A negative electron affinity (NEA) was found both experimentally by photoemission spectroscopy and theoretically by ab initio calculations for the 2 X 1 reconstructed diamond (100) surface. This surface is the dominant growth surface for thin diamond films and can be obtained by chemical-vapor deposition. Various surface-preparation methods which result in a NEA are described. Theoretical results indicate that the observed NEA is associated with a monohydride-terminated surface, while the hydrogen-free surface exhibits a positive electron affinity.
ABSTRACT:We describe the development and applications of a new electronic structure method that uses a real-space grid as a basis. Multigrid techniques provide preconditioning and convergence acceleration at all length scales and therefore lead to Ž . particularly efficient algorithms. The salient points of our implementation include: i new compact discretization schemes in real space for systems with cubic, orthorhombic, Ž . and hexagonal symmetry and ii new multilevel algorithms for the iterative solution of Kohn᎐Sham and Poisson equations. The accuracy of the discretizations was tested by direct comparison with plane-wave calculations, when possible, and the results were in excellent agreement in all cases. These techniques are very suitable for use on massively Ž . parallel computers and in O N methods. Tests on the Cray-T3D have shown nearly Ž . linear scaling of the execution time up to the maximum number of processors 512 . The above methodology was tested on a large number of systems, such as the C molecule, 60 diamond, Si and GaN supercells, and quantum molecular dynamics simulations for Si. Large-scale applications include a simulation of surface melting of Si and investigations of electronic and structural properties of surfaces, interfaces, and biomolecules.
The results of an extensive theoretical study of native defects in GaN and of vacancies in AlN are presented. We have considered cation and anion vacancies, antisites, and intersti-tials. The computations were carried out using quantum molecular dynamics, in supercells containing 72 atoms. Due to the wide gap of nitrides, the formation energies of defects depend strongly on the position of the Fermi level. The N vacancy in GaN introduces a shallow donor level that may be responsible for the n-type character of as-grown GaN.Other defects introduce deep states in the gap, with strongly localized wave functions.
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