Charge Transport Asymmetry in Cryogenic High Purity Germanium

The SuperCDMS experiment relies on detection of free charges generated in high purity germanium (HPGe) crystals by particle interactions. To better understand long-term trapping effects which make carriers unavailable for such rapid ionization measurements, we used infrared LEDs (\(\lambda \) = 940 nm) to create electron-hole pairs near each face of a HPGe SuperCDMS detector operated under applied electric field at 400 mK. By alternating the polarity of an applied electric field, we were able to study propagation of each carrier through the crystal separately. Asymmetry in the resulting current transients revealed differences in trapping characteristics between electrons and holes at these low temperatures.