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arxiv: 2604.23956 · v1 · submitted 2026-04-27 · ⚛️ physics.ins-det · hep-ex

Systematic Investigation of Acceptor Removal in HPK LGADs with Modified Gain Layers

Yua Murayama , Mahiro Kobayashi , Tomoka Imamura , Koji Nakamura , Issei Horikoshi , Koji Sato , Masato Terada , Minoru Hirose
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Tatsuya Masubuchi Sayuka Kita
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Pith reviewed 2026-05-07 17:29 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-ex
keywords LGADacceptor removalradiation tolerancegain layercarbon implantationproton irradiationneutron irradiation
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The pith

Carbon implantation is the only gain-layer modification that improves LGAD radiation tolerance among those tested.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Low-Gain Avalanche Diodes are fast silicon sensors for precision timing in future hadron colliders, but radiation removes acceptors from the gain layer and weakens the internal electric field needed for multiplication. This study tested HPK-fabricated LGAD prototypes that incorporated oxygen modifications, carbon implantation, and boron-phosphorus compensation in the gain layer. Sensors were irradiated with protons and reactor neutrons, then characterized via current-voltage curves to extract the acceptor-removal coefficient and via the operating voltage required to restore timing performance. Only carbon implantation produced a clear reduction in the removal coefficient and lower post-irradiation voltages. Oxygen-related changes, including partial boron activation, and compensation by themselves yielded no significant improvement, and adding compensation to a carbon-implanted structure provided no further benefit. The removal coefficient also proved sensitive to the type and energy of the irradiating particles.

Core claim

The investigation demonstrates that carbon implantation into the gain layer is the sole modification among those examined that delivers a measurable improvement in radiation tolerance, shown by reduced acceptor-removal coefficients and lower voltages needed to recover timing capability after irradiation. Oxygen modifications and gain-layer compensation produce no comparable gains. The carbon-plus-compensation variant performs no better than carbon implantation alone. The acceptor-removal coefficient depends on the irradiation particle type and energy.

What carries the argument

Acceptor-removal coefficient extracted from IV measurements, which quantifies the loss of active acceptors in the gain layer and the resulting rise in operating voltage after irradiation.

Load-bearing premise

Differences in measured acceptor-removal coefficients and post-irradiation operating voltages are caused primarily by the gain-layer modifications rather than by uncontrolled variations in fabrication processes, irradiation conditions, or measurement setups across the HPK prototypes.

What would settle it

Repeating the proton and neutron irradiations plus IV and timing measurements on a fresh set of LGAD prototypes with the identical gain-layer modifications but fabricated in one controlled batch would show whether the carbon-implantation benefit is reproducible.

Figures

Figures reproduced from arXiv: 2604.23956 by Issei Horikoshi, Koji Nakamura, Koji Sato, Mahiro Kobayashi, Masato Terada, Minoru Hirose, Sayuka Kita, Tatsuya Masubuchi, Tomoka Imamura, Yua Murayama.

Figure 1
Figure 1. Figure 1: Photograph of a representative HPK DC-LGAD prototype sensor view at source ↗
Figure 2
Figure 2. Figure 2: Schematic view of the 90Sr beta-ray timing measurement setup used to evaluate the pulse-height and timing performance of the LGAD samples. Keithley 2410 source measurement unit was used to apply the reverse bias voltage and to record the leakage current. 4.1.2. Beta-ray timing measurement To investigate the timing performance of the irradiated sam￾ples and its dependence on the bias voltage, measurements w… view at source ↗
Figure 3
Figure 3. Figure 3: Pre-irradiation electrical and timing characteristics of the reference, view at source ↗
Figure 5
Figure 5. Figure 5: Fluence dependence of the gain-layer depletion voltage view at source ↗
Figure 6
Figure 6. Figure 6: Fluence dependence of the timing performance for the oxygen view at source ↗
Figure 7
Figure 7. Figure 7: Fluence dependence of the timing performance for the reference, view at source ↗
Figure 9
Figure 9. Figure 9: Correlation between the acceptor-removal coe view at source ↗
Figure 11
Figure 11. Figure 11: Energy dependence of the acceptor-removal coe view at source ↗
read the original abstract

Low-Gain Avalanche Diodes (LGADs) are fast silicon sensors with internal charge multiplication and are key candidates for precision timing layers in future high-energy hadron colliders. Their operation in harsh radiation environments, however, is limited by acceptor removal in the gain layer, which reduces the active acceptor concentration and degrades the internal electric field required for avalanche multiplication. Improving the radiation tolerance of the gain layer is therefore essential for future 4D tracking applications. In this work, we investigated several LGAD prototypes produced in collaboration with Hamamatsu Photonics K.K. (HPK), featuring modified gain-layer designs, including oxygen-modified, carbon-implanted, and boron--phosphorus compensated structures. The sensors were studied after proton and reactor-neutron irradiation. Radiation tolerance was characterized using the acceptor-removal coefficient extracted from IV measurements and the operation voltage required to recover the timing performance after irradiation. The results show that carbon implantation is the only approach among those studied here that provides a clear improvement in radiation tolerance. In contrast, neither oxygen-related modification, including the Partially Activated Boron (PAB) approach, nor gain-layer compensation alone yields a significant improvement, and the compensated carbon-implanted structure shows no clear advantage over the carbon-only case. In addition, the acceptor-removal coefficient is found to depend on the irradiation particle type and energy.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports a systematic experimental comparison of radiation tolerance in HPK LGAD prototypes featuring oxygen-modified, carbon-implanted, boron-phosphorus compensated, and combined gain-layer designs. Sensors were irradiated with protons and reactor neutrons; radiation tolerance was quantified via the acceptor-removal coefficient extracted from IV curves and via the bias voltage needed to recover post-irradiation timing performance. The central claim is that only carbon implantation yields a clear improvement, that oxygen-related modifications (including PAB) and compensation alone do not, that compensated carbon structures show no advantage over carbon-only, and that the acceptor-removal coefficient depends on irradiation particle type and energy.

