Gallium oxides (Ga₂O₃) has five different crystal structures, namely, α-, β-, γ-, ε-, and δ-phases^[1]. Among them, monoclinic β-Ga₂O₃ structure is the thermodynamically most stable under atmospheric pressure. As a new ultra-wide bandgap semiconductor, β-Ga₂O₃ has a band gap as large as 4.8–4.9 eV^[2–5] and a critical breakdown electric field of 8 MV/cm, much higher than SiC and GaN. It also has excellent chemical, mechanical and thermal stabilities at elevated temperatures. As a result, β-Ga₂O₃ is considered a suitable and promising ultra-wide bandgap semiconductor for a variety of applications. Current interests of β-Ga₂O₃ focus on three areas: (1) UV emission/detection devices for disinfection purposes and solar-blind sensor^[6], (2) substrates for high-brightness vertically-structured and flip-chip GaN-LEDs^{[7, 8]}, and (3) high-power devices because the large band gap of β-Ga₂O₃ is advantageous for the realization of high-efficiency transistors^[9–11]. At present, although the research on Ga₂O₃ is still at the initial stage, Ga₂O₃, which is suitable for large-scale production and has excellent chemical properties and thermal stability, will become one of the candidate materials for power electronic devices in the future.

One of the great advantages of β-Ga₂O₃ over GaN and SiC lies on the fact that high-quality single crystal wafers can be produced from melt by Czochralski (CZ)^[12], floating-zone (FZ)^{[13, 14]} and edge-defined film-fed growth (EFG)^[15]. Up to date, EFG-grown 4-inch β-Ga₂O₃ wafers are commercially available. However, problems such as large size and easy cracking of Ga₂O₃ crystal are still not solved. The melt growth method enables easy growth of single crystal with less crystal defects and large diameter with relatively low cost. However, the EFG growth of β-Ga₂O₃ from melt requires the use of very expensive noble-metal crucibles (such as Ir). Besides, the melt methods require effective and precise control of the growth conditions, such as temperature distribution, oxidation atmosphere, and air pressure. At the same time, Ga₂O₃ is easy to decompose at high temperature to generate GaO, Ga₂O, Ga and etc., which will volatilize and corrode the precious metal crucible seriously. The melt growth method can also produce multiple crystals, double crystals, Mosaic structure, screw dislocation, cracking and other problems, such that it is difficult to grow large-area and high quality Ga₂O₃ crystals. In addition, there are also many difficulties for the control of crystal orientation, cutting and polishing processes of Ga₂O₃ crystals for device epitaxy because of the easy cleavage of Ga₂O₃.

One of the solutions for the above-mentioned problems is the realization of heteroepitaxial β-Ga₂O₃ film on large-area foreign substrates, such as sapphire. Thin film growth of β-Ga₂O₃ has also been realized by using molecular beam epitaxy(MBE)^{[9, 10, 16]}, pulsed laser deposition (PLD)^[4], and metal-organic vapor phase epitaxy (MOVPE)^[17]. However, it is usually difficult to grow high-purity β-Ga₂O₃ at high growth rates by these methods. To ensure sufficiently high device breakdown voltage, thick Ga₂O₃ layers need to be grown with controllable thickness, electrical conductivity in a cost-effective production manner. Therefore, the growth of β-Ga₂O₃ by halide vapor phase epitaxy was proposed, similar to the growth of high-purity GaN, ZnO and AlN by HVPE^[18–20].

HVPE is widely used in the III–V semiconductor industry, especially for the preparation of free-standing GaN substrates in recent years. As a non-organic chemical vapor deposition technique, HVPE method is not only characterized by a fast growth rate and the high quality of thick epitaxial layers than other methods, but also characterized by the versatility for the fabrication of both substrates and devices. In this paper, the growth and device application of HVPE method for Ga₂O₃ are reviewed in details.

HVPE growth of β-Ga₂O₃ thin films is generally carried out at ~1050 °C by using gallium chloride (GaCl) and O₂ as the precursors. N₂, He or Ar can be used as the carrier gas. Nomura^[21] has reported thermodynamic aspects of the growth of β-Ga₂O₃ by HVPE. GaCl and O₂ were determined to be the appropriate precursors for the HVPE growth of β-Ga₂O₃. The theoretical estimations agreed well with their experimental results on the homoepitaxial growth, indicating that the HVPE growth of β-Ga₂O₃ can be thermodynamically controlled^[21].

In the HVPE system, HCl is more commonly used than chlorine gas due to the purity and operability. The GaCl was formed upstream in the reactor by the reaction between Ga metal and HCl gas. The main chemical reactions are as follows.

