Morphological Evolution of Ag/Mica Films Grown by Pulsed Laser Deposition
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Morphological Evolution of Ag/Mica Films Grown by Pulsed Laser Deposition Jeffrey M. Warrender & Michael J. Aziz Division of Engineering and Applied Sciences Harvard University ABSTRACT Many vapor-deposited metal-on-insulator films exhibit a morphological progression with increasing thickness consisting of several distinct stages: (1) nucleation of 3-dimensional nanocrystalline islands; (2) elongation of the islands; (3) film percolation. Here we report a study of this progression during Pulsed Laser Deposition (PLD), a technique for film deposition that differs from thermal deposition in that the depositing species arrive in short energetic bursts, leading to instantaneous deposition fluxes orders of magnitude higher than can be achieved in thermal growth. Atomic Force Microscopy reveals that advancement through this same morphological progression occurs at lower thickness in PLD films relative to films grown under comparable conditions by thermal deposition, with PLD films having lower RMS roughness at a given thickness. We also observe that for a constant amount deposited per pulse, films deposited at higher laser pulse frequency are further advanced in morphological state. Kinetic Monte Carlo simulations reveal that PLD nucleation behavior differs from that of thermally deposited films, and this can account for the observed differences. Simulations also reveal a scaling of the percolation thickness with pulse frequency that is consistent with experiment. INTRODUCTION Metal-on-insulator thin films typically grow according to the Volmer-Weber growth mode, in which atoms grow in 3D islands on the surface.1-5 As the islands grow larger, they impinge upon other islands. When this occurs, the islands may begin to coalesce, with atoms being exchanged between them, driving them to become a single island. Such is the case for liquid droplet growth.6 Metal nanocrystalline “droplets” also appear to undergo a “liquidlike” coalescence; however, rather than being much faster than deposition and surface processes, as is the case for liquid droplets, the time scale for nanocrystal coalescence is thermally activated, and thus occurs on the same time scale as deposition and surface diffusion. For low substrate temperatures, no redistribution of material whatsoever may be observed. At higher temperature, some restructuring takes place. The time scale for this process increases as the participating islands grow larger.7 As such, as discussed by Jeffers et al. 3, at some point, given a system of two coalescing islands, the time for a third island to grow to impinge with one of these islands becomes less than the time for the islands to coalesce. It is at this point that clusters of coalescing islands remain elongated on the surface; they have become kinetically frozen. Further deposition joins these elongated clusters, forming a tortuous network of channels that continues to fill in with subsequent deposition.2 The extensive knowledge base on metal-on-insulator film growth by thermal deposition or Molecular Beam Epitaxy (MBE
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