Geometry Evolution of HPGe Detectors with Amorphous Ge Dual Blocking Contacts: From Planar to Inverted-Coaxial Toward Ring-Contact Designs using USD home-grown HPGe crystals.

High-purity Germanium (HPGe) detectors with amorphous Germanium (a-Ge) dual-blocking contacts were fabricated and characterized as part of a geometry-evolution study from planar to inverted-coaxial designs toward future ring-contact configurations. Four detectors: square planar (SAP12), planar point contact (SAP14), cylindrical point contact (SAP15), and inverted-coaxial point contact (ICPC, SAP17), were fabricated using home-grown, zone-refined crystals (~3 x10¹⁰ cm^-3 impurities). The a-Ge layers were RF-sputtered in 7 %H₂ environment and overcoated with Aluminum at 100% Argon, forming bipolar blocking contacts. The ICPC detector exhibited ~4.62 pA at 500 V with ~ 0.50 pF capacitance, while the planar detector achieved ~1.06 pA at 1600V leakage and required ~1300 V for full depletion. Arrhenius analysis of ln(I/T²) versus 1/T indicated thermally activated conduction with an effective barrier near 0.1 eV that increased slightly with bias, consistent with weak field-assisted emission through the a-Ge layer. Hall-effect data confirmed high hole mobility and validated detector-grade purity. The C-V results and electrostatic simulations in ICPC detector revealed partial depletion near the coaxial edge even at 500 V, motivating geometry optimization for different bore sizes and toward ring-contact designs for uniform electric fields. All detectors tested for gamma-ray spectroscopy using ²⁴¹Am (59.5 keV) and ¹³⁷Cs (662 keV) demonstrated excellent charge collection. Angular-response measurements showed enhanced efficiency for 59.5 keV energy at 30-40º from the coaxial axis, while the 662 keV response remained nearly constant, confirming the ICPC detector’s directional sensitivity and low-noise performance. These detectors are therefore particularly suitable for rare-event searches such as dark matter and neutrinoless double-beta decay experiments, where ultra-low electronic noise and high energy resolution.