Characterization of Micropipes and Other Defect Structures in 6H-SiC Through Fluorescence Microscopy
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W. M. Vetter,* M. Dudley,* and J. SUNY T. Fredrich*** *Dept. of Materials ScienceT.-F. and Wong** Engineering, at Stony Brook, Stony Brook, NY 11794-2275 "**Dept. of Earth & Space Sciences, SUNY at Stony Brook, Stony Brook, NY 117942100 ***Geomechanics Dept., Sandia National Laboratories, Albuquerque, NM 87185-0751 ABSTRACT
Crystals of silicon carbide, and other polytypic materials often have micropipes associated with screw dislocations of large Burgers vectors running along their axial dimensions. These defects are considered the most deleterious to the performance of SiC semiconductor devices. Optical micrographs of micropipes in silicon carbide crystals are ordinarily faint. To obtain micrographs showing higher contrast and detail, laser scanning confocal microscopy (LSCM) and simple fluorescence microscopy were used on 6H-SiC single crystals after infiltrating them with a low-viscosity epoxy containing a fluorescent dye. "Staining" the micropipes rendered them much more visible both in fluorescence and conventional optical microscopies. Details of their structures and shapes were revealed, and their dimensions were measured accurately, using LSCM and other, less sophisticated, fluorescence microscopies. Other voids present, such as microcracks, were also visualized. Observations by this optical technique were related to information obtained by synchrotron white beam x-ray topography. INTRODUCTION
Various optical methods have been devised whereby dislocations in transparent crystals may be characterized. Birefringence topography and light scattering tomography have been the most important of these[I]. In the former, optical inhomogeneities in the crystal induced by strain fields are viewed between crossed polars. In the latter, dislocations decorated by the precipitation of small particles scatter light, rendering their course through the crystal visible. Here, we present a more recently developed method, which is based on making voids associated with crystal dislocations fluorescent. Crystals of silicon carbide, and other polytypic materials often have screw dislocations running along their axial dimensions whose Burgers vectors are multiples of the c-parameters of their unit cells[2]. These have been termed superscrew dislocations. Typically, when superscrew dislocations have a Burgers vector of more than several c, hollow tubes, referred to as micropipes, can be observed coinciding with the position of their dislocation cores. Such hollow cores were theoretically predicted for dislocations with large Burgers vectors by Frank[3]. Micropipes present in SiC crystals are of particular importance, being considered the most deleterious to the performance of semiconductor devices fabricated from this material. On various occasions, optical micrographs of micropipes present in SiC crystals have been presented[4]. The contrast of these cylindrical voids has always been faint. To obtain micrographs showing higher contrast and detail, we have borrowed a method used in the petrography of geomaterials such as sandstone
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