Emission spectroscopy of laser-ablated Si plasma related to nanoparticle formation

Abstract

We report on the laser ablation of Si in vacuum, and in the presence of helium ambient at 1 and 10Torr, respectively. The silicon nanoparticles were deposited on silicon substrate at room temperature by ablating silicon wafer in ambient atmosphere of helium at 1Torr. The mean cluster size ranging from 1.8 to 4.4nm is observed depending on the laser intensity. Optical emission spectroscopy and images of the plume are used to study the spatial and temporal variation of the silicon plasma. The electron density, measured by the Stark-broadening of Si I transition 3p21S–4s1P0 at 390.55nm and temperature, assuming thermal equilibrium, were found to be 1.2×1018cm−3 and 2eV, respectively. The temporal variation of Si I transition 3p21S–4s1P0 at 390.55nm showed a shift in peak position attributed to collisions at an early stage of plasma formation. The relative concentration of Si II/Si I estimated by using the Saha–Boltzmann relation showed abundance of Si I. Time resolved images of the plume were used to investigate the dynamics of the expanding plasma plume, estimating the vapor pressure, vapor temperature, velocity, and stopping distance of the plume. The photoluminescent spectra of the Si thin films showed three distinct emission bands at 2.7, 2.2 and 1.69eV, the origin of these bands is attributed to defects and quantum confinement.