Characterization of mc-Si directionally solidified in travelling magnetic fields

Solar-grade boron doped silicon has been directionally solidified in a vertical gradient freeze-type furnace equipped with KRISTMAG®-heaters to study the influence of travelling magnetic fields (TMFs) on the ingot quality. As-grown silicon ingots of 22×22×13 cm3 in volume were cut vertically and analysed. Information was obtained on the curvature of the melt–solid interface, the grain size distribution, the content of SiC and Si3N4 particles and the electrical activity of defects. TMFs were used to enhance melt stirring and to control the growth interface shape. Primarily inclusion-free ingots were solidified with grains of several centimetres in size. Minority carrier lifetimes of τ=20–30 μs were measured on polished surfaces of cuts from as-grown ingots. The concentrations of carbon, oxygen and nitrogen were determined by FTIR spectroscopy to (3–4)×1017 atoms/cm3, (2–3)×1017 atoms/cm3 and (0.6–2)×1015 atoms/cm3, respectively. Mean etch pit densities were evaluated on vertical cuts as low as (2–3)×103 cm−2.

► Application of travelling magnetic fields.
► Control of growth interface shape and morphology.
► Unseeded directional solidification process and large grains.
► Inclusion-free silicon ingots with low mean etch pit densities.