Systematic study of massive germanium pin diode detectors at 20 mK

To investigate the properties and improve the performances of bolometers with simultaneous measurement of ionization for dark matter search, we have realized 6 g and 70 g germanium PIN diodes with thin (35-80 nm) B and P implanted faces on 4 and 10 mm thick ultrapure germanium plates. The faces were characterized by LT sheet resistance measurements, ellipsometry, SIMS and RES, and the bulk by carriers lifetime and capacitance measurements. We found a diode behaviour at 20 mK and 4.2 K. We made a systematic study of the response to photons from 5 to 105 keV at 0.02, 0.08 and 4.2 K, for polarization voltages from 0 to 10 V. Our main purpose was to study the slow time evolutions of the pulse heights which characterize the behaviour of such detectors at very low temperatures. We obtained a threshold of 4 keV, a resolution of 0.76 keV (rms), and a stability better than 1 hour, with a polarization voltage V = 0.2 V on a 4 mm thick diode. Similar resolutions were obtained with a 10 mm thick diode, but the time stability was worse. We found that most of the carriers are drifting across the 4 mm diode for V = 0. A possible contribution of a built-in potential to the pulses time evolution is evoked. We also studied the incomplete carriers collection close to the edges.