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

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.

💡 Research Summary

This paper presents a comprehensive experimental and theoretical investigation of charge transport and avalanche multiplication in lateral amorphous selenium (a‑Se) photodetectors operated at cryogenic temperatures. The motivation stems from the need for large‑area, low‑cost photon sensors that can provide internal gain and stable performance in high‑energy physics experiments, especially those employing noble‑liquid time‑projection chambers.

Device architecture and fabrication
The authors fabricated interdigitated electrode (IDE) structures with 20 µm finger width and 20 µm gap on a quartz‑glass substrate. A 200 nm polyimide (PI) blocking layer was spin‑coated over the entire IDE region to suppress charge injection from the metal contacts. Subsequently, a 600 nm thick stabilized a‑Se layer was thermally evaporated through a shadow mask, forming a 1.6 mm‑diameter active area centered on the IDE array. The geometry ensures that photogenerated holes drift laterally across the gap, while electrons have negligible contribution due to their much lower mobility (μ_h ≈ 0.13–0.14 cm² V⁻¹ s⁻¹ versus μ_e ≈ 0.005–0.007 cm² V⁻¹ s⁻¹). COMSOL simulations confirm that the electric field is essentially uniform across the central portion of the gap, with only modest edge effects.

Experimental setup
Measurements were performed in a vacuum‑tight, liquid‑nitrogen‑cooled optical cryostat at four discrete temperatures: 93 K, 165 K, 200 K, and 297 K. A 401 nm picosecond diode laser (4 Hz repetition, 1.56 × 10⁶ photons per pulse, ≈0.74 pJ) illuminated the a‑Se dot through a fused‑silica window. The incident photon flux was calibrated to a 5 % systematic uncertainty. Bias voltages were applied to generate electric fields ranging from 10 to 120 V µm⁻¹. The resulting transient currents were captured with a charge‑sensitive preamplifier and digitized on an oscilloscope for offline analysis.

External quantum efficiency (EQE) analysis
EQE is defined as the product of photogeneration efficiency η(E,T) and charge‑collection efficiency ξ(E,T). The authors model η using the Onsager theory of geminate‑pair dissociation, which accounts for field‑assisted diffusion of the initially bound electron‑hole pair. By fitting the measured EQE versus field at each temperature, they extract an effective initial separation r₀ that decreases from ~1.2 nm at 297 K to ~0.8 nm at 93 K, reflecting reduced thermal agitation and enhanced field‑driven separation at low temperature.

Field‑assisted detrapping and transport regimes
Below the avalanche threshold, the current–field characteristics reveal three distinct regimes: (i) field‑assisted detrapping, (ii) transport‑limited conduction, and (iii) avalanche. To discriminate among competing transport models, the authors compare (a) Poole‑Frenkel‑type emission, (b) multiple‑trapping‑and‑release (MTR) with temperature‑dependent detrapping rates, and (c) a combined field‑assisted hopping plus thermally‑assisted tunneling framework. The data favor model (c): at temperatures ≤165 K, hopping between localized tail states dominates, with a field‑dependent hopping rate that scales as exp(β_h E^½). At higher temperatures, thermally‑assisted tunneling becomes significant, leading to a smoother field dependence.

Avalanche multiplication and Lucky‑drift modeling
At 93 K, the current exhibits a sharp upturn at ≈80 V µm⁻¹, indicating the onset of impact ionization. The authors adopt the Lucky‑drift (LD) model, originally developed for disordered semiconductors, to extract the ionization coefficient α(E). In LD, carriers experience rapid momentum‑relaxing collisions (mean free path λ) interspersed with rarer energy‑relaxing events (characterized by an energy‑relaxation length λ_E). The probability of reaching the ionization threshold E_I without an energy‑relaxing event is calculated analytically, yielding α(E) = (1/λ_E) exp