Characterization of the binding energy of cluster dipole states in an n-type germanium detector operated at cryogenic temperature for low-mass dark matter searches.*

We have studied the application of n-type germanium detectors operated at 5.2 K for low-mass dark matter searches.  The characterization of the detector is through measuring the binding energy of cluster dipole states.  The detector is operated in two different modes: the first method is to deplete the detector at 77k and then decrease the temperature to 5.2K and the second method is directly to decrease the detector temperature to 5.2 k and then apply different bias voltages. It is found that the binding energy of cluster dipole states is lower in the second method than in the first method. It indicates the different charge states, to begin with in the detector. The binding energy of the trapped charge carriers in cluster dipole states is found in the range of ~5-8 meV. Such small binding energy allows the charge to be released by absorbing very small energy from low-mass dark matter interacting with Ge atoms.