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Article

Effects of Substrate Polarity and Pre-Growth Treatments on Plasma-Assisted Molecular-Beam Epitaxy of β-Ga2O3 on 4° Off-Axis 4H-SiC

by
Raouf Hayyak
1,
Trong Si Ngo
1,
Taswar Iqbal
1,
Mee-Hi Choi
1,
Soon-Ku Hong
1,*,
Im-Gyu Yeo
2,
Moonkyong Na
3 and
Tai Hee Eun
2
1
Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
2
Advanced Material Research Group, Research Institute of Industrial Science & Technology (RIST), Pohang 37673, Republic of Korea
3
Semiconductor Research Center, Korea Electrotechnology Research Institute, Changwon 51543, Republic of Korea
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(7), 467; https://doi.org/10.3390/cryst16070467
Submission received: 2 June 2026 / Revised: 16 July 2026 / Accepted: 17 July 2026 / Published: 21 July 2026
(This article belongs to the Section Inorganic Crystalline Materials)
Browse Figures
Versions Notes

Abstract

This study reports the growth and structural analysis of β-Ga2O3 films on 4° off-axis (000±1) Si- and C-face 4H-SiC substrates by plasma-assisted molecular-beam epitaxy (PAMBE). Pre-growth treatments of 4H-SiC substrates were conducted by employing: (I) unintentional oxygen exposure, (II) intentional Ga pre-exposure, (III) a Ga flash-off process followed by Ga pre-exposure, and (IV) intentional oxygen-plasma pre-exposure prior to β-Ga2O3 growth, which led to different growth behaviors. The intentional Ga pre-exposure and Ga flash-off followed by Ga pre-exposure treatments modified the initial Si-face surface condition and were consistent with the mitigation of oxygen-induced surface degradation, including possible SiOx-related effects. In contrast, unintentional oxygen exposure and intentional oxygen-plasma pre-exposure produced RHEED evolution consistent with substantial surface disordering and possible amorphous SiOx formation on the Si-face 4H-SiC substrate, leading to disordered nucleation and degraded film growth. Growth on the C-face 4H-SiC substrate resulted in more ordered β-Ga2O3 films, suggesting that the C-face surface is less susceptible to SiOx-related degradation under the present oxygen-containing growth environment. The combined in situ RHEED, AFM, and HRXRD results indicate that substrate polarity and pre-growth surface treatments strongly influence the initial nucleation, morphology, and crystalline quality of β-Ga2O3 films on 4H-SiC. These findings provide a useful strategy for controlling oxide film growth on easily oxidized substrates under oxygen environments.

