Propagation and Density Reduction of Threading Dislocations in SiC Crystals during Sublimation Growth
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1069-D07-01
Propagation and Density Reduction of Threading Dislocations in SiC Crystals during Sublimation Growth Ping Wu, Xueping Xu, Varatharajan Rengarajan, and Ilya Zwieback Wide Bandgap Materials Group, II-VI Incorporated, 20 Chapin Rd., Suite 1005, Pine Brook, NJ, 07058 ABSTRACT SiC single crystal wafers grown by sublimation exhibit relatively high dislocation densities. While it is generally known that the overall dislocation density tends to decrease throughout crystal growth, there has been a limited quantitative analysis of such trend. In this study, we measured the density of threading dislocations in the wafers sliced from several SiC boules. Although the dislocation density in the wafers sliced from different boules could differ by orders of magnitude, a consistent empirical relationship was found between the dislocation density (ρ) and the axial wafer position within the crystal (w): ρ ∝ w −0.5 . Monte Carlo simulations were performed based on two assumptions: (i) during growth the threading dislocations move randomly in the lateral directions, and (ii) two dislocations of opposite sign annihilate when they come within a critical distance between them. Good agreement was achieved between the model and experimental results. The critical distance determined from the simulations was in the range between a few hundred Ă and a micron. INTRODUCTION Silicon carbide crystals grown by sublimation are used for the development and manufacturing of SiC and GaN semiconductor devices of new generation. In order to produce low-defect epilayers and high-quality devices, especially for SiC homoepitaxy, the substrate must have good crystal quality, including, low dislocation density. Previously it has been reported [1,2] that threading screw dislocations as well as overall dislocation density decreases during growth. Generally, this was attributed to the generation of dislocations, especially screw dislocations, at the growth interface, followed by a gradual reduction. While the threading dislocations propagate towards the dome of the SiC crystal, those having opposite Burgers vectors can interact with each other and annihilate. However, there are limited data to quantitatively characterize this trend. In this study, we studied changes in the threading dislocation densities through the growth of several 6H SiC crystals. Monte-Carlo simulations based on a simple annihilation model have been carried out to provide additional information on this trend. EXPERIMENT 6H SiC single crystals were grown by the PVT technique and sliced into wafers normal to the c-axis using a multi-wire diamond saw. All wafers were nominally 400 micron thick, with
the spacing between two adjacent wafers corresponds to 650 micron in the direction of crystal growth. The wafers were numbered in the direction from seed to the boule dome, that is, wafer # 1 was the one adjacent to the seed. The 6H crystals used in this study were either undoped or doped with vanadium to ~1×1017 cm–3. Characterization of the dislocation density was performed using et
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