Measuring the Impact Ionization and Charge Trapping Probabilities in SuperCDMS HVeV Phonon Sensing Detectors

A 0.93 gram $1{\times}1{\times}0.4$ cm$^3$ SuperCDMS silicon HVeV detector operated at 30 mK was illuminated by 1.91 eV photons using a room temperature pulsed laser coupled to the cryostat via fiber optic. The detector's response under a variety of specific operating conditions was used to study the detector leakage current, charge trapping and impact ionization in the high-purity Si substrate. The measured probabilities for a charge carrier in the detector to undergo charge trapping (0.713 $\pm$ 0.093%) or cause impact ionization (1.576 $\pm$ 0.110%) were found to be nearly independent of bias polarity and charge-carrier type (electron or hole) for substrate biases of $\pm$ 140 V.