David J. Lees

Photomultiplication with low excess noise factor in MWIR to optical fiber compatible wavelengths in cooled HgCdTe mesa diodes

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.

Large-format MWIR focal plane arrays

Medium wavelength IR arrays have been develoepd which have 1024×768 pixels on a 26 micron pitch. The arrays are made from epitaxially grown indium antimonide, the use of which confers two advantages over conventional InSb owing to the ability to exercise atomic level control of dopants and material thicknesses. Firstly, the photodiodes can be grown on degenerately doped InSb substrates which have a high degree of transparency, so the requirement for the substrate to be thinned is much reduce dleading to simplified manufacture. Secondly, it offers the potential for an increase in operating temperature of many tens of degrees, through elimination of contact leakage currents, though we focus on 80K performance here for comparison with conventional structures. We present initail results form arrays which indicate high operability, despite the need to stitch reticles in the fabrication of the silicon read-out circuit, and temperature sensitivity close to the theoretical limit. Imaging from the arrays compares very favorably with that taken using generation II cameras and gives confidence that this technology offers a cost effective route to large format MWIR systems.

Dual-waveband infrared focal plane arrays using MCT grown by MOVPE on silicon substrates (Invited Paper)

Dual-waveband, Focal Plane Arrays (FPAs) based on Hg1-xCdxTe multi-layer structures have previously been produced by the Molecular Beam Epitaxy (MBE) growth technique. It is shown that the multi-layer structures required for dual-waveband devices can also be grown by Metal Organic Vapor Phase Epitaxy (MOVPE). The MOVPE growth process allows excellent control of both the composition and doping profiles and has the advantage of allowing growth on a range of substrates including silicon. Previous research on back-to-back diodes for dual-waveband has concentrated on npn structures. The design of the alternative pnp structures is discussed and a model is developed which gives a good fit to the measured spectra. We report on the design and characterization of dual-waveband detectors including current-voltage and spectral cross talk for the case of two close sub-bands within the 3-5 μm mid-wave infrared (MWIR) spectral range. The mechanisms for spectral cross talk are discussed including incomplete absorption, transistor action and radiative coupling. A custom readout circuit (ROIC) has been designed. This allows the capture of data from the two bands which is spatially aligned but sequential in time.

Epitaxial InSb for elevated temperature operation of large IR focal plane arrays

The use of epitaxially grown indium antimonide (InSb) has previously been demonstrated for the production of large 2D focal plane arrays. It confers several advantages over conventional, bulk InSb photo-voltaic detectors, such as reduced cross-talk, however here we focus on the improvement in operating temperature that can be achieved because more complex structures can be grown. Diode resistance, imaging, NETD and operability results are presented for a progression of structures that reduce the diode leakage current as the temperature is raised above 80K, compared with a basic p+-n-n+ structure presented previously. These include addition of a thin region of InAlSb to reduce p-contact leakage current, and construction of the whole device from InAlSb to reduce thermal generation in the active region of the detector. An increase in temperature to 110K, whilst maintaining full 80K performance, is achieved, and imaging up to 130K is demonstrated. This gives the prospect of significant benefits for the cooling systems, including, for example, use of argon in Joule-Thomson coolers or an increase in the life and/or decrease in the cost; power consumption and cool-down time of Stirling engines by several tens of per cent.

HOTEYE: a novel thermal camera using higher operating temperature infrared detectors

Conventional high performance infrared (IR) sensors need to be cooled to around 80K in order to achieve a high level of thermal sensitivity. Cooling to this temperature requires the use of Joule-Thomson coolers (with bottled gas supply) or Stirling cycle cooling engines, both of which are bulky, expensive and can have low reliability. In contrast to this, higher operating temperature (HOT) detectors are designed to give high thermal performance at an operating temperature in the range 200K to 240K. These detectors are fabricated from multi-layer mercury cadmium telluride (MCT) structures that have been designed for this application. At higher temperatures, lower cost, smaller, lighter and more reliable thermoelectric (or Peltier) devices can be used to cool the detectors. The HOTEYE thermal imaging camera, which is based on a 320x256 pixel HOT focal plane array, is described in this paper and performance measurements reported.

