A diode structure consisting of a polar epilayer on a nonpolar substrate grown by metalorganic vapor phase epitaxy often faces problems of antiphase domain formation in the polar semiconductor and cross diffusion across the heterointerface. Ge outdiffusion into GaAs epilayers was studied by low temperature photoluminescence spectroscopy after etching the film from the surface. The absence of p-n junction formation inside the Ge substrate from interdiffusion of Ga and As has been studied by current-voltage characteristics using mesa diodes. These observations were confirmed by electrochemical capacitance voltage polaron profiler and secondary ion mass spectroscopy techniques. To understand the material quality and current conduction mechanism across the GaAs/ Ge heterojunction, I-V characteristics of Si-doped n-GaAs/n-Ge isotype heterojunctions using Au Schottky diodes have been studied for different doping densities. A plethora of growth conditions appear in the literature concerning the attempt to grow antiphase domain ͑APD͒-free GaAs on Ge. In the present case, even though the growth temperature regime is close to reported values, the main difference in minimizing APD formation may arise from the growth rates ͑ϳ3 m/h͒ and the V/III ratio ͑ϳ88͒.

Multiferroic BiFeO3 (BFO) nanotubes have been successfully fabricated by the modified sol–gel method within the nanochannels of porous anodic aluminum oxide (AAO) templates. The morphology, structure and composition of the nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM, (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). Postannealed (650 °C for 1 h), BFO nanotubes were polycrystalline and X-ray diffraction study revealed that they are of the rhomohedrally distorted perovskite crystal structure. The results of SEM and TEM revealed that BFO nanotubes possessed a uniform length (up to 60 μm) and diameter (about 200 nm), which were controlled by the thickness and the pore diameter of the applied AAO template, respectively and the thickness of the wall of the BFO nanotube was about 15 nm. Y-junctions in the BFO nanotubes were observed. EDX analysis demonstrated that stoichiometric BiFeO3 was formed. HRTEM analysis confirmed that the obtained BFO nanotubes made up of nanoparticles (3–6 nm). The possible formation mechanism of BFO nanotubes was discussed.

BaTiO3 films have been grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) for the first time. The BaTiO3 films were grown on Pt-coated Si substrates in an inverted vertical reactor with a remote O2 plasma at 450 mTorr. Plasma powers exceeding 10 W reduced or eliminated the presence of CO3 in the as-grown films at 525 and 600 °C, respectively. Films deposited at 600 °C by PE-MOCVD were polycrystalline BaTiO3 with perovskite structure and partial texture in the [100] direction; films grown at 525 °C were amorphous. The polycrystalline BaTiO3 films had dielectric constants as large as 300, loss tangents of 0.02, and resistivities exceeding 109 Ω cm at room temperature.

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