Charge Transport and Multiplication in Lateral Amorphous Selenium Devices Under Cryogenic Conditions

TL;DR

This work investigates field- and temperature-dependent charge transport and avalanche in cryogenic lateral amorphous selenium detectors, using 401 nm pulsed excitation to span 93–297 K and fields up to 120 V/μm. The authors combine Onsager geminate-pair dissociation for photogeneration, multiple transport models (trap-limited drift, hopping, and field-enhanced emission), and the Lucky-drift avalanche framework to interpret pre-avalanche and multiplication regimes. They find that pre-avalanche transport is dominated by field-assisted detrapping and thermally assisted tunneling at higher temperatures, with hopping prevailing at low temperatures, while avalanche is well described by LD with decreasing ionization efficiency at lower temperatures and a mid-temperature anomaly around 165–200 K. Overall, the study provides a quantitative, mechanism-based map of cryogenic avalanche in lateral a-Se devices, offering design rules for stable, high-gain low-temperature photon detectors and highlighting geometry- and field-dependent considerations in assessing gain and efficiency.

Abstract

Cryogenic photon sensing for high-energy physics motivates photosensor technologies that combine large-area scalability with internal gain and stable operation at low temperature. Amorphous selenium is a promising photoconductor, yet its field- and temperature-dependent transport and avalanche response in lateral geometries have not been systematically established. This work reports field-resolved photocurrent measurements of lateral a-Se devices from 93 K to 297 K under 401 nm excitation at fields up to 120 V/um. Below avalanche onset, the external quantum efficiency was described by the Onsager model, yielding effective post-thermalization separations that decrease with decreasing temperature. The field-assisted detrapping region was evaluated using several transport models, with the data favoring field-assisted hopping and thermally-assisted tunneling as the mechanisms that best capture the temperature evolution of the photocurrent. The boundaries between field-assisted detrapping, transport-limited conduction, and avalanche shift with temperature; at 93 K the response transitions directly from detrapping into avalanche. Avalanche multiplication was analyzed using the Lucky-drift model. These results provide the first systematic characterization of cryogenic avalanche behavior in lateral a-Se detectors and establish quantitative trends relevant to low-temperature, high-gain photodetector design.

Figures (8)

• Figure 1: (a) Fabricated detector showing a 1.6 mm diameter, 600 nm-thick a-Se dot thermally evaporated onto a 200 nm-thick spin-coated PI layer covering the entire IDE region. (b) Schematic of the IDE geometry highlighting the electrode finger width and gap separation, G. (c) Simulated electric field distribution in the a-Se photodetector at a nominal applied field of 100 V/µ m. Field lines are shown only within the active a-Se region for clarity. Adapted from Rooks2025.
• Figure 2: Diagram of the optical cryostat test setup. A picosecond-pulsed diode laser at 401 nm is directed through a fused-silica window to illuminate the a-Se detector mounted on a copper block at the cold finger. The detector is biased through external feedthroughs, and the output is routed to a charge-sensitive preamplifier and oscilloscope for signal acquisition. Adapted from Rooks2025.
• Figure 3: Response data expressed as EQE versus applied field for detectors operated at four temperatures with Onsager fits over the field-assisted detrapping region, as discussed in Sec. \ref{['sec:transport_results']}. Starred points indicate field screening.
• Figure 4: Top: Multiplication factor calculated from EQE response data in the avalanche regime. Bottom: Ionization coefficients from this work together with data extracted from Tsuji Tsuji1989, both fit with the LD model for comparison.
• Figure 5: Measured photocurrent $I$ (left) and diagnostic $I/E$ (right) as functions of applied field for detectors operated at 297 K, 200 K, 165 K, and 93 K. The plots illustrate transitions between FAD, TL and AV regimes. The inset highlights the knee at 35.3 V/µ m in the 297 K data.