Optical coherence tomography (OCT) has developed rapidly and is widely used in different fields such as biomedicine and optometry. The characterization and calibration of OCT systems is essential when testing the system and during normal use to ensure that there is no misalignment or distortion that could affect clinical decisions. Imaging distortion is a significant challenge for OCT systems when viewing through non-planar surfaces. Here we present a new multi-purpose plano-convex OCT phantom which is designed to be used for OCT characterization and calibration as well as to validate the post-processing algorithm for the imaging distortion of the OCT systems. A femtosecond laser direct writing technique is used to fabricate this phantom which consists of a landmark layer with radial lines at a 45-degree angular spacing inscribed at 50μm in apparent depth (AD) underneath the planar surface. Below that there are a further 8 layers of a spherical inscription pattern which has a 150μm (in AD) separation between each layer. The first spherical layer is located at 150μm (in AD) underneath the planar surface. Due to the laser power loss when travelling through the deeper layer, an increased power is applied to the deeper layers. The spherical pattern overcomes orientation issues seen with existing calibration phantoms. The landmark layer is applied so that it can easily tell the exact location when scanning which will also benefit the image distortion correction process.
Optical coherence tomography (OCT) is increasingly used in areas such as ophthalmology and contact lens metrology. However, in such cases, image distortion can occur due to the non-planar nature of the measured sample. Postprocessing algorithms can be implemented to correct this distortion. Here we present an OCT phantom designed to confirm the validity of post-processing algorithms used for measuring curved surfaces. A multi-purpose OCT phantom has been created within a fused silica plano-convex lens using the direct femtosecond laser writing technique. This phantom can be used to calibrate and quantitatively assess the performance (e.g. resolution, sensitivity and distortion) of OCT systems and associated post-processing algorithms for curved structures such as lenses. This novel OCT phantom has been characterized using an optical microscope and OCT systems.
There is an increasingly important requirement for day and night, wide field of view imaging and tracking for both
imaging and sensing applications. Applications include military, security and remote sensing. We describe the
development of a proof of concept demonstrator of an adaptive coded-aperture imager operating in the mid-wave infrared
to address these requirements. This consists of a coded-aperture mask, a set of optics and a 4k x 4k focal plane array
(FPA). This system can produce images with a resolution better than that achieved by the detector pixel itself (i.e. superresolution)
by combining multiple frames of data recorded with different coded-aperture mask patterns. This superresolution
capability has been demonstrated both in the laboratory and in imaging of real-world scenes, the highest
resolution achieved being ½ the FPA pixel pitch. The resolution for this configuration is currently limited by vibration
and theoretically ¼ pixel pitch should be possible. Comparisons have been made between conventional and ACAI
solutions to these requirements and show significant advantages in size, weight and cost for the ACAI approach.
Coded aperture imaging has been used for astronomical applications for several years. Typical implementations used a
fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. Recently applications have emerged in
the visible and infra red bands for low cost lens-less imaging systems and system studies have shown that considerable
advantages in image resolution may accrue from the use of multiple different images of the same scene - requiring a
reconfigurable mask.
Previously reported work focused on realising a 2x2cm single chip mask in the mid-IR based on polysilicon micro-optoelectro-
mechanical systems (MOEMS) technology and its integration with ASIC drive electronics using conventional
wire bonding. It employs interference effects to modulate incident light - achieved by tuning a large array of asymmetric
Fabry-Perot optical cavities via an applied voltage and uses a hysteretic row/column scheme for addressing.
In this paper we report on the latest results in the mid-IR for the single chip reconfigurable MOEMS mask, trials in
scaling up to a mask based on a 2x2 multi-chip array and report on progress towards realising a large format mask
comprising 44 MOEMS chips. We also explore the potential of such large, transmissive IR spatial light modulator arrays
for other applications and in the current and alternative architectures.
Adaptive coded aperture imaging (ACAI) has the potential to enhance greatly the performance of sensing systems by
allowing sub detector pixel image and tracking resolution. A small experimental system has been set up to allow the
practical demonstration of these benefits in the mid infrared, as well as investigating the calibration and stability of the
system. The system can also be used to test modeling of similar ACAI systems in the infrared. The demonstrator can use
either a set of fixed masks or a novel MOEMS adaptive transmissive spatial light modulator. This paper discusses the
design and testing of the system including the development of novel decoding algorithms and some initial imaging
results are presented.