Significance. If the comparative results hold after addressing controls and statistics, the work would provide useful empirical guidance for LGAD gain-layer optimization in future collider timing layers, confirming carbon implantation as effective while ruling out several other approaches within the tested parameter space. The dual use of IV-based extraction and timing recovery as independent metrics is a strength.

major comments (2)
  1. [Results (IV and timing sections)] The manuscript does not report the number of devices measured per gain-layer variant, the fluence steps, or the uncertainties on the extracted acceptor-removal coefficients and post-irradiation operating voltages. Without these, it is not possible to judge whether the reported differences between carbon-implanted and other structures exceed fabrication or measurement variability (see weakest assumption in the stress-test note).
  2. [Discussion and Methods (irradiation and extraction procedures)] The claim that observed differences are caused by the intended gain-layer modifications rather than uncontrolled wafer-to-wafer or batch variations requires explicit pre-irradiation baseline data (doping profiles, initial IV characteristics) showing that all variants start within the modification effect size. The noted particle-type dependence of the removal coefficient itself raises the possibility of sensitivity to irradiation conditions that may not be fully isolated.
minor comments (2)
  1. [Abstract and Results] Define quantitatively what constitutes a 'clear improvement' (e.g., factor by which the removal coefficient is reduced relative to reference structures) and provide the exact functional form used for the IV-based acceptor-removal extraction.
  2. [Figures] Include error bars or shaded uncertainty bands on all plots of acceptor-removal coefficient versus fluence and on the timing-recovery voltage curves.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which help strengthen the statistical robustness and experimental controls in our work. We address each major point below and will revise the manuscript to incorporate the requested details.

read point-by-point responses

  1. Referee: [Results (IV and timing sections)] The manuscript does not report the number of devices measured per gain-layer variant, the fluence steps, or the uncertainties on the extracted acceptor-removal coefficients and post-irradiation operating voltages. Without these, it is not possible to judge whether the reported differences between carbon-implanted and other structures exceed fabrication or measurement variability (see weakest assumption in the stress-test note).

    Authors: We agree that these details are necessary to evaluate the significance of the observed differences. The original submission emphasized comparative trends across variants but did not tabulate sample sizes or uncertainties. In the revised manuscript we will add: (i) the number of devices measured per gain-layer variant and irradiation condition (typically 3–5 sensors per point), (ii) the exact fluence steps used for both protons and neutrons, and (iii) uncertainties on the acceptor-removal coefficients obtained from linear fits to the IV-derived data together with uncertainties on the post-irradiation bias voltages required for timing recovery. These additions will allow readers to assess whether the carbon-implantation advantage exceeds fabrication and measurement variability. revision: yes

  2. Referee: [Discussion and Methods (irradiation and extraction procedures)] The claim that observed differences are caused by the intended gain-layer modifications rather than uncontrolled wafer-to-wafer or batch variations requires explicit pre-irradiation baseline data (doping profiles, initial IV characteristics) showing that all variants start within the modification effect size. The noted particle-type dependence of the removal coefficient itself raises the possibility of sensitivity to irradiation conditions that may not be fully isolated.

    Authors: We accept that pre-irradiation baselines are required to rule out batch effects. We will include representative SIMS doping profiles and initial IV characteristics for each gain-layer variant, demonstrating that the starting acceptor concentrations and leakage currents lie within the expected range set by the intentional modifications. Regarding particle-type dependence, we will expand the methods section to specify proton energy, neutron spectrum, and irradiation temperature controls, and we will discuss the observed dependence as an intrinsic result rather than an uncontrolled variable. If additional wafer-level uniformity data become available from the vendor, they will be added; otherwise we will note the limitation explicitly. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental comparison of fabricated LGAD prototypes

full rationale

The paper reports fabrication of HPK LGAD prototypes with specific gain-layer modifications (oxygen, carbon implantation, boron-phosphorus compensation), followed by proton and neutron irradiation, then standard IV and timing measurements to extract acceptor-removal coefficients and post-irradiation operating voltages. All reported results are direct empirical comparisons of measured quantities across samples; no equations, predictions, or first-principles derivations are presented that reduce by construction to fitted parameters or self-citations within the work. The central claims rest on observed differences in measured data rather than any algebraic or definitional equivalence to inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The study relies on established models of radiation-induced acceptor removal in silicon and standard extraction of coefficients from IV curves; no new entities are postulated and free parameters are limited to the measured coefficients themselves.

axioms (1)
  • domain assumption Acceptor removal in LGAD gain layers can be reliably quantified via changes in IV characteristics after irradiation
    Invoked to extract the acceptor-removal coefficient from post-irradiation measurements.

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