<i>In-situ</i> annealed epitaxial (001) Ni films MBE grown on MgO substrates exhibit a missing-row (2x1) surface reconstruction due to oxygen adsorption. Thus, the resulting nano-patterning of the surface consists of self-assembled NiO nano-wires. Correlated RHEED, STM, XTEM, PNR and MOKE studies indicate that there is intermixing of the Ni film and the substrate with NiO formation at the interface between film and substrate and also on the surface of the films. This leads to the presence of an additional uniaxial magnetic anisotropy superimposed to the expected 4-fold magneto-crystalline anisotropy as determined with longitudinal MOKE.
Epitaxial heterostructures constitute a wide variety of modern microelectronics devices. In the limit of ever decreasing feature dimensions, now entering the nanoscale in some cases, the interfaces of such devices are crucial to their operation and performance. In general the properties of the interfaces will differ significantly from those of the bulk structure of either the substrate or the heteroepitaxial film. To date, direct, non-destructive characterizations of the atomic-level structure of films and interfaces have not been readily available and this has hampered the design and optimization of heteroepitaxial devices. We describe here a novel x-ray interference method which is useful for imaging such structures with sub-Ångstrom spatial resolution while also providing chemical composition information from a map of the electron density. We illustrate the method, known as Coherent Bragg Rod Analysis (COBRA), with recent results on GaSb-InAs heterostructures of interest as infrared sources and detectors. We show that, with detailed knowledge of the interfaces from COBRA, it is now feasible to correlate specific molecular beam epitaxy growth conditions with desired electronic characteristics associated with the interface bonding. The COBRA method is quite general and only requires an epitaxial relationship between the substrate and the nanostructure that is deposited on it.
Lithium promises to give refractive x-ray optics the highest possible
transmission, aperture and intensity gain. Room-temperature embossing of lithium with parabolic dies from polypropylene produces lenses that focus well but are not yet good enough for imaging. X-ray measurements suggest two causes of problems, one of which one can be solved easily.
Lithium is the best material for refractive x-ray lenses, with peak performance around 8 keV. To date we have built a prototype of Cederstrom's so-called alligator lens, and have tested the lens with beamline 7ID's 10 keV x-rays on the Advanced Photon Source at Argonne National Laboratories. To date we have attained only a threefold gain, most likely limited by surface roughness that is avoidable with more careful manufacturing techniques.
The development of ultrahigh-brightness x-ray sources makes time-resolved x-ray studies more and more feasible. Improvements in x-ray optics components are also critical for obtaining the appropriate beam for a particular type of experiment. Moreover, fast parallel detectors will be essential in order to exploit the combination of high intensity x-ray sources and novel optics for time-resolved experiments. A CCD detector with a time resolution of microseconds has been developed at the Advanced Photon Source (APS). This detector is fully programmable using CAMAC electronics and a MicroVax computer. The techniques of time- resolved x-ray studies, which include scattering, microradiography, microtomography, stroboscopy, etc., can be applied to a range of phenomena (including rapid thermal annealing, surface ordering, crystallization, and the kinetics of phase transition) in order to understand these time-dependent microscopic processes. Some of these applications are illustrated by recent results performed at synchrotrons. New powerful x-ray sources now under construction offer the opportunity to apply innovative approaches in time-resolved work.
We discuss the application of high-resolution x-ray diffractometry to studies of semiconductor heterostructures. A new technique has been devised which extends structural measurements into the time domain. Using x-ray synchrotron radiation in conjunction with dispersive optics and fast x-ray area detectors we have been able to study for the first time the structure of heterointerfaces undergoing thermal processing. The techniques are illustrated with results on the strain kinetics of SQW''s and ion-implanted InAli_As layers.
The advent of extremely bright x-ray beams from new low-emittance sources such as ESRF and APS offers new opportunities for materials research. One of the most exciting and technologically demanding areas is likely to be time-resolved x-ray studies. Our recent experiments at NSLS Brookhaven (Beamline X-16B) explore some of the challenges for time-resolved x-ray scattering combining developments in x-ray optics (dispersive geometry) area detectors (CCD''s) and fast data acquisition. The techniques are illustrated with results on the rapid thermal annealing of electronic materials including strained-layer InGai_As quantum-well structures. We describe the application of a new virtual-phase CCD detector for real-time diffraction studies at the microsecond time scale. 1.