Coded aperture imagery poses a challenge for traditional image tracking algorithms because of the highly distributed
nature of the coded imagery. Traditional algorithms would require this imagery to undergo a computationally expensive
decoding operation before subsequent processing and tracking. In this paper, a novel tracking algorithm is described that
can track point-source targets at sub-pixel accuracy without requiring the coded aperture imagery to be decoded.
Furthermore, it is shown that the algorithm is robust to changes in the coded aperture mask pattern, and so is suitable for
use in adaptive coded aperture imaging systems. Some results of the algorithm on synthetic and initial MWIR
experimental data will be given.
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
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.
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.
One of the main advantages of increasing the operating temperature of infrared focal plane arrays (FPAs) is to take
advantage of lower cost cooling options such as thermoelectric coolers. However the maximum reduction in temperature
available from the current generation of coolers (e.g. 4-stage) is around 110 K. For a maximum operating temperature of
70 oC, this means that the FPA needs to operate above 233 K. In this region, the performance becomes a strong function
of array temperature and designing a system becomes a trade-off between the performance of the fpa; the speed of the
optics; the maximum temperature of operation; and the cooler power and complexity.
In this paper, previous results will be extrapolated to estimate the FPA performance across this trade space by varying
cut-off wavelength. Possible techniques to enhance the performance of the FPAs by reducing low frequency noise or
adding optical concentrators will also be considered. These extrapolated results indicate that in an f/2 system at 210 K,
an NETD of around 30 mK could be achievable. Potential applications for the technology are in systems where long
lifetime; no moving parts; or reduced weight are an advantage. Ideally the maximum ambient temperature should be
limited to maintain the best thermal sensitivity. Suitable applications could include sensors which operate from UAVs or
in space.
Coded aperture imaging has been used for astronomical applications for several years. Typical implementations used a
fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. Recently applications have emerged in
the visible and infra red bands for low cost lens-less imaging systems and system studies have shown that considerable
advantages in image resolution may accrue from the use of multiple different images of the same scene - requiring a
reconfigurable mask.
Previously we reported on the realization of a 2x2cm single chip mask in the mid-IR based on polysilicon micro-opto-electro-mechanical systems (MOEMS) technology and its integration with ASIC drive electronics using conventional
wire bonding. The MOEMS architecture employs interference effects to modulate incident light - achieved by tuning a
large array of asymmetric Fabry-Perot optical cavities via an applied voltage and uses a hysteretic row/column scheme
for addressing.
In this paper we present the latest transmission results in the mid-IR band (3-5μm) and report on progress in developing
a scalable architecture based on a tiled approach using multiple 2 x 2cm MOEMS chips with associated control ASICs
integrated using flip chip technology. Initial work has focused on a 2 x 2 tiled array as a stepping stone towards an 8 x 8
array.
In a previous paper we presented initial results for sub-detector-pixel imaging in the mid-wave infra-red (MWIR) using
an imager equipped with a coded-aperture based on a re-configurable MOEMS micro-shutter. It was shown in laboratory
experiments that sub-pixel resolution is achievable via this route.
The purpose of the current paper is to provide detail on the reconstruction method and to discuss some challenges which
arise when imaging real-world scenes. The number of different mask patterns required to achieve a certain degree of
super-resolution is also discussed. New results are presented to support the theory.
Following the development of 1st Generation systems in the 1970s, thermal imaging has been in service with the UK
armed forces for over 25 years and has proven itself to be a battle winning technology. More recently the wider
accessibility to similar technologies within opposing forces has reduced the military advantage provided by these 1st
Generation systems and a clear requirement has been identified by the UK MOD for thermal imaging sensors providing
increased detection, recognition and identification (DRI) ranges together with a simplified logistical deployment burden
and reduced through-life costs.
In late 2005, the UK MOD initiated a programme known as "Albion" to develop high performance 3rd Generation single
waveband infrared detectors to meet this requirement. At the same time, under a separate programme supporting higher
risk technology, a dual waveband infrared detector was also developed. The development phase of the Albion
programme has now been completed and prototype detectors are now available and have been integrated into
demonstration thermal imaging cameras. The Albion programme has now progressed into the second phase,
incorporating both single and dual waveband devices, focussing on low rate initial production (LRIP) and qualification
of the devices for military applications.
