Formation of SiH3 Radicals and Nanoparticles in SiH4–H2 Plasmas Observed by Time-Resolved Cavity Ringdown Spectroscopy

Abstract

The growth of nanoparticles between electrodes in a pulsed very high frequency (VHF) silane–hydrogen (SiH4–H2) plasma under hydrogenated microcrystalline silicon (µc-Si:H) growth conditions has been studied by time-resolved cavity ringdown (τ-CRD) spectroscopy. The light absorption of silyl (SiH3) radicals and the light scattering and absorption of nanoparticles have been measured in the UV spectral range (220 and 280 nm). The contribution of the SiH3 radicals and nanoparticles to the measured cavity loss could be distinguished by 1) varying the wavelength of the probe laser pulse, 2) using time-resolved information of the SiH3 radicals and the nanoparticle density, 3) the measured spatial distribution of the species between the electrodes, and 4) the dependence of these distributions on the electrode temperature. From the time evolution of cavity loss related to the nanoparticles growing in plasma, we can determine whether the light losses of the nanoparticles are in the Rayleigh or Mie regime. Subsequently, these measurements also provide information on typical particle size. Additional scanning electron microscopy (SEM) analyses, which reveal the evolution of the nanoparticle size distribution with time, corroborate the results obtained by τ-CRD. Finally, the spatial distribution of the SiH3 radical density and the electrode temperature dependence of the nanoparticles between the electrodes have been studied. These results are discussed in terms of the dominant forces acting on nanoparticles in plasma and the nanoparticle growth mechanism.