Gallium oxide (Ga₂O₃) has received extensive research attention recently as an ultra-wide bandgap material. Among all the five polymorphsα-, β-, ε-, δ-, γ-Ga₂O₃,β-Ga₂O₃ is the thermal stable phase^[1]. Possessing numerous excellent properties including the wide band gap of 4.8–4.9 eV^[2], the advanced breakdown voltage is expected to be 8 MV/cm^[3], as well as high sensitivity in deep-UV^[4], whereβ-Ga₂O₃ are widely used in many fields^[5]. For instance, in the application of the Schottky barrier diode (SBD), higher breakdown voltage can be achieved by depositing high purityβ-Ga₂O₃ films with low carrier density. Thus, the SBDs with higher quality can be achieved^[6]. However, excellent performance power device applications require not only the quality but also the thickness of the epitaxial layer material^[7]. It has been widely investigated howβ-Ga₂O₃ films are grown by a variety of techniques, including molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and halide vapor phase epitaxy (HVPE)^[8-19] and etc. Nevertheless, these methods have some limitations. According to the numerous papers related to the epitaxy of differentβ-Ga₂O₃ polymorphs by the most common approaches mentioned above, the gas flows, the ratio of the raw materials used in the deposition and the growth temperature are the factors that can impact the quality of growth^[20-23]. Recently, we demonstrate that the carbothermal reduction rapid growth method can be used to grow heteroepitaxial films ofβ-Ga₂O₃ on c-plane sapphire substrates^[24]. It is a method which was widely used in synthesizingβ-Ga₂O₃ nanostructure^[25]. It is a simple, practical techniques for the synthesis and this method allowed for the rapid and inexpensive epitaxy^[26,27]. There are two stages in the carbothermal reduction reaction, in the first stage, suboxide Ga₂O was formed as the Ga₂O₃ powder react with graphite powder, and they further oxidation to Ga₂O₃ in the next process^[28,29]. In the second period, the Ga₂O vapor reacts further with CO to produce Ga and CO₂^[24]. As the vapor pressure of Ga₂O is significantly higher than Ga^[30], its desorption limits the growth rate due to the following three factors: (1) insufficient O-flux was provided to oxidize all formed suboxide^[29], (2) high growth temperatures at which the thermally activated desorption of Ga₂O outperforms its oxidation even under O-rich growth conditions^[2], and (3)β-Ga₂O₃ substrate orientations that provide only weak bonds to the adsorbed Ga₂O, resulting in a low activation energy for its desorption^[28]. Hence, it is powerfully suggested to investigate the O₂ flux and growth temperature to enhance the first phase of the carbothermal reduction and impede the second period. In this work, we explored three main factors on the influence of high qualityβ-Ga₂O₃ films, including the lower part’s furnace temperature, the O₂ flow, the molar ratio between Ga₂O₃ powder and graphite powder.

Pure Ga₂O₃ powder with a purity level of 99.999% was mixed with graphite powder used as the precursors, meanwhile, argon was used as protective gas. The mixtures were put into a corundum crucible. The substrate was c-plane sapphire. Carbothermal reduction was the method used to deposit Ga₂O₃ films. The deposition time was 2 h and the gas pressure for the samples treated in this research was fixed at 3.0 × 10³ Pa. Since the facility was a vertical dual temperature zone furnace, the diagram was shown inFig. 1. There were two settings for the lower and upper parts of the furnace, respectively. A lower part setting temperature of the furnace was the primary factor affecting the quality of the films. The growth reaction mechanism could be explained as follows^[24]:

12C(s)+Ga2O3(s)→Ga2O(g)+2CO(g),

2Ga2O(g)+O2(g)→Ga2O3(s).

The surface morphologies of the as-grown samples were examined using scanning electron microscopy (SEM, FEI Nova Nano 450). The Ga₂O₃ thick films were investigated by X-ray diffraction (XRD, Bruker D8 Advance), and standard XRD powder profiles (θ–2θ) were measured. We present the factors that determine the crystal phases and orientations based on the experiments results.

