CI. Observations on carborundum of growth spirals originating from screw dislocations

Abstract

Abstract Numerous growth spirals with shapes ranging from circular to regular hexagonal have been observed on carborundum crystals of types I. and II., by coating the crystal faces with a thin film of silver of reflectivity nearly 90 per cent and then working in reflection, using phase contrast illumination. The shapes of these spirals, in relation to the crystal structure, are shown to be in accordance with the predictions of Frank's theory. On a type II. crystal, hexagonal interlaced spirals have been observed. The annihilation of growth fronts where they meet, and other properties of growth fronts are illustrated and hence the observed patterns for two or more screw dislocations are explained. The observed density of dislocations ranges from small values up to a maximum of 104 per sq. cm. For a typical circular spiral the calculated values of the radius of the critical nucleus is 2μ and supersaturation is 0·2 per cent. By the application of multiple-beam interference, using both Fizeau fringes and fringes of equal chromatic order, the step heights in spirals have been measured accurately for type II. crystal and found to be 15 Å. This is equal to the height of the unit cell, proving that these are growth spirals originating from screw dislocations, in exact accordance with theoretical prediction.