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This paper gives an overview of infrared array detectors which have been tested and used at ESO. The performance of arrays using Reticon type readouts, CCD readouts and switched FET multiplexers have been evaluated for both InSb and Hg1−xCdxTe detectors. Performance limitations specific to the NICMOS3 256 × 256 Hg1−xCdxTe detector installed in the ESO infrared array camera IRAC2 are addressed. The first test results with a high well capacity SBRC 256 × 256 InSb array are also presented.
Advanced readout techniques for image sharpening tested on a 2.2-m telescope are discussed briefly. A new generation of instruments being built for the VLT, the very large telescope project of ESO, is designed to house large format 1024 × 1024 IR arrays. A fast data acquisition system is currently being developed at ESO. The system is capable of handling the high data rates generated in the thermal infrared by large format low well capacity arrays. It can also cope with the low read noise required for flux levels of ≤ one photon/sec. It will first be installed in ISAAC, the Infrared Array Camera and Spectrometer built for the VLT (Moorwood 1993). The present status of both the detector developments and the data acquisition system is reviewed.
Because individuals develop dementia as a manifestation of neurodegenerative or neurovascular disorder, there is a need to develop reliable approaches to their identification. We are undertaking an observational study (Ontario Neurodegenerative Disease Research Initiative [ONDRI]) that includes genomics, neuroimaging, and assessments of cognition as well as language, speech, gait, retinal imaging, and eye tracking. Disorders studied include Alzheimer’s disease, amyotrophic lateral sclerosis, frontotemporal dementia, Parkinson’s disease, and vascular cognitive impairment. Data from ONDRI will be collected into the Brain-CODE database to facilitate correlative analysis. ONDRI will provide a repertoire of endophenotyped individuals that will be a unique, publicly available resource.
In the focal article, Ree, Carretta, and Teachout (2015) address a common error in research methods, in which researchers neglect the shared variance between facets of a multidimensional construct. We agree with the need to attend to the entire factor structure of constructs when using measures, whether in research or application. The objective of this commentary is to elaborate on useful practices when a dominant general factor (DGF), as defined by the focal article, is found to be present and, in particular, to explore cases of DGF results under research paradigms not considered by the focal article.
Flash-lamp annealing (FLA) has been investigated for the crystallization of a 60 nm amorphous silicon (a-Si) layer deposited by PECVD on display glass. Input factors to the FLA system included lamp intensity and pulse duration. Conditions required for crystallization included use of a 100 nm SiO2 capping layer, and substrate heating resulting in a surface temperature ∼ 460 °C. An irradiance threshold of ∼ 20 kW/cm2 was established, with successful crystallization achieved at a radiant exposure of 5 J/cm2, as verified using variable angle spectroscopic ellipsometry (VASE) and Raman spectroscopy. Nickel-enhanced crystallization (NEC) using FLA was also investigated, with results suggesting an increase in crystalline volume. Different combinations of furnace annealing and FLA were studied for crystallization and activation of samples implanted with boron and phosphorus. Boron activation demonstrated a favorable response to FLA, achieving a resistivity ρ < 0.01 Ω•cm. Phosphorus activation by FLA resulted in a resistivity ρ ∼ 0.03 Ω•cm.
In this paper we report on the 532 nm Nd:YAG laser-induced crystallization of 10 nm thick boron-doped hydrogenated amorphous silicon thin films deposited on flexible polyimide and on rigid oxidized silicon wafers by hot-wire or by plasma-enhanced chemical vapor deposition. The dark conductivity increased from ∼10-7 Ω-1cm-1, in the as-deposited films, to ∼10 and 50 Ω-1cm-1 after laser irradiation, on rigid and flexible substrates, respectively. Depending on type of substrate, laser power and fluence, a Raman crystalline fraction between 55 and 90 % was measured in HWCVD films, which was higher than observed in rf-PECVD films (35-55 %). Crystallite size remained small in all cases, in the range 6-8 nm. Due to a very high conductivity contrast (>7 orders of magnitude) between amorphous and crystallized regions, it was possible to define conductive paths in the a-Si:H matrix, by mounting the sample on a X-Y software-controlled movable stage under the laser beam, with no need for the usual lithography steps. The resistors scribed by direct laser writing had piezoresistive properties, with positive gauge factor ∼1. The details of the laser interaction process with the Si film were revealed by scanning electron microscopy imaging.
