For several years, CEA-LETI has worked on the development of IR MCT detectors for low flux applications in scientific imaging. MCT has remarkable material properties that allow for highly performing detection, with high QEs and minimal dark currents. Currently, both Teledyne US and LETI-Lynred in France achieve dark currents in the range of a fraction of e/s/pixel for NIR imaging, utilizing the p-on-n extrinsic diode structure, for astronomy applications. As part of the NEOCAM project, Teledyne has demonstrated the ability to extend the cut-off wavelength to significantly longer wavelengths (10.3μm and recently 13μm) while keeping the dark current in the e/s range. Furthermore, LETI is presently enhancing its p-on-n technology to satisfy the demanding specifications of low-flux applications. Achieving very low dark current leakage and large diode polarisation plateaus is required to achieve SFD input stage ROIC operation and to reach such low dark current values. This entails the mitigation of tunnelling currents that appear in low-gap materials. In this report, we present the production of a 15μm pitch TV-size prototype array that operates in the LW range (8μm @ 35K). Dark current values as low as 0.5 e/s/pixel have been measured below 40K, and will be discussed.
A 4-quadrant large area HgCdTe APD detector module have been developed and characterized in view of application in deep space optical communications. Single photon detection capacity has been demonstrated on each of the four channels of the detector module, associated with a bandwidth close to 400 MHz. The performance for pulse position modulation (ppm) has been estimated from the detection of strongly attenuated laser pulses and were found to be close to the system performance specifications given by ESA: the pulse detection probability in a time slot of 800 ps was measured to be higher about 90 % for a signal of 7 photons focused on the center on one channels, associated with a false alarm rate below 1 %, although the sensitivity of the full detector module was limited by a low quantum efficiency and a high dark count rate. With a 16-ary ppm modulation, this corresponds to a data rate of 320 Mbps at less than 2 photons per bit.
LiDAR remote sensing of the atmosphere requires a photons detection chain with a high sensitivity and a high dynamic range associated with a good temporal resolution. To meet these requirements, a specific CMOS read out circuit (ROIC) has been studied, design and realized at CEA/Leti within the frame of the H2020 project HOLDON. This ROIC is based on a CTIA amplifier with four different current/voltage conversion gain thanks to capacitance ranging from 10 fF to 10 pf. Other functionalities like on chip sampling, auto-reset or programmable low pass filtering have been implemented to optimize the detector for different measurement chain. Based on this circuit architecture, two versions of the ROIC with different way to connect the circuit to the photodiodes have been manufactured and tested. The first one is design to be directly hybridized to a small array of photodiodes with a format imposed by the ROIC design in term of pixel pitch and array size. For the second ROIC version, the photodiode array is hybridized to an interconnection circuit used as a fan-out of electrical connections to a bonding pad. This module is then wire-bonded to the ROIC to get the final detector assembly. This configuration allows us to couple the ROIC with different APD geometry adapted to a specific application need. The performances of one of the first hybridized devices were previously presented during ICSO 2020[1]. For this review, we focus on the second version of the ROIC. The tested detector module is made of an array of 76 HgCdTe APD in parallel with a pixel pitch of 15 µm. The array forms a 150 µm diameter macro-photodiode. The device was tested at 78K within a liquid nitrogen cooled cryostat. Under photon flux, we have obtained a linear response of the device with an incoming flux varying over more than six orders of magnitude without varying the APD gain. This wild dynamic is associated with a high sensitivity with a noise standing below the unique photon. The response to a brief laser pulse gives a rise time ranging from 175 ns for the highest CTIA gain to 6 ns for the lowest CTIA gain.
With the development of SWIR sources and sensors for LIDAR and telecom applications, diffuse reflectance spectrometry (DRS) in the SWIR range (900-1600 nm) appears as a promising method for non-invasive measurement of glucose concentration in biological tissue. In particular, spatially resolved DRS (SRDRS) should address the main issue of such measurements, namely the layered nature of biological media. The following paper studies the feasibility of a SRDRS probe in the SWIR range for glucose measurement.
The program Astronomy European Infrared Detector (ASTEROID), funded by the European Union through H2020 (under Grant Agreement n°730161), aims at enabling Europe to acquire the technology and knowledge necessary to manufacture 2k² high performance IR detectors. To reach these goals 9 detectors have been manufactured at Lynred and characterized at the Astrophysics Department of CEA. ASTEROID detectors are 640×512 pixels arrays with a pixel pitch of 15 μm. The detectors are p-on-n technology, with 15 μm pixel pitch, with a cut-off wavelength of 2.1 μm. In the detector architecture, the MCT light-sensitive layer is hybridized on a Source Follower Detector (SFD) Read Out Integrated Circuit (ROIC) via indium bumps. In this paper, the characterization results of ASTEROID detectors will be presented. The best detectors show extremely low dark current around 0.001 e-/s/pix, which is equivalent to standard H2RG IR detector (widely used in the IR domain for astrophysics applications). The quantum efficiency (QE) of these detectors has also been measured on a dedicated test bench and will be presented. ASTEROID detectors demonstrated a QE of 70 %.
