Analysis of the Characterization of Impurities and Defects in Cast Monocrystalline Silicon with Shadows

The shadows phenomenon often appears in the infrared flaw detection map due to the accumulation of impurities in the growth process of cast monocrystalline silicon(CMC-Si). Shadows reduce the quality of silicon crystal and the conversion efficiency of the solar cells. This paper focuses on impurities and crystal defects in shadows and studies them in detail. Firstly, the distribution of carbon, oxygen and iron impurity in the CMC-Si brick with shadows are analyzed. Three samples were cut from the brick with shadows at the height of 75 mm, 145 mm, and 185 mm, respectively. The results of Fourier Transform Infrared Spectrometer (FTIR) show that the substitutional carbon concentration shows a trend of increasing first and then decreasing, and the substitutional carbon concentration in the area with shadows is significantly high. The interstitial oxygen concentration shows a decreasing trend, and carbon impurities promote the formation of oxygen precipitation. The results of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) show that the impurity iron increased first and then decreased, and the iron concentration in the area with shadows is significantly high. In addition, SEM & EDS results show that the dislocations in the area with shadows have two morphologies: ordered dislocations and disordered dislocations, and silicon carbide precipitates are observed at dislocation etch pits and subgrain boundaries. EBSD results show that there are many wide strip-shaped grains with growth plane of (122) and narrow strip-shaped grains with growth plane of (313) in the area with shadows and they are formed sigma 3 grain boundary with the matrix (100) plane, respectively.

Automated summary

This study analyzes the impurities and crystal defects in the shadows of cast monocrystalline silicon and provides detailed insights. The results show that the concentration of substitutional carbon and iron in the shadow area is significantly high, while carbon impurities promote the formation of oxygen precipitation. Additionally, the study reveals the presence of two types of dislocations and the observation of silicon carbide precipitates at dislocation etch pits and subgrain boundaries in the shadow area.