Dislocation formation at Σ= 27a boundaries in multicrystalline silicon for solar cells

Investigations with optical microscope and SEM have been carried out in order to study the microstructure of silicon wafers from the bottom parts of a silicon block, where the main focus was to study dislocation formation at Σ=27a boundaries. A Σ=27a boundary was found to dissociate into relatively large and stable triangular grains consisting of Σ=3 and Σ=9 boundaries, where one of the Σ=9 boundaries was an asymmetric CSL-boundary. The asymmetric Σ=9 boundary lowered its energy by dissociating further into Σ=3 boundaries. Dislocations were produced in the junctions between the symmetric and asymmetric Σ=9 boundaries. Some of these will be geometrically necessary due to translations at the facet junctions. Further dislocation formation is expected to be caused by stress concentrations which develop at these boundary defects during crystallization and cooling of the silicon ingot. Also, different etching behaviour at boundaries is discussed.