Linked by ESA’s Astronomy Large Format Array for the near-infrared ("ALFA-N") technology development program, CEA and Lynred aim at setting up the fabrication of very large IR focal plane arrays (FPA) for astronomy needs. Prior to this project, dark current and image persistence are under investigation for achieving the high level of performance needed by astronomers. During previous characterization of this kind of detector, the FPA appeared particularly sensitive to ROIC electro-luminescence, preventing to observe fainter effects such as persistence. With the mitigation of the glow, the first measurements showed that dark current was dominated by persistence instead of classical diffusion, Auger or ShockleyRead-Hall mechanisms. We propose a dedicated test protocol in order to electrically characterize persistence and an empirical modelling tool to describe it in terms of amplitude and characteristic time constant. The first step consists in removing the residual persistence, allowing to characterize the intrinsic photodiode’s dark current, down to 0.03e-/s at 90K on four tested devices. From this reference, the persistence contribution is dramatically minimized and experimental conditions are reproducible, enabling further investigation on persistence to be carried out. Applied on detectors manufactured in the CEA-LETI clean rooms, this protocol aims at a better understanding of the phenomenon. Using an array containing different diode flavours (ie variations in the technological parameters such as diode geometry, passivation…), the characterization scheme described above should bring information about technological contributions on persistence.
CEA and Lynred develop very large focal plane arrays (FPA) in the short wave infrared range (SWIR) with ultra-low dark current for space and astronomy applications. The structure of such arrays is based on a HgCdTe sensitive layer flipchipped onto a Si ROIC. This ROIC is based on a source follower per detector input and output stage giving access to very high gains for very low flux (below 1ph/s) and very low noise (11.5e-) measurements. However, during previous characterisations, these FPAs appeared particularly sensitive to electro-luminescence emitted by ROIC source follower output stage transistors in the saturation regime. Indeed, the emitted photons are in the sensitive wavelength range of the HgCdTe layer (2.1 μm cut-off). They are then collected by the photodiodes thus degrading the measured dark current. This phenomenon, called ROIC glow, is the limiting mechanism of dark current at low temperature for such arrays. We describe here a solution to reduce to ground level this ROIC glow. MOSFET drain to source voltage is a major parameter for limiting glow and our results show good agreement with hot electrons light emission models. Transfer function characterisation of the ROIC was also performed to highlight the limits of the proposed procedure, which is that the source follower MOSFETs must stay in the saturated regime. Measurements carried out on different characterisation benches and several detectors at CEA-LETI and CEA-DAP show dark currents below 0.03e/s/pixel after glow mitigation.
The Laboratoire Electronique et Traitement de l’Information (LETI) of the Commissariat à l’Energie Atomique (CEA, Grenoble, France) has been involved in the development of infrared detectors based on HgCdTe (MCT) material for over 30 years, mainly for defence and security programs . Once the building blocks are developed at LETI (MCT material process, diode technology, hybridization, …), the industrialization is performed at SOFRADIR (also in Grenoble, France) which also has its own R&D program .
In past years, LETI also developed infrared detectors for space astrophysics in the mid infrared range – the long wave detector of the ISOCAM camera onboard ISO – as well as in the far infrared range – the bolometer arrays of the Herschel/PACS photometer unit –, both instruments which were under the responsibility of the Astrophysics department of CEA (IRFU/SAp, Saclay, France).
Nowadays, the infrared detectors used in space and ground based astronomical instruments all come from vendors in the US. For programmatic reasons – increase the number of available vendors, decrease the cost, mitigate possible export regulations, …– as well as political ones – spend european money in Europe –, the European Space Agency (ESA) defined two roadmaps (one in the NIR-SWIR range, one in the MWIR-LWIR range) that will eventually allow for the procurement of infrared detectors for space astrophysics within Europe.
The French Space Agency (CNES) also started the same sort of roadmaps, as part of its contribution to the different space missions which involve delivery of instruments by French laboratories. It is important to note that some of the developments foreseen in these roadmaps also apply to Earth Observations.
