GROWTH OF NANOMETER-SIZE METALLIC PARTICLES ON CAF2(111)

Fe, Co, and Ag particles grown on various CaF2 substrates have been st udied using ultrahigh vacuum scanning electron microscopy with nanomet er resolution. Fe and Co show a very high nucleation density which is remarkably independent of deposition temperature in the range 20<T(300 degrees C, on both bulk CaF2(111), and on thin CaF2(111) films grown on Si(111). This feature is characteristic of nucleation at defect sit es with a h-gh trapping energy. An atomistic nucleation model has been extended to cover this case. The comparison with experiment requires adsorption, pair binding, and defect trapping energies all to be aroun d 1 eV. The trapping sites occupy 1% of the surface, and are thought t o be chemical (F-vacancy, oxide, or hydroxide) in nature. In contrast, the growth of Ag on the same substrates shows a more usual nucleation and growth pattern, though the growth of Ag on Fe islands shows inter esting features which are discussed. A self-similar coalescence model is tested using the data obtained. The agreement is excellent for Ag, while Co and Fe show the expected deviations due to limited surface di ffusion around the islands. (C) 1996 American Institute of Physics.