(Invited) Study on the Role of Thermal Stress on Prismatic Slip of Dislocations in 4H-SiC Crystals Grown by PVT Method

Abstract

Recent advances made towards the improvement of the quality of 4H silicon carbide (4H-SiC) have enabled many successful applications in the power device industry. Nevertheless, there is still a great interest in understanding the formation mechanism of crystal defects such as dislocations as well as their behavior during crystal growth. While basal plane slip is most frequently observed in 4H-SiC crystals grown by PVT method, prismatic slip of dislocations has also been reported before. [1] The glide of threading edge dislocations (TEDs) on the prismatic planes ({1-100}), for example, leaves long segments of prismatic dislocations in their wake, some of which, if adopting enough screw component, become screw type dislocations. On one hand, these screw dislocations can cross-slip onto the basal planes, generating more BPDs in the substrate crystals. On the other hand, the existence of screw BPDs segments with Burgers vector 1/3[11-20] in the substrate will be harmful for the quality of epilayers since such dislocations, if intersecting with substrate surface, will replicate into the epilayer without conversion to TEDs. In this study, we have observed three types of straight prismatic dislocation segments in a commercial wafer. These are three different types of dislocations with either screw or mixed characters and are formed by the prismatic slip of different types of TEDs with Burgers vectors and line directions along one of the three 1/3<11-20> directions. The three types of prismatic dislocations are each distributed non-uniformly in the wafer. We believe this non-uniformity is a result of the resolved shear stress distribution caused by the radial temperature gradient in the growing crystal boule, since TEDs only start gliding on the prismatic planes in regions where the critical resolved shear stress is reached. From thermal model simulation, we obtained the distributions of the resolved shear stress for all three types of prismatic dislocations by assuming an appropriate profile for the radial temperature variation. Based on these results, a simulated map can be generated to predict the spatial distributions of prismatic dislocations in the wafer. The map is then compared with X-ray topographic images and correlation is found between the two. It is shown that the prismatic dislocations are mainly distributed on the periphery of the boule and the exact distribution of each type depends highly on the difference in radial temperature profile. [1] Huanhuan Wang, Shayan Byrappa ,Fangzhen Wu, Balaji Raghothamachar, Michael Dudley, Edward Sanchez, Darren Hansen, Roman Drachev, Stephan G. Mueller, Mark J. Loboda, Materials Science Forum Vols. 717-720 (2012) pp327 Figure 1