Dislocations in directionally solidified crystalline silicon: Generation mechanisms and interactions

Abstract

The growth of crystalline silicon from crucibles by directional solidification will invariably cause structural defects in the material. In addition, the casting process leads to the incorporation of impurities, and the combination of these defects will severely degrade the material. It is therefore highly desirable to develop new growth processes to control and depress the defect level. However, without a comprehensive knowledge of the formation mechanisms and interactions of dislocations, any attempt to remove the defects is likely to be ineffective or unsuccessful. In the present work dislocation structures on polished and etched samples were investigated to provide insight into some of the unanswered questions on the dislocation behaviour in crystalline silicon.

One of the main questions that arises is what causes the dislocation generation. Investigations of a 10 mm stack of multicrystalline wafers showed dislocation clusters that originate at grain boundaries. The investigations induced a grain boundary study, which illustrated some characteristics of the special boundaries in silicon. Dissociation and faceting of the grain boundaries into lower energy CSL-boundaries reduces the total GB energy. However, the presence of GB defects such as steps and junctions can form stress concentrators on the boundaries, which in turn can act as dislocation nucleation centres.

A high temperature anneal of bent multicrystalline silicon rods demonstrated the difficulties in the recovery of dislocations in silicon wafers. High temperatures and long annealing times are necessary to achieve a homogenous polygonizated structure. Polygonized dislocation structures were therefore found to be in a very early stage of development. Y-shaped etch-pit structures illustrated the coarsening mechanism of polygonization. In addition, it was found that in regions with more than one activated slip plane dislocations interactions result in a square shaped cell pattern.

An investigation of quasi-monocrystalline silicon produced by seeded growth revealed that the CZ-seeds had a high density of dislocations after the growth process. Furthermore, the structural differences of the dislocation clusters imply that they are consequences of different generation mechanisms. A cellular dislocation structure which is different from the structures found in as-grown crystals, suggests the importance of image forces on the structures formed during crystal growth. The cell structure is assumed to form due to surface defects on the seeds at temperatures above 900 °C. Clusters around the seed interface are the result of an indentation mechanism, and dislocation rosettes are found on both seed surfaces in contact. The same generation mechanism also takes place at the bottom surface where similar dislocation rosettes are formed.