Transport and Two-Species Capture of Electrons and Holes in Ultrapure Germanium at MilliKelvin Temperature

This work describes the transport and capture processes of electrons and holes in ultrapure germanium \(\langle 100 \rangle \) at low temperature and low electric field. Dynamic space-charge effects are responsible for the most significant systematics of the Cryogenic Dark Matter Search. To understand the relationship of transport dynamics to space charge evolution, it is important that we understand charge carrier transport under these nonequilibrium operating conditions. Here, we present measured data and a consistent model of electron and hole dynamics and capture. A charged and a neutral capture probability for both electrons and holes appears to explain the multiple, field-dependent power laws observed in data. This offers a quantitative understanding why germanium crystals operating under these conditions should “neutralize” when grounded, but accrue space charge while under bias.