One of the main goal of the ESA and CNES roadmaps is to reduce the level of dark current in MCT devices at all wavelengths. The objective is to use the detectors at the highest temperature where the noise induced by the dark current stays compatible with the photon noise, as the detector operating temperature has a very strong impact at system level. A consequence of reaching low levels of dark current is the need for very low noise readout circuits.
CEA and SOFRADIR are involved in a number of activities that have already started in this framework. CEA/LETI does the development of the photo-voltaic (PV) layers – MCT material growth, diode technologies–, as well as some electro-optical characterisation at wafer, diode and hybrid component levels, and CEA/IRFU/SAp does all the electro-optical characterisation involving very low flux measurements (mostly dark current measurements). Depending of the program, SOFRADIR can also participate in the development of the hybrid components, for instance the very low noise readout circuits (ROIC) can be developed either at SOFRADIR or at CEA/LETI.
Depending of the component specifications, the MCT epitaxy can be either liquid phase (LPE, which is the standard at SOFRADIR for production purposes) or molecular beam (MBE), the diode technology can be n/p (standard at LETI and SOFRADIR) or p/n (under development for several years now) , and the input stage of the ROIC can be Source Follower per Detector (SFD for very low flux low noise programs) or Capacitive Trans Impedance Amplifier (CTIA for intermediate flux programs) .
This paper will present the different developments and results obtained so far in the two NIR-SWIR and MWIR-LWIR spectral ranges, as well as the perspectives for the near future. CEA/LETI is also involved in the development of MCT Avalanche Photo Diodes (APD) that will be discussed in other papers [5,6].
Detection for space application is very demanding on the IR detector: all wavelengths, from visible-NIR (2- 3um cutoff) to LWIR (10-12.5um cutoff), even sometimes VLWIR (15um cutoff) may be of interest. Moreover, various scenarii are usually considered. Some are imaging applications where the focal plane array (FPA) is used as an optical element to sense an image. However, the FPA may also be used in spectrometric applications where light is triggered on the different pixels depending on its wavelength. In some cases, star pointing is another use of FPAs where the retina is used to sense the position of the satellite.
In all those configurations, we might distinguish several categories of applications:
• low flux applications where the FPA is staring at space and the detection occurs with only a few number of photons.
• high flux applications where the FPA is usually staring at the earth. In this case, the black body emission of the earth and its atmosphere ensures usually a large number of photons to perform the detection.
Those two different categories are highly dimensioning for the detector as it usually determines the level of dark current and quantum efficiency (QE) requirements. Indeed, high detection performance usually requires a large number of integrated photons such that high QE is needed for low flux applications, in order to limit the integration time as much as possible. Moreover, dark current requirement is also directly linked to the expected incoming flux, in order to limit as much as possible the SNR degradation due to dark charges vs photocharges. Note that in most cases, this dark current is highly depending on operating temperature which dominates detector consumption. A classical way to mitigate dark current is to cool down the detector to very low temperatures.
This paper won't discuss the need for wavefront sensing where the number of detected photons is low because of a very narrow integration window. Rigorously, this kind of configuration is a low flux application but the need for speed distinguishes it from other low flux applications as it usually requires a different ROIC architecture and a photodiode optimized for high response speed.
HgCdTe is very unique material system for infrared (IR) detection. In combination with its lattice matched native substrate CdZnTe, this semiconductor alloy allows to address the whole infrared (IR) band, from the near IR (NIR, 2?m cutoff) to the middle wave IR (MWIR, 5μm cutoff), the long wave IR (LWIR, 10μm cutoff), up to the very long wave IR (VLWIR, cutoffs larger than 14μm).
