Confinement of Screw Dislocations to Predetermined Lateral Positions in (0001) 4H-SiC Epilayers Using Homoepitaxial Web
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Confinement of Screw Dislocations to Predetermined Lateral Positions in (0001) 4H-SiC Epilayers Using Homoepitaxial Web Growth Philip G. Neudeck, David J. Spry1, Andrew J. Trunek1, J. Anthony Powell2, and Glenn M. Beheim NASA Glenn Research Center, 21000 Brookpark Road, M.S. 77-1, Cleveland, OH 44135, U.S.A. 1 OAI, 21000 Brookpark Road, M.S. 77-1, Cleveland, OH 44135, U.S.A. 2 Sest, Inc., 21000 Brookpark Road, M.S. 77-1, Cleveland, OH 44135, U.S.A. ABSTRACT This paper reports initial demonstration of a cantilevered homoepitaxial growth process that places screw dislocations at predetermined lateral positions in on-axis 4H-SiC mesa epilayers. Thin cantilevers were grown extending toward the interior of hollow pre-growth mesa shapes etched into an on-axis 4H-SiC wafer, eventually completely coalescing to form roofed cavities. Each completely coalesced cavity exhibited either: 1) a screw dislocation growth spiral located exactly where final cantilever coalescence occurred, or 2) no growth spiral. The fact that growth spirals are not observed at any other position except the central coalescence point suggests that substrate screw dislocations, initially surrounded by the hollow portion of the pre-growth mesa shape, are relocated to the final coalescence point of the webbed epilayer roof. Molten potassium hydroxide etch studies revealed that properly grown webbed cantilevers exhibited no etch pits, confirming the superior crystal quality of the cantilevers. INTRODUCTION AND BACKGROUND Axial screw dislocations are unpredictably distributed in high densities across all commercial 4H- and 6H-SiC wafers [1]. In agreement with Frank’s theory, screw dislocations with large Burgers vectors form hollow core “micropipes”, while hollow cores are absent from screw dislocations with smaller Burger’s vectors. Micropipe defects are well established to be unacceptable in SiC power devices, as junctions containing micropipe defects are unable to standoff high electric fields without excessive leakage current and microplasma formation [2]. Therefore, considerable efforts were undertaken with reasonable success to reduce micropipe densities in commercial SiC wafers to around 10 micropipes per square centimeter of wafer area [1]. However, much less attention has been paid to reducing the density of closed core screw dislocations, whose densities are more than 100 times more plentiful than micropipes in commercial SiC wafers. Closed core axial screw dislocations have also been shown to degrade the blocking properties of SiC high-field junctions, but to a much lesser degree than micropipes [3,4]. Therefore, commercialization of some SiC power devices containing closed core screw dislocation defects has been possible, as non-ideal device behavior has been largely dealt with by appropriate device design, yield screening, and blocking voltage de-rating [5]. Despite the successes in reducing micropipe densities and realizing modestly rated SiC power Schottky diodes on commercial 4H-SiC wafers, there is nevertheless an increasing body of
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