${\rm{Ga}}\left( {\rm{s}} \right) + {\rm{HCl}}\left( {\rm{g}} \right) \to {\rm{GaCl}}\left( {\rm{g}} \right) + {{\rm H}_2}\left( {\rm g} \right),$  ()

${\rm{GaCl}}\left( {\rm{g}} \right) + {\rm O_2}\left( {\rm g} \right) \to {\rm G{a_2}{O_3}}\left( {\rm s} \right) + {\rm C{l_2}}\left( {\rm g} \right).$  ()

Gallium chloride has a variety of different valence states, e.g. GaCl, GaCl₃, etc. GaCl is stable at high temperatures, and will decompose into GaCl₃ below a certain temperature. This feature enables temperature adjustment of the reaction chamber to realize GaCl generation, transportation and deposition of Ga₂O₃. As a result, a typical HVPE growth system contains two reaction chambers at different temperatures, as shown in Fig. 1.

As a cost-effective growth technique, HVPE supports production of high quality crystals with a faster growth rate. The fastest growth rate reported to date was ~250 μm/h for the growth of homoepitaxial layer on β-Ga₂O₃ (001) substrate^[22]. However, the as-grown Ga₂O₃ films suffer from large surface roughness due to the high growth rate, and an additional polishing procedure is required prior to device processing. Besides, the Ga₂O₃films show the presence of Cl-induced impurities and defects because of the use of GaCl as the Ga source.

To realize n-type doping of β-Ga₂O₃ thin films grown by HVPE, either SiCl₄ or SnCl₄ can be used as the efficient dopant source^[23]. Besides, semi-insulating Ga₂O₃ substrates can be achieved by using Fe or Mg dopants^{[24, 25]}. In the HVPE system, Fe-doped Ga₂O₃ can achieved by HCl flowing through the metal Fe to form FeCl₂ as the dopants. Of course, organic iron compound can be used as the dopants.

Note that the β-Ga₂O₃ structure is the most stable phase of Ga₂O₃, among the five different phases (α, β, γ, ε, and δ)^{[1, 2]}. So, HVPE growth for β-Ga₂O₃ have already been studied. However, the interests in other Ga₂O₃ phases have also arisen in recent years, particularly the metastable rhombohedral α- and hexagonal ε-Ga₂O₃ phases, both of which have been observed to grow epitaxially on oriented substrates. The α- and ε-phases are of particular interest because of their higher symmetry and simpler epitaxial relationships with c-plane sapphire. As a result, α, β, ε-Ga₂O₃ have all been obtained by HVPE. All reported films containing only α- and ε-Ga₂O₃ were grown at temperatures of ≤ 800 °C, and the growth temperature of β-Ga₂O₃ is generally above 850 °C.

The first HVPE growth of β-Ga₂O₃ has been reported by Matsumoto et al.^[26]. Small flakes or needle-like β-Ga₂O₃ were synthesized by the reaction between GaCl gas and O₂ at 1100–1150 °C. Foreign substrates, such as sapphire^[27–32], MgO^{[27, 33]} and etc, have been used for the growth of β-Ga₂O₃. The substrate symmetry would affect the formation of in-plane rotational domains, which results in the growth of β-Ga₂O₃ thin-films with highly textured surface and single out-of-plane orientations. For example, (-201) oriented β-Ga₂O₃ layers on sapphire (0001) consist of six in-plane rotational domains with six-fold symmetry in accordance with the substrate symmetry^{[16, 27–29, 31]}. Similarly, (100) and (102) oriented β-Ga₂O₃ layers are obtained on MgO (100) and (110) with four- and two-fold symmetric in-plane rotational domains, respectively^{[27, 30]}. Y Oshima et al.^[22] reported that the rotational domains could be suppressed in β-Ga₂O₃ grown on off-angled (0001) sapphire substrates, which shows no in-plane rotational symmetry. The growth rate can reach over 250 μm/h. However, the fast growth rate also leads to thin films with rough surface. In fact, even at low growth rates, the surface roughness of β-Ga₂O₃ layers grown by HVPE is relatively large. In 2017, Xiu et al. from Nanjing University, had successfully grown 2 and 4 inch β-Ga₂O₃ films on sapphire substrate by HVPE^[32]. Fig. 2 shows the optical photograph and the SEM images of HVPE grown β-Ga₂O₃ layers^[32]. The β-Ga₂O₃ films are transparent with roughened surface at micrometer scale. For β-Ga₂O₃ film grown at 850 °C by HVPE, the root mean square (RMS) value by atomic force microscopy is larger than 3 nm (shown in inset of Fig. 2). And the RMS value of β-Ga₂O₃ films becomes larger with increasing the growth temperature.

