Heteroepitaxial films of Ga 2 O 3 were grown on c-plane sapphire (0001). The stable phase β-Ga 2 O 3 was grown using the metalorganic chemical vapor deposition technique, regardless of precursor flow rates, at temperatures between 500 • C and 850 • C. Metastable α-and ε-phases were grown when using the halide vapor phase epitaxy (HVPE) technique, at growth temperatures between 650 • C and 850 • C, both separately and in combination. XTEM revealed the better lattice-matched α-phase growing semi-coherently on the substrate, followed by ε-Ga 2 O 3 . The epitaxial relationship was determined to be [1100] IMPACT STATEMENTThis study demonstrates one of the first epitaxial growths of multiple polymorphs of Ga 2 O 3 on sapphire (0001) substrates, including its β-, α-, and ε-phases. Epitaxial relationship is confirmed through HRTEM. ARTICLE HISTORY

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There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges. V C 2013 AIP Publishing LLC.

Electrical tunability of molecular doping of graphene has been investigated using back-gated field effect transistors. Variation of the gate voltage from positive to negative values resulted in reduced p-type doping by NO2, which decreased below detection limit at −45 V. A reverse trend was observed for NH3, where its n-type doping increased with more negative gate voltage, becoming undetectable at 5 V. Our results indicate that adsorption induced molecular doping of graphene could not be detected when the Fermi level coincides with the adsorption induced defect states, which yields NO2 acceptor energy level of ∼320 meV below the Dirac point.

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