Fig. 2 displays the influence on the growth rate of the Ga₂O₃ films regarding temperatures of the lower part of the furnace. The growth rate shows a tendency of accelerated growth rate from 5.43μm/h at 1000 °C and reaches 7.47μm/h at 1050 °C. However, in the range of 1050 to 1150 °C, the growth rate is sharply reduced to 1.14μm/h, equivalents to the thickness of 2.28μm. When temperatures were set at 1150 and 1250 °C, it appears that the rate of growth of the epitaxy films remained relatively stable, measuring 1.14 and 1.006μm/h, respectively. This phenomenon can be explained by too high temperatures leading to the desorption of Ga₂O outperforming its oxidation under O-rich growth^[22].

A comparison of X-ray diffraction (XRD) patterns for films grown at different temperatures are depicted inFig. 3. Peaks of 18.9°, 38.4°, and 59.2° correspond to the diffraction of (−201), (−402), and (−603) planes ofβ-Ga₂O₃, respectively^[31-33].Fig. 3(a) representing the setting temperature of the lower part of the furnace at 950 °C, no obviously dominated peaks can be seen which means the film is anisotropic growth. Meanwhile, inFig. 3(b), the intensities of other diffraction peaks that belong toβ-Ga₂O₃ are lower, which indicate that the films are pureβ-Ga₂O₃ with (−201) plane orientation. However, comparing the diffraction peaks that are indexed to the {−201} plane family, the films obtained at 1150 and 1250 °C show considerably lower intensity, as shown inFigs. 3(c) and3(d).

According toFig. 4(a), the film grown at 950 °C exhibits a special microrod like structure, and not able to be measured in terms of thickness. Columns made up the majority of the film.Fig. 4(b) depicts the small area of the film with no obvious columns covering it. This is the evidence of the generation of crystal structures in this region at this temperature. An example of a film produced at 1050 °C is shown inFig. 4(c), this is a surface that exhibits a high degree of uniformity. The top view of the SEM image of the film showed flat morphology, with almost no visible protrusions across the entire image area for the films obtained under 1050 and 1350 °C. However, the film grown at 1350 °C has fewer grains than other films, as shown inFig. 4(d). In conclusion, this result confirms that high temperatures can contribute to the desorption of Ga₂O at a high rate. In this case, lower temperatures caused a decrease in the diffusion length of Ga₂O^[34]. Moreover, as the diffusion energies of the deposited atoms are different according to the varied temperatures^[35]. When the temperature was lower, the growth of the crystal was restrained due to the atoms not having sufficient energy to move to the substrate during the crystallization process. Whereas, with the temperature of 1050 °C, the atoms acquire enough energy to move and bond with adjacent nucleation sites. However, when the temperature rises further, the epitaxy speed decreases sharply owing to the migration of the atoms being too fast to form the regular bonds of a single crystal^[32].

InFig. 5, the growth rates of films grown under different O₂ flow conditions are summarized. Increasing the O₂ flow from 5 to 20 sccm resulted in an increased growth rates from 3.1225 to 7.4745μm/h. In the case of O₂ flow greater than 20 sccm, the rate of Ga₂O₃ films epitaxy decreased significantly. Growth rate increased when more O₂ flow was used since more Ga₂O vapor can fully react with O₂. O₂ fluxes of 50 and 100 sccm contributed to a reduction in film thickness as a result of more graphite powder reacting with O₂, less Ga₂O₃ powder was able to react adequately.

XRD patterns ofβ-Ga₂O₃ crystallized during deposition on c-plane sapphire under O₂ flow ranged from 5 to 50 sccm and are shown inFig. 6. Diffraction peaks of (−201), (−402), (−603) appeared for deposition O₂ flow between 5 and 100 sccm. However,Fig. 6(a) shows the film grown at 50 sccm is not oriented properly in any plane. InFigs. 6(c) and6(d), the intensity of the (−201) peaks are lower compared to those peaks obtained at O₂ flow of 20 sccm (Fig. 6(b)), this indicates that theβ-Ga₂O₃ films have inferior [−201]-oriented crystallinity when 20 sccm O₂ is used in the experimental process. In other words, the film grown at 20 sccm O₂ has the greatest crystallization considering the relationship of parallel betweenβ-Ga₂O₃ (−201) and c-plane sapphire (0001). The extension mode should to be domain matching epitaxy because the oxygen atoms in the (−201) equivalent plane ofβ-Ga₂O₃ have the same arrangement as the oxygen atoms of the c-plane (0001) plane^[36,37]. However, the too high oxygen pressures cause the poor property of crystalline of the films^[38].

