Investigation of high purity Si detectors with ohmic contacts at low temperature for dark matter searches

The development of dark matter ionization/thermal hybrid detectors using silicon requires substrate material in which good charge collection can be achieved at low applied voltages (typically <3 V) and low temperature (typically <300 mK). Presented are results from studies of the effect of material purity on the charge collection efficiency and sensitive volume of Si ionization detectors, fabricated with low resistance contacts and operated at low temperature and bias. Samples of float-zone hypercube Si with four-point room temperature resistivities of 6, 15, 90, and 140 kΩ-cm were used. At applied voltages below 3 V improved detector characteristics were observed with the higher resistivity material. In particular, at 1 V bias a detector of 0.5-mm thickness using 140-kΩ-cm material achieved a factor of two times greater charge collection than a similar detector using 6-Ω-cm material