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 QinetiQ's 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
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.
We have previously discussed the potential of using an Hg1-xCdxTe (MCT) source as a reference plane for the nonuniformity
correction of thermal imagers. Due to the fast switching speed, the apparent temperature can be changed on a
frame to frame basis. This allows multipoint correction data to be obtained without having to wait for temperatures to
stabilize as with a Peltier reference source. Also, the operation of the device can be synchronized to the integration
period of the camera to reduce the mean power requirements by the ratio of the frame to the integration time and hence
thermal heating effects are also reduced.
In this paper, we discuss a practical implementation of this concept in a thermal imaging camera, which is being
developed as part of the UK MOD Albion program. This development has involved increasing the device size,
increasing the effective temperature range and matching the drive requirements to typical camera power supplies. The
factors determining the achievable effective temperatures are discussed, together with modifications to the device design
that have been implemented to obtain a useful temperature range. The drive requirements have been improved by
developing a series connected structure. This has reduced the peak current by a factor of 4 and allows the devices to be
controlled with conventional Peltier reference electronics rather than a custom unit. The improved devices have now
been incorporated into a state-of-the-art infrared camera and their performance in this system will be discussed.
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.
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.
We have previously discussed the potential of using a Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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 Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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/cm<sup>2</sup> 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
Negative luminescent (NL) devices, which to an IR observer can appear colder than they actually are, have a wide range of possible applications, including use as modulated IR sources in gas sensing systems and as thermal radiation shields in IR cameras. A further important use would be a calibration source for IR focal plane arrays where there are many potential advantages over conventional sources, including high speed operation (for multi-point correction) and lower power consumption. Such applications present considerable technological challenges as they require large area uniform devices (>1cm<sup>2</sup>) with a large apparent temperature range.
In this paper we report on recent progress in fabricating large area (1.5cm × 1.5cm) negative luminescence devices from Hg<sub>1-x</sub>Cd<sub>x</sub>Te grown on silicon substrates using a segmented device architecture.
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 3x10<sup>4</sup> Ω cm<sup>2</sup> at 77K. This is background limited in f/2 and demonstrates that wafer scale production is achievable.
Dual-waveband, Focal Plane Arrays (FPAs) based on Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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.
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.
Negative luminescent devices, which absorb more light than they emit when reverse biased, have a large number of applications including, reference planes for thermal cameras, infrared (IR) sources and IR scene projection. This paper describes devices made from mercury cadmium telluride grown on silicon substrates, focusing on large area arrays with reduced operating powers. Novel growth structures and device designs have been investigated in order to reduce the series resistance. Results from the first dry etched, LW MCT on Si, 1 cm<sup>2</sup> device with optical concentrators are presented.
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 R<sub>0</sub>A~1x10<sup>3</sup>Ohm cm<sup>2</sup> at zero bias and R.A~1x10<sup>4</sup>Ohm cm<sup>2</sup> at 0.1V reverse bias. Arrays from this material have been demonstrated with operabilities up to 99.7%.
Infrared LEDs and negative luminescent devices, where less light is emitted than in equilibrium, have been attracting an increasing amount of interest recently. They have a variety of applications, including as a 'source’ of IR radiation for gas sensing; radiation shielding for and non-uniformity correction of high sensitivity starring infrared detectors; and dynamic infrared scene projection. Similarly, IR detectors are used in arrays for thermal imaging and, discretely, in applications such as gas sensing. Multi-layer heterostructure epitaxy enables the growth of both types of device using designs in which the electronic processes can be precisely controlled and techniques such as carrier exclusion and extraction can be implemented. This enables detectors to be made which offer good performance at higher than normal operating temperatures, and efficient negative luminescent devices to be made which simulate a range of effective temperatures whilst operating uncooled.
In both cases, however, additional performance benefits can be achieved by integrating optical concentrators around the diodes to reduce the volume of semiconductor material, and so minimise the thermally activated generation-recombination processes which compete with radiative mechanisms. The integrated concentrators are in the form of Winston cones, which can be formed using an iterative dry etch process involving methane/hydrogen and oxygen. We will present results on negative luminescence in the mid and long IR wavebands, from devices made from indium antimonide and mercury cadmium telluride, where the aim is sizes greater than 1cm x 1cm. We will also discuss progress on, and the potential for, operating temperature and/or sensitivity improvement of detectors, where very higher performance imaging is anticipated from systems which require no mechanical cooling.
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<sup>+</sup>-n-n<sup>+</sup> 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.
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.