*3.1. Gallium Oxide Bulk Crystal Growth*

Commonly used growth techniques of bulk *β*-Ga2O<sup>3</sup> crystal are (Table 1): Verneuil method [21,29], Czochralski (CZ) method [30–33], floating-zone (FZ) method [34], edgedefined film fed (EFG) method [16,17], and Bridgman (horizontal or vertical, HB and VB) method [35,36],summarizing the basic features of melt growth methods reported so far.

**Table 1.** Overview of *β*-Ga2O<sup>3</sup> bulk crystal growth methods.

The Verneuil method, being a crucible-free technique, enables both oxidizing and reducing of growth conditions [21]. The synthesis under a reducing condition benefited electron conductivity [49]. *N*-type doping was realized by Harwig et al. [37], the free carrier concentration was determined to be ~10<sup>19</sup> cm−<sup>3</sup> by Mg doping, and ~10<sup>21</sup> cm−<sup>3</sup> by Zr doping at 900 ◦C. The *β*-Ga2O<sup>3</sup> bulk crystal grown by this method has poor quality, and it was used mainly last century, as other more efficient techniques were well developed. The FZ method is also a crucible-free technique, it was recently used to grow bulk *β*-Ga2O<sup>3</sup> crystal to investigate the scintillation features [50,51] as it can be employed in an air atmosphere, which may allow for creation of fewer oxygen defect centers being the emission origin of Ga2O<sup>3</sup> [52]. Tomioka et al. [41] analyzed the residual impurities of *β*-Ga2O<sup>3</sup> grown by the FZ method by inductively-coupled plasma mass spectroscopy; besides Si or Sn, Al, Mg, and Fe have also been detected with a concentration of ~10<sup>16</sup> cm−<sup>3</sup> . Al was presumed to be a neutral impurity, while Mg and Fe were considered as deep ionized acceptors and could compensate Si donors. To our knowledge, the lowest FWHM reported is ~22 arcsec

for the peak *β*-Ga2O<sup>3</sup> (400) by Hossain et al. [39], in this work, the Laue diffraction patterns also confirmed that the grown *β*-Ga2O<sup>3</sup> crystal has a good crystallinity. However, FWHM of *β*-Ga2O<sup>3</sup> rocking curves larger than 100 arcsec has also been measured [38,53]. However, both these techniques mentioned above suffer from small crystal size (wafer is no more than 1 inch so far, as summarized in Table 1.

Using an Ircrucible, the CZ method has been predicted to be a potential candidate for large boule, but thermal instability is an issue at high temperature that leads to decomposition of Ga2O3. Thus, this technique requires atmosphere control. Being a crack-free technique, the *β*-Ga2O<sup>3</sup> crystal grown by the CZ method has small or even no boundaries. Several works reported by Galazka et al. [32,43,44] evidenced that the FWHM of the X-ray rocking curve could be as low as 22–50 arcsec on average, and the dislocation density was ~103 cm−<sup>2</sup> . Moreover, Galazka et al. [31] recently reported that bulk Ga2O<sup>3</sup> grown by the CZ method has an electron mobility of 80–152 cm2V −1 s <sup>−</sup><sup>1</sup> with a low residual Si impurity concentration of ~10<sup>16</sup> cm−<sup>3</sup> . Similar to the CZ method, the EFG method has the same technique issue. However, this technique is available for a 4-inch wafer and recently became commercially available. Commonly observed twin-boundaries in the EFG grown *β*-Ga2O<sup>3</sup> were efficiently avoided by optimizing the growth process (the so-called shouldering process). Different from the traditional growth direction (010), Oshima et al. [54] demonstrated that the (001) oriented *β*-Ga2O<sup>3</sup> grown by the EFG is more suitable than (010) for a Schottky barrier diode (SBD). A weak correlation between pits and electrical properties has been revealed [27,54]. The use of the VB method allows withstanding of high oxygen concentrations as a Pt-Rh (70–30%) alloy crucible. Additionally, this crucible also facilitates the pulling-up process as the grown crystal does not adhere to the wall. The major residual impurities are generally Rh (~several tens wt.ppm) from the crucible, Sn and Si (~several wt.ppm) from raw materials, and Fe and Zr (~several wt.ppm) from the furnace [36,48].This technique recently became *n*-type doping available by using a resistance heating VB furnace, and electron concentration and electron mobility were determined to be 3.6 <sup>×</sup> <sup>10</sup><sup>18</sup> cm−<sup>3</sup> and 60 cm2<sup>V</sup> −1 s −1 , respectively, by 0.1 mol% Sn-doped [35,48]. As the CZ, EFG, and VB method use the crucible, they all have a high level of scalability.
