**4. Conclusions**

In this work, we systematically investigated the influence of different carrier gases (O2, N2, and air) on the grown film quality on c-plane sapphire substrates by using the mist-CVD method. Although XRD and Raman measurements show that the pure corundum-structured α-Ga2O3 was successfully obtained on the c-plane sapphire substrates with single (0006) plane orientation for all three different carrier gases, the crystal quality could be greatly affected by the carrier gas. When O2 was used as the carrier gas, the sample showed the smallest FWHM of about 72 arcsec obtained from the XRD rocking curves. The UV-VIS measurement also showed that the sample grown with O2 as the carrier gas showed a very sharp absorption cutoff edge. It is proposed that the oxygen content in carrier gas should be responsible for all of these results. More oxygen in the carrier gas can greatly increase growth rate, and at the same time guarantee the film quality. This is further verified by the smoother surface and rapid growth rate (10.3 nm/min) for O2 as the carrier gas compared to air and N2 as the carrier gases (5.3 nm/min and 2.4 nm/min respectively). XPS analysis confirmed that more oxygen element can be included in epitaxial film and thus help improve the crystal quality. The proper carrier gas is essential for high quality α-Ga2O3 growth.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/22/3670/s1, Figure S1: TEM images of the sample grown with air as the carrier gas. (1) Cross-sectional α-Ga2O3/α-Al2O3 interface, (2) diffraction spots of α-Ga2O3/α-Al2O3, Figure S2: TEM images of the sample grown with N2 as the carrier gas. (1) Cross-sectional α-Ga2O3/α-Al2O3 interface, (2) diffraction spots of α-Ga2O3/α-Al2O3, Figure S3: X-ray photoelectron wide spectra for the α-Ga2O3 sample grown with air as the carrier gas, Figure S4: X-ray photoelectron wide spectra for the α-Ga2O3 sample grown with N2 as the carrier gas.

**Author Contributions:** Conceptualization, C.Z.; methodology, C.Z. and Y.X.; validation, Y.C. (Yaolin Cheng), Z.L., Y.C. (Ya'nan Cheng); formal analysis, Y.X. and Y.C. (Yaolin Cheng); investigation, C.Z. and J.Z.; resources, Y.H.; data curation, D.C.; writing—original draft preparation, Y.X.; writing—review and editing, C.Z.; visualization, Q.F.; project administration, C.Z.; funding acquisition, Y.H.

**Funding:** This research was funded by National Key R&D Program of China, gran<sup>t</sup> number 2018YFB0406504.

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