1. Introduction

β-Ga2O3 is regarded as a representative ultrawide-bandgap semiconductor for high-power, high-temperature, and harsh-environment electronic devices because of its wide bandgap and high critical breakdown field of approximately 6–8 MV cm−1, which is larger than those of SiC and GaN [1,2,3]. These properties give β-Ga2O3 a high Baliga figure of merit for power-device applications [2,3]. Ga2O3 has five known polymorphs, namely α-, β-, γ-, δ-, and κ(ε)-Ga2O3, among which the β-phase has received the most attention because of its high thermodynamic stability [1,4]. In addition, β-Ga2O3 is the only Ga2O3 polymorph for which large bulk single crystals can be grown via melt-growth techniques, owing to its thermal stability and high melting point [5,6,7,8]. β-Ga2O3 also allows controllable n-type conductivity through intentional doping [9,10,11]. Furthermore, β-Ga2O3 has attracted attention for radiation-tolerant and harsh-environment applications because several studies have reported its stability under irradiation by γ-rays, electrons, neutrons, and other energetic particles [1,12,13,14].
Besides melt-growth techniques for Ga2O3 bulk crystals [6,7,8], homoepitaxial and heteroepitaxial β-Ga2O3 thin films have been grown using various deposition techniques, including metal–organic chemical vapor deposition [15,16], pulsed laser deposition [10,17], halide vapor phase epitaxy [18], magnetron sputtering [19,20], plasma-enhanced atomic layer deposition [21], plasma-enhanced chemical vapor deposition [22], and molecular-beam epitaxy, including plasma-assisted molecular-beam epitaxy (PAMBE) [23,24,25,26]. Among these techniques, MBE-based approaches, including PAMBE, have been widely used in early β-Ga2O3 thin-film research because they enable precise control of the Ga flux, oxygen supply, and growth conditions.
However, the low thermal conductivity of β-Ga2O3 can cause severe self-heating in β-Ga2O3 power devices, leading to an increase in device temperature and accelerated degradation under high-power operation [27,28]. Therefore, effective thermal management is crucial for β-Ga2O3-based power devices. One approach to solving this challenge and enhancing the performance of high-temperature Ga2O3-based devices is to use 4° off-axis c-plane 4H-SiC substrates miscut toward [11–20] as wide-bandgap and thermally conductive substrates for heteroepitaxial β-Ga2O3 growth. 4H-SiC provides excellent thermal conductivity and a relatively small lattice misfit for β-Ga2O3 heteroepitaxy [29,30], and it can also serve as a platform for heterojunction structures with improved heat dissipation. Therefore, β-Ga2O3/4H-SiC heteroepitaxy is of interest for high-power and high-temperature device applications, where the 4H-SiC substrate can provide improved heat dissipation and a mechanically robust wide-bandgap platform for β-Ga2O3-based heterostructures.
Furthermore, both Si-face (000+1) and C-face (000−1) 4H-SiC substrates are available for Ga2O3 growth, and substrate polarity can significantly affect the crystallinity of the resulting β-Ga2O3 films. However, although the lattice misfit between 4H-SiC and β-Ga2O3 is smaller than that between c-plane sapphire and β-Ga2O3, Nepal et al. reported that β-Ga2O3 films grown on Si-face 4H-SiC showed limited improvement compared with those grown on c-plane sapphire. This was attributed to the unintentional formation of metastable polycrystalline SiOx at the β-Ga2O3/SiC interface under similar PAMBE conditions [29]. Since β-Ga2O3 growth via PAMBE generally requires a high growth temperature and an oxygen-containing environment, oxide growth on Si-face 4H-SiC may induce surface oxidation and possible formation of an SiOx-related interfacial layer. The oxidation behavior of Si- and C-face 4H-SiC has been studied theoretically and experimentally [31,32,33], and previous results indicate that SiOx-related interfacial formation can affect the crystal quality of heteroepitaxial oxide films [29].
Therefore, there are challenges in preparing the surface of 4H-SiC substrates for growing oxide films, such as β-Ga2O3 films, due to the limited understanding of initial nucleation behavior on the (000±1)Si, C 4H-SiC substrates and engineering the interface in growing β-Ga2O3 films. Hence, for oxide epitaxy on 4H-SiC substrates, it is essential to consider the substrate polarity of 4H-SiC substrates. Moreover, in the case of (000+1)Si 4H-SiC substrate, further optimization is required to achieve a sharp interface in β-Ga2O3/(000+1)Si 4H-SiC, similarly to the interface in β-Ga2O3/c-plane sapphire under the same experimental parameters.
Interface engineering in Oxide-MBE systems is a key factor in achieving improved crystal quality and morphology in oxide epitaxy. Hong et al. reported that pre-growth surface treatments significantly impact the interfacial phase formation, crystallite quality of epi-films, and growth mode [34]. On the other hand, Brown et al. have studied the role of (000+1)Si 4H-SiC substrate pre-treatment for GaN epitaxy [33].
Additionally, introducing slightly off-axis substrates has been shown to play a key role in controlling step-flow growth, thereby achieving β-Ga2O3 film with excellent crystalline properties and improved electronic properties [35]. The atomic step structure on an off-axis substrate acts as a preferred nucleation site for attaching atoms [35,36,37]. Jichao Hu et al. have demonstrated that the electrical properties, morphology, surface structure, and crystallinity of β-Ga2O3 films were improved by introducing a 4° off-axis 4H-SiC substrate, resulting in a reduction in the required energy for nucleating adatoms and absorbing atoms at the dangling bond positions of the steps [35,37].
Substrate surface preparation before epitaxial growth is a critical factor for controlling the crystalline quality and surface morphology of homoepitaxial and heteroepitaxial β-Ga2O3 films. For β-Ga2O3 homoepitaxy, the substrate preparation method depends strongly on the surface orientation, because the surface energy, cleavage behavior, and step formation differ for the (100), (010), and (001) planes. Typically, chemical-mechanical polishing is followed by organic cleaning, wet chemical treatment, and thermal annealing. For example, H3PO4-based treatment and high-temperature O2 annealing have been used to remove polishing damage and form step-and-terrace structures on β-Ga2O3 substrates [38,39,40]. Commercial epi-ready β-Ga2O3 substrates are generally prepared by CMP and final polishing processes designed to minimize polishing-induced subsurface damage. Subsequent wet chemical treatment and thermal annealing can then be used to remove residual surface damage and obtain well-ordered step-and-terrace surfaces. However, the effectiveness of thermal annealing depends on the crystal orientation: atomically flat stepped surfaces can be formed on β-Ga2O3 (100) substrates by annealing at elevated temperatures, whereas β-Ga2O3 (010) and (001) substrates require different preparation conditions because of their different surface stability and etching behavior [38,39,41,42]. For c-plane sapphire substrates, organic solvent cleaning, acid-based wet etching, and high-temperature annealing in oxygen or vacuum have been widely used to obtain smooth and ordered surfaces before β-Ga2O3 heteroepitaxy [40,43,44]. In addition, plasma-assisted or low-temperature annealing treatments, including Ar plasma annealing, have been reported to affect Ga2O3 crystallization and surface evolution in other deposition processes [45]. More specifically, Gogova et al. proposed an alternative method for preparing epi-ready β-Ga2O3 (010) substrate surfaces using low-temperature Ar annealing below 600 °C for a short annealing time of approximately 1 min [46]. This approach was reported as an efficient and cost-effective alternative to conventional high-temperature annealing processes in oxygen-containing atmospheres, which are typically performed at around 1000 °C for oxide substrates such as sapphire and β-Ga2O3 [38,39,40,43,44]. In contrast, for 4H-SiC substrates used in this study, organic cleaning, HF treatment, and in-vacuum thermal cleaning were combined with Ga or oxygen pre-exposure in the PAMBE chamber to control the initial oxide-growth environment.
To the best of our knowledge, a systematic investigation combining substrate polarity, oxygen/Ga pre-exposure treatments, and real-time in situ RHEED evolution for β-Ga2O3 growth on 4° off-axis 4H-SiC by PAMBE has not been reported in detail. In this study, we employed a series of pre-growth surface treatments, including (I) unintentional oxygen exposure, (II) intentional Ga pre-exposure, (III) a Ga flash-off process followed by Ga pre-exposure before β-Ga2O3 growth, and (IV) intentional oxygen-plasma pre-exposure. We evaluate how these treatments affect the initial nucleation behavior, surface morphology, and crystalline quality of β-Ga2O3 films on Si-face and C-face 4H-SiC substrates under identical PAMBE growth conditions. In particular, the combined RHEED, AFM, HRXRD, and φ-scan results are used to clarify how substrate polarity and pre-growth treatments modify the early growth stage and rotational-domain-related diffraction features. The proposed interpretation of SiOx-related surface degradation and its mitigation by Ga pre-exposure is based on indirect experimental evidence from in situ RHEED evolution, ex situ structural characterization, and the previous literature reports.

