Micropipes in silicon carbide crystals: Do all screw dislocations have open cores?
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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 dislocations 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 densities of these micropipes vary between 10 and 103 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 between 103 and 105 cm−2. These possess a range of Burgers vectors. Those with the smallest, which have no optical-microscopically visible micropipes, are most prevalent. These are not considered as detrimental to device performance as the micropipes, without the micropipes’ combination of larger Burgers vector and hollow dislocation core. Therefore, it is an important question whether or not these smallest dislocations, whose Burgers 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 conditions, using various approximations, over past decades. J. Mater. Res., Vol. 15, No. 8, Aug 2000
The first and simplest was published by Frank, who calculated the radius of a cylinder that a screw dislocation 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 increase in free energy used to create the hollow core, the diameter is D ⳱ b2/42␥
,
where is the shear modulus and ␥ is the surface energy of the material. Quantities der
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