All of the detectors have been fabricated using cadmium mercury telluride material (CMT), grown by metal organic
vapour phase epitaxy (MOVPE) on low cost, gallium arsenide (GaAs) substrates and bump bonded to the silicon read
out circuit (ROIC). This paper discusses the design features of the 3rd Generation detectors developed in the UK together
with the results obtained from the prototype devices both in the laboratory and when integrated into field deployable
thermal imaging cameras.
An earlier paper [1] discussed the merits of adaptive coded apertures for use as lensless imaging systems in the thermal
infrared and visible. It was shown how diffractive (rather than the more conventional geometric) coding could be used,
and that 2D intensity measurements from multiple mask patterns could be combined and decoded to yield enhanced
imagery. Initial experimental results in the visible band were presented. Unfortunately, radiosity calculations, also
presented in that paper, indicated that the signal to noise performance of systems using this approach was likely to be
compromised, especially in the infrared.
This paper will discuss how such limitations can be overcome, and some of the tradeoffs involved. Experimental results
showing tracking and imaging performance of these modified, diffractive, adaptive coded aperture systems in the visible
and infrared will be presented. The subpixel imaging and tracking performance is compared to that of conventional
imaging systems and shown to be superior. System size, weight and cost calculations indicate that the coded aperture
approach, employing novel photonic MOEMS micro-shutter architectures, has significant merits for a given level of
performance in the MWIR when compared to more conventional imaging approaches.
Coded aperture imaging has been used for astronomical applications for several years. Typical implementations used a
fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. Recently applications have emerged in
the visible and infra red bands for low cost lens-less imaging systems and system studies have shown that considerable
advantages in image resolution may accrue from the use of multiple different images of the same scene - requiring a
reconfigurable mask.
Previously we reported on the early feasibility of realising such a mask based on polysilicon micr-opto-electromechanical
systems (MOEMS) technology and early results in the visible and near IR bands. This employs interference
effects to modulate incident light - achieved by tuning a large array of asymmetric Fabry-Perot optical cavities via an
applied voltage whilst a hysteretic row/column addressing scheme is used to control the state of individual elements.
In this paper we present transmission results from the target mid-IR band (3-5μm), compare them with theory and
describe the scale up from a 3x3 proof-of-concept MOEMS microshutter array to a 560 x 560 element array (2cm x 2cm
chip) with the associated driver electronics and embedded control - including aspects of electronic design, addressing
control and integration. The resultant microsystem represents a core building block to realise much larger reconfigurable
masks using a tiled approach with further integration challenges in the future.
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 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.
Coded aperture imaging (CAI) has been used extensively at gamma- and X-ray wavelengths, where conventional
refractive and reflective techniques are impractical. CAI works by coding optical wavefronts from a scene using a
patterned aperture, detecting the resulting intensity distribution, then using inverse digital signal processing to
reconstruct an image.
This paper will consider application of CAI to the visible and IR bands. Doing so has a number of potential advantages
over existing imaging approaches at these longer wavelengths, including low mass, low volume, zero aberrations and
distortions and graceful failure modes. Adaptive coded aperture (ACAI), facilitated by the use of a reconfigurable mask
in a CAI configuration, adds further merits, an example being the ability to implement agile imaging modes with no
macroscopic moving parts. However, diffraction effects must be considered and photon flux reductions can have
adverse consequences on the image quality achievable.
An analysis of these benefits and limitations is described, along with a description of a novel micro optical electro
mechanical (MOEMS) microshutter technology for use in thermal band infrared ACAI systems. Preliminary
experimental results are also presented.
Previous applications of coded aperture imaging (CAI) have been mainly in the energetic parts of the electro-magnetic
spectrum, such as gamma ray astronomy, where few viable imaging alternatives exist. In addition, resolution
requirements have typically been low (~ mrad).
This paper investigates the prospects for and advantages of using CAI at longer wavelengths (visible, infrared) and at
higher resolutions, and also considers the benefits of adaptive CAI techniques. The latter enable CAI to achieve
reconfigurable modes of imaging, as well as improving system performance in other ways, such as enhanced image
quality. It is shown that adaptive CAI has several potential advantages over more traditional optical systems for some
applications in these wavebands. The merits include low mass, volume and moments of inertia, potentially lower costs,
graceful failure modes, steerable fields of regard with no macroscopic moving parts and inherently encrypted data
streams.
