Gallium oxide (Ga₂O₃), a representative ultra-wide bandgap (UWBG) semiconducting material, has attracted considerable attention in the applications of power electronic devices and solar-blind photodetectors owing to its unique properties, including an ultra-wide bandgap of about 4.9 eV and a high breakdown electric field of 8 MV/cm^{[1, 2]}. As is well known, Ga₂O₃ has six different phases, among which, the monoclinic β-phase is the most thermodynamically stable phase and most studies in the last decade have focused on the use of β-Ga₂O₃^{[3, 4]}. Various growth techniques have been proved to be effective methods for the preparation of β-Ga₂O₃ in forms of bulk^[5], thin films^[6–10] and nanostructures^[11]. Power devices, including MOSFET^[2], and Schottky diode^[12], as well as solar-blind photodetectors^[11] based on β-Ga₂O₃ have been reported with improving performance. In contrast, the other metastable phases are less studied but recently have drawn increasing attention due to their interesting properties. For instance, corundum-like α-phase Ga₂O₃ exhibits similar hexagonal structure and relatively small lattice mismatch with GaN, ZnO and sapphire substrates. It also offers a relatively large bandgap of about 5.1 eV, small electron effective mass, a higher breakdown field and a larger Baliga’s figure of merit^{[13, 14]}. The superior physical properties of α-Ga₂O₃, together with easy integration with other corundum structure functional oxides, such as Al, Cr, Fe oxides^[15], allows the design and delivering of high performance solar-blind photodetector and power electronic devices.

The growth of α-Ga₂O₃ thin films have been realized by various means of metal–organic vapor phase epitaxy (MOCVD)^[16], halide vapor phase epitaxy (HVPE)^[17], mist-chemical vapor deposition and molecular beam epitaxy (MBE) techniques^[18]. In particular, a high performance Schottky diode with a breakdown voltage over 1 kV and a small specific on-resistance of 2.5 mΩ·cm² has been achieved and a normally-off MOSFET has been first demonstrated based on α-Ga₂O₃ material grown by a mist-CVD system^[19]. Therefore, the growth of highly crystalline α-Ga₂O₃ on an inexpensive sapphire substrate provides an alternative platform to realize high performance power devices. However, the in-plane lattice mismatch of 4.6% between α-Ga₂O₃ and the sapphire substrate, as well as the mismatch in the coefficient of thermal expansion, will build up strain within α-Ga₂O₃ epilayers, which will lead to lattice distortion and introduce dislocations. Owing to the heteroepitaxial growth, a thick α-Ga₂O₃ epilayer over 3 μm often suffers from the generation of cracks, which is not desirable as the drift layer of the Schottky diode in vertical configuration, which can operate at high voltages^[20]. To this end, we have optimized the growth condition to produce a highly crystalline, crack-free α-Ga₂O₃ layer with a thickness up to 8 μm. The resultant thick α-Ga₂O₃ exhibits a relatively low screw dislocation density, a large crystalline domain size, and a large bandgap of about 5 eV. The lattice dynamics indicate the strain is fully relieved despite lattice mismatch.

The growth procedures were conducted using a hot-wall type mist-CVD system as developed by Shinohara and Fujita^[21]. The schematic of the mist-CVD system is shown in Fig. 1. The sapphire (0001) substrate was cleaned by acetone, ethanol and deionized water and dried by nitrogen. Gallium acetylacetonate [(C₅H₇O₂)₃Ga] solved in DI water (0.05 mol/L) with 1.5% HCl was used as Ga source. The solution was atomized into micron-sized particles by a 1.7 MHz ultrasonic transducer and then carried into the growth chamber by N₂ gas with a flow rate of 700 sccm. To eliminate the carbon contamination, a small fractional oxygen with a flow rate of 50 sccm was also introduced into the carrier gas. A right-trapezoid shaped quartz liner was designed to form a fine channel to improve the reaction efficiency of the reactants^[22]. The atmospheric pressure was maintained for the growth period and the substrate temperature was kept at 500 °C. The thickness of the grown sample was determined to be 8 μm from the cross-section SEM and the corresponding growth rate was about 1 μm/ h.