Space applications are requiring low dark current in the long wave infrared at low operating temperature for low flux observation. The applications envisioned with this type of specification are namely scientific and planetary missions. Within the framework of the joint laboratory between Sofradir and the CEA-LETI, a specific development of a TV format focal plane array with a cut-off wavelength of 12.5μm at 40K has been carried out. For this application, the p on n technology has been used. It is based on an In doped HgCdTe absorbing material grown by Liquid Phase Epitaxy (LPE) and an As implanted junction area. This architecture allows decreasing both dark current and series resistance compared to the legacy n on p technology based on Hg vacancies. In this paper, the technological improvements are briefly described. These technological tunings led to a 35% decrease of dark current in the diffusion regime. CEA-LETI and Sofradir demonstrated the ability to use the p on n technology with a long cutoff wavelength in the infrared range.
HgCdTe (MCT) is a very versatile material for IR detection. Indeed, the ability to tailor the cutoff frequency as close as
possible to the detection needs makes it a perfect candidate for high performance detection in a wide range of
applications and spectral ranges. Moreover, the high quality material available today, either by liquid phase epitaxy
(LPE) or molecular beam epitaxy (MBE) allows for very low dark currents at low temperatures and make it suitable for
very low flux detection application such as science imaging. MCT has also demonstrated its robustness to aggressive
space environment and faces therefore a large demand for space application such as staring at the outer space for science
purposes in which case, the detected photon number is very low This induces very strong constrains onto the detector:
low dark current, low noise, low persistence, (very) large focal plane arrays. The MCT diode structure adapted to fulfill
those requirements is naturally the p/n photodiode. Following the developments of this technology made at DEFIR and
transferred to Sofradir in MWIR and LWIR ranges for tactical applications, our laboratory has consequently investigated
its adaptation for ultra-low flux in different spectral bands, in collaboration with the CEA Astrophysics lab. Another
alternative for ultra low flux applications in SWIR range, has also been investigated with low excess noise MCT n/p
avalanche photodiodes (APD). Those APDs may in some cases open the gate to sub electron noise IR detection.. This
paper will review the latest achievements obtained on this matter at DEFIR (CEA-LETI and Sofradir common
laboratory) from the short wave (SWIR) band detection for classical astronomical needs, to the long wave (LWIR) band
for exoplanet transit spectroscopy, up to the very long waves (VLWIR) band.
This paper presents recent developments done at CEA-LETI Infrared Laboratory on processing and characterization of p-on-n HgCdTe (MCT) planar infrared focal plane arrays (FPAs) in LWIR and VLWIR spectral bands. These FPAs have been grown using liquid phase epitaxy (LPE) on a lattice matched CdZnTe substrate. This technology presents lower dark current and lower serial resistance in comparison with n-on-p vacancy doped architecture and is well adapted for low flux detection or high operating temperature. This architecture has been evaluated for space applications in LWIR and VLWIR spectral bands with cutoff wavelengths from 10μμm up to 17μm at 78K. Innovations have been introduced to the technological process to form a heterojunction with a LPE growth technique. The aim was to lower dark current at low temperature, by decreasing currents from the depletion region. Electro-optical characterizations on p-on-n photodiodes have been performed on QVGA format FPAs with 30μm pixel pitches. Results show excellent operabilities in current and responsivity, with low dispersion and noise limited by current shot-noise. Studies performed on dark current show that dark current densities are consistent with the heuristic prediction law "Rule07" at 78K. Below this temperature, dark current varies as a pure diffusion current.