Among the challenges associated with this new imaging approach are the effects of diffraction, interference, photon
absorption at the mask and the low scene contrasts in the infrared wavebands. The paper analyzes some of these and
presents the results of some of the tradeoffs in optical performance, using radiometric calculations to illustrate the
consequences in a mid-infrared application. A CAI system requires a decoding algorithm in order to form an image and
the paper discusses novel approaches, tailored to longer wavelength operation. The paper concludes by presenting initial
experimental results.
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 (>1cm2) 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 Hg1-xCdxTe grown on silicon substrates using a segmented device architecture.
The drive towards improved target recognition has led to an increasing interest in detection in more than one infrared band. This paper describes the design, fabrication and performance of two-colour and three-colour infrared detectors made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out integrated circuits (ROICs). Each mesa diode has one connection to the ROIC and the colours are selected by varying the applied bias. Results will be presented for both two-colour and three-colour devices.
In a two-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the doping and composition of the p-type layer are chosen to prevent transistor action. The bias polarity is used to switch the output between colours. This design has been used to make MW/LW detectors with a MW band covering 3 to 5 μm and a LW band covering 5 to 10 μm.
In a three-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the p-type absorber, which has an intermediate cut-off wavelength. The absorbers are separated from each other by electronic barriers consisting of wide band-gap material. At low applied bias these barriers prevent photo-electrons generated in the p-type absorber from escaping and the device then gives an output from one of the n-type absorbers. At high applied bias the electronic barrier is pulled down and the device gives an output from both the p-type absorber and one of the n-type absorbers. Thus by varying the polarity and magnitude of the bias it is possible to obtain three-colours from a two-terminal device. This design has been used to make a SW/MW/MW detector with cut-off wavelengths of approximately 3, 4 and 6 μm.
The first generation of high performance thermal imaging sensors in the UK was based on two axis opto-mechanical
scanning systems and small (4-16 element) arrays of the SPRITE detector, developed during the 1970s. Almost two
decades later, a 2nd Generation system, STAIRS C was introduced, based on single axis scanning and a long linear
array of approximately 3000 elements. The UK has now begun the industrialisation of 3rd Generation High Performance
Thermal Imaging under a programme known as "Albion". Three new high performance cadmium mercury telluride
arrays are being manufactured. The CMT material is grown by MOVPE on low cost substrates and bump bonded to the
silicon read out circuit (ROIC). To maintain low production costs, all three detectors are designed to fit with existing
standard Integrated Detector Cooling Assemblies (IDCAs). The two largest focal planes are conventional devices
operating in the MWIR and LWIR spectral bands. A smaller format LWIR device is also described which has a smart
ROIC, enabling much longer stare times than are feasible with conventional pixel circuits, thus achieving very high
sensitivity. A new reference surface technology for thermal imaging sensors is described, based on Negative
Luminescence (NL), which offers several advantages over conventional peltier references, improving the quality of the
Non-Uniformity Correction (NUC) algorithms.
The first generation of high performance thermal imaging sensors in the UK was based on two axis opto-mechanical scanning systems and small (4-16 element) arrays of the SPRITE detector, developed during the 1970s. Almost two decades later, a 2nd Generation system, STAIRS C was introduced, based on single axis scanning and a long linear array of approximately 3000 elements. This paper addresses the development of the UK's 3rd Generation High Performance Thermal Imaging sensor systems, under a programme known as "Albion". Three new high performance detectors, manufactured in cadmium mercury telluride, operating in both MWIR and LWIR, providing high resolution and sensitivities without need for opto-mechanical scanning systems will be described. The CMT material is grown by MOVPE on low cost substrates and bump bonded to the silicon read out circuit (ROIC). All three detectors are designed to fit with existing standard Integrated Detector Cooling Assemblies (IDCAs). The two largest detectors will be integrated with field demonstrator cameras providing MWIR and LWIR solutions that can rapidly be tailored to specific military requirements. The remaining detector will be a LWIR device with a smart ROIC, facilitating integration times much longer than can typically be achieved with focal plane arrays and consequently yield very high thermal sensitivity. This device will be demonstrated in a lab based camera system.
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.
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 cm2 device with optical concentrators are presented.
Gavin Bowen, Ian Blenkinsop, Rose Catchpole, Neil Gordon, Mark Harper, Paul Haynes, Les Hipwood, Colin Hollier, Chris Jones, David Lees, Chris Maxey, Daniel Milner, Mike Ordish, Tim Philips, Richard Price, Chris Shaw, Paul Southern
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.