The microstructures of α-Ga₂O₃ epilayers were characterized by high resolution X-ray diffraction (HRXRD) using a D8 advance system with a Cu Kα X-ray source and a high resolution of 0.0001°. Optical transmission spectra were recorded by a UV-visible near-IR scanning spectrophotometer (Lambda 950, PerkinElmer). Raman scattering and photoluminescence spectroscopy measurements were performed at room temperature using a Micro-Raman spectrometer system (Horiba JY T64000) in a backscattering configuration with a 514 nm Ar⁺ laser as the excitation source. The laser was focused using a 100× objective for a spot size of ~2 μm in diameter and the excitation power was from 0.2 mW to avoid the laser heating effect. The surface morphology was investigated using atomic force microscopy (Asylum Research AFM).

Fig. 2 shows the X-ray diffraction (XRD) 2θ/ω scan spectrum of the thick α-Ga₂O₃ epilayer. The spectrum is dominated by the diffraction peaks at 40.34° and 41.76°, which correspond to the (0006) planes of α-Ga₂O₃ epilayer and sapphire substrate, respectively. The calculated lattice constant along the c-axis is 13.40 Å, consistent with that of the other reported values for α-Ga₂O₃. There are no other distinct diffraction peaks observed in the XRD pattern, indicating phase-pure single crystalline α-Ga₂O₃ with (0001) out-of-plane orientation has been achieved. The inset of Fig. 2 illustrates the ω scan rocking curves of (0006) and (10-14) planes under symmetric and skew-symmetric scan configuration, respectively. The full-width at half maximum (FWHM) of the ω scan rocking curves for (0006) planes is as narrow as 0.024° (86 arcsec). Similar to the hexagonal III-nitride and ZnO epilayers grown on sapphire substrates, lattice twist and tilt are directly related to the dislocation density and can be roughly evaluated from the broadening feature of the ω scan^[23]. Normally, ω-scans of (000l) reflections are used to measure the lattice tilt from mixed or screw dislocations, while edge dislocations do not distort the (000l) planes as their Burgers vectors lie within those planes. Assuming the dislocations are randomly distributed, the dislocation density can be obtained from the relationship of , where β is the FWHM of ω scan and b is the length of the Burgers vector^[24]. This model has been used to calculate the density of edge and of screw dislocation densities in III-nitride and ZnO films, with separate equations involving the ω-FWHM of symmetric and asymmetric planes^[23]. Following the same approach, the c-type screw dislocation density is estimated to be 2.24 × 10⁶ cm⁻², given that the length of the Burgers vector is equal to the lattice constant of c for (0006) orientation. The low screw dislocation density indicates a small lattice tilt from the (0006) orientation^[25]. In contrast, the ω-FWHM of (10-14) plane exhibits a larger value of 0.24°, and the corresponding dislocation density is estimated to be 1.63 × 10⁹ cm⁻² with a Burgers vector of 1/3(-2110), which is much higher than the screw dislocation density^[25]. It can be understood that the in-plane lattice mismatch of 4.6% between α-Ga₂O₃ and α-Al₂O₃ will introduce strain within the initial growth stage and the strain relaxation in thick α-Ga₂O₃ leads to the generation of a large amount of a-type edge dislocations with twist crystalline domains.

Fig. 3 shows the result of XRD Φ-scan measurement for the (10–14) plane of the α-Ga₂O₃ epilayer and α-Al₂O₃ substrate. Predominant peaks with 120° intervals appear at the same rotational angle of Φ, suggesting the epitaxial α-Ga₂O₃ film has the same corundum structure as the substrate. The epitaxial relationship is [0001]_α-Ga2O3//[0001]_α-Al2O3 in the out-of-plane orientation and [10-10]_α-Ga2O3//[10-10]_α-Al2O3 in the in-plane orientation. Aside from the distinct peaks of three-fold rotational symmetry, very weak peaks are seen in the α-Ga₂O₃ epilayer (marked by the red arrows in Fig. 3) at the positions rotated by 60° with respect to the predominant peaks. It means that a very small amount of 60°-twisted domains are formed during the epitaxial growth, which is expected to generate edge dislocations surrounding the twist boundaries of the rotational domains. The crystalline domains can be observed by the optical microscopic and atomic force microscopic (AFM) characterization as shown in Figs. 4(a) and 4(b), respectively. Determined from the optical microscopic image, the surface is crack-free but dominated by pyramid structures with different size distribution. The AFM image in Fig. 4(b) also clearly shows that a bunch of tetrahedron-structured grains dominates the surface morphology and the size of lateral coherent domain is in micron-scale. The cross-sectional profile of AFM shown in Fig. 4(c) shows that the angle between the side facet of the tetrahedron and the surface is about 6.05°, which is very close to the angle of 6.11° between the atomic planes of (0006) and (10-16). It indicates the side facets of tetrahedrons are in 10-16 orientation and exhibit a small roughness of 2.9 nm as shown in Fig. 4(d).