Since 2005, in the scope of “DEFIR”, the joint laboratory between CEA-LETI and SOFRADIR, p-on-n photodiodes and FPAs (Focal Plane Arrays) have been developed and optimised. This p-on-n architecture, obtained by As implantation into an In doped base layer, offered a significant decrease of the dark current compared to our n-on-p standard architecture. Following these developments, this p-on-n technology has been successfully transferred to SOFRADIR for industrial production . Results obtained on TV format, 15μm pitch, showed that this first architecture has reached its maturity with excellent results in LWIR and MWIR. In parallel, further developments and studies are still in progress at CEA-LETI in order to improve the photodiode performance and understanding of the physical mechanisms. In this way, new p-on-n architectures have been studied on LPE (Liquid Phase Epitaxy) in the VLWIR spectral band. Using this new architecture, the transition temperature, where the dark current shifts from diffusion limited regime to another one, has been lowered by more than 10K. Extremely low dark current has been obtained, down to 50 e-/s/pixel. The p-on-n technology also been studied at DEFIR in SWIR range specifically for space applications were 2Kx2K MCT arrays are required with dark current below 0.01e-/s at 18μm pitch in the 80-140 K. Finally in the MWIR and LWIR spectral bands, the reduction of production cost and the increase of resolution call for smaller pixel pitches with larger format. In this way, first results have been obtained on test diodes with pixel pitch as low as 5 μm. The I(V) and R(V) plots illustrate the very good characteristic of our p-on-n diodes. These photodiodes present large reverse breakdown voltage, witnessing the quality of our device fabrication procedure.
The actual trend in quantum IR detector development is the design of very small pixel pitch large arrays. From previously 30μm pitch, the standard pixel pitch is today 15μm and is expected to decrease to 12μm in the next few years. Furthermore, focal plane arrays (FPA) with pixel pitch as small as small as 10μm has been demonstrated. Such ultra-small pixel pitches are very small compared to the typical length ruling the electrical characteristics of the absorbing materials, namely the minority carrier diffusion length. As an example for low doped N type HgCdTe or InSb material, this diffusion length is of the order of 30 to 50μm, i.e. 3 to 5 times the targeted pixel pitches. This has strong consequences on the modulation transfer function (MTF) for planar structures, where the lateral extension of the photodiode is limited by diffusion. For such aspect ratios, the self-confinement of neighboring diodes may not be efficient enough to maintain optimal MTF. Therefore, this issue has to be addressed in order to take full benefits of the pixel pitch reduction in terms of image resolution. This paper aims at investigating the MTF evolution of HgCdTe and InSb FPAs decreasing the pixel pitch below 15μm. Both experimental measurements and finite element simulations are used to discuss this issue. Different scenarii will be compared, namely deep mesa etch between pixels, internal drift, surface recombination, thin absorbing layers.
In this paper, we report on results obtained both at CEA/LETI and SOFRADIR on p-on-n HgCdTe (MCT) grown by liquid phase epitaxy (LPE) Infra-Red Focal Plane Arrays (IR FPAs) for the Long-wave (LW) and the Very-long-wave (VLW) spectral ranges. For many years, p-on-n arsenic-ion implanted planar technology has been developed and improved within the framework of the joint laboratory DEFIR. Compared to n-on-p,p-on-n technology presents lower dark current and series resistance. Consequently, p-on-n photodiodes are well-adapted for very large FPAs operating either at high temperature or very low flux. The long wave (LW) spectral ranges have been firstly addressed with TV/4, 30 µm pitch FPAs. Our results showed state-of-the-art detector performances, consistent with "Rule 07" law , a relevant indicator of the maturity of photodiode technology. The low dark current allows increasing the operating temperature without any degradation of the performances. The subsequent development of p-on-n imagers has produced more compact, less energy consuming systems, with a substantial resolution enhancement. Space applications are another exciting but challenging domains and are good candidates for the p-on-n technology. For this purpose, TV/4 arrays, 30 µm pixel pitch, have been manufactured for the very long wave spectral range. For this detection range, the quality of material and reliability of technology are the most critical. Detectors with different cutoff wavelength have been manufactured to aim 12.5 µm at 78K, 12.5 µm at 40K and 15 µm at 78K. Electro-optical characterizations reveal homogeneous imagers with excellent current operabilities (over 99.9% at best). The results highlight the very good quality of p-on-n technology with carrier diffusion limited dark current, fitting the "Rule 07" law, and high quantum efficiency. Further process developments have been made to improve photodiodes performances. Especially the transition temperature where the dark current shifts from diffusion limited regime to another one, has been lowered by more than 10K. Extremely low dark current has been obtained, down to 50 e-/s/pixel.
