  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.
