The Heteroepitaxy of Thick β-Ga₂O₃ Film on Sapphire Substrate with a β-(Al_xGa_1−x)₂O₃ Intermediate Buffer Layer

Abstract

A high aluminum (Al) content β-(Al_xGa_1−x)₂O₃ film was synthesized on c-plane sapphire substrate using the gallium (Ga) diffusion method. The obtained β-(Al_xGa_1−x)₂O₃ film had an average thickness of 750 nm and a surface roughness of 2.10 nm. Secondary ion mass spectrometry results indicated the homogenous distribution of Al components in the film. The Al compositions in the β-(Al_xGa_1−x)₂O₃ film, as estimated by X-ray diffraction, were close to those estimated by X-ray photoelectron spectroscopy, at ~62% and ~61.5%, respectively. The bandgap of the β-(Al_xGa_1−x)₂O₃ film, extracted from the O 1s core-level spectra, was approximately 6.0 ± 0.1 eV. After synthesizing the β-(Al_xGa_1−x)₂O₃ film, a thick β-Ga₂O₃ film was further deposited on sapphire substrate using carbothermal reduction and halide vapor phase epitaxy. The β-Ga₂O₃ thick film, grown on a sapphire substrate with a β-(Al_xGa_1−x)₂O₃ buffer layer, exhibited improved crystal orientation along the (-201) plane. Moreover, the scanning electron microscopy revealed that the surface quality of the β-Ga₂O₃ thick film on sapphire substrate with a β-(Al_xGa_1−x)₂O₃ intermediate buffer layer was significantly improved, with an obvious transition from grain island-like morphology to 2D continuous growth, and a reduction in surface roughness to less than 10 nm.

1. Introduction

β-Ga₂O₃ has garnered significant attention in recent years due to its wide bandgap, high breakdown electrical field, and Baliga’s figure of merit [1,2]. The synthesis of β-Ga₂O₃ films on various substrates, such as Ga₂O₃ [3,4], sapphire [5,6], and GaAs [7], has been widely reported. Among these substrates, homoepitaxial β-Ga₂O₃ films exhibit a smooth surface without cracks or dislocations, making it the most favorable substrate currently available. However, the cost of β-Ga₂O₃ substrate is relatively high, which limits its market application. Compared with β-Ga₂O₃ substrate, sapphire is an economic and cost-effective substrate for heteroepitaxy. Several groups have attempted to synthesize β-Ga₂O₃ on sapphire substrate using various methods, including pulsed layer deposition (PLD) [8], molecular beam epitaxy (MBE) [4,9], halide vapor phase epitaxy (HVPE) [3,10], carbothermal reduction [11], metal organic chemical vapor deposition (MOCVD) [12,13], and low-pressure chemical vapor deposition (LPCVD) [14]. PLD and MBE, allowing for precision controllability, are able to achieve high-quality β-Ga₂O₃ films on sapphire. Nevertheless, their growth rate is not sufficient for fast growth of thick films. HVPE, carbothermal reduction, and MOCVD methods show competitive growth rates over 5 μm/h. Among these, carbothermal reduction is a promising technique for the growth of thick β-Ga₂O₃ films, as it avoids the use of corrosive precursor gases. However, the crystal quality is significantly degraded under fast growth because of the lattice mismatch between the corundum structure of sapphire (α-Al₂O₃) and the monoclinic structure of β-Ga₂O₃. One solution for heteroepitaxy on sapphire is to use a larger bandgap material as a buffer layer to mitigate the lattice mismatch-induced strain, as is commonly known for growth of GaN on AlN/sapphire template. In the case of β-Ga₂O₃, we employed the ((Al_xGa_1−x)₂O₃) film as a buffer layer to indirectly mitigate the lattice mismatch. Y. Cheng et al. grew β-(Al_xGa_1−x)₂O₃ on a sapphire substrate and used it as an intermediate buffer layer to epitaxially grow β-Ga₂O₃ thin films. The results showed that β-(Al_xGa_1−x)₂O₃, as an intermediate buffer layer, can reduce lattice mismatch and improve the crystal quality of β-Ga₂O₃ thin films [15].

Currently, molecular beam epitaxy (MBE) [16], pulsed laser deposition (PLD) [17], and metal–organic vapor deposition (MOCVD) [18] are common methods used to grow (Al_xGa_1−x)₂O₃ alloys. However, achieving a growth strategy for β-(Al_xGa_1−x)₂O₃ film remains a challenge. This obstacle is associated with synthesizing a stable high-Al-content β-(Al_xGa_1−x)₂O₃ by direct growth techniques, such as MOCVD, MBE, and PLD, due to the phase transformation from β to γ for (Al_xGa_1−x)₂O₃ when the Al content exceeds 30% [19,20]. Additionally, these methods are based on expensive vacuum equipment. Instead of direct epitaxial β-(Al_xGa_1−x)₂O₃ on sapphire, we utilized the gallium (Ga) diffusion method, which has previously been used to fabricate Ga-diffused waveguides in sapphire [21].

