Characterization of Lateral Amorphous Selenium Photodetectors for Low-Photon and VUV Detection at Cryogenic Temperatures

TL;DR

This work investigates lateral amorphous selenium photodetectors as cryogenic, VUV-sensitive devices suitable for scalable integration in pixelated liquid-argon TPCs. Using interdigitated-electrode devices with a blocking layer, the authors characterize performance across cryogenic temperatures (87–93 K) under low and high photon flux, and directly test sensitivity to 130 nm VUV light. They demonstrate reliable single-shot detection in the low-photon regime, observe linear-to-avalanche transitions at cryogenic temperatures, and confirm direct VUV detection, with tellurium-doped variants showing earlier avalanche onset and higher gain but reduced quantum efficiency. The results indicate a viable path toward high-field, low-noise, VUV-capable photodetection in large-area TPC readouts, with material tuning (a-Se vs a-SeTe) offering routes to optimized performance.

Abstract

The performance of amorphous selenium (a-Se) as a cryogenic photodetector material is evaluated through a series of experiments using laterally structured devices operated in a custom optical test stand. These studies investigate the response of a-Se detectors to low-photon fluxes at high electric fields near avalanche conditions, the linearity of the photoconductive response over a wide dynamic range and the direct detection of narrowband 130 nm vacuum ultraviolet (VUV) illumination. At 87 K, matched-filter analysis shows reliable single-shot detection with efficiencies greater than or equal to 80 percent and area under the curve (AUC) greater than or equal to 0.85 using as few as approximately 6800 incident 401 nm photons, corresponding to approximately 3400 photons within field-active regions after accounting for geometric constraints. Measurements are performed at cryogenic temperatures using calibrated photon fluxes derived from a silicon photomultiplier reference and a characterized optical filter stack. Additional experiments using a tellurium-doped a-Se (a-SeTe) device explore the material's behavior under identical test conditions and demonstrate that avalanche is achievable in a-SeTe at cryogenic temperatures. The results demonstrate reproducible low-noise operation, VUV sensitivity and field-dependent gain behavior in a lateral a-Se architecture, representing the first reported observation of avalanche multiplication in laterally structured a-Se and a-SeTe devices at cryogenic temperatures. These findings support the potential integration of laterally structured a-Se devices into next-generation pixelated liquid-argon time projection chambers (TPCs) requiring scalable, high-field-compatible photon detection systems.

Figures (18)

• Figure 1: Schematic of a lateral amorphous selenium photodetector showing electric field lines simulated using COMSOL Multiphysics® software with an applied bias of 2 kV corresponding to an electric field of 100 V/µ m comsol. For visual clarity, field lines are shown only within the regions active for charge transport and collection. Simulations use relative permittivities of 6.6 for amorphous selenium and 2.9 for polyimide. Incident photons generate electron–hole pairs and under the applied bias, holes drift toward the signal electrode to produce the photocurrent signal.
• Figure 2: Photograph of the fabricated detectors: IDE-A (left) and IDE-B (right). Overall substrate dimensions are indicated for each device. Both detectors have a centrally located 1.6 mm diameter, 600 nm thick a-Se dot, thermally evaporated onto a 200 nm spin-coated polyimide layer that spans the entire interdigitated region of 50 finger pairs. The field-active region of a single finger pair in IDE-A is illustrated in Fig. \ref{['fig:lateral_sketch']}.
• Figure 3: Schematic of the cryogenic optical test system. The optical cryostat features a rotatable liquid nitrogen–cooled cold head with a sample mount that holds the a-Se detector on one side and a SiPM reference on the other. Two optical viewports admit light from a picosecond pulsed laser and an argon flashlamp. Rotating the cold head positions the a-Se detector in the path of the selected light source for optical excitation
• Figure 4: Thermal response of photodetectors in the cryogenic setup. Temperature difference between the detector surface and the sample holder plotted against detector surface temperature for IDE-A and IDE-B with polynomial fits.
• Figure 5: Left: Heat map of the collimated laser beam profile at the detector plane. The two-dimensional intensity distribution shows a central peak, with standard deviations of 0.27 mm and 0.26 mm in the $x$- and $z$-directions respectively, determined from Gaussian fits to the one-dimensional intensity projections. The full beam diameter at the detector plane is 1.2 mm. Right: Photograph of the experimental alignment setup showing the laser, micrometer stage, lens tube assembly and optical cryostat interface.