| The high-current pulsed electron beam (HCPEB) technique features electron beams of low-energy～30keV, high peak current-103A, and short pulse duration～μs. The instantaneous energy density can be as high as1-10J/cm2, and the heating and cooling rates in the treated material can reach109K/s. Additionally, due to the high electron beam penetration capability of micrometers in depth, it can produce preferential sublayer melting and crater eruption, and eventually surface evaporation. Due to the differences of heat capacities, conductivities and other physical parameters, the electron beam energy is deposited preferentially on secondary phases, defects and grain boundaries. So the high-current pulsed electron beam technique is an effective method for surface purification via its unique crater eruption mechanism.High-purity silicon is widely demanded due to the rapid growth of the photovoltaic industries. A variety of purification techniques are being developed, and in all these purification processes, all the silicon atoms, together with the impurity ones, have to be refined a few times, which is the main cause of the high energy cost. Ideally, only the impurity part is removed, leaving the pure silicon matrix untouched. This is a selective purification route towards obtaining high-purity silicon. Owning to the crater eruption mechanism that removes impurity precipitates over a depth of a few micrometers, the HCPEB treatment makes the purification process energetically efficient and cost-effective. The high-current pulsed electron beam technique may become a new process for raw silicon purification.In this article,99%,99.999%polycrystalline silicon and99.9999%single crystal silicon are treated by the high-current pulsed electron beam. Surface and cross-section microstructures were observed by using optical microscopy (OM) and scanning electron microscopy (SEM). As expected, the craters are appeared on the surface, and the crater densities are decided by the silicon’s purities, the energy densities and pulses. Due to the high viscosity of molten silicon and its low purities contents, the crater densities are smaller and the size of crater is much bigger. The crater diameter in the cross-section profile is about322μm. Annular corrugations around the crater are often observed, which are presumably induced by the thermal stress generated by the eruption on the molten surface. The melting depth at3J/cm2,5pulses can reach about3.3μm, which is larger than depth of the simulation,2.4μm. It illustrates that more pulses can increased the purification depth. Raman spectroscopy shows red shifts, up to2.2cm"1, caused by the tensile stress due to impurity re-dissolution.The accompanied transient temperature field and stress field during the electron beam treatment were simulated through the finite difference and finite-element methods. At2.0J/cm2and3.0J/cm2, the highest temperatures can reach1416℃and1555℃respectively, the surface is completely melted. Also the sublayer can melt firstly, which supports the crater eruption mechanism. The highest temperature is only1412℃at1.8J/cm2, the surface is not completely melted and the thermal stress can’t be fully released, which could induce micro-cracks on the surface. During the treatment, there exist compressive stress on the surface, and tensile stress in the inner. The maximum compressive stress and tensile stress are260MPa and57MPa respectively. There exists stress concentration at the edge of the sample, and the stress release when the surface melt. |
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