Effects of Neutron Irradiation on LGADs with Broad Multiplication Layer and varied Carbon-Enriched Doses: A Study on Timing Performance and Gain Deterioration

TL;DR

The study evaluates LGADs with carbon-enriched, broad multiplication layers under neutron irradiation up to $2.5\times10^{15}\,n_{eq}\,\text{cm}^{-2}$ to assess timing performance and gain deterioration. Using IV/CV measurements and beta-source timing, it quantifies acceptor removal via the depletion of the gain layer, finding that carbon enrichment reduces the acceptor-removal coefficient by roughly a factor of two. Time resolution below 50 ps is achievable at high fluences, with carbonated devices showing improved radiation tolerance in both timing and collected charge, albeit with noise limitations at the highest doses. Comparisons between production runs reveal that Si–Si wafer devices with optimized inter-pad and JTE structures offer higher gain and lower operating voltages, reinforcing the viability of carbonated LGADs for HL-LHC timing layers.

Abstract

In this radiation tolerance study, Low Gain Avalanche Detectors (LGADs) with a carbon-enriched broad and shallow multiplication layer were examined in comparison to identical non-carbonated LGADs. Manufactured at IMB-CNM, the sensors underwent neutron irradiation at the TRIGRA reactor in Ljubljana, reaching a fluence of $2.5 \times 10^{15}n_{eq}cm^{-2}$. The results revealed a smaller deactivation of Boron and improved resistance to radiation in carbonated LGADs. The study demonstrated the potential benefits of carbon enrichment in mitigating radiation damage effects, particularly the acceptor removal mechanism, reducing the acceptor removal constant by more than a factor of two. Additionally, time resolution and collected charge degradation due to irradiation were observed, with carbonated samples exhibiting better radiation tolerance. A noise analysis focused on baseline noise and thermally generated pulses showed the presence of spurious thermal-generated pulses attributed to a excessive small distance between the gain layer end and the p-stop implant at the periphery of the pad for the characterized LGAD design; however, the operation performance of the devices was unaffected.

Figures (13)

• Figure 1: Description of the cross-sectional structure of a Low Gain Avalanche Detector. Components like the Collector Ring, Channel Stopper, P-Stop, Multiplication Layer, and the Junction Termination Extension (JTE) are illustrated. Note that the thickness of the active volume, low resistivity wafer, and component distributions are not scaled proportionally.
• Figure 2: The leakage currents of the main diode before irradiation are presented as a function of reverse bias before irradiation. The left-hand plot (a) shows the complete IV curve, while the right-hand plots (b) it is an enlarged view of the region where the gain layer is depleted.
• Figure 3: The leakage currents of the main pad after irradiation as a function of the reverse bias is shown. Samples irradiated at 4e14n_eq ^-2, 8e14n_eq ^-2, 15e14n_eq ^-2 and 25e14n_eq ^-2 are shown in (a), (b), (c) and (d) respectively.
• Figure 4: Pad capacitance before irradiation as a function of the reverse bias. The characteristic kinks in the curves due to the gain layer and bulk depletion can be observed.
• Figure 5: Pad capacitance after irradiation as a function of the reverse bias. Standard and carbonated devices are shown in (a), (b), (c) and (d), according to the fluence points: 4e14n_eq ^-2, 8e14n_eq ^-2, 15e14n_eq ^-2 and 25e14n_eq ^-2. Displacement of the $V_{GL}$ (start of the peak in the curve) as result of the irradiation at the four fluence points is observed.