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TT2.02 - Pixel Detectors with CdTe and Si Sensors Applied in Material Science for Diffraction Tomography and Imaging Experiments 
December 1, 2014   2:00pm - 2:15pm

This presentation will describe results recently obtained with pulsed electron beam deposition (PED) of GaN on c-plane sapphire, silicon (111), and 2 nm germanium coated silicon (111) substrates. The PED technique is potentially useful for growth of III-nitrides at lower substrate temperatures, a capability that can allow use of new buffer layer materials, introduction of chemically dissimilar lattice-matched materials, and help solve wafer bowing and cracking problems during growth. The introduction of this technique could lead to improvements in device quality and fabrication of vertical LED structures. In this study, GaN was deposited on sapphire at a substrate temperature of 750°C, and on silicon (111) and Ge/Si (111) at 600°C in a UHP N2 (15 mTorr) environment, without any surface pre-treatment such as pre-nitridation. A high power electron gun pulse (Neocera, Inc) was used to ablate the GaN target (1” dia. x 0.250” thick, 99.99% pure) stationed at a 5 cm vertical distance from the substrate. The electron pulses were generated at 15KV, 0.3 J/pulse at 1 Hz for initial few nm of growth, and then increased to a 3 Hz pulse rate. Scanning electron microcopy (SEM), X-ray diffraction (XRD), Rutherford backscattering (RBS), and optical absorption characterization were performed. SEM imaging confirmed a rough surface morphology with the presence of 30 nm to 300 nm scaled GaN crystallites for the GaN/Sapphire sample, while smaller but more coalesced crystallites of 30-50 nm size were observed for the GaN/Si (111) and GaN/Ge/Si (111) samples. The average film thickness was 350 nm, yielding a growth rate of 0.16 angstrom/pulse. XRD θ-2θ scans from 2θ = 0° to 2θ = 70° for the GaN on sapphire sample showed only two other peaks, besides the peaks from the sapphire, near 2θ = 34.6°. The peaks near 2θ = 34.6° consist of a stronger peak at 34.668° and a much weaker peak at 36.903°. These peaks correspond to the (0002) and (10-11) orientations for GaN, respectively. XRD scans for the GaN/Si (111) and GaN/Ge/Si (111) samples showed only a polar GaN (0002) peak at 34.7°. The XRD results clearly show that the deposited GaN material is not polycrystalline. Optical absorption spectroscopy over a 1.2 eV to 6.2 eV spectral range, for the GaN/Sapphire sample, showed an abrupt absorption edge at 3.4 eV, a clear indication of interband transitions in binary GaN. These results confirm that our PED-grown GaN is highly c-axis oriented and suitable for the initial growth of GaN on various substrate materials.

A large-area CdTe HPC detector suitable for high-energy synchrotron applications at X-ray energies from about 8 keV to 80 keV has been developed. The detector has 300k pixels with a sensitive area of 83 mm x 106 mm and a pixel size of 172 ?m x 172 ?m. It was built in hybrid-pixel technology by bumpbonding PILATUS3 CMOS readout ASICs [1] to six large pixelated CdTe sensors with a size of 42 mm x 34 mm and a thickness of 1000 ?m. The PILATUS3 ASIC with instant retrigger technology (count-rate capability of up to 107 photons per second per pixel) is operated in its modes specifically implemented for compatibility with CdTe, which are 'negative bias polarity' for electron collection and 'reduced shaper gain' for processing the high-energy X-ray pulses. The detector has been thoroughly characterized in terms of energy resolution, long-term stability of CdTe material, quantum efficiency, point-spread function and count rate using an X-ray tube setup and at a synchrotron (BESSY). These characterization results together with first X-ray experiments at the synchrotron confirm the suitability of this large area CdTe detector for high-energy synchrotron applications such as X-ray diffraction tomography [2]. We will present first studies on test objects demonstrating the advantages of this technology in material science.

The properties of HPC detectors are also ideal for diffractive and phase-contrast imaging. The newly developed EIGER HPC detector [3] features a high frame rate of 3 kHz and a pixel size of only 75 ?m x 75 ?m. Tomographic applications from synchrotron-based small-angle x-ray scattering (SAXS) imaging [4] and coherent diffractive ptychographic imaging [5] profit from the new improvements. The first experiments with this technology will be presented.


[1] T. Loeliger et al., IEEE Nucl Sci Symp Conf Rec, pp. 610-615 (2012)
[2] S. D. M. Jacques et al., Angew. Chem. Int. Ed. 50, 10148 (2011)
[3] R. Dinapoli et al., Nucl. Instrum. Meth. Phys. Res. A 650, 79 (2011)
[4] O. Bunk et al., New J. Phys. 11, 123016 (2009)
[5] M. Guizar-Sicairos et al., Opt. Express 22, 14859 (2014)

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