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

TL;DR

This work demonstrates the fabrication and comprehensive characterization of two p-type inverted coaxial point contact HPGe detectors (SAP16, SAP17) employing thin amorphous-Ge dual-blocking contacts. The ICPC geometry yields picoampere-level leakage currents and sub-picofarad capacitances at 76 K, enabling robust low-energy spectroscopy as shown by clear 59.5 keV and 662 keV gamma peaks. SAP16 achieves superior energy resolution (1.44 keV at 59.5 keV and 2.39 keV at 662 keV) compared with SAP17, with electrostatic simulations attributing performance differences to geometry-induced weak-field regions near bore and wing transitions. The angular-response measurements reveal strong low-energy geometry sensitivity, consistent with field maps, underscoring the need for careful geometric optimization and passivation uniformity for next-generation low-background ICPC HPGe detectors. Collectively, the results offer practical design guidance to maximize depleted volume, charge-collection uniformity, and energy resolution for low-threshold, rare-event applications.

Abstract

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 76~K demonstrate stable operation with picoampere-level leakage currents and sub-pF capacitance: SAP17 reached $\sim 4.62$~pA at the maximum tested bias (500~V) and operated stably at 400~V with $C\simeq 0.503$~pF. \emph{Meanwhile,} SAP16 achieved superior spectroscopic performance, with energy resolutions of 2.42\% at 59.5~keV and 0.36\% at 662~keV. 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 59.5~keV, 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.

Figures (18)

• Figure 1: Schematic diagrams of both (a) SAP16 and (b) SAP17 inverted coaxial detectors.
• Figure 2: Fabrication steps of ICPC detectors: (a) as-grown HPGe crystal from Czochralski method, (b) machining of the groove and wing structures on the top surface using a lathe machine, and (c) cold drilling of the central coaxial bore.
• Figure 3: Surface finishing steps of the ICPC HPGe detectors: (a) detector after groove and bore cutting, (b) detector after mechanical polishing of all surfaces, and (c) detector during chemical etching.
• Figure 4: Fabrication steps associated with contact deposition for the ICPC HPGe detectors: (a) detector loaded into the sputtering chamber after surface treatment, (b) amorphous germanium (a-Ge) deposition for surface passivation, and (c) aluminum contact deposition by DC magnetron sputtering under an argon plasma.
• Figure 5: Chemical etching steps of the ICPC HPGe detector: (a) masked detector before etching, (b) surface condition immediately after HF:HNO$_3$ etching, and (c) detector after cleaning and drying.