Fig. 1. (Color online) Monoclinic unit cell of β-Ga₂O₃ and relative lattice parameters.

Fig. 2. (Color online) Bulk β-Ga₂O₃ crystals obtained by the following melt methods: (a) EFG, (b) VB, (c) Czochralski, and (d) OFZ. Copyright 2017 ECS Journal of Solid State Science and Technology^[28]. Copyright 2016 J Cryst Growth^[13]. Copyright 2014 J Cryst Growth^[47]. Copyright 2004 J Cryst Growth^[11].

Fig. 3. (Color online) Theoretical limits of on-resistance versus breakdown voltage in unipolar power devices for β-Ga₂O₃ and other major semiconductors. Copyright 2014 Physical Status Solidi^[29].

Fig. 4. Cross-sectional TEM image of a dislocation viewed from [ ]. Copyright 2015 Jpn J Appl Phys^[38].

Fig. 5. (Color online) (a) Optical micrograph of surface of ( ) plane after etching. (b) Enlarged image of the etch pits. An XTEM sample is indicated by a white line. Copyright 2016 Jpn J Appl Phys^[39].

Fig. 6. Cross-sectional TEM image of a hollow nanopipes defect viewed from [ ]. Copyright 2015 Jpn J Appl Phys^[38].

Fig. 7. (a) A cross-sectional FIB-SIM image of an PNPs defects. Copyright 2016 Jpn J Appl Phys^[39]. (b) SEM image of three NSGs in an array along the [001] direction. Copyright 2016 Jpn J Appl Phys^[40].

Fig. 8. (Color online) DICM images showing line-shaped voids exposed on the surface etching for (a) 5 min and (b) 25 min. (c) Length histogram of the 625 line-shaped voids observed on the 5-min etched surface. Copyright 2017 Jpn J Appl Phys^[44].

Fig. 9. (a) An SEM image of etched surface of ( ) plane including a twin boundary. (b) The atomic structure of a region including a twin lamellae in β-Ga₂O₃ crystal viewed from the ( ) direction. Copyright 2016 Jpn J Appl Phys^[39].

Fig. 10. (Color online) AFM images of an arrow-shaped etch pit. Copyright 2017 Jpn J Appl Phys^[45].

Fig. 11. (Color online) AFM image of a gourd-shaped etch pit. Copyright 2017 Jpn J Appl Phys^[45].

Fig. 12. SEM images of six etch pits of different shapes (Types A, B, C, D, E, and F) according to shapes of the etch pits on β-Ga₂O₃ (010) single crystal surface. Copyright 2016 Jpn J Appl Phys.^[43]

Fig. 13. SEM images of the same pit at the same position on the same β-Ga₂O₃ (010) single crystal surface etched for the etching times of 0, 2, 4, and 10 min. Copyright 2016 Jpn J Appl Phys^[43].

Fig. 14. (Color online) (a) Distributions of number of dislocations and reverse leakage current (I_R) in SBD. (b) Distributions of number of dislocations and forward leakage current (I_F) in SBDs. (c) Distributions of number of voids and reverse leakage current (I_R) in SBDs. (d) Distributions of number of voids and forward leakage current (I_F) in SBDs. Copyright 2016 Jpn J Appl Phys^[48].

Fig. 15. (a) Relationship between the reverse leakage current and the number of arrow-shaped etch pits in SBD. (b) Relationship between the reverse leakage current and the number of gourd-shaped etch pits in SBD. The area of each SBD (S) is 1 × 10⁻³ cm². Copyright 2017 Jpn J Appl Phys^[45].

Fig. 16. Schematic of a subgrain boundary. Copyright 2015 Jpn J Appl Phys^[38].

Table 1. All kinds of defects observed on different β-Ga₂O₃ crystal plane.