The use of high mobility channel materials such as Ge and III/V compounds for CMOS applications is being explored.
The introduction of these new materials also opens the path towards the introduction of novel device structures which
can be used to lower the supply voltage and reduce the power consumption. The results illustrate the possibilities that are
created by the combination of new materials and devices to allow scaling of nanoelectronics beyond the Si roadmap.
We describe the principles of operation of the STJ spectrometer baselined for XEUS (X-ray Evolving Universe Spectroscopy mission), and describe some of the practical implementation issues that are being developed or trade-offs that are still being studied actively. The instrument is to be optimized for good energy resolution (<2eV FWHM) in the energy range between 0.1 - 3keV. The field of view should be maximized to allow spectroscopic mapping of modestly extended objects (e.g. cluster cores) and to maximize the number of point source objects analysed. This remains the greatest challenge for the instrument design.
We report the results of a series of experiments designed to assess the relative radiation hardness of a range of compound semiconductor X-ray detectors. The specific compounds tested were GaAs, InP, CdZnTe, HgI<sub>2</sub> and TlBr, along with an elemental Si device. To allow meaningful comparisons, all devices were of a similar size and, with the exception of the InP detector, had sub-keV energy resolution at 5.9 keV. The irradiations were carried out using the University of Helsinki’s Cyclone 10/5 10 MeV proton cyclotron. Each detector was given six consecutive exposures - the integral fluences being; 2.66 x 10<sup>9</sup> p cm<sup>-2</sup>, 7.98 x 10<sup>9</sup> p cm<sup>-2</sup>, 2.65 x 10<sup>10</sup> p cm<sup>-2</sup>, 7.97 x 10<sup>10</sup> p cm<sup>-2</sup>, 1.59 x 10<sup>11</sup> p cm<sup>-2</sup>, and 2.65 x 10<sup>11</sup> p cm<sup>-2</sup>, respectively. In Si, these correspond to absorbed radiation doses of 2, 6, 20, 60, 120 and 200 krads. During the exposures, the detectors were kept unbiased and at room temperature. After each irradiation, the effects of the exposure were assessed, both at room temperature and at a reduced temperature using <sup>55</sup>Fe, <sup>109</sup>Cd and <sup>241</sup>Am radioactive sources. It was found that with the exception of the HgI<sub>2</sub> and TlBr detectors all materials showed varying degrees of damage effects.
Superconducting tunnel junctions are being developed for application as photon detectors in astronomy. We present the latest results on the development of very high quality, very low critical temperature junctions, fabricated out of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic demagnetisation refrigerator. The contacts to the top and base electrodes of these junctions are fabricated either out of Nb or Ta, which has strong implications on the loss time of the quasiparticles. The Nb contacted junctions show quasiparticle loss times varying between 5 and 80 μsec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 μV, because of the set-in of strong non-equilibrium quasiparticle multiplication currents at higher bias voltages. The Ta-contacted junctions, on the other hand, show quasiparticle loss times in excess of 200 μsec. These long loss times lead to very strong quasiparticle multiplication, which prevents the stable biasing of the junctions even at very low bias voltages. Junction fabrication and characterisation are described, as well as the response of the detectors to monochromatic light with wavelengths varying from 250 to 1000 nm. The energy resolution of the detectors is discussed.
Measurements of quantum efficiencies are presented for three epitaxial gallium arsenide detectors with nominal depletion depths of 40, 325, and 400 μm. Attempts to measure the depletion as a function of bias indicated that the apparent depletion depth was much less than the intrinsic layer thickness. Expectation that the of the detection efficiency could be increased using a larger intrinsic layer could not be met. The largest value of a depletion depth measured by X-rays was determined at about 100 μm for the 400 μm device.
We present an experimental study of the performance of one-dimensional Distributed Read-Out Imaging Devices (DROIDs), based on two Ta/Al-based STJs placed on either side of a Ta absorber strip. We focus our discussion on the prospects of building large-format photon-counting imaging spectrometers for applications at soft X-ray energies. Tunnel-limited spectroscopical resolutions have already been demonstrated for optical photons. With a 20 x 100 micrometers <SUP>2</SUP> absorber we have measured an intrinsic energy resolution of 2.1 eV FWHM for 500 eV photons. This demonstrates that at soft X-ray energies resolutions close to the tunnel limit are also feasible for these type of detectors. A detailed analysis of pulse-shapes with analytical models allows us to assess the main parameters that determine the performance of these detectors. In particular, we discuss the dependence of the quasiparticle diffusion constant on the temperature of the absorber. Extrapolation of these models indicates that it is possible to extend the length of the absorber to 1.5 mm, without a serious degradation of the detector's performance.
We present preliminary results of X-ray measurements on three small format compound semiconductor arrays. The devices, a 4x4 pixel GaAs array fabricated on 325 micrometers epitaxial material, a 4x4 pixel CdZnTe array fabricated on a 4X4X1 mm<SUP>3</SUP> mono crystal and a 3x3 TlBr array fabricated on a 2.7 x 2.7 x 1.0 mm<SUP>3</SUP> mono crystal. The pixel size for all arrays is 350x350micrometers <SUP>2</SUP>. Results are presented of <SUP>55</SUP>Fe and <SUP>241</SUP>Am measurements at 5.9 keV and 59.54 keV. For detector temperatures <+5 degree(s)C typical FWHM energy resolutions of 410 eV, and 600 eV at 5.9 keV and 640 eV and 1.4 keV at 59.54 keV were recorded for the GaAs, and CdZnTe arrays, respectively. Unlike the GaAs and CdZnTe arrays, the TlBr array showed a much wider variation in pixel performance and was difficult to operate with all pixels at a common bias. For example, biasing the detector so that all pixels worked within the operating envelope of the preamplifiers resulted in average energy resolutions of 20 keV at 59.54 keV. However, optimizing the operating conditions of individual pixels resulted in a marked improvement to ~2keV.
We present preliminary results of X-ray measurements on two small format compound semiconductor arrays. The devices, a 5 X 5 gallium arsenide array and a 3 X 3 cadmium zinc telluride array, were produced specifically to address the material, electronic and technological problems that need to be solved in order to develop mega-pixel, Fano limited spectroscopic arrays. The GaAs device was fabricated on 40 micrometer epitaxial material and has a pixel size of 200 X 200 microns<SUP>2</SUP> with pitch 250 micrometer. The CdZnTe array was fabricated on a 5 X 5 X 1.6 mm <SUP>3</SUP> single crystal of spectroscopic quality. The pixel sizes were 350 X 350 microns<SUP>2</SUP> with a pixel pitch of 250 micrometer. Measurements from 5.9 keV to 100 keV were carried out both in our laboratory and at the HASYLAB synchrotron research facility in Hamburg, Germany. The typical FWHM energy resolutions recorded at 5.9 keV by the GaAs and CdZnTe arrays were 394 eV and 900 eV, respectively.
We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), 1- and 2-D photon-counting imaging spectrometers, based on Ta/Al-based STJs placed on a Ta absorber. Results obtained with highly collimated illumination with 10 keV X-ray photons clearly demonstrate the imaging capabilities of 2-D DROIDs. The derived spatial FWHM resolution is 7 micrometers for a 200 X 200 micrometers <SUP>2</SUP> absorber. With a 1-D DROID we have measured an intrinsic energy resolution of 15 eV FWHM for 6 keV photons. At high energies (E > 6 keV) the resolution is limited by spatial fluctuations in the qp recombination rate.