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

TL;DR

This work demonstrates a physics-driven, scalable fabrication path for large-area HPGe detectors by combining a Li-painted, diffusion-driven n+ backside contact with a thin amorphous-Ge/Al blocking top contact and a-Ge sidewall passivation in a planar KL01 device. The measured depletion at ~1.3 kV, pA-level leakage at 77 K, and energy resolutions of 1.57 keV at 59.5 keV and 2.57 keV at 662 keV validate the viability of the hybrid Li/a-Ge approach for low-noise spectroscopy and robust HV operation, while highlighting the near-contact tailing associated with the Li-diffused inactive/transition layer. A quantitative comparison with a fully active bipolar detector shows expected improvements in peak sharpness for the bipolar case but confirms the practical advantages of Li-diffused contacts for large-mass, low-capacitance modules due to HV robustness and surface-background suppression. The study establishes a repeatable fabrication and metrology workflow, benchmarks diffusion-profile-informed dead-layer characteristics, and outlines a clear path to ring-contact geometries that can scale HPGe detectors toward next-generation rare-event experiments such as LEGEND-1000.

Abstract

Rare-event searches including dark matter, coherent elastic neutrino--nucleus scattering (CE$ν$NS), and neutrinoless double-beta decay (0$νββ$) 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.

Figures (13)

• Figure 1: Simplified fabrication process-flow diagram for the KL01 hybrid-contact planar HPGe detector. The detailed procedures for each stage are described in Secs. 3.4--3.9.
• Figure 2: Cross-sectional schematic of the hybrid planar HPGe detector.
• Figure 3: Simulated electric-field magnitude and field lines for the realized hybrid planar HPGe detector geometry at an applied bias of 1600 V, obtained with SolidStateDetectors.jl (SSD.jl) Abt2021_JINST_SSD.
• Figure 4: Lithium-in-oil preparation stages. Panel (a) shows the lithium-in-oil mixture in a ceramic crucible during ultrasonication, and panel (b) shows the concentrated lithium-in-oil suspension transferred to an amber glass bottle for storage prior to use.
• Figure 5: Evolution of the detector bottom surface during the lithium-paint diffusion process: (a) detector on a graphite plate inside the glovebox before lithium deposition; (b) detector after application of lithium paint; (c) detector after diffusion and cooling with uniform brick-red appearance.