According to the top view SEM of the films, the typical growth pattern of grains have excellent uniformity of the film grown at 50 sccm O₂ flow, as shown inFig. 7(a). It should be noted, however, that the film obtained under 5 sccm O₂ did not reveal a very flat surface (Fig. 7(c)). It is due to the decrease of the O₂ flow, the growth rate increased. Poor surface morphology is not guaranteed by a low O₂ component. A haphazard pattern of surface islands of Ga₂O₃ has been observed in this case. With an O₂ flow of 20 sccm, the fastest growth rate was achieved, and good surface morphology was ensured with an appropriate O₂ fraction (Fig. 7(b)).

To determine the effect of the molar ratio between graphite powder and Ga₂O₃ powder on the quality of Ga₂O₃ films, the graphite powder was adjusted in weight. It can be concluded fromFig. 8 that the growth rate was 25μm/h when the molar ratio was 5 : 1. By doubling the graphite powder usage, the deposition rate declined to 12μm/h, meanwhile, the thickness decreased to 25.32 from 50.23μm. In order to confirm this tendency, fourfold graphite powder was used, the film growth rate decreased by nearly half to 7.4745μm/h. This phenomenon can be explained by the fact that the newly introduced O₂ will react with the excess graphite powder, where there is insufficient O₂ to react with Ga₂O in the second period reaction.

InFig. 9, XRD patterns ofβ-Ga₂O₃ films crystallized during deposition on c-plane sapphire are illustrated. The peak at 18.9° belongs to (−201), the other (−201) plane family peaks appear with considerable intensities. However, the diffraction signal at 2θ = 30.3° and 57.6° is assigned to (110) and (−313) inFig. 9(a). It was found inFig. 9(c) that the film grown at the molar ratio of 5 : 1 possessing the (310) and (−601) planes. Compared to the {−201} peak ofβ-Ga₂O₃, these peaks have lower intensity. Nonetheless, at the molar ratio of 10 : 1, no obvious subpeaks are seen inFig. 9(b). Meanwhile, the intensity of the diffraction peak gets its maximum at a molar ratio of 10 : 1. Hence, the ideal molar ratio between graphite powder and Ga₂O₃ powder is 10 : 1 for obtaining goodβ-Ga₂O₃ films.

InFig. 10, we show SEM images ofβ-Ga₂O₃ films that were deposited on c-plane sapphire substrates. With a 20 : 1 molar ratio between graphite powder and high purity Ga₂O₃ powder,β-Ga₂O₃ films with good uniformity were obtained (seeFig. 10(a)). When the molar ratio was 5 : 1,β-Ga₂O₃ islands developed were observed to be more vertical to the substrates (Fig. 10(c)). This may be a result of too high a growth rate. It is impossible for these islands to become regular at a high growth rate. On the other hand, as shown by the SEM images, the size of Ga₂O₃ islands was larger when the molar ratio between graphite powder and Ga₂O₃ powder was 10 : 1 (Fig. 10(b)). In addition, the islands did not have sharp edges. Accordingly, the crystalline quality ofβ-Ga₂O₃ films can be enhanced by properly balancing the molar ratio between two types of powders.

In summary, the heteroepitaxy ofβ-Ga₂O₃/c-plane sapphire was investigated. The monoclinic films with the (−201) preferred orientation can be obtained indicating carbothermal reduction is a promising growth technology for growing high qualityβ-Ga₂O₃ films. According to the results of SEM and XRD, the growth rate, the orientation, and the morphology of the surface were strongly influenced by the temperature of the lower part of the furnace, the O₂ flow, and the molar ratio between Ga₂O₃ powder and graphite powder. Higher growth rate, the best crystallinity and surface morphology can be realized by the growth temperature of 1050 °C. In addition, our results reveal that an increasing O₂ flux helps maintain the quality of morphology. However, the results of XRD indicate that the 20 sccm O₂ flow can ensure good crystalline property. As the molar ratio between Ga₂O₃ powder and graphite powder declined, the growth rate increased. The highest deposition rate of 25μm/h has been achieved with sacrificing the crystalline quality of theβ-Ga₂O₃ layers. The best quality of crystalline was achieved when the molar ratio was 10 : 1. These results indicate promising properties for the future development of the carbothermal reduction method.