Effect of thermal annealing on defects of upgraded metallurgical grade silicon

Effect of thermal annealing on the upgraded metallurgical grade(UMG)-Si was investigated under different conditions. The dislocation, grain boundaries and preferred growth orientation of Si ingot were characterized by optical microscopy, electron back scattering diffraction (EBSD) and X-ray diffractometry (XRD), respectively. The arrange order of dislocation density of Si ingot is from the lowest in the middle to the lower in the bottom and low in the top before and after annealing. And it decreases gradually with increase of the annealing temperature. The number of small angle grain boundaries declines gradually until disappears whereas the proportion of coincidence site lattice (CSL) grain boundaries increases firstly and then decreases. The twin boundary Σ3 reaches the highest proportion of 28% after annealing at 1 200 °C for 3 h. Furthermore, the crystal grains in different positions gain the best preferred growth orientation, which can promote the following machining of Si ingot and the conversion efficiency of solar cells.