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<sup>-4</sup> at 200 ns after the detection of short light impulse, which is compatible with demanding LIDAR applications such as bathymetric profiling.
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.
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.
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.