Adatom Adsorption and Diffusion on the GaAs(100)-c(4x4) Surface

We have modeled the microscopic processes of Ga and As adatom diffusion on the reconstructed GaAs(100)-c(4x4) surface using the recently developed Car-Parrinello Projector Augmented Wave method (CP-PAW).( P. E. Blöchl, Phys. Rev B 50), 17953 (1994) This new method is capable of ab-initio molecular dynamics, (2) can treat atoms across the periodic table with equal ease, (3) allows direct access to the full valence and core wave functions, and (4) has been implemented as a fully parallel code. A simple repeated slab model of a GaAs surface was used: each unit cell consisted of a thin film of GaAs sandwiched between a single passivating H layer on one side and approximately 10 Åof vacuum on the other. The efficiencies inherent in the PAW formalism and its parallel implementation allowed us to conduct a series of large scale convergence tests (with respect to system size and plane wave cutoff) using unit cells with as many as twenty GaAs layers and cutoffs as high as 30 Rydbergs. We find that diffusion for both Ga and As is strongly anisotropic, primarily due to the presence of rows of As dimers on this surface. Supported by NSF, DOE, the Cornell Theory Center, and the Ohio Supercomputer Center.