**4. Conclusions**

In summary, we investigated the correlation between the impurity concentrations and PL properties of single 4H-SiC crystals prepared with the PVT method. The single 4H-SiC crystals grown with the PVT process were evaluated by using Raman spectroscopy, XRD, ICP-OES, UV-Vis spectroscopy, and PL measurements. As a result, XRD peaks at two prominent peaks at 36◦ and 76◦, which are reflections from the (0004) and (0008) planes, respectively, were found to correspond to 4H-SiC. After purification (S2 and S3), a few peaks of weak intensity were also separately observed at almost equal intervals, although the S1 sample had no small periodic peaks due to the large number of impurities. However, with XRD results, it is somewhat difficult to analyze the concentrations of trace impurities using peak shifts or weak minor peaks. Thus, we investigated the 4H-SiC crystal samples in terms of the trace impurity doping level by using the Raman and PL techniques. The Raman spectra of the 4H-SiC crystals that were grown were observed to have three characteristic peaks, which were detected at approximately 795, 800, and 970 cm<sup>−</sup>1. In particular, the LO peak intensities of S2 and S3 decreased, and the LO peak shifts of the S2 and S3 samples occurred at lower wavenumbers than those of S1. It may be considered that reason was that the LO phonon mode also caused a shift in peaks toward lower frequencies, which could probably be attributed to the decrease in the grain size, internal stress from impurities, and the atomic size effect. For the PL spectra at a low temperature (50 K), two prominent PL emission peaks were observed at 420 and 580 nm. These DAP emission peaks were attributed to the impurity concentration caused by the doping of N and B in the single 4H-SiC crystals. The value of 2*C*B/(*C*N − *C*B) was employed to evaluate the correlation between the N-B concentrations and PL. As the ratio of 2*C*B/(*C*N − *C*B) was less than 0.01, there were no emissions in the S3 sample. Since Al and N existed in the S1 sample, N-B DAP emissions were not observed due to due to the luminescence quenching by the N-Al DAP emissions at 420 nm. These results show that N-B DAP luminescence quenching can be extinguished depending on the concentrations of N and B impurities in 4H-SiC. When the 2*C*B/(*C*N − *C*B) ratio was 0.01 or less, the 4H-SiC samples with high *C*Al values showed no N-B DAP emissions at 420 nm. Thus, the PL technique is a useful technique for detecting the lower trace impurity doping levels in 4H-SiC crystal samples.

**Author Contributions:** Conceptualization, S.-K.K., E.Y.J. and M.-H.L.; methodology, S.-K.K. and M.-H.L.; software, S.-K.K. and E.Y.J.; validation, S.-K.K., E.Y.J. and M.-H.L.; formal analysis, S.-K.K., E.Y.J. and M.-H.L.; investigation, S.-K.K., E.Y.J. and M.-H.L.; resources, S.-K.K. and M.-H.L.; data curation, S.-K.K., E.Y.J. and M.-H.L.; writing, original draft preparation, S.-K.K., E.Y.J. and M.-H.L.; writing—review and editing, S.-K.K., E.Y.J. and M.-H.L.; visualization, S.-K.K. and E.Y.J.; supervision, E.Y.J. and M.-H.L.; project administration, M.-H.L.; funding acquisition, M.-H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Technology Development Program (No. S2797799) from the Ministry of SMEs and Startups.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Sang-Geul Lee at the Korea Basic Science Institute (Daegu) for their useful discussions and for providing the XRD data. In addition, the authors would like to thank Hongtaek Kim at Horiba Korea Ltd. for his useful discussions and for providing the PL data.

**Conflicts of Interest:** The authors declare no conflict of interest.
