Observation of Impact Ionization of Shallow States in Sub-Kelvin, High-Purity Germanium

Abstract

We report on the observation of impact ionization processes involving shallow impurity states in a sub-Kelvin, high-purity n-type germanium detector similar to those used by direct detection dark matter experiments such as the Cryogenic Dark Matter Search. An optical fiber is used to generate packets of charge carriers near one surface of the detector. The charge carriers drift to the opposite surface by application of an electric field. The resulting drift current is measured by a high-speed charge amplifier. The onset of impact ionization for both electron and hole transport is clearly observed in the drift current as the applied electric field is increased above $\approx$5 V/cm. We present the effective charge collection efficiency and trap** length as a function of applied electric field for electrons and holes. We estimate the impact ionization cross section to be on the order of $5\times 10^{-13}\,\mathrm {cm}^2$.