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.
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.
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%.
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.
We present results from indium antimonide and mercury cadmium telluride IR detector arrays operating at temperatures above 80K, whilst retaining high performance. Multi-layer epitaxial growth is employed to minimize thermally generated leakage currents, through the use of structures designed to control transport of charge generated outside of the active region to the diode junction and to minimize Auger generation within the active region. This enables an increase in operating temperature of a few tens of degrees in the case of background limited III-V devices, and thermoelectric operation of MCT detectors sensitive to the MWIR band. We also discuss the effects of reverse bias on diodes to actively suppress the Auger generation, and the consequent introduction of 1/f noise. Optical concentrators can be used to minimize the volume of detector material in order to gain further increases in temperature. The concentrators, based on Winston cone designs, are fabricated at each pixel by reactive ion etching directly into the detector material and its substrate, and allow a theoretical reduction in volume of a factor of up to 16. This translates into a potential additional increase in temperature of several tens of degrees.
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+-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.
Recent advances in MOVPE growth and heterostructure fabrication technology mean that infrared detector arrays based on Hg1-xCdxTe now have the potential to produce high performance imagery when operated in the temperature range 150-200 K. This has a number of system advantages including reduced cooler power consumption and increased cooler life. This paper reports the fabrication and assessment of a MW staring array with a cut-off of 4 μm at 150 K for intermediate temperature operation. Near background limited (BLIP) performance was achieved at temperatures up to 180 K with a median NETD better than 12 mK. Above this temperature, the array still operates normally however there is an exponential increase in the number of noisy pixels, and the median NETD degrades more rapidly than predicted from Shot noise. This behavior is consistent with increased low frequency or 1/f noise at the higher temperatures. This excess noise is not a fundamental limitation and if it could be eliminated, the array would remain close to BLIP up to 200 K.
The high gains in performance predicted for optical immersion are difficult to achieve in practice due to total internal reflection at the lens/detector interface. By reducing the air gap at this interface optical tunneling becomes possible and the predicted gains can be realized in practical devices. Using this technique we have demonstrated large performance gains by optically immersing mid-infrared heterostructure InA1Sb LEDs and photodiodes using hypershperical germanium lenses. The development of an effective method of optical immersion that gives excellent optical coupling has produced a photodiode with a peak room temperature detectivity (D*) of 5.3 x 109 cmHz½W-1 at λpeak=5.4μm and a 40° field of view. A hyperspherically immersed LED showed a f-fold improvement in the external efficiency, and a 3-fold improvement in the directionality compared with a conventional planar LED for f/2 optical systems. The incorporation of these uncooled devices in a White cell produced a NO2 gas sensing system with 2 part-per-million sensitivity, with an LED drive current of <5mA. These results represent a significant advance in the use of solid state devices for portable gas sensing systems.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as IR sources in gas sensing systems and as thermal radiation shields in IR cameras. Additionally these devices can be used as calibration sources for very large IR focal plane arrays and have many potential advantages over conventional calibration sources, including high speed operation and low power consumption. For many of these applications a large area device which displays as large area device which displays as large as possible apparent temperature range is required. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum dynamic range. We have therefore used a novel micromachining techniqe to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced by resist reflow. Inductively coupled plasma etchign is then used to alternatley etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Recent results from large area medium wavelength devices with integrated optical concentrators are presented, together with a description of the continuing optimization of the process and progress towards the fabrication of large area long wavelength devices.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as thermal radiation shields in IR cameras, and as IR sources in gas sensing systems. For many of these applications a large area (>1cm2) device is required, together with as large as possible apparent temperature range. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. These may lead to current heating of the device, which in turn reduces the apparent temperature range. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators parabolic resist masks are first produced, which are approximately 10 μm high and approximately 53 μm wide, by resist reflow at 120 degrees C. Inductively coupled plasma (ICP) etching is then used to alternately etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of approximately 15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.
