NW14A is a newly constructed undulator beamline for 100 ps time-resolved X-ray experiments at the Photon Factory Advanced Ring. This beamline was designed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved diffraction, scattering and X-ray absorption fine structure. Its versatility is allowed by various instruments, including two undulators, three diffractometers, two pulse laser systems and an X-ray chopper. The potential for the detection of structural changes on the 100 ps time scale at NW14A is demonstrated by two examples of photo-induced structural changes in an organic crystal and photodissociation in solution.

We report the experimental results of the valence band offset at a ZnO/GaN (0001) heterointerface. The ZnO/GaN (0001) heterointerface is prepared by growing a ZnO layer on (0001) GaN/Al2O3, in which the ZnO layer is epitaxially deposited by plasma-assisted molecular-beam epitaxy, while the GaN template is prepared by metalorganic chemical-vapor deposition. Ex situ ultraviolet and x-ray photoelectron spectroscopy have been used to measure the valence band offset ΔEV. The photoelectron spectroscopy measurements are done before and after Ar+ ion cleaning of the surfaces. Type-II band alignments with band offsets of ΔEV=1.0 eV (before cleaning) and 0.8 eV (after cleaning) with the valence band maximum of GaN being placed above that of ZnO are obtained.

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We report on the growth of polarity-controlled ZnO epilayers by plasma-assisted molecular-beam epitaxy and the measurement of valence-band offset at the ZnO/GaN heterointerface. The polarity of ZnO epilayers is determined by coaxial-impact-collision ion-scattering spectroscopy. The band offset is determined by ultraviolet and x-ray photoelectron spectroscopy. The high-resolution transmission electron microscopy study reveals the formation of an interface layer between the ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is formed in Zn preexposed samples. Zn preexposure prior to ZnO growth results in Zn-polar ZnO epilayers (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO epilayers (O face). The interface layer is identified to be single-crystalline, monoclinic Ga2O3. The estimated valence band offset at the ZnO/GaN(0001) heterojunction with Zn preexposure is 0.8 eV with a type-II band alignment.

The main purpose of this study is to grow an organic thin film consisting of aromatic molecules with enhanced π-conjugations. For this purpose, an anthracene derivative, 9,10-bis (methylthio) anthracene, was deposited on a CaF2∕Si(111) substrate using a hot-wall epitaxy technique. The crystal structure of this anthracene derivative consists of molecular stacks with a face-to-face configuration due to the sulfur-sulfur interactions between neighboring molecules, which enhances π-electron conjugation. The hot-wall epitaxy technique made precise control of the growth conditions possible. Atomic force microscopy and x-ray diffraction measurements demonstrated that a polycrystalline thin film with the molecules in the face-to-face configuration was obtained, and that, depending on the growth conditions, the thin-film structure was controllable.