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Nakagomi and Kokubun (pp. 1738–1744) investigated the micro-structure of β-Ga2O3 thin films formed on c-plane and a-plane sapphire substrates prepared by gallium evaporation in an oxygen plasma. Cross-sectional transmission electron microscopy (TEM) images and electron diffraction patterns were obtained from a cross section of β-Ga2O3 on a sapphire substrate. The results indicate that TEM images and electron diffraction patterns can be classified into four types according to the rotation angle of the unit cell of (-201)-oriented β-Ga2O3. The authors explain the reason for the appearance of the four types of cross sectional TEM image using crystal orientation. The classification was summarized in a table that is expected to be useful for further studies on β-Ga2O3.