Crystallization Kinetics of Plasma-Produced Amorphous Silicon Nanoparticles

Abstract

The use of a continuous flow non-thermal plasma reactor for the formation of silicon nanoparticles has attracted great interest because of the advantageous properties of the process [1]. Despite the short residence time in the plasma (around 10 milliseconds), a significant fraction of the precursor, silane, is converted and collected in the form of nanopowder. The structure of the produced powder can be tuned between amorphous and crystalline by adjusting the power of the radio-frequency excitation source, with higher power leading to the formation of crystalline particles. Numerical modeling suggests that higher excitation power results in a higher plasma density, which in turn increases the nanoparticle heating rate due to the interaction between ions, free radicals and the nanopowder suspended in the plasma [2]. While the experimental evidence suggests that plasma heating may be responsible for the formation of crystalline powder, an understanding of the mechanism that leads to the crystallization of the powder while in the plasma is lacking. In this work, we present an experimental investigation on the crystallization kinetic of plasma-produced amorphous powder. Silicon nanoparticles are nucleated and grown using a non-thermal plasma reactor similar to the one described in [1], but operated at low power to give amorphous nanoparticles in a 3-10 nm size range. The particles are then extracted from the reactor using an orifice and aerodynamically dragged into a low pressure reactor placed in a tube furnace capable of reaching temperatures up to 1000°C. Raman and TEM have been used to monitor the crystalline fraction of the material as a function of the residence time and temperature. It is expected that for a residence time in the annealing region of approximately ∼300 milliseconds, a temperature of at least 750 °C is needed to observe the onset of crystallization. A range of crystalline percentages can be observed from 750 °C to 830 °C. A discussion of particle growth and particle interaction, based on experimental evidence, will be presented with its relation to the overall effect on crystallization. Further data analysis allows extrapolating the crystallization rate for the case of this simple, purely thermal system. We conclude that thermal effects alone are not sufficient to explain the formation of crystalline powder in non-thermal plasma reactors.