Hitherto, two distinct families of multielement detector arrays have been used for infrared (IR) imaging system applications: linear arrays for scanning systems (first generation) and two-dimensional arrays for staring systems (second generation). Nowadays, third-generation IR systems are being developed which, in the common understanding, provide enhanced capabilities such as larger numbers of pixels, higher frame rates, better thermal resolution, multicolor functionality, and/or other on-chip signal-processing functions. In this paper, fundamental and technological issues associated with the development and exploitation of third-generation IR photon detectors are discussed. In this class of detectors the two main competitors, HgCdTe photodiodes and quantum-well photoconductors, are considered. This is followed by discussions focused on the most recently developed focal plane arrays based on type-II strained-layer superlattices and quantum dot IR photodetectors. The main challenges facing multicolor devi...

Third-generation infrared photodetector arrays

In the last two decades, several new concepts for improving the performance of infrared detectors have been proposed. These new concepts particularly address the drive towards the so-called high operating temperature focal plane arrays (FPAs), aiming to increase detector operating temperatures, and as a consequence reduce the cost of infrared systems. In imaging systems with the above megapixel formats, pixel dimension plays a crucial role in determining critical system attributes such as system size, weight and power consumption (SWaP). The advent of smaller pixels has also resulted in the superior spatial and temperature resolution of these systems. Optimum pixel dimensions are limited by diffraction effects from the aperture, and are in turn wavelength-dependent. In this paper, the key challenges in realizing optimum pixel dimensions in FPA design including dark current, pixel hybridization, pixel delineation, and unit cell readout capacity are outlined to achieve a sufficiently adequate modulation transfer function for the ultra-small pitches involved. Both photon and thermal detectors have been considered. Concerning infrared photon detectors, the trade-offs between two types of competing technology-HgCdTe material systems and III-V materials (mainly barrier detectors)-have been investigated.

http://iopscience.iop.org/article/10.1088/0034-4885/79/4/046501/pdf

Challenges of small-pixel infrared detectors: a review

HgCdTe 2D arrays are needed in both medium (MW) and long (LW) wavebands for imaging, search, and track and guidance applications. Often the detector is the performance-limiting component in the system, and it is necessary to use detectors with very low excess noise and few defective pixels. Normally the detector is cooled sufficiently to freeze-out thermally generated leakage currents, so the main interest is to understand the mechanisms that determine the general detector performance and the cause of defective pixels. This paper describes the detector technology and the ion beam junction-forming process. The fundamental performance limits of homojunction HgCdTe technology and the doping levels needed to produce a detector with impact-ionization limited performance are discussed. Extensive studies have been made on defective pixels in long wavelength arrays and some technologies for reducing them are described here. Defective pixels have been found to be associated with material dislocations crossing the p-n junction and a model has been proposed for the noise-generating mechanism.

Summary of HgCdTe 2D array technology in the U.K.

The only way to overcome the CMOS noise barrier of near infrared sensors used for wavefront sensing and fringe tracking is the amplification of the photoelectron signal inside the infrared pixel by means of the avalanche gain. In 2007 ESO started a program at Selex to develop near infrared electron avalanche photodiode arrays (eAPD) for wavefront sensing and fringe tracking. In a first step the cutoff wavelength was reduced from 4.5 micron to 2.5 micron in order to verify that the dark current scales with the bandgap and can be reduced to less than one electron/ms, the value required for wavefront sensing. The growth technology was liquid phase epitaxy (LPE) with annular diodes based on the loophole interconnect technology. The arrays required deep cooling to 40K to achieve acceptable cosmetic performance at high APD gain. The second step was to develop a multiplexer tailored to the specific application of the GRAVITY instrument wavefront sensors and the fringe tracker. The pixel format is 320x256 pixels. The array has 32 parallel video outputs which are arranged in such a way that the full multiplex advantage is available also for small subwindows. Nondestructive readout schemes with subpixel sampling are possible. This reduces the readout noise at high APD gain well below the subelectron level at frame rates of 1 KHz. The third step was the change of the growth technology from liquid phase epitaxy to metal organic vapour phase epitaxy (MOVPE). This growth technology allows the band structure and doping to be controlled on a 0.1μm scale and provides more flexibility for the design of diode structures. The bandgap can be varied for different layers of Hg( 1-x )Cd x Te. It is possible to make heterojunctions and apply solid state engineering techniques. The change to MOVPE resulted in a dramatic improvement in the cosmetic quality with 99.97 % operable pixels at an operating temperature of 85K. Currently this sensor is deployed in the 4 wavefront sensors and in the fringe tracker of the VLT instrument GRAVITY. Initial results will be presented. An outlook will be given on the potential of APD technology to be employed in large format near infrared science detectors. Several of the results presented here have also been shown to a different audience at the Scientific Detector Workshop in October 2013 in Florence but this paper has been updated with new results [1].