In this paper, the β-(Al_xGa_1−x)₂O₃ film was grown on c-plane sapphire substrate in a high-temperature tubular furnace by the gallium (Ga) diffusion method, serving as an intermediate buffer layer for the subsequent heteroepitaxial growth of the thick β-Ga₂O₃ film. The obtained β-(Al_xGa_1−x)₂O₃ film displayed a high crystal quality, with a thickness of approximately 750 nm. The distribution of Al components in the film was homogenous, with an Al content of approximately 62%. After synthesizing the β-(Al_xGa_1−x)₂O₃ buffer layer, the β-Ga₂O₃ thick film was further deposited on the β-(Al_xGa_1−x)₂O₃/sapphire template using two methods: carbothermal reduction, reported recently by our group [11], and HVPE. Finally, we characterized the properties of the β-Ga₂O₃ thick film on sapphire with and without the β-(Al_xGa_1−x)₂O₃ buffer layer. The results showed that the β-Ga₂O₃ thick film, grown on a sapphire substrate with a β-(Al_xGa_1−x)₂O₃ buffer layer, improved crystal orientation and surface quality.

2. Experiments

The (Al_xGa_1−x)₂O₃ film was synthesized using a high-temperature tubular furnace, as illustrated in Figure 1. Ga2O3 powder with purity of 99.999% as the source material was put in a corundum crucible. The sapphire substrate was inserted into the corundum crucible, and the system was subjected to setting temperature of 1450 °C. The growth process took place in an anoxic atmosphere, where 1.5 slm argon (Ar) was maintained at the pressure of 3 × 10⁴ Pa for 2 h. In a neutral gas atmosphere, Ga₂O₃ powder underwent decomposition into volatile Ga₂O(g), which further decomposed into gaseous Ga, as illustrated below:

$$\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)\to \mathrm{G}{\mathrm{a}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(1)

$$\mathrm{G}{\mathrm{a}}_2\mathrm{O}\left(\mathrm{g}\right)\to 2\mathrm{G}\mathrm{a}\left(\mathrm{g}\right)+1/2{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(2)

The Ga species diffused into α-Al₂O₃, resulting in the formation of a β-(Al_xGa_1−x)₂O₃ buffer layer according to the Al₂O₃-Ga₂O₃ phase diagram.

The Ga₂O₃ thick film was grown using the carbothermal reduction method in a home-made growing system as shown in Figure 2. During the growth process, 20 sccm of O₂ and 500 sccm of Ar were kept for 2 h at a pressure of 3 × 10⁴ Pa. The growth condition for HVPE was as follows: the ratio of flow rate between HCl and O₂, setting growth temperature, and pressure were 10/30, 1060 °C, and 5 × 10⁴ Pa, respectively. The growth was kept for 2 h for HVPE.

3. Results and Discussions

The thickness of the (Al_xGa_1−x)₂O₃ film was investigated using cross-sectional scanning electron microscopy (FEI Nova Nano SEM 450), as shown in Figure 3a, which presents a clear interface between (Al_xGa_1−x)₂O₃ and sapphire substrate. The β-(Al_xGa_1−x)₂O₃ film thickness was measured to be 750 nm, corresponding to a growth rate of 375 nm/h. The surface morphology and roughness of the (Al_xGa_1−x)₂O₃ film were characterized by top-view scanning electron microscopy and atomic force microscopy (AFM, Dimension Icon, Bruker, Germany). Figure 3b shows the equilateral triangular morphology of the (Al_xGa_1−x)₂O₃ film, which corresponds to the arrangement of oxygen atoms (equilateral triangles) on a c-plane sapphire substrate. When β-Ga₂O₃ is grown on a c-plane sapphire substrate, the oxygen atoms on the surface between the β-Ga₂O₃ (-201) plane and the c-plane sapphire are arranged in equilateral triangles, leading to (-201)-oriented growth. The AFM image corroborates the equilateral triangular morphology and corresponds to a root mean square roughness (RMS) of around 2.10 nm.

The crystalline orientation was characterized using high-resolution X-ray diffraction (HRXRD, Bruker D8 Advance). Figure 4a depicts a θ–2θ scan XRD result, showing three diffraction peaks at 19.3°, 39.12°, and 60.2°, corresponding to the (-201), (-402), and (-603) planes of monoclinic β-(Al_xGa_1−x)₂O₃, respectively. The growth of β-(Al_xGa_1−x)₂O₃ film on (0001) sapphire substrate along the (-201) crystal plane is attribute to the similar arrangement of oxygen atoms(equilateral triangles) at the surface monoclinic structure β-Ga₂O₃ (-201) plane and corundum structure α-Al₂O₃ (0001) plane. The diffraction peaks of β-Ga₂O₃ (PDF#43-1012) were used as a standard. As shown in

Table 1. The diffraction peak positions of β-(Al_xGa_1−x)₂O₃ film and the corresponding β-Ga₂O₃ standard diffraction peak positions.