Cooled IR technologies that offer high performances are at the top of DEFIR’s priority list. We have been
pursuing further infrared developments on future MWIR detectors, such as the VGA format HOT detector that
operates at 150K and the 10μm pitch IR detector which gives us a leading position in innovation In the same
time Scorpio LW expands Sofradir's line of small pixel pitch TV format IR detectors from the mid-wavelength
to the long-wavelength, broadening the performance attributes of its long wave IR product line. Finally, our dual
band MW-LW QWIP detectors (25μm, 384×288 pixels) benefit to tactical platforms giving an all-weather
performance and increasing flexibility in the presence of battlefield obscurants.
These detectors are designed for long-range surveillance equipment, commander or gunner sights, ground-toground
missile launchers and other applications that require higher resolution and sensitivity to improve
reconnaissance and target identification. This paper discusses the system level performance in each detector
In this paper, we report on results obtained both at CEA/LETI and SOFRADIR on p-on-n Infra-Red Focal Plane Arrays
(IR FPAs) from the Short-Wave (SW) to the Very-Long-Wave (VLW) spectral range. For many years, p-on-n arsenicion
implanted planar technology has been developed and improved within the framework of the joint laboratory DEFIR,
a collaborative effort bringing together the expertise of both teams. Compared to n-on-p architecture, p-on-n technology allows to lower dark current density and series resistance by means of respectively long-lifetime minority carriers (hole) and high-mobility majority carriers (electrons). As a consequence, p-on-n photodiodes are well-adapted for very large FPAs operating either at high temperature or very low flux. The Mid-Wave (MW) and Long-Wave (LW) spectral ranges have been firstly addressed with TV/4 FPAs, 30 μm pitch, principally for defence and security applications. Our results showed state-of-the-art detector performances, consistent with “Rule 07” model, a relevant indicator of the p-on-n technology maturity. The subsequent developments of p-on-n imagers have produced more compact, less energy consuming systems, with a substantial resolution enhancement. In this way, MW and LW FPAs, TV format, with 15 μm pixel pitch have been designed. First results obtained in MW (λc=5.3 μm @80 K) for High Operating Temperature (HOT) applications have showed highly promising Electro-Optical (EO) performances.
Space applications are another exciting but challenging domains where p-on-n is a good candidate. In this way, imagers dedicated to low-flux detection have first been realized as TV/4 FPAs, with 15μm pitch in the SW spectral range (2 μm). The dark currents obtained are coherent with those published in the literature. Finally, TV/4 arrays, 30 μm pixel pitch, have been manufactured for the very long wave spectral range. For this detection range, the quality of material and reliability of technology are the most critical. The measured dark current fits “Rule 07” well, with homogeneous imagers. In conclusion, DEFIR team have developed, improved and characterized p-on-n IR FPAs from SW to VLW spectral range. In all spectral ranges, we have demonstrated state-of-the-art results, which highlight the quality of material and viability of our p-on-n technology. This technology, currently industrialized by SOFRADIR, opens new ways for next generation of imagers.
Developments made last years at CEA-LETI on p-on-n planar HgCdTe (MCT) photodiodes technology on long-, midand
short-wavelength led to the manufacture of focal plane arrays (FPA) demonstrators with high performances.
