Growth of Silicon Crystals Free from Dislocations

Abstract

The dislocations which occur in silicon crystals grown by the Czochralski method with and without a crucible are studied by etching and copper decoration techniques. A representative crystal with about 500 dislocations per cm² is examined in detail to provide general information about their distribution. Sources of dislocations which are important at particular stages of growth are isolated and means found to minimize or eliminate them. The sources include generation by thermal shock from surface damage or residual dislocations in the seed; poor epitaxy at the seed; plastic generation in the bulk of the crystal; effects of segregation of impurities. Plastic generation is the only source found in the bulk of a crystal. ``Spontaneous'' formation of dislocations from excess vacancies is not found. Impurity segregation causes some generation at the tip end of crystals. For certain crystallographic directions it is possible to eliminate residual dislocations and thenceforth grow a dislocation‐free crystal. This is interpreted in terms of climb. In a super‐saturation of vacancies, an edge dislocation can be forced completely out of a crystal; a screw will remain straight or may be distorted into a coaxial helix. By growth along 〈100〉 or 〈111〉 axes under conditions which favor a vacancy supersaturation, screw as well as edge and mixed dislocations can propagate to the surface and be eliminated. The observations are consistent with this interpretation. When the dislocations are eliminated, the growing crystal can withstand large temperature fluctuations without further generation of dislocations. A procedure is outlined for the growth of dislocation‐free silicon crystals. Germanium crystals free of dislocations have been grown by a direct application of the method. It is believed that the conclusions reached during the course of this investigation may be applied to the development of methods for growing dislocation‐free crystals of other substances.