The superb image quality that is predicted, and even demanded, for the next generation of Extremely Large Telescopes
(ELT) presents a potential crisis in terms of the sheer number of detectors that may be required. Developments in
infrared technology have progressed dramatically in recent years, but a substantial reduction in the cost per pixel of these
IR arrays will be necessary to permit full exploitation of the capabilities of these telescopes. Here we present an outline
and progress report of an initiative to develop a new generation of astronomical grade Cadmium Mercury Telluride
(HgCdTe) array detectors using a novel technique which enables direct growth of the sensor diodes onto the Read Out
Integrated Circuit (ROIC). This technique removes the need to hybridise the detector material to a separate Silicon
readout circuit and provides a route to very large monolithic arrays. We present preliminary growth and design
simulation results for devices based on this technique, and discuss the prospects for deployment of this technology in the
era of extremely large telescopes.
SMART focal plane arrays have in-pixel signal processing circuits that improve the performance of electro-optical
sensors and extend their functionality. This paper describes two types of SMART focal plane array that have been
developed at QinetiQ aimed at improved sensitivity and long range object identification. A novel in-pixel adaptive
circuit is described which improves sensitivity by removing the background photo-signal. This allows the detector stare
time to be increased resulting in lower noise bandwidth and an increase in signal-to-noise ratio. The second type of
SMART focal plane array described in this paper is designed to detect time varying signals generated, for example, by
helicopter blades, jet turbine engines and hot exhaust plumes. The detection of temporal signatures enables objects to be
identified at significantly longer ranges than conventional focal plane arrays.
The standard process for manufacturing mercury cadmium telluride (MCT) infrared focal plane arrays (FPAs) involves hybridising detectors onto a readout integrated circuit (ROIC). Wafer scale processing is used to fabricate both the detector arrays and the ROICs. The detectors are usually made by growing epitaxial MCT on to a suitable substrate, which is then diced and hybridised on to the ROIC. It is this hybridisation process that prevents true wafer scale production; if the MCT could be grown directly onto the ROIC, then wafer scale production of infrared FPAs could be achieved. In order to achieve this, a ROIC compatible with the growth process needs to be designed and fabricated and the growth and processing procedures modified to ensure survival of the ROIC. Medium waveband IR detector test structures have been fabricated with resistance area product of around 3x104 Ω cm2 at 77K. This is background limited in f/2 and demonstrates that wafer scale production is achievable.
Infrared avalanche diodes are key components in diverse applications such as eye-safe burst illumination imaging systems and quantum cryptography systems operating at telecommunications fiber wavelengths. HgCdTe is a mature infrared detector material tunable over all infrared wavelengths longer than ~850nm. HgCdTe has fundamental properties conducive to producing excellent detectors with low noise gain. The huge asymmetry between the conduction and valence bands in HgCdTe is a necessary starting point for producing impact ionization with low excess noise factor. Other factors in the band structure are also favorable. The low bandgap necessitates at least multi-stage thermoelectric cooling. Mesa diode structures with electron initiated multiplication have been designed for gains of up to around 100 at temperatures at or above 80K. Backside illuminated, flip-chip, test diode arrays have been fabricated by MOVPE using a process identical to that required for producing large imaging arrays. Test diode results have been obtained with the following parameters characterized, dark current vs. voltage and temperature, gain vs. voltage, and spectral response as a function of wavelength and bias. The effect of changing active region cadmium composition and active region doping is presented along with an assessment of some of the trade-offs between dark leakage current, gain, operating voltage and temperature of operation.
We have demonstrated the successful growth of mercury cadmium telluride (MCT) infrared detector material on silicon substrates. Growth on silicon increases the maximum achievable array size, reduces manufacturing costs, and paves the way for infrared detector growth directly on multiplexing circuits. In addition, the thermal match with multiplexing circuits eliminates the requirement for complex thinning procedures. Since the crystal lattice of MCT is not matched to that of silicon, an intermediate buffer layer is required. We have developed a buffer layer technique that is compatible with MCT grown by Metal Organic Vapour Phase Epitaxy (MOVPE). Long-wavelength heterostructure device designs were grown using this technique. Test devices and 128x128 focal plane arrays were fabricated by wet etching mesa structures and passivating the mesa side-walls with a thin layer of CdTe. An indium flip-chip technique was used to form interconnects between the detector material and test or multiplexing circuit. At 77K, 50x50μm test devices with a 10.2μm cut off wavelength have been measured with R0A~1x103Ohm cm2 at zero bias and R.A~1x104Ohm cm2 at 0.1V reverse bias. Arrays from this material have been demonstrated with operabilities up to 99.7%.