Improvements have been done on both technology and process to index very long-wavelength spectral band. Such
detectors is currently being evaluated for space applications such as IASI-NG  or EChO  as they give the
opportunity to address low flux detection conditions or higher operating temperature (lowering the electrical power
consumption). Various process settings were tested to find optimized conditions in order to obtain the best detector
performances. Cutoff wavelength of the manufactured detectors ranges from 9.5 to 15.5 μm at 78K. MCT base layer has
been grown by liquid phase epitaxy (LPE) on lattice matched CdZnTe. The n-type doping is achieved during epitaxy by
Indium incorporation. Planar p-on-n photodiodes were manufactured by Arsenic incorporation using ion-implantation
and activation is done by post-implantation annealing under Hg overpressure. Electro-optical characterizations were
performed both on test arrays and FPAs. Results show excellent operabilities (over 99.9% with ±0.5×mean value
criterion) in responsivity and NETD. Measured RMS noise of the photodiodes is comparable to calculated current shot
noise. The dark current is following the well-known Rule07 for every component and on a large temperature range.
This paper describes the recent developments of Mercury Cadmium Telluride (MCT) infrared technologies in France at Sofradir and CEA-LETI made in the frame of the common laboratory named DEFIR. Among these developments, one can find the crystal growth of high quality and large Cadmium Zinc Telluride (CZT) substrates which is one of the fundamental keys for high quality and affordable detectors. These last years, a great effort was done on this topic and also on MCT epilayer process from Short Waves (SW) to Very Long Waves (VLW). These developments about the quality of the material are needed for the challenge of the High Operating Temperature (HOT). Over these lasts years, the operating temperature of n/p MCT detectors was increase of several tens of Kelvin. In addition the development of the p/n MCT technology that reduces dark current by a factor ~100 saves about twenty Kelvin more. The next step for the increase in operating temperature will be the complex photodiodes architectures using molecular beam epilayer. The reduction of the pixel pitches is another challenge for infrared technologies for Small Weight and Power (SWAP) detectors. Moreover, this reduction allows the increase in the resolution and consequently in the detection range of the systems. In addition, last results on 3rd generation detectors such as multicolor focal plan arrays, 2D, 3D, low noise and high images rate focal plane array using Avalanche Photodiose (APD) are described.
In this paper, we present the design of the MWIR channels of EChO. Two channels cover the 5-11 micron spectral
range. The choice of the boundaries of each channel is a trade-off driven by the science goals (spectral features of key
molecules) and several parameters such as the common optics design, the dichroic plates design, the optical materials
characteristics, the detector cut-off wavelength. We also will emphasize the role of the detectors choice that drives the
thermal and mechanical designs and the cooling strategy.
Developments made last years at CEA-LETI on p-on-n planar HgCdTe (MCT) photodiodes technology on long-, midand
short-wavelength led to the manufacture of focal plane arrays (FPA) demonstrators with high performances. This
technology has been successfully transferred to SOFRADIR for industrial production. Improvements have been done on
both technology and process to index very long-wavelength spectral band. MCT base layer has been grown by liquid
phase epitaxy (LPE) on lattice matched CdZnTe. The n-type doping is achieved during epitaxy by Indium incorporation,
as In is naturally active as a donor in MCT. Planar p-on-n photodiodes were manufactured by Arsenic doping. As
incorporation is achieved by ion-implantation and activation is done by post-implantation annealing under Hg
overpressure. Multiples process settings were tested to find optimized conditions in order to obtain the best detector
performances. Cutoff wavelength increase from LWIR at 9.2 μm at 77K was done in two steps, by adjusting technology
process to get firstly 12.3 μm cutoff and then 15 μm at 77K. The second step was funded by french National Space
Studies Center (CNES) to evaluate p-on-n IRFPAs performances for very long-wavelength detection for space
applications such as IASI-NG. Electro-optical characterizations were performed both on test arrays and FPAs. Results
show excellent operabilities (over 99.9% with ±0.5×mean value criterion) in responsivity and NETD, and current shot
noise limited photodetectors. R0A figure of merit is very high and at the state of the art.
This paper describes the recent developments of Mercury Cadmium Telluride (MCT) infrared technologies in France at
Sofradir and CEA-LETI made in the frame of the common laboratory named DEFIR.
