Quantitative analysis of screw dislocations in 6H-SiC single crystals

Screw dislocations along the [0001] axis in 6H-SiC single crystals have been studied extensively by Synchrotron White-Beam X-ray Topography (SWBXT), Scanning Electron Microscopy (SEM), and Nomarski Optical Microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: I) the diameter of dislocation images in back-reflection topographs; 2) the width of bimodal dislocation images in transmission topographs; 3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and 4) the magnitude of the tilt of lattice planes in section topographs. The four methods; show good agreement. SEM results reveal that micropipes in the form of hollow tubes run through the crystal emerging as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice constant along the [0001] axis). There is no empirical evidence that Ic dislocations have hollow copes. The Burgers vector magnitude of screw dislocations, b, and the diameter of associated micropipes, D, were fitted to Frank's prediction for hollow-core screw dislocations: D = mu b(2)/4 pi(2) gamma, where mu is the shear modulus, and gamma is the specific surface energy. Statistical analysis of the relationship between D and b(2) shows that it is approximately linear, and the constant, gamma/mu, obtained from the slope, ranges from 1.1 x 10(-3) to 1.6 x 10(-3) nm.