The use of heater-magnet module for Czochralski growth of PV silicon crystals with quadratic cross section

Abstract

Conventionally grown Czochralski (Cz) silicon crystals for photovoltaic (PV) application have an unfavourable cylindrical shape leading to essential material loss during the wafer cutting process. Additionally, the typical high oxygen concentration promotes solar cell degradation. In this paper a new pulling technology for growth of silicon crystals with both quadratic cross section and relatively low as-grown oxygen content is presented. A dynamic-magnetic-field-assisted Cz growth of facetted crystals is reported. At [001]-oriented growth in very low radial temperature gradient holding steady by a special traveling magnetic field (TMF) the growing crystal body becomes self-profiling by four {110} facets parallel to the pulling direction. To keep down costs the KRISTMAG˜® principle was used whereupon the Lorentz field and heat are simultaneously generated within a graphite heater design supplied by alternating (AC) multiphase current of various frequencies and phase shifts.The first experimental results show single crystalline Si crystals with reproducible square cross sections up to 91×91mm2 including rounded corners. Until now TMF frequencies of f=180 and 300Hz and a phase shift of φ=90° were applied. For high-purified material an average facet undercooling of ΔT≈2K has been deduced from the observed rectangular side plane widths. According to high-resolution transmission electron microscopy (HRTEM) the four macroscopically flat faces are microscopically composed of {110} sub-facets and {111} macrosteps. Etch pit densities (EPD) between 0 and 104cm−2 were ascertained. Due to the magnetically induced high-speed melt flow toroid around the growing crystal a relatively low and homogeneously distributed oxygen concentration can be achieved. A minimum value of 7.5×1017cm−3 was measured in high-purity as-grown crystals at a TMF frequency of f=300Hz.