RAPID COMMUNICATIONS The purpose of this Rapid Communications section is to provide accelerated publication of important new results in the fields regularly covered by Journal of Materials Research. Rapid Communications cannot exceed four printed pages in length, including space allowed for title, figures, tables, references, and an abstract limited to about 100 words. Micropipes in silicon carbide crystals: Do all screw dislocations have open cores? William M. Vetter and Michael Dudley Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275 (Received 16 December 1999; accepted 12 May 2000) Micropipes in a 6H–SiC semiconductor wafer were studied by scanning electron and atomic force microscopy. The screw dislocations intersecting the wafer’s surface were located by etch pitting, and their Burgers vectors determined by x-ray topography. The etch pits were eroded into smooth craters by ion beam etching to expose levels of dislocation line from inside the sample’s bulk. There a micropipe’s diameter is distant from surface relaxation effects. Hollow cores (micropipes) were observed at the base of the craters whose screw dislocations had Burgers vectors of magnitude three multiples of the c-lattice parameter and higher. Screw dislocations with 1c and 2c Burgers vectors had no associated micropipes. Silicon carbide devices are primarily fabricated through deposition of SiC epilayers on SiC substrates. 1 The most important defects that invariably occur in physical vapor transport (PVT)-grown SiC boules are micropipes. 2 These are the hollow cores of screw dislo- cations with large Burgers vectors, integral multiples of the c-lattice parameter of the SiC crystal. 3 The hollow cores extend along the growth direction of the crystal. Semiconductor wafers cut from these boules have holes in them, visible under an optical microscope. The densi- ties of these micropipes vary between 10 and 10 3 cm -2 . 4 The presence of micropipes in high voltage diodes has been shown to cause their failure under reverse bias conditions. 5 X-ray topography shows screw dislocations occurring in commercially available SiC wafers in densities be- tween 10 3 and 10 5 cm -2 . These possess a range of Burg- ers vectors. Those with the smallest, which have no optical-microscopically visible micropipes, are most prevalent. These are not considered as detrimental to de- vice performance as the micropipes, without the micro- pipes’ combination of larger Burgers vector and hollow dislocation core. Therefore, it is an important question whether or not these smallest dislocations, whose Burg- ers vectors equal the c-lattice parameter of the crystal, have hollow cores whose widths lie below the resolution limit of optical microscopy. There has been a series of theoretical calculations of widths of micropipes under different equilibrium condi- tions, using various approximations, over past decades. The first and simplest was published by Frank, who calculated the radius of a cylinder that a screw disloca- tion should have in an infinite crystal at equilibrium. 6 According to Frank’s pediction, when the surface free energy of the interior cylindrical surface equals the in- crease in free energy used to create the hollow core, the diameter is D 4 mb 2 /4p 2 g , where m is the shear modulus and g is the surface energy of the material. Quantities derived from this relation are, of course, dependent on the value of m/g. Later, various other theoreticians predicted that, below a critical strain level or supersaturation, the formation of a hollow core is not thermodynamically favored and that the core radius varies with the degree of supersatu- ration during growth conditions. 7–9 Srolovitz calculated that where a dislocation intersects a surface, the hollow core will flare into a catenoid, or trumpet-shaped pit; the equilibrium radius changing with the distance from its surface intersection. 10 Funnel-shaped pits have been seen to occur on the growth surfaces of SiC crystals. 11 It is important to note that Srolovitz’ calculations show that smaller trumpet-shaped pits may appear at a growth surface–dislocation intersection in cases where the Burg- ers vector of the dislocation is too small to form a hollow core in the bulk of the crystal. J. Mater. Res., Vol. 15, No. 8, Aug 2000 © 2000 Materials Research Society 1649