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.
In the present communication, the characterization results of an in-house developed four-quadrants detection module based on HgCdTe APDs and a Si-CMOS ROIC pre-amplifier is discussed. The module has been designed to be employed as high data rate ground-segment detector for 1.55 μm long-distance free-space optical communication links in the framework of a project funded by the European Space Agency. The detector is characterized by a multiplication gain in excess of M = 150, a ROIC input referred noise of Ne = 45 electrons rms and a measured bandwidth of BW = 450 MHz. These characteristics enable the linear-mode detection of meso-photonic states ranging from tens of photons per pulse down to the single-photon level at high count rates exceeding 500 MHz per quadrant (and 2 GHz if the signal is dispatched over all four-quadrants). For the present module, the performance for PPM and OOK modulation formats was estimated and its potentiality for long-distance free-space optical communications employing these modulation formats was validated. In particular, for the PPM format, a detection probability of 0.9 and a false alarm probability of 10-2 , a minimum PPM slot width of 500 ps and a temporal jitter with a FWHM ~ 160 ps were estimated, for an incident photonic state with 10 photons/pulse. The potentiality of the detector for 625 Mbps OOK modulation format was also evaluated and compared with a quantum limited situation. In this case, a -3.9 dB penalty from the quantum limited BER was obtained. A new generation of detectors is currently in development, which is expected to further improve the performance.
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.
In the last decade, IR imaging detector trend has gone for smaller pixels and larger formats. Most of the time, this scaling is carried out at given total sensitive area for a single focal plane array (FPA). As an example, QVGA 30µm pitch and VGA 15µm pitch exhibit the exact same sensitive area. SXGA 10µm pitch tends to be very similar as well. This increase in format is beneficial to image resolution. However, this scaling to even smaller pixels raises questions because the pixel size becomes similar to the IR wavelength but also to typical transport dimensions in the absorbing material. Hence, maintaining resolution for such small pixel pitches requires a good control of the modulation transfer function (MTF) and quantum efficiency (QE) of the array, while decreasing the pixel size. This might not be obtained just scaling the pixel dimensions. As an example, bulk planar structures suffer from excessive lateral diffusion length inducing pixel-to-pixel cross talk and thus degrading MTF. Non-bulk semiconductor materials such as colloidal quantum dots might exhibit much smaller cross talk due to weak transport properties, but it usually strongly degrades the accessible QE. On the other side, mesa structures might minimize cross-talk physically separating pixels, but also tends to degrade the QE due to non-negligible pixel fill factor shrinking down the pixel size. This paper intend to discuss those issues, taking into account different material systems and structures, in the perspective of expected future pixel pitch IR FPAs. This paper also introduces an important issue in this context: how to reliably measure the MTF of those small pitch detectors. As an answer to this question we will share our first investigations of MTF measurement using the electron beam of an SEM instead of a photon beam (EBIC measurement)
HgCdTe APD detector modules telecommunication are developed at CEA/Leti for atmospheric LIDAR and free space optical (FSO). The development is driven by the design and manufacture of generic sub-assemblies that can be adapted in each detector module to meet the specific detector requirements of each application. The optimization of such subassemblies is detailed in perspective of the challenges that are set by the specifications for detector modules currently developed for atmospheric LIDAR, in the scope of an R&T CNES project for Airbus and an H2020 project HOLDON, and FSO, in the scope of an ESA project and in collaboration with Mynaric Lasercom GmbH. Two detector modules have recently been delivered to Airbus DS for extensive LIDAR simulation tests. Initial characterization of these modules shows that the input noise, NEP=10-15fW/√Hz (5 photons rms) have been reduced by a factor three compared to previously developed large area detectors although the bandwidth have been increased to 180 MHz in order to respond to the requirements of high spatial depth resolution. The temporal remanence was 10-4 at 200 ns after the detection of short light impulse, which is compatible with demanding LIDAR applications such as bathymetric profiling.
This LETI/Sofradir/Defir study aims at realizing sub-10 μm pitch HgCdTe infrared FPAs. To cope with the different diode process issues related to pitch reduction-morphologic realization, short-circuits, FTM optimization - a parametric study was carried out - contact size, passivation properties, doping levels, diode processing conditions-. A wafer-level test campaign was conducted to evaluate the process window. It revealed functional MWIR diodes from 15 μm to 3 μm pitch. 7.5 μm pitch 640×512 and 5 μm pitch 64×152 FPA were characterized and turned out to be functional.
We present CEA-LETI’s recent work on very small pitch HgCdTe focal-plane-arrays (FPA): materials, diode processing, readout circuit (ROIC) optimization and hybridization, done in the context of the common laboratory with SOFRADIR called DEFIR. We report on a 7.5μm pitch 640×512 FPA and a smaller 5μm pitch 64×152 FPA operating in middle wave infrared range (MWIR). The diode technology is n-on-p processed onto LPE grown HgCdTe. We will describe the two readout integrated circuits (ROIC) developed for 7.5μm and 5μm pitches and present the characterization of the IRFPAs hybridized to those ROICs. For these very small pitch detectors, we designed classic snapshot Direct-Injection (DI) Integrate-While-Read (IWR) ROICs that maximize the charge handling capacity by significantly increasing the dynamic range. For the 7.5μm ROIC, dedicated electronics has been embedded to measure the ROIC cross-talk. The 7.5μm pitch IRFPA operating at 110K displays nonlinearity under 0.5% across the maximum dynamic range, a full-well of 3.1 Me- with a 3.8V dynamic range, a ROIC noise of 210μV and SNR of 62 dB and NETD (Noise-Equivalent Temperature Difference) of 25 mK for an average current of 30 pA, and a responsivity of 1.3 pA/K.
HgCdTe avalanche photodiodes offers a new horizon for observing spatial or temporal signals containing only a few infrared (IR) photons, enabling new science, telecommunication and defence applications. A large number of HgCdTe APD based detectors have been developed at CEA LETI to address the increasing number of applications in which a faint photonic information needs to be extracted from the noise of the proximity electronics used to sample the signal. The performance of HgCdTe APDs is directly related to the multiplication process and the dark current generation in the APDs. The impact of these parameters is presented as a function of the Cd composition and geometry of the APDs. The obtained and expected performance of HgCdTe APD detectors is reported for applications ranging from very low flux observations with long observations times to high data rate telecommunications with up to single photon resolution.
HgCdTe avalanche photodiode single element detectors have been developed for a large scope of photon starved
applications. The present communication is dedicated to use of these detectors for free space optical communications. In
this perspective we present and discuss the sensitivity and bandwidth that has been measured directly on HgCdTe APDs
and on detector modules. In particular, we report on the performance of TEC cooled large area detectors with sensitive
diameters ranging from 30- 200 μm, characterised by detector gains of 2- 20 V/μW and noise equivalent input power of
0.1-1 nW for bandwidths ranging from 20 to 400 MHz. One of these detectors has been used during the lunar laser
communication demonstration (LLCD) and the results The perspectives for high data rate transmission is estimated from
the results of impulse response measurements on HgCdTe APDs. These results indicate that bandwidths close to 10 GHz
can be achieved in these devices. The associated sensitivity at an APD gain of 100 is estimated to be below 4 photons
rms (NEP<10 nW) for APDs operated at 300 K.
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