Sample	(-201)	(-402)	(-603)
β-Ga2O3 (reference)	18.95	38.404	59.19
(AlxGa1−x)2O3	19.3	39.12	60.2

, the diffraction peak position of β-(Al_xGa_1−x)₂O₃ film shifted to a higher diffraction angle. This phenomenon arose because the Ga³⁺ ion was replaced by the smaller-radius Al³⁺ ion, causing the lattice spacing to shrink and the diffraction peak to move to a higher angle. Considering the monoclinic structure, the Al compositions in β-(Al_xGa_1−x)₂O₃ film was determined using following expression [22,23].

$$\frac{1}{d^2}=\frac{h^2}{a^2\mathrm{s}\mathrm{i}{\mathrm{n}}^2\beta }+\frac{k^2}{b^2}+\frac{l^2}{c^2\mathrm{s}\mathrm{i}{\mathrm{n}}^2\beta }-\frac{2hl\cos \beta }{ac\mathrm{s}\mathrm{i}{\mathrm{n}}^2\beta }$$

(3)

where h = −4, k = 0, and l = 2. Based on the (-402) diffraction peak position and Equation (3), we obtained the Al compositions in films to be around 62%. The crystalline quality of β-(Al_xGa_1−x)₂O₃ film was characterized by ω rocking curve spectra. As shown in Figure 4b, the full width at half maximum (FWHM) of the (-201) plane was around 0.42°, indicating that the β-(Al_xGa_1−x)₂O₃ film had high crystalline quality, although the crystalline quality of β-(Al_xGa_1−x)₂O₃ films prepared by PLD, MBE, and MOVD deteriorated when the Al composition exceeded 30% [19,20].

The content of Al in β-(Al_xGa_1−x)₂O₃ films was further determined by X-ray photoelectron spectroscopy (XPS, K-alpha+). A wide survey spectrum clearly showed peaks of Al 2s and Al 2p for the β-(Al_xGa_1−x)₂O₃ film, along with β-Ga₂O₃ crystal as a reference, as shown in Figure 5a. The Al compositions in the film were estimated from the Al 2p and Ga 2p core-level peak areas, considering the sensitivity factors of the elements. Figure 5b,c show the Al 2p and Ga 2p core-level spectra for the film. The Al 2p peak in the β-(Al_xGa_1−x)₂O₃ film displayed a binding energy of 73.96 eV. This shift towards a lower binding energy compared with the Al 2p peak in the sapphire substrate (74.5 eV) can be attributed to the formation of Al-O-Ga bonds [24]. Based on the XPS results, the Al composition in the β-(Al_xGa_1−x)₂O₃ film was around 61.5%, which is consistent with the quantitative results obtained from the XRD analysis.

The bandgap of β-(Al_xGa_1−x)₂O₃ film was determined by analyzing the O 1s core-level spectra in XPS. This approach has been established by previous studies [25,26,27,28]. The bandgap energy can be derived from the difference between the core-level peak energy and the initial inelastic losses [29]. Figure 6 presents the O1s core-level spectra obtained from XPS analysis of the β-(Al_xGa_1−x)₂O₃ film. The O1s peak energy was 530.8 eV, while the initial inelastic losses were 536.8 eV. The bandgap of β-(Al_xGa_1−x)₂O₃ film extracted from the O1s spectra was approximately 6.0 ± 0.1 eV, which is consistent with the bandgap energy estimated by absorption spectra in previous literature [30]. The impurity in the β-(AlxGa1-x)2O3 film was investigated using time-of-flight secondary ion mass spectrometry (SIMS, IONTOF 5). Figure 7 shows the TOF-SIMS depth profile for the β-(Al_xGa_1−x)₂O₃ film, which revealed that the impurity in the film was negligible. Additionally, the β-(Al_xGa_1−x)₂O₃ film exhibited a homogenous distribution of Al.

High-resolution field emission transmission electron microscopy (HRTEM, JEM F200) was used to investigate the interface microstructure of β-(Al_xGa_1−x)₂O₃/Al₂O₃ heterojunction, as shown in Figure 8. The crystal lattice spacing of β-(Al_xGa_1−x)₂O₃ was 0.456 nm, corresponding to the (-201) crystal plane spacing. The transition layer thickness in the vicinity of the interface on sapphire substrates was approximately ~3.3 nm. As shown in the inset, the fast Fourier transform (FFT) diffraction patterns describe the monoclinic β-phase crystal structure and the high crystalline quality of the β-(Al_xGa_1−x)₂O₃ film.

