High temperature deformation behaviour of ultra-high purity polycrystalline silicon

Ultra-high purity polycrystalline silicon was deformed by compression at te mperatures from 1123 to 1643 K and at strain rates from 1 x 10(-5) to 1 x 1 0(-4) s(-1), and the dislocation structures developed during high temperatu re deformation were also observed by transmission electron microscopy to ob tain the fundamental knowledge on the high temperature deformation of polyc rystalline silicon. It was found that the flow stress for the polycrystalline silicon deformed at 1123 K and at a strain rate of Ix 10(-4) s(-1) was much greater than tha t for the single crystalline silicon (FZ-Si) deformed under the same condit ion. The observed difference in the flow stress was considered to be attrib uted to the existence of stable and fine twin boundaries, which could opera te as the barriers for dislocation motion. The steady-state deformation, wh ere the flow stress was almost independent of strain, was observed at tempe ratures over 1473 K. The stress exponents, n, were found to be about 7 at 1 473 K and about 5 at 1573 and 1643 K, respectively. The activation energy f or deformation at temperatures, where the stress exponent was about 5, was in good agreement with that for the lattice self-diffusion in silicon. Ther efore, it is concluded that the recovery controlling mechanism governed by the lattice diffusion is the rate controlling mechanism for the deformation of polycrystalline silicon at these temperatures.