X. D. Wang

Morphology and orientation of iron oxide precipitates in epitaxial BiFeO3 thin films grown under two non-optimized oxygen pressures

Microstructures of multiferroic BiFeO3 thin films epitaxially grown on SrRuO3-buffered SrTiO3 (001) substrates by laser molecular-beam epitaxy under two non-optimized oxygen pressures were characterized by means of transmission electron microscopy. The results showed that the films grown under oxygen pressures of 1 Pa and 0.3 Pa contain a secondary phase embedded in the BiFeO3 matrix. High-angle annular dark-field imaging, elemental mapping and composition analysis in combination with selected area electron diffraction revealed that the parasitic phase is mainly antiferromagnetic α-Fe2O3. The α-Fe2O3 particles are semi-coherently embedded in the BiFeO3 films, as confirmed by high-resolution transmission electron microscopy. In addition to the α-Fe2O3 phase, ferromagnetic Fe3O4 precipitates were found in the BiFeO3 films grown under 0.3 Pa and shown to accumulate in areas near the film/substrate interfaces. In our heteroepitaxy systems, very low density misfit dislocations were observed at the interfaces between the BiFeO3 and SrRuO3 layers implying that their misfit strains may be relieved by the formation of the secondary phases. Using X-ray photoelectron spectroscopy it was found that Fe exists in the +3 oxidation state in these films. The possible formation mechanisms of the secondary phases are discussed in terms of film growth conditions.

Preparation and wear resistance of Ti–Zr–Ni quasicrystal and polyamide composite materials

Ti–Zr–Ni icosahedral quasicrystal powders (Ti-QC), prepared by mechanical alloying and then annealing in a vacuum furnace, were used as a novel filler material in polyamide 12 (PA12). The melt processability of the composite was studied using a Haake torque rheometer. This indicates that PA12/Ti-QC composites can be melt-processed into a wear-resistant material. Further, these composites, fabricated by compression molding, were tested in sliding wear against a polished bearing steel counterface. The results from wear testing show that the addition of Ti-QC filler to PA12 enhances wear resistance and reduces volume loss by half compared with neat PA12. Furthermore, it is found that the hardness of the composite increases with increasing content of Ti-QC filler. In addition, PA12/Ti-QC composites exhibit a slightly higher crystallization temperature and better thermal stability than PA12. These combined results demonstrate that Ti-QC filler may be a desirable alternative when attempting to increase the wear resistance of PA12.

Photoresponse simulation of visible blind GaN/AlGaN p-i-n photodiode

The spectral photoresponse characteristics for visible blind GaN/AlGaN p-i-n photodiode have been numerically studied. Effects of the absorption layer thickness and the n-layer thickness on the photoresponse spectra have been investigated. Our work shows that the absorption layer thickness and n-layer thickness have important impact on the peak value of photoresponse spectra and rejection ratio of short-wavelength side, respectively.

Electro-optical characteristics for AlGaN solar-blind p-i-n photodiode: Experiment and simulation

The fabrication and modeling for solar-blind AlGaN-based p-i-n photodiode have been presented. The simulated dark current characteristics are in good agreement with the experiments. It is found that the peak responsivity of 0.005A/W can be achieved at 265nm corresponding to the cutoff wavelength of the Al0.45Ga0.55N absorption layer. The transmission spectra drop to nearly zero due to the intense light absorption of n-type Al0.65Ga0.35N layer.

Electro-optical characteristics of separate absorption and multiplication GaN avalanche photodiode

The fabrication and electro-optical characteristics for separate absorption and multiplication GaN avalanche photodiode have been presented. The multiplication gain as a function of reverse bias at room temperature is also obtained. It is found that multiplication gain monotonically increases with increasing reverse bias, high multiplication gain of 3×105 at 110V is achieved