For several years now, LYNRED, CEA-LETI and CEA-IRFU have been involved in the development of large area, very high performance NIR retinas for astronomy, in the context of the ALFA program (Astronomical Large Format Array). It aims at demonstrating the ability to produce in Europe low flux 2kx2k arrays exhibiting the very high performances required by science applications. In this context, high performance means very low dark current (below 0.1 e/s/px) with high QE (above 80%). LETI and LYNRED succeeded this year in the fabrication of a 2kx2k array, with very high uniformity as characterized at IRFU. One of those arrays will be used on the CAGIRE camera of the SVOM mission, aiming at observing afterglows of gamma ray bursts. Additional studies are ongoing on test arrays manufactured with the same technology to assess the behavior of this technology in terms of persistence and radiation hardness for space use.
HgCdTe Avalanche Photo Diodes (APDs) are developed at CEA/Leti to enable applications that require the detection of information contained in a low number of photons in each spatial and/or temporal bin, such as LiDAR and free space optical communications. The requirements for such detectors are strongly application dependent, which is why both the HgCdTe APD technology and the proximity electronics, used to extract the detected photocurrent, needs to be optimized for each application. The present communication reports results obtained from the development of detectors for high dynamic range LiDAR applications, made within the scope of the H2020 project HOLDON, and high data rate FSO, made in collaboration with Mynaric Lasercom AG. For FSO applications, we have measured 10 GHz bandwidth at unity gain for APDs with 10 μm diameter. At higher APD gain and diameter, the BW is presently limited by carrier transit and by resistance-capacitance product in small and large area APDs, respectively. For LiDAR we have developed APDs with an made of an array of diodes in parallel with a diameter up to 200 μm and large avalanche gain, M<100, that will be hybridized with a dedicated CMOS amplifier. This circuit was designed to enable photon shot noise limited linear detection over a dynamic range of 6 order of magnitude of signal for observation times ranging from ns up to μs. First characterizations made at unity APD gain shows that the HOLDON detector will meet most of the required performance parameters in terms of sensitivity and linear dynamic range.
KEYWORDS: Sensors, Mercury cadmium telluride, Infrared detectors, Radiation effects, Short wave infrared radiation, Space operations, Pollution, Monte Carlo methods, Luminescence, Renewable energy
Experimental study of the influence of the CdZnTe substrate thickness on the response of IR detectors under irradiation has been performed. Two detectors, with different substrate thickness 800 μm and 50 μm, were submitted to low flux 62 MeV proton irradiation.Images acquired under irradiation were analyzed. Whereas the detector with 800 μm substrate thickness has shown background signal increase under irradiation, the detector with the substrate partially removed has not shown any background signal elevation. The elevation of the background signal under irradiation in the detector with intact substrate is attributed to the large extension of one transient event as reveal by the derivation of one transient event. The results of the irradiation campaign are further compared to simulations. This comparison shows very good agreement and highlights the role of emission of low energy photons inside the substrate which are in turn detected by the light sensitive layer.
We report on the development of short wave infrared (SWIR) imaging arrays for astronomy and space observation in Europe. LETI and Sofradir demonstrated 640×480 SWIR HgCdTe (MCT) arrays geared at low flux, low dark noise operation. Currently, we are developing 2048×2048 arrays mated to a newly developed ROIC. In parallel, the European Space Agency and the European Commission are funding the development and industrialization of 4" CdZnTe substrates and HgCdTe epitaxy. These large wafers are needed to achieve the necessary economies of scale and address the need for even larger arrays. HgCdTe SWIR detector performance at LETI/Sofradir is known from previous programs and will be discussed here. However, we will only be able to summarize the features and specifications of the new 2048×2048 detectors which are still at a prototype stage.
CEA and Sofradir have been involved for 7 years in studies related to a large format detector development for science and astronomy applications. These studies are linked with ESA's Near Infrared Large Format Sensor Array roadmap which aims to develop a 2Kx2K large format low flux low noise device. The ALFA (Astronomical Large Focal plane Array) detector is currently at design, manufacturing and validation phase at CEA and Sofradir. This paper will present the very last achievements of the ALFA development with a specific focus on the readout integrated circuit design itself. Features and specification of the 2048x2048 15μm pitch with Source Follower Detector (SFD) input stage will be described. Apart from ESA development, European Commission is also contributing to the large detector development thanks to ASTEROID (AStronomical TEchnology EuROpean Infrared detector Development) program founded by REA (Research European Agency). ASTEROID main objectives are to develop very large raw materials (CdZnTe substrate, HgCdTe epilayer…) compatible with the manufacturing of very large detectors in volume keeping the same level of performance. Organization and status of this program will be presented where high synergy with 2K² ALFA detector are included.
