Fabrication and characterization of high-purity germanium detectors with amorphous germanium contacts

Abstract

Large, high-purity, germanium (HPGe) detectors are needed for neutrinoless doublebeta decay and dark matter experiments. Currently, large (> 4 inches in diameter) HPGe crystals can be grown at the University of South Dakota (USD). We verify that the quality of the grown crystals is sufficient for use in large detectors by fabricating and characterizing smaller HPGe detectors made from those crystals. We report the results from eight detectors fabricated over six months using crystals grown at USD. Amorphous germanium (a-Ge) contacts are used for blocking both electrons and holes. Two types of geometry were used to fabricate HPGe detectors. As a result, the fabrication process of small planar detectors at USD is discussed in great detail. The impact of the procedure and geometry on the detector performance was analyzed for eight detectors. We characterized the detectors by measuring the leakage current, capacitance, and energy resolution at 662 keV with a Cs-137 source. Four detectors show good performance, which indicates that crystals grown at USD are suitable for making HPGe detectors.

Characterization of high-purity germanium detectors with amorphous germanium contacts in cryogenic liquids

“…Their leakage current measurement results were shown together with those mea- The side surface leakage currents of USD-R02 were typically higher than its bulk leakage currents through the central contact around operational voltages in both environments. These results are consistent with more thorough investigations done in vacuum at USD with more sample detectors [38,39], that is, the performance of the detectors made at USD has yet to be improved to match that of the detectors made at LBNL by Mark Amman, in particular, the quality of the side surface. Nevertheless, the performance of USD-8-4-15 in both cryogenic liquid is very encouraging.…”

“…Detector USD-8-4-15 was operated in LAr once. Below 800 V, the leakage current was below 1 pA. Its significantly lower leakage current is a clear evidence that the quality of the amorphous germanium surface made at LBNL [35,46] was better than that made at USD [38,39]. The quick rise of the leakage current above 800 V was due to damage to the detector when it fell from the PTFE stage during the preparation of the fifth thermal cycle in LN 2 .…”

“…3. The fabrication procedure at USD was almost identical to the one used at LBNL [38], with only minor adjustments to accommodate for different devices.…”

“…A dozen mini planar detectors have been fabricated at the University of South Dakota (USD) using the technique developed at LBNL and HPGe crystals produced at the USD crystal pulling facility [38]. They perform well in a traditional vacuum cryostat [39].…”

Investigation of amorphous germanium contact properties with planar detectors made from USD-grown germanium crystals

“…This recipe consisted of depositing the a-Ge with a 14 mTorr pressure of Ar with 7% H 2 gas mixture at a power of 100 W. The thickness of the a-Ge film produced by this recipe is about 300 nm. Please refer to our work [30] for the fabrication details for all detectors fabricated in our lab.…”