Papers by Author: Kui Xian Wei

Abstract: The multicrystalline silicon wafers purified by directional solidification route were used to introduce copper impurities. The resistivity and minority carrier lifetime of multicrystalline silicon wafers were investigated by four-point probe resistivity tester and μ-PCD, respectively. Annealing temperature, atmosphere and cooling rate were researched. It was found that copper contaminants have a greater impact on the electrical properties of multicrystalline silicon. Research results showed that copper impurities tend to exist at defect sites at high temperature, and high annealing temperature, argon atmosphere and slow cooling conditions make more impact on the electrical properties of multicrystalline silicon than low annealing temperature, air atmosphere and fast cooling.

Abstract: The structural defects including dislocations and grain boundaries (GBs) in upgraded metallurgical grade silicon (UMG-Si) prepared by vacuum directional solidification were investigated. The results demonstrated that higher withdrawal rates increased the dislocation density. The state of melt growth changed from quasi-equilibrium to non-equilibrium, and the GB type was also highly related to the withdrawal rate, especially for ∑3 boundary. The change of total interfacial energy and increase of carbon concentration may be a possible driving mechanism for this phenomenon.

Abstract: Annealing and Al gettering were performed on upgraded metallurgical grade multicrystalline silicon (UMG multi-Si) wafers with a purity of 99.999%. The dislocation and grain boundaries of samples were characterized by optical microscopy and electron back scattering diffraction (EBSD), respectively. The minority carrier lifetime and resistivity of the Si wafers were measured using microwave photoconductance decay and four-point probe techniques, respectively. The results show that the number of dislocations in Si wafers reduced obviously after annealing and Al gettering for 2 hours at 600～1100°C. The proportion of Σ3 grain boundary increases. But the minority carrier lifetime and resistivity of the Si wafers after annealing decreases. However, the minority carrier lifetime and resistivity of the Si wafers after Al gettering increases firstly and then decreases with increasing of the annealing temperature. It is considered that the metal impurities determine electrical properties of UMG multi-Si wafers rather than dislocations and grain boundary. However, Al gettering can enhances the properties of Si wafers effectively and the optimal effect of Al gettering has been achieved at 800°C.

Abstract: The high ash coke breeze was leaching by two-stage chemical leaching. X-ray diffraction (XRD) and X-Ray Fluorescence (XRF) were employed to characterize the mineral phase and the element content respectably. More than 80% ash has been removed from the coke breeze by two-stage leaching. The removal of Si and Al was significantly affected by the temperature of alkali leaching. The mineral phase of the result coke breeze is similar to that of the raw ash except with cristobalite and sodium mica.

Abstract: The pot material of Sb-doped n-type mono-crystal silicon was purified by our selfassembled vacuum directional solidification furnace. In the experimental, the pulling rate was 7μm/s, 10μm/s, 20μm/s, 30μm/s and 40μm/s, respectively. The experimental concentration of Sb, Al and Fe at 0.3 proportion of silicon ingot bottom to the top is detected and the theoretical concentration is calculated. The experimental results show that the pulling rate causes a great effect to concentration of Sb and a small effect to concentration of Fe. At the same time, the theoretical distribution concentration and experimental distribution concentration were compared. The experimental route provided a promising idea for complex utilization of the pot material.

Abstract: The main raw material of solar energy is multi-crystalline silicon. Directional solidification technique is one important technological process of metallurgy purification technology for multi-crystalline silicon. It can purify metallurgical grade silicon by removing metal impurities and control crystal growth at the same time. In experiment, metallurgical grade silicon by acid leaching pre-treatment, was purified by our self-assembled directional solidification furnace. The sample was analyzed by electron-prode micro analysis (EPMA). According to the results, the removal efficiency of Fe and Al is 96.3% and 96.7%, respectively. The removing mechanism of metal impurities and the difference between theory value and experiment value were also discussed. The segregation effect in directional solidification is the reason of removing Fe, but analgesic effects of the segregation effect combined with vacuum volatilization are that of removing Al. When the silicon ingot was cooled down, lengthways section of silicon ingot was cut and etched, crystal growth was studied. The results indicate that columnar crystal growth shows diverging tendency from the bottom to the top of silicon ingots, and solidification interface shape is convex. The reasons may be the nucleation of new crystals on crucible sidewall is very serious and the pulling rate is too high.