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We report across-bandgap p-type and n-type control over the Seebeck coefficients of semiconducting single-wall carbon nanotube networks through an electric double layer transistor setup using an ionic liquid as the electrolyte. All-around gating characteristics by electric double layer formation upon the surface of the nanotubes enabled the tuning of the Seebeck coefficient of the nanotube networks by the shift in gate voltage, which opened the path to Fermi-level-controlled three-dimensional thermoelectric devices composed of one-dimensional nanomaterials.

Epitaxial growth of ε-Ga2O3 is demonstrated for the first time. The ε-Ga2O3 films are grown on GaN (0001), AlN (0001), and β-Ga2O3 (2¯01) by halide vapor phase epitaxy at 550 °C using gallium chloride and O2 as precursors. X-ray ω-2θ and pole figure measurements prove that phase-pure ε-Ga2O3 (0001) films are epitaxially grown on the three kinds of substrates, although some minor misoriented domains are observed. High temperature X-ray diffraction measurements reveal that the ε-Ga2O3 is thermally stable up to approximately 700 °C. The optical bandgap of ε-Ga2O3 is determined for the first time to be 4.9 eV.

Ga2O3 is an ultrawide bandgap semiconductor with a bandgap energy of 4.5–5.3 eV (depending on its crystal structure), which is much greater than those of conventional wide bandgap semiconductors such as SiC and GaN (3.3 eV and 3.4 eV, respectively). Therefore, Ga2O3 is promising for future power device applications, and further high-performance is expected compared to those of SiC or GaN power devices, which are currently in the development stage for commercial use. Ga2O3 crystallizes into various structures. Among them, promising results have already been reported for the most stable β-Ga2O3, and for α-Ga2O3, which has the largest bandgap energy of 5.3 eV. In this article, we overview state-of-the-art technologies of β-Ga2O3 and α-Ga2O3 for future power device applications. We will give a perspective on the advantages and disadvantages of these two phases in the context of comparing the two most promising polymorphs, concerning material properties, bulk crystal growth, epitaxial growth, device fabrication, and resulting device performance.

The Ge doping of β-Ga2O3(010) films was investigated using plasma-assisted molecular beam epitaxy as the growth method. The dependences of the amount of Ge incorporated on the substrate temperature, Ge-cell temperature, and growth regime were studied by secondary ion mass spectrometry. The electron concentration and mobility were investigated using Van der Pauw Hall patterns. Hall measurement confirmed that Ge acts as an n-dopant in β-Ga2O3(010) films. These results were compared with similar films doped by Sn. The Hall data showed an improved electron mobility for the same electron concentration when Ge is used instead of Sn as the dopant.