Silicon carbide (SiC) is a promising material for next-generation power device.  The performance of the SiC power device strongly depends on the crystal quality of SiC substrate.  State of the art bulk growth of SiC crystals is carried out by the seeded sublimation growth method.  In the early stage of the development, the unique macro-defect i.e. “micropipe,” was a critical problem.  4 inch wafers are commercially available with almost zero micropipe density, today  In current growth technology, however, crystals include the other extended defects (threading dislocations and basal plane dislocation) affecting the reliability.

Recently we have achieved the growth of ultra-high quality SiC crystal with top-seeded solution growth method utilizing the conversion phenomenon from threading dislocations to basal pane defects.  For instance, the density of threading screw dislocation (TSD) in the crystal grown by the solution method (20-30 cm^-2) was much lower than that of the crystal produce by using conventional sublimation method (~1000 cm^-2).  Moreover, it is easier to decrease the defect density in the short-term growth compared to the RAF (repeated a-face) method which is well-known as the specially modified sublimation method.

The key to high-quality crystal growth is to utilize the defect conversion due to the macrostep flow. Figure 1 shows the X-ray topographic images of 4H-SiC layer grown on vicinal (0001) Si-face taken at the same position before and after growth.  Before the growth, we can observe many rice-shape images corresponding to TSDs.  After the growth, most of the TSD images changes to line images extending towards the step-flow direction corresponding to the Frank-type plane defects on basal planes.  The conversion phenomenon is induced by the macrostep flowing over the TSD on the surface, and the frequency of the conversion depends on the height of macrostep.  In addition, the macrostep flowing leads to the conversion from threading edge dislocations (TEDs) to defects on basal planes as well.  These conversion phenomena can improve the quality of SiC crystal.  As shown in Fig. 2, the threading dislocations propagate to the grown layer, while the defects lying on basal planes are finally wiped out.  Actually, we have successfully achieved the ultra-high quality SiC crystal utilizing the conversion phenomenon.

Several years ago, it was believed that the bulk growth of solution method was impossible because of low growth rate, the difficulty of polytype control, enlargement of crystal size and so on.  Recently, many groups can overcome these problems.  Each solution is still independent now, but we cannot realize them together.  The last remaining problem that is to merge these solutions and develop an unified technique.  We firmly believe that it will be completed within a few years.

Fig. 1: X-ray topographic images of 4H-SiC grown on a vicinal (0001) seed crystal before and after growth.  The rice-shape image (TSD) was changed to the line image (defect on basal plane).

 Fig. 2: (a) Without macrostep-flow, most of threading dislocations propagate to the grown layer.  (b) With macrostep-flow, threading dislocations are converted to defects on basal planes and they are wiped.  Consequently high-quality crystal is achieved.

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