After preparing the buffer layer, the β-(Al_xGa_1−x)₂O₃/sapphire template was transferred to a fast epitaxial β-Ga₂O₃ thick film through carbothermal reduction and HVPE techniques, respectively. A reference sample of β-Ga₂O₃ on sapphire without a buffer layer was also grown under the same conditions for comparison. The growth rate for all samples was approximately 4~6 μm/h, depending on the film thickness measured by cross-sectional SEM image as shown in Supplementary Figure S1. Figure 9 presents the θ–2θ scan XRD characterization of β-Ga₂O₃ thick film on sapphire with and without a β-(Al_xGa_1−x)₂O₃ buffer layer, respectively. The samples grown on the buffer layer showed a clear appearance of dominant (-201) and high-order β-Ga₂O₃ diffraction peaks, as shown in Figure 9b,c for the growth carried out by carbothermal reduction and HVPE, respectively. However, the β-Ga₂O₃ on sapphire without a buffer layer revealed a competitive crystal orientation of (400), (002), (-403), and (-313) peaks marked in Figure 9a, except for the peak of (-201) orientation planes. This competitive crystal orientation is due to the lattice mismatch related anisotropic growth, which leads to the existence of rhombic prism faces, as marked in the rectangle area indicated in Figure 10a. The XRD results demonstrate that miscellaneous crystalline facets were strongly inhibited for β-Ga₂O₃ thick film on sapphire by means of a β-(Al_xGa_1−x)₂O₃ buffer layer, which implies a much-improved crystalline quality.

The surface properties of β-Ga₂O₃ thick film were investigated by SEM. As shown in Figure 10a, the surface morphology of β-Ga₂O₃ on sapphire without a buffer layer displayed the pseudo hexagonal shape with an average grain size of 4 μm and well-defined boundaries. The details of coalesced β-Ga₂O₃ grain were composed of rhombic prism faces, which contribute to the sub-peak in XRD measurement as we mentioned above. Taking into account the β-Ga₂O₃ thick film on sapphire with β-(Al_xGa_1−x)₂O₃ buffer layer, the SEM image explicitly unveiled a significant improvement in surface quality through the obvious transition from grain island-like morphology to 2D continuous growth, as shown in Figure 10b,c, respectively.

The surface roughness was finally identified by AFM measurements of 5 × 5 μm. The AFM results displayed highly correlated morphology with the high magnification SEM images for all three samples. The RMS surface roughness was 74 nm for the β-Ga₂O₃ thick film on sapphire without a buffer layer, as shown in Figure 11a. By contrast, the use of the β-(Al_1−xGa_x)₂O₃ buffer layer resulted in a much smoother surface, as confirmed by the RMS values of 9 nm and 5 nm for the epitaxial film prepared by carbothermal reduction and HVPE, respectively, as shown in Figure 11b,c. Further effort would be to optimize the growth parameter to obtain an even smoother surface without additional pits.

4. Conclusions

In this paper, we described the heteroepitaxy of a thick β-Ga₂O₃ film on c-plane sapphire substrate employing a larger bandgap β-(Al_xGa_1−x)₂O₃ buffer layer to improve the growth quality while maintaining a comparable growth rate (~5 μm/h). We used the gallium (Ga) diffusion method to create a β-(Al_xGa_1−x)₂O₃ buffer layer on c-plane sapphire substrate. The Al composition in the β-(Al_xGa_1−x)₂O₃ film estimated by XRD was ~62%, which is comparable to the result of ~61.5% estimated by XPS. The bandgap of the β-(Al_xGa_1−x)₂O₃ film derived from the O 1s core-level spectra was around 6.0 ± 0.1 eV. SIMS results indicated a homogenous distribution of the Al element in the film. After the formation of the β-(Al_xGa_1−x)₂O₃ buffer layer, the β-Ga₂O₃ thick film was deposited on the β-(Al_xGa_1−x)₂O₃/sapphire template by carbothermal reduction and HVPE, respectively. The β-Ga₂O₃ thick film on the sapphire substrate with the β-(Al_xGa_1−x)₂O₃ buffer layer exhibited an evident appearance of dominated (-201) crystalline facets and inhibited (400), (002), (-403), and (-313) miscellaneous crystalline facets, regardless of the growth method. The surface quality of the β-Ga₂O₃ thick film on the β-(Al_xGa_1−x)₂O₃/sapphire template was significantly improved compared with that without a buffer layer, as measured by SEM and AFM, with the obvious transition from a grain island-like morphology to 2D continuous growth and a reduction of surface roughness to less than 10 nm.