2. Experimental Details

In this study, PAMBE was used to grow (−201)-oriented β-Ga2O3 films on 4° off-axis Si-face and C-face 4H-SiC substrates. Before these 4H-SiC growth experiments, preliminary β-Ga2O3 growth on on-axis c-plane sapphire was carried out only to optimize the baseline PAMBE growth parameters, including the growth temperature, Ga cell temperature, oxygen-plasma power, oxygen flow rate, and growth pressure. The optimized conditions were then applied to β-Ga2O3 growth on Si-face and C-face 4H-SiC substrates. Since the present study focuses on the effects of substrate polarity and pre-growth treatments on 4H-SiC substrates, the sapphire calibration results are not discussed in this manuscript. Both faces of 4H-SiC substrates, i.e., 4° off-axis (000±1)Si, C 4H-SiC, were initially cleaned in acetone (99.9%) and methanol (99.9%) using an ultrasonic cleaner for 10 min, respectively. This was followed by immersion in concentrated HF (49% vol) for 5 min to remove the oxide layer on the substrates. Subsequently, the substrates were rinsed in deionized water for 10 min and dried with pure N2 (99.99%) gas blowing.
In the next step, the cleaned substrates were placed inside the growth chamber. Before starting growth, they were annealed at 800 °C for 30 min as a further cleaning step to remove any possibility of contamination. A 6N-purity Ga source and an RF oxygen-plasma source were employed to supply Ga and active oxygen radicals, respectively. A Veeco dual-filament effusion cell with a SUMO source was used as the Ga source, and an SVT Associates RF-4.5 plasma source equipped with a quartz plasma chamber and capable of RF power up to 600 W was used as the oxygen-plasma source. The base pressure of the growth chamber during the process was nearly ~10−10 torr, while the growth pressure was kept at 4.2 × 10−5 torr during the growth processes. In situ real-time monitoring by using reflection high-energy electron diffraction (RHEED) was applied to investigate the growth evolution and the atomic step on (−201) β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates. Surface morphology and roughness were characterized using an atomic force microscope (AFM). Crystal structure of Ga2O3 film, orientation, and quality were examined by employing several high-resolution X-ray diffraction (HRXRD) techniques, consisting of θ–2θ, φ-scan, and Ω-rocking curve (Ω-XRC) analysis. The substrate was continuously rotated by ~3 rpm during the growth process to produce uniform films [47].
The (−201)-oriented β-Ga2O3 films on 4° off-axis (000±1) Si- and C-face 4H-SiC substrates were investigated by employing four different pre-growth treatments. Table 1 summarizes the experimental conditions in detail. The following four schemes describe the growth processes investigated in this study.
I.
Unintentional oxygen exposure: By supplying the oxygen gas to turn on the oxygen-plasma cell, the substrate is naturally exposed to the oxygen gas. Just after supplying the oxygen gas, RF oxygen plasma was turned on with an RF power of 300 W and an oxygen flow rate of 2 SCCM, while both the plasma shutter for the plasma cell and the main shutter for the substrate were closed. Then, both oxygen and Ga shutters were opened simultaneously followed by the opening of the main shutter for the growth of β-Ga2O3 films. Recording the RHEED patterns was assessed when plasma was turned on, while both the plasma shutter and main shutter were closed.
II.
Intentional Ga pre-exposure: Ga beam (TGa cell = 820 °C) pre-exposure treatment was intentionally performed. The thermally cleaned substrate was initially subjected to Ga flux (TGa cell = 820 °C) for a short time [two samples were grown: S2 with Ga exposure for 1 min and S3 with Ga exposure for 5 min]. The oxygen plasma was then stabilized while Ga was continuously supplied to the substrate. After plasma stabilization, the oxygen plasma shutter was opened to start β-Ga2O3 growth, while the main shutter had already been opened for Ga exposure.
III.
Ga flash-off, followed by Ga pre-exposure: In this process, to mitigate possible residual oxide-related surface effects on the Si-face 4H-SiC substrate, the thermally cleaned substrate was exposed to Ga flux at a low substrate temperature of 650 °C (Tsubstrate = 650 °C). After this step, the Ga pre-exposure procedures in II were carried out.
IV.
Intentional oxygen-plasma pre-exposure: In this process, when the oxygen-plasma was stabilized, the oxygen shutter and the main shutter were opened and substrates were exposed to energetic oxygen for 5 min, while the Ga shutter was closed. Then, the Ga shutter was opened to start the growth of the film.

3. Results and Discussion

3.1. In Situ RHEED Characterization of Atomic Steps on 4° Off-Axis 4H-SiC Substrate

This section covers in situ RHEED observations, which can be performed to investigate the surface morphology in real time as a kinetics signature. Figure 1 illustrates the evaluation of (−201) β-Ga2O3 film grown on 4° off-axis (000+1)Si 4H-SiC. The RHEED patterns revealed the development of a steps and terraces structure on the (−201) β-Ga2O3 film grown on the 4° off-axis 4H-SiC substrate. The in-plane orientation relationship was assigned from azimuth-dependent RHEED observations during substrate rotation. The crystallographic azimuths of the 4H-SiC substrate were first identified from the substrate RHEED patterns. After β-Ga2O3 growth, the azimuthal positions of the film-related RHEED streaks were compared with those of the substrate. The film streaks corresponding to the [010]β-Ga2O3 direction appeared along the [11–20]4H-SiC substrate azimuth, whereas those corresponding to the [102]β-Ga2O3 direction appeared along the [1–100]4H-SiC substrate azimuth. Based on this azimuthal correspondence, the in-plane orientation relationship was assigned as [010]β-Ga2O3 // [11–20]4H-SiC and [102]β-Ga2O3 // [1–100]4H-SiC, consistent with previously reported β-Ga2O3/4H-SiC epitaxial relationships. Similarly, previous studies for β-Ga2O3/α-Al2O3 heteroepitaxial growth showed the 6-fold symmetry [43]. The (0001) 4H-SiC substrate shows RHEED patterns expected to be repeated by every 60° due to the underlying hexagonal 6-fold symmetry of the substrate. However, the RHEED observations in Figure 1 were obtained from the β-Ga2O3 on 4° off-axis substrate and the symmetry of repeating RHEED pattern of each direction is disrupted as given in Figure 1a. Therefore, the film-related RHEED features were not observed at equally spaced 60° intervals during substrate rotation. Among the possible in-plane orientations, the [010]β-Ga2O3 // [11–20]4H-SiC relationship was most clearly observed at 180° intervals, as shown in Figure 1a. This suggests that the substrate surface developed an atomic step structure, resulting in the growth of film in the favorable off-axis direction.
The formation of steps and terraces on the 4° off-axis substrate can give rise to a weak splitting or inclination of the RHEED streaks when the electron beam is coherently incident along the step edges. In the present case, the splitting feature is weak because of the small off-axis angle and the relatively wide terrace spacing. Therefore, the RHEED splitting shown in Figure 1b is used only as an indication of step-terrace-related diffraction from the 4° off-axis surface. The in-plane lattice spacing is related to the distance between RHEED streaks, whereas the terrace width is inversely related to the spacing of the split streaks [48,49,50,51].

