(Invited) In-Situ Observation of SiC Solution Growth: Observation Principle and Effect of Al Addition to Si-Cr Solvent
Authors: Kawanishi Sakiko , Yamada Tomohiro , Daikoku Hironori , Shibata Hiroyuki , Yoshikawa Takeshi 
Published: December 2024

Backgrounds
Solution growth of SiC bulk single crystals has been studied as a potential method for growing high-quality crystals with keeping high growth rate. The use of Si-Cr alloy solvent has led to rapid growth rates, and reduction of dislocations using macrosteps has been reported. On the other hand, when the degree of supersaturation is increased to achieve high-speed growth, the distribution of supersaturation causes step bunching, which leads to solvent and polytype inclusions. Therefore, we have established an in-situ observation method at temperatures above 1300 °C to clarify various phenomena at the growth interface. This observation method is based on the transmission property of SiC against visible light. Bright-field observation using a white LED light enables us to obtain the step distributions at the interface, and internal interference observation using a He-Ne laser (632.8 nm) enables to measure the step height and growth rate. In this study, we evaluated temperature dependence of the optical absorption properties of the major SiC polytypes with the aim of enabling the identification of the polytypes during the in-situ observation method. We also focused on Al, which is known to suppress bunching when added in small amounts to the solvent, and investigated the effect of Al on the SiC solution growth interface by in-situ observation.
Polytype identification during in-situ observation
The polytypes that easily occur at 1300-2000 °C are 3C, 4H, and 6H-SiC, each of which has its own band gap. Therefore, if the temperature dependence of the optical absorption property of each polytype is known, it is possible to determine the polytype from the observed image. Based on this, we measured the temperature dependence of the optical absorption properties of each polytype and evaluated the band gap. As a result, the band gap of each polytype decreased linearly with temperature, and was found to be larger in the order of 3C, 6H, and 4H-SiC in any temperature range. By referring to the optical absorption properties, it became possible to capture the distribution of different polytypes by using the difference in transmittance when they have a thickness of ~1 μm or more.
Effect of Al addition to Si-Cr solvent on SiC growth interface
It is known that the solvent system has a significant effect on the step structure of the growth interface in terms of polytype inclusions and step bunching formation. Especially, addition of a small amount of Al is considered to have advantage in improving crystal quality. Therefore, in-situ observation of the growth interface on an on-axis 4H-SiC(000-1) at 1600 °C was performed using Si-40 mol%Cr and Si-40 mol%Cr-2 mol%Al solvents. Without Al, formation of step bunching was observed during the progress of step flow growth, which was induced by the small off-angle of the seed crystal. Eventually, the terrace expanded and bunching of the surrounding steps progressed, resulting in significant roughening of the interface. In contrast, with Al, the growth interface was covered by aligned steps, and step flow growth continued with steps of 10–20 nm in height. Therefore, it was found that Al prevents the formation of step bunching, leading to the suppression of macrostep formation.
Conclusions
We have established in-situ observation method of SiC solution growth interface for clarifying various phenomena at the growth interface. Polytype identification during the in-situ observation was proposed as a further technique, and the temperature dependence of the optical absorption properties of SiC polytypes were summarized as fundamental data. It was also revealed that the addition of Al to the Si-Cr solvent has an effect to suppress the formation of step bunching at the SiC growth interface.