Among these developments, one can find the crystal growth of high quality and large Cadmium Zinc Telluride (CZT)
substrates which is one of the fundamental keys for high quality and affordable detectors. These last years, a great effort
was done on this topic and also on MCT epitaxy layer process from Short Waves (SW) to Very Long Waves (VLW).
These developments about the quality of the material are needed for the challenge of the High Operating Temperature
(HOT). Over these lasts years, the operating temperature of n-on-p MCT detectors was increase of several tens of
Kelvin. In addition the development of the p-on-n MCT technology that reduces dark current by a factor ~100 saves
about twenty Kelvin more. The next step for the increase in operating temperature will be the complex photodiodes
architectures using molecular beam epitaxy layer.
The reduction of the pixel pitches is another challenge for infrared technologies for Small Weight and Power (SWAP)
detectors. Moreover, this reduction allows the increase in the resolution and consequently in the detection range of the
In addition, last results on 3rd generation detectors such as multicolor focal plan arrays, 2D, 3D, low noise and high
images rate focal plane array using Avalanche Photodiode (APD) are described.
Cooled IR technologies are challenged for answering new system needs like the reduction of power consumption. This
reduction is requested in new IR system design in particular for cooled IR detection. The goal is to reduce system sizes,
to increase system autonomies and reliabilities and globally to reduce system costs. One of the key drivers for cooled
systems is the cooler and the operating temperature. As far as operating temperature is concerned, Sofradir and CEALETI
LIR put a lot of efforts to increase the operating temperature of IR MCT detectors. The n/p and p/n MCT
technologies are improved to operate at high temperature with good performances and particularly with low rate of
defective pixels. These detectors operate in the MW blue band, MW and LW. In addition complex structures like nBn
structures are developed to go further in the high operating temperature. Results are presented and discussed.
CEA-Leti has developed a 320x256 FPA for 3D flash LADAR active imaging. The readout IC (ROIC) performs time-of-flight
(TOF) measurement in addition to 2D intensity imaging with a single emitted laser pulse. The FPA consist of a
ROIC hybridized to a 30 μm pitch HgCdTe avalanche photodiode (APD) array. The illuminator used for testing this FPA
is a 1.57 μm laser producing 8 ns pulses with a maximum energy of 8 mJ per pulse. This paper describes the readout IC
pixel architecture and presents ranging performances obtained in laboratory conditions. The first 2D and 3D active
images obtained during the first field trial of our prototype LADAR system are presented.
This paper presents an overview of the very recent developments of the MCT infrared detector technology developed by
CEA-LETI and Sofradir in France. New applications require high sensitivity, higher operating temperature and dual
The standard n on p technology in production at Sofradir for 25 years is well mastered with an extremely robust and
reliable process. Sofradir's interest in p on n technology opens the perspective of reducing dark current of diodes so
detectors could operate in lower flux or higher operating temperature.
In parallel, MCT Avalanche Photo Diodes (APD) have demonstrated ideal performances for low flux and high speed
application like laser gated imaging during the last few years. This technology also opens new prospects on next
generation of imaging detectors for compact, low flux and low power applications.
Regarding 3rd Gen IR detectors, the development of dual-band infrared detectors has been the core of intense research
and technological improvements for the last ten years. New TV (640 x 512 pixels) format MWIR/LWIR detectors on
20μm pixel pitch, made from Molecular Beam Epitaxy, has been developed with dedicated Read-Out Integrated Circuit
(ROIC) for real simultaneous detection and maximum SNR.
Technological and products achievements, as well as latest results and performances are presented outlining the
availability of p/n, avalanche photodiodes and dual band technologies for new applications at system level.
This paper presents recent development made at CEA-LETI on manufacturing and characterization of planar p-on-n
HgCdTe photodiodes on long-, mid- and short-wavelength. HgCdTe (MCT) layer was grown both by liquid-phase
epitaxy (LPE) and by molecular beam epitaxy (MBE) on lattice matched CdZnTe (CZT). The n-type MCT base layer
was obtained by indium doping. Planar p-on-n photodiodes were manufactured by arsenic doping, which has been
activated by post-implanted annealing in Hg overpressure. As incorporation is achieved either by implantation or by
incorporation (during MBE growth). Electro-optical characterizations on these p-on-n photodiodes were made on FPAs.