α-Ga₂O₃ is also a very promising wide-bandgap semiconductor. α-Ga₂O₃ has been reported to have a large bandgap energy (E_g = 5.3 eV)^[34] and its electrical conductivity can be well-controlled^[35–36]. Moreover, α-Ga₂O₃ has been proposed to fabricate novel functional materials through the formation of solid solutions with other corundum-structured oxides, such as α-Al₂O₃^[37].

Although trigonal corundum-structured α-Ga₂O₃ exists in a meta-stable phase compared to the β-Ga₂O₃ and cannot be obtained from the melt method, α-Ga₂O₃ has been hetero-epitaxially grown on sapphire substrates by HVPE^[38], which is the first report about the successful high-speed growth of high-purity (0001) α-Ga₂O₃ by HVPE.

α-Ga₂O₃ with the same corundum structure is the best lattice-matched polymorph with sapphire. The lattice mismatches between α-Ga₂O₃ and sapphire along the a- and c-axes are ~4.5 and ~3.3%, respectively. Although the lattice mismatch along the c-axis is considerably large, the pseudomorphic growth of α-Ga₂O₃ on α-Al₂O₃ can be stabilized because they share a common corundum structure. Once the first layer of α-Ga₂O₃ is formed, the structural similarity will promote the pseudomorphic growth.

It has been discovered that the deposition temperature was within the range of 525 to 650 °C^[38] and the growth rate can reach approximately 150 μm/h, over two orders of magnitude larger than those of conventional mist CVD or MBE. It means that HVPE is an excellent candidate for growing thick α-Ga₂O₃ films or even free-standing α-Ga₂O₃ wafers. The films are twin-free and have no detectable formation of β-Ga₂O₃. The optical bandgap is estimated to be 5.16 eV. The epitaxial relationships are also determined to be [10-10]α-Ga₂O₃||[10-10] sapphire and (0001)α-Ga₂O₃|| (0001) sapphire.

Epitaxial growths of the metastable ε-Ga₂O₃ phases have also been reported recently. Oshima^[39] reported the epitaxial growth of ε-Ga₂O₃ by HVPE for the first time. The pure phase ε-Ga₂O₃ films are grown on (0001) GaN, (0001) AlN, and (-201) β-Ga₂O₃ by HVPE at 550 °C, although some minor misoriented domains are observed. The ε-Ga₂O₃ is thermally stable up to approximately 700 °C. The optical bandgap of ε-Ga₂O₃ is determined for the first time to be 4.9 eV.

Yao et al.^[40] also reported metastable α- and ε-phases grown by HVPE between 650 °C and 850 °C. The epitaxial relationship was revealed to be [-1100] ε-Ga₂O₃||[11-20] α-Ga₂O₃||[11-20] α-Al₂O₃, as shown in Fig. 3.

Currently, HVPE growth of Ga₂O₃ focus on two areas. One is to grow the drift-layer for β-Ga₂O₃ Schottky barrier diodes, the other is to grow β-Ga₂O₃ films as the substrate or the buffer layers for III-nitrides epitaxy.

β-Ga₂O₃ is expected to have much larger Baliga’s figure of merit than SiC and GaN due to the predicted high breakdown electric field (~ 6–8 MV/cm) and high electron mobility (μ ~ 200–300 cm²/(V·s)). Currently, Schottky barrier diodes (SBDs) and FETs are being intensively investigated as the fundamental device components for power conversion systems. β-Ga₂O₃ based Schottky barrier diodes (SBDs) have been demonstrated, showing the reasonably good device characteristics^[41–45]. The reverse blocking voltage of field-plated lateral β-Ga₂O₃ SBD on sapphire is now more than 3 kV^[46].

In the early years, the development Ga₂O₃ SBDs was behind FETs, due mainly to a lack of suitable epitaxial growth techniques for the growth of thick n-Ga₂O₃ drift layer. In order to ensure the sufficient break down voltage, relatively thick n-Ga₂O₃ layers need to be grown with controlled thickness, good electrical conductivity in a cost-effective manner. HVPE method offers the possibility of growing this thick drift layer with high quality. Recently, the full-scale Ga₂O₃ SBDs were fabricated on epitaxial wafers with HVPE grown Si-doped n-Ga₂O₃ drift layers on n⁺-Ga₂O₃ (001) substrates^[45]. A Ga₂O₃ SBD power rectifier with a breakdown voltage of 400 V was successfully demonstrated. In 2016, Higashiwaki et al.^[47] improved the breakdown voltage up to 500 V by increasing the drift layer thickness and doping concentration. The device structure is shown in Fig. 4. The Ron values included a substrate resistance as large as 1.0 mΩ·cm², which is near to the ideality factors of the SBDs (~1.03–1.07).

