Quantitative Modelling of Nucleation Kinetics in Experiments for Poly-Si Growth on SiO 2 by Hot-Wire Chemical Vapor Depo
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QUANTITATIVE MODELLING OF NUCLEATION KINETICS IN EXPERIMENTS FOR POLY-Si GROWTH ON SiO2 BY HOT-WIRE CHEMICAL VAPOR DEPOSITION Maribeth Swiatek, Jason K. Holt and Harry A. Atwater Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology Pasadena, CA, 91125, USA
ABSTRACT We apply a rate-equation pair binding model of nucleation kinetics [1] to the nucleation of Si islands grown by hot-wire chemical vapor deposition on SiO2 substrates. Previously, we had demonstrated an increase in grain size of polycrystalline Si films with H2 dilution from 40 nm using 100 mTorr of 1% SiH4 in He to 85 nm with the addition of 20 mTorr H2. [2] This increase in grain size is attributed to atomic H etching of Si monomers rather than stable Si clusters during the early stages of nucleation, decreasing the nucleation density. Atomic force microscopy (AFM) measurements show that the nucleation density increases sublinearly with time at low coverage, implying a fast nucleation rate until a critical density is reached, after which grain growth begins. The nucleation density decreases with increasing H2 dilution (H2:SiH4), which is an effect of the etching mechanism, and with increasing temperature, due to enhanced Si monomer diffusivity on SiO2. From temperature-dependent measurements, we estimate the activation energy for surface diffusion of Si monomers on SiO2 to be 0.47 ± 0.09 eV. Simulations of the temperature-dependent supercritical cluster density lead to an estimated activation energy of 0.42 eV ± 0.01 eV and a surface diffusion coefficient prefactor of 0.1 ± 0.03 cm2/s. H2-dilution-dependent simulations of the supercritical cluster density show an approximately linear relationship between the H2 dilution and the etch rate of clusters. INTRODUCTION Hot wire chemical vapor deposition (HWCVD) is an attractive method for growth of polycrystalline silicon thin films for thin film photovoltaics applications [3,4]. A key issue is to identify growth conditions that enable the largest possible grain size at a given growth temperature with low intragranular defect density. Hydrogen is known to play a critical role in the development of a crystalline microstructure in both polycrystalline [5,6] and epitaxial [7] films grown by HWCVD at low temperatures. The role of atomic hydrogen produced by the wire in the etching of Si and its effect on the resulting film microstructure are investigated through experiments and quantitative modelling of the nucleation kinetics of Si on SiO2. EXPERIMENT All experiments were performed at pressures of no higher than 1x10-6 Torr. A W wire of 0.25 mm diameter was resistively heated to 2000ºC and positioned 2.5 cm from the substrate. The wire radiatively heated substrates consisting of 100 nm SiO2 on Si to 300ºC; higher substrate temperatures were achieved by heating with a resistive substrate heater in combination with the wire. H2 dilutions are referenced to 1 mTorr of SiH4 in 99 mTorr He; all gases used are ultrahigh purity. A translatable shutter between the wire and substrate enabled
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