*3.2. Gallium Oxide Thin-Film Growth*

Bulk devices and subsequent epitaxy of *β*-Ga2O<sup>3</sup> layers could be provided by bulk growth, while high-quality epitaxial growth technologies are still required in order to study and fabricate more complex devices. Halide vapor phase epitaxy (HVPE), metal-organic vapor phase epitaxy (MOVPE), pulsed laser deposition (PLD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), mist-chemical vapor deposition (CVD), and metalorganic chemical vapor deposition (MOCVD) are all involved in thin-film growth of Ga2O3.

Vapor phase epitaxy is a commercially promising technique for mass production of *β*-Ga2O3. Based on VPE, the halide vapor phase epitaxy (HVPE) method enables a growth rate as high as 250 µm/h [55] and the wafer size from 2 to 6 inches [56], it is thus a suitable technique for thick films with high purity for high voltage vertical switching devices. Furthermore, with the presence of chlorine catalyst in the growth chamber, this technique exhibits the growth of metastable phases of Ga2O3, such as α and *ε* [57]. The HVPE method suffers from a high level of roughness on the surface even at a relatively low growth rate [56,58]; an electrical mechanical [59] or a chemical mechanical [60] polishing can be employed to remove further deep surface pits formed during the growth. Leach et al. [61] reported a vast difference in surface morphology and XRD full-width half-maximum (FWMH), between sufficiently and insufficiently CMP polished (discriminated by the polishing times of the various polishing steps) *β*-Ga2O<sup>3</sup> wafers grown by HVPE. Despite the poor morphology, the FWHM of the films grown on on-axis substrate were as narrow as 28 arcsec. Moreover, Murakami et al. [62] revealed that effective donor concentration without intentional doping could reach as low as 10<sup>13</sup> cm−<sup>3</sup> .

Metal-organic vapor phase epitaxy (MOVPE)can provide a highly scalable growth as its deposition areas are large. Triethylgallium (TEGa), trimethylgallium (TMGa), and O2are

most commonly the precursors for gallium and oxygen, respectively. The homoepitaxial growth of *β*-Ga2O<sup>3</sup> by MOVPE can be strongly affected by substrate orientation. The growth rate is approximately 1.6–2.0 nm/min on the (100) plane, 0.65–1 µm/h on the (010) plane, and 1.6–4.3 nm/min on the (00-1) plane with miscut angles [63]. Recently, the growth rate can be elevated to 3.6 nm/min on the (100) plane [64] by tuning the growth pressure. A high-quality homoepitaxial growth on *β*-Ga2O<sup>3</sup> the (100) with an FWMH of 43 arcsec has been reported by Gogova et al. [65]. The study of residual donor source is still in progress [66] while an electron concentration of 8 <sup>×</sup> <sup>10</sup><sup>19</sup> cm−<sup>3</sup> by Si-doping was realized by Baldini et al. [67], which is the highest doping level by this technique so far.

Pulsed laser deposition (PLD) has often been used for doped layers of Ga2O<sup>3</sup> as it can transport materials from the target to the substrate stoichiometrically, thus the thickness of layers can be incisively controlled. It also has a relatively low operating temperature compared to other techniques. However, the quality of the materials deposited and the deposition rate are relatively low compared with other CVD and MBE methods. The roughness measured on the surface of Ga2O<sup>3</sup> films had a root mean square between 1 and 7 nm [68–70].

A growth rate of 10.8 nm/min could be reached without oxygen, while it decreased to 6.5 nm/min by increasing oxygen pressure to 50 mbar [71]. Indeed, oxygen partial pressure and temperature are considered as the dominant parameters for properties of materials grown by the PLD [72]. The crystallinity was enhanced by increasing oxygen pressure at either low deposition temperature (250 ◦C [71]) or high deposition temperature (780 ◦C [68]). A higher oxygen partial pressure also leads to self-trapped holes at O1*s* and between two O2*s* sites [68], which could further act on the transport properties. Unlike the influence of oxygen pressure, a higher temperature does not always lead to a better film quality [73,74]. While, as expected, a higher annealing temperature could improve the crystallinity, as it helps the re-arrangement of Ga and O atoms to their optimal sites [75,76]. The highest *<sup>n</sup>*-type doping level achieved by the PLD is 1.7 <sup>×</sup> <sup>10</sup><sup>20</sup> cm−<sup>3</sup> by Si doping [69].

