Thermal stress as the major factor of defect generation in SiC during PVT growth
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Thermal stress as the major factor of defect generation in SiC during PVT growth D. I. Cherednichenko, R. V. Drachev1, I. I. Khlebnikov, X. Deng2 and T. S. Sudarshan Electrical Engineering Department, University of South Carolina, Columbia, SC 29208, USA 1 Band Gap Technologies Inc. Columbia, SC 29208, USA. 2 Mechanical Engineering Department, University of South Carolina, Columbia, SC 29208, USA. ABSTRACT Numerical simulations of the thermal stress distribution in a SiC boule 2” in diameter and 1” long grown by conventional PVT technique were performed based on the temperature field distribution in a resistively heated growth reactor that was simulated using the GAMBIT2.0.4/FIDAP-8.6.2 software package. Analysis of the simulation results revealed the existence of a thermal stress, which was excessively nonuniform in distribution and whose magnitude exceeded the value of the critical resolved shear stress of 1.0 MPa by a factor of 2. The high stress initiated plastic deformation and the high temperature provoked the intense self-diffusion processes. The combination of these factors alters the mechanism of plastic deformation, significantly affecting the structural quality of the growing crystal. The influence of selfdiffusion processes initiating the formation of interstitial atoms and vacancies; stacking fault formation as a result of the nonconservative motion of the basal plane dislocations; and micropipe formation from the dislocation groups piled up at silicon and carbon second phase inclusions are also discussed. INTRODUCTION The presence of micropipes and a high density of dislocations in PVT grown SiC still represent major obstacles for the commercial production and stable operation of high power SiC electronic devices. The mechanisms of formation of these defects are not completely understood [1]. It has been demonstrated that the growth rate of SiC PVT is positive and stable only if sufficient heat dissipation through the growing crystal volume is provided [2, 3]. This basically implies correspondingly increasing temperature gradients (axial and radial), which result in the associated enhancement of thermal stresses in the growing crystal. In this work the generation of threading dislocations, micropipe formation and nucleation of polytype heterogeneities in SiC during PVT growth are considered in detail as thermal stress induced phenomena. THERMAL STRESS DISTRIBUTION Numerical simulation of thermal field distribution in the furnace of conventional resistively heated design [4] suited for 2” diameter and 1” long SiC ingot growth was performed using the GAMBIT-2.0.4/FIDAP-8.6.2 software package. An analysis of the simulation results (Fig1(a)) showed that the radial temperature distribution throughout the crystal volume is significantly nonlinear, while the temperature gradients in the axial direction are nearly constant. Consequently, a calculation of thermal stresses in the 2” × 1” SiC boule was conducted using ANSIS-5.7.1 software package and the FIDAP simulated temperature distribution in the
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