The effects of gas temperature gradient, pulse discharge modulation, hydrogen dilution, gas flow, and substrate materials on growth of clusters below about 10 nm in size in silane parallelplate RF discharges are studied using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method. Thermophoretic force due to the gas temperature gradient between the electrodes drives neutral clusters above a few nm in size toward the cool RF electrode. Pulse discharge modulation is much more effective in reducing the cluster density when it is combined with the gas temperature gradient, and clusters above a few nm in size cannot be detected by the PCLLS method even for the discharge over a few hours. Hydrogen dilution and gas flow are also effective in suppressing growth of clusters, when the H2/SiH4 concentration ratio is above about 5 and the flow velocity is above about 6 cm/s, respectively. Cluster growth rate with a glass or Si substrate is found to be considerably higher than that without the substrate.