[Show abstract][Hide abstract] ABSTRACT: Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of -oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called delta-doped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6 K < T < 450 K). Depending on the sample, metallic or non-metallic behavior was observed. A hopping conduction mechanism with an anomalous hopping exponent was detected in the non-metallic samples. All metallic delta-doped layers exhibited the same mobility value, around 3.6 ± 0.8 cm2/Vs, independently of the layer thickness and the substrate type. Comparison with previously published data and theoretical calculations showed that scattering by ionized impurities explained only partially this low common value. None of the delta-layers showed any sign of confinement-induced mobility enhancement, even for thicknesses lower than 2 nm.
[Show abstract][Hide abstract] ABSTRACT: The ID21 Scanning X-ray Microscope (SXM) is optimized for micro-spectroscopy with submicron resolution in the 2 to 9.5 keV energy range. After a brief description of the microscope setup, we present here recent developments, in particular, the latest version of the compact Wavelength Dispersive Spectrometer and the refurbished cryo-stage.
Journal of Physics Conference Series 03/2013; 425(18):182004. DOI:10.1088/1742-6596/425/18/182004
[Show abstract][Hide abstract] ABSTRACT: Single-crystal diamond is a material with great potential for the fabrication of X-ray photon beam-position monitors with submicrometre spatial resolution. Low X-ray absorption combined with radiation hardness and excellent thermal-mechanical properties make possible beam-transmissive diamond devices for monitoring synchrotron and free-electron laser X-ray beams. Tests were made using a white bending-magnet synchrotron X-ray beam at DESY to investigate the performance of a position-sensitive diamond device using radiofrequency readout electronics. The device uniformity and position response were measured in a 25 microm collimated X-ray beam with an I-Tech Libera ;Brilliance' system. This readout system was designed for position measurement and feedback control of the electron beam in the synchrotron storage ring, but, as shown here, it can also be used for accurate position readout of a quadrant-electrode single-crystal diamond sensor. The centre-of-gravity position of the F4 X-ray beam at the DORIS III synchrotron was measured with the diamond signal output digitally sampled at a rate of 130 Msample s(-1) by the Brilliance system. Narrow-band filtering and digital averaging of the position signals resulted in a measured position noise below 50 nm (r.m.s.) for a 10 Hz bandwidth.
[Show abstract][Hide abstract] ABSTRACT: Over the energy range 5∼30 keV a suitably contacted, thin (∼100 μm) diamond plate can be operated in situ as a continuous monitor of X‐ray beam intensity and position as the diamond absorbs only a small percentage of the incident beam. Single crystal diamond is a completely homogeneous material showing fast (ns), spatially uniform signal response and negligible (<pA) leakage currents . Due to its unsurpassed thermal conductivity, it is the only semiconductor material that can be used in intense synchrotron white beams. We report on tests made at ESRF and DESY using diamond beam position monitors of simple quadrant electrode designs with metal contacts, operated using wideband electronic readout corresponding to the RF accelerator frequency. The instrumentation for these monitors must cover a large range of operating conditions: different beam sizes, fluxes, energies and time structure corresponding to the synchrotron fill patterns. Sophisticated new RF sampling electronics can satisfy most requirements: using a modified Libera Brilliance readout system, we measured the center of gravity position of a 25 μm beam at the DORIS III F4 beam line at a rate of 130 Msample∕s with narrowband filtering of a few MHz bandwidth. Digitally averaging the signal further provided a spatial resolution ∼20 nm.
[Show abstract][Hide abstract] ABSTRACT: The development of a wavelength-dispersive spectrometer for microfluorescence analysis at the X-ray Microscopy ID21 beamline of the European Synchrotron Radiation Facility (ESRF) is reported. The spectrometer is based on a polycapillary optic for X-ray fluorescence collection and is operated in a flat-crystal geometry. The design considerations as well as operation characteristics of the spectrometer are presented. The achieved performances, in particular the energy resolution, are compared with the results of Monte Carlo simulations. Further improvement in the energy resolution, down to approximately eV range, by employing a double-crystal geometry is examined. Finally, examples of applications requiring both spatial and spectral resolutions are presented.
[Show abstract][Hide abstract] ABSTRACT: The properties of coplanar propagation waveguides (CPW) on various diamond substrates are investigated. We demonstrate that on such material, the physico-chemical surface treatments may be fundamental to obtain good microwave properties. CPW were processed on single crystal CVD diamond samples that were grown either as bulk substrate or as a thin epitaxial layer on a type IIa natural diamond substrate. These CPW exhibited losses around 0.15 dB/mm at 10 GHz and 0.25 dB/mm at 40 GHz. Temperature-dependant measurements imply that structural defects may involve parasitic current conduction. The loss angle tangents of both samples have then been extracted from a quasi-TEM numerical modelling, they are about 0.025 for each kind of substrate.