One of the most important factors limiting the optical efficiency of LEDs is total internal reflection of generated light, where photons incident to the surface at angles greater than the critical angle are reflected back into the semiconductor and absorbed. Most semiconductors have a large refractive index and hence a small critical angle. Narrow gap semiconductors, such as InSb, have particularly large refractive indexes and corresponding smaller critical angles. Additionally, strong absorption of light in the 3-5(mu) m range means that epoxy immersion lenses, which are used for GaAs Ir LEDs, cannot be used in InSb based IR LEDs. We have therefore used a novel micromachining technique to fabricate optical concentrators inInSb and HgCdTe layers. Inductively coupled plasma (ICP) etching is used to alternatively eatch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with parabolic profiles. Continuing optimization of the process to reach the theoretical limits of optical gain is described together with some of the main issues associated with the fabrication process.
The incorporation of non-imaging optical concentrations in uncooled mid-IR LEDs is described. Novel micromachining methods are used to produce optical concentrators in the growth substrate of epitaxial InSb/InAlSb heterostructures. Resultant large area LED arrays, displaying both positive and negative luminescence, are shown to have optical gains of 3.5 over conventional mesas made form the same material. The LED technology shown also relies on the micromachined substrate being transparent to 3-5 micrometers radiation and to act as a low resistance common contact. The use of degenerate doping in InSb is described, resulting in a shift in the room-temperature transmission to the 3-5micrometers atmospheric window and providing high electrical conductivities.
IR detectors are normally cooled to 80K or below to obtain the highest, background limited performance. We present results for indium antimonide/indium aluminium antimonide and mercury cadmium telluride detectors grown by epitaxial processes in order to facilitate high performance with reduced cooling requirements. The epitaxial growth enables structures to be grown which offer precise control of carrier generation and current leakage mechanisms so that the maximum temperature can be achieve din a photodiode operated in a conventional manner, near zero bias. These types of structure offer even greater operating temperature when reverse biased to suppress non-radiative generation mechanisms. The epitaxial growth also has advantages for conventional, 80K operation, which are described.
We describe uncooled mid-IR light emitting and negative luminescent diodes made form indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The application of these devices to gas sensing, improved thermal imagers and imager testing is discussed.
Cadmium mercury telluride (Hg1-xCdxTe or MCT) non- equilibrium detector structures which allow room temperature operation have been grown by metal-organic vapor phase epitaxy (MOVPE). These devices suppress the auger generation by reducing the intrinsic electron and hole concentrations in the active region of the device. The MCT characteristics in this region should then be determined by the extrinsic doping concentration. In order to minimize the remaining generation processes within this so called (pi) -region, it is best formed from low acceptor doped (low X1015 cm-3) MCT, with as low a trap density as possible. The p+(pi) n+ device structure which is required to achieve the non-equilibrium phenomena requires stringent control on acceptor and donor doping, as well as composition. Acceptor doping studies with trisdimethylamino arsine (DMAAs) have been performed using GaAs and CdZnTe substrates. Minority carrier lifetime results have been obtained which are near rotatively limited and comparable to As-doped, Hg-rich liquid phase epitaxy (LPE) grown layers on CdZnTe substrates. Ambient temperature, auger-suppressed devices have levels of 1/f noise which currently limit their use in imaging applications. However, they are of great interest in other applications such as approximately equals 10 micrometer negative luminescence emitter devices and heterodyne detection of 10.6 micrometer infrared (IR) radiation from carbon-dioxide lasers. Reduction in the series resistances has been achieved by utilizing a device design with a n+ MCT common which should improve the frequency response of these devices. Another design modification, predicted to reduce the leakage current, has been the introduction of low doped, wide band gap regions either side of the (pi) -region. In practice these structures have produced over an order of magnitude improvement in the leakage current characteristics.
Recent advances in the growth of cadmium mercury telluride (Hg1-xCdxTe or MCT) by metal organic vapor phase epitaxy (MOVPE) allow the fabrication of advanced device structures where both the alloy composition x and the doping concentration can be accurately controlled throughout the epitaxial layer. For p-type doping using arsenic, the acceptor concentration can be varied from 5 X 1015 cm-3 to 4 X 1017 cm-3 and for n-type doping using iodine, the donor concentration can be varied from 1 X 1015 cm-3 to 2 X 1017 cm-3. A number of diode arrays have been fabricated in this material and their properties assessed at 77 K, 195 K and 295 K. It has been found that the diffusion currents are at least ten times lower than in homojunctions. In addition, the devices exhibit negative resistance at temperatures above 190 K due to auger suppression. The successful demonstration of auger suppression in these structures has greatly improved the diode leakage currents at room temperature and will enable the development of new devices such as a room temperature laser detector.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.