Two talks

Germanium CCDs

speaker:  Christopher Leitz, MIT Lincoln Laboratory

Abstract: 
The short-wave infrared (SWIR) band, spanning approximately 1 – 3 μm, is of great importance for night vision, wavefront sensing, and hyper-spectral imaging.  While silicon is the material of choice for visible imaging, it is insensitive to wavelengths longer than 1.1 μm.  The dominant imaging technology for wavelengths from 0.9 – 1.7 μm consists of InGaAs diode arrays bump-bonded to silicon readout integrated circuits (ROICs).  In contrast to silicon, these detectors are expensive, fragile, and limited to 150-mm diameter or smaller substrates.  Bump bonding limits the minimum pixel pitch to about 15 µm.  Furthermore, back-illumination is performed on the chip- rather than wafer-level, and response cuts off below 920 nm due to absorption in the InP substrate.  Finally, the largest InGaAs devices are limited to 1.3 Mpixel.  Arrays based on HgCdTe are also available and cover a larger portion of the SWIR band but require substantial cooling and are not as widely used as those of InGaAs.  
 
Compared to these imager materials, bulk germanium offers several compelling advantages.  Germanium is commercially available in wafer diameters up to 200-mm and can be processed in the same semiconductor fabrication tools used to build silicon imaging devices.  This enables fabrication of large-format imaging devices with a narrow pixel pitch.  Recent advances in surface passivation have yielded the high-quality metal-oxide interface required for fabricating CCDs, providing the same advantages (uniformity, noiseless binning, TDI) of silicon CCDs.  Next, germanium offers wider sensitivity than any of these materials, with high quantum efficiency from 0.3 – 1.8 μm, enabling both SWIR and hyper-spectral imaging.  Finally, because it can be grown ultra-pure and has a high atomic number, germanium is also an efficient X-ray absorber with sensitivity to 30 keV and beyond, enabling further applications in medical and scientific imaging.
 
In this talk, we describe our progress in fabricating germanium CCDs, outline our performance goals for a germanium imaging device, and highlight potential applications of these devices.

Host:  Mark Bautz

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The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters

speaker:  Remco van der Burg, CEA Saclay, France

Abstract: 
Recent observations have shown that Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giants) are surprisingly abundant in local clusters of galaxies. The origin of UDGs remains puzzling, as the interpretation of these studies has been hindered by the (partly) subjective selection of these galaxies, and the limited study of only the Coma and Virgo clusters. I’ll present results of the first systematic search for UDGs in eight nearby clusters. I’ll focus on the abundance of UDGs as a function of halo mass, and their radial distribution in the clusters. The latter indicates that they were likely accreted by the cluster several Gyrs ago, but are still hosted by massive dark-matter haloes. I’ll discuss implications and future prospects to learn more about their properties.

Host:  Rongmon Bordoloi