DIFFRACTION CHARACTERIZATION OF ROUGH FILMS FORMED UNDER STABLE AND UNSTABLE GROWTH-CONDITIONS

Characterizing the roughness of epitaxial films by diffraction techniq ues with respect to the step density and the rms roughness is well est ablished. For self-affine surfaces the morphology of growing films, ho wever, is often characterized by the correlation length xi of the heig ht-height correlation and the roughness exponent a governing the behav ior at small lateral distances. Recently, it has been emphasized that for unstable growth conditions, characteristic lengths (average pyrami d sizes) appear that produce an oscillating character of the height-he ight correlation. Here we investigate the influence of both kinds of c orrelations on the diffraction spots. The oscillating correlation caus es a splitting of the diffuse shoulder into satellites. The satellite position and half-width show characteristic oscillations depending on the scattering condition. From the latter one can determine the roughn ess exponent a. The correlation length 5 and the characteristic length can be evaluated from the satellite half-width and position at the ou t-of-phase scattering condition taking into account the rms height w. This model has been applied to the statistical growth of Ag adlayers o n Ag(lll) at low temperatures where the satellites of the diffuse shou lder point to the formation of pyramids. From the phase dependence we obtain the roughness exponent alpha=1/2. The step density and the corr elation length 5 increase with increasing coverage while no coarsening of the pyramid sizes is observed.