One of Many Sources of Defect Generation in SiC
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One of Many Sources of Defect Generation in SiC Igor I. Khlebnikov, Tangali S. Sudarshan University of South Carolina, College of Engineering and Information Technology, Columbia, SC 29208, U.S.A. Yuri I. Khlebnikov Bandgap Technologies Inc., Columbia, SC, U.S.A. Colin Wood Office of Naval Research, Arlington, VA, U.S.A. ABSTRACT Silicon carbide is a unique material for the study of process of defect generation and crystallization. In this paper, for the first time, we report the observation of the entrapment of whiskers and dendrites (tree-like defects) within the volume of the growing monocrystalline SiC. The encapsulation of the tree-like defects in the volume of the grown crystal leads to solid state polytype transformation. According to Oswald rule, the most probable transformation sequence is as follows: 2H→3C→xR→4H, 6H until a stable phase is established for the given conditions of crystal growth. We observe that the entrapments of tree-like defects are the source of SiC defects such as micropipes and planar defects. It is very likely that the above process is also the source of dislocations. Practically, every branch of the tree-like structure generates the above mentioned defects (micropipes, planar defects, etc.). Our investigation (by EDAX) shows that the chemical composition of the tree-like defect is the same as that of bulk SiC. In this paper, we will present the mechanism of the entrapment of the tree-like defects in the bulk crystal. INTRODUCTION The initial stage of crystallization on the seed during PVT bulk SiC growth occurs during non-equilibrium conditions, when all the process parameters are changing. The transition process of establishing stationary conditions initiates such negative processes as the replication of seed micropipes, polytype inheritance, polytype transformation, generation of dislocations, planar defects and inclusions. Our investigation shows that processes occurring on the seed-boule interface and the first ~1mm of grown material determine the quality of the grown SiC ingot. Defects generated at the initial stage of crystallization propagate in the direction of boule growth. Usually, the first 2 to 10 mm of grown crystal is defective and not suitable for device applications. Consequently, boules >10 mm in length must be grown to obtain good bulk crystal; hence longer boules are desired. However, our investigations show that optimization of the conditions of crystallization at the initial stage may significantly reduce the transition phase at the initial stage of growth which may significantly reduce the transition layer thickness and defect concentration within the transition layer This will maximize the amount of usable boule volume, and result in near 100% yield. In this report we will focus on the characteristics and the nature of defects generated within the transition layer.
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EXPERIMENT
Using the modified Lely technique [1], bulk crystals both 6H and 4H were grown in a reactor with flexible temperature control in the crystallization zone during the growth proce
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