The optical transmittance spectrum of the α-Ga₂O₃ epilayer was recorded by using a c-plane sapphire substrate as reference and is shown in the inset of Fig. 5(a). A sharp absorption edge is observed at about 245 nm while the transmittance is low without the observable interference oscillations in the spectral range from near-ultraviolet to the visible region, which may suffer from a rough surface morphology. Typically, the optical bandgap can be evaluated by using the well-known Tauc rules of (αhν)ⁿ = A(hν − E_g), where the value of the exponent, n denotes the nature of the optical transitions^[26]. Good linearity of (αhν)ⁿ versus hν is expected when n equals 1/2 for indirect allowed transition or n equals 2 for direct bandgap^[10]. Fig. 5(a) shows the (αhv)^1/2 curve as a function of photon energy, hv for all samples derived from the original transmittance spectra. It is clear that the plot of (αhv)^1/2 versus hv exhibits an excellent linearity relationship with a smaller deviation error. The analysis unambiguously identified the indirect band structure of α-Ga₂O₃, in good agreement with the theoretical calculation results discussed earlier^[10]. The indirect optical bandgap is determined to be 5.03 eV, which is larger than that of β-Ga₂O₃.

Raman scattering is a powerful and nondestructive approach to probe lattice dynamics and evaluate crystalline quality and strain status. Fig. 5(b) shows the Raman scattering spectra of α-Ga₂O₃ epilayer obtained in a backscattering configuration together with that of α-Al₂O₃ substrate for reference. The Raman peaks located at 218.1, 288.4, 328.5, 431.4, 570.3, 690.3 cm⁻¹ are the Raman-allowed vibration modes of A_1g, E_g, E_g, E_g, E_g, A_1g and E_g, respectively, consistent with the theoretical calculation in Ref. [27]. As reported by R. Cusco et al., the α-Ga₂O₃ with a rhombohedral structure belongs to the R3c space group and Raman active optical modes in the Brillouin zone center include 2A_1g + 5E_g in terms of Raman selection rule. The low-frequency A_1g mode at 218.1 cm⁻¹ corresponds to Ga atoms vibrating against each other along the c-axis while the high-frequency A_1g mode at 570.3 cm⁻¹ mainly involves the vibration of oxygen atoms perpendicular to the c-axis. The linewidth of the low-frequency A_1g is as narrow as 4.3 cm⁻¹, strongly suggesting the highly crystallinity of the epilayer. As compared with the frequencies obtained from DFT calculations, the measured Raman frequencies are higher^[27]. This could be the result of the compressive residual strain in the α-Ga₂O₃ epilayer at the interface due to the in-plane lattice mismatch.

In summary, the 8-μm thick single-crystalline crack-free α-Ga₂O₃ epilayers have been heteroepitaxially grown on sapphire (0001) substrates via the mist chemical vapor deposition technique. The resultant α-Ga₂O₃ has a low screw dislocation density of 2.24 × 10⁶ cm⁻² with very tiny lattice tilt from the (0006) orientation, while the 60°-twisted domains formed during the epitaxial growth leads to the generation of edge dislocations with a large density of about 1.63 × 10⁹ cm⁻². The surface is crack-free but dominated by pyramid structures with domain size in micron-scale. The optical transition exhibits an indirect nature with an optical bandgap of 5.03 eV. Lattice dynamics reveal the high crystallinity of the thick epilayer with a compressive residual strain not fully relaxed at the interface. The achieved high quality α-Ga₂O₃ with unique properties may be an alternative to β-Ga₂O₃ in the application of power devices and solar-blind photodetectors.

Acknowledgements

This work was supported by the National Key Research and Development Project (No. 2017YFB0403003), Shenzhen Fundamental Research Project (Nos. 201773239, 201888588), the National Natural Science Foundation of China (Nos. 61774081, 61322403), State Key Laboratory of Wide-Bandgap Semiconductor Power Electric Devices (No. 2017KF001), the Natural Science Foundation of Jiangsu Province (No. BK20161401), the Six Talent Peaks Project in Jiangsu Province (Mo. 2014XXRJ001), the Fundamental Research Funds for the Central Universities (Nos. 021014380093, 021014380085).