The Relationship Between Micropipes and Screw Dislocations in PVT Grown 6H-SiC
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ABSTRACT The growth surface of a 6H-SiC boule, grown by physical vapor transport, was examined using scanning force microscopy. The dimensions of surface/micropipe intersections and screw dislocation Burgers vectors have been determined from topographic data. All micropipes are positioned along the lines of super screw dislocations with a Burgers vectors of at least 4 times the c-axis repeat distance (15.2 A). Perfect c-axis screw dislocations with Burgers vectors of only 15.2 A are stable and do not have open cores. Measurements show that micropipe core radii, determined indirectly from the width of the craters formed at the surface/micropipe intersections, increase with the square of the dislocation Burgers vector. INTRODUCTION Micropipes are cylindrical voids with diameters in the 1-10 l1m range that are found oriented along the c-axis of SiC crystals grown from the vapor phase. For example, in crystals grown by the physical vapor transport (PVT) process, these defects occur with a density of 50103 cm-2 and extend throughout most of the boule volume [1,21. Because micropipes limit the operating range of high voltage SiC devices, their elimination has important practical implications [3]. The objective of this paper is to specify the nature of this extended defect. A possible mechanism for the stabilization of such a long, narrow cylindrical void was proposed by Frank in 1951 [4]. If a dislocation with a sufficiently large Burgers vector threads the length of the crystal, it can be energetically favorable to replace the most highly strained part of the crystal in the vicinity of the dislocation line with an empty cylinder. Frank demonstrated that a state of local equilibrium can be achieved by balancing the elastic energy of the dislocation against the surface energy of facets bounding a narrow cylinder. One of the central predictions of Frank's theory is that the pipe radius (ro) should be proportional to the square of the Burgers vector (b); the relevant physical parameters that determine the proportionality are the surface energy (y) and the shear modulus (G): b22 - 7' 8 n2 y
r0
(1)
G
There are several pieces of evidence supporting the idea that micropipes are, indeed, empty core screw dislocations. First, Verma [5] noted that micropipes intersect the growth surface at the origins of optically visible spiral steps. Also, measurements based on optical micrographs of the growth surface have been used to show that the core radius increases with 539 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
the height of the spiral step [6]. More recently, white beam synchrotron topography was used to show that micropipes in PVT grown 6H-SiC crystals were empty core screw dislocations with Burgers vectors 3 to 7 times the c lattice constant (15.2 A) [7]. In the present paper, we present quantitative scanning force microscopy (SFM) measurements of the dislocation Burgers vectors and micropipe radii based on topographic images of the growth surface of a 6H SiC crystal produced by the PVT method. Our
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