Morphology and size distributions of islands in discontinuous films

Abstract

We have investigated sizes and shapes of islands in discontinuous gold films. These were UHV deposited in an electric field parallel with the surface of the glass substrates. Films with average thickness t in the range 10⩽t⩽45Å have been studied. The two-dimensional island shapes are represented by ellipses where, from experiments, both the major and minor axes obey log-normal distribution functions. These facts can be reconciled with a statistical model of island coalescence. Empirically, the median island diameter is approximately proportional to t, whereas the geometric standard deviation σ falls in the range 1.25⩽σ⩽1.30, irrespective of thickness. We define a mean eccentricity ē as an average over the ratio between major and minor axes; as expected ē increases with increasing t. Supplementing the size determinations with evaluations of area fraction of the substrate covered with metal we find the three-dimensional form of the islands to be accurately approximated by prolate spheroids (symmetry axes parallel with substrate plane). Finally, we have studied the role of field-induced coalescence which appears to be significant already at t=20 Å.