3.2. Effects of Pre-Growth Treatments on the Morphology of Ga2O3 Films Grown on 4° Off-Axis Surfaces

3.2.1. Growth on On-Axis and 4° Off-Axis Substrates

Growth behaviors of β-Ga2O3 films on on-axis and 4° off-axis 4H-SiC are schematically illustrated in Figure 2. Thin film growth process in single crystalline substrates is significantly affected by the dynamics of surface diffusion of adatom and the kinetics of step incorporation [52]. Growth of the film on a singular surface with an infinite terrace width greater than the surface diffusion length of Ga atoms leads to random nucleation and incorporation of Ga atoms on the substrate surface, resulting in a 3-dimensional (3D) characteristic growth mode [35,53,54], because there are no kink positions for incorporating the Ga adatoms, as illustrated in Figure 2a.
On the other hand, as illustrated in Figure 2b, employing the off-axis substrate introduces high density of active dangling bonds on the surface through the supply of kink positions (step edges) as being ideal sites for Ga adatom incorporation, which can lead to improving the crystallinity and morphology of the β-Ga2O3 films [35,36,55]. The diffusion length of the adatom compared to the terrace width affects growth behaviors [56,57,58,59,60]. The diffusion length of adatoms is varied by growth conditions [54,61], off-axis substrates [38], and the lattice misfit between the film and substrate [62]. However, sustaining the desired morphology and crystallinity of β-Ga2O3 films on off-axis 4H-SiC substrates is more complex, especially for oxide growth on the Si-face, because the Si-face surface can be susceptible to oxygen-induced surface degradation and possible SiOx-related interfacial formation during growth.

3.2.2. Growth Evaluation by In Situ RHEED Observations

Growth evaluation based on the RHEED observations in three different pre-treatments, as mentioned in the experimental details section, was performed. The RHEED patterns showed sharp streaky features from the (000±1)Si, C 4H-SiC substrate, suggesting that substrate surfaces were well-prepared and clean for the growth of β-Ga2O3 films, as shown in Figure 3.
The RHEED patterns for the pre-treatment procedure I are displayed in Figure 3a. Before the growth start, the RHEED pattern was not changed when the oxygen-plasma shutter was closed, which is likely due to no active oxygen radicals to the substrate surface. Natural oxygen gas was supplied through the plasma cell into the growth chamber, as checked by the pressure change in the growth chamber, however, the natural oxygen gas on the substrate has a negligible effect because it is not energetic. Growth was started by opening both oxygen plasma and Ga source shutters simultaneously, followed by opening the main shutter when the oxygen plasma was turning on. After 5 min of growth, the streaky RHEED pattern defused a little bit, and by increasing growth time the defusing increased. It was observed that spots were appearing on the streaky lines of the pattern after 5 min growth (3D spotty + streaky pattern). After 90 min of growth, additional spots and ring-like features appeared, indicating a polycrystalline character. The RHEED patterns became weaker and weaker and almost disappeared after 240 min growth, which implied poor crystalline properties. This behavior may be associated with oxygen-induced surface degradation and possible formation of an amorphous SiOx-related layer on the Si-face 4H-SiC surface, which can disturb ordered nucleation and promote surface roughening. Consistently, Nepal et al. reported TEM evidence of SiOx formation at the β-Ga2O3/SiC interface under PAMBE growth conditions [29]. The 3D growth was maintained until the completion of the growth process.
In the case of the β-Ga2O3 film on the (000−1)C 4H-SiC as shown in Figure 3b, strong streaky RHEED patterns were observed until the end of growth. This may be attributed to the C-face being less prone to stable SiOx-related surface degradation than the Si-face is under the present oxygen-containing growth environment. The relatively preserved crystalline surface order on the C-face is expected to promote better wetting and more ordered nucleation of Ga and oxygen species, resulting in improved crystallinity and surface morphology. Hence, we can achieve a smoother surface on the (000−1)C 4H-SiC.
The RHEED patterns for pre-treatment procedure II are displayed in Figure 3c. When the substrate surface was exposed to Ga flux for 5 min, the streaky RHEED pattern remained almost unchanged. Importantly, the streaky RHEED pattern without spotty features was maintained throughout the 240 min growth, indicating improved crystalline quality after Ga pre-exposure. This behavior is significantly different from that observed for pre-treatment procedure I.
The RHEED patterns for pre-treatment procedure III are displayed in Figure 3d. In this process, Ga was first supplied to the Si-face substrate at a low substrate temperature of 650 °C to modify the initial surface condition and mitigate possible oxide-related surface effects. After 5 min, the streaky RHEED pattern from the substrate nearly disappeared, indicating that the supplied Ga covered or strongly modified the substrate surface. By increasing the substrate temperature to 800 °C, the streaky RHEED pattern recovered, indicating desorption of excess Ga from the surface. Once the streaky RHEED pattern had fully recovered, the Ga pre-exposure procedure described in procedure II was carried out. After growth, the RHEED pattern became sharper and more well-defined than those observed for procedures I and II, suggesting a smoother growing surface and improved crystalline quality. These observations are consistent with the interpretation that Ga pre-exposure modifies the initial Si-face surface condition and mitigates oxygen-induced surface degradation during the early growth stage. Although RHEED does not provide direct chemical identification of SiOx, the observed RHEED evolution indicates that pre-growth treatment can effectively modify the initial interface condition and improve the crystalline quality of β-Ga2O3 films on non-oxide substrates. Several studies have reported interface engineering in oxide-material epitaxy on non-oxide substrates during MBE processes [34,63,64,65].
According to previous studies, the lowest relaxation energy corresponds to the Si-O interface, which has the greatest adhesion energy (Wad) among the other terminations, including C-O, C-Ga, and Si-Ga. In addition, the strongest and most stable bonds correlate with Si-O. The oxidation mechanism between (000+1)Si 4H-SiC and (000−1)C 4H-SiC differs because the C-O bond is weaker than the Si-O bond [31,32]. Therefore, possible SiOx-related surface or interfacial formation during the synthesis of the 4H-SiC/β-Ga2O3 interface can significantly affect β-Ga2O3 growth in PAMBE. If an amorphous SiOx-related layer forms on the Si-face surface, it may restrict the mobility of Ga and oxygen species on the growing surface. Such surface degradation is expected to promote non-uniform nucleation and growth, leading to a transition from smoother, more ordered growth to a rougher 3D growth mode.

