Novel diffusion pathways in low temperature self-organization of nanostructures

Surface diffusion is the key process responsible for mass transport on surfaces. Experiments can be carried out to measure surface diffusion either under equilibrium or non-equilibrium conditions. Equilibrium experiments can be used to measure the coverage and temperature dependence of the diffusion coefficient Dc(θ,T) which in turn is useful to deduce thermodynamic information about the system (adatom–adatom interactions, phase transitions, collective effects, etc.). Non-equilibrium experiments are more relevant to systems grown epitaxially because of the lower temperatures and higher deposition rates commonly used. Under these conditions metastable self-organized structures can be prepared within surprisingly short times, which suggests that some novel but still unclear kinetic pathways must exist at low temperatures. Such effects will be illustrated with the formation of uniform height Pb islands on Si(111) because of quantum size effects (QSE) and the formation of numerous “Devil's Staircase” phases in the dense Pb/Si(111)-α3×3 phase. In addition, the failure of the quasi-equilibrium description of nanostructure relaxation for sufficiently small sizes will be discussed to stress the need for better knowledge of the kinetic barriers controlling the relaxation.