Hybrid-Contact Planar HPGe Process Vehicle Toward Ring-Contact Designs

TL;DR

This paper demonstrates a hybrid-contact process for high-purity germanium detectors, combining lithium-suspension paint and a-Ge/Al contacts, enabling scalable, low-noise, high-voltage operation suitable for rare-event searches.

Contribution

It introduces a novel hybrid-contact fabrication process for planar HPGe detectors, compatible with future ring-contact designs for large-scale rare-event detection.

Findings

Achieved low leakage current at 77 K under kV bias

Demonstrated energy resolutions of 1.57 keV at 59.5 keV and 2.57 keV at 662 keV

Validated the process compatibility with thin-film a-Ge/Al contacts

Abstract

Rare-event searches including dark matter, coherent elastic neutrino--nucleus scattering (CE$\nu$NS), and neutrinoless double-beta decay (0$\nu\beta\beta$) require high-purity germanium (HPGe) detectors with ultralow noise, stable backgrounds, and electrode geometries that can scale to larger single-crystal masses. Ring-contact (ring-and-groove) designs address scalability by shaping the electric field to preserve low-capacitance readout, but their nonplanar topology motivates a lithium-contact process that is compatible with conformal deposition and robust high-voltage operation. As a process demonstration toward future ring-contact prototypes, we fabricate and characterize a hybrid-contact planar HPGe device, KL01. Here, ``hybrid'' denotes an $n^{+}$ contact formed by an in-house lithium-suspension paint followed by controlled thermal diffusion, combined with an AJA-developed a-Ge/Al $p^{+}$ contact and a-Ge sidewall passivation. At 77~K the device exhibits pA-scale leakage current under kV bias, a depletion plateau near $V_{\mathrm{dep}}\approx 1300$~V, and energy resolutions of 1.57~keV FWHM at 59.5~keV and 2.57~keV FWHM at 662~keV. These results validate the compatibility of the paint-and-diffuse lithium process with thin-film a-Ge/Al contacts and establish a practical fabrication workflow to be extended to ring-and-groove electrodes for next-generation rare-event HPGe modules.