3.2.3. Morphological Characteristics

Along with the in situ RHEED observation, surface morphologies of all the grown films were analyzed using AFM. Figure 4a shows the surface morphology of the reference sample (S1) without any substrate pre-treatments, exhibiting a non-uniform surface with 3D growth mode characteristics and an RMS roughness of 7.25 nm. This morphology is consistent with non-uniform nucleation associated with oxygen-induced surface degradation and possible SiOx-related interfacial formation on the Si-face substrate. The samples (S2 and S3) grown with intentional Ga pre-exposure, i.e., procedure II, showed smoother surface morphology with increasing Ga exposure time, producing a more coalesced surface and growth transitions from a non-uniform growth surface to a more uniform morphology, as shown in Figure 4b,c. The RMS roughness of the samples with Ga exposure times of 1 and 5 min were 5.29 and 4.23 nm, respectively. The RMS roughness was further decreased to 3.63 nm for the pre-treatment procedure III, as shown in Figure 4d. Interestingly, the surface morphological features of the β-Ga2O3 films on Si-face showed an isotropic morphological feature, which is different from the typical anisotropic elongated morphology observed frequently from the homoepitaxial growth of β-Ga2O3 [66]. In the case of sample S5, which was grown on the C-face SiC without any pre-treatments, the lowest RMS roughness was 1.80 nm, as shown in Figure 4e,f.
The RMS roughness values of the β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates are summarized in Table 2. The roughness systematically decreased from 7.25 nm for S1 to 3.63 nm for S4 on the Si-face substrate as the Ga-related pre-growth treatments were applied, indicating that modification of the initial Si-face surface conditions improves the subsequent β-Ga2O3 morphology. C-face sample S5 showed the lowest RMS roughness of 1.80 nm, suggesting that the C-face surface provides a more favorable initial condition for β-Ga2O3 nucleation under the present PAMBE conditions. Compared with homoepitaxial β-Ga2O3 growth on properly prepared β-Ga2O3 substrates, where atomically stepped surfaces can be obtained, heteroepitaxial growth on foreign substrates such as sapphire and 4H-SiC is more strongly affected by lattice mismatch, substrate polarity, and interfacial reactions. Therefore, the roughness trend observed in this study indicates that the surface polarity and pre-growth treatment of 4H-SiC play key roles in controlling the morphology of heteroepitaxial β-Ga2O3 films.

3.2.4. Intentional Oxygen-Plasma Pre-Exposure

Now we discuss the extreme case of oxygen pre-exposure, that is, the intentional oxygen-plasma pre-exposure (procedure IV). The 4° off-axis (000±1)Si, C 4H-SiC substrates were exposed to the intentional oxygen plasma for 5 min. Thermally cleaned substrates revealed strong streaky RHEED patterns, indicating well-ordered clean surfaces, as shown in Figure 5. During the unintentional natural oxygen gas exposure while turning on the oxygen plasma, changes in the RHEED patterns were negligible for both Si- and C-face SiC substrates. Upon exposure to energetic oxygen plasma, the streaky RHEED pattern from the Si-face surface nearly disappeared, whereas the C-face RHEED pattern became only slightly fainter and retained discernible crystalline features. The severely degraded RHEED pattern from the Si-face was maintained until the end of growth. This RHEED evolution is consistent with strong oxygen-induced surface disordering and possible formation of an amorphous SiOx-related layer on the Si-face at the initial stage of treatment, in agreement with the strong tendency for Si-O bond formation reported in previous studies [31,32].
On the other hand, the RHEED patterns from the C-face surface gradually degraded with increasing growth time, as indicated by the appearance of spotty patterns and ring-like features, as shown in Figure 5. This implies that energetic oxygen radicals also modified the C-face SiC surface, although severe SiOx-related surface degradation, such as that observed on the Si-face, was not evident under the same exposure conditions, which is different from the unintentional oxygen exposure treatment on the C-face surface (sample S5) discussed in Figure 3b. Initially, it was observed that the strong streaky RHEED pattern revealed a partial loss of crystalline order, resulting in the RHEED pattern fading, and a diffuse (faint) streaky line was still visible. This meant that the surface still had some degree of atomic arrangement. This behavior may be related to partial oxidation of the C-face surface and possible formation/desorption of volatile COx species. However, the degradation of surface did not happen completely; therefore, the less degraded parts of the surface made polycrystalline β-Ga2O3 grow as observed in real-time monitoring evaluation of Figure 5b. Figure 5c,d show surface morphology observed via AFM. On the oxygen-plasma exposed Si-face, the surface morphology is relatively smooth with an RMS = 3.72 nm; however, the oxygen-plasma exposed C-face showed a rougher surface with an RMS roughness of 7.37 nm.