Diamond and Related Materials 05/2009; 18(5-8):881-883. DOI:10.1016/j.diamond.2009.02.002 · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The hard x-ray microprobe beamline, ESRF-ID22, is primarily devoted to x-ray fluorescence spectroscopy. New requirements in terms of detection limits and acquisition rates fostered the installation and commissioning of a new Gresham multielement Si(Li) detector. It is based on a concentric and oriented array of 13 Si(Li) crystals viewing the same emission point. Beyond the energy resolution, linearity tests, and measurements of detection limits, the influence of the geometrical arrangement of the 13 elements has been investigated. In particular, the relationship between the scattering contribution to the fluorescence spectra and the angular position of individual Si(Li) crystals has been assessed. After optimization, the acquisition time can be reduced by a factor of 10 compared to what can be achieved with a single-element Si(Li) detector.
[Show abstract][Hide abstract] ABSTRACT: The 3rd and th generation X-ray sources are characterised by their high brilliance. This may induce high heat loads and high local power densities on the beamline optical elements such as monochromators, filters, phase plates, beam splitters, lenses, vacuum windows and on beam position monitors. Optical components are often made of silicon (available in large dimensions, grown with high crystal perfection and with very good surface quality). Instead of silicon we can use diamond, a material that has excellent thermal characteristics and thus considerable advantages compared to silicon. Nevertheless, up to now, the diamond material available had small dimensions and many defects in the bulk and often an insufficient surface quality. Nowadays, the diamond industry is developing an improved High Pressure High Temperature growth process for highly pure type IIa single crystal diamond. This results in a considerable reduction of the number of crystallographic defects within the material. The key parameters of a diamond crystal for the majority of X-ray applications are crystalline perfection of its bulk and also of its surface. In the most advanced applications it should conserve the coherence of the X-ray beam. Industry efforts must be focused on these directions as well as on increasing the crystal dimensions. In a successful collaboration project between the authors, many diamond samples have been studied over the past few years. At the ESRF, X-ray topography is the most important and effective experimental method used to characterise the defects structure in diamond crystals. Some of the topographs are presented below. It appears that for most of the future practical diamond applications the efficiency of crystal cooling methods has to be improved; this will reduce the thermo-mechanical deformation of the crystal. We are working at the ESRF on two complementary approaches. The first is to increase the surface of thermal exchange with the cooled support. The size of available single crystal diamond plates is about 7x7 mm2 (with a perfect central region of about 4x4 mm2, (100)-orientation). Such single crystal diamonds can be brazed on larger CVD diamond plates. Brazing tests have been initiated in order to qualify the different brazing techniques and to measure the stress induced within the single crystal material by this process. The second approach is to increase the efficiency of the cooling support structure. The design and manufacture of dedicated supports are in progress.
[Show abstract][Hide abstract] ABSTRACT: The ID21 ESRF beamline is dedicated to micro-X-ray fluorescence (µXRF) and micro X-ray absorption spectroscopy (µ-XANES) in the X-ray energy domain between 2 keV and 7 keV . The scanning x-ray microscope enables the localization and speciation of trace elements down to the ppm range with a submicrometer beam size. It provides access to the absorption edges of a wide range of elements of interest in the areas of Environmental Sciences, Earth and Planetary Sciences, Life Sciences and Cultural Heritage. Recently, several developments in the field of x-ray detection have been made to enlarge the measurement possibilities available at the beamline and to improve the detection limits for x-ray fluorescence. The scanning X-ray microscope (SXM) of ID21 was completely refurbished in 2008. It is now a highly versatile instrument in terms of focusing optics, detectors, and sample environment . The X-ray beam spot size ranges from ~20 µm (obtained with a polycapillary optics) to ~500 nm (obtained with focusing zone plates or a Kirk-Patrick Baez mirror system). The SXM employs several detectors, which are complementary in terms of count-rate throughput, solid-angle collection efficiency and spectral resolution. For experiments demanding high sensitivity, a single-or a seven-element Ge detector provide large fluorescence detection solid angle, absence of detector K-escape peak contributions, and detector peak-to-valley ratio >5 k. For the case of highly concentrated samples, or for samples where the matrix signal dominates the fluorescence signal, either 30 mm 2 or 100 mm 2 silicon drift diode (SDD) detectors can be used at count rates approaching 1 Mcps. Finally, in order to resolve overlapping or closely spaced fluorescence lines, a polycapillary optic based, scanning wavelength-dispersive spectrometer (WDS) has been developed . In many applications the WDS makes possible detailed elemental and chemical analyses that are simply not accessible with the lower resolution solid state detectors. We present the operational characteristics, the construction details and the performance achieved with the detectors of ID21. In particular the throughput count rates, spectral resolutions and achievable detection limits are compared. We describe the application of different detection systems for two-dimensional elemental mapping and micro-XANES analysis. The x-ray fluorescence spectra obtained are compared to theoretical simulations and discussed in terms of absorption, re-absorption and scattering phenomena.