An ideal emitter of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)A surface. The emitted photons reveal a fidelity to the Bell state as high as 86(±2)% without postselection. We show a violation of Bell's inequality by more than five times the standard deviation, a prerequisite to test a quantum cryptography channel for eavesdropping. Due to the strict nonlocal nature the source can be used for real quantum processing without any postprocessing. The remaining decoherence channel of the photon source is ascribed to random charge and nuclear spin fluctuations in and near the dot.
GaNAs films grown on GaAs͑001͒ substrates by metalorganic molecular beam epitaxy were studied by high-resolution x-ray diffraction ͑XRD͒ mapping measurements. The lattice constants of epitaxial films are usually estimated from symmetric and asymmetric XRD 2 Ϫ measurements. In this study, it is pointed out that the consideration of the tilt angle between the GaAs͑115͒ and GaNAs͑115͒ planes caused by elastic deformation of the films is crucial to determine the lattice constants of the GaNAs films coherently grown on GaAs substrates. Mapping measurements of ͑115͒ XRD (2 Ϫ )Ϫ⌬ were performed for this purpose. The band gap energy of the films was determined by Fourier transform absorption spectroscopy measurements. The band gap energy bowing measured up to the N composition of 4.5% will be discussed by comparing with other measurements and theoretical calculations.
A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.
Vicinal 4H and 6H-SiC 0001 surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed the characteristic selfordering of nanofacets on any surface, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes are typically revealed: (0001) and high index 112n that are induced by equilibrium surface phase separation. A 112n plane may have a free energy minimum due to attractive step-step interactions. The differing ordering distances in 4H and 6H polytypes imply the existence of SiC polytypic dependence on nanofaceting. Thus, it should be possible to control SiC surface nanostructures by selecting a polytype, a vicinal angle, and an etching temperature.
Photosynthesis of porous silicon is reported for the first time with visible-light irradiation in a hydrofluoric acid solution, which does not need any electrodes for anodization. The photosynthesized porous layer consisted of microparticles, and the photoluminescence spectra were very close to those of anodized porous silicons. The formation of the porous layer was dependent on the wavelength of the incident light, and the simultaneous irradiation of an ultraviolet light did not form the porous layer. The mechanism is discussed with the quantum confinement model of the porous layer.
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