A measurement of electron and hole drift velocities in a germanium <100> CDMS detector, at a temperature of 31 millikelvin

Abstract

The Cryogenic Dark Matter Search (CDMS) uses utoapure germanium detectors at a temperature of approximately 40 mK. These detectors are maintained in a state where shallow impurities are well neutralized. Free electrons and holes, generated by particle events, are "hot" and remain out of equilibrium in the bulk. In this system, carrier drift and diffusion occur simultaneously with the generation of free carriers, and with recombination to localized bulk and surface states. These processes jointly affect the space charge distribution in the bulk and at the surfaces of our detectors. The impact of space charge on the electric field likely accounts for much of the phenomenology found in our detectors. Any ability to predict the evolution of the space charge distribution requires a fundamental understanding of carrier transport processes under our operating conditions. To this end, we have measured carrier drift velocities as a function of electric field in an high-purity CDMS detector of germanium < 100> at a temperature of 31 mK. We adapted CDMS electronics to perform a high-bandwidth measurement of charge propagation times for near-surface events in our detector under test. We fmd that measured velocities coincide reasonably well with predictions made in our previous theory treatment. This indicates that carrier mobility is limited solely by phonon emission. © 2009 American Institute of Physics.