Mercury cadmium telluride (HgCdTe or MCT) is the material of choice for infrared avalanche photodetectors (APDs) owing to its desirable qualities including high quantum efficiency and low excess noise factor. Recent advancements in growth techniques have allowed for bandgap engineered MCT films that further enhance the performance of MCT APDs. Monte Carlo has been a widely used method for simulating the multiplication process within avalanche photodiodes (APDs) due to its ability to accurately simulate non-equilibrium transport. In this work, we demonstrate how the gain, excess noise, and bandwidth of bandgap engineered MCT APDs can be accurately modeled in 3-D using Monte Carlo.
HgCdTe material has been grown on GaAs substrates using Metal Organic Vapour Phase Epitaxy (MOVPE) and 64 x 64 arrays were subsequently manufactured. HgCdTe was grown on 12 wafers and 6 wafers continued through processing. Companion 320 x 256, 24 μm pitch arrays were also manufactured on the same wafer. These 320 x 256 arrays are hybridized to an existing imaging ROIC. Signal and noise data are collected as a function of bias to determine Gain vs Bias and operability of the companion detector arrays. The existing 320 x 256 ROIC was designed for astronomy applications and precludes measurements at bias values < ~ 10 V since the amplified signal from the detector saturates the well of the ROIC. Gain was measured for bias values up to ~ 10 V and extrapolated to determine gain at higher bias values. This ROIC also does not permit fast pulse measurements. An alternate ROIC has been designed for fast pulse measurements but will not be presented here. Based on the 320 x 256 array signal, noise, Gain vs Bias and morphology data all 6 processed wafers yielded 64 x 64 detector arrays that are available for hybridization to ROICs. 320 x 256 arrays had operability < 99.9% based on the signal and noise data. Response and noise histograms have mean and median values within 1% of each other. The noise histogram is near Gaussian in shape. APD arrays hybridized to fanout chips are in assembly and APD gain vs bias, noise and transient response measurements are being measured directly without going through a ROIC.
A germanium charge-coupled device (CCD) offers the advantages of a silicon CCD for X-ray detection – excellent uniformity, low read noise, high energy resolution, and noiseless on-chip charge summation – while covering an even broader spectral range. Notably, a germanium CCD offers the potential for broadband X-ray sensitivity with similar or even superior energy resolution than silicon, albeit requiring lower operating temperatures (≤ 150K) to achieve sufficiently low dark noise due to the lower band gap of this material. The recent demonstration of high-quality gate dielectrics on germanium with low surface-state density and low gate leakage is foundational for realization of high-quality imaging devices on this material. Building on this advancement, MIT Lincoln Laboratory has been developing germanium CCDs for several years, with design, fabrication, and characterization of kpixel-class front-illuminated devices discussed recently. In this article, we describe plans to scale these small arrays to megapixel-class imaging devices with performance suitable for scientific applications. Specifically, we discuss our efforts to increase charge-transfer efficiency, reduce dark current, improve fabrication yield, and fabricate backside-illuminated devices with excellent sensitivity.
Silicon charge-coupled devices (CCDs) are commonly utilized for scientific imaging in wavebands spanning the near infrared to soft X-ray. These devices offer numerous advantages including large format, excellent uniformity, low read noise, noiseless on-chip charge summation, and high energy resolution in the soft X-ray band. By building CCDs on bulk germanium, we can realize all of these advantages while covering an even broader spectral range, notably including the short-wave infrared (SWIR) and hard X-ray bands. Since germanium is available in wafer diameters up to 200 mm and can be processed in the same tools used to build silicon CCDs, large-format (>10 MPixel, >10 cm2 ) germanium imaging devices with narrow pixel pitch can be fabricated. Furthermore, devices fabricated on germanium have recently demonstrated the combination of low surface state density and high carrier lifetime required to achieve low dark current in a CCD. At MIT Lincoln Laboratory, we have been developing germanium imaging devices with the goal of fabricating large-format CCDs with SWIR or broadband X-ray sensitivity, and we recently realized our first front-illuminated CCDs built on bulk germanium. In this article, we describe design and fabrication of these arrays, analysis of read noise and dark current on these devices, and efforts to scale to larger device formats.
Although HgCdTe imagers are a well-established technology, photodetectors fabricated using the same process still yield a large variation in their performance characteristics, largely stemming from hard-to-control pecu- liarities at the interface between the surface passivation and the active region of each photodiode. This work investigates the dark current characteristics of long-wave IR (cutoff wavelength of 10um) Hg0.774Cd0.226Te mesa photodiodes, which have been passivated with a CdTe film. We use a 2-D model of a p-on-n device structure to study how interface states and Cadmium diffusion at the passivation interface can influence the photodiode dark current.
The Transiting Exoplanet Survey Satellite (TESS) is an Explorer-class mission dedicated to finding planets
around bright, nearby stars so that more detailed follow-up studies can be done. TESS is due to launch in
2017 and careful characterization of the detectors will need to be completed on ground before then to
ensure that the cameras will be within their photometric requirement of 60ppm/hr. TESS will fly MITLincoln
Laboratories CCID-80s as the main scientific detector for its four cameras. They are 100μm deep
depletion devices which have low dark current noise levels and can operate at low light levels at room
temperature. They also each have a frame store region, which reduces smearing during readout and allows
for near continuous integration. This paper describes the hardware and methodology that were developed
for testing and characterizing individual CCID-80s. A dark system with no stimuli was used to measure the
dark current. Fe55 and Cd109 X-ray sources were used to establish gain at low signal levels and its
temperature dependence. An LED system that generates a programmable series of pulses was used in
conjunction with an integrating sphere to measure pixel response non-uniformity (PRNU) and gain at
higher signal levels. The same LED system was used with a pinhole system to evaluate the linearity and
charge conservation capability of the CCID-80s.
We report on two recently developed charge-coupled devices (CCDs) for adaptive optics wavefront sensing, both designed to provide exceptional sensitivity (low noise and high quantum efficiency) in high-frame-rate low-latency readout applications. The first imager, the CCID75, is a back-illuminated 16-port 160×160-pixel CCD that has been demonstrated to operate at frame rates above 1,300 fps with noise of < 3 e-. We will describe the architecture of this CCD that enables this level of performance, present and discuss characterization data, and review additional design features that enable unique operating modes for adaptive optics wavefront sensing. We will also present an architectural overview and initial characterization data of a recently designed variation on the CCID75 architecture, the CCID82, which incorporates an electronic shutter to support adaptive optics using Rayleigh beacons.
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