High temperature deformation behaviour of ultra-high purity polycrystalline silicon

Ultra-high purity polycrystalline silicon was deformed by compression at temperatures from 1123 to 1643 K and at strain rates from 1×10^-5 to 1×10^-4 s^-1, and the dislocation structures developed during high temperature deformation were also observed by transmission electron microscopy to obtain the fundamental knowledge on the high temperature deformation of polycrystalline silicon. It was found that the flow stress for the polycrystalline silicon deformed at 1123 K and at a strain rate of 1×10^-4 s^-1 was much greater than that for the single crystalline silicon (FZ-Si) deformed under the same condition. The observed difference in the flow stress was considered to be attributed to the existence of stable and fine twin boundaries, which could operate as the barriers for dislocation motion. The steady-state deformation, where the flow stress was almost independent of strain, was observed at temperatures over 1473 K. The stress exponents, n, were found to be about 7 at 1473 K and about 5 at 1573 and 1643 K, respectively. The activation energy for deformation at temperatures, where the stress exponent was about 5, was in good agreement with that for the lattice self-diffusion in silicon. Therefore, 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.