3.3. Structural Investigations by XRD

3.3.1. Crystallographic Orientation of the β-Ga2O3 Films and Crystal Quality

Figure 6a shows the HRXRD θ–2θ scan of the β-Ga2O3 films on 4° off-axis on (000±1)Si, C 4H-SiC substrates with different pre-treatments. The peaks at 17.59, 26.52, 35.6, 44.94, 54.57, 64.72, 75.36 and 86.87 degrees correspond to (0002), (0003), (0004), (0005), (0006), (0007), (0008), and (0009) diffractions from the 4H-SiC substrate, respectively (JCPDS card No. 01-073-1664) [29,67,68,69]. The diffraction peaks observed at 18.91, 38.33, 58.9 and 81.98 degrees correspond to the (−201), (−402), (−603), and (−804) planes of the β-Ga2O3 film, respectively (ICDD card No. 01-082-3838). No additional diffraction peaks were detected; therefore, phase-pure (−201)-oriented β-Ga2O3 films were grown under the present conditions. Similarly, the same crystal orientation has been observed in previous studies [29,32,35,47,53,55,69]. The FWHM values of the (−201) β-Ga2O3 diffraction peak are plotted in Figure 6b. The decrease in FWHM from S1 to S5 indicates that the crystalline quality of the β-Ga2O3 films is improved by Ga pre-exposure, Ga flash-off followed by Ga pre-exposure, and the use of the C-face substrate.
In order to investigate crystal quality of β-Ga2O3 films with various pre-treatments of the substrate surfaces, Ω-XRC analysis was performed as shown in Figure 7. The FWHM values revealed that the crystalline quality of β-Ga2O3 films was affected significantly by pre-treatments. The β-Ga2O3 film on C-face SiC with no pre-exposure (S5 sample) showed the lowest FWHM values of 1.41 and 2.51 degrees for (−402) and (−401) Ω-XRC, respectively. The β-Ga2O3 films on Si-face SiC substrates with various pre-treatments showed higher Ω-XRC FWHM values, which are likely associated with stronger surface/interface degradation and possible SiOx-related effects on the Si-face. The FWHM values ranged from 2.48 to 2.09° and from 2.96 to 2.69° for the (−402) and (−401) Ω-XRCs, respectively. Because the c-plane sapphire growth was used only for preliminary calibration of the PAMBE growth parameters, the crystallinity comparison in this study focuses on β-Ga2O3 films grown on Si-face and C-face 4H-SiC substrates. These results suggest that pre-growth treatment of the SiC substrate surface is a critical parameter for determining the crystal quality of oxide films grown on SiC under oxygen-containing conditions, in addition to the lattice misfit effect.
To provide a clearer quantitative comparison of the effects of substrate polarity and pre-growth treatment, the RMS roughness values obtained from AFM measurements, the HRXRD θ–2θ FWHM values of the (−201) β-Ga2O3 reflection, and the Ω-XRC FWHM values of the (−402) and (−401) reflections are summarized in Table 2. The results show a systematic decrease in RMS roughness and XRD peak broadening from S1 to S5, indicating that Ga pre-exposure, Ga flash-off followed by Ga pre-exposure, and the use of the C-face substrate improve the surface morphology and crystalline quality of the β-Ga2O3 films.

3.3.2. Rotational-Domain-Related φ-Scan Features in β-Ga2O3 Films

X-ray φ-scan analysis was carried out to investigate the rotational-domain-related diffraction features of β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates. The reduced number of visible diffraction peaks at a χ angle of 20° may seem to suggest fewer rotational-domain-related diffraction features. However, the φ-scan results are strongly dependent on the selected χ angle and diffraction geometry.
As shown in Figure 8, φ-scans were measured at χ = 20°, 24°, and 28° to examine the χ-angle dependence of the {−401} β-Ga2O3 diffraction peaks. At χ = 20°, the β-Ga2O3 films showed four visible {−401} peaks over a full 360° rotation. However, when the χ angle was changed to 24°, six peaks associated with rotational-domain-related diffraction were observed. This indicates that χ = 20° does not capture all diffraction peaks from the rotational domains and that the apparent number of peaks depends on the χ angle and diffraction geometry.
The six {−401} peaks, separated by approximately 60°, are commonly observed for β-Ga2O3 films with rotational domains grown on on-axis c-plane sapphire and on-axis 4H-SiC substrates [32,60,70]. Therefore, the observation of six peaks at χ = 24° indicates that rotational-domain-related diffraction features are still present in the β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates. Accordingly, the present φ-scan results should not be interpreted as direct evidence for complete suppression of rotational domains. Instead, the results indicate that the 4° off-axis substrate modifies the rotational-domain-related diffraction features, including the relative intensity and visibility of the peaks as a function of χ angle.
The unequal peak intensities observed in the φ-scan patterns suggest that the rotational-domain-related diffraction features are affected by the off-axis substrate geometry. However, further quantitative analysis, such as detailed pole-figure measurements or direct structural characterization, would be required to determine the rotational-domain fraction more precisely. Further optimization of the off-axis angle may be useful for controlling rotational-domain formation, as previous studies have reported a reduction or elimination of rotational domains in β-Ga2O3 films grown on large off-axis sapphire substrates [60,70,71].

4. Summary

β-Ga2O3 films were grown on 4° off-axis Si-face and C-face 4H-SiC substrates by PAMBE, and the effects of substrate polarity and pre-growth surface treatments were investigated using in situ RHEED, AFM, and HRXRD. On Si-face 4H-SiC, unintentional oxygen exposure and intentional oxygen-plasma exposure caused RHEED evolution consistent with oxygen-induced surface disordering and possible SiOx-related surface degradation, resulting in degraded nucleation and film quality. In contrast, Ga pre-exposure and Ga flash-off followed by Ga pre-exposure improved the initial Si-face surface condition and reduced the RMS roughness from 7.25 nm to 3.63 nm. Among the main comparison samples, growth on C-face 4H-SiC without pre-exposure produced the smoothest film with an RMS roughness of 1.80 nm and the lowest Ω-XRC FWHM values, indicating improved morphology and crystalline quality. The φ-scan results showed χ-angle-dependent, rotational-domain-related diffraction features rather than direct evidence of complete rotational-domain suppression. These results demonstrate that substrate polarity and pre-growth surface treatments are critical parameters for controlling β-Ga2O3 growth on 4H-SiC under oxygen-containing PAMBE conditions.