Until now, the highest reported breakdown voltage is ~1.1 kV for β-Ga₂O₃ drift-layer based field-plated SBD grown by HVPE^[48]. Yang et al.^[43] reported that the reverse breakdown voltage of β-Ga₂O₃ rectifiers without edge termination over 1 kV and the ideality factor increased from 1.08 to 1.28 with decreasing the Schottky barrier height. The schematic Ga₂O₃ SBD device structures is shown in Fig. 5.

The above-mentioned results show that β-Ga₂O₃ Schottky rectifiers fabricated by HVPE technology are promising candidates for high power devices.

β-Ga₂O₃ is also promising as substrate or buffer layer for epitaxial growth of GaN and related III-nitrides. Native GaN substrates are the best choice for the fabrication of III-nitride devices as only homoepitaxial growth provides the lowest defect density, zero lattice and thermal mismatch. However, native GaN substrates are costly and difficult to obtain despite recent progress in bulk GaN single crystal synthesis. For this reason, the majority of GaN-based devices are produced by heteroepitaxial growth on foreign substrates such as sapphire, Si or SiC. However, a lot of challenges still exist because of the insulating nature of sapphire, high price and high UV absorption of SiC, large lattice mismatch and chemical interaction of Si with GaN. As a result, bulk GaN remains prohibitive for the majority of device applications.

Sapphire is generally used as the substrate for GaN growth, but it is still limited because of poor lattice match (13.9%) and insulating nature of sapphire. In contrast, the lattice mismatch between β-Ga₂O₃ and GaN is significantly lower. β-Ga₂O₃ is excellent to serve as substrate for the growth of III-nitrides. It also combines the advantages of being transparent and conductive. Especially, high quality bulk β-Ga₂O₃ crystals can be produced by melt-growth techniques at low cost and high throughput.

As a result, there have been several reports published recently on the growth of GaN epilayers on differently oriented β-Ga₂O₃ substrates by MOVPE, MBE and HVPE^[49–57]. For GaN films grown on β-Ga₂O₃, the epitaxial relationship of (011) β-Ga₂O₃||(10-10) GaN only shows a lattice mismatch of ~2.6%^[7]. M Muhammed et al.^[52] studied GaN growth on (-201) β-Ga₂O₃ substrates. They found that the epitaxial relationship is (010) β-Ga₂O₃||(11-20) GaN with a lattice mismatch of 4.7%. Recently, the growth of GaN-based light emitting diodes on β-Ga₂O₃ substrates has been demonstrated by Xie et al.^[51].

Nikolaev et al.^[57] reported GaN growth on bulk crystal Ga₂O₃ substrates by HVPE technique. Earlier, pseudo-HVPE growth of bulk GaN directly on single crystalline (100) β-Ga₂O₃ were reported, where the β-Ga₂O₃ substrate was in situ heat-treated in NH₃ ambient before GaN epitaxy^[56]. After the nitridation of Ga₂O₃ substrate, a porous GaN buffer layer would be formed on the surface of β-Ga₂O₃ substrate^[58] and hence the crystal quality of subsequent GaN epilayers were significantly improved.

In these cases, high quality bulk crystal β-Ga₂O₃ have been used as the substrate for GaN epitaxy. However, bulk crystal β-Ga₂O₃ is still very expensive. Recently, we also have grown β-Ga₂O₃ thin films by HVPE^[33] and in situ regrown GaN thick films, and successfully obtained free-standing GaN substrate^[58]. It should be noted that the growth and nitridation of β-Ga₂O₃ and re-growth of GaN can all be in situ completed in the HVPE system, which facilitates the growth process of GaN. Self-separation of GaN from the substrate can be realized due to easy cleavage of the Ga₂O₃ buffer layer. As a result, we expect that HVPE could be developed as a universal equipment for the epitaxy of GaN, Ga₂O₃ and other related novel hetero-structures.

In this paper, we have reviewed the HVPE growth and the latest progress in the area of the ultra-wide bandgap Ga₂O₃. It is concluded that HVPE is a promising candidate for the epitaxy of large-area Ga₂O₃ substrates and for the fabrication of Ga₂O₃-based high power devices. We also present discussions on the technological advantages and aspects of HVPE for epitaxy growth and high-power devices (such as SBDs) of Ga₂O₃. We expect that HVPE can be further developed as an all-purpose equipment for the large-scale, low-cost epitaxy of GaN and Ga₂O₃.

Acknowledgements

This work was supported by the National Key R&D Program of China (No. 2017YFB0404201), the Solid State Lighting and Energy-Saving Electronics Collaborative Innovation Center, PAPD, and the State Grid Shandong Electric Power Company.