Development of HPGe detectors with amorphous Ge contacts for rare-event physics searches*

Large high-purity germanium (HPGe) detectors are needed for dark matter and neutrinoless double-beta decay experiments. Currently, large (up to 10 cm in diameter) HPGe crystals with impurity concentration below 5*10$^{\mathrm{10}}$/cm$^{\mathrm{3\thinspace }}$can be grown at the University of South Dakota (USD). The quality of USD-grown crystals has been verified to be sufficient for use in large detectors by converting those crystals into small planar HPGe detectors and characterizing the detector performance. We report the testing results from ten good detectors fabricated so far with amorphous germanium (a-Ge) contacts, which can block both electrons and holes. We investigated an important property of the a-Ge contacts, the energy barrier to charge injection, to optimize the a-Ge contacts and thus minimize the detector leakage current. We also studied the charge trapping processes utilizing nine of our planar detectors to have a better understanding of the crystals we grow at USD.