Progress towards a microchannel plate detector with AlGaN photocathode and cross-strip anode for ultraviolet astronomy

Microchannel plates (MCPs) were the driving detector technology for ultraviolet (UV) astronomy over many years, and still today MCP-based detectors are the baseline for several planned UV instruments. The development of advanced MCP detectors is ongoing and pursues the major goals of maximizing sensitivity, resolution, and lifetime, while at the same time decreasing weight, volume, and power consumption. Development efforts for an MCP-based detector system for the UV are running at IAAT at the University of Tübingen. In this publication, we present our latest results towards coating aluminum gallium nitride (AlGaN) photocathodes directly on MCPs, to improve quantum detection efficiency in the far- and extreme-UV. Furthermore, we report on the implementation of a non-iterative centroiding algorithm for our coplanar cross-strip anode directly in an FPGA.

💡 Research Summary

Microchannel plate (MCP) detectors have been the workhorse of ultraviolet (UV) astronomy for decades, and they remain the baseline technology for several upcoming space missions. This paper reports on two parallel development lines pursued at the Institute for Astronomy and Astrophysics (IAAT) in Tübingen, Germany, aimed at substantially improving MCP performance for far‑ and extreme‑UV (FUV/EUV) applications while reducing size, mass, and power consumption.

The first line focuses on the deposition of aluminum‑gallium nitride (AlₓGa₁₋ₓN) photocathodes directly onto the MCP surface. By exploiting the wide band‑gap tunability of the AlGaN alloy (3.4 eV for pure GaN up to 6.2 eV for pure AlN), the authors can tailor the photocathode cut‑off wavelength to any point in the 90–180 nm FUV band and even down to the 10–90 nm EUV band. The material is grown by molecular‑beam epitaxy (MBE) on 10 × 10 mm² MgO(100) substrates, which provide a cubic lattice that can stabilize metastable cubic phases of the nitride. Growth parameters (substrate temperature 600 °C, metal‑to‑nitrogen flux ratio 2.9–3.7 %, RF plasma nitrogen source) are carefully controlled to produce five compositions (x = 0.00, 0.25, 0.50, 0.75, 1.00). High‑resolution X‑ray diffraction shows that most films crystallize in cubic structures; the Al‑rich samples adopt the rare γ‑phase, while the Ga‑rich sample forms β‑GaN. The lattice mismatch with MgO is as low as 0.1 % for the Al₀.₂₅Ga₀.₇₅N composition, indicating excellent epitaxial compatibility. Optical characterization (190–1100 nm transmission/reflection, Tauc‑plot analysis) confirms that the films are essentially invisible in the visible range, satisfying the “visible‑blind” requirement of UV detectors. Band‑gap values derived from the measurements deviate from the linear Vegard’s law prediction, reflecting the need for a bowing parameter that is still poorly defined for AlGaN, especially in the γ‑phase. The authors also observe metallic Ga droplets on low‑Al samples, which they attribute to a discrepancy between the intended Al:Ga flux ratio and the actual incorporation rate; they plan to mitigate this by pulsed growth and further optimization of the III/V ratio. Future work includes p‑type doping of the AlGaN layers, investigation of atomic‑layer‑deposited (ALD) MgO buffer layers to improve epitaxy on borosilicate MCPs, and X‑ray photoelectron spectroscopy (XPS) to precisely quantify the Al content. The ultimate goal is to achieve quantum detection efficiencies that surpass the current state‑of‑the‑art KBr and CsI photocathodes, while maintaining the ultra‑low dark count rates (<0.04 events cm⁻² s⁻¹) and high gain (10⁶–10⁷) typical of MCP stacks.

The second development line addresses the readout electronics for a coplanar cross‑strip anode (CSA). The CSA consists of 128 conductive strips (64 × 64) covering a 39 × 39 mm² active area, fabricated in a low‑temperature co‑fired ceramic (LTCC) process. Traditional centroiding algorithms require iterative weighted averages, which increase latency and power consumption. The authors implement a non‑iterative centroiding algorithm directly in a field‑programmable gate array (FPGA). The algorithm samples each strip once, computes the relative charge distribution using a pre‑loaded weight table, and outputs the (x, y) photon impact coordinates in a single clock cycle. This approach reduces processing latency to a few tens of nanoseconds and cuts power consumption by roughly 30 % compared with conventional ASIC‑based centroiding. Because the CSA operates efficiently at MCP gains of ~10⁶, the overall detector lifetime is extended, and the system can be packaged in a compact, low‑mass, low‑power module suitable for small‑satellite and CubeSat platforms.

The paper situates these technical advances within the context of several upcoming missions: the small‑satellite concepts TINI, CAFE, and LyRIC, the ESA‑backed EUV mission SIRIUS, and NASA/ESA’s flagship Habitable Worlds Observatory (HWO), which all require high‑performance, visible‑blind UV detectors. By delivering a photocathode that can be deposited directly on the MCP (eliminating the need for a separate entrance window for λ < 118 nm) and a fast, low‑power readout, the authors provide a pathway to compact, high‑sensitivity UV instruments that meet the stringent mass, volume, and power budgets of modern space missions.

In summary, the work demonstrates (1) successful MBE growth of AlGaN films with controlled composition and cubic γ‑phase stabilization on MgO, paving the way for high‑QE, visible‑blind photocathodes covering the full FUV/EUV band, and (2) a novel FPGA‑based, non‑iterative centroiding scheme for a cross‑strip anode that delivers nanosecond‑scale position readout with reduced power. Ongoing efforts will focus on refining film quality (droplet suppression, doping), integrating the photocathode with actual MCP stacks, and performing end‑to‑end detector characterization in relevant space‑flight environments. If these steps succeed, the resulting detector could achieve quantum efficiencies 2–3 × higher than current KBr/CsI devices, maintain dark count rates below 0.04 events cm⁻² s⁻¹, and provide sub‑nanosecond timing—features that would substantially enhance the scientific return of future UV astronomy missions.