Author Contributions

Conceptualization, R.H. and S.-K.H.; methodology, R.H., S.-K.H. and T.S.N.; validation, R.H., S.-K.H., T.I. and M.-H.C.; formal analysis, R.H. and S.-K.H.; investigation, R.H., M.N., I.-G.Y. and T.H.E.; resources, S.-K.H.; data curation, R.H. and S.-K.H.; writing—original draft preparation, R.H.; writing—review and editing, S.-K.H.; visualization, R.H., S.-K.H., T.I. and M.-H.C.; supervision, S.-K.H.; project administration, S.-K.H.; funding acquisition, S.-K.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) (Grant No. 2020R1I1A3073787) and by the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Ministry of Trade, Industry and Energy (MOTIE) (Grant No. RS-2026-25544170).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Correction Statement

There was production error, that was corrected now.

Abbreviations

The following abbreviations are used in this manuscript:
PAMBEPlasma-assisted molecular-beam epitaxy
RHEEDReflection high-energy electron diffraction
AFMAtomic force microscope
HRXRDHigh-resolution X-ray diffraction
Ω-XRCΩ-x-ray rocking curve

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Figure 1. (a) RHEED evolution during the rotation of substrate with film (sample S1) on the 4° off-axis Si-face 4H-SiC substrate. The RHEED patterns on red and blue circles are from the film and substrate, respectively. (b) RHEED splitting feature observed from β-Ga2O3 grown on off-axis 4H-SiC. The azimuthal positions of the substrate and film RHEED patterns were used to assign the in-plane orientation relationship between β-Ga2O3 and 4H-SiC.
Figure 1. (a) RHEED evolution during the rotation of substrate with film (sample S1) on the 4° off-axis Si-face 4H-SiC substrate. The RHEED patterns on red and blue circles are from the film and substrate, respectively. (b) RHEED splitting feature observed from β-Ga2O3 grown on off-axis 4H-SiC. The azimuthal positions of the substrate and film RHEED patterns were used to assign the in-plane orientation relationship between β-Ga2O3 and 4H-SiC.
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Figure 2. Schematic illustration of the β-Ga2O3 film growth behaviors on 4H-SiC substrates. (a) On-axis and (b) 4° off-axis substrates.
Figure 2. Schematic illustration of the β-Ga2O3 film growth behaviors on 4H-SiC substrates. (a) On-axis and (b) 4° off-axis substrates.
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Figure 3. Evolution of RHEED patterns during the β-Ga2O3 film growth on 4° off-axis 4H-SiC substrates employing different pre-treatment procedures. (a) unintentional oxygen exposure, Si-face, (b) unintentional oxygen exposure, C-face, (c) intentional Ga pre-exposure for 5 min, Si-face, (d) Ga flash-off, followed by Ga pre-exposure for 5 min, Si-face. Each row corresponds to a certain pre-growth treatment. Each column corresponds to specific experimental steps described at the bottom, for example, before growth, after exposure, and 5, 90, 180, and 240 min of growth. In case of Ga flash-off, the RHEED pattern subjected to Ga exposure at Tsubstrate = 650 °C was additionally shown.
Figure 3. Evolution of RHEED patterns during the β-Ga2O3 film growth on 4° off-axis 4H-SiC substrates employing different pre-treatment procedures. (a) unintentional oxygen exposure, Si-face, (b) unintentional oxygen exposure, C-face, (c) intentional Ga pre-exposure for 5 min, Si-face, (d) Ga flash-off, followed by Ga pre-exposure for 5 min, Si-face. Each row corresponds to a certain pre-growth treatment. Each column corresponds to specific experimental steps described at the bottom, for example, before growth, after exposure, and 5, 90, 180, and 240 min of growth. In case of Ga flash-off, the RHEED pattern subjected to Ga exposure at Tsubstrate = 650 °C was additionally shown.
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Figure 4. AFM images of the β-Ga2O3 films with different pre-treatments and substrate surface orientations. (a) No pre-treatment, Si-face, (b) Ga pre-exposure for 1 min, Si-face, (c) Ga pre-exposure for 5 min, Si-face, (d) Ga flash-off followed by Ga pre-exposure for 5 min, Si-face, (e) no pre-treatment, C-face, (f) plot of the RMS roughness changes.
Figure 4. AFM images of the β-Ga2O3 films with different pre-treatments and substrate surface orientations. (a) No pre-treatment, Si-face, (b) Ga pre-exposure for 1 min, Si-face, (c) Ga pre-exposure for 5 min, Si-face, (d) Ga flash-off followed by Ga pre-exposure for 5 min, Si-face, (e) no pre-treatment, C-face, (f) plot of the RMS roughness changes.
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Figure 5. Growth evolution by RHEED observations with intentional oxygen-plasma pre-exposures for the β-Ga2O3 films on (a) Si-face and (b) C-face SiC substrates. AFM images for the β-Ga2O3 films on (c) Si-face and (d) C-face SiC substrates.
Figure 5. Growth evolution by RHEED observations with intentional oxygen-plasma pre-exposures for the β-Ga2O3 films on (a) Si-face and (b) C-face SiC substrates. AFM images for the β-Ga2O3 films on (c) Si-face and (d) C-face SiC substrates.
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Figure 6. HRXRD measurements of β-Ga2O3 films grown on 4° off-axis Si-face and C-face 4H-SiC substrates with various pre-growth treatments. (a) θ–2θ scans displayed using the same y-axis scale, with indexed 4H-SiC substrate and β-Ga2O3 film reflections. (b) FWHM values of the (−201) β-Ga2O3 reflection.
Figure 6. HRXRD measurements of β-Ga2O3 films grown on 4° off-axis Si-face and C-face 4H-SiC substrates with various pre-growth treatments. (a) θ–2θ scans displayed using the same y-axis scale, with indexed 4H-SiC substrate and β-Ga2O3 film reflections. (b) FWHM values of the (−201) β-Ga2O3 reflection.
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Figure 7. HRXRD Ω-XRC measurements from the β-Ga2O3 films on 4° off-axis (000±1)Si, C 4H-SiC substrates with various pre-treatments. (a) Symmetric (−402) and (b) asymmetric (−401) reflections. (c) Plot of the FWHM values for the (−402) and (−401) reflections.
Figure 7. HRXRD Ω-XRC measurements from the β-Ga2O3 films on 4° off-axis (000±1)Si, C 4H-SiC substrates with various pre-treatments. (a) Symmetric (−402) and (b) asymmetric (−401) reflections. (c) Plot of the FWHM values for the (−402) and (−401) reflections.
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Figure 8. HRXRD φ-scan measurements of the {−401} β-Ga2O3 reflection for films grown with various pre-growth treatments on 4° off-axis 4H-SiC substrates. φ-scans were measured at χ = 24° and 28° in (a), and at χ = 20° in (b), showing χ-angle-dependent, rotational-domain-related diffraction features.
Figure 8. HRXRD φ-scan measurements of the {−401} β-Ga2O3 reflection for films grown with various pre-growth treatments on 4° off-axis 4H-SiC substrates. φ-scans were measured at χ = 24° and 28° in (a), and at χ = 20° in (b), showing χ-angle-dependent, rotational-domain-related diffraction features.
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Table 1. Growth conditions and parameters of PAMBE-grown β-Ga2O3 films in this work.
Table 1. Growth conditions and parameters of PAMBE-grown β-Ga2O3 films in this work.
Sample CodeSurface PolarityPre-TreatmentTGrowth (°C)TGa cell (°C)Plasma-O (W-SCCM)Growth Time (Min)Growth Pressure (Torr)
S1Si-face 4H-SiCNo pre-exposure800820300-22404.2 × 10−5
S2Si-face 4H-SiCGa pre-exposure in 1 min800820300-22404.2 × 10−5
S3Si-face 4H-SiCGa pre-exposure in 5 min800820300-22404.2 × 10−5
S4Si-face 4H-SiCGa flash off + Ga pre-exposure in 5 min800820300-22404.2 × 10−5
S5C-face 4H-SiCNo pre-exposure800820300-22404.2 × 10−5
S6Si-face 4H-SiCO pre-exposure in 5 min800820300-22404.2 × 10−5
S7C-face 4H-SiCO pre-exposure in 5 min800820300-22404.2 × 10−5
Table 2. Quantitative comparison of surface roughness and crystalline quality of β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates with different substrate polarities and pre-growth treatments. RMS roughness values were obtained from AFM measurements. The HRXRD θ–2θ FWHM values correspond to the (−201) β-Ga2O3 reflection, and the Ω-XRC FWHM values correspond to the (−402) and (−401) β-Ga2O3 reflections.
Table 2. Quantitative comparison of surface roughness and crystalline quality of β-Ga2O3 films grown on 4° off-axis 4H-SiC substrates with different substrate polarities and pre-growth treatments. RMS roughness values were obtained from AFM measurements. The HRXRD θ–2θ FWHM values correspond to the (−201) β-Ga2O3 reflection, and the Ω-XRC FWHM values correspond to the (−402) and (−401) β-Ga2O3 reflections.
SampleRMS (nm)HRXRD θ–2θ -FWHM (Deg.)Ω-XRC -FWHM (Deg.)
(−201)(−402)(−401)
S17.250.261712.479392.9623
S25.290.247782.268722.84795
S34.230.242852.08162.70323
S43.630.240922.094232.68799
S51.800.233551.410132.51486
Note: S1: Si-face without pre-exposure; S2: Si-face with Ga pre-exposure for 1 min; S3: Si-face with Ga pre-exposure for 5 min; S4: Si-face with Ga flash-off, followed by Ga pre-exposure for 5 min; S5: C-face without pre-exposure.
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MDPI and ACS Style