Integrated infrared detectors and readout circuits

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.

Dual waveband MW/LW focal plane arrays grown by MOVPE on silicon substrates

The use of silicon substrates has been very successful for producing large area focal plane arrays operating in the MWIR waveband using the MBE growth process. More recently, promising results have been obtained in the LWIR waveband using a MOVPE growth process on a buffered silicon substrate. The MOVPE growth process is also suitable for more complex multi-layer structures and we have now used this technique to produce our first MW/LW dual waveband focal plane arrays. In this paper we show that close to background limited performance can be achieved in both wavebands, however the main challenge with arrays grown on silicon is to obtain low defect counts. These first arrays are promising in this respect and operabilities of 99.4% and 98.2% have been achieved in the MWIR band and LWIR band respectively. The availability of dual waveband arrays allows the correlation of defects in the two wavebands to be compared. In general, we find that the correlation is low and this suggests that defect generation mechanisms which would affect both bands (such as threading dislocations) are currently not the main source of defective devices in MOVPE grown devices on silicon.

High-performance 2D MWIR Hg 1-x Cd x Te array operating at 220K

Infrared detectors based on Hg1-xCdxTe and grown by the MOVPE process can be designed to have very low dark currents, even for temperatures above 200K. These low dark currents are compatible with achieving background-limited performance at a temperature of 200K in f/2. However, in practice the detectors suffer from high 1/f noise. In this paper, a novel approach is explored in which most of the low frequency noise can be eliminated by operating the arrays at near zero bias. Using this technique, imaging arrays have been demonstrated at temperatures up to 220K giving a NETD of around 60mK in f/2.

Wide-band (2.5 - 10.5 µm), high-frame rate IRFPAs based on high-operability MCT on silicon

We have previously presented results from our mercury cadmium telluride (MCT, Hg1-xCdxTe) growth on silicon substrate technology for different applications, including negative luminescence, long waveband and mid/long dual waveband infrared imaging. In this paper, we review recent developments in QinetiQs combined molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE) MCT growth on silicon; including MCT defect density, uniformity and reproducibility. We also present a new small-format (128 x 128) focal plane array (FPA) for high frame-rate applications. A custom high-speed readout integrated circuit (ROIC) was developed with a large pitch and large charge storage aimed at producing a very high performance FPA (NETD ~10mK) operating at frame rates up to 2kHz for the full array. The array design allows random addressing and this allows the maximum frame rate to be increased as the window size is reduced. A broadband (2.5-10.5 μm) MCT heterostructure was designed and grown by the MBE/MOVPE technique onto silicon substrates. FPAs were fabricated using our standard techniques; wet-etched mesa diodes passivated with epitaxial CdTe and flip-chip bonded to the ROIC. The resulting focal plane arrays were characterized at the maximum frame rate and shown to have the high operabilities and low NETD values characteristic of our LWIR MCT on silicon technology.

A high-speed, MWIR reference source for FPA non-uniformity correction using negative luminescence

We have previously discussed the potential of using a Hg1-xCdxTe source as a reference plane for the non-uniformity correction of thermal imagers and which is being developed as an option for the UK 3rd generation, high performance thermal imaging program (Albion). In this paper we will present our first results on a large area (1.5 cm x 1.5 cm) source which was grown on a silicon substrate and can simulate a range of temperatures from -10 °C to +30 °C. Due to the fast switching speed, the apparent temperature can be changed on a frame by frame basis. Also, the operation of the device can be synchronized to the integration time of the camera to reduce the mean power requirements by a factor of 10 and reduce thermal heating effects. The main applications for Hg1-xCdxTe devices as high-performance, cryogenically-cooled detectors typically require very low drive currents. The use of this material for large-area LEDs has generated new challenges to deal with the high peak currents. These are typically in the range 1-2 A/cm2 for a MWIR waveband source and have led to a need to reduce the common impedance, reduce the contact resistances and consider the effects of current crowding.