Polytype Stability and Defect Reduction in 4H-SiC Crystals Grown via Sublimation Technique
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T.Iakimov , M.Syvdijarvi , H.Jacobsson , P.Raback , A.Vehanen , E.Janzdn
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aDept of Physics and Measurement Technology, Linkoping University, S-581 83 Link6ping, Sweden bOkmetic AB, Box 255, 17824 Eker6, Sweden CCenter for Scientific Computing, P.O. Box 405, FIN-02101 Espoo, Finland dOkmetic Ltd., PO Box 44, FIN-01301 Vantaa, Finland ABSTRACT Reproducible growth of 4H-SiC with good crystalline quality has been obtained in a temperature interval around 2350'C and on 4H-SiC C-face seeds. It has been observed that morphological instability may appear at the initial stage of growth, causing formation of defects. Experimental evidence has been found that supersaturation and surface kinetics are responsible for the polytype stability, while growth front and growth mode address defect reduction. An explanation of the findings has been suggested. It has been shown that starting the growth with a relatively low growth rate ( = 100 jim/h ) can be beneficial for the crystal quality. INTRODUCTION Silicon Carbide (SiC) is a material of expectation for high temperature power switching and high frequency power generation. While SiC may offer an exclusive combination of physical and electronic properties for many applications, the high temperature and chemical stability of this material, as well as the variety of stacking sequence along the c-direction in the close-packed structure of SiC, cause difficulties for growth of device quality crystals, especially of large single crystals. Among different crystallographic modifications of SiC, 4H polytype is the most interesting for power device applications. However due to the low stacking fault energy it is difficult to restrict syntaxy (parasitic polytype formation) during bulk crystal growth and thus to grow a single polytype material. Another well known problem is the large number of structural defects such as micropipes, mosaicity and dislocations. Moreover, these problems are interrelated to a large extent; defects easily result in polytype disturbances [1], while polytype inclusions may lead to defect formation [2]. However, little is known about the kinetics and thermodynamics of polytype formation, growth stability, and also the mechanism that produces the periodic sequences. As discussed in Reference [3] 3C-SiC may be the initial polytype that forms at virtually all growth temperatures and thus acts as a necessary precursor for the phase transformation to other polytypes. Several growth parameters, such as the growth temperature [4,5], supersaturation [3,4], vapor phase stoichiometry and impurities [3,6] and polarity of seed surface [7] have been discussed to influence the polytype stability. Seeded sublimation growth has been the most successful method to date for growth of large 4HSiC boules that can be sliced into wafers. 4H wafers of 50 mm diameter are commercially available and 35 mm wafers with 7 micropipes (0.7 cm 2 ) have been reported [8]. Generally, it is more difficult to grow 4H polytype in comparison with 6H-SiC, considering the size of
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