SAPHIRA detector for infrared wavefront sensing

Active systems, using a near-infrared pulse laser and a fast, gated detector, are now adopted for most long range imaging
applications. This concept is often called laser-gated imaging (LGI) or burst-illumination LIDAR (BIL). The SELEX
solid state detector is based on an array of HgCdTe avalanche photodiodes, and a custom-designed CMOS multiplexer to
perform the fast gating and photon signal capture. This paper describes two recent developments. The first is aimed at
reducing the size, weight, power and cost of steerable platforms which often have to contain a large number of electrooptic
tools such as lasers, range finders, BIL, thermal imaging and visible cameras. A dual-mode infrared detector has
been developed with the aim of shrinking the system to one camera. The detector can be switched to operate as a passive
thermal imager, a laser-gated imager or a solar flux imager. The detector produces a sensitivity in the MW thermal band
of 16-18mK and a sensitivity in the BIL mode as low as 10 photons rms, in other words close to the performance of
dedicated imagers. A second development was to extend the current BIL capability to 3D. In complex scenes, with camouflage and concealment, the ability to generate 3D images provides a signal-to-clutter advantage. Also in airborne applications, especially, it is useful to have 3D information to provide agile, feedback control of the range gating in a dynamic environment. This report describes the development of the 3D detector and camera, and the results of field trials
using a prototype system.

http://client.blueskybroadcast.com/spie/dss08/pdf/6940-83-a3_baker.pdf

Advanced infrared detectors for multimode active and passive imaging applications

This paper describes a generic focal plane technology which has been developed to serve a range of second generation infrared system applications in the UK and Europe. These applications call for both two-dimensional and long linear arrays, and spectral sensitivities from 2.5 to 12.5 micrometers . The infrared sensor technology is based on CdHgTe material grown by the tellurium rich, liquid phase epitaxy (LPE) process and lateral collection photodiode arrays. The CdHgTe arrays are mounted on custom designed CMOS integrated circuits which provide the multiplexing and signal processing functions required by the system. The technical directions chosen for the LPE growth process and the hybrid fabrication process to produce the highest performance after multiplexing are described, along with a discussion of some of the performance limits for this technology. The producibility, environmental stability and typical radiometric performance are presented with respect to one focal plane type, a 1024 element long linear array. Recently, the availability of denser CMOS processes has enabled some special techniques to be developed for advanced infrared search- and-track and high performance imaging applications. Such applications require the highest possible performance, and call for special functions such as: signal-to-noise enhancement by time-delay and integration (TDI), defective element deselection (DED) for redundancy, large numbers of elements, and low image crosstalk. The technical routes chosen for long wavelength, long linear arrays are outlined, and prototype devices with up to 12 elements in TDI, user-definable DED, and a pixel size of 30 micrometers square are described.

CdHgTe-CMOS hybrid focal plane arrays: a flexible solution for advanced infrared systems

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CdHgTe-CMOS hybrid focal plane arrays: a flexible solution for advanced infrared systems