in the case of III–V (110) surfaces, it is in contradiction to a previous calculation which predicted that the bond‐rotation angle would decrease with increasing ionicity, going to zero for CuCl. However, our results are in qualitative agreement with a recent dynamical low‐energy electron diffraction analysis of the CuCl (110) surface. Despite the presence of a large‐angle bond rotation for all three surfaces, the local atomic geometries depend on ionicity as follows: both the total energy gain for a given bond‐rotation angle and the deviation of the surfa...

High-resolution 3-D imaging of surface damage sites in fused silica with Optical Coherence Tomography

In this work, we present the first successful demonstration of a non-contact technique to precisely measure the 3D spatial characteristics of laser induced surface damage sites in fused silica for large aperture laser systems by employing Optical Coherence Tomography (OCT). What makes OCT particularly interesting in the characterization of optical materials for large aperture laser systems is that its axial resolution can be maintained with working distances greater than 5 cm, whether viewing through air or through the bulk of thick optics. Specifically, when mitigating surface damage sites against further growth by CO2 laser evaporation of the damage, it is important to know the depth of subsurface cracks below the damage site. These cracks are typically obscured by the damage rubble when imaged from above the surface. The results to date clearly demonstrate that OCT is a unique and valuable tool for characterizing damage sites before and after the mitigation process. We also demonstrated its utility as an in-situ diagnostic to guide and optimize our process when mitigating surface damage sites on large, high-value optics.

DOE accelerated strategic computing initiative: challenges and opportunities for predictive materials simulation capabilities

In response to the unprecedented national security challenges emerging from the end of nuclear testing, the Defense Programs of the Department of Energy has developed a long-term strategic plan based on a vigorous Science-Based Stockpile Stewardship (SBSS) program. The main objective of the SBSS program is to ensure confidence in the performance, safety, and reliability of the stockpile on the basis of a fundamental science-based approach. A central element of this approach is the development of predictive, ‘full-physics’, full-scale computer simulation tools. As a critical component of the SBSS program, the Accelerated Strategic Computing Initiative (ASCI) was established to provide the required advances in computer platforms and to enable predictive, physics-based simulation capabilities. In order to achieve the ASCI goals, fundamental problems in the fields of computer and physical sciences – of great significance to the entire scientific community – must be successfully solved. Foremost among the key elements needed to develop predictive simulation capabilities, the development of improved physics-based materials models is a cornerstone. We indicate some of the materials theory, modeling, and simulation challenges and illustrate how the ASCI program will enable both the hardware and the software tools necessary to advance the state-of-the-art in the field of computational condensed matter and materials physics.