In-flight calibration sources for Herschel-SPIRE

SPIRE, the Spectral and Photometric Imaging Receiver, will be a bolometer instrument for ESA's Herschel satellite. The instrument comprises a three-band imaging photometer covering the 250-500 μm range, and an imaging Fourier Transform Spectrometer (FTS) covering 200-670 μm. This paper presents the requirements for and design of the photometer and spectrometer calibration/illumination sources, and the results of laboratory tests on prototypes. The photometer calibrator is an electrically heated thermal source of submillimetre radiation, the purpose of which is to provide a repeatable signal for in-flight monitoring of health and responsivity of the SPIRE photometer detectors. It is not required to provide absolute calibration or uniform illumination of the arrays, but it may be used as part of the overall calibration scheme. The spectrometer calibrator is located at a pupil at the second input port of the FTS. It is designed to enable matching of the telescope emission for a range of telescope temperature (60-90 K) and emissivity (2% - 10%). By matching the telescope emission at this port, the high background from the Herschel telescope emission can be nulled to a high degree, resulting in an interferogram in which the contribution from the astronomical source is not overwhelmed by the telescope offset. The flexibility for telescope matching inherent in the design is important, as the exact telescope parameters will be unknown until the satellite is in operation. The FTS calibrator will also be used to assist in the absolute calibration scheme for SPIRE FTS observations

Electron beam crystallised thin film silicon solar cell on floatglass

Sufficient optical thickness and a minimum of recombination losses are the requirements for efficient silicon thin film solar cells with efficiencies above 15 . A capable solution would be a 5 10 m thick polycrystalline Si absorber [1] grown on a low cost substrate and completed to a solar cell by an amorphous a Si H heteroemitter [2]. This article describes a polycrystalline thin film solar cell with grain sizes of 5 10 m at a layer thickness of 5 7 m prepared on an easily available low cost float glass substrate. The grain growth is achieved by zone melting crystallisation ZMC of amorphous silicon with a line shaped electron beam [3]. An intermediate layer is necessary to protect the glass softening point 700 C from the melt zone and to prevent the silicon from being contaminated by elements from the glass. Presently, a combination of silicon carbide SiC and silicon oxide SixOy is used as diffusion barrier and structural fitment. The silicon carbide is deposited by an RF magnetron sputtering process at room temperature with a typical layer thickness of 200 nm, silicon oxide by a PECVD process with a thickness of 300nm. For the purpose of absorber doping by diffusion during the crystallization, a thin layer of spin on glass, containing either phosphorus or boron was applied on top of the SiC SiO sandwich before the absorber layer was deposited. Three alternative technologies have been used for absorber deposition PECVD at 600 C, LPCVD at 600 C and high rate vacuum e beam evaporation with growth rates up to 2 m min with typical absorber thicknesses from 5 to 15 m. The succeeding crystallisation is carried out after preheating the substrate to a temperature near 600 C. Scan velocities are varying from 5 up to 30 mm s through the focus of the line shaped electron source. Typical energy densities are around 450 mJ mm2. The silicon morphology before crystallization does not affect the crystallisation process. After crystallization the poly Si layers exhibit crystalline grain sizes comparable to the layer thickness, i. e. up to 10 m as shown by Scanning Electron Microscopy SEM investigations figure 1 . The surface morphology is smooth shingle type modulated due to the grains. XRD measurements indicate lt;111 gt;, lt;110 gt; and lt;311 gt; crystal orientations figure 2 . Raman spectroscopy analysis indicates that the structure of the poly Si films is crystalline over the whole area figure 3 ; no amorphous phase was detected. Furthermore, the line shapes are symmetrical no Fano effect . Therefore, a defect concentration below 1019cm 3 can be expected. These results are supported by photoluminescence spectra which display a detectable maximum at about 1150nm figure 4 . This indicates that the fast non radiative recombination does not completly annihilate the excess charge carriers. The minority carrier lifetime was directly investigated by photoconductance decay. These first measurements are resulting an effective carrier lifetime of about 40 s figure 5 . This result can be improved for example by passivating the grain boundaries with hydrogen. The surface photovoltage SPV of the poly Si absorber and its decay was measured. Surface band bending modulation via field dependent SPV was analysed. Such an modulation is an indispensable precondition for the formation of a built in voltage. Finally, heterojunction solar cells were prepared by PECVD deposition of 10 30nm thin a Si H n,p emitter on top of the crystallised c Si p,n absorber. The solar cell was then characterized by taking current voltage curves under AM1.5 conditions, measuring internal quantum efficiency and open circuit voltages by Suns Voc. [1] R. B. Bergmann. J.H. Werner, Thin Solid Films 403 404, p. 162 2002 [2] J. Poortmans et. al., Proceedings IEEE 4th WCPEC, p.1449 2006 [3] F. Gromball et. al., Rev. Sci. Instrum. 76, p. 063901 200

