AB-INITIO CALCULATIONS OF ENERGIES AND SELF-DIFFUSION ON FLAT AND STEPPED SURFACES OF AL AND THEIR IMPLICATIONS ON CRYSTAL-GROWTH

Using density-functional theory we investigate properties of Al(111), Al(100), Al(110), and stepped Al(111) surfaces, including formation en ergies of surfaces, steps, adatoms, and vacancies. For adsorption and diffusion of Al on flat regions of Al(111) surfaces the hcp site is en ergetically slightly preferred over the fee site. The energy barrier f or self-diffusion on Al(111) is very low (0.04 eV). Close to either of the two sorts of close packed, monoatomic steps on Al(111), Al adatom s experience an indirect attraction of less than or similar to 0.1 eV with the edge of the step, which has a range of several atomic spacing s and is of electronic origin. At the lower step edge, an adatom attac hes with no barrier at a low-energy fivefold coordinated site. Coming from the upper terrace, it incorporates into the step by an atomic exc hange process, which has a barrier below 0.1 eV for both sorts of clos e-packed steps. The barrier for diffusion along the lower edge is 0.32 eV at the {100}-faceted step and 0.39 eV at the {111}-faceted step. U nexpectedly, the latter diffusion process proceeds by an exchange mech anism. Diffusion by an exchange mechanism is also found for the ''easy '' direction on the Al(110) surface, i.e., along the channels. We show that Al(110) is a model system for diffusion at the {111}-faceted ste p on Al{111} because of its similar local geometry. We estimate temper ature ranges for different modes of homoepitaxial growth on Al(111). O f particular importance are the rather low barriers for diffusion acro ss the descending steps and the rather high barriers for diffusion alo ng the steps. We discuss island shapes on Al(111) during growth and in thermodynamic equilibrium. Depending on the temperature the growth sh apes can be fractal, triangular, or hexagonal and mainly determined by kinetics; in equilibrium the island shape is hexagonal and determined by the different step formation energies. Many of these phenomena hav e been seen experimentally for other metals.