In this study, ε-Ga2O3 thin films were grown by mist chemical vapor deposition on a hexagonal (0001) GaN template and a cubic (111) SrTiO3 (STO) substrate. By analyzing the obtained X-ray diffraction (XRD) φ-scans, it was found that the ε-Ga2O3 epitaxial thin films grown on both GaN and STO exhibited an orthorhombic structure. In addition, a method was proposed to distinguish between hexagonal and orthorhombic structures on the basis of the relationships between the 2θ and χ angles for these structures, obtained from the XRD studies. Finally, a mechanism was discussed on the basis of angle relationships, where three rotational domains were observed for orthorhombic ε-Ga2O3 on GaN and STO. Transmission electron microscopy was then employed to determine whether the ε-Ga2O3 thin films on the GaN template and STO substrate consisted of columnar ε-Ga2O3 comprising small domains and intermediate layers between the ε-Ga2O3 film and the substrate.

In this study, single-phase ε-gallium oxide (Ga2O3) thin films were heteroepitaxially grown on c-plane sapphire substrates.

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.

This study demonstrates the formation of the van der Waals epitaxy of the ε-gallium oxide (Ga 2 O 3 ) thin film on cleaved synthetic mica via mist chemical vapor deposition. Orthorhombic ε-Ga 2 O 3 (001) was epitaxially grown on synthetic mica (001). The analysis using transmission electron microscopy revealed an in-plane orientation of ε-Ga 2 O 3 [010] || synthetic mica [010]. However, the most thermodynamically stable β-Ga 2 O 3 was grown at the film-substrate interface. The optical direct bandgap of the ε-Ga 2 O 3 thin film grown by van der Waals epitaxy was estimated to be 5.0 eV, which was the same as for the heteroepitaxially grown ε-Ga 2 O 3 thin film on other substrates. Besides, after ε-Ga 2 O 3 thin film was grown on the synthetic mica substrates that are cleaved sufficiently thin, the sample could be bent or cut with scissors. These results denote that the epitaxial ε-Ga 2 O 3 thin films grown by van der Waals epitaxy can be applied to flexible electronics.

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.

We studied the phase diagram of (In x Ga1−x )2O3 thin films with a composition of x = 0 to 1 on Aluminum Nitride (AlN) templates grown using mist chemical vapor deposition. From X-ray diffraction results, we observed that the (In x Ga1−x )2O3 thin films exhibited three different single-phase crystal structures depending on the value of x: orthorhombic (κ)-(In x Ga1−x )2O3 for x ≤ 0.186, hexagonal (hex)-(In x Ga1−x )2O3 for 0.409 ≤ x ≤ 0.634, and body-centered cubic (bcc)-(In x Ga1−x )2O3 for x ≥ 0.772. The optical bandgap of (In x Ga1−x )2O3 was tuned from 3.27 eV (bcc-In2O3) and 4.17 eV (hex-InGaO3) to 5.00 eV (κ-Ga2O3). Moreover, hex-(In x Ga1−x )2O3 exhibited a wide bandgap (4.30 eV) and a low resistivity (7.4×10‒1 Ω·cm). Furthermore, hex-(In x Ga1−x )2O3 thin films were successfully grown on GaN and AlGaN/GaN templates. Therefore, hex-(In x Ga1−x )2O3 can be used in transparent conductive films for deep-ultraviolet LEDs.

The resistive switching temperature associated with the metal−insulator transition (MIT) of epitaxial VO 2 thin films grown on flexible synthetic mica was modulated by bending stress. The resistive switching temperature of polycrystalline VO 2 and V 2 O 5 thin films, initially grown on synthetic mica without a buffer layer, was observed not to shift with bending stress. By inserting a SnO 2 buffer layer, epitaxial growth of the VO 2 (010) thin film was achieved, and the MIT temperature was found to vary with the bending stress. Thus, it was revealed that the bending response of the VO 2 thin film depends on the presence or absence of the SnO 2 buffer layer. The bending stress applied a maximum in-plane tensile strain of 0.077%, resulting in a high-temperature shift of 2.3 °C during heating and 1.8 °C during cooling. After 10 4 bending cycles at a radius of curvature R = 10 mm, it was demonstrated that the epitaxial VO 2 thin film exhibits resistive switching temperature associated with MIT.

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