Daisuke Tahara

Heteroepitaxial growth of ε-Ga2O3 thin films on cubic (111) MgO and (111) yttria-stablized zirconia substrates by mist chemical vapor deposition

In this study, epitaxial e-Ga2O3 thin films are successfully grown on cubic (111) MgO and (111) yttria-stablized zirconia (YSZ) substrates by mist chemical vapor deposition. Pure-phase hexagonal e-Ga2O3 thin films are grown on the two substrates with a c-axis orientation determined by X-ray diffraction (XRD) 2θ–ω scanning. XRD pole figure measurements reveal that the in-plane orientation relationship between the (0001) of e-Ga2O3 and the (111) of the two substrates is e-Ga2O3 ∥ substrates . Using (111) MgO substrates with a 2.5% lattice mismatch, the epitaxial e-Ga2O3 films are successfully grown at a low temperature of 400 °C. The optical direct and indirect bandgaps of pure e-Ga2O3 thin films are estimated as 5.0 and 4.5 eV, respectively.

Stoichiometric control for heteroepitaxial growth of smooth ε-Ga2O3 thin films on c-plane AlN templates by mist chemical vapor deposition

Epitaxial e-Ga2O3 thin films with smooth surfaces were successfully grown on c-plane AlN templates by mist chemical vapor deposition. Using X-ray diffraction 2θ–ω and scans, the out-of-plane and in-plane epitaxial relationship was determined to be (0001) e-Ga2O3 ∥ (0001)AlN. The gallium/oxygen ratio was controlled by varying the gallium precursor concentration in the solution. While scanning electron microscopy showed the presence of large grains on the surfaces of the films formed for low concentrations of oxygen species, no large grains were observed under stoichiometric conditions. Cathodoluminescence measurements showed a deep-level emission ranging from 1.55–3.7 eV; however, no band-edge emission was observed.

Incorporation of indium into ε-gallium oxide epitaxial thin films grown via mist chemical vapour deposition for bandgap engineering

Epitaxial e-gallium oxide (Ga2O3) thin films incorporated with In were successfully grown by mist chemical vapour deposition (CVD) on c-plane sapphire substrates for bandgap tuning. In was successfully incorporated into epitaxial e-(InxGa1−x)2O3 films at an In composition of x = 0.2 without inducing phase separation. Phase separation originated from the (400) bixbyite structure of (InxGa1−x)2O3 when x > 0.2. The solubility limit of In incorporated into e-Ga2O3 on sapphire substrates via mist CVD was therefore x = 0.2. Transmission electron microscopy measurements revealed that e-(InxGa1−x)2O3 consisted of polycrystalline phases observed in the interface of the sapphire substrate and e-phases located above the polycrystalline phase. The pole figure of e-(InxGa1−x)2O3 thin films revealed that the epitaxial relationship between the e-(InxGa1−x)2O3 thin film and the α-Al2O3 substrate is (001) e-(InxGa1−x)2O3 [130]||(0001) α-Al2O3 [11−20]. The optical bandgap of the e-(InxGa1−x)2O3 thin films was tuned from 4.5 to 5.0 eV without inducing phase separation.

Heteroepitaxial growth of ε-(AlxGa1−x)2O3 alloy films on c-plane AlN templates by mist chemical vapor deposition

In this study, e-(AlxGa1−x)2O3 alloy films were grown on c-plane AlN templates by mist chemical vapor deposition. The Al content of two samples was determined by Rutherford backscattering analysis. The lattice constant of the e-(AlxGa1−x)2O3 alloy films followed Vegards law, and the Al contents of other samples were determined to be as high as x = 0.395 by Vegards law. The direct bandgap was obtained in the range of 5.0–5.9 eV by transmittance measurements. The valence-band offset between e-(Al0.395Ga0.605)2O3 and e-Ga2O3 was analyzed to be 0.2 eV, and the conduction-band offset was calculated to be 0.7 eV by X-ray photoelectron spectroscopy. The e-(AlxGa1−x)2O3/e-Ga2O3 interface band discontinuity was type I. Our experimental results will be important for the actual application of e-(AlxGa1−x)2O3/e-Ga2O3 heterojunction devices.

Heteroepitaxial growth of single-phase ε-Ga2O3 thin films on c-plane sapphire by mist chemical vapor deposition using a NiO buffer layer

In this study, single-phase e-gallium oxide (Ga2O3) thin films were heteroepitaxially grown on c-plane sapphire substrates. When the Ga2O3 films were directly grown on c-plane sapphire substrates, they tended to grow with a mixture of α-, β-, and e-Ga2O3. However, with the insertion of a cubic NiO buffer layer, single-phase e-Ga2O3 thin films were successfully grown at temperatures from 400 °C to 800 °C. Furthermore, e-Ga2O3 thin films grown at 750 °C exhibited a smooth surface. Transmission electron microscopy observations revealed that the (111) plane-oriented NiO buffer layer prevented the growth of both polymorphs other than e-Ga2O3 and the intermediate layers. The direct bandgap was estimated to be 4.9 eV in thin films in which e-Ga2O3 was predominant and 5.3 eV in thin films dominated by α-Ga2O3.

Heteroepitaxial growth of ε-Ga 2 O 3 thin films on cubic (111) GGG substrates by mist chemical vapor deposition

We demonstrated that heteroepitaxial ε-Ga<inf>2</inf>O<inf>3</inf> thin films can be grown on (111) GGG substrates by mist CVD. The out-of-plane and in-plane epitaxial relationship between the ε-Ga<inf>2</inf>O<inf>3</inf> thin film and the (111) GGG substrate was determined to be (0001) ε-Ga<inf>2</inf>O<inf>3</inf> [10-10] // (111) GGG [-1-12] by XRD 2θ-ω and φ scanning. This is the first report on heteroepitaxial growth of ε-Ga<inf>2</inf>O<inf>3</inf> thin films on (111) GGG substrates by mist CVD.