As we enter this new space age where the barrier to access space has never been lower, the technologies that enable various space-based missions are being reevaluated in light of evolving requirements and constraints. For example, event-based sensors show great promise for executing tracking functions with higher timing resolution and reduced power consumption and datalink demands, a great benefit for larger sensor network architectures that may be enabled by recent reductions in launch costs. Currently, the vast majority of event-based sensors on the market are designed to operate for visible wavelength applications using silicon-based photodetectors, however, operation in the infrared is essential for many space-based sensing applications. Evaluation of how the event-based read-out integrated circuit will interact with smaller bandgap photodetectors and how typical infrared photogenerated signal levels will propagate through the event-based sensor pixel unit cell will be necessary to extend the utility of event-based sensing into the mid- to long-wavelength infrared. To evaluate the functionality of the event-based sensor pixel unit cell, the circuit is implemented on a custom-designed printed circuit board using discrete devices selected to tailor the functionality to operate a mid-wave infrared photodetector. The measurements conducted provide understanding of merits such as photoresponse, latency, and general operation of the unit cell alongside possible limitations of the unit cell.
In many type-II superlattice infrared detector architectures, performance at low temperatures is limited due to the dependence on minority hole conduction for operation, with holes having a high effective mass in the vertical direction. This inherently results in a decreased (especially as temperature is reduced) carrier diffusion length in the absorber, which can limit the detector quantum efficiency. The alternative pBpn architecture utilizes minority electrons for detection which have a higher mobility and enhanced collection. A general limitation of the pBpn design is that surface currents are often found to dominate the dark current density. This paper explores the effect of varying the absorber region pn junction growth parameters in an attempt to influence the surface current magnitude. An analysis of the surface vs. bulk contributions of the dark current is made as a function of absorber design, and potential sources of the surface current are presented. Ultimately, it is determined that the surface current magnitude is independent of the bulk absorber properties varied, implying that the surface properties, especially of the p-type absorber, must be altered to effectively mitigate the surface current.
HgCdTe has been called the ideal infrared detector material for good reason: high absorption coefficients and very long Shockley-Read-Hall (SRH) recombination lifetimes lead to the highest performance infrared detectors today for space applications. III-V materials, such as InAsSb, are currently limited by short SRH recombination lifetimes due to defects, and their performance is still relatively lacking for space applications where sensitivity requirements are extremely high. However, the performance of III-V superlattice infrared detectors has improved such that it is sufficient for tactical applications, which can now take advantage of the manufacturing benefits of III-V (greater uniformity and yield). With the growing NewSpace movement, there is a need for higher-volume, lower-cost infrared detectors capable of operating in space for applications such as environmental monitoring, space-based weather, and planetary science. One way to increase volume and lower cost is to grow the detectors on large-format substrates, such as 6-inch silicon or GaAs, but lattice-matched large substrates are not available for HgCdTe or InAsSb. Here a comparison between mid-wavelength infrared HgCdTe and InAsSb infrared detectors grown on non-lattice-mismatched substrates and designed for increased proton radiation tolerance, as compared to previous designs on mismatched substrates, is given. The comparison of these recent HgCdTe photodiode and InAsSb bariode designs for space applications shows that the InAsSb bariode has an order of magnitude better dark current density proton radiation tolerance while the HgCdTe photodiode has an order of magnitude better quantum efficiency proton radiation tolerance operating at 130 K. Therefore, the choice of detector material and architecture is not clear and will depend on the required performance for a specific space application.
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