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Today's most frequently used production path for polysilicon and solar grade silicon (SG-Si) is based on thermal decomposition of trichlorosilane (SiHCl3) in a Siemens-type reactor. Due to the batch-wise mode of its operation and its requirement for heating and cooling, the Siemens technology is an energy demanding procedure. As an alternative to this process, SG-Si can be produced through the thermal decomposition of monosilane (SiH4), for example in a fluidized bed reactor or a centrifugal chemical vapour deposition reactor. In order to avoid production of fine particulates and related clogging, this process requires knowledge of the nucleation properties of silane. In this work, the critical concentration for particle nucleation from SiH4, diluted in H2 has been determined for temperatures in the range 400 - 700°C. For this purpose, we used a free space reactor and an optical particle detector of own design for in-situ monitoring of the reactor exhaust. In a plot of logarithmic critical nucleation concentration against inverse temperature, an almost linear relation was found for temperatures below approximately 500°C and above approximately 600°C. Between 500 and 600°C, a less temperature dependent behavior is observed. Apparent activation energies are estimated to be 32kcal/mol for temperatures below 500°C, 8kcal/mol for temperatures between 500 and 600°C and 23 kcal/mol for temperatures above 600°C. The first and last of these values agree well with existing literature. The low temperature dependence in the range 500°C to 600°C, however represents a deviation from earlier studies. Investigations of reactor exhaust by GC-MS (gas chromatography - mass spectrometry), allowing detection of the concentrations of higher order silanes (disilane (Si2H6), trisilane (Si3H8), etc) as function of reactor temperature, suggest that the low temperature dependence of the nucleation concentration for some conditions is related to an increase of the concentration of Si2H6 and Si3H8 at the same temperature range

Critical Nucleation Concentration for Monosilane as Function of Temperature Observed in a Free Space Reactor