Stable, solar-blind ultraviolet III-nitride photocathode imagers with high quantum efficiency

Currently, many detectors used in ultraviolet (UV) instruments utilize a photocathode to convert UV photons into electrons that are subsequently detected by microchannel plate or CCD. UV photocathodes provide low dark count rate combined with high out-of-band rejection. The performance of these detectors critically depends on the efficiency and stability of their photocathodes. In particular, photocathode instability is responsible for many of the fabrication difficulties commonly experienced with this class of detectors. In this effort, we will advance to TRL-4 a new type of cesium-free photocathode using III-nitride materials (GaN, AlN, and their ternary alloy Al(1-x)Ga(x)N) to achieve highly efficient, solar blind, stable UV response. In recent years, group-III/nitride (in particular GaN) photocathodes have been demonstrated with high quantum efficiency (QE) (>50%) in the UV spectral range. In particular, GaN photocathodes have promise to substantially surpass the QE of photocathodes fabricated from other materials. Moreover, due to the tunable wide bandgaps of III-nitride materials, photocathode response can be tailored, and can be made intrinsically solar-blind. However, these photocathodes still rely on reactive materials (such as cesium) for activation, necessitating in many cases all-vacuum fabrication and sealed-tube operation. In addition to the expense, mass, and fragility of the sealed-tube configuration, QE has been observed to degrade with cesiated photocathodes, even in the vacuum environment. The photocathode/microchannel plate structure proposed here achieves activation through methods for band structure engineering such as delta-doping and polarization field engineering. Compared to the current state-of-the-art microchannel plate sealed tubes, they will provide high QE and significantly enhance stability, fabrication yield, and reliability. We have demonstrated III-nitride photocathode operation without the use of cesium. We have implemented III-nitride polarity control, polarization charge engineering, and alloy fraction control to demonstrate high-QE UV photocathodes. However, further work is needed to provide improved stability over time, even higher QE, and truly tunable response enabled by AlGaN heterostructures. These improvements will be achieved by employing novel designs, surface passivation methods, and heterostructure growth techniques, and by continued optimization of placement of internal fixed charge. We will demonstrate the performance of these detectors in a system that includes either conventional micro-channel plate instruments (MCPs) or solid-state arrays with gain such as electron-bombarded CCD arrays. This technology will be enabling for future large UV spectroscopic and imaging missions such as HabEx and LUVOIR, as well as smaller orbital and sub-orbital missions where low mass, high QE, high out-of-band rejection, low dark count rates, and robustness are of primary importance.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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