Characterization of polycrystalline silicon grown in a fluidized bed reactor

Megan Mureen Dahl

Due to an increase in demand for photovoltaic (PV) materials, the fluidized bed reactor (FBR) has become a popular alternative to the traditional and expensive method of producing and acquiring photovoltaic grade polycrystalline silicon (polysilicon). Traditional polysilicon acquisition for the PV industry relied on waste materials from the semiconductor industry that used the expensive Siemens reactor to produce the ultra pure silicon. Due to the recent application of fluidized bed technology to the silicon industry, it becomes necessary to characterize the product in order to fully understand its growth mechanisms as well as the most suitable handling techniques. The fluidized bed reactor makes use of pyrolytic decomposition of silane gas onto seed particles to form high purity polysilicon. The polysilicon granules were analyzed with various characterization techniques to establish their macro and microstructures. Optical microscopy reveals a ringed structure with a closer electron microscopy examination detecting that the dark contrast rings consist of a less dense, porous structure. Transmission electron microscopy and X-Ray diffraction analysis determined that the as received silicon has an average grain size of 30 nm. A minor amount of grain growth is observed with subsequent annealing cycles under 1050°C; significant abnormal grain growth is observed at higher annealing temperatures. Hardness data was collected to establish mechanical property details and their effects of annealing on them. Hardness of granules was determined to be approximately 10% less than that of single crystal material. Heat treatments had no significant effect on the hardness of the material at annealing times used in this thesis. In addition to the polysilicon granules produced in the FBR, a homogeneously nucleated material referred to as fines is produced. Fines are often considered a parasitic material due to the consumption of silane gas to form the particles. Analysis of the fines by TEM and Fourier transform infrared (FTIR) spectroscopy reveals that the mean and median diameters are 78 nm and that the particles are hydrogenated amorphous silicon. Annealing these particles causes non-diffusion limited crystallization determined by crystallization vs. amorphous ratios of various sized particles.

Characterization of polycrystalline silicon grown in a fluidized bed reactor

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