Fabrication and Characterization of p-type Inverted Coaxial Point Contact (ICPC) Detectors with a-Ge Dual-Blocking Contacts

We report the fabrication and characterization of two p-type inverted coaxial point contact (ICPC) high-purity germanium (HPGe) detectors, SAP16 and SAP17, produced from USD-grown crystals with net impurity concentrations of $\sim 3\times10^{10},\mathrm{cm^{-3}}$. Both devices employ \emph{thin} amorphous-germanium (a-Ge) dual-blocking contacts, implemented here for the first time on ICPC detectors, to provide bipolar charge blocking while limiting dead-layer thickness. Electrical tests at 76K demonstrate stable operation with picoampere-level leakage currents and sub-pF capacitance: SAP17 reached $\sim 4.62$pA at the maximum tested bias (500V) and operated stably at 400V with $C\simeq 0.503$pF. \emph{Meanwhile,} SAP16 achieved superior spectroscopic performance, with energy resolutions of 2.42% at 59.5keV and 0.36% at 662keV. Gamma-ray spectroscopy with $^{241}$Am and $^{137}$Cs shows that modest geometric differences lead to measurable changes in depletion behavior and charge-collection uniformity, consistent with electrostatic modeling. Angular-response measurements further reveal pronounced directional sensitivity at 59.5keV, whereas the 662~keV response is essentially isotropic over the measured range. These results validate thin a-Ge dual-blocking contacts for ICPC HPGe detectors and highlight geometry-driven trade-offs among leakage current, depletion, and energy resolution relevant to low-background and low-threshold applications.

💡 Research Summary

The authors present a comprehensive study on the fabrication, electrical performance, and spectroscopic characterization of two p‑type inverted coaxial point‑contact (ICPC) high‑purity germanium (HPGe) detectors, designated SAP16 and SAP17. Both detectors are cut from zone‑refined, Czochralski‑grown crystals produced at the University of South Dakota, with net impurity concentrations of approximately 3 × 10¹⁰ cm⁻³. The key technological innovation is the use of thin (~600 nm) amorphous‑germanium (a‑Ge) dual‑blocking contacts on all exposed crystal surfaces. Unlike conventional lithium‑diffused n⁺ outer contacts, the a‑Ge layers provide bipolar charge blocking (suppressing both electron and hole injection) without introducing a millimetre‑scale dead layer or a transition layer, thereby preserving active mass and simplifying processing (no high‑temperature diffusion or post‑deposition annealing).

The detector geometry follows the ICPC concept: a right‑cylindrical crystal (≈ 14 mm diameter, 7 mm height) with a shallow coaxial bore machined from the bottom and a small point contact (≈ 1.5 mm diameter) on the top. SAP16 and SAP17 differ slightly in bore diameter, crystal height, and wing thickness, allowing the authors to probe geometry‑dependent effects on depletion, leakage, and resolution. Additional design features include a machined wing for handling and a circumferential groove near the top contact to lengthen the surface leakage path and electrically isolate the point contact.

Fabrication proceeds through meticulous mechanical polishing, HF:HNO₃ etching, a‑Ge sputter deposition (7 % H₂/Ar, 100 W RF, base pressure 3 × 10⁻⁶ Torr), and aluminum metallization (≈ 120 nm) for the outer electrode and point contact. The point contact is defined by a localized 1 % HF etch through a Kapton mask, ensuring a clean, well‑isolated electrode. No thermal cycling or annealing follows deposition, preserving the integrity of the thin a‑Ge film.

Electrical testing at 76 K (liquid‑nitrogen temperature) reveals picoampere‑level leakage currents: SAP17 reaches 4.62 pA at 500 V, while SAP16 remains below 4 pA up to 400 V. Capacitance measurements using a calibrated charge‑injection pulser give sub‑pF values (0.498 pF for SAP16, 0.503 pF for SAP17). C‑V analysis shows full depletion occurring between 350 V and 380 V, consistent with electrostatic simulations that model the field distribution within the coaxial bore and the surrounding crystal volume. The low capacitance directly translates into reduced electronic noise, a critical factor for low‑energy spectroscopy.

Gamma‑ray spectroscopy with 241Am (59.5 keV) and 137Cs (662 keV) sources demonstrates the detectors’ energy‑resolution capabilities. SAP16 achieves 2.42 % FWHM at 59.5 keV and 0.36 % at 662 keV; SAP17 shows slightly poorer resolution, reflecting the subtle influence of geometry on field uniformity and charge‑collection efficiency. The authors decompose the total resolution into statistical (Fano), charge‑collection, and electronic‑noise components using standard quadrature subtraction with pulser‑derived noise measurements. At low energies, electronic noise dominates, while at higher energies the contribution from charge‑collection non‑uniformity becomes more apparent.

Angular‑response measurements further elucidate detector behavior. By rotating the detector relative to a collimated source, the authors find a pronounced dependence of relative efficiency on angle at 59.5 keV, whereas the 662 keV response is essentially isotropic. This is attributed to the shallow interaction depth of low‑energy photons, which makes the signal sensitive to the exact electric‑field configuration near the point contact. In contrast, high‑energy photons generate charge throughout the bulk, averaging out field variations. The observed anisotropy at low energy is advantageous for discriminating surface α/β backgrounds in rare‑event searches.

Electrostatic simulations (COMSOL) reproduce the measured I‑V, C‑V, and depletion characteristics. The a‑Ge contacts are modeled as high‑resistivity layers that block carrier injection while allowing the external bias to establish a uniform field across the bulk. Simulated capacitance values agree within a few percent of the experimental data, validating the design methodology. The simulations also show that the field intensity peaks near the bore wall, which can enhance pulse‑shape discrimination (PSD) by producing distinct charge‑collection times for events near the bore versus those near the outer surface.

In the discussion, the authors compare the a‑Ge dual‑blocking approach to traditional lithium‑diffused contacts, emphasizing the elimination of dead layers (improving active mass for enriched ⁷⁶Ge), the stability of leakage currents over time (no lithium migration), and the simplified fabrication flow (room‑temperature sputtering, no high‑temperature diffusion). They acknowledge trade‑offs: thin contacts provide less intrinsic shielding against surface α/β particles, but this can be turned into a feature for background tagging when combined with active vetoes (e.g., liquid argon).

Overall, the work demonstrates that thin a‑Ge dual‑blocking contacts are compatible with the ICPC geometry, delivering sub‑pF capacitance, picoampere leakage, and excellent energy resolution across a broad energy range. The results support the adoption of this contact technology in next‑generation low‑background experiments such as LEGEND, where kilogram‑scale detectors with sub‑keV thresholds and robust PSD are required. The paper provides a detailed roadmap for scaling the approach to larger masses and integrating it into multi‑detector arrays, highlighting the balance between geometry, electrical stability, and spectroscopic performance.