(254b) Rates of Silicon Nucleation and Condensation during Silane Pyrolysis

Photovoltaic devices based on crystalline silicon hold the largest (>70%) market share.   To continue the tremendous decrease in the cost to produce silicon solar cells that has occurred over the last decade, the silicon refining process must be made more energy efficient.  A key energy intensive step in silicon refining is the Siemens process, by which a gaseous silicon precursor reacts to form polycrystalline silicon by a chemical vapor deposition (CVD) reaction.  A more energy efficient alternative to the Siemens process is fluidized-bed CVD, which is often carried out using silane as the gaseous silicon precursor.  Fluidized-bed CVD reactors have demonstrated significant savings compared to the Siemens process, however the reactors are plagued by issues related to undesirable homogenous nucleation of fine particles, which occurs in parallel to the desired CVD reaction.  Despite decades of research on silane pyrolysis, the reaction mechanism under industrially relevant conditions, which is the key to understanding fine particle nucleation, is still under debate.  In this work we report on particle size distribution measurements under systematically controlled silane pyrolysis conditions.  Particle size distributions were measured by online techniques such as scanning mobility particle spectrometry (SMPS) as well as offline techniques such as electron microscopy for primary particle analysis.  The evolution of the particle size distribution and silicon mass concentration in the aerosol as a function of reactor space time were used to determine relative rates of CVD and homogenous nucleation under different reactor conditions.  This information will be coupled to discrete-sectional modeling of the aerosol dynamics to describe the different mechanistic pathways of silane pyrolysis.