Results show excellent operabilities (99.95% with ±0.5×mean value criterion) in responsivity and NETD and
background limited photodetectors. For long-wavelength FPAs, dark current is very low, leading to a R0A product
comparable to the state of the art at cut-off wavelength of λc = 9.2 μm. MBE mid-wavelength FPAs present very low
responsivity dispersion, reaching 1.1%. Comparisons are made between implantation and growth incorporation As
In this paper, we report the fabrication and electro-optical characterization of both long-wavelength (LWIR) and middle-wavelength
(MWIR) p-on-n infrared photodiodes in HgCdTe. LWIR and MWIR HgCdTe epitaxial layers were grown
by liquid phase and molecular beam epitaxy respectively. p-type doping was obtained by arsenic implantation and n-type
doping by indium incorporation during growth.
The arsenic concentration profile determined by Secondary Ion Mass Spectroscopy showed multi-component diffusion
after Hg post-implant annealing. The process yields an arsenic activation efficiency of around 50%, estimated from
MEMSA (Maximum Entropy Mobility Spectrum Analysis) measurements. The damage induced by arsenic implantation
into HgCdTe have been examined by transmission electron microscopy (TEM) and suggest the formation of an array of
dislocations loops after arsenic implantation. However, after annealing under Hg overpressure, the impact of
implantation falls below the sensitivity of the TEM, suggesting that annealing effectively suppresses most of the defects.
The p-on-n photodiodes showed low leakage currents (shunt resistance>100 MOhms) and typical RoA values
comparable to the state of the art (RoA>4000 Ω.cm2 for λc=9.2 μm at 77K). Finally, first results on p-on-n focal plane
arrays realized at CEA-LETI will be presented.
We report the latest developments of MW HgCdTe electron initiated avalanche photo-diodes (e-APDs) focal plane
arrays (FPAs) at CEA-LETI. The MW e-APD FPAs are developed in view of ultra-sensitive high dynamic range
passive starring arrays, active 2D/3D and dual-mode passive-active imaging, which is why both the passive imaging
performance and the gain characteristics of the APDs are of interest. A passive mode responsivity operability of 99.9%
was measured in LPE and MBE e-APDs FPAs associated with an average NETD=12mK, demonstrating that dual mode
passive-active imaging can be achieved with LETI e-APDs without degradation in the passive imaging performance. The
gain and sensitivity performances were measured in test arrays and using a low voltage technology (3.3V) CTIA test
pixel designed for 3D active imaging. The CTIA and test arrays measurements yielded comparable results in terms of
bias gain dependence (M=100 at Vb=-7V), low excess noise factor (=1.2) and low equivalent input current
(Ieq_in<1pA). These results validated the low voltage CTIA approach for integrating the current from a HgCdTe e-APD
under high bias. The test array measurements demonstrated a relative dispersion below 2% in both MBE and LPE e-
APDs for gains higher than M>100, associated with an operability of 99%. The operability at Ieq_in<1pA at M=30 was 95%. A record low value of Ieq_in=1fA was estimated in the MBE e-APDs at M=100, indicating the potential for using the MW e-APDs for very low flux applications. The high potential of the MW e-APDS for active imaging was
demonstrated by impulse response measurements which yielded a typical rise time lower than 100ps and diffusion
limited fall time of 900ps to 5ns, depending on the pixel pitch. This potential was confirmed by the demonstration of a
2ns time of flight (TOF) resolution in the CTIA e-APD 3D pixel. The combined photon and dark current induced
equivalent back ground noise at f/8 with a cold band pass filter at λ=1.55μm was 2 electrons rms for an integration time