Hayyak, R.; Ngo, T.S.; Iqbal, T.; Choi, M.-H.; Hong, S.-K.; Yeo, I.-G.; Na, M.; Eun, T.H. Effects of Substrate Polarity and Pre-Growth Treatments on Plasma-Assisted Molecular-Beam Epitaxy of β-Ga2O3 on 4° Off-Axis 4H-SiC. Crystals 2026, 16, 467. https://doi.org/10.3390/cryst16070467

AMA Style

Hayyak R, Ngo TS, Iqbal T, Choi M-H, Hong S-K, Yeo I-G, Na M, Eun TH. Effects of Substrate Polarity and Pre-Growth Treatments on Plasma-Assisted Molecular-Beam Epitaxy of β-Ga2O3 on 4° Off-Axis 4H-SiC. Crystals. 2026; 16(7):467. https://doi.org/10.3390/cryst16070467

Chicago/Turabian Style

Hayyak, Raouf, Trong Si Ngo, Taswar Iqbal, Mee-Hi Choi, Soon-Ku Hong, Im-Gyu Yeo, Moonkyong Na, and Tai Hee Eun. 2026. "Effects of Substrate Polarity and Pre-Growth Treatments on Plasma-Assisted Molecular-Beam Epitaxy of β-Ga2O3 on 4° Off-Axis 4H-SiC" Crystals 16, no. 7: 467. https://doi.org/10.3390/cryst16070467

APA Style

Hayyak, R., Ngo, T. S., Iqbal, T., Choi, M.-H., Hong, S.-K., Yeo, I.-G., Na, M., & Eun, T. H. (2026). Effects of Substrate Polarity and Pre-Growth Treatments on Plasma-Assisted Molecular-Beam Epitaxy of β-Ga2O3 on 4° Off-Axis 4H-SiC. Crystals, 16(7), 467. https://doi.org/10.3390/cryst16070467

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