Young-Soo Ahn

Directional Solidification Behaviors of Polycrystalline Silicon by Electron-Beam Melting

The advanced electron beam melting (EBM) system with the combination of vacuum refining and directional solidification (DS) performed the purification of large amounts of metallurgical grade silicon (MG-Si). In order to increase grain size or to align columnar grains being parallel to DS pulling direction in Si ingots, non-irradiated inner diameters in an EB pattern in the DS process were varied at a range of 5–35 mm. Average grain size increased with increasing non-irradiated inner diameter due to a smaller temperature gradient during the solidification of Si melts. However, the slope of the grain boundary inclined towards the ingot axis, which led to the formation of a triple junction in the ingot center in the case of large non-irradiated inner diameter. This happened despite there being a large temperature gradient due to the turbulent flow in the pool. This work reported that a purity of 99.8% for MG-Si was improved to above 99.999% with an ingot yield of 90% for 1 h.

Impurity segregation behavior in polycrystalline silicon ingot grown with variation of electron-beam power

Electron beam melting (EBM) systems have been used to improve the purity of metallurgical grade silicon feedstock for photovoltaic application. Our advanced EBM system is able to effectively remove volatile impurities using a heat source with high energy from an electron gun and to continuously allow impurities to segregate at the top of an ingot solidified in a directional solidification (DS) zone in a vacuum chamber. Heat in the silicon melt should move toward the ingot bottom for the desired DS. However, heat flux though the ingot is changed as the ingot becomes longer due to low thermal conductivity of silicon. This causes a non-uniform microstructure of the ingot, finally leading to impurity segregation at its middle. In this research, EB power irradiated on the silicon melt was controlled during the ingot growth in order to suppress the change of heat flux. EB power was reduced from 12 to 6.6 kW during the growth period of 45 min with a drop rate of 0.125 kW/min. Also, the silicon ingot was grown under a constant EB power of 12 kW to estimate the effect of the drop rate of EB power. When the EB power was reduced, the grains with columnar shape were much larger at the middle of the ingot compared to the case of constant EB power. Also, the present research reports a possible reason for the improvement of ingot purity by considering heat flux behaviors.

Polycrystalline silicon wafer with columnar grain structure grown directly on silicon carbide coated graphite substrate

Concerning horizontally direct growth technology, graphite material with excellent thermal and chemical resistance is frequently used as both the crucible and the substrate in equipment operated at high temperatures. However, this can cause a major problem, namely, contamination due to the carbon and metallic impurities derived from graphite components. This study presents the experimental findings on a method of preventing the incorporation of unintended impurities into the silicon wafer by modifying the surface of the graphite. Coating the crucible and substrate with silicon carbide (SiC) could significantly reduce carbon (1.5 × 1017 atom/cm3) and metallic (undetected under analysis resolution) contamination by suppressing physical contact between the silicon melt and the graphite. It is expected that the purity of the silicon wafer will be improved to meet the demand for next-generation solar cell production with the optimum growth conditions required for obtaining the desired property of the silicon wafer.

Recovery of 4N-grade copper from photovoltaic ribbon in spent solar module

The study on the purification of copper from spent photovoltaic ribbon was conducted using zone-melting furnace to fabricate high-purity copper with 4N grade. In order to recover copper from spent photovoltaic ribbon, oxidation process was first conducted at the temperature range from 300 to 900 °C to remove coating layer. After oxidation, the oxidized coating layer consisting lead of 68.99 wt.% and tin of 31.21 wt.% was taken off from the substrate at room temperature. The chemical composition of copper ribbon after oxidation was analysed using ICP-MS (inductively coupled plasma mass spectrometry), and the purity of copper obtained was found to be about 99.5 wt.%. Further process using zone-melting furnace was consequently carried out, and 4N grade of copper (≥ 99.99%) was finally obtained.

Microstructure control of columnar-grained silicon substrate solidified from silicon melts using gas pressure

A silicon substrate with the dimensions of 100 × 140 × 0.3mm was grown directly from liquid silicon with gas pressure. The silicon melt in the sealed melting part was injected into the growth part at applied pressure of 780-850 Torr. The solidified silicon substrate was then transferred by the pull of the cooled dummy bar. A desirable structure with a liquid-solid interface perpendicular to the pulling direction was formed when the mold temperature in the solidification zone of the growth part was much higher than that of the dummy bar, as this technique should be able to overcome thermal loss through the molds and the limited heat flux derived from the very narrow contact area between the silicon melt and the dummy bar. In addition, because the metallic impurities and expansion of volume during solidification are preferably moved to a liquid phase, a high-quality silicon substrate, without defects such as cracks and impurities in the substrate, could be manufactured in the interface structure. The present study reports the experimental findings on a new and direct growth system for obtaining silicon substrates characterized by high quality and productivity, as a candidate for alternate routes for the fabrication of silicon substrates.

Effect of Processing Parameters on Thickness of Columnar Structured Silicon Wafers Directly Grown from Silicon Melts

In order to obtain optimum growth conditions for desired thickness and more effective silicon feedstock usage, effects of processing parameters such as preheated substrate temperatures, time intervals, moving velocity of substrates, and Ar gas blowing rates on silicon ribbon thickness were investigated in the horizontal growth process. Most of the parameters strongly affected in the control of ribbon thickness with columnar grain structure depended on the solidification rate. The thickness of the silicon ribbon decreased with an increasing substrate temperature, decreasing time interval, and increasing moving velocity of the substrate. However, the blowing of Ar gas onto a liquid layer existing on the surface of solidified ribbon contributed to achieving smooth surface roughness but did not closely affect the change of ribbon thickness in the case of a blowing rate of ≥0.65 Nm3/h because the thickness of the solidified layer was already determined by the exit height of the reservoir.