Atomic layer deposition (ALD), initially called atomic layer epitaxy (ALE), is a sub-set of the chemical vapor deposition (CVD) technique based on self-saturation, sequential surface reactions. ALD is a more general deposition containing ALE and molecular layering (ML) techniques [77]. The highly controlled thickness of films and conformal coverage are the main advantages of ALD over other techniques, it also allows a relatively lower deposition temperature compared to MBE and CVD techniques and a lower growth rate (generally less than 0.1 nm/cycle). Sn-doped Ga2O<sup>3</sup> grown by ALD was investigated by Siah et al. [78], however the concentration of Sn was estimated as 2 <sup>×</sup> <sup>10</sup><sup>20</sup> cm−<sup>3</sup> , with the free electrons determined to be 4 <sup>×</sup> <sup>10</sup><sup>18</sup> cm−<sup>3</sup> . This was due to the low growth temperature.

Thus, post-annealing is generally also required to improve the crystalline quality. Additionally, the temperature during growth depends mainly on the gallium precursor chosen [79,80]. Besides the conventional ALD, the plasma-enhanced atomic layer deposition (PEALD) further permits a lower deposition temperature and better Ga2O3film properties with very smooth surface roughness (<1 nm) [81–83].

Molecular beam epitaxy (MBE)suits research purposes better than commercial use, as it enables the growth of high structural quality *β*-Ga2O<sup>3</sup> with a relatively low growth rate (<1 µm/h) and high production cost, while high voltage vertical devices often require thick drift regions (dozens of microns). The orientation of growth has been found to be one factor that influences the growth rate [84]. Mazzolini et al. [85] further demonstrated the growth rate of different orientations <sup>Γ</sup>(010) (2.3 nm/min) > <sup>Γ</sup>(001) > <sup>Γ</sup>(−201) > <sup>Γ</sup>(100) of Incatalyzed *β*-Ga2O3layers;this phenomenon was believed to be associated with the surface free energy related to the binding energy of the In ad-atom. Nepal et al. [86] reported a heteroepitaxial growth on SiC with (−402) having a relatively high FWMH (694 arcsec), which can be reduced to 30–60 arcsec by homoepitaxial growth [87]. The thin films grown by MBE also benefit a smooth surface with a roughness of less than 1 nm [88,89]. The densities of the threading dislocation etch pits was determined to be ~10<sup>5</sup> cm−<sup>2</sup> for the film grown at 850 ◦C [89]. An electron concentration of 10<sup>20</sup> cm−<sup>3</sup> has been achieved by Sn doping [90].

Techniques based on chemical vapor deposition (CVD) have also been employed for the growth of Ga2O3. Scalability and mass production are the most advantageous characteristics of the mist-CVD technique, as it is a vacuum free, low-cost, and solutionprocessed approach. This technique is also often used for epitaxial growth of α-Ga2O<sup>3</sup> on sapphire [91–94]. Morimoto et al. [94] also pointed out the facilities of mist-CVD for Ga2O3by F doping. Both homoepitaxial [95,96] and heteroepitaxial [97] growth of *β*-Ga2O3have been successfully performed. It is also worth noting that the FWMH of rocking curves was 39–91 arcsec for homoepitaxial growth with growth rate of 0.5–3.2 µm/h [96,98]. An electron concentration was measured as 5 <sup>×</sup> <sup>10</sup><sup>20</sup> cm−<sup>3</sup> by Sn doping [98].

The metal-organic chemical vapor deposition (MOCVD) technique uses Ga-based organic material as metal precursors, such as trimethylgallium (TMGa) and triethylgallium (TEGa), which usually leads to C-contamination of the as-grown film (relatively less carbon by using TEGa than TMGa). It is well-known that such contamination can be efficiently reduced by high growth temperature, and eliminated by post-annealing. Li et al. [99] reported a high-quality homoepitaxially grown film with FWMH and surface roughness of 21.6 arcsec and 0.68 nm, respectively. The growth rate is generally from several hundred nm/h [100,101] to10 µm/h [102–104]. This technique is also available for both *n*- and *p*-type dupability [24,105] (Figure 4).

**Figure 4.** Ga2O<sup>3</sup> and related oxides have been demonstrated to exhibit some remarkable features, such as (**a**) ultra-high critical electric field, (**b**) potential bipolar operation due to its demonstrated *n*-type and *p*-type conductivity, (**c**) ultra-stable interfaces that may host a 2D electron gas, (**d**) extended transparency into the UV-A region for transparent conducting oxide (TCO) applications (tail state density is located deeper in the ultraviolet than conventional TCOs). Panel (**a**) adapted with permission from Chikoidze et al. [24] © 2022 Elsevier Ltd. All rights reserved. Panel (**b**) adapted with permission from Chikoidze et al. [106] Copyright © 2022, American Chemical Society. Panel (**c**) adapted with permission from Chikoidze et al. [107]. © 2022 Elsevier Ltd. All rights reserved. Panel (**d**) adapted with permission from Perez-Tomas et al. [108,109] © 2022 WILEY-VCH Verlag GmbH & Co. KGaA. Adapted with permission from [12] © 2021 COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE).
