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A9.33.1

Enhanced Surface Diffusion in Low-temperature a-Si:H Processing. George T. Dalakos1, Joel L. Plawsky2, and Peter D. Persans3 General Electric Global Research Center, Niskayuna, NY 2 Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 3 Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 1

ABSTRACT Glow discharge amorphous hydrogenated silicon (a-Si:H) prepared at near room temperature typically results in an inhomogeneous morphology that is undesirable for a number of thin film applications. The most commonly observed features of this include columnar morphology and surface roughness. This usually results from anodic deposition, where substrates are placed on the grounded electrode. We have discovered that placing substrates on the RF-powered electrode (referred to as cathodic deposition) offers a much wider processing range for homogenous growth than anodic growth. We have also found that the magnitude of the surface roughness and the bulk void fraction of both anodic and cathodic a-Si:H thin films processed at low-temperatures is proportional to ~D/F, where D is the surface diffusivity and F, the adatom flux, though anodic and cathodic deposition affect these global parameters differently. Surface processes unique to cathodic deposition can enhance adatom surface diffusion, while diffusion during anodic deposition is fixed and cannot attain homogeneous growth at high adatom fluxes. Processing a-Si:H on the cathode, associated with enhanced adatom surface diffusion, allows for homogeneous growth even at high deposition rates that has benefits for a number of applications. INTRODUCTION The growth of amorphous thin films by vapor condensation typically occurs far from equilibrium. Under such conditions, kinetics, rather than thermodynamics, drives formation of the final morphology. The morphology formed will depend upon the competition between the film precursor flux to the surface and adsorbed species' lateral movement upon the surface. This is observed in low-temperature epitaxy studies, as the incident species flux is increased, lateral motion is interrupted by new island nucleation resulting in 3-D mounded growth [1]. On the other hand, decreasing the incident flux results in longer diffusion lengths on the surface eventually resulting in layer-by-layer growth. This may be extended to the deposition of a-Si:H by plasma-enhanced chemical vapor deposition (PECVD) near room temperature. The final morphology under these conditions usually exhibits a columnar-type structure [2]. In PECVD, a higher incident flux is driven by the gas-phase generation of film precursors (e.g., varying the RF input power) while the surface diffusion is a function of the substrate temperature. This means that PECVD process parameters can be used to control the final material morphology. The diffusion of adsorbed species (sometimes referred to as an “adatom”) on a surface depends on the energy threshold for site hopping, E D and the surface temperature, T. This is shown by the surface diffu