A fast moving infrared excess source (G2) which is widely interpreted as a core-less gas and dust cloud approaches Sagittarius A* (Sgr A*) on a presumably elliptical orbit. VLT Ks-band and Keck K′-band data result in clear continuum identifications and proper motions of this ∼19m Dusty S-cluster Object (DSO). In 2002-2007 it is confused with the star S63, but free of confusion again since 2007. Its near-infrared (NIR) colors and a comparison to other sources in the field speak in favor of the DSO being an IR excess star with photospheric continuum emission at 2 microns than a core-less gas and dust cloud. We also find very compact L′-band emission (<0.1″) contrasted by the reported extended (0.03″ up to ∼0.2″ for the tail) Brγ emission. The presence of a star will change the expected accretion phenomena, since a stellar Roche lobe may retain a fraction of the material during and after the peri-bothron passage.
It is well established that controlled high-temperature annealing of hydrogen silsesquioxane leads to the formation of small spherical silicon nanocrystals (∼3 nm). The present study outlines an investigation into the influence of annealing time and temperature. After prolonged annealing, crystal surfaces thermodynamically self-optimize to form a variety of faceted structures (e.g., cubic, truncated trigonal and hexagonal structures).
We report on the measurement of defect densities and minority carrier lifetimes in nanocrystalline Si samples contaminated with controlled amounts of oxygen. Two different measurement techniques, a capacitance-frequency (CF) and high temperature capacitance-voltage techniques were used. CF measurement is found to yield noisy defect profiles that could lead to inconclusive results. In this paper, we show an innovative technique to remove the noise and obtain clean data using wavelet transforms. This helps us discover that oxygen is creating both shallow and deep/midgap defect states in lieu with crystalline silicon. Minority carrier lifetime measured using reverse recovery techniques shows excellent inverse correlation between deep defects and minority carrier lifetimes through which hole capture cross section can be evaluated.
We have performed an analysis on three hydrogenated nanocrystalline silicon (nc-Si:H) based solar cells. In order to determine the impact that impurities play in shaping the material properties, the XRD and Raman spectra corresponding to all three samples were measured. The XRD results, which displayed a number of crystalline silicon-based peaks, were used in order to approximate the mean crystallite sizes through Scherrer's equation. Through a peak decomposition process, the Raman results were used to estimate the corresponding crystalline volume fraction. It was noted that small crystallite sizes appear to favor larger crystalline volume fractions. This dependence seems to be related to the oxygen impurity concentration level within the intrinsic nc-Si:H layers.
We report on the use of coplanar transient photoconductivity and post-transit time-of-flight spectroscopy techniques in the study of carrier transport in microcrystalline silicon films prepared over a range of crystallinities. Coplanar samples are susceptible to post-deposition oxidation and reversible adsorption of atmospheric gases, which may alter the apparent density of states. Coplanar measurements suggest lower deep defect densities in more highly crystalline films, but this is due at least in part to an increased occupancy of these states. A comparison of results obtained using both techniques suggests anisotropic transport, with reduced band tailing (greater structural order) along the direction of film growth, a larger defect concentration around the column boundaries, and a higher defect density within the amorphous tissue than in optimised single-component amorphous silicon films.
A study of the effects of light-soaking and atmospheric adsorption (aging) on the dark- and photo-conductivity of a series of microcrystalline silicon films of varying crystallinity is presented. Light-soaking in vacuum slightly reduces photoconductivity in films close to the amorphous – microcrystalline transition, and there is also a reduction in dark current. Aging increases the dark current, and thus unless due care is taken during light-soaking experiments to eliminate or compensate for aging, the apparent effect of light-soaking may be reduced or even reversed in sign. Transient photocurrent decays confirm the presence of a large density of metastable light-induced defects. A shift in the apparent distribution of defects occurs on prolonged aging, which may be due either to changes in the DOS or a shift in the Fermi level.
We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical room-temperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been reported for electrons and for holes in amorphous silicon, and suggesting that this magnitude is a basic characteristic of mobility-edges, at least in silicon-based materials. The attempt-frequency is about 109 s-1; this value is substantially smaller than the values 1011 - 1012 s-1 typically reported holes in amorphous silicon, but the physical significance of the parameter remains obscure.