Ultrathin SiO2 layers on Si 111 preparation, interface gap states and the influence of passivation

Essential prerequisite for successful application of Si SiO2 nanostructures in photovoltaics is the realization of well defined and abrupt interfaces with low densities of interface gap states. Here, a complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near UV photoelectron spectroscopy without breaking ultrahigh vacuum UHV conditions is introduced. It is demonstrated, that by RF plasma oxidation of Si 111 substrates with thermalized neutral oxygen atoms ultrathin SiO2 layers with compositionally and structurally abrupt Si SiO2 interfaces with minimal amount of intermediate oxidation states bridging the transition from Si to SiO2 can be realized. Plasma oxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidation at 850 C. Interface gap state densities were further reduced by in situ hydrogen plasma passivation with nearly thermalized H atoms. The resulting reduction of interface recombination velocity and the increase of effective majority and minority carrier lifetimes are revealed by constant photocurrent measurements and quasi steady state photoconductance, respectivel

A novel high efficiency buried grid rear contact amorphous crystalline silicon heterojunction solar cell concept

A novel high efficiency Silicon wafer based solar cell concept, combining the amorphous crystalline silicon heterojunction solar cell technology with a specifically adopted rear contact scheme is presented The Buried Grid RECASH Rear Contact Amorphous Crystalline Silicon Heterojunction Solar Cell. This newly developed solar cell design patent pending [1] consists of an electrically insulated contact grid buried within a rear side a Si H emitter layer and needs no structuring processes of the solar cell absorber or emitter. The efficiency potential of the concept is discussed by means of numerical computer simulation, and first experimental results of the RECASH technology are presente

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

A variety of Hg-1223 high-T-c superconducting: systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples. (C) 2002 Elsevier Science B.V. All rights reserved

Temperature behaviour of irreversibility fields of various Hg- based superconducting ceramics

A variety of Hg-1223 high-T-c superconducting: systems, partly substituted by lead or fluorine, are investigated by means of magnetic susceptibility measurements. As already observed earlier, the double peak structure in the imaginary parts of the ac signals below about 1 T is confirmed indicating different pinning mechanisms. Irreversibility lines are determined from the upper peak values and analysed applying the theory of Matsushita which is based on a depinning mechanism caused by thermally activated flux creep. It leads to a suitable description in the whole temperature range investigated and allows a discussion of differences of the various samples. (C) 2002 Elsevier Science B.V. All rights reserved

Electrical Conductance of Bolted Copper Joints for Cryogenic Applications

International audienceWe present the results of electric contact resistance measurements at low temperatures on copper-to-copper bolted joints. Our accurate and systematic data display a rather small dispersion, and maybe a useful tool for cryogenic applications like pulse-tubes, dilution refrigerators and nuclear refrigerators

The Herschel-SPIRE instrument and its in-flight performance

The Spectral and Photometric Imaging REceiver (SPIRE), is the Herschel Space Observatory`s submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier-transform spectrometer (FTS) which covers simultaneously its whole operating range of 194–671 μm (447–1550 GHz). The SPIRE detectors are arrays of feedhorn-coupled bolometers cooled to 0.3 K. The photometer has a field of view of 4´× 8´, observed simultaneously in the three spectral bands. Its main operating mode is scan-mapping, whereby the field of view is scanned across the sky to achieve full spatial sampling and to cover large areas if desired. The spectrometer has an approximately circular field of view with a diameter of 2.6´. The spectral resolution can be adjusted between 1.2 and 25 GHz by changing the stroke length of the FTS scan mirror. Its main operating mode involves a fixed telescope pointing with multiple scans of the FTS mirror to acquire spectral data. For extended source measurements, multiple position offsets are implemented by means of an internal beam steering mirror to achieve the desired spatial sampling and by rastering of the telescope pointing to map areas larger than the field of view. The SPIRE instrument consists of a cold focal plane unit located inside the Herschel cryostat and warm electronics units, located on the spacecraft Service Module, for instrument control and data handling. Science data are transmitted to Earth with no on-board data compression, and processed by automatic pipelines to produce calibrated science products. The in-flight performance of the instrument matches or exceeds predictions based on pre-launch testing and modelling: the photometer sensitivity is comparable to or slightly better than estimated pre-launch, and the spectrometer sensitivity is also better by a factor of 1.5–2.Peer reviewe