Bayer filter arrays are commonly added to visible detectors to achieve multicolor sensitivity. To extend this approach to the infrared range, we present frequency selective surfaces that work in the mid-infrared range (MWIR). They are easily integrated in the device fabrication process and are based on a simple operating principle. They consist of a thin metallic sheet perforated with apertures filled with a high-index dielectric material. Each aperture behaves as a separate resonator. Its size determines the transmission wavelength λ. Using an original approach based on the temporal coupled mode theory, we show that metallic loss is negligible in the infrared range, as long as the filter bandwidth is large enough (typically <λ/10). We develop closed-form expressions for the radiative and dissipative loss rates and show that the transmission of the filter depends solely on their ratio. We present a prototype infrared detector functionalized with one such array of filters and characterize it by electro-optical measurements.
Multicolor detection capabilities, which bring information on the thermal and chemical composition of the scene, are desirable for advanced infrared (IR) imaging systems. This communication reviews intra and multiband solutions developed at CEA-Leti, from dual-band molecular beam epitaxy grown Mercury Cadmium Telluride (MCT) photodiodes to plasmon-enhanced multicolor IR detectors and backside pixelated filters. Spectral responses, quantum efficiency and detector noise performances, pros and cons regarding global system are discussed in regards to technology maturity, pixel pitch reduction, and affordability. From MWIR-LWIR large band to intra MWIR or LWIR bands peaked detection, results underline the full possibility developed at CEA-Leti.
We report on the progress achieved in the molecular beam epitaxy of 3" and 4" HgCdTe on CdTe(211)B/Ge composite substrates, and the subsequent fabrication of high performance focal plane arrays. We first describe the growth of the heterostructures, and their characterization. Then we examine the fabrication of a 1280x1024 small-pitch focal plane array, which shows operability in excess of 99% for both the responsivity and the noise-equivalent thermal difference.
Specially designed mercury cadmium telluride (Hg1-xCdxTe) p-ν-n+ heterostructures were grown by molecular beam epitaxy (MBE) on CdTe/Si and CdZnTe (211)B-oriented substrates for infrared photo-detector operation at near room temperature. Growth of this structure requires precise control over the crystal quality, compositional profiles, and donor and acceptor doping levels. The doping levels and density of Shockley-Read-Hall centers in the absorber layer must be low enough to realize the benefits of Auger suppression under non-equilibrium device operation. In order to avoid possible contamination from chemical compounds used in traditional substrate mounting methods, non-contact (In-free) substrate mounting was used to grow the structures. High-energy electron diffraction (RHEED) was implemented to develop a substrate thermocouple temperature ramping curve that maintains a constant epilayer temperature. The structures were characterized by FTIR, x-ray diffraction, and temperature dependent Hall measurements. High operating temperature (HOT) detectors were fabricated on these materials and showed good room-temperature response.
A Woollam M88 spectroscopic ellipsometer was used to characterize the molecular beam epitaxy growth nucleation of Hg1-xCdxTe layers on CdZn0.035Te substrates and the substrate temperature prior to the growth. We developed a new approach to ellipsometry data analysis to better determine the substrate temperature. It is based on the accurate determination of the critical point energies and linewidths, which display strong temperature dependence in the CdZnTe system. The new model was able to resolve temperature differences of the order of +/-2.5oC. We also show that ellipsometry can be used to characterize the nucleation of Hg1-xCdxTe on CdZnTe substrates. More work is in progress to assess the run-to-run reproducibility of our temperature measurement, and to further investigate Hg1-xCdxTe nucleation.
The annealing and electrical properties of extrinsic in situ doped mercury cadmium telluride epilayers grown by molecular beam epitaxy (MBE) on B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers of CdTe mole fraction in the range of mid-wavelength IR are grown at substrate temperatures of 175-185 degrees C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. The analysis of the Hall coefficient in the temperature range of 40-300 K with a fitting based on a three-band non-parabolic Kane model, a fully ionized compensating donor concentration, and tow independent discrete acceptor levels is reported. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed-type characteristics whereas annealed samples showed p-type characteristics. Activation annealing at different temperatures was performed. Conversion to p-type at lower than conventional annealing temperatures was achieved. Theoretical models are utilized to understand the dependence of the activated arsenic concentration on the annealing temperature.
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