1H NMR has been employed to study the local environments of bonded hydrogen and trapped molecular hydrogen (H2) in a series of a-Si1−xGex:H alloys. There is a monotonic decrease of bonded hydrogen with increasing x from ≈ 10 at. % at x = 0 (a-Si:H) to ≈ 1 at. % at x = 1 (a-Ge:H). The amplitude of the broad 1H NMR line, which is attributed to clustered bonded hydrogen, decreases continuously across the system. The amplitude of the narrow 1H NMR line, which is attributed to bonded hydrogen essentially randomly distributed in the films, decreases as x increases from 0 to ≈ 0.2. From x = 0.2 to x ≈ 0.6 the amplitude of the narrow 1H NMR line is essentially constant, and for x ≥ 0.6 the amplitude decreases once again. The existence of trapped H2 molecules is inferred indirectly by their influence on the temperature dependence of the spin-lattice relaxation times, T1. Through T1, measurements it is determined that the trapped H2 concentration drops precipitously between x = 0.1 and x = 0.2, but is fairly constant for 0.2 ≤ x ≤ 0.6. For a-Si:H (x = 0) the H2 concentration is ≈ 0.1 at. %, while for x ≥ 0.2 the concentration of H2 is ≤ 0.02 at. %.
Using a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.
The influence of deposition temperature on hydrogen incorporation in a-Si:H prepared by VHF glow discharge at 70 MHz is investigated using hydrogen evolution, elastic recoil detection analysis and infrared spectroscopy.The films were further characterized by dark- and photoconductivity and by photothermal deflection spectroscopy. While the electronic film properties deteriorate in the usual manner with decreasing substrate temperature it is found that the total hydrogen content CH and the degree of microstructure that can be directly correlated to CH increase only moderately. It is concluded that a higher flux of low energy ions in the VHF plasma plays a key role in this context, possibly by increasing the surface mobility of the H atoms and thereby preventing the build-in of a large amount of hydrogen at low substrate temperatures.
We have investigated the influence of substrate temperature on the optoelectronic and structural properties of heavily doped μc-Si:H, prepared with the Very High Frequency Glow Discharge process. At substrate temperatures as low as 160°C we obtain, for films with 0.5μm thickness, maximum conductivities of 100 S/cm and 20 S/cm for <n> and <p> material, respectively. Starting from these values the deposition parameters were optimised for ultrathin layers having thicknesses in the range of 100 to 500Å. We observe that boron doping plays a critical role in the crystallisation of ultrathin films. The thinnest layers investigated so far show conductivities of 0.2 S/cm at d=100Å for <n>, and 0.2 S/cm at d=250Å for <p> material. These properties make μc-Si:H films attractive candidates to form tunnel junctions in tandem solar cells.
We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AIGaAs material system. The use of a metallized surface relief grating for feedback allows a fabrication process without regrowth. A feature of this laser is that either single mode or double mode emission at λ ≍ 10 µm is achieved, which is typical for index coupled lasers. The coupling coefficient is measured from the mode spacing for double mode emission of a strong overcoupled laser to be κ ≍ 24 cm"−1. The emission wavenumber can be continuously tuned with the temperature at a rate of dv / dT ≍ 0.048 cm−1/K, which is in close agreement to the temperature dependence of the refractive index of GaAs.
Post-transit time-of-flight spectroscopy has been used to study the density of states distribution in hot-wire CVD microcrystalline silicon pin solar cell structures. For an absorber layer Raman scattering intensity ratio ICRS of 0.4 or less, behaviour consistent with multiple-trapping carrier transport is observed and may be interpreted in terms of a conduction-band tail of some 18 meV slope plus a broad defect bump of order 1017 cm-3 centered at 0.55 eV relative to the mobility edge. As ICRS is increased beyond 0.4, the temperature-dependence of the photocurrent transient becomes inconsistent with multiple-trapping and above 0.6 the decays are almost temperature-independent. By comparing data taken at 300 K, it may be inferred from the multiple-trapping model that localised states between 0.35 and 0.5 eV are associated with the presence of columns or clusters of nanocrystals and those deeper than 0.5 eV with the amorphous tissue. Results are compared with previous work on coplanar and sandwich structures.