LiNbO3 and LiTaO3 substrates are used in wide-bandwidth applications such as surface acoustic wave filter and show structural similarity to α-Ga2O3. In this study, we demonstrated the phase control of Ga2O3 epitaxial thin films, grown by mist chemical vapor deposition, on the (0001) planes of LiNbO3 and LiTaO3 substrates using α-Fe2O3 buffer layers. κ-Ga2O3 thin films were grown epitaxially on bare LiNbO3 and LiTaO3 substrates. Conversely, the insertion of the α-Fe2O3 buffer layer led to the preferential growth of the α-Ga2O3 epitaxial thin films. X-ray diffraction (XRD) φ-scan results revealed that the α-Ga2O3 thin films were grown along the same in-plane direction as that of the substrates. Besides, the XRD φ-scan results indicated that twin-free α-Ga2O3 was grown on the LiNbO3 and LiTaO3 substrates with the α-Fe2O3 buffer layers. The x-ray rocking curve scans of the asymmetric plane of α-Ga2O3 showed that the full width at half maximum values of α-Ga2O3 on the LiNbO3 and LiTaO3 substrates with the buffer layers were smaller than that of the c-plane α-Al2O3 substrate with and without the buffer layer. In addition, we grew the (11-20) and (1-100) planes of the α-Ga2O3 epitaxial thin films on the (11-20) and (1-100) planes of LiNbO3 substrates with the α-Fe2O3 buffer layer, respectively. This study showed that LiTaO3 and LiNbO3 are promising substrates for the epitaxial growth of α-Ga2O3 and κ-Ga2O3.

AbstractBendable p-type NiO and n-type In2O3 thin films were epitaxially grown on synthetic mica using mist chemical vapor deposition. It was found that at a growth temperature of 400 °C, epitaxially grown cubic (111) NiO thin films developed twin rotational domains, and the epitaxial relationship between each domain and the substrate was (111) NiO [1-10] or [10-1] || (001) synthetic mica [100]. In the visible light region, the epitaxial NiO thin films showed high transparencies, and their cut-offs appeared in the UV region. Additionally, at a growth temperature of 500 °C, cubic (111) In2O3 thin films with and without Sn doping were epitaxially grown on synthetic mica. As a result of the plasma oscillation of free carriers, Sn-doped In2O3 thin films exhibited reflection characteristics in the infrared region, while maintaining their visible light transmission characteristics. Furthermore, compared with non-doped In2O3, Sn doping decreased the sheet resistance by two digits.

This study demonstrated homoepitaxial growth of Ge-doped β-Ga2O3 thin films on β-Ga2O3 substrates via mist chemical vapor deposition (CVD) using GeI4, a water-soluble Ge precursor. The carrier concentration of the Ge-doped β-Ga2O3 thin films was controlled by varying the Ge precursor concentration in the solution. A mobility of 66 cm2V-1s-1 was obtained at a carrier density of 3.4×1018 cm-3 using oxygen gas. X-ray diffraction (XRD) scans revealed that homoepitaxial Ge-doped β-Ga2O3 thin films were grown on β-Ga2O3 without phase separation. However, the XRD rocking curves revealed that the mist CVD- grown Ge-doped β-Ga2O3 was slightly degraded compared to the substrate as the Ge concentration increased. The surface morphologies of the Ge-doped β-Ga2O3 exhibited atomically flat surfaces with a root mean square roughness of less than 1 nm. These results indicate that the Ge-doped β-Ga2O3 thin films prepared by mist chemical vapor deposition are promising for device applications.

Gallium oxide (Ga 2 O 3 ) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga 2 O 3 . Here, we demonstrate the epitaxial growth of metastable δ-Ga 2 O 3 thin films by mist chemical vapor deposition using β-Fe 2 O 3 buffer layers. X-ray diffraction (XRD) 2θ−ω scan pattern revealed that (004) κ-Ga 2 O 3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In 2 O 3 buffer layer, whereas (222) δ-Ga 2 O 3 grew on (222) β-Fe 2 O 3 . The β-Fe 2 O 3 buffer layer led to the epitaxial growth of the δ-Ga 2 O 3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe 2 O 3 and δ-Ga 2 O 3 triggered this growth. XRD analysis shows that δ-Ga 2 O 3 grew epitaxially on the β-Fe 2 O 3 buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga 2 O 3 grown on β-Fe 2 O 3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga 2 O 3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga 2 O 3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga 2 O 3 epitaxial thin film for the first time. KEYWORDS: δ-Ga 2 O 3 , β-Fe 2 O 3 , bixbyite structure, epitaxial growth, mist CVD

Corundum structured α-Ga2O3 is a promising semiconductor material for power switching devices due to its large bandgap (5.3 eV). The high dislocation density of α-Ga2O3 caused by a large lattice mismatch between the α-Ga2O3 and a sapphire substrate is a significant issue to a high-reliability operation. We featured LiTaO3 substrates exhibiting a near corundum structure as growth substrates to decrease the lattice mismatch. Our previous study revealed that the growth of α-Ga2O3 on LiTaO3 substrates required α-Fe2O3 buffer layers. In this study, the impact of α-Fe2O3 buffer layer growth time on the growth of α-Ga2O3 was investigated. X-ray diffraction 2θ-ω analysis revealed that α-Ga2O3 was successfully grown by inserting α-Fe2O3 buffer layer regardless of the buffer layer growth time. The growth time of the α-Fe2O3 buffer layer affected the number of edge dislocations examined by XRD rocking curve (XRC) measurements. The smallest full width at half maximum of XRC measurements at (0006) plane of the α-Ga2O3 with the buffer layer growth time of 1min was 60 arcsec. The α-Ga2O3 with the buffer layer growth time of 1 min had a smooth surface. This study contributes power switching device application of α-Ga2O3 grown on LiTaO3 substrates.