Mechanical properties of polycrystalline silicon solar cell feed stock grown via fluidized bed reactors

Abstract

Polysilicon granular beads grown via a fluidized bed reactor, a feedstock for silicon solar cell production, were annealed, sectioned, and indented using a combination of nanoindentation and microhardness testing to determine the mechanical response of this commercially available raw material. The granular material, with macroscopic dimensions on the order of millimeters and an internal grain size on the order of 20 nm, has an indentation modulus of approximately 160 GPa, and a hardness prior to fracture of 9.6 GPa; these values are relatively insensitive to annealing at temperatures between 600 and 1100 °C. Indentation fracture testing suggests the toughness of this material is on the order of 0.6 MPa m^1/2. The fracture sequence has been verified using acoustic emission testing during indentation. Annealing in air at 600 °C for 3 days increases the toughness by approximately 50% with little change in grain size. The as grown material contains solute hydrogen, identified by infrared spectroscopy, from the growth process; annealing in air tends to remove solute hydrogen from the material at temperatures above 1050 °C. The removal of solute hydrogen appears to cause slight increases in toughness, while grain growth at elevated annealing temperatures or the formation of hydrogen complexes in the silicon appears to decrease toughness. The results suggest thermal